 light to perform photomish experiments and so the idea to have an experimental talk here in this theoretical school is to remind you first of all that physics is an experimental science so of course we like to study the theory but we need to start from experiments and then Polina will give us the experimental view on the bus structure and the properties of materials so it's important to keep that in mind okay good morning everyone I would like to thank the organizer for inviting me and for giving me the possibility to make this talk I am an experimentalist so my idea is to give you a broad idea what is the experimental photomission spectroscopy I will introduce the photomission spectroscopy from an experimentalist point of view or give you some details not too much details of the photomission experiments some examples of the angle assault photomission of simple two-dimensional electronic bands and we talk about the potential potentiality of arpes yeah okay I can take out the mask so arpes the photomission as you know probably is based on the photoelectric effect when the light with sufficient energy is illuminating illuminating the sample surface sorry Polina I think we have an issue because the on zoom we are not looking at the slides so I have to check with the guy we're just seeing the video but not the slides I think we're yeah okay thank you okay so now we also see the slides thank you sorry yeah okay so yeah when the box sample is eliminated with the some life with sufficient energy can leave the electrons so this effect has been discovered in 1886 for the first time it was explained by Albert Einstein in 1905 so the kinetic energy of the emitted electrons does not depend on the intensity of the light it depends on the on the photon energy of the beam and on some parameters for example also the work function that is the potential that electrons need to overcome while living the surface so the here is the simple relation between the electronic structure inside the solid and the outside of the solid of the automation spectrum so it's just the simple the energy scales just simply translated from the binding energy to the kinetic energy by using this relation so we can see also that there is a sudden drop of the intensity above the thermal level the maximum energy that can maximum kinetic energy that the electrons can have so and just below the thermal level we can see this complex structure this is derived from the balance band electrons that are weakly bounded then deeper and the energy we can see the core levels which are have easier shapes somehow and there is also a exponential growing background that can that is derived from the inelasticly scattered electrons and also the information of this background can also be used for analyzing the electronic properties of the materials for example can be used to determine the work function of the element the core levels are typically used to analyze the chemical composition of the materials also the its atomic composition stoichiometry and so on and the one of the important part is also the electronic structure electron butt structure which is close to the thermal level which is given by the valence band electrons so the main scope of the photomission analysis is to reconstruct the behavior of the electron inside the sample from its photomission spectrum so the typical photomission setup or just need these three elements that is a photon source that can be for example a ultraviolet lamp it can be x-ray tube it can be synchrotron can be laser a free electron laser so on it needs a sample that need will be eliminated by light and it needs an electron analyzer that the most common usually electron analyzer are the hemispherical analyzers and then there are some more exotic types of knowledge like momentum microscope type of light microscope and so on and the sample for the photo emission need to be ordered so it like might be bulk material might be thin film might be multilayer or molecular will rail and so on so these are typical photon sources that are used for the photo emission the first two they got discharge lamps and x-ray tubes these are commonly used in the laboratories in the laboratory photo emission they are say relatively they have relatively low cost they are more available than the rest and they but they have say somehow lower resolution typically the resolution much lower and they have large spot size and then there is also they also have fixed photon energy to for studying the one is bent one normally use the gaseous charge lamps that provide this typical to photon energies and then also the x-ray tubes are commonly used to study the core levels then the synchrotron light which is say the one of the commonly used techniques to study the electronic structure it provides a continuously tunable light between the very broad range of energy it has also high intensity brilliance it has variable polarization and can also have smaller spot size and many other advantages then there are also laser sources to provide the light on the sample and they typically have a limited range of energies but their advantage is that they can permit the time result studies and they also have much higher energy resolution and there are also some more exotic things which are more hard to access as a free electron laser and this is a typical example of a photo emission beam line this is all this is needed just to bring the light on the sample and to study the photo emission so for the photo emission beam line is typically the synchrotron you need all these optics like undulator you need this mirror you need a monochromator in order to produce a monochromated light and to give a touch of a very high energy resolution and always mirrors and so on so synchrotron light based photo emission so far as considered one of the most powerful tools in studying the mass structure of the elements in the photo emission but the disadvantage of this technique is that you typically need a synchrotron and so to access an experiment of synchrotron one needs to write an experimental proposal which is a sort of project and this project are need to be approved on the proposal committee so it typically to perform an experiment at synchrotron you need from three to 12 months from the time you decide to do an experiment in case your proposal is successful so the typically the photo emission is made in this range of energies that is from 10 to 1000 electron volt it's so-called surface sensitive regime so if you look on the inelastic mean three parts of the outcome in electrons that means the the maximum inelastic mean three parts is not more than two nanometers that means that compared to the typical thickness of monatomic layers of elemental materials means at maximum you will prop 10 layers in the surface in the same like if you use five to 100 electron volt you would be it would be even smaller so we'll prop one to two monatomic layers and this particularity in asks for a very clean surfaces and also it asks for a ultra high vacuum conditions in the way that your sample surface can be measured without being destroyed within several hours so and also for photo emission the samples need to be conductive so the samples might be for the photo emission can be either prepared exito like bulk crystals like several topological materials they can be prepared outside they can bring be brought in the in the experimental station they can be clipped in order to get the clean surface they can be exflated like graphene samples either it can also be pre-prepared in situ the same topological insulators might be grown by in big rows the graphene can be prepared also in situ by dosing some gas by chemical vapor deposition and so on and also the clean surfaces can be prepared by spatter in annealing techniques so this is typical layout of experimental setup for study in the photo emission so you need a place from where you insert the samples then you need all these experimental facilities to prepare the samples to dose the gases to spatter to anneal to cool to heat up the samples and also for making some basic checks of the sample surface and then sorry then you can finally when you are think your sample is radiocontransfer it in front of the beam and you can take the your photo emission with the normally with the analyzer so and this is how in the real in reality all this stuff looks like so this big piece is the hemispherical analyzer that measures the photo emission spectra this is the these are the experimental chambers these are all the facilities to prepare like gas bottles and so on these are manipulators in order that you can transfer the samples here and there prepare them and there is also all these big features that are designated to create and maintain the UHV condition ultra high vacuum condition so that the sample surface may remain clear clean sorry so what is the typical how does the typical photo emission spectrum look like so for example if I talk about the XPS or ESCA when we use somehow normally people talk about XPS they are talking about somehow high energies high photon energies but XPS spectrum can be also taken in surface sensitive regime and you can take it also even with helium lamp you can take this spectrum so normally in this type of experiment you are not interested in the angle distribution of electrons you are typically taking the intensity of the limited electrons as a function of their kinetic or binding energies so the conversion of this type of spectra is very easy you just scale the kinetic energy into the binding energy that's all so this is a typical spectrum that is taken on topological insulator tin bis mode telluride these are his it's these sharp peaks are the core levels from which we can understand the sample composition also we can understand the analyzed the ratio between the peaks and so we can check the sample composition whether the sum it's has the correct stick geometry and close to the formal level we can see the valence states which in this particular case are very very weak with respect to the core levels and the this XPS analysis can also provide many useful information on the samples for example this is the study on the molybdenum carbide Maxine and if it has so this study shows two peaks that are related for the molybdenum and carbon which are intrinsically the elements of which the Maxine is built and we can estimate the from these two cores we can estimate the ratio between the two elements we can say whether there is some excess of carbon and say that the material synthesis was correct or not correct to be can also see some adsorbites which are on top of the materials normally these core levels are fitted with different procedure in order to to find different components in the spectrum from these components from the binding energy of the components you can understand something about the type of the chemical chemical state of these elements and finally you can just also without information about the angle angle or distribution of the electrons you can also see the valence band and from just from the shape of the valence band you can say where the material is is elating or it is metallic okay and when one talks of the valence band normally the people study the valence band with angle is also the emission that means that also the emission angles of the electron is taken into account so basically for these reasons either one can use a single channel analyzer that can just analyze one emission angle of the electron and then you can rotate the analyzer but the result is always the same as the intensity of the immediate electron versus kinetic energy either in the modern analyzers you can take in parallel several angles on the on the detector you can take like several hundred of channels at the same time you always same the same spectrum you will have some 600 spectra at the same time and conventionally these spectra are shown like image plots where the brighter color corresponds to higher intensity and the darker color corresponds to lower intensity in this particular plot but there are also some very exotic colors case that are used in Arpe's so then this one need also to link the information about this spectra that is measured to the bands inside the structure for what regard is regards regarding the two-dimensional bands and the simple system it's the relation is rather easy because the parallel momentum of the electron that is parallel to the surface and conserved so it can be directly directly converted from the angle using this formula and the binding energy of the electrons can be also get from its kinetic energy using these simple relations they can be simply relate to the Fermi level if the sample is metallic and what is particular also of this type of conversion that the spanded range of the angular of the wave vector depends strongly also on the photon energy so the lower photon energy you use the smaller information you will get in it can also be the the field of you can also be widened but in the case you have to take several spectra rotating the sample and so on and then in case one wants to study the anisotropy of the sample one can take the Fermi surface mapping of the sample either he can rotate the sample in front of the analyzer or he can adjust like here tilt the sample in front of the analyzer also in some more than analyzers this procedure can be done from the analyzer optics and then one can get a three-dimensional set of data if from there one can build one can cut and get for example the Fermi surface if he cut this cube at the Fermi level are the some constant range of cuts and he can understand something more about the bands anisotropy so the band structure in the case of simple two-dimensional systems can also be very easily compared to the LDI DFT calculations for example here I show an example of silver thin films and it's electronic band structure which is an analog of the particle in a box picture so we can see this number of quantized states which are quantum well states in the thin films that are due to the electron confinement in thin film and we can see that Arpus can very nicely show very similar parabolic like states which are which have most formal so it's very similar to what DFT calculations predict next there is a some example which we did these are example on quantum well stage which we are observed on iridium or on a thermium in films so we can see that for sufficiently thick films we can observe and I'm sorry a number of states that cross the Fermi level and this number of states increase with the film thickness from where we can deduce that all these states are coming indeed from some bulk state which is supposed to cross the Fermi level but they initially the DFT calculation was not able to explain our findings and because it was providing some very different quantum very different electronic structure with later states and they were all located above the Fermi level and in this case it was shown that one can improve the agreement by taking it out the self-energy correction in this way he can get very good agreement with the band structure so not only after including this self-energy correction it was found that a thermium also have some HCP thermium also have some topological properties which emerge after one takes into account this self-energy correction by Hubbard correction so it's in this way the R-PACE and DFT were nicely collaborating and found also sandwich which was not not there before that was the first evidence for a direct nodal line in Atlanta with metal so then also there are some examples of other simple band structure like graphene for example for the fringe state on graphene there is a very good agreement between the band structure the calculations and also graphene on a low interacting substrate like graphene or silicon carbide and you can find this diracon very close to the Fermi level and the only difference with the calculation might be just the energy shift of the bands due to the doping and differently in graphene that is grown on some interacting substrate the R-PACE can provide the information where there are strong interaction with the substrate if in case you see the now for example this diracon that comes to close to the Fermi surface you can judge that there is the materials is more free-standing like but when it is for example the nickel is intercollated below the graphene you can see that graphene is neural destroyed and so R-PACE can say that there is now there is more interaction with the substrate in the same way it can work in the reverse way so when the graphene is strongly interacted one can intercollate the silver below the graphene level and restore the linear shape of the graphene and say that okay now the graphene is mostly free-standing so R-PACE can probe also the degree of the interaction of the material as a substrate. Another example of the study of the two-dimensional material is the study of silicene. Silicene is also the analog of graphene that is built of silicene atom arranged in the Haudencombe lattice with a difference with graphene these atoms are slightly this sheet is likely buckled so it's expected to induce some particular properties in the graphene band structure in low buckled configuration the graphene is expected or silicene is also expected to show a diracon close to the thermal level very similar to that of graphene and since silicene was grown on a silver 111 surface there was an early R-PACE report that they also observed something which was a linear band close to the thermal level since they were expecting a linear band from the calculations this okay it was reported as evidence of the diracon like state in the silicene but indeed then if one analyze carefully the band structure in two dimensional it take about the anisotropy it can be seen that this band was not having the correct topology does what not echo it was just said the like point that means you see maximum along one direction and the mean my along another direction and indeed the calculations that we are taking an account as substrate we're shown that there is a very strong interaction with the substrate so the original diracon is destroyed and so there is no more diracon and also the states which are seen they are mostly the silver states that they are modified by silicon okay so just just to to be sure that not every linear band is necessary a diracon and they always requires a 3d analysis okay so shall I go to go fast okay then I can skip to probably this part just I will tell you about the importance of the topology of the photon energy-dependent arpus that is provided by synchrotron the band structure is a previous lecture thought is very sensitive to photon energy in particular you can see different features in a different manner for example this is topology conciliator bismuth selenite and you can see that there is much more intensity on the surface state with respect to the bulk states in the value of the photon energy you can choose mostly the feature which you want to see it can be also very crucial since for example in this case there are some selected photon energy where you can see that topological surface state while it's some a different energy you do not see it in the initially it was claimed that there is no topological surface state in this type of anti-flare magnetic topological insulator and also the photon energy dependent arpus can provide the information on the bulk bands of the states that can they can also be converted to the wave vector of the electron inside the material the relation is a bit more complicated and also needs to need several assumption like for example like the final states in the form of free electron like states then from there one can also relate the variation in the intensity of the states in their position with the bulk states that are shown by calculations and okay there are also okay I believe I do not have much time okay there is also the one of the most advances of the arpus that can allows to measure the spin result band structure it can be also done in not only for one spin channel spin direction but also for all three dimensions one can take the vectorial spin analysis for example the calls also be done in the form of angle is always for the mission together with the spin resolution can provide these bands the time which is required to take this type of maps is due to the sensitivity of spin resolved arpus is much higher like this might take several hours with respect that this might take several minutes it's there just this order of magnitude higher the time which is needed this can be done also for topological surface state they are spin resolved arpus can be also used to resolve between the individual bands that are closely learned as they're so close that even the band width is higher than the peak separation using the spin resolution they can finally resolve also this be the features which are not possible to resolve otherwise okay there are also the possibly the laser biased arpus can probe the non-occupied band structure and also can allow for studying on the dynamics of the states that are excited there are also I have to skip probably a bit faster not because then there are also some extensions to far because they can use the summer focus in elements and then allowed to reduce the spot size on the samples to several microns in this way one can study and in homogeneous sample and then can find very very small pieces of the elements and study their band structure one can also use the electrical gating for some layer of materials in this way apply the electronic voltage to the electrical water to the sample roughly speaking and also move the thermal levels of material and access the non-occupied state this is some very new experiments which were recently done also tele-synchrotron and one and okay in the last part is that one can also use another type or completely another type of electron spectroscopy that instead of measuring the spectra it can measure the inverse it can take take all the electrons from the surface and they are all projected on the image plane in the so instead of spectra you are getting this directly you get these constant energy cuts and then with this is called my mito microscope because they are not resolved in angle but they are resolved in inverse angstrom space directly in this structure can also be coupled to spin but in this case so far the spin is measured only along one spin quantization axis in this way you can also get directly the constant energy counters that spin resolved and you can take them in form is photon energy and you can build all the three dimensional one structure of the element with spin resolution and okay and okay so I come to the conclusions of automations petroscope is one of the most direct experimental method to prove the electronic structure of solids and there are rich variety of light sources and detection systems and the arpes is there are a lot of new developments in the field of arpes new much new instrumentation is coming in the last years and okay the possibility of arpes are continuously growing thank you very much for your attention so thank you very much Polina I think there were very nice examples from which to start for the discussion so let me just start with a comment for the example of silly scene on silver was the example and I think I mean I guess that one of the points of this talk is that arpes is what is a very powerful technique to get the most structure and to directly connect with what we do but there are a lot of technicalities which Polina was discussing the photon energy the sensitivity to the surface the substrate etc and so we have a really to pay attention to when we do a calculation not to jump to the conclusion that we can directly describe what is measured experimentally and I think the example of silly scene is is nice because I'm sure there was some theoretician who was ready to say yeah this is exactly the band of silly scene I'm able to compute it and I get the same but in the end it was the band of the substrate yeah it was just as far as I remember it was depending also on the thickness of the slab one was used for the calculation when one was using like a five layers lab one was shown a linear band but also these linear bands are coming from silver itself exactly so the point is that pay attention not to just directly jump to the conclusion yeah okay so then we can start with the questions maybe yeah and first of all if there is someone in the audience okay so I just want to know please can you please comment on this the layer resolved measurements how deep can you measure sorry the layer resolved yes so how deep you can go in in the sample how deep we can go on the sample it depends on the photon energy basically if you look oh sorry if I look on this photo this one so basically you look in depends on the photon energy in this photon engine where a photo emission probes it can go like two nanometers that means up to 10 atomic layers hello I just want to ask about the part that AR PES plus spin that you introduced is it possible to find the lifetime of the this electron that just excited and going to the higher state I know that there is an example for also spin resolved like these are I just introduced this time result studies I know that these can be also preformed with spin resolution okay the problem of this time result studies it was in my experience that you always normally you have very low resolution so you have already low resolution when you study the time result structure plus when you are reading the spin you are normally decreased by 100 of times so the so you increase and by 100 of times all the acquisition times so these are some particular measurements I have seen some of them in the literature but it can be done as far as I know okay thank you and then pulling in the study measurement you can also inspect the line with to say something about the lifetime no yeah probably yes yeah okay yeah okay we have four questions from from the chat okay can one say that there is been crossing if the absurd band gap is of one to ten millilitron volt oh it depends on the resolution of your system normally so there are some dedicated beam lines there we study very high resolution you have to go to very low temperature probably you have to increase your resolution by several times probably with a laser laser basing techniques like here you can effectively do this and the second question is that is how can we choose the exact probing energy in order to foot to eject the electrons from the materials I think is the question is how you select the photon energy basis of what normally what you select in the in the photon energy is when you better see the features so you can when you have a tunable energy you can just take several energies and see select what you are interested in where you have better contrast there to see when you can have a mission from the sample you just need to have a photon energy that is higher than the work function it's typically I don't know four five six seven electron volt and there is action question interesting about the minima in the plot of the main free path as a function of kinetic energy is wondering why there is a minimum attendee 100 eV yeah from 50 to 100 eV yeah because okay I'm not ready to answer this question I know that the lower energies there's electrons can penetrate deeply into the sample that that what I know but it was I believe it is also not available for the curve is not also is not available for the materials yeah I believe the theoretician may not answer better this question can we detect electron phonon coming by arpus yes basically we can I'm not expert of that because our beam line is not some very high resolution we do say a lot of a bit of everything so I know that electron phonon coupling for example can be nicely seen here even in the copper 111 surface state when you use the laser by start but with has a very good beam stability and the very good energy resolution and so on so you can see this kink sorry the electron photon company can be seen that you do not see a straight bonds which comes directly to the thermosethos you can see there is this change in the band slope if from there one can estimate these different parameters that are related to the electron phonon coupling okay the last question is that is how tricky is to prepare these samples for performing arpus measurements can we control the environment to avoid for instance oxidation at the surface yeah of course that we say in our system we can do that because in our for the mission beam line we can go very high in the energy we can okay here it's not written but we can go up to one thousand of electron was that means we are covering binding core levels of the oxygen so we can also follow okay like it was in this example we can also see what happens to the oxygen how it's difficult to prepare the sample it depends also on the sample some sample like topology consulate or you can just bring into the vacuum you cleave the surface and you can measure it sometimes you can spend several days just trying to prepare the surface and you are not getting it it depends a lot okay so any more question from the audience so may sound super stupid but how do you separate the band structures of the interested material and the substrates for example how do you post process these things one can distinguish between the surface related features and the bulk related features by taking the photon energy dependence if you have a synchrotron then you can tune the light and basically the surface related features like here you see this sorry this cut the surface which is the surface state they remind fix it they do not change their position with the photon energy while the bike derives feature they normally change it also is not always like this for example in the case of silly scene that feature that she was reported for silly scene as a diracon it was not changing with the photon energy but because this is some particular silver states that stays there because the density of states is high there okay so thank you very much Pauline again I guess it's time for lunch yeah it's ready to go to it and okay so now for lunch we