 Svoje cilidrom sem možemo počkej, da grimo pristih, zato sem tko včasno zelo kaj je zelo, nebo prejzavljar, kak je vstupen. E.g. Iron Silinium, ki je iznačitiva, zato sem da je počkala, da imamo na teba, da se kovakarujem vzelo. Vzelo se v sej progrupov, Laura van Frihlen, Valen Valenzuela, zelo, ki sem počkala, da se neko se zelo, Manuele Cappelluti včešljič v rovnih vseh, in izvečnju, kar so pričo, comunikajne z venikimi izvahčiti v Peresce. The first point I would like to try to convince you that it was a good idea to start this workshop with session in Iron Selenium. Because even if not everybody here live and breath Iron Selenium as some of the speakers in the people in the audience, this is a nice system, it belongs to the family Iron base superconductor, kaj ne vseh ne zelo koralitivne materijale. V seboju, da je to metalice sistem, ne zelo koralitivne, ali je začel v realnih moutonče, ki smo zelo vseh zelo v zelo, ki je zelo v fermiči, in vseh zelo v fermiči, predivno z dencijnjelja teori, je prizvečen in akulativno izvah, zelo v izvečenih režim, je to vseh hradi vzela vseh superkonduktor, in tudi spolivno. Je to ne spolivno, da se počutimo in se nešto izgleda. Počutimo, da se je, da je ne spolivno, da se je, ta je metallik, nešto nematik. In potem zelo pa vse izgleda, nematik, kot nekaj nekaj, da se pa vse izgleda. Kaj je rejst, da je vse, da je vse izgleda, nekaj rejst, da je izgleda. kako se izgleda, da je je tudi veliko mlače, da se je predizvano. And then this LOTC superconduktor is the one in the family highroom printize, where it's much more difficult to explain the gap anisotopi. And this is one issue that several theory groups are fighting on recently. So in some sense, iron selenium exasperates all the passing effect that are there in general in iron-based superconductor. And this is why it's interesting and this is why there are so many people in imeli smo je zelo všeč da vsezpene v terzy več jaz. So, da vsezpnega se, kako izgleda in silenja in silenja, da sem zelo vsezpnega in silenja v fizijske oprike tajs. Vsezpega se, da se napravite na komentarje, in pri studenti ne različite vse, zelo pa bo se nekih idej, nekaj ne bi vsezpega vsi, nekaj nekaj da se napravite na komentarje. Taj so materijal, kaj bo vsezpega na izgleda, na izgleda, in silenja in silenja, in in vseh lejer. Faze diagram je seminarne z vrstvenim in otroj koralitivnimi sistemami. Kaj da je sistem, da je najbolj manjetične vse, nekaj je to obržen, ta superkondativitje vse. Takaj sistem je z vzbim v multi band. Kaj da se vzbim v terkome uniji cel, ne ste imali vse vsevrje pohleda in elektroniko pohleda, ali ta trasnja vse se pohleda in nokrča, in da je ta vse pohleda in elektroniko. Ekatera taj poslutni unit sel je pomejnit z dvej ironi imel vjavnih atom in tako, bo isuprati, in silinijtih atom vse je prizvene vza oblad, kjer se jah bi ir ne stranje. Tudi, odprav je reelo unit sel buenoj, tako, da je svoj vse taj neštrunstvar na konev reprodukulje in zetnih atomov, in tukaj še v zelo všem vsega je vsobit. Na vsega experimentujega je, da bi se na tukaj vsega vsega vsega na gamma, na m, in vsega teori vsega vsega, da bi lahko ta časna vsega veča in zelo. Protože je lepo, da se vsega vsega vsega vsega, da se teori in teori počusti. Nekaj, da se vsega vsega ta vsega vsega vsega vsega, in elektroni. Nisem vsega vsega vsega, da se vsega vsega vsega vsega vsega vsega vsega, vsega vsega in teori je počusti. Tako je vznik, da majske vsožite. Vznik njega je in elektronika poživna. Vznik z njega je veliko zelo. Vznik njih je užil, zato, da načinem ta začočen spetul, da prunem začočen, da prunem zelo, zato je minus zelo. Vznik njega je užil, da prejvoje začočen vsega njega, da je to vznik in sega. Vznik njega je vznik zelo, da je to, kaj je vzivnega logaritmijska in vzivnačenja. Tukaj, da vzivnega in vzivnega vzivnačenja je zelo vzivnačenja na rpa z vsej tukaj, na svečenju sektoru. Zato je tukaj vzivnačenja, kaj je vzivnačenja vzivnačenja in vzivnačenja na vzivnega vzivnačenja vzivnačenja vzivnačenja, vzivnačenja vzivnačenja in elektroničnja. Zelo je zelo, zelo je zelo. In, when you adopt the system, the Fermi level moves, so, you go out from nesting, so the spin variability is gone. But there is residual interaction between the whole, like, electron-like pockets, which are mediated by residual spin fluctuations. And this can be a glue for pairing. And this pairing will lead to s plus, minus superconducting or a parameter in the sense that if you now want to solve this couple VCSE equation k je to neko lahko v dakle v elektroni, k ki se se neko neko lahko tu nako lepo, incelja celja in rupalsi, skazal jo si odjeljenj vzor v t斌idov prameter in zopraviti sem ovega nekaj vzaj. Zelo, to je nekaj, neko da je vički argument, da je toedge, niči argument, ko je preprostao v ter laj putov, kaj tudi za outfiti v kaseli. Roshne, da se je res rečeno, samo da, da je skupana, da je kar spremno vzelo, zaato, da je učnila, začetno, po vrstej naši energij, začen je je zelo vzeljeno. Tudi je izgleda v tega včasna mikroskopična modela, kaj je vse obojezaj, oj či zelo celinu. Zelo ki se odstavljajo, da se obojezaj obiže, tudi zelo je običan izgled. Zelo je začetno v Krištelju, jezak je nekaj energij, jezak je zelo v zelo zelo zelo zelo zelo zelo zelo zelo zelo zelo zelo zelo zelo zelo zelo. Zelo, da se je začetno, načo je izgleda izgleda, s ko poču, občise je, that this pictures work more or less well. One that you overall renormalize bandwidth of the electronic state in rispective DFT calculation. So, there is a C, that you can squeeze your bands, and then you can end up one with the low energy model, which is the one that I was showing before in the ơi rek podridaux effect v Milwaukee. This somehow provides a good description of the Fermi levels physics. Drisko retali Flash, 녹ißen iz NATO PER SET Assemble vsk音 pack nekaj je, da se prišli, da vidimo, da je bilo dobro. Če však. Dej, da je bilo dobro. Tako, pošli, da je, v FES-E, nekaj le bo izvečil, o druh ponekv. Tak, tak. V štih, da je, sem tako, da je neko... Nekaj neč je neko skupovati vzvečki, nekaj nekaj pravda nekaj. Nelj ta tukaj nekaj, pa vse partirimo in da prvi, posledajtečne spočrjevce n replace ateli. Pa počekaj do zelovačneho stručnjavih, zapravim, da sem češa, da ne kreče orošnje. Zelovačne korolje prej pomečujemo skorotno z pravi za zelovačna očke in elektronice. Najlepši v mostu, ta smutna zelovačne zelovačnega istrano. nešto prej pahne, či ne bo in nešto presično. Zato bomo mene zelo nešto poživati, neče, da bi priješel, zelo nešto priješel, nešto poživati, nešto je. Poznačite, da je iznovu na svakom tega, da je nešto iznovu, da je nešto iznovu, da je iznovu nešto tega, kde je je, nešto poživati, nešto je. Zato bomo nešto poživati, nešto je. Veseli, da je tudi tudi tudi, da je izgleda drtveno vzelo do 3 elektroni. Vse je to nezatelnit, da je vse z 2,3, moj da nezatelnite, in bilo do 1 elektroni. Vsreč sem izgleda, da se je vse začunil do 500 mili elektroni. Vse je to, da se nimi se zelo na ovo energijstje. Tudi je to vzela vzela, da je to vzela vzela vzela, vzela na matiziji in sovno. Eno, nače ne zelo, da se deluje, da sovno vzela, bo to možete čempeljone napakovati, ali pa je o jebadešen诚eng, v načaj node objezavaj, dar na eventualnosti da ne napakovate v milijetumbotni, nerej včeljem師ni, ne težel, tako da všeč srednje bo potatoes, da je tukaj k pola. Mi narrujem, da nam je vseところ težel, zadebeno pravje, neko težel, nekaj težel, nekaj je zelo, sprinflatacj vzivo RPA. Daj je prihodnju modu. Pačiš sprinflatacj in kredite vzivne brrati sa mene povrste. Vzivno ne比較 pojave, če se v gripsimitri in s plusovim su papravih parametrov. Ta neče vzivna, nečo bolj, vzivač po kašku andi quantity. To zelo konog sprem, da je počet kez linstva s nãoo početna mene je prihodnju. Je zelo skupno, in vso prijasne koncepti je zprenostil, na which I Together will discuss later in particular for Island Silenium. So far, so good. Everything is simple. Even within this very simple scheme where we have a whole electric and electron-like pockets which are interacting thanks to spring fluctuations between them, what can have very interesting physics? So having whole electric and electron-like bands in to je zelo inžel, ne zelo na vsej dobrojj vsej superkondakter. Taj je vsej zelo, ki smo tega daj o zvonju, kjer bi splah v vsej vsej fizikljno vseči, kaj je benššinj, nekaj preselj, nekaj vsej inžel, za superkondakterin. Ko bo me da ga vsežel na nekaj del, je to iz vsej benšinj, ker je prej se izvonj, ki se je začeli, in da ne vsej na njela vsej superkondakter. Tako, ki je tudi, Or DFT more or less reproduz the topology of the Fermi surface on these materials. But the real band, as they are measured, for example in the Azvan Alpha, have pockets much smaller than prediction. So this shrinking of the Fermi surface has nothing to do with the band normalization. So if you change the mass of the band, it doesn't mean that you change the Fermi surface area. You're just changing the slope, which is the Fermi velocity. So this is a picture that is not usually captured by the dynamic and material theory. And this is in the Azvan Alpha. It's also observed in arc-based measurements. So it looks like your bands are shifted up and down, giving rise to smaller Fermi surface. And of course, since this is a semi-metal, you still preserve the number of electrons, so this effect doesn't violate the number conservation. So a very simple way, somehow to explain this effect, is to consider the coupling of this electron with this residual spring fluctuation. And this can be done within the Liageberg approach, where essentially you compute the self-energy of the electrons interactive with the collective bosonic mode, which is here model as the typical spring fluctuation spectrum. But what is important and specular to plink ties, as compared to other systems, is that the bands here are almost empty, which means that you are doing a Liageberg calculation, relaxing the usual assumption of infinite bands width. And this give rise to a real part of the self-energy at finite zero frequency, which leads to essentially to a band shift of the electrons. And again, the main mechanism will be still whole interacting with the electrons, because the main exchange is the interband one. And it is very interesting, because actually, when you try to do this calculation, so we do full numerical calculation, where you can estimate in approximate way how large is the shrinking of the self-energy, you see it depends on the asymmetry of the band. So the distance of the band at the top or bottom from the Fermi level. The sign of the shrinking depends on the character of interaction in this sense. It should start from a whole like band, then you ask yourself, what is the effect of the self-energy on the change of the Fermi wave vector? You have two possibilities. If the band has a strong interband interaction, so somehow the band fills itself, this can be a typical mechanism for interacting with the phonons, for example, the total sign of the shift is positive, so the band would expand. And this is, for example, a mechanism that we recently invoked to explain some pump probe experiments in GB2. But if the interaction has a dominant interband character, so the whole like band sees somehow the effect in the electron-like band, and then this number changes sign. It's negative. And then you have a shrinking of the Fermi surface. So having expansion of shrinking of the Fermi surface somehow gives you indication if the dominant interaction are interband or interband. And in the case of pinning times, which I tried to do calculation in very simple model, so parabolic bands in orbital character, this is a work back in 2009. And then we modeled the interaction as interband once. And then you see essentially the shrinking of the Fermi surface, the white lines are the bare bands, the color plot is the spectral function for the normalized ones. And then you see this shrinking of the Fermi surface. So somehow the most likely candidate for this interband interaction are, of course, pin fluctuation. Or if you want, the systematic observation of shrinking in pinning times is an indication of relevance of interband interaction, and the most likely candidate are spin fluctuation. This effect is temperature dependent, because the spin fluctuation itself are temperature dependent. If you move up in temperature, somehow the spin mosque go over, and then the self-energy correction becomes smaller. And then the effective size of the Fermi surface becomes larger. So somehow the pockets are expected to expand as the temperature increases. And then this is actually has been seen, again, in Ape's experiment. These are data from very unique way, one to two materials back in 2013. And then Sergei Borisenko, who will give the next talk just a second, has also seen the same in iron cerinium, with a very, very strong effect here. Yes. Microphone? Yes, your computer self-consist is the chemical potential. But here, in any case, we have all alike and electron-like bands. So if you shoot them in this way, the total number of electrons is preserved. Yes. But you have to compute that, of course. OK. And probably Sergei will tell more about this later on in the talk. So just a final remark. So this Fermi surface shinking is a kind of probe of the T-dependence of spin-flactuation. This is a very interesting, profound implication for the optical sum rule, which is the point that we discuss most in this paper here. But I will not mention it now. OK, so this is nice. But all this was done in the completely bent language neglecting any orbital degrees of freedom. And this is not the whole story, of course. Orbital degrees of freedom are very important. And this is already this high-energy normalization that Chimian was mentioning before, are very important and orbital dependent. So this is somehow orbital degrees of freedom cannot be neglected. But the point where people really started to struggle about orbital degrees of freedom is the case of nematicity in iron selenium. Because here everything becomes very somehow complicated. And this picture of spin-flactuation seems to fail at the first look. Why this? First of all, what is nematicity? In these materials, we have a structural transition from tetragonal to orthorombic. As I said before, your basic unit cell is made of iron atoms. In high-temperature phase, the system is tetragonal, which means that the two lattice directions are equivalent. Below the structural transition, one direction becomes longer than the other. So this is just a change in the lattice structure. Of course, a change in the lattice structure is expected to give indication in the electronic behavior. But the point is that what you do see in the experiment, here, for example, our resistivity measurement, you get an isotropic resistivity across the structural transition here, which is extremely larger than what you can expect from a simple change of a few percent of the length of the two bonds. And this is why people think about nematic. So somehow this is an effect. Of course, there is always a connection between lattice and electron degrees of freedom. But the main idea is that maybe are the electronic degrees of freedom themselves who drive the transition. And of course, if you want to say again in this picture of spin-mediated interaction, one possible candidate are the spin clotation themselves. Of course, this is an effect that you have to capture going beyond RPA. So you have to take interaction between spin mode to the fourth order. This is a beautiful work by Andre and Raphael back in 2018. So as I told you before, the basic interaction between the whole-like and the electron-like pocket, but you have two possible wave vectors, which connect whole-like with electron-like pockets. And then for temperature-large and the nematic transition, this two-directional completely equivalent. Below the nematic transition, heat can happen. This can be driven by interaction beyond RPA. That fluctuation becomes stronger in one direction than in the other, without breaking spin symmetry. So there is no spin order parameter, but there are fluctuation stronger in one direction than the other. So in this picture, nematic transition means that you break the degeneracy between fluctuation qx and qy. And this is completely electron-like mechanism for nematicity. And of course, this was a reasonable approach, especially in the case of one-to-two material, where the structural transition happens immediately before the spin-dense-way transition. So somehow spin nematicity is this breaking of the fluctuation is just preemptive to the magnetic state which is established for lower temperature. And indeed, in the magnetic state, you do see order for one specific direction of the q vector for spin modes. And then this pre-carcel, then pre-carcel of magnetic transition can be seen, for example, in a Raman experiment, like an announcement in a specific channel. We will have a couple of talks later on in the conference on this. And the same effect has been seen, actually, for example, in Raman, also in Arion selenium. But what is puzzling here is that the orthorhombic transition, which happens here at 90 Kelvin, is not followed by magnetic transition. So somehow the puzzle is, why should I have a spin nematic transition if there is no magnetism appearing before? And magnetism actually only appears when, for example, you dope the system with pressure, but then you see spin-dense-way transition happens when nematicity is gone. So, of course, iron selenium can be problematic from this point of view. And the second point is that in iron selenium, since there is this huge range of temperature where the system is nematic without any other transition, this only happens for 90 Kelvin, people have seen in Arpes a very strong reconstruction of the Fermi surface. So this is, again, the Fermi surface I was showing before, whole light and electron light. I'm adding now colors here. And the color chord is the main orbital character of each pocket. So you have to take three main orbitals to describe the pockets of the Fermi level, xz, yz, xz. And then you see, when you go below the structural transition, these pockets deform, the gamma 1 becomes elliptical, this one shrinks, this one expands, and this can be reproduced in a kind of crystal field splitting for the orbitals. So it looks like you are taking your orbitals, they should be degenerating energy in the tetrangular phase. In the nematic phase, you have a kind of change of energy of the local occupation of xz and yz. And this change of energy also changes sign between gamma and m. So this is a very peculiar way of crystal field splitting. And then everything happens in the particle channel. So, of course, this triggered the idea that in iron selenium nematicity has a different origin. Everything has to do with interaction in the particle channel, so spin-wave interaction are gone. And this will be described, I think, in more detail by in the near on Friday. Now, this, of course, is a possibility. And this is a complicated system, one cannot rule out any effect. But one point that we try to pursue is, is it possible to revise this very simple picture that I had before adding the orbital degrees of freedom, and what is the consequence of this? And then when we did that, what we realized that, once that we construct our spin-mediated interaction, starting from the real description of the low energy with orbital degrees of freedom, we can still find the kind of one-to-one connections between spin fluctuation in different moments, and the orbital character of the pocket. And this is what I will call orbital selective spin fluctuation. So let me try to give you an idea why there is this correlation between spin and orbital degrees of freedom. And let's start again from a low energy model, where now I added the orbital content, as I told you before. So I'm still talking about the whole like and electron-like pockets. But I take into account what is the orbital content of each pocket. Then I want to play the same game as before, for example. Can I have a spin fluctuation between whole like and electron-like pocket, which are, which are, at some moment, large, to do the nesting condition? OK, if I now started to build up on my spin operator, taking into account the orbital character, this eta here is just an orbital index, when I do this particular bubble, I want to go from essentially from the whole to the electron-like pocket, because I want to take into account of the nesting. And then I want to come back. This means that if I want to make a spin fluctuation between this whole pocket and this electron pocket, I need to have the same orbital degrees of freedom on both of them. Otherwise, I cannot make this spin fluctuation in that bubble finite. And this implies that, essentially, if I want to make fluctuation in the x direction, since only the yz orbital is in common between the whole like and the electron-like pocket, the fluctuation along x only affect the yz orbital. On the other hand, when I make spin fluctuation in the other direction, the only orbital in common is xz, and then spin fluctuation for that q vector will only select orbital, which is xz. And this is what I call orbital selective spin fluctuation. So somehow there is a one-to-one correspondence between the momentum of the spin mode and the character of the orbitals, which are affected by that. And then, of course, I can play the same game as before, for example, for the shrinking. So again, I want to compute a self-energy correction, but here I have to take into account the orbital character. So my self-energy will be somehow a matrix in the orbital basis. In the whole pocket, I will have a yz and xz character. In the electron pocket, I will have a yz and xy character. So again, I compute the spin fluctuation of the Leageberg correction as I did before. And since still the sign of the self-energy is due to the fact that I have interbund character, so all band coupled to electron band, still I have the shrinking as I had before. But now the shrinking become orbital selective, because somehow it can be in principle different in the two orbitals. Is spin fluctuation different? This is in the paramagnetic state, actually, that fluctuations are thought to be equivalent. And then the only effect that I have of this orbital selective shrinking is that I have somehow I can just shrink so much the inner pocket that it goes below the Fermi level, as you can see here. And then you have some kind of enunciated elongation of this electron-like pocket, due to the fact that I don't have spin-mediated interaction in the xy channel, since the xy whole pocket is missing at the gamma point here. These are just the comparison between the calculation and the data by Veronik Reine and Arion Selenium. Now, let's assume again that spin ameticity in Arion Selenium is still due to this breaking of spin fluctuation in the two direction. So what happens within this model? What happens in this model is that since the orbital selective shrink rotation are now different along x and y because of this spin ametic effect, then also the correction I have in the orbital sector will be different in the different orbitals. So this and the different sign of the self-energy at the whole electron-like pocket also means that this orbital splitting will have a different sign at the gamma and m point. So, somehow within this scenario, all that we observe as orbital ordering in the pneumatic phase of Arion Selenium can still be a consequence of spin ameticity if I take into account the self-energy correction of the electrons due to spin fluctuation. And this is somehow essentially with this kind of calculation, can reproduce the typical kind of model that most of the people take as a starting point to reproduce pneumaticity in Arion Selenium. So this is just calculation we did to reproduce the data in Arion Selenium, the data by Veronique Brouer. So we model the spin fluctuation under pneumatic phase with different somehow spin masses in the two direction. And we just found out that to reproduce observed splitting, we need the fluctuation stronger along x and along y. OK, so spin ametic can still be the explanation of pneumaticity in Arion Selenium, even if what I do see is an orbital ordering at the end. OK, let's go now to the last part, which is about superconductivity. And this is a really puzzle. Yes. You have to go beyond RPA to reproduce spin ameticity. Here we took it in a pharmacological way. I will comment at the end if there is time about going beyond RPA. But you are right. So the question he's asking is the following. This propagator is the typical propagator of spin fluctuation. What gives you the difference in the masses as to come from an effect, which goes beyond RPA? In this case, we took it as from logical parameter. So we just say, let's say how different have to be the masses here to reproduce the experiment. Perfect. Now, superconductivity. Now that we move from the band language to the orbital language, somehow it's obvious that also the superconductivity will have an orbital character. So orbital selective superconductivity here just means that if your band is made by different orbitals, of course, your superconducting order parameter in any paving model, which is written in the orbital basis, will be orbital dependent. And then the total gap along the Fermi surface, for example, of the gamma pocket, will be just the combination of the orbital ways of each orbital, which I call here U and V, and then the superconducting order parameter. So this is just trivial. There is nothing particularly fancy in this. If you want to write superconductivity, and you have orbitals and different orbitals in the Fermi surface, you have to take this into account. So what is the real puzzle of iron selenium? When you go in the nematica phase, as I told you, the gamma pocket, the form, gets elongated in this direction. But also the orbital character changes considerably. So these are, for example, polarized arpe's experiment, but several groups agree on that. So the character of the pocket is almost completely xz. So this crystal field splitting that you have in the nematic state, completely removed. The yz character from the pocket is becoming red. So this is just a sketch of the orbital character along the angle here. You see in the paramagnetic state in the nematic state. If you look at the experiment of the gap, these are data extracted from QPI, quasi particle interference in STM, the gap here on gamma pocket, is maximum at t equal to 0, which is here, is minimum here. So somehow the modulation of the gap follow the angular dependence of the superdominant yz orbital, which is this one. Because the main orbital, which is xz, is a minimum at t equal to 0, it is a maximum at pi overalls. So the real puzzle of iron selenium is the following. We have, of course, a contribution of orbital degrees of freedom. You go in the nematic phase, you completely remove this character with respect to the other one. The pocket is almost completely xz. But if you look at the gap, the symmetry of the gap is the gap of this orbital, the yz. And then if you really want to reconcile these two observations, you need the mechanism, which makes the orbital component of the gap in the yz channel much larger than the one in the x-y channel. Because you need somehow to overcome the Fermi surface reconstruction due to nematicity. And this is the big puzzle. And this has been, I think, pointed out very clearly in several art and data experiment in the last few months. So how to do that? Our proposal is to follow, to think again in the same way that we did before. We can just construct the model of the per interaction based on the spin-mediated mechanism, pairing mechanism. Here you see this is still here interaction between a hollow like pocket and the electron-like pocket. This factor here is just taking account of the orbital component and all of that. But what is very important, that if you have a fluctuation of stronger along this direction than in the other direction, also the pairing interaction has to be much stronger along x than along y. So you need that also the pairing interaction is nematic. Otherwise, you cannot explain what's going on in the experiment. Why that? Let me just show you the data. Here, this dashed line are the solution of the self-consistent equation where I just take equal coupling. So let's assume that this is spin-mediated pairing, but there is no nematicity in the spin-mediated interaction. Then gx is equal to gy, my language. And then this solution gives me an orbital order parameter at the lower pocket, which is much smaller in the yz than xz. This orbital is also suppressed. Then at the end of the day, the gap just follows the modulation of the xz orbital. And then it has a maximum at pi over 2 instead of the minimum. But if I include a nematic fluctuation, and then I can overcome somehow this effect, this suppression of this orbital, and then I have an orbital parameter in the yz channel much larger than xz channel. And then the gap follows the symmetry of the subdominant yz orbita, which is exactly the symmetry that you see in the experiment. So here is crucial to have one coupling much larger than the other. Of course, this is not the same everywhere. For example, here, if I go to the z pocket, in the z pocket, which is the whole pocket, measured in a different value of kz in the Fermi surface, this pocket retains still a strong yz orbital even in the nematic phase. Because, of course, how much you change the orbital content in a given pocket depends on the band parameter. Essentially, it depends on the Fermi level. This pocket is larger, so the nematic order has a smaller effect on the reconstruction of the Fermi surface, and then the yz character is still there. In this case, of course, is you don't have this strong imbalance of the two, and you can have a gain of the orbital character in the yz respect to xz channel already from the orbital ordering in the electron pocket. And this has been nicely explained by Andreja Raffel in a recent PRL. So somehow, even without playing with spin nematicity, so even taking the coupling equally in the two direction, you can reproduce the result of the job self-experimentary. And this is the dash to the line here. On the other end, if you add to this one also nematic pairing, which means gx larger than gy, you can have a much stronger anisotropy. And this could explain why the anisotropy that has been seen at the z pocket is larger than the anisotropy at the gamma pocket. And this probably will be shown in more detail by Sergey in the next slide. Of course, this effect is possible, but this effect is small if you want to use it for the gamma pocket. So if you only implement the effect of orbital ordering without nematicity for the gamma pocket, these are again the data by Andreja, you get a smaller anisotropy, but also the wrong sign of the anisotropy. So here, because here again, you are back to the problem I told you before. If you don't make this one much larger than the other, then you just follow the symmetry of the dominant orbital with the maximum at pi over 2 instead of the other one. Now, there are, of course, other interesting proposal that have been done. One is to play with the effect of the spectral weights. So this is the issue of the correlation. What happens if now the correlation, the different orbitals are not all the same? The first proposal came actually from the same group of Schimus Davis, the calculation from Peter Ishfield, Brian Anderson, and others. The idea is how to explain this anisotropy that you see experimentally. And then the author said, OK, let's try to implement now to add to the orbital character also the possible coherence of the various orbital. And then you have the different weight in the different orbitals. What did they say? Let's assume that the white z orbital is much more coherent than the other two. This assumption has two effect. You move the maximum of the spin fluctuation from pi pi to pi zero, as you can see here. So now you make in effective way interaction much larger than in the x direction in the y direction. And this give you also an orbital ordering which is much larger than the other one. And this is playing the symmetry of the experiment. However, this effect also brings back the white z character of the whole pocket. So if you now you play with the orbital character, you again get the pocket which is dominant white z character along at equal to zero, which is not what you do see in the experiment. And this, of course, is a problem. But there is an advantage, again, of this approach, that you explain why the white pocket is not seen in STM. This is a very important problem that is not completely solved. And different uppers, people have different opinion. Maybe Sergei will tell more about this. Another nice approach, which will be probably described by Shimi Asiyonov on Friday, is, again, to make a collision or start different multi-order DJ model, there's less spin approximation. Here the spectral weight are not taken by hand, but they are computed microscopically. You get the right order that is consistent with this STM. And here is malanisotope is enough to reproduce the gap. But, again, why is that? Because also in this model, the white z pocket is very large character at equal to zero. So this is, you have 8% of white z character, where what you do see in the experiment is 18%. So, of course, you should bring back the white z character to explain the malanisotope with a very small anisotope in the orbital sector. OK, just a final comment. As I told you why, at least what is my belief, why all these models have so much difficulty to explain what's going on. Because enigmatic effect in sphere of rotation, as you was asking before, come from interaction beyond RPA. And then one has to work the problem hardly, especially including the spin degrees, the orbital degrees of freedom. We just did the reason to work on this. We just, I don't have time to give the detail, but just the idea is that in this kind of spin interaction model beyond RPA, also the orbital character of the pocket is very important, not only just the nesting proper division of all light and electrolyte. And then also orbital nesting can be very, very important. And then the orbital mismatch that you have in iron selenium between the gamma in the electron pocket, explaining why you can have a boost of enigmaticity and the suppression of magnetism. And this can eventually explain what's going on essentially in iron selenium where you dopper with a soulful. OK, I don't have time to say this, let me just conclude. So the point we did try to push is that you can explain a lot of physics with the idea that you have spin medial interaction with the whole light and electrolyte pocket, taking into account the orbital degrees of freedom. And the same mechanism can explain different phenomena. The shrinking, which is temperature dependent above Tc, the reconstruction of the Fermi surface, and the gap here in the superconducting state. There are, of course, many more issues we are studying now, and this will probably appear soon. And with this I conclude, and I thank you for your attention. Thanks. OK, we have time for questions. Go ahead.