 Okay, good morning everyone. I wanted to discuss our latest results, but Lawrence twisted my arm saying that not everyone is expert in this audience, so I'll start with a usual line. The first time transparency is how great graphene is and so on and then okay switch probably 80% of my talk will be the latest results from our group in Manchester. So that's the storyline. So what's so special about so simple material which is just only carbon and the structure is one of probably most simple one could imagine. Yeah, seems to be not much to be gained, but when you start looking for the properties, okay, some of you have already seen many times the list of superlatives which I compiled over the last couple of years. Yes, it's of course the thinnest material you can imagine. And yeah, it covers a surface area with one gram. You can cover a football pitch easily in Manchester. We measure everything in football pitches, as you know. And it's according to Columbia group, it's the strongest material ever measured. I do agree with this statement. Although carbon nanotube shows similar strengths, of course, probably more defective usually. It's stiffest material. It's stiffer than diamond. That's for sure. At the same time, as you know, it's pliable material and you can elastically stretch it by 20% as several groups including our groups have shown. It shows record thermal conductivity outperforming graphite and diamond. Unfortunately, now we know that it has to be suspended graphene on the substrate. Flexural phonons are suppressed, so conductivity is no longer records, but still very high. It shows it at room temperature can sustain current density, million times of copper at room temperature. It's really the record. It's impermeable even for helium. So those rings you see on the screen there, they're so densely packed with electron wave functions that they do not allow with electron waves that they do not allow even helium atoms to squeeze through. And it shows pretty good electronic properties which I'm going mostly to concentrate in my research. I thought what else to add there and from time to time I get some people, some other similarities. For example, there are groups who claim that it shows the best married figure for semi-electrical conductance and recently MPL group has shown that it's the best material for quantum Hall effect standard. So, okay, out of those two, our group and probably most people in the audience are interested in quality and electronic tunability of the material. Let me remind what this is about. You put graphene on the substrate and make substrate conductive or top gate until you can change properties of the material by sucking in or pulling out electrons and holes from the system. Usually for silicon and gallium arsenide you can sweep very small concentration of the carriers but in graphene you can do practically from 10 to the 13 very much nearly half electron wall concentration of electrons to the same concentration of holes. If you try a little bit harder, in suspended devices we could reach one electron or one hole per square micron which actually no other material allows to do remaining conductive at the same time. And if you try liquid gate you can go to really high concentration. Columbia group and several other groups have shown that you can dope really high. Probably the most amazing property of graphene that despite being placed on a rough substrate and being covered with at the base and so on, it shows quite remarkable electronic quality so electrons can shoot sub micron distances routinely in the devices. Okay, made by whatever technique it is, either transfer from metals or exfoliation or silicon carbide, it's still the same sub micron distances. And already a few years ago it was argued that if you eliminate those scatters you get really a record intrinsic mobility in the material which all translates that you can study properties very nicely and can find as a cleaner material they usually the more pronounced properties are. So let me a little bit update on what currently current status of quality of graphene, mostly it's refers to exfoliated graphene which still had as a proof of concept devices and for fundamental studies. It was known for quite some time that the problem is a substrate and observates and we probably tried half a dozen or more than that of different substrate instead of silicon oxide. The breakthrough came from Jim Hones and Philip Kim's group who used a specific type of boron nitride. We also tried highly oriented pyrolytic graphite but it didn't show any improvement but there is hexagonal monocrystals of boron nitride. You can get from actually couple of different sources and it was a dramatic improvement, it's really, really, really very important achievement. So that's our data using these substrates graphene placed on boron nitride, structure, whole bar and as I will do usually we get mobility around 100,000 but sometimes we reach half a million mobility at low temperature and couple of 100,000 at room temperature. Another way of getting good graphene was demonstrated by Ivan Ray, Philip Kim and Amir Yukobi's group. It's suspended graphene, you put those fingers and you etch away half of silicon oxide and it's suspended and you need to anneal it in liquid helium or in ultra high vacuum and it shows good quality, typically to 100,000 to 200,000. We managed, don't know why but our devices sometimes shows couple of millions mobility. This is an example, Schumnik of the gas oscillation started 50 gauss and level degeneracy you see splitting already at 500 gauss. People sometimes in literature they mention that suspended devices show very high mobility at room temperature indeed. Philip reported I think 150,000 or even 200,000 at room temperature there was for some time disagreement because our devices have showed very strong temperature dependence and at room temperature we typically see only 20,000 mobility 10 times or so less than Philip so what we think this is actually intrinsic mobility for graphene is lower at room temperature and this is due to flexural phonons due to this vibration out of plane like a drum. Vibrations and there is no disagreement because our samples actually also show some spread in room temperature mobility and we attribute these differences between different samples to strain so if you pull your sample a little bit between those fingers then you suppress those flexural phonons and then you can get high mobility even at room temperature. As another reminder of why we are interested in graphene okay of course it's electronic structure. Practically everyone has seen this picture I believe it's in graphene it's not Schrodinger electrons it's okay. Dirac like equation is what is used to describe low energy dynamics of charge carriers in graphene instead of spin you have pseudo spin which is coupled to orbital motion. It's essentially the weight of the wave function on one of those coven sub latitudes and in bilayer graphene you get another very interesting matrix like equation which is a mixture of Schrodinger equation and Dirac like equation and who knows what's happening in trilayer. Trilayer it's already rather complicated story so all this sort of this picture allows you to think how we can compete with with another branch of physics like particle physics or nuclear physics and there are phenomena and two of them shown on this transparency which have been known for 70 years like Klein tunneling or relativistic fall on the center which were dimmed by particle physics or nuclear physics not to be accessible in any reasonable way experiment for the next century in graphene those phenomena at least Klein tunneling is routine and this hopefully supercritical regime when coupling is really strong between impurity and electron hopefully micromy one day will report. There are phenomena which also contradict to what we know from condensed metaphysics usually it's still badly understood phenomena of minimal metallic conductivity why with such high resistivity of the order of H over E square graphene is still metal in this regime I will discuss it later in my talk. There are new results to contribute to this one but it's very unusual that you can go all the way from electrons to holes from metallic regime and conductivity and at optical frequencies is considered to be universal with some many body corrections. So when you look through the graphene you assess not only its opacity by your eye you assess five structure constant because its opacity is given by pi multiplied by alpha which is given by this universal conductivity. So it's sort of examples of the phenomena you can study in graphene and it's the main interest of probably graphene community there is also interest in applications everyone who is doing graphene is sort of thinking what sort of applications could come from particular research and each superlative of course offers an idea what to think about and there is a huge potential for applications and I'm not going to discuss those except showing this one transparency applications or ideas of applications range from something which can be called only dreams like graphene as the next silicon or DNA sequences to quite reasonable applications. High frequency optoelectronics and electronics I believe Fiedin will be speaking about this graphene instead of ITO at least this is proven in academic and in some industrial labs that it's a feasible it's still a long way to consumer products and some applications like conductive inks and batteries. Allegedly graphene already there also people by graphene usually mean platelets of graphite so at the moment okay we're still gearing up to go into applications and don't expect anything after five six years of even of such intensive research has been done. So okay with this one I'll give you I'll overview some topics we were studying during the last year since okay since last summer I would say so the first subject is the question of this Dirac spectrum how linear the spectrum actually is okay. You saw the picture Dirac cones and so on so let's remind you how we know that okay. In good old times where there were few competitors okay like Philip and ourselves okay we measured Schumnik of the gas oscillations as a as a carrier concentration measures their temperature dependence analyze it extracted cyclotron mass and from cyclotron mass we found that cyclotron mass. Is function square root function of concentration which actually translates into this Dirac like spectrum and the slope or Fermi velocity which is confusingly called Fermi velocity actually it just velocity slope of this spectrum is this number with some accuracy both our groups reported. There were many other measurements which essentially gave the same number for for velocity in this range of concentration 10 to the 12. Now we have those suspended high mobility samples and what is most important that they allow us to go to really very small range of gate voltages and concentration. And study how this Fermi velocity or slope how they slope changes so that's we did the same routine and the first thing we have found that's experimental data and if Fermi velocity would remain the same as as typical numbers we extracted at high density. That's where we would expected that corresponding curves to fit the data we need in this particular case twice higher Fermi velocity so and this is not marginally it's okay factor of two two difference in in slopes and in temperature dependence it's it's qualitatively large effect so. Fermi velocity changes with concentration slope changes with concentration and that's for one of the samples we we we get this data that's the previous value and that's for high concentration for concentration like like here on this scale would be and it's it's goes higher. So what what's what's the origin for this the explanation has been long time in literature so electrons are usually in metals they they interact with each other and so only when you have a large concentration of electrons they're screen enough by other electrons so you can you can use them as a single particle picture. Picture Lundau Fermi liquid theory what's happening in graphene here near the Dirac on neutrality for in concentration goes down and interaction becomes extremely strong and that's what we expect from previous series so renormalization of interaction one has to be careful. How to interpret this picture because the spectrum doesn't actually change itself it's still linear spectrum the spectrum depends how many electrons or holes in your in your system each time you change your concentration the Fermi velocity changes. But but it remains constant underneath underneath the Fermi surface so it's sort of dynamic this slope is dynamic it's when you probe your spectrum each at each concentration you probe different Fermi velocity according to to theoretical predictions which go back to particle physics actually well before the first papers considering this theoretical. That's a collection let's see how it matches with with the series this is a collection of data for four different samples for suspended devices and those two curves that's where we expected and previously measured for concentrations somewhere here that's the slope. I specially give this in logarithmic scale because of the scatter and to cover three orders of magnitude changes in concentration and this is where our lowest concentration data goes three times high so it's a big effect but only as you change your concentration by three orders of magnitude. Those pink curves that's theoretical predictions from this 90s paper because there is a fitting parameter in those series which is self screening at dielectric constant of graphene we don't see any anomalies. It behaves as it should be in this case and but we did a little bit more theoretically in this work here and we incorporated self consistently into into this theory that graphene self screening changes with concentration so it's reasonably good fit to all the data. One thing I have to mention there were couple of papers. 2008 recent paper which worked in this regime larger than 10 to the 12 and they reported deviations from a constant Fermi velocity by 25% and if you extrapolate those deviations they they would go much much bigger effect that we observe so. In our case it's how it should be not big not small one might notice that we do not reach in our suspended samples the value 10 to the 6 which is we certainly know that happens for graphene on silicon oxide and in other systems. We know the origin this is describing this paper that's because we have suspended graphene when we put graphene on boron nitride we'll see that Fermi velocity actually moves upwards due to dielectric screening again in very good agreement with with theory. So a message to take away from from this particular research that there are renormalization effects that they are modest one can call them weak but if you go to concentration less than 10 to 11 they become quite clear and pronounced as this dense Disney as a direct point and should be taken into account. Lawrence asked me to make it more interactive so if someone wants to get any questions concerning this part of research you're welcome to ask now or at any moment you just. Can shout rubbish okay and I'm happy to confront you in order to see to see Schumnik of the gas oscillations the sample should be bigger than cyclotron orbit automatically when we see Schumnik of the gas oscillations we. I in the regime when the sample is larger than cyclotron orbit otherwise we won't see anything and if you estimate that's in some cases we do see cut off of Schumnik of the gas by the size of the sample but so far the quality is not as good to go into this regime yeah. That's what Nelson Mandela would call I have a dream so okay so that's what what. I like to to share with you okay so somewhere okay five six years ago we reported that it's not only graphene and I have shown this transparency ad nauseam for for some people for many many years. So except for graphene many other materials are lead and a single layer can be extracted or few less boron nitride bisco di calcogenides and so on that was at this very slow burner for quite some time only recently boron nitride has come into play. And few papers on other di calcogenides were were published but okay not not as popular as graphene so what I love I always pointed out that some of those materials are insulators some metals some semiconductors some superconductors some fair magnets. So there is a huge range of of different materials you can play with so the dream is something like that okay to make a new lead compound on demand and see what would what would happen. How do we do that how could we do this in principle this is what already people in Manchester in a Columbia have been doing them in several other places. So okay you prepare graphene okay that's one of techniques we are using okay on double layer PMMA with release layer. There are variants of this technique you can use then lift it off then place face down align with another layer on the substrate diesel. And you get two layered systems consistent of different different different monolayer for example few layers of different materials to make this stack of course you have to repeat this procedure many many times. And this procedure is not simple simple and straightforward but we know if we one day find say room temperature superconductor let's fall for the sake of gravity let's say putting those materials together than someone like from something like beyond he next day will make roll on production of this material. So unfortunately this dream is sort of a little bit difficult because certainly we know that boron nitrite and bisco those two stable materials not superconducting at one layer sickness but they're stable enough. Other materials are less stable some sometimes quality is not that good so you have to deal with few layers of those materials so if you were going to single layer we mostly limited to insulators why do we need insulators. Okay for those who are in semiconductor physics they know that insulators can be used in a variety of structures in tunneling and resonant tunneling devices specially put here for Lauren Sieves who is an expert in this sort of devices spin tunneling is also another important application for for tunneling devices. So the question is okay usually you do evaporation and so on why we wouldn't use one of those insulators as atomically seen barrier something what MBE can't do that what we have done last year we learn how to isolate boron nitrite in single bilayer and other quality that's way more complicated. And difficult than doing the same with graphing the contrast is extremely weak it's in constant it's in grayscale and so on but we can do it reasonably reliably in our experiments. So what we started with is making gold finger putting boron nitrite say single layer or few layer put contents on top so you we try to assess properties of boron nitrite as a as a barrier if you put graphing here you don't see anything we tried before the resistance is incredibly small you can't you can't see a barrier. But with boron nitrite lay seven seven less it's about three nanometers it's an insulator and then you start seeing tunnel current at high voltages before it breaks down down for two layers you start no longer see any insulating state it's just high resistivity everywhere we're not exactly sure about this number because interfaces might contribute because we do not know the exact area because interface of gold is roughish and for single layer from mega ohms we go to kilo ohms and it's linear weekly temperature dependent IV characteristics and what they can be translated in high gap and effective sickness actually larger than what you expect three layers we don't we're not sure why it is so but it's requires okay first principle probably analysis what effective sickness of one monolayer and but most importantly what we learn from this one is that there are no pink holes in the system which is very good news for for the material to use as a tunnel barrier. We can do a little bit better use conductive FM that's cost this result results here with his pulse dogs and we measure if we put at a particular point we measure IV characteristics which are non-linear again it's room temperature measurements and you can see that okay. It's tunnel kind of behavior which is dependent on the number of the last and we can scan reasonably large areas many many microns and again we do not find any pink holes which brings me to conclusion to this part that born nitride can be used not only. As a substrate which is currently using but as a high quality tunneling barrier and then you can start thinking what people have done in two five in two six three five semiconductor physics about thinking about vertical various vertical devices as well going out of plane transistors but doing something else. That the simplest case. I'm trying to because those okay futuristic devices are too complicated for the moment so the first example you might think of this one is encapsulating graphene in born nitride actually there are advantages with respect to putting in on top of born nitride as. Philip came out Jim home. Did recently. We covered with another graphite layer and it turned out not to be a marginal advantage because really for those who work with graphing know that each thermal cycle exposure to air changes the device you need to anneal and that sort of so we find those last really protected there much much. Better and more stable and in addition there is an advantage you can put a gate on the top if you have more than three layers and control concentration from the top so it's a pretty good top gate. Dialectic so. That's an example of one of my. Our structure that's okay born nitride substrate and me the edge the weight in graphing and this line indicates that here there is a sick ish layer I don't remember 1020 nanometers sick born nitride on the top in some cases we put. A gate top gate on top of this structure so usually we get for those devices we they show pretty nice characteristics mobility up to 150 I would say say at concentration 10 to 11 similar to what. The Columbia groups reported and. And but in some devices it goes better those devices are usually characterized as a square root dependence on gate voltage rather than linear voltage so in those devices okay also the. Although mobility is still sort of a hundred hundred fifty okay as in other devices many of those okay say we studied 10th of the well. 20 devices by by the moment her for the moment so okay we in those devices we use our favorite band geometry putting. Current through these two electrons and measure voltage here and then we'll find out that resistance for for terminal resistance becomes negative which tells you that electrons from this contact can go all the way through and reach this contact and then. The chair the reversal of science that's possessed up to 250 probably room temperature in some devices devices and we know how to interpret this it's negative bent resistance it's responses to magnetic field as it should be. By applying magnetic fields you quench this negative. Resistance so everything what has been seen. What 10 20 years ago in Gully aluminum arsenide to dimensional electron gases what what you can do from this negative resistance and it behavior you can estimate. Reasonably accurate within say 50% you can estimate mean free pass and extract mobility for typical concentration and it's goes at low temperature to half a million. Of those we're now pretty sure that this is the case because we do see devices with the same mobility. Measured by normal way but they don't not show this. Negative bent resistance so and numbers okay for room temperature so pretty pretty pretty large so it's probably we don't know okay some devices only show this behavior but it gives you an idea that for that sort of structures mobility. Can be really high and we believe in capsulations really helps at least it's makes more comfortable to work with this sort of devices. Since I mentioned the top gate okay that's an example that top gate does work okay it changes the banter resistance some electrons no longer go into this contact by reflected weekly by reflected and so it's. You can see fabric or interference on to the gate 100 nanometers without any problems we didn't investigate this but similar what. And and the young Philip Kim reported so it just tells you that that the gate does work pretty nicely when born nitrite is using the game. My my last. Subject is a little bit more complicated so the structure along the same lines of compounds lead materials but in this case we use two layers of. Graphene and they both everything okay encapsulated okay maybe sometimes without the top layer but. It's born nitrate born nitrate born nitrate on top so it's double less structure similar to those which were described in. Gully. Asana asana it's heterostructure business but okay let's see what the difference graphene makes for that line of research so how to make it I'm here. I like to show that it's really okay very involving procedure so first we deposit on silicon oxide. A crystal of born nitrate a sickish say 20 nanometers then a layer of graphene edge it into a whole bar structure we want. Then another layer of born nitrite with a chosen sickness say from one nanometer to 20 nanometers whatever we want then graphene. Aged again deposition of the contact so it's involves three dry crystal transfer okay with with crystallized are not touching and nearly in this at. Three hundred four rounds of electron beam lithography three plasma etching two metal deposition and hundred times of cleaning structure and three moving resistant so. So good things that it does work okay that's an example of the structures we have made let's look at this one for example that's the bottom layer. Of born nitrite that's the bottom line false color orange brownish color is the bottom layer here on top of this bottom layer there is a flag. Shaded here by a sin counter that's a flake of born nitrite sin is sort of 10 nanometers then another layer of graphene a line on top with accuracy 10 nanometers or so on top of another another structure. Typically we got to okay mobility is 100,000 low way in top layer which remains usually exposed and actually a little dope in separation with rights three to 20 nanometers so far as a it's top layer usually deteriorates after exposure to air. On this bottom layer remains okay pretty stable for very long period of time. What we can do because those last individually contacted and there is no for sick born nitrite there is no leakage between those two we can apply it back gate from the substrate which is color. Colors are wrong here okay we okay red is missing okay. So we can we can buy back gate we pump electrons mostly in the bottom layer and because of some screening in the top layer we also get a small concentration when we apply interlayer voltage we push them in different direction electrons in one layer. Holes in another layer so we have we have control one has to be careful this is very unusual case because of finite screening of the system okay. We can't know we cannot relate or that's better colors we we cannot we cannot relate voltages to concentration by linear equation quantum capacitance which was looked for many many years actually a dominant phenomena here no screening at low concentration everything goes there and because of these distances. Relation between concentration and those voltages is strongly nonlinear. So the first experiment you can do you can see how properties of your bottom layer. Influence what's happening in the top layer let's put it pretty large distance 10 nanometer for example and see how this layer changes properties of another layer. If you are at 70 Kelvin or something like that and distances like that okay that's a typical curve which we usually measure changing concentration in the bottom layer and then we add something to the top layer and nothing happening what you as you see here as if the top layer doesn't do any influence. This is not the case at low temperatures what you see you what you find out that okay resistance in the high mobility layer usually diverges when you put carriers in the top layer it's better seen here that high density in the top layer and that's our characteristics. But now at different temperatures if you if you are at low essentially very low concentration no electrons here that's a typical temperature dependence a little bit freeze out of electrons but then there is a metal insulator transition. If you put a lot of electrons screening in the top layer that's better seen here on this picture on this picture when you put a space closer than the phenomena becomes diverging you can go to mega ohm regime but it's difficult to work in this region because. Because some specific to this double layer structure phenomena occurring but what is it is it a gap state or is it an insulating metal insulator transition at this resistivity. The answer is if you apply to this state here magnetic field you'll find out that so that you quench this insulating state which is a clear indication that what we're dealing is a sort of interference under some of strong localization regime. This is confirmed by the fact that these divergence between sort of quasi-metallic to insulated regime happens when resistivity per carrier type is about h e square over e square it has nothing to do with the metallic resistivity which was discussed many times before in graphene. That's the number comes when usually people see metal insulating transition the transition in any system in graphene for example how do we explain this knowing that this is insulating state and this explanation is thanks to Valodia Falco. We know that in graphene whether it's on silicon oxide in boron nitrate there are puddles we have known this for many many years and it has been argued that within each puddle. Graphene remains metallic and conductivity of this complex system is just a percolating problem between electron and hole puddles from here here so conductivity is not given what's happening within the puddle it's given by this barrier between two puddles and it happens to give a conductivity of the order of h over. For e square or something like that so within each puddle we have a metal so and it's probably extends to boron nitrate where puddles are bigger and shallow but still have a pretty large density. Of the order of 10 to 11 or something like that what we believe that the top graphene acts as a metal plate and this metal plate screens out those puddles and push the system into its intrinsic regime which is where some okay yeah where is. You exceed this value h over e square and you are becoming an under some insulator so that's okay conclusion from this part of the research and the only one thing I would like to add let me skip it because I'm running out of time. Time let me skip this transparency so let's let's take this as a conclusion so you can do double layer maybe triple layer for layer okay heterostructures with graphene and boron nitrate and extra layer offers new flexibility. An example is my last few slides is that you can do interaction experiments between those how influence of one gas goes on to another beyond metallic screening I discussed so in this case we typically get a case smaller separations for nanometers. We push current through the bottom or top layer and measure voltage induced on the top layer remember there is no any tunnel current so any current induced here is due to an interaction or coolant drag sometimes it's called okay. That's how it's typically behave you measure resistances it's linear response drag resistance as a function of how you put electrons and holes in the system and when. The system has the same sign of charges charges the drag is negative it becomes positive where charges have opposite have the opposite sign and this sort of regime has recently somewhere here from here to here has been. Studied by a Texan group and using instead of boron nitrate they use silicon oxytocin insulate and mobility is below low and they couldn't go into say from one. I'm bipolar regime was not possible in the experiment it's very easy to achieve the now experiment and what we actually like to simplify the situation and study drug in this symmetric regime when you put. Calls in one system electrons in another system and they have the same concentrations that there is a beautiful behavior which shows that the drug decays when you have more. Charge carriers and it shows that deep at zero concentration as it should be whenever one of the systems goes through zero it has to be zero so qualitatively behavior is understood we don't know how how it picks up here and what the value. Is there and actually that says the series has already been produced even for Dirac fermions not very different from the case of just electronic gas that's a simplified formula for this particular case of equal concentration in top and bottom layer and since we do not know what's. Happening here this is most interesting situation but let's try to understand what happening on this slope away from the neutrality point if you look for the temperature dependence concentration 10 to 11 to 10 to the 12. You'll find out that it's T square behavior it's essentially fluctuations in one gas give you T fluctuations in another gas give you T and it's T square behavior so as it should be Dirac fermions or Schrodinger fermions doesn't matter it's. If we go to concentration dependence how it decays here will we find out that it's not that quick decays a decays much slower like one over and Q sorry square squared high concentration to less than one over and at low. Concentration or one over and so and it's it's for a big a range of various temperatures what's going on why Siri doesn't work in this case it's very simple in fact that the interaction between the two system is described separation between the. The parameter is separation divided over wave lengths of electrons in any of those systems so usually people who studied two dimensional gases in conventional systems and. Sunker as well they thought okay it would be the same weekly interacting regime as. Has always been studied before but in our case we can go to low separation and even for hundred nanometer separation we are no longer in this weekly interacting regime and we are actually in strongly interacting regime and those. Different concentration dependence shows after we presented this at a couple of conferences of course several groups came up with predictions what we should observe in our experiment and it's pretty good agreement agreement with with Siri. And so okay to finish this part okay it's. You can do with graphene boron nitrite and other samples okay very complex hetero structures quantum wealth vertical devices not only in plain devices what. Mb can do it you can't do a single layer continuous layer by mb or something like that and to conclude everything I think okay it's. A general my view would be that after we can probably consider graphene research quite mature after seven years so many people. At this conference and so many conferences every year but as for me I don't see any sign of. This gold mine being exhausted okay it's it's it's really good and finally I like to acknowledge your collaborators especially Roma. Gorbachev who who make all those hundred washes of of the structures three guys who measured the devices and other people who were involved in this particular research I talked in today and finally thank you for listening me without interruptions. For this very low density regime that you can access that in the first part of the talk I'm just curious. I'm not a big expert on these interacting systems but I'm just curious like I know that in sort of the. Gallium arsenate to dig people they talk about creating Vigner crystals you know when you get into these very low density regimes and electrons start isolating informing these patterns due to these interactions is it possible to get into this kind of regime in these graphene. I'm most certain that we'll find all those phenomena okay usually Vigner crystals require low temperatures okay we we limited ourselves so far to temperatures of the water okay high temperature. Yeah but this is graphene man. I expect room temperature. Thank you for reminding. Yeah what we certainly see interlayer excitonic features so essentially electron in one layer couples with hole in another layer and this is what you can see in drug and these features I didn't I don't have enough data to present those but they're seen. Vigner crystal for with in suspended samples we didn't see anything there is a paper we I presented it a year ago we it's not on archive where we studied by layer at a very at a very low density and we don't see any Vigner crystals we see we see reconstruction of the spectra and so on but but nothing as Vigner crystals but interaction. That's become yes they become very strong and this is this would be one of the major topics I believe within the next 10 years I believe. Jeremy good. I'm crunching the you talked about the puddles and I didn't quite understand the origin of the puddles was this from the substrate or where do the puddles come from. Yeah. I think it's I didn't mention this because it's sort of consensus in the war say even before Amir Yakobi mentioned by STM so there is a saying silicon oxide or in any other substrate there is a distribution of charges and this distribution of charges created a random varying potential. And the electrons feel if you wish it's a random gate sometimes it puts electrons sometimes calls and this is this is when this is why we call not the rock point we call it neutrality point because it's usually consist of puddles of different signs of electrons. Electrostatic potential just to follow up on Jeremy's point. Have you had time to apply magnetic field in this percolation crossover. If you have what will that tell you in which in the bilayer in double layer structures bilay it's a different system sorry double yes we tried but it's so complicated system so. You see many things okay what they mean. We are focusing at the moment in in zero field regime we we do apply puddles are. We see the drug it's increases which one of indications of interlayer excitons and so on okay something going on but they are probably not related to puddles because Warren nitride is much nicer system and the size of the puddles is larger than the interlayer separation okay. The film came from Columbia. Here yeah the other questions on on this and this localization limits that when you have the top gates screens out what is the role of the pseudo spinning the case say. Don't we okay yeah localization because of pseudo spin yeah that's that's a good that's a good question so essentially what Philip is referring to to. Get localization in the system you need to restore. Time reversal symmetry and for long time. One of the explanation. For the absence of localization of weak localization was that we have a broken time reversal symmetry broken by the fact that two valleys do not speak to each other. When they are completely separate then there is no localization so there is no localization in this case so to get localization you need to add some. Into valley scatter for example we don't see any sign of metal insulating transition in suspending devices they go straight to a finite conductivity. Insuspended devices straight to a finite conductivity because they're ballistic but if there is a some minor. Number of scatterers which. Kick electrons between the valley present then you can can restore this. Localization regime there has been argument we don't know we can't measure the concentration we have we can estimate is 10 to 11 from the transition behavior but for graphene on silicon oxide that's a typical. Number of into valley scatter is which are a minor contribution but still present there so the answer is you need to have into valley scatter is. Born nitrate is transparent it's a it's a wide gap five electron volts. We don't see any absorbance all those okay you can visualize just due to interference like phenomena rather than absorbance this is why it's hard to see quantization if you wish you can see one layer. Absorbed twice or two layers of sorts twice three last try last three times in this case there is quantization the number pi alpha is reasonably accurate it seems to be there are corrections due to some. Exitonic phenomena at support what they are at three at five ev and there is a tail. Goes to visible light but it's in suspended devices it's usually small on substrate it's usually higher but otherwise two three percent accuracy for this for this number. You talked about tunneling and resonant tunneling for these heterostructures that you're building but you have got this lattice mismatch between say boron boron nitride and. And the graphene yeah so you haven't got translation symmetry across that interface as you would have in a in a lattice match three five heterostructure do you envisage any complications of that I mean that's going to presumably. Make the you've got it normally resonant tunneling you've got momentum and energy conservation. Yeah it's a good point for example with reference to Philip Kim's question about okay that you need to break down time reversal symmetry maybe this interaction with boron nitride gives another channel for. Breaking the symmetry because it's atomic scale potential of boron nitride which can scatter we probably don't need it but it might be important in inducing metal insulating transition on the other hand. We know that this phenomenon is pretty small people studied turbostratic graphene okay where which is called confusingly epitaxial graphene where where planes are also randomly rotated and we know if the. Angle is reasonably large there is a very weak interaction between those those less and so even when you have a perfect match in the in the constant just rotation already decouples less sufficiently. And with the boron nitride you haven't looked at the effect of angle yet it's your dream.