 Yes, we are live okay. Good evening everybody and I use students and everybody else present here It's a great pleasure to have with us now professor S. Ramakrishnan Who is the director of the top institute of fundamental research Mumbai? And he has kindly agreed to find to take time out of his busy schedule to come and talk to you so this is a special lecture and Professor Ramakrishnan of course needs to know introduction is a very well-known Experimental physicist in the area of convenience matter his main research interests are in the areas of superconductivity and vortices and magnetic fields and we will hear basically low-temperature physics that is what his specialty is and He has been at TFR for more than 40 years now, I think and There's a huge volume of work that he has carried out here with more than 200 publications and Guided many students over many years. So many of the people at various institutes across the country are his students at some point of time. So We are eagerly waiting for professor Ramakrishnan to tell us more. So over to you, professor Ramakrishnan Thank you Anvesh First of all, good evening to everybody. I'm sure you will be tired at the end of the day So what I'm going to tell you is a kind of a story, you know, which Which of course involves the basic research So as you must have seen that TFR is a place where we build the equipment at large scale, small scale to address one, you know, one or two specific problems so I am a condensed matter experimentalist and We have developed a certain world-class system, which I will tell you about to go to very ultra low-temperature To look at one specific problem So instead of first telling you how do we go to ultra low-temperature Let me address the problem which made us to go to this ultra low-temperature domain okay, so See the low-temperature the most interesting Things many things happens at low-temperature. The one of the most interesting things happened When Camerling on a school the liquefied helium He lealy helium. So he He's the first one to liquefied at 4.2 Kelvin at 1908 Immediately he you know those days, you know hundred years ago That is the lowest temperature achieved anywhere in the world and then he decided to Look for properties of materials at low-temperature so to his surprise he found this Mercury which is the found to be really going to a Resistant less state at 4.2 Kelvin which we call which he called as superconductivity So this is what the basis of people You know to go to very lower and lower temperatures because those tiny interaction which leads to such a macroscopic phenomena Will not you know it didn't happen at higher temperatures so that is the that is the important aspect of Any low-temperature on ultra low-temperature resist so you want to get into a temperature field regime where nobody has gone before and See whether those tiny interactions are there give rise to new interesting physical properties So so what do we do? So many times the novel phenomena occurs at temperatures only at low temperatures The perfect example is superconductivity Once this phenomena is known in mercury for instance, then you know It's a kind of a metallurgy and some intuitive idea To choose Multiple elements to raise this phenomena which was happening at 4.2 Kelvin to at higher temperatures Where it is can be of some practical importance so as I told you earlier mercury was superconducting at 4.2 Kelvin and in 1911 then This high-temperature superconductor based on mercury and barium calcium copper oxide was superconducting at 140 Kelvin in 1993, which is at ambient pressure very recently this year they squeezed the Hydrogen sulfide coupled with the carbon. It's it is called CH8X material at 2.7 megabar. It's a very high pressure and They achieve superconductivity at 288 Kelvin already. It is at 15 degrees centigrade So you don't have to go to lower temperature to look at this So what is the methodology you go to the low temperatures find a phenomena user? user You know your ideas of judicious choice of alloys then possibly the same phenomena could happen at higher temperatures of course You know this is at a pressure such a high pressure This will not have any practical use but then that doesn't deter people to try and see Whether we can achieve the holy grail is the room temperature superconductivity. We are far from it right now so so the The important aspect of superconductivity, you know when camberling owners discovered this superconductivity He presented to the the big society at large in the in the Lightened community. So one of them asked what is the use of going to such low temperatures? So camberling owners typical answer is only time will tell So then you see now you see an mri You know magnetic resonance imaging which is a in every major hospital you have this to you know As a medical diagnosis This is the magnetic field is provided by a superconducting magnet Then you have power generators transportation, you know this mega layer train which goes at 500 kilometer hour It's also based on superconductivity quantum computing medical. So the application of superconductivity has become widespread But you know if All of them requires one drawback is that low temperature. So if one can find a material at room temperature That is the holy grail of superconductivity. So many of the researchers are still not given up And there are people who are squeezing the material and getting a room temperature superconductivity There are people who are looking at superconductivity with a different mechanism which can raise this temperature To a practical way whether things can be used. So this game is on For the last hundred years or so So what is where are we coming? So we wanted to know That you know once the superconductivity was discovered in mercury The race was on to test the superconductivity of almost all the elements of the periodic table So then we see what you see in the green Are the ones which are superconducting at ambient pressure So it varies from let's say 9.2 kelvin to naobium to 350 micro kelvin for rhodium and the ones in yellow Are either superconducting under only under pressure or in some thin film form Not in the normal ambient conditions So you see that superconductivity is a widespread phenomena. I mean it is you know Many of the metals when you cool down Probably the ultimate ground state is Superconductor There are notable exceptions like magnesium sodium potassium also copper gold silver and so on Which refused to become superconductor. I will come to this little later in my talk In this case the bismuth is a really curious Curious element See unlike most metals the carrier concentration the electron concentration is about 10 to the power of 22 electrons per centimeter cube bismuth Is a semi-metal with the equal number of electrons and holes and it has five orders of magnitude smaller electron concentration So our aim is to see whether Does bismuth superconduct if it's superconductor What is the mechanism for superconductivity? So that is what our aim is to start so With this in mind we decided to Look for superconductivity in bismuth, which I will explain to you. Why is it so important? Okay, before that So as I as I told you that once the mercury started superconducting people looked at all the elements of the periodic table to see superconductivity some found success but as the TC the superconducting transition temperature goes lower and lower things became a little difficult For instance, you know superconducting iridium, which is an element. It's at 113 millikelvin Then tungsten, which is at 15 millikelvin Barely because of its toxicity We couldn't purify later found to be superconductor in 1967 at 27 millikelvin Then it took a long time, you know from beryllium to rhodium. It was almost 16 years The the cooling technique got better The what is known as dilution refrigerator, which I will explain to you came into existence Then people went up to 50 micro kelvin to see the superconductivity of rhodium Then it took another 24 years to find the superconductivity in lithium So what we have found out for this superconductivity this month after nine years So so that is the trajectory in which the superconductivity of pure elements have been found out So the people involved this is already published some some time back in science and then It's going to come in nature physics soon I think next year. So the people who are involved are Om Prakash who did his PhD thesis on this and then we have an excellent scientific staff Anil Kumar and then the single crystals are grown in professor Tamil Vail's lab I think professor Tamil Vail must have given you a lecture or it's going to come soon. I think on crystal growth So what I'm going to tell you is briefly on the theory and tell you that why this theory is not useful for BISMA. So the As I told you that superconductivity in periodic table Most of them can be explained by what is known as Biden, Cooper and Schriefer's theories called the DCS theory Happened in 1957 for which they got the Nobel Prize in 1972 So what they did was they took the idea of frolic Who said that see what is a metal? Metal you have a periodic arrangement of atoms and then you have a conditional electrons moving around So when electron passes through this positive ions which are neatly arranged on a lattice There is a lattice distortion very tiny lattice distortion. What I've shown you is an enlarged version of it So as the conditional electron moves through the lattice The positive charge cloud accompanies these electron You know because there is a local distortion the positive ions get attracted to these And then so you have a positive charge So this electron passes away And the next electron Sees this positive charge and then gets attracted to it So in some sense this electron gets attracted to the other one. Why are this? mediated what we call as phonon interaction because it's a quantized lattice vibration So directly they won't get attracted because you know like charges will repel This is a cool home repulsion is coming to the picture, but indirect way they get attracted It's called that's why this electron electron interaction is called. It's retarded in time You know one electron goes there is a positive charge cloud created the other one comes in gets attracted to it So this is okay If you if your lattice vibrations are very very Small compared to the velocity of the electrons. See the ions have to move slowly Compared to the electrons which happens in most of the metals why The fermi energy of the most of the metals are in electron volts Whereas the thermal energy, which is the debi energy of the atoms are of the order of middle electron volts So the condition of bcs theory is well met. You know, it's a theory of adiabatic adiabaticity So the electrons move faster the ions move so slowly that they can be an electron electron interaction This is working correctly for almost all the metals, but not in the case of this much This much you will see because the carrier concentrations are small The fermi energy is much much smaller than the lattice energy So the bcs theory if bismuth is formed to be superconducting Cannot explain this it is not bent for bismuth. We will look into this a little more So this is the basic thing. So what do we what is a metal? So in metal at absolute zero you have a well defined fermi surface in the reciprocal space So the electrons have energy energy states Which fills this fermi sphere and at t is equal to zero. There's nothing after this fermi This fermi sphere. So what happens when you have a so the energy is a It's a function of density of the electrons And these are constants blank constants as well as the mass of the electron the to the power of two third So if the n is small the e f gets smaller Since it is for metals It is the of the order of 10 to the power of n is of the order of 10 to the power of 22 So you have e f is of the order of 10 9 10 electron volts So you will see that for bismuth. It is 9.5 electron volts for tin It is 11.7 electron volts aluminium. It is 10.2. You see this month Bismuth is only 25 milli electron volts almost room temperature, right room temperature is something like 26 milli electron volts So it's extremely small compared to all the other other metals so What happens? How do we say that? You know, what is responsible for superconductivity? So that we can see, you know, this small plot energy as a function of density of states density of state is the number of states available in a unit energy And you can see that the one which is shaded is the what happens in absolute zero So when you have a finite temperature of kbt Then the the the electrons move from the region one to region two And the electrons which is close to this fermi available are the ones which is responsible for all its electronic properties including superconductivity So how does one get superconductivity out of these conditional electrons which are close to the fermi level? So the person who tried to do that was Leon Cooper So what he asked he asked a question? Suppose you have conditional electrons Which are just outside this fermi surface And they have an attractive interaction Just like this attractive interaction. I told you about so what happens to the system He showed that if such things happen Then the fermi surface is unstable. That is if you have an electron which is spin up you know at Bound to a spin down electrons with a different momentum And this fermi sphere is unstable with respect to because energetically it is better to form this Pass as compared to simply filling up this energy levels of the Fermi sphere So what happens is that then The Bardin and Schriefer took this idea and showed that And showed that you know, if you look at a thin window of energy scale Close to this fermi level. I have just very much exaggerated that It's cross omegas 2 is tiny compared to the ek and ek2 So in a tiny shell We can have an attractive interaction and it is zero otherwise If you assume such a small window, which is responsible for superconductivity You can find an expression that is the superconducting tc is related to the divide temperature density of states at the fermi level and this is the interaction between the two electrons, which are just outside the close to the fermi surface and You can also then have an energy gap between the ground state and the Exited state which is called delta, which is the universal relation And the density of state is related to the electron cone on coupling constant Instead of a simple Coulomb parameter now we are talking about not two electrons large number of electrons So they are screened. So that is why it's called mu star. It's called the screen Coulomb parameter So forget about this equation right now. Just apply this equation to the system which we have so Let's say it's sodium We know that the debate temperature of sodium is 153 kelvin. So the lambda is 0.16 So if you plug in these values into that expression, which I gave you you get one nano kelvin for tc Of course, nobody is interested in sodium. So nobody has able to hold it To such a low temperature to see whether it is superconducting or not aluminum The tc given by bcs is 2.5. The tc observed is 1.2 k So it is not very surprising the important thing people many people miss out Is that bcs theory never predicts the correct temperature bcs theory tells you What is the mechanism which leads to superconductivity? So people blame the bcs theory because they thought that it should accurately predict the tc You know because see each material has its own Fermi surface all other complications coming in which is not being considered by bcs. They just put the You know the interaction as minus v. That's all and it is zero elsewhere. It's a very simple assumption The idea of the beauty of the bcs theory is to show That how the attractive mechanism comes between the electrons and how it is coming how the superconductivity is produced They were never said that you should take this as a God-given formula and try to get the dc out of it but remarkable even with such a Drastic assumptions. We are not very far, you know 10.2 theory Experiment says 7.2 magnesium of course is a question where People predict 30 milli k. We have gone down to 100 micro Kelvin. We don't see anything This myth is the other way around The theory predicts it should be at 0.2 nano Kelvin Whereas the experiment I will show you it's that 500 micro k 0.5 milli Kelvin. So it's Ulta here. That is because the theory is not applicable here So you can't use the same theory for a case where you know things are not applicable I told you right in bismuth you have a lattice energy which is comparable to the fermi energy So the adiabatic approximation that you know the ions will relax And then the electrons come in and get attract that won't happen that will not happen. So the basic assumption of the pcs is gone So So we come back to the bismuth again So as I told you that you know, we have bismuth is five orders of magnitude in carrier concentration Less compared to the normal metals So what is so special about bismuth? Actually, if you look at the periodic table, bismuth is the one which is studied for a long time and still being studied You know, it's more than 100 what 100 Let me just go back And you know, it's almost now what? 250 years or 220 years or so. We still There are still interesting aspects of bismuth people are looking at it. We started with dam activism Attributed to faraday in 1945 Then you have c-back effect nerds effect capitals All these things are textbook cases, you know, all the things see the c-back effect was first discovered and then c-back put a put for the theory Same is the case with the nerds. So all these things came because of bismuth so and around 2000 2002 there was a lull because people thought that they have You know discovered what is about bismuth all all are over then came with the vengeance in the last 15 years There are very very new aspects of bismuth including, you know Topological properties and so on and so forth. It's it's it's gone into a different Dimensions now it's still this much is being pursued in different aspects in thin films and so on The most interesting thing about bismuth is came from our high energy colleagues You know, they found a this much which was used for medicine, you know for treating stomach disorder and so on They found alpha radioactivity which is at 3.1 m e v and Many a good thing about alpha activity is that it dies before it comes out of the Bismuth atom. It's such a short range But then the half life is about 10 to the power of 19 years, you know, which came as an age paper some 18 years ago So this is billion times the age of the universe So you can safely say that this much is stable and then move away from that point of view But it's an interesting twist which came several years ago So what is again special about this month? So I told you that Unlike the other metals the carry where the carrier concentration is flat, you know There is no temperature dependence of the carrier concentration Bismuth is really fun. It still has a temperature dependence of carrier concentration as you go from room temperature to the Low temperature it goes by a factor of almost 10 This result has been a beautiful result has been there for the last 50 years Still, there's no decent theory which tries to explain why the carrier concentration should go down in Bismuth And as I said the low carrier density And low Fermi energy Which means its mean free path is extremely and it has a lower effective mass. So that means Because of the carrier concentration is small it has a very large mean free path even at room temperature It's about two micrometers and it can go to a few millimeters at two kelvin So imagine if you have a Completely large mean free path system like say five millimeters in Bismuth Which means the electrons are ballistic electrons are not having any collision in that material So to measure transport and so on it is a huge huge task Because most of the contribution is coming from the boundary scattering not inside the material So it's it's it's it is a really difficult to determine its absolute resistance of the system The intrinsic boundary resistance you can compute So that is another puzzle. I mean that that is another interesting thing about Bismuth Silicon of course has also has a carrier concentration difference But that is because it is being doped by other elements. So that is the reason why Okay, so little more on the atomic physics So what is Bismuth? Let's look at its Outracell configuration. It is 6x to 6p3 so h and p levels are brown here. So if you bring two Bismuth atom Actually, you should have seen an insulator with a cubic structure You know the electrons get paired nicely the p level as well as level nothing is there no And you know no unfilled shells and so on. So there should not be any any Conductivity, but that doesn't happen in nature. It undergoes a small lattice distortion Instead of cubic it becomes a Robohedral And the important thing is that at two different symmetry points the Bottom of the conduction band You know it is it is lower than the top of this valence band So if there is a bond, you know energy overlap if you consider these two these two These two Valence and conduction band and that is the important criteria of Bismuth the unusual properties Stems from this fact that you know the conditional electron is lower than the valence band in this system Yeah, there are a few basic questions. Maybe you can hear them. Yeah. Yeah, so the first one is what do you mean by carrier concentration? Yeah, so see the metals There are some things which conducts the heat as well as electrical conductivity which conducts that The electrons close to the familial are the ones which conducts this So the carrier concentration of you know per unit volume the number of electrons per unit volume is what is known as a carrier concentration So in metals electrons are the carrier concentration in semiconductors You will have both metals as well as holes in semi metals. It's the same thing. You will have Electrons and holes holes is a Excitation where the electrons leave the place and you have a positive charge So they are there in semi metals semi conductors and so on So metals we are talking about electrons the carrier concentration means number of carriers per Centimeter cube per unit volume Okay, thank you. There are a few questions, which I think you are in the process of answering but I will still ask them So several students are asking that what theory explains the behavior of this mark. I think you are in the middle of it perhaps I think you are in the middle of it perhaps. Yeah, that is that they have to wait till the end There is not a one single theory is there right now. There are many Proposals the jury is not out yet on the the right theory for this But I think we can zero into one of the theories. I will come to that later Okay, okay. So you have to wait till the end of my talk. Yeah There is also a basic question that how do we precisely measure such low temperatures? Correct. I'm coming to that. See, I have not told you about how to reach and how to measure So you have to wait the later part of my talk. So what I'm setting up is the physics problem Now I want to set up an experiment to solve this physics problem So I'm just telling you what is so special about bismuth Then we will cool this bismuth and show you with the what kind of cooling apparatus We have how we measure the temperature and get to the superconductivity of this month So that is the plan of my talk Yeah, so I'll let you continue some other questions we can take at the end again So as I said bismuth should have been an insulator But because of this lattice distortion something similar to the piles distortion, which you will see You have this Semi-metal behavior, you know unusual band structure and semi-metal behavior So what is bismuth? Is it a metal or a semiconductor or an insulator? So the the best way to check is what is known as Mott's limit This was Proved by sir Neville Mott late civil mod who was also a noble orate who got the Nobel Prize for this discovery Of metal insulator transition. So what he said was if take a star a star is the It's something similar to the Bohr's radius in the system a naught is the Bohr's radius Epsilon r is the effective dielectric constant m naught is the effective mass of the electrons in that system and me is the Electron effect the electron effective mass So if a star to the power of n by 3 is greater than 0.26, then he says that that is a metal So it turns out that this empirical rule Which seems to satisfy a host of oxides Intermetallic compounds all kinds of things Which can distinguish between metal Semiconductor and an insulator So you apply for bismuth By plugging the values of a bismuth you see that, you know, it's of the order of 34. And so we are still Safe it is it bismuth can be considered as a metal In this case it is semi-metal because we have both electrons and holes, but the important thing is that unlike the normal metals Where each atom share one electrons In bismuth one conduction electron is shared by really 100 000 atoms That is because its carrier concentration is only 10 to the power of 17 five orders of magnitude smaller So this is the energy level diagram, which I showed you before, you know, there are different There are different energies for Different Fermi surface Fermi energies one Fermi energy for electrons Fermi energy for heavy holes and light holes, which are there in the system Forget this for a moment right now All I want to tell you is that the Fermi energy is extremely small It's about 25 milli electron volt as you can see from there Which is comparable to its lattice energy So bismuth since you didn't study it for 100 years everything is known Its Fermi surface is known and it's so happened that it has a very tiny Fermi surface You see this small blobs, which I am pointing out with my thing See that these are hole packets extremely small and these are electron pockets There are three electron pockets and one hole pocket Which occupies only just, you know, 10 ppm of this Berloin source So it's a tiny Fermi surface And I'm going to show you that such a tiny Fermi surface Both electrons and hole together is what is responsible for superconductivity Yeah Hello Okay, so now let me go to the practical points So bismuth can be grown very easily You know, it can be grown as in air by just pulling the things that in the in the heat You have a small furnace and you have a small pulley with a motorized pulley You can just pull it out slowly In fact, you will see nice crystals with the shining colors and so on Which is basically an oxide, which gives you an interesting colors So but for our experiment, we need 69 pure bismuth, that is 99.999% pure bismuth So it has to be grown in VACCO And that is what down in Professor Thammeljee's lab where we grow these crystals And it has a rhombohedral structure as I said Its unit cell is a rhombohedral with 4.75 angstrom And it is distorted rhombohedral structure I told you this distortion is coming from piles like Distortion in the system And you can do an x-ray diffraction to see confirm its structure Quality of the crystal, you know, by x-ray backscattering measurements So what has been done in bismuth before we entered Bismuth under pressure is superconducting That is everybody knows and this result has been there for a long time You know, 3.9 Kelvin at 26 kilobytes It has a monoclinic structure 7.2K it goes to tetragonal And then it goes to cubic at very high pressures But all this and our amorphous bismuth is superconducting at 6 Kelvin Granular or nanoparticles is between 1 and 2 Kelvin In all cases, you know, all the 3 cases The carrier concentration changes You are not no longer dealing with the virgin bismuth See the structure changes Its formula will change Its carrier concentration changes When you do things like this And most of these things can be explained by BCS theory Or variation of it But no superconductivity in bismuth was done down to 10 Kelvin People search for it It's not that they didn't search for it They search for it down to 10 Kelvin And they thought that since the carrier concentration is very low Bismuth is never going to become a superconductor So we decided to test this hypothesis By doing the experiment Because that is the best way to do it So now I am coming to this low temperature part So how does one cool systems? So typically you have two isotopes Helium-4 and Helium-3 And by pumping over Helium-4 You can get down to 1.2 Kelvin And cooling with Helium-3 By pumping over this you can get down to about 0.3 Kelvin But if you want to go well below that You have to do what is known as the dilution refrigerator Which is commercially available And if you want to go further down Below 5 mil Kelvin Then you have to build your system So we built a nuclear demagnization fridge in TFR Which is capable of reaching 40 micro Kelvin For the electrons and lattice Important thing is that we can keep this 40 micro Kelvin For nearly 3 days 72 hours So now My roadmap is explain you the dilution refrigerator How it works Which is commercially available Then tell you about what we do in adiabatic demagnization So it's not enough to produce this temperature As somebody asked me How do you measure this temperature? So that is another important aspect which you will touch upon Then I will tell you about the superconductivity of this month So as I told you Pumping over Helium-4 Or Helium-3 You can reach to 1.2 K Or 1.3 Kelvin You can't do much below that Because the vapor pressure goes down Tremendously down You can see that For Helium-4 Even at something like 0.55 Kelvin The vapor pressure is so small That you know it's very difficult to go below 1.2 K Because there is a super fluidity comes in And the film creeps in So the practical limit is 1.2 K For Helium-3 You can get down to about 0.3 Kelvin And that's all Because the vapor pressure is so small That you know Any cooling you can achieve It is compensated by the heat leak into the system So all practical purpose These are the two limiting factors on this You can also write a simple Classics Crapron equation You know if you look at Heat and Thermodynamics by Zimanski It gives you all this nicely And tells you how the pressure is related to the Latent heat and temperature So if you want to go below 1.3 Kelvin You need to know what is known as a dilution refrigerator This was proposed by a theorist Called Heinz London in 1950 Then it took about 14 years And it was built first in Camerlingon's laboratory to show that Such a dilution refrigerator can be made So what is the basic principle? So I told you an interesting phenomena Which occurs at low temperature Is superconductivity There's another interesting phenomena Which happens for Helium You cool Helium below 2.1 Kelvin It becomes a super fluid I'm talking about Helium-4 It becomes a super fluid What do you mean by super fluid? It has no viscosity It can flow through anything So it's a quantum liquid And it has its own Very interesting properties So now if you put Helium-3 In that mixture See Helium-3 is a normal fluid At these temperatures So when you put Helium-3 The lambda point The transition to super fluid state Continuously At this concentration Something like 0.75 You see that It splits into two phases One is rich in Helium-3 here The other is poor in Helium-3 So it so happened That you can use this To Do Produce low temperature Basically because the energetic Concentration Helium-3 Dilute phase Because the density of Helium-4 is larger So it just floats on it So how do we understand this cooling? Simple diagram is that In the evaporation cooling what you do You take Helium-3 You pump over the bath So you know the classes Capron equation Tells you that you know Evaporation to reduce pressure You can get lower temperatures So the hot gas is again put back And then you can have a continuous Helium-3 Here In Helium-3, Helium-4 mixture The energetic particle is Helium-3 So you are going to Take away this energetic particle Across this surface You know this red blue surface You have to take it across this By a suitable pump Which is operating at room temperature So unfortunately at these pressures No pump can operate And pull this Helium-3 across this Interface So what do we do Is So this is the heat of mixing So when you pull this Helium-3 Energy to Helium-3 atoms across The surface you can achieve cooling So that's why as opposed To evaporation cooling This is a dilution cooling Because you are diluting Helium-3 From this rich phase Across this mixture This is here it is diluted Helium-3 So that's why the name So what is the pump which we use So the pump which we use Is a ingenious pump In a sense that we have what is known as Still Still came from distilling So what you do here You keep this at higher temperatures Like 0.7 Kelvin Here the vapor pressure of Helium-3 Is As you know vapor pressure of Helium-3 Is much larger than the vapor pressure of Helium-4 So when you connect a pump At room temperature it will predominantly pump The Helium-3 across this mixture So the concentration of Helium-3 Here will be larger than The concentration of Helium-3 Helium-3 here So there is an osmotic pressure The osmotic pressure drives this Helium At the bottom to the middle chamber To the still So then it gets pumped at the room temperature So this circulation And will cause this cooling And you can reach down to about In an ideal Frigerator you can get about 2-3 Helium-3 So the one at TAFR Which was bought in 2005 So that has a Helium-3 dump of 176 liters of gas Which is about quarter liter of liquid And the Helium-4 dump Is about 600 liters of gas Which is about 0.86 liters Of liquid So these are the two Mixtures which we use And then we can reach down to about 5 milliliters in our system So Since because the Covid The lab is open but sometimes When there is this Covid restrictions Are removed I would like you to Come and see What we have set it up Because all this have to be The crusted Helium-3 And the rest Nuclear stage everything we need to build So this is the lab which is at CG-35 Where we have taken care of Electromagnetic radiation Vibrations and so on and so forth So And this is resting on a sand fill You know caught sand fill Pillars to avoid vibration So then as I said The dilution takes down To about 5 milliliters And if we go well below 5 milliliters Then you do an adiabatic Demagnetization So what you do here You are talking about adiabatic Demagnetization of nuclear spins So it is similar to the Vapor cycle refrigeration What you do in vapor cycle refrigeration You take a refrigerant Put a pressure So when you apply a pressure You have a heat And you have to have a medium And then now the heat of compression Is taken it is at temperature And at 5 pressure Release the pressure So it takes away the temperature from the surrounding So you get T-delta T So this is what the vapor pressure refrigeration Similarly You do the same thing for nuclear spins Spins are aligned random At paramagnetic at some temperature Apply a strong feel Align the field along the Magnetic field direction There is a heat of magnetization So use your dilution fridge To take away this heat of magnetization Now you have System at temperature T And at finite Same temperature at some Feel Then you do adiabatically reduce the magnetic Feel And that again takes away the heat from the surrounding So you can have what is known as Magnetic refrigeration So with this It is possible to reach Very low temperatures So what I am showed here is the entropy As a function of nuclear spin temperature So you start at 10 mK In a field of 8 Tesla Take away the heat Produced by this application of 8 Tesla Then do Adiabatic demonetization You reach 8 mT And then at that 8 mT Your temperature goes down to 10 mK So what is the Schematically done So this is your dilution fridge You have a copper foil And then you have a superconducting Switch with the magnetic field And this is your nuclear stage Your sample is attached to this nuclear stage So what you do You cool the system to Let's say 10 mK You magnetize it with 9 Tesla There will be heat of magnetization Keep this switch Closed Heat which is produced here is taken away To the dilution refrigerator Then you close this Open the switch So what is this switch This is again an ingenious thing Because superconductor Though the electrical conductivity Is infinite Its thermal conductivity Will become very very small So you know It will be a good electrical conductor Can be a bad thermal conductor For electrical interpreters So use that as a thermal switch So when you do Demagnization here You make this switch Superconducting So there is no heat flow Practically no heat flow from here to that place So that is achieved By just Switching off the field of this superconducting magnet When you want to thermally connect these two You put on a small field So this becomes normal This aluminium becomes normal So it conducts the heat When it becomes superconducting It cuts off the heat Leak from here to the mixing chamber So you can do a simple Reduce the field slowly And you can do an adipatic demagnization Reach Very low temperature In our case we reach about So this is just to tell you How it is done The thermal conductivity of a superconductor The aluminium It is coming very close to a polymer at low temperatures It is as bad A thermal conductivity as This epibond which is Insulating polymer Basically because The Thermal conduction from the Normal electrons are reduced Because they go into a superconducting state So we will come To this part a little later So a superconductor can be used As an excellent thermal switch And that is what we do In dilution refrigerators So this is the one we have In my laboratory This is the dilution refrigerator So that is commercially available So what we have done is We have used copper as a nuclear refrigerant Copper has a Nuclear spin 3 half So we use About 5 kg of copper Which is attached to this When we cool Nuclear demeritization We cool this 5 kg copper Down to 40 micro Kelvin The whole 5 kg is at 40 micro Kelvin Why we need such a large Weight Basically because we need to retain that heat Remember Room temperature is more than a sun To this small Experimental space Room temperature is 300 K Whereas we are sitting at 40 micro Kelvin Heat leak is inevitable Some heat leak will come to the system So we need to have Large cooling power To that system To withstand small heat leaks Which is coming from there The heat leak from the top to the stage Is estimated to be one Picoat in our system So Even that So that is the reason why we need to cool 5 kg of copper at such Low temperatures So somebody then asked How do you measure these temperatures So the measurement of temperature Depends on the temperature range Which we use So normally at high temperatures Like above 1 Kelvin We have various sensors Resistant sensors Wafer pressure sensors Gas thermometer and so on Then slightly lower temperatures Use this superconducting fix points Standard superconductors who have Different TCs which can be used As a calibration point Then you have a melting curve of helium 3 which can be used So what we use in our experiment Is we use this platinum NMR That is the only thing which can go down To 10 micro Kelvin So you measure the Magnetic susceptibility Nuclear magnetic susceptibility of platinum Which is related to inverse of temperature To get the Lowest temperature which is possible And intermediate range We use the noise thermometer Noise thermometer actually we have Extended down to 0.6 micro Kelvin So the two sensors which we use Are the noise thermometer And the PtNMR So just briefly tell you what the noise thermometer is So this is the Beautiful illustration of non-equilibrium Thermodynamics It is called Nyquist noise Or Johnson's noise Depending on whether you are a theorist Or an experimentalist So the point is that It was discovered Earlier by Johnson And the Theory was provided by Nyquist Who said that At any temperature There is a voltage fluctuation Caused by the thermal agitation In this case electrons In an unbiased conductor Because of the temperature T Where voltage is related To the temperature resistance As well as the frequency range And by measuring This mean square voltage Or the later on the power spectrum And knowing the value of R And the delta F You can always determine The temperature So this works beautifully From 4.2 Kelvin Now we have extended down to about 600 micro Kelvin We can hear this The resistance noise Is pretty small So we use what is known as a Superconducting transformer Coupled to a Superconducting quantum Interferon device I cannot explain to you It is a magnetometer It is very very sensitive to the flux Changes So the resistor noise Is coupled to this inductor It is actually coupled to this quid Which will measure this The tiny Voltage which is developed here So the quid can be Thought of a current transformer It just amplifies the signal At low temperature And then it picks up By the room temperature So what we do in this You measure the power spectrum At a reference temperature And then measure it At a reference temperature So take the ratio of this And usually the T Relative is The fixed point is 4.2K itself So you must the system In 4.2K Then you know you have calibrated this And that works down to about Something like 1 mK So this is how And this is commercially available You can buy this So this is the plot You can fit it to a nice Curve And then you can get the temperature out And this is how the temperature goes down You know when you demagnetize it It goes to about Something like 70 mK 70 mK And then it slowly comes down It takes about 36 to 48 hours To cool down From 70 mK To about 17 mK So if you want to measure Below So as I said all of these Stop said 5 mK Or even 1 mK If you want to measure microkelvin The only option for you is to use the nuclear Suscibility So you know that the suscibility goes as 1 by T to the point T minus theta n Our impractical purpose theta n is very small So it goes as C by Nt So by measuring the suscibility You can measure this temperature By calibrating this with respect To again a noise thermometer So what we do is We do a standard NMR measurements In NMR we apply a steady Feel along the Z axis Then an RF feel along the Y axis And then stop the feel Let it process in the XY plane And you measure the voltage Induce the coil in the Z direction Again you take the two voltages 1 by U2 is Mt1 It's proportional to the magnetization So that is equal to T2 by T1 So by measuring the voltages At the standard temperature With respect to the noise thermometer You calibrate this Then use the NMR System to measure temperature Right from let's say 5 mK to 50 mK So That's what we do To measure the temperature So just to tell you the complexities Involved So what we do here See the thermal switch Takes the new nuclear spins You put the thermal switch off You cool the nuclear stage So by cooling the nuclear stage You are cooling the nuclear spins Adiabatic demeritation cools only the Copper nuclear spins Hyperfine interaction Cools the electrons And the electron phonon interaction And how do we measure the temperature The cool lattice Via electron phonon interaction Cools the electrons The cool electrons via hyperfine interaction Measure the nuclear spins So Here you are using a platinum NMR thermometer To measure Here you are using the copper stage To go down to lower temperatures So this process As you go down lower and lower temperatures You Your measurements time takes a long time Because the thermometer Has to come to equilibrium So at something like 100 mK You can only take Two readings per day 24 hours you can have two data points So When the fridge runs We run it for three to four weeks Higher temperatures of course Then you will have more data points But at the lowest temperature At the backstations everything has to come To the equilibrium temperature You need to wait for a long time So the experiments are not for the faint hearted ones So what we did was We showed that we can cool the system Down to 40 mK Important thing is that We can keep this temperature More than 48 hours for experiments So Once we develop such a facility You want to compare with the best in the world So at present we are third in the list In terms of the lowest temperature Not only that In terms of the Cooling power So The fridge is Available to cool systems Down to such a low temperature So now Let's come back to this month So now we have a Fantastic ultra low temperature system And can we try to see What is the case of this month One of the major problems With the superconductive Discovery is that When you have such a low TC superconductor Its critical field is also going to be small The magnetic field Will destroy the superconductor Which is well known So it turns out that As you go lower and lower In superconductive transition temperature You need to shield it from the earth's magnetic field Because the earth's magnetic field Is you know 40 to 45 micro tesla And that is Good enough to clear the superconductivity Even if you cool down to One micro kelvin If you don't shield the system You are not going to see it So the second task for us Is to shield the earth's magnetic field So luckily There are systems like You know cryo-perms It's a high High new metal shield at low temperatures So you make two of them Then use lead You know lead is a good superconductor It expels the magnetic field So that also will act as a shield So we made a shield Out of this And made sure that the field inside Is less than 4 nano tesla So you know already We have got 1,000 times Or 10,000 times less than the earth's Magnetic field The magnetic field is 40 micro tesla So you need to make a chamber Of such a low Magnetic field environment To test the superconductivity So Ohm built that You know it's almost a zero gas chamber Not only that Then we need to measure the magnetization So we have to again Employ a squid Magnetometer Where the buy the basic magnetometer To assemble all the The The magnetometer part Only the sensor we buy it from outside The rest is all built ours So what we see here is the bismuth With the pickup coil which goes into a spit Then you can Measure its magnetization So as I said one of the Second fundamental property of a superconductor Is that it acts like a diameter So when you have a normal metal Above its transition The field penetrates When you cool it It expels the magnetic flux Inside the field is zero So when you measure the susceptibility As a function of temperature What you see is a kick like this You know it's The diamagnetic susceptibility is almost Very very small And then you see a huge diamagnetic susceptibility Almost it is minus one So this is what the TC is all about So if you do a magnetization experiment With a signal like this So how do we test our system whether it works So you take the lowest Superconducting transitive pressure Element like this rhodium Which was done More than 30 years ago And we took that Rhodium and showed that It is indeed a superconductor At 325 mil micro Kelvin Slightly less than what they have reported Because there is a much purer Sample than compared to ours probably Important thing is that you measure Its critical field as a function Of temperature that matches With this paper which was A land map paper In this superconductivity of rhodium So which means that you know Our macro meter works And so we started Working with this math So you know we took Two crystals and you see That the magnetization is Showing diamagnetism And also this diamagnetism As a function of field gets decreased Lower in course to lower And lower temperatures Then you can see that critical field As a function of temperature The extrapolated value of this critical field Is about 5.2 micro tesla So you can see that you know This is almost one tenth of the earth's Magnetic field So if the earth's magnetic field is present You will never see the superconductivity Of this math There are you know The bcs theory tells you that the ratio Of the critical field to the temperature Should be this And it is you know 10 times lower Than what we also The point is that the bcs theory is not applicable But we just wanted to see Whether what happens with the numbers The numbers also clearly Don't match So what is the superconductivity Here so as I said there are two Approach to the high tc problems One is you try to Look for the same mechanism Electrophone interaction Apply pressure the way people have achieved 288 Kelvin in this year At 2.7 million megawatt Then try to Work with these compounds You know do an alchemy And try to see whether you can get the Superconductivity at reduced pressure Or you go away from the bcs Look for a new mechanism For superconductivity So what happens you have You know the phonons are there The electron phonon interaction is the Hallmark of the bcs superconductivity But there are systems Where The phonons won't work Especially in non adiabatic system I told you that you know If you look at this You know there is The fermi energy is comparable to lattice energy This is where the Dismuth comes in There are systems like strontium titanate Where the omega D is even larger than EF You know so there are some theories Proposed to that And unfortunately what is developed for The strontium doesn't work For the bismuth So what is the What are the theories in the Market There is certain low Energy acoustic plasmons This is what proposed by The late professor As well as And so on There are very tiny excitations Which is there in the system But the spectroscopy measurements Rule out that such a thing doesn't exist In the smut Whereas patrick lee suggests that Sorry patrick lee suggests that There could be Heavy holes in the system Which is responsible for superconductivity And Has this resonating well spawn Theory and so on So is a large From my MSc fluctuating So it's Not clear which of these Is going to explain Because many of them Their predictions don't match With some of the recent experimental Reserves except that this one Because it shows that The Superconductivity seems to come The presence of heavy holes in the system That seems to be closer To the truth as of now As I said it is still Not Confirm but most possible Explanation is the model by patrick lee So To some us summarize I will say that it's an Unconditional superconductor So it has to go beyond the Standard models of superconductivity And As I told you most of the things I told you About is magnetization We didn't do a transport Typically Did the resistance measurement Because that was the easier measurement To do at that time But the transport At 100 micro kelvin Requires completely different techniques Because you can't put a leads there That will conduct heat to the system You will never get the micro kelvin So what you have to do Is by inductive technique Just like temperature we are measuring This has never been done before So we are Trying to see whether we can see The resistance going to zero Where transport So that is a Difficult experiment at these temperatures But we also want to see It's normal state properties Just about tc The theories wanted So we still Are interested to do this transport And Since I am nearing my limits Of Time in My student who did this Work is continuing To do these transport measurements Elsewhere So hopefully in a year's time We will see those results as well Then what about superconductivity in other Semimetals like antimony and arsenic So that has not been Seen before It is just that we need to have a very Pure antimony and arsenic To look for Beginning of a new class of superconductors Which whose mechanism of Superconductivity is certainly not pcs So finally The role to ULT is difficult But there is interesting and enough Physics at the bottom The important question one should ask Who the fate of metals Atleast the non-magnetic metals Atro temperature Do they become superconductor or do they become something else Then there are these Granular superconductors You know if you take platinum is never Superconductor Even down to about 10 micro kelvin But when you make a powdered particle And then compress it And make a small ingot And then there is the superconductor Below 1 millikelvin Still not sure what is causing the Superconductivity Nanomaterials Granular material Superconductivity happens in magnetic system Is still not understood And then you know you are really into the Realum of unknown Temperature range Where as I said There could be tiny interaction Which leads to new Phase transition like the superconductor Atropeochismat There are many things possible Many exciting things possible at such low temperatures And So far I have told you Only You know ultra low temperatures can be done in Two different ways One is this brute force cooling Which we do And our laser colleagues Are more you know Smarter so they use the laser To stop the motion of the atoms And get down to low temperatures But if you want nuclear refrigeration You have no option But to do You know brute force cooling So I stop here and Feel free to ask me any questions Either now or later as well I left my email I.D. so I will have some free time during weekend So I will certainly answer your questions Thank you for your patience Thank you Thank you Professor Ramgashan It's a very nice talk There are some questions already And which I will ask And in the meantime students can Write your questions More questions in the chat So Sorry Some questions are of Basic nature About the theory So maybe those things I will ask first So somebody has asked That what is the original Of the forces between electrons To form bound states Is it some other kind of fundamental force Or part of any fundamental force Yeah so there is no new force here It is just that In the case of BCS Superconductor I explained to you It is the lattice vibration Which is called the phonons are responsible For the bound The binding of two electrons In the system It is not very clear You know One of the theories Which says that These holes Changes the dielectric nature Of the bismuth This has a very large dielectric constant Of the order of 100 So there is There is a Electrostatic disturbance Because of these holes And there is A mechanism which seems to bind These holes Which is not simply phonons But it is coming because of Its self energy correction So this The glue for this Hole binding is still have to be found out Because All this had to come through the experiments You know this pairing In the case of normal BCS Superconductor the pairing came because People found that it is How paired and the charge of the Cooper pair is 2e and so on and so forth These came from experiments Unfortunately It is easy to do experiments at One kelvin or above But it is very very difficult to do Those kinds of experiments At micro kelvin So we need to do more To understand what the glue is But this is the thing That because of its High dielectric constant Can bind together Overcome the Coulomb repulsion Okay Thank you There is another question Is it possible to achieve Absolute zero temperature in lab And what would happen if Third law of thermodynamics tells you That you can never get absolute zero So the point here Is that Even in a practical sense You will The best engine we can get Is the Carnot engine So I will leave this as an exercise To you if you can't then I will Provide the answer So you allow a Carnot engines to work Between two temperatures And then try to ask T1 And T2 keep reducing the T1 And T2 And you will see that You need as much energy As when you go from 10 kelvin To 1 kelvin And 0.1 kelvin to 0.01 kelvin You will see that same amount Of energy you have to spend And when you go T tends to infinity You can see that the amount of energy You have to spend is almost infinite You cannot get down to this So this is A small exercise you do Otherwise I will give you the answers later So you will see that practically You can never reach absolute zero You can very come close But never go to zero Yeah so there is a related question Which is immediately popped up Is it possible to achieve arbitrary low temperatures Or is there some practical limit Yeah so typically the problem Is the heat leak See it's the practical limit which tells you We can go down to very low temperatures So what is happening is that Below let's say 100 micro kelvin You have to be careful Because you are Using the nuclear spins to cool Electrons and lattice If the spin lattice relaxation times are large The nuclear spin will cool To very low temperatures Whereas the electrons and lattice Will be at much higher temperatures So nuclear spins can be Cool to even picot kelvin You know but that is No relevance to the electrons and lattice You know it's not the equilibrium temperature It is the nuclear spin temperature In fact you can achieve negative Temperatures also Because you can populate the levels You know the higher energy levels You can populate more the lower energy Populate you can less Using magnetic field and so on So that is possible Negative temperature is possible with nuclear spins But in real life You want to see nuclear refrigeration You want to cool other materials So you use the nuclear spins To cool the electrons and lattice There the practical limit is about 5 micro kelvin But that is on a smaller system You know it's about Something like 150 grams of platinum Wire is cooled down to 5 micro kelvin The basic problem is the heat leak The heat leak from the From the room temperature to this Cryostat You know there is vibration There is electromagnetic radiation Then there is all these wires Which is coming from the top So the practical limit People have achieved for platinum I think 150 grams of platinum Cool down to 5 micro kelvin The large systems where The experiments are done like the one Which is at TFR and so on So people have reached up to about 20 micro kelvin And we reached about 40 So that is the That is the limit Okay Somebody asked Does the zero point energy Play any role in the minimum temperature You know unfortunately We are not getting into that So zero point energy Is there even at absolute zero So that comes from the answer to the principle So we can never reach to such Such a low temperatures to do that So We'll never go to that place Yeah What other phenomena are known Which occur at ultra low temperatures Apart from superconductivity And do they have any future application So the question is that Whatever happens in ultra low temperature Have no future application That much I can tell you that Straight away It's like this high pressure 2.7 megabar Superconductivity Right now there is no application But that doesn't stop people Because they are tinkering with the parent Material to see whether they can see The same phenomena at Low pressure and ultimately Ambient pressure At ultra low temperature You find some phenomena And see that Then use a clever metallurgy To push this temperature To a reasonably manageable one So that it can find some application Superconductivity born that way The other interesting thing which happens In such a low temperature Is the magnetism of nuclear spins See the The magnetism of electronic spin is well known It is thousand years known But when you try to Analyze with theories There are complications There is disorder, there is phonon Thermal vibrations and so on and so forth So the theorist can It's difficult problem for the theorist To solve Magnetism of Compounds or elements and so on Electronic magnetism Because the other effects come in Whereas if you look at the nuclear spins You know The present example is produced in Nickel 5 PRNFI where the PR nuclear spins Order at low temperatures At such low temperatures The lattice vibrations are ruled out Because it is so small So you are talking about ideal Nuclear spins So if you want to test your theory To understand how the nuclear spins Get down How the spins get down to magnetic ordering The best place is to look for Nuclear ordering At such low temperatures Because you are dealing with pure system Most of the theorist can be If it doesn't work out You throw away the theory If it doesn't explain the experiment Then you can throw away theory So most of the models can be Tested there to see their validity At such low temperatures That is another useful thing Which will come The second thing is that I told you about granular superconductivity So it happens right now At millikelvin temperatures We don't know how the superconductivity Comes in What if you can cook up such granular materials At higher temperatures Once you find the reason for it Then it's possible to cook up materials Which will occur the same phenomena At higher temperatures So that is the whole motivation Apart from curiosity Of studying unusual phase Transition at low temperatures To find out that And then find the real reason for it Then cook up materials Which will show the same phenomena At higher temperatures So that is the case But there are systems right now Operating not at microkelvin But at millikelvin temperatures Like superconductivity Used in quantum computing You know if you look at the The one One The plausible quantum computer Is this based on Josephson effect I think there is TFR Pijerao and I don't know whether he is going to Top on this In this meeting But so this is Something which is unique At least at millikelvin temperature To work with this system You know both of the system Like IBM and all those people have this But this is at much Higher temperatures So But the basic understanding is that You find a phenomena then Try to find the theoretical reasons for it Correct explanation for it Then cook up materials to push this Temperature, transition temperature At higher temperatures Then possibly you will see See it's like as I said When camel owners discovered mercury Then people said Why do you want to do it at such low Temperature it's practically impossible But you saw that after 50 years After about Let's say after 70 years The first magnet came from Superconductivity then after 20 years Something like 80s You saw the MRI machines come into the Fold so one doesn't know So the question is that find a phenomena Then you try And see how we can push the transition Temperature out so that is the motivation Okay There are a bunch of questions regarding Bismuth but all in the same Vane so basically They're asking does Bismuth Come under the same class of Superconductors that you mentioned Towards the end for arsenic Or is there any other Absolutely no other element which Bees like Bismuth These kind of things Yeah so Bismuth is a unique element So nothing is You know come close to Bismuth It has its very unusual properties Because its thermal energy is comparable To the lattice energy The other elements like anti-money And arsenic has the same structure As Bismuth And also it has They're also semi-metals But then their carrier concentration Doesn't change if you look at this I showed you the plot right from room To pressure do this So if one sees Superconductivity There and if that is First of all nobody has seen Between arsenic and anti-money down to Let's say 100 micro Kelvin And if one sees Superconductivity then One can see How to distinguish the Superconductivity which we've seen in Bismuth Vizav is the Superconductivity there Because the unusual Band structure of Bismuth All this carrier concentration temperature dependence Is not seen in Anti-money and arsenic So we can try to distinguish Something which has a similar structure Crystal structure But it has different properties So one can try to isolate The reasons for it Whether it is coming from the structure Or it is coming from somewhere else So that is the reason why One would like to see Superconductivity in anti-money and arsenic Okay Then somebody has asked that Even though silver is a very good conductor At room temperatures So the silver See copper gold and silver So the only Choice of the material is gold Because see if I told you this electron phonon Interaction Why is the resistance is slow Why small Because the scattering from the phonons Is much less in silver As compared to the rest of the metal So if the electron phonon Scattering is less The electron phonon coupling Between the phonon Electrons and phonon Is very very small So that is the reason Silver is predicted to be a Superconductor at sub nano Kelvin I think So it is around 2030 picocelvin So that is the reason Why silver and copper Will never superconduct Gold there is a chance Silver is predicted to be a superconductor At 50 micro Kelvin So we have tried to See that but It didn't show Superconductivity And we know the reason for it Because it has 0.5 PPM of manganese So these magnetic elements Are really bad for such superconductors So this 0.5 Parts per million of manganese In pure Pure silver Pure gold Kills its superconductivity So that is the reason Why all these noble metals Because the electron phonon coupling is very weak That is the reason why they have Very high conductivity So they will never become a superconductor At low temperatures Isn't it possible That while trying to Measure the noise The measuring apparatus itself adds to the noise Yeah so the good question See the point here is that When we measure the noise We use the squid to amplify it So that The internal noise of the system Which we measure is smaller than Ultimately that's what limits it So why can't we Measure the noise below 1 mK Or 0.6 mK It's basically because the noise Of the thermometer comes into the picture So it levels off Is about 1 mK Or 0.6 mK The important thing is that Everything is amplified by the squid Sitting at very very low Temperatures Okay I mean again they are wondering That things like about absolute zero That theoretically If we are somehow able to control the vacuum Fluctuations and able to Aim it in such a way that Negative energy ends up at our apparatus I don't understand fully Can we reach absolute zero for some practical time? No no never So it violates All the thermodynamic Laws so You know One of these days maybe I Think I will ask him to send it I will tell them how the thermodynamics Prevents that you can never go To absolute zero So even if you go in the Non-liquidium state You cannot reach absolute zero You can go to negative temperatures And so on For nuclear spins which is isolated You know it's not something which You can realize in the In the equilibrium condition But in a non-equilibrium Condition you can Have a negative temperature as well In nuclear spins because you populate See the temperature Is a measure of how it is populated So if the high energy state Is populated more compared to the Lower energy state You have a negative temperature so we can define that But you can never reach Absolute equilibrium temperature Absolute zero Are there any other conditions Where we observe superconductivity Where Apart from low temperatures I think that's the intent of the question Yeah no no yes You have superconductivity in stars You know they are in Very large temperatures Ten thousand or even Billion temperatures and so on There you are talking about different kind Superconductivity of different neutron stars You know you have a Superfluidity there which is close To the superconductivity which you see here So that is something Which is totally Different and a totally different Phenomenon occurs there Right In the beginning you said In addition to choices of elements We can make alloys that have higher Tc but how do we make such choices Yeah good question See this is the million dollar Question because People have been doing you know When two elements are Superconducting you try to mix them Try to do a band structure calculation And see whether you know Ultimately what is the Criteria which pushes the Tc If you believe BCS You have to make sure that the Theta D which comes in the Tc Debye temperature has to be large So all these high pressure People that's what they are trying to do They are taking hydrains You know hydrogen, metallic Hydrogen is supposed to have a Debye temperature of the order of thousand So it all started With them the theory said You make a metallic hydrogen under pressure Then it will become a superconductor In room temperature So the whole thing started Because of that So initially they tried to squeeze hydrogen gas Became a solid But then unfortunately it was Metastable and so on and so forth So it was not possible The whole thing was abandoned In 70s and late 80s In the late 80s Then people started working on hydrides And in 2000 People really started working on hydrides Different hydrides When they were squeezing hydrogen They squeezed the hydrides And this Hermits in In Germany He is the first guy who tried these hydrides To get into a You know Transition temperature in some of these Sulfurous hydrides where he showed Around 200-220 Kelvin At 2 megabar And not only that He showed that You know it is electron-phonon interaction He duetriated the hydride And then applied the pressure Showed that that has a lower Tc And because of that We know that the electron-phonons are involved there And the important thing there Is that the debate temperatures Are large in that case So that is how they achieve this high Tc So you know There is no straight, hard and fast Rule on this So it depends on how ingenious you are And you try to see That is how it has been happening But the progress In ambient superconductivity Is quite low Till this high Tc oxides came You know that came by accident If you look at this paper In 1986 by Zedphysic You know they talk about some oxide They talk about some resistivity Going to very low value And so on It's a badly written paper But it's a very low price for it Because they were on to something Which is very very fundamental So you know it changed the landscape People never thought that they are going to have A superconductivity Above the liquid nitrogen temperature So they provided the path for it So that was just an accident I would say And they were working on magnesium And then they landed on superconductivity So that is As I said there is no straight rule for this And it has its own idea of how to go about this The high pressure guys are ruling In the last 5-6 years Because every day there is a note That Tc has gone from 200 to 220 220 to 240 Now it's at 288 They certainly crossed 300 by the end of next year The way it is going But the important thing is that That is an important piece of research But the important thing there Is that they need to find Materials again Which will show the same phenomena At lower pressures at ambient pressure Just like you know our case We find something at ultra temperature Ultimately if you want to be a practical Significance it has to Work at room temperature So that is the goal So they are trying to reduce the pressure To go down to Workable Practically applicable superconductors So we are trying to raise this temperature Something interesting So there are two different approaches To the superconductivity One is this way The other one is looking for an exotic mechanism Other than phone-on interaction To see whether we can reach High temperature superconductors So as Camberlingon has famously put it Time only can tell Yeah Okay I think Last two questions I will just So one is that Is there a correlation Between ambient pressure and high pressure And can one make predictions By extrapolation No not in this case Of this High pressure superconductors Because for them You know when we Reduce the pressure At The ones which are superconducting Let's say at 250 Kelvin So when they reduce the pressure Down to 0 They are unable to Keep the stability of that system It goes to 0 I mean the TC goes to 0 So if we extrapolate it You always land up with the 0 temperature See the only thing which they can Try to do is try to find Metastable structures So you squeeze this Thing up to largest pressure Keep it superconducting Release the pressure so that It is in a metastable state For a longer time, period of time Where application can be thought of So that the whole game Is now towards that You know they have raised the TC Now they have to find out metastable states By introducing possibly Another chemical element onto this Parent compound Try to see whether you can achieve A long term metastability of this So that when the pressure is removed It's still, the structure Still survives So that is the game there Okay Someone just asked Why do we consider Whole current Why do we have whole current She has never understood it properly So if you can say if you want See as you go down To See in the metals There are no excitations Across the Gaps So for semiconductors You have a small band gap So when the electrons get excited Across the gap It leaves this positive Charge at the At the valence band So they are what is known as the holes If you go to Let's say Charles Kittel Or some solid state physics book It tells you why the holes are created Because it has to be The system has to be neutral So when the electrons get excited You have negative charge And then it has to be compensated By a positive charge which is the hole So that is what is happening In Bismuth you have a direct energy gap Although in some Bands are overlapping but you have this So you have both Holes as well as electrons So it's called a compensated metal You know they are both They are there at the equal concentration So My advice to you just take up Kittel and see how The holes are coming into the picture In semiconductors and so on You will get clear Okay Finally somebody Has asked just out of curiosity Do any species like super Insulators exist And if so they can Couple them with superconductors Yeah see super insulators Exists but they you know You don't have to cool them to get it They exist in many form I mean the best There are two kinds of super insulators One is electrically super insulating Diamond is the best example for that And If you want a super Thermal insulator then there are Polymers which are Really thermally insulating And so on but you cannot Couple insulated to the superconductor Because how the charge carriers will Come across So the only way one can do But people have done that in a way That you take a sandwich You take two You know oxides Which are you know insulating The interface for some reason Becomes a superconductor because You know this is something which has Happened ten years ago Became a very unusual thing Because you can have a 2d electron which is Convined between these two insulators And though Those undergo superconductivity So this is called the interface Superconductivity whereas the In between the Two insulators So there I think you can study Then people have made devices and so on And so forth but that is In the thin film region not Okay Thank you I think I will send you the chat And you can look at the question And if you feel like you can answer On the Moodle or by email And I request everybody To unmute and please applaud Round of applause for Prasar Abaghashnan Thank you so much for taking out Time to do this And that's again So I think once this Activation hopefully it comes all right I would like you to visit the labs And see when all things are being Done there so that is The that is that would be More interesting also for you Some of you at least thank you Thank you for you Thank you I just remind the students that The Open room will be on From 8 o'clock 8pm Thank you and believing now Thank you