Okay. Good morning, everyone. I wanted to discuss our latest results, but Lawrence

twisted my arms saying that not everyone is expert in this audience, so I'll start with a usual line. The first ten transparencies, how great graphene is and so on, and then our case, which 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. Yes, 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. 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. So we measure everything in football pitches, as you know, and it's according to Columbia Group, it's the strongest material ever measured, and 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 percent

as several groups, including our groups, have shown. It shows rapid 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

record, 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

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 squeezed 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 superlatives. For example, there are groups

who claim that it shows the best merit figure for some electrical conductance, and recently MPL group has shown that it's the best material for quantum hall effect standard.

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 put top gate, and

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 large nearly half electron volt 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 scatterers, you get really record intrinsic mobility in the material, which all translates that you can study properties very nicely

and can find the cleaner materials that usually the more pronounced properties are. So let me a little bit update on what currently current status of quality of graphene, mostly it

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 we probably tried half a dozen, no 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 pyrrolytic graphite but it didn't show any improvement but there is hexagonal mono-crystals of boron nitride,

you can get from actually a 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 mobilities around 100,000 but sometimes we reach half a million mobility at low temperature and a couple of hundred thousand at room temperature. Another way of getting good graphene was

demonstrated by Ivan, Ray, Philip, Kim and Amir. You call this 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 200,000. We managed, don't know why but our devices sometimes shows couple of millions mobility. This is an example, Shubhnikov-Degas oscillations start

at 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

least 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 low 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, of course it's electronic structure.

Practically everyone has seen this picture. I believe it's in graphene. It's not shredding 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 carbon 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. It's already a rather complicated story. So all these sort of this picture allows you to think how we can compete 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 deemed by particle physicists or nuclear physicists

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 super-critical regime when coupling is really strong between impurity and electron. Hopefully, Mike Cromy

one day will report. There are phenomena which also contradict to what we know from condensed matter physics. 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 through metallic regime and conductivity 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 it's

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. 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, anti-electronics. I believe Fiden will be speaking about this graphene instead of ITO. At least this is proven in academic and in some industrial labs that it's feasible. It's still a long way to consumer products

and some applications like conductive ink and batteries allegedly graphene already there although 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 out

of five, six years even of such intensive researchers 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

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 a few competitors, okay, like Philip and ourselves, okay, we measured shumik of the gas oscillations as a carrier concentration measures their temperature dependence, analyze it, extract it, cycle

it on mass and from cycle it on mass we found that cycle it on mass is a 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's just velocity slope of the spectrum is this number with some accuracy both our groups reported this. There were many other measurements which essentially gave the same number for velocity in this range of concentration 10 to the 12th. Now we have those suspended

high mobility samples and what is most important that they allow us to go to really a very small range of gate voltages and concentration and study how this Fermi velocity

or slope, how the 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 typical numbers we extracted at high density, that's where we would expected the corresponding

curves. To fit the data we need in this particular case twice higher Fermi velocity so and this is not marginal it's okay a factor of two difference in slopes and in temperature

dependence it's qualitatively large effect. So Fermi velocity changes with concentration, slope changes with concentration and so that's for one of the samples we get this data that's the previous value and that's for high concentration for concentration like here

on this scale would be and it goes higher. So what's the origin for this? The explanation has been long time in literature so electrons are usually in metals they interact with each

other and so only when you have a large concentration of electrons they're screened enough by other electrons so you can use them as a single particle picture. Picture Landau Fermi liquid theory. What's happening in graphene here near the Dirac or neutrino

reality? Point concentration goes down and interaction becomes extremely strong and that's what we expect from previous theories 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 system. Each time you change your concentration the Fermi velocity changes but it remains constant

underneath the Fermi surface so it's sort of dynamic this slope is dynamic. When you probe different Fermi velocity according to theoretical predictions which go back to particle

physics actually well before the first papers considering this theoretically. That's a collection. Let's see how it matches with the theory. This is a collection of

data for four different samples for suspended devices and those two curves. This is a 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 higher. 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 theories which is cell 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 incorporate itself consistently into this theory that graphene cell screening changes with concentration. So it's reasonably good fit to all the data. One thing I have

to mention there were a couple of papers, 2008 and a recent paper which worked in this regime larger than 10 to the 12 and they reported deviations from a constant Fermi velocity

by 25 percent and if you extrapolate those deviations they would go much, much bigger effect that we observed. 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 described in 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 theory.

So a message to take away from this particular research that there are renormalization in effect that they're modest or 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. I'm happy to confront

you. In order to see Shubhnik of the gas oscillations the sample should be bigger than cyclotron orbit. Automatically when we see Shubhnik of the gas oscillations we are in the regime when the sample is larger than cyclotron orbit otherwise we wouldn't

see anything and if you estimate that in some cases we do see cut off of Shubhnik of the gas by the size of the sample but so far the quality is not as good to go into this regime. That's what Nelson Mandela would call I have a dream. So that's what

I like to share with you. Somewhere five, six years ago we reported that it's not

only graphene and I have shown this transparency ad nauseum for some people for many, many years. So, except for graphene many other materials are led and tested. Single layer can be extracted or a few layers boron nitride, bisco, dicalcogenides and so

on. That was at a very slow burner for quite some time only recently boron nitride has come into play and few papers on other dicalcogenides were published but okay not

as popular as graphene. So what I always pointed out that some of those materials are insulated, some metals, some semiconductors, some superconductors, some ferromagnets. So there is a huge range 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 happen. So how do we do that? How could we do this in principle? This is what already people in Manchester, in Columbia have been doing in several other places. 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 and lift it off, then place face down

a line with another layer on the substrate dissolve and you get two layered systems consisting of different monolayers for example or 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 for the sake of gravity let's say putting

those materials together then someone from Samsung 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 thickness but they are stable enough. Other materials are less stable sometimes. Quality is not that good so you have to deal with few layers of those materials. So if you were go to single layer we mostly limit

it 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 tunneling devices. So the question is okay usually you do evaporation and so on why we wouldn't use one of those insulators as a atomically thin barrier something what MBE can't do. That

what we have done last year we learn how to isolate boron nitrite in single by layer and other qualities that's way more complicated and difficult than doing the same with graphene

the contrast is extremely weak it's enhanced and it's in gray scale and so on but we can do it reasonably reliably in our experiments. So what we started with is making gold finger

put in boron nitrite say single layer or few layer put contents on top so we try to assess properties of boron nitrite as a barrier if you put graphene here you don't

see anything we tried before the resistance is incredibly small you can't see a barrier but with boron nitrite lay seven layers it's about three nanometers it's an insulator and then you start seeing tunnel current at high voltages before it breaks down and

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 a linear weakly temperature dependent dependent IV characteristics and what they can be translated in high gap and effective

thickness actually larger than what you expect three layers we don't we're not sure why it is so but it requires okay first principle probably analysis what effective thickness of one monolayer and but most importantly what we learn from this one

is that there are no pinholes in the system which is very good news for the material to use as a tunnel barrier we can do a little bit better use conductive FM that's cost this result here with his pulse docs 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 a tunnel kind of behavior which is dependent on the number of the layers and we can scan reasonably large areas many many microns and again

we do not find any pinholes which brings me to conclusion to this part that boron 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's 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 boron nitride actually there are advantages with respect to putting in

on top of boron nitride as Philip Kim and Jim Hone did recently we covered it with another graphite layer and it turns out not to be a marginal advantage

because really for those who work with graphene know that each thermal cycle exposure to air changes the device you need to anneal and etcetera so we find those layers really protected they're 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 our structures that's okay boron nitride substrate and these are etched

away in graphene and this line indicates that here there is a sickish layer I don't remember 10, 20 nanometers thick boron 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 they show pretty

nice characteristics mobility up to 150 I would say at concentration 10 to 11 similar to what the Columbia groups reported 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 although the mobility is still sort of 100, 150 okay

as in other devices many of those okay we'll say we studied tenths of the well 20 devices by the moment for the moment so okay we in those devices we use our favorite band

geometry putting currents through these two electrodes and measure voltage here and then we'll find out that resistance for four 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 reversal of sign that's persist up to 250 and probably room temperature

in some devices and we know how to interpret this its negative band resistance its 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 galley aluminum arsenide two dimensional electron gases what you can do from this negative resistance and its behavior you can estimate reasonably accurate within say 50% you can estimate

mean free pass and extract mobility for typical concentration and it 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 do not show this

negative band resistance so and numbers okay for room temperature are so 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 encapsulations really helps at least it 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 band resistance some electrons no longer go into this contact by reflected weakly 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 Andy Yana and Philip Kim reported so it just tells you that the gate does work pretty nicely when boron nitride is used in the gate my last subject is a little

bit more complicated 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 boron nitride boron nitride boron nitride on top so it's double layer structure similar to those which were described in galley arsenide 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 across crystal of boron nitride a sickish say 20 nanometers then a layer of graphene etch

it into a whole bar structure we want then another layer of boron nitride with a chosen sickness say from one nanometer to 20 nanometers whatever we want then graphene etched again deposition of the contact so it involves three dry crystal transfer

okay with with crystallites they're not touching annealing this at 300 four rounds of electron beam lithography three plasma etching two metal deposition and a hundred times of cleaning

structure and removing the resistance so so good thing 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 boron nitride that's the bottom lane false color orange brownish

color is the bottom layer here on top of this bottom layer there is a flake shaded here by a sink counter that's a flake of boron nitride sinish 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 get okay mobility is a hundred thousand lower in top layer which remains usually exposed and actually a little dopey in separation

which rides three to 20 nanometers so far as its top layer usually deteriorates after after exposure to air and this bottom layer remains okay pretty stable for very long period of time what we can do because those layers individually contacted and there

is no for sick boron nitride there is no leakage between those two we can apply back gate from the substrate which is colors are wrong here okay we okay red 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 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 all that's better colors we we can we 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 non-linear 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 add 70 kelvin or something like that and distance is large then okay that's a typical curve which we usually measure change in concentration in the bottom

layer and then we add something to the top layer and nothing happen in what view 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 electron screening in the top layer that's better seen here on this picture on this picture when you put

the spacer 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

uh metal insulator transition of 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 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 a 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 valoia falco we know that in graphene whether it's on silicon oxide in boron nitride 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 probe a 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 four e square or something like that so within each

puddle we have a matter so and it's probably extends to boron nitride 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 from this part of the research and

the only one thing i would like to add let me skip it because i'm i'm running out of time time let me skip this transparency so let let's take is this as a conclusion so you can do

double layer maybe triple layer for layer okay heterostructures with graphene and boron nitride 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 discuss so in this case we typically get okay smaller separations four 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 resistance is its 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 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 nitride they use silicon oxide as an

insulate and mobilities were low low and they couldn't go into say from one ambipolar regime was not possible in the experiment it's very easy achieved in our experiment and what we actually like to simplify the situation and study tracking in this symmetric regime when you put holes in one system electrons in another system and they have

the same concentrations that there is a beautiful behavior which shows that the drag decays when you have more charge carriers and it shows a dip 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's the theory 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 and this is most interesting situation but let's try to understand what's 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 the dirac fermions or schrodinger fermions doesn't matter it's single if we go to concentration dependence how it decays here well we find out that it's not that quick decay the decay is much slower like one over and q sorry a square squared highest concentration

to less than one over n 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 theory 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 wavelengths of electrons in any of those systems so usually people who started two-dimensional

gases in conventional systems and Sankar as well they thought okay it would be the same weakly interacting regime as has always been studied before but in our case we can go to low separation and even 400 nanometer separation we are no longer in this weakly interacting

regime and we are actually in strongly interacting regime and those different concentration dependencies shows okay after we presented this at a couple of conferences of course several groups came up with predictions what we should observe in our

experiments and it's pretty good agreement agreement with with siri so okay to finish this part okay it's uh you can do with graphene boron nitride and other samples okay

very complex heterostructures quantum wells vertical devices not only in-plane devices what mbe can't do it you can't do a single layer continuous layer by mbe 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 all collaborators especially roma garbachev 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 talking today and finally thank you for listening me

without interruptions for this very low density uh 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 i i know that in sort of the uh gallium arsenide two dig people they talk about creating vigner crystals you know when you get into these very low density regimes and the electrons start isolating and forming these patterns due to these interactions is it possible to get into this kind of regime in these graphene uh 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 uh what what i 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 drag and these features

i didn't i i don't have enough data to present those but they are seen vigner crystal with in suspended samples we didn't see anything there is a paper we i presented it a year ago with it's not an archive where we study bilayer 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 has vigner crystals but interaction effects 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 yeah

germany good um kranjic 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 uh i think it's i didn't mention this because it's sort of consensus in the was say even before amir yakobe mentioned by stm so there is a say in 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's full electrons sometimes holes and this is this is when this is why we call not dirac point we call it neutrality point because it's usually consists of puddles of

different signs of electrons holes electrostatic potential just to follow up on jeremy's point have you had time to apply magnetic field in this percolation crossover uh or if you have what will that tell you in which when in the bilayer in by in double layer

yeah structures by the way it's a different system sorry yes we tried but uh it's so complicated system so you see many things okay what they mean uh we are focusing at the moment

in in zero field regime we we do apply uh puddles are we see the drag it increases which one of indications of uh interlayer excitons and so on okay something going on but and

probably not related to puddles because boron nitride is much nicer system and the size of the puddles is larger than the interlayer separation okay thank you the philipp came from columbia um here yeah yeah the i have questions on uh on this and this localization limits that

uh when you have the top gates is cleansed out uh what is the role of the pseudo spin in that case say um don't we okay no collision because of pseudo spin yeah that's that's a good that's a good question so essentially what philipp 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 or 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 scatterings for example we don't see any sign of metal insulating transition in suspending devices they go straight to a finite conductivity in suspended 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 presence 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 inter-valley scatterers which are a minor contribution but still present there so the answer is you need to have inter-valley scatterers boron nitride is transparent it's uh 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 absorbs twice two layers absorbs twice

three layers tri-layer 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 uh excitonic phenomena at what what they are at three at five ev and there is a tail

goes to visible light but it's uh in suspended devices it's usually small on substrate it's usually higher but otherwise two three percent accuracy for this uh 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 them i mean you've got it normally resonant tunneling momentum and and energy conservation yeah it's it's it's a good point so for example uh with

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, which is called confusingly epitaxial graphene where planes are also

randomly rotated and we know if the angle is reasonably large there is a very weak interaction between those layers and so even when you have a perfect match in the constant, just

rotation already decouples less efficiently. And with the boron nitride, you haven't looked at the effect angle yet? It's your dream.