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We calculate the interactions between the Wannier functions of the 8-orbital model for twisted bilayer graphene (TBG). In this model, two orbitals per valley centered at the AA regions, the AA-p orbitals, account for the most part of the spectral weight of the flats bands. Exchange and assisted-hopping terms between these orbitals are found to be small. Therefore, the low energy properties of TBG will be determined by the density-density interactions. These interactions decay with the distance much faster than in the two orbital model, following a 1/r law in the absence of gates. The magnitude of the largest interaction in the model, the onsite term between the flat band orbitals, is controlled by the size of the AA regions and is estimated to be ~ 40 meV. To screen this interaction, the metallic gates have to be placed at a distance smaller than 5 nm. For larger distances only the long-range part of the interaction is substantially screened. The model reproduces the band deformation induced by doping found in other approaches within the Hartree approximation. Such deformation reveals the presence of other orbitals in the flat bands and is sensitive to the inclusion of the interactions involving them., Comment: 9 pages, including 6 multiple figures and one appendix

Moir\'e systems displaying flat bands have emerged as novel platforms to study correlated electron phenomena. Insulating and superconducting states appear upon doping magic angle twisted bilayer graphene (TBG), and there is evidence of correlation induced effects at the charge neutrality point (CNP) which could originate from spontaneous symmetry breaking. Our theoretical calculations show how optical conductivity measurements can distinguish different symmetry breaking states, and reveal the nature of the correlated states. In the specific case of nematic order, which breaks the discrete rotational symmetry of the lattice, we find that the Dirac cones are displaced, not only in momentum space but also in energy, inducing finite Drude weight at the CNP. We also show that the sign of the dc conductivity anisotropy induced by a nematic order depends on the degree of lattice relaxation, the doping and the nature of the symmetry breaking., Comment: 6 pages, including 4 multiple figures + Suppl. Information. with text and 3 figures

The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments call for a prominent role of local correlations and place iron superconductors at the entrance of a Hund's metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hund's coupling. We show that Hund's physics strongly affects the nematic properties of the system. It severely constrains the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the zx/yz orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes overlooked so far in the interpretation of experiments. As notable examples the splittings between zx and yz bands at Γ and M points are modified, with important consequences for angle-resolved photoemission spectroscopy measurements., E.B. acknowledges funding from Ministerio de Economía, Industria y Competitividad via Grants No. FIS2011-29689 and No. FIS2014-53218-P and from Fundación Ramón Areces. M.C. acknowledges funding from SISSA/Consiglio Nazionale delle Ricerche project “Superconductivity, Ferroelectricity and Magnetism in bad metals” (Protocollo 232/2015) and Ministero dell'Istruzione, dell'Università e della Ricerca through the PRIN 2015 program (Protocollo 2015C5SEJJ001).

Undoped iron superconductors accommodate n=6 electrons in five d orbitals. Experimental and theoretical evidence shows that the strength of correlations increases with hole doping, as the electronic filling approaches half filling with n=5 electrons. This evidence delineates a scenario in which the parent compound of iron superconductors is the half-filled system, in analogy to cuprate superconductors. In cuprates the superconductivity can be induced upon electron or hole doping. In this work we propose to search for high-Tc superconductivity and strong correlations in chromium pnictides and chalcogenides with n, Funding from Ministerio de Economía, Industria y Competitividad (Spain) via Grants No. FIS2012-33521, No. FIS2014-53218-P, No. FIS2015-64654-P, and No. MAT2015-66888-C3-1-R and from Fundación Ramón Areces is gratefully acknowledged. J.L. acknowledges the financial support of the National Natural Science Foundation of China (Grants No. 11374186 and No. 51231007) and the China Scholarship Council.

[EN] High-temperature superconductivity in iron pnictides and chalcogenides emerges when a magnetic phase is suppressed. The multi-orbital character and the strength of correlations underlie this complex phenomenology, involving magnetic softness and anisotropies, with Hund's coupling playing an important role. We review here the different theoretical approaches used to describe the magnetic interactions in these systems. We show that taking into account the orbital degree of freedom allows us to unify in a single phase diagram the main mechanisms proposed to explain the (π, 0) order in iron pnictides: nesting-driven superconductivity, exchange between localised spins, and Hund-induced magnetic state with orbital differentiation. Comparison of theoretical estimates and experimental results helps locate the Fe superconductors in the phase diagram. In addition, orbital physics is crucial to address the magnetic softness, the doping-dependent properties, and the anisotropies., [FR] La supraconductivité à haute température dans les pnictures et les chalcogénures de fer émerge quand une phase magnétique est supprimée. Le caractère multi-orbital et la force des corrélations sont sous-jacents à cette phénoménologie complexe, faisant appel à la douceur magnétique et aux anisotropies, dans lesquelles le couplage de Hund joue un rôle important. Nous passons ici en revue les différentes approches théoriques utilisées pour décrire les interactions magnétiques dans ces systèmes. Nous montrons que la prise en compte du degré de liberté orbital nous permet d'unifier dans un seul diagramme de phases les principaux mécanismes proposés pour expliquer l'ordre dans les pnictures de fer : supraconductivité nesting-driven, échange entre spins localisés et état magnétique induit par couplage de Hund avec différenciation orbitale. La comparaison des estimations théoriques et des résultats expérimentaux nous permet de localiser les supraconducteurs à base de fer dans le diagramme de phases. De plus, la physique orbitale est cruciale pour approcher la douceur magnétique, les propriétés dépendant du dopage ainsi que les anisotropies., We acknowledge funding from Ministerio de Economía y Competitividad (Spain) via grants No. FIS2011-29689, No. FIS2012-33521 and No. FIS2014-53219-P and from Fundación Ramón Areces.

Oral presentation given at the 20th International Conference on Magnetism, held in Barcelona (Spain) on July 5-10th, 2015. Friday, July 10. FR.J.1_THEORY OF STRONGLY CORRELATED MATTER 09:00-10:30 (ROOM E1-E3)., A new concept has emerged in the last years in correlated electron systems: the Hund metal. This name was coined within the context of iron superconductors and refers to a strongly correlated metallic state found in multi-orbital systems with Hund¿s coupling playing a key role in setting the correlations [1]. Until very recently [2] the nature of correlations in Hund metals has been controversial and not well understood. When the atoms become spin polarized the overlap between the atomic and single particle states decreases suppressing the quasiparticle weight. Nevertheless the system seems to be far from a Mott transition as the transition is not found for moderate increase of interactions. On the other hand the strong increase of correlations as half-filling is approached has led to a description of Hund metals as doped Mott insulators. This has led to a discussion on the role of Mott physics in these systems. We have addressed the correlations in multi-orbital systems as a function of interactions, number of orbitals and electronic filling. We have clarified the nature of correlations in Hund metals and solved the controversy between Hund and Mott systems [2]. Our calculations clearly show that the suppression of coherence in Hund metals is directly connected to the Mott insulating state at half-filling. However correlations due to Hund¿s coupling are intrinsically different to those in Mott states. This is observed in the charge fluctuations, which are only weakly reduced or even enhanced in Hund metals as the quasiparticle weight is suppressed, contrary to what happens in Mott correlated systems. We have shown that the dependence of correlations, energy scales, charge and spin fluctuations can be understood by looking at the hopping processes which are suppressed or promoted by Hund's coupling.

The role of non-equilibrium effects in the conductance through quantum dots is investigated. Associated with single-electron tunneling are shake-up processes and the formation of excitonic-like resonances. They change qualitatively the low temperature properties of the system. We analyze by quantum Monte Carlo methods the renormalization of the effective capacitance and the gate-voltage dependent conductance. Experimental relevance is discussed., Comment: 10 pages, 8 postscript figures

Charge, spin, and lattice degrees of freedom are strongly entangled in iron superconductors. A neat consequence of this entanglement is the behavior of the A1g As-phonon resonance in the different polarization symmetries of Raman spectroscopy when undergoing the magnetostructural transition. In this work, we show that the observed behavior could be a direct consequence of the coupling of the phonons with the electronic excitations in the anisotropic magnetic state. We discuss this scenario within a five-orbital tight-binding model coupled to phonons via the dependence of the Slater-Koster parameters on the As position. We identify two qualitatively different channels of the electron-phonon interaction: a geometrical one related to the Fe-As-Fe angle α and another one associated with the modification upon As displacement of the Fe-As energy integrals pdσ and pdπ. While both mechanisms result in a finite B1g response, the behavior of the phonon intensity in the A1g and B1g Raman polarization geometries is qualitatively different when the coupling is driven by the angle or by the energy integral dependence. We discuss our results in view of the experimental reports. © 2013 American Physical Society., We acknowledge funding from MINECO-Spain through Grants FIS2008-00124, FIS2009-08744, FIS2011-29689, and FIS2012-33521. S.C. acknowledges support from Spanish Education Ministry programme SAB2010-0107. E.C. acknowledges support from the European FP7 Marie Curie project PIEF-GA-2009-251904 and Italian Project PRIN “GRAF” No. 20105ZZTSE.

We analyze the metallic (π,0) antiferromagnetic state of a five-orbital model for iron superconductors. We find that with increasing interactions the system does not evolve trivially from the pure itinerant to the pure localized regime. Instead we find a region with a strong orbital differentiation between xy and yz, which are half-filled gapped states at the Fermi level, and itinerant zx, 3z2−r2, and x2−y2. We argue that orbital ordering between the yz and zx orbitals arises as a consequence of the interplay of the exchange energy in the antiferromagnetic x direction and the kinetic energy gained by the itinerant orbitals along the ferromagnetic y direction with an overall dominance of the kinetic energy gain. We indicate that iron superconductors may be close to the boundary between the itinerant and the orbital differentiated regimes and that it could be possible to cross this boundary with doping., We acknowledge funding from MINECO-Spain through Grants FIS2008-00124, FIS2009-08744, and FIS2011-29689.

We have analyzed the magnetic interactions of a five orbital model for iron superconductors treated both within Hartree-Fock and Heisenberg approximations. We have found that the exchange constants depend non-trivially on the Fe-As-Fe angle and on the charge and orbital filling. Within the localized picture, columnar ordering is found for intermediate Hund's coupling J_H. At smaller J_H, an unusual orbital reorganization stabilizes checkerboard ordering. Ferromagnetism appears at large J_H. Ferromagnetic correlations are enhanced with electron doping while large hole doping stabilizes checkerboard antiferromagnetism, explaining the change in magnetic interactions upon substitution of Fe by Co or Mn. For intermediate and large values of U, Hartree-Fock shows similar results as strong coupling though with a double stripe phase instead of ferromagnetism. Itinerancy enhances the stability of the columnar ordering. Comparison of the two approaches reveals a metallic region of the phase diagram where strong coupling physics is determinant., 8 pages including 7 figures. As accepted in Phys. Rev. B

Recent experiments on iron pnictides have uncovered a large in-plane resistivity anisotropy with a surprising result: the system conducts better in the antiferromagnetic x direction than in the ferromagnetic y direction. We address this problem by calculating the ratio of the Drude weight along the x and y directions, Dx/Dy, for the mean-field Q=(��,0) magnetic phase diagram of a five-band model for the undoped pnictides. We find that Dx/Dy ranges between 0.3 < D_x/D_y < 1.4 for different interaction parameters. Large values of orbital ordering favor an anisotropy opposite to the one found experimentally. On the other hand D_x/D_y is strongly dependent on the topology and morfology of the reconstructed Fermi surface. Our results points against orbital ordering as the origin of the observed conductivity anisotropy, which may be ascribed to the anisotropy of the Fermi velocity., 4 pages, 3 pdf figures. Fig 1(b) changed, one equation corrected, minor changes in the text, references updated

We propose a five-band tight-binding model for the Fe-As layers of iron pnictides with the hopping amplitudes calculated within the Slater-Koster framework. The band structure found in DFT, including the orbital content of the bands, is well reproduced using only four fitting parameters to determine all the hopping amplitudes. The model allows to study the changes in the electronic structure caused by a modification of the angle $\alpha$ formed by the Fe-As bonds and the Fe-plane and recovers the phenomenology previously discussed in the literature. We also find that changes in $\alpha$ modify the shape and orbital content of the Fermi surface sheets., Comment: 12 pages, 6 eps figures. Figs 1 and 2 modified, minor changes in the text. A few references added

Recent experiments in underdoped hole-doped cuprates have shown the presence of two energy scales in the Raman spectrum in the superconducting state. This feature has a natural explanation in some models in which pseudogap and superconductivity compete. In electron-doped cuprates antiferromagnetic correlations are believed to survive in the superconducting state, and to produce a pseudogap above the critical temperature. Contrary to hole-doped systems, in electron-doped compounds only one energy scale appear since the pair breaking Raman intensity peaks in both B$_{1g}$ (antinodal) and B$_{2g}$ (nodal) channels at a frequency of a few meV, typical of the superconducting order parameter. In this paper we analyze the different effect in the Raman spectrum of the competition between pseudogap and superconductivity in electron and hole-doped cuprates. The difference in energy scales in both systems is explained in terms of the different truncation of the Fermi surface induced by the pseudogap. For electron-doped cuprates we also analyze the spectrum with antiferromagnetism and a non-monotonic superconducting order parameter., 7 pages, 4 postscript figures, minor changes in the text, Fig. 3 and Fig. 4 replaced to better visualize them

We study the dependence of the electronic structure of iron pnictides on the angle formed by the arsenic-iron bonds. Within a Slater-Koster tight binding model which captures the correct symmetry properties of the bands, we show that the density of states and the band structure are sensitive to the distortion of the tetrahedral environment of the iron atoms. This sensitivity is extremely strong in a two-orbital (d_xz, d_yz) model due to the formation of a flat band around the Fermi level. Inclusion of the d_xy orbital destroys the flat band while keeping a considerable angle dependence in the band structure., 5 pages, including 5 figures. Fig. 5 replaced. Minor changes in the text

In the pseudogap state the spectrum of the autocorrelation of angle resolved photoemission (AC-ARPES) data of Bi2212 presents non-dispersive peaks in momentum space which compare well with those responsible of the checkerboard pattern found in the density of states by Scanning Tunneling Microscopy. This similarity suggests that the checkerboard pattern originates from peaks in the joint density of states, as the dispersive peaks found in the superconducting state do. Here we show that the experimental AC-ARPES spectrum can be reproduced within a model for the pseudogap with no charge-ordering or symmetry breaking. We predict that, because of the competition of superconductivity and pseudogap, in the superconducting state, the AC-ARPES data of underdoped cuprates will present both dispersive and non-dispersive peaks and they will be better observed in cuprates with low critical temperature. We finally argue that the AC-ARPES data is a complementary and convenient way to measure the arc length.

We study the properties of superconducting bridges of Pb of nanoscopic dimensions fabricated by an STM under magnetic fields. The bridges behave as weak links of variable dimensions whose form and smallest contact radius can be varied in-situ. We find that these structures have upper critical fields several times the bulk critical field of Pb, forming at large fields an N - s - junction - s - N system whose superconducting part s is of mesoscopic dimensions. We could find no evidence for the nucleation of vortices in these structures, in agreement with the estimation for the geometry which we can get from R-z curves and the model of Ref... Therefore, their phenomenology cannot be explained by the usual type II superconducting behavior. The physical properties of the system are then expected to be dominated by the pair breaking effect of the magnetic field. For instance, in the case of a bridge whose smallest contact radius is of the order of a single atom, we can follow the subharmonic gap structure as a function of the pair breaking parameter. By contrast, in bridges formed by two long cone like structures with a smallest contact of the order of 20Aor larger a new phenomenology appears. We observe two well defined conduction regimes as a function of the field and under strong current flow. In an intermediate field regime, strong peaks appear in the dV/dI curves which disappear at large fields, and at the highest fields, an new anomaly in dV/dI appears. This behavior can be understood in terms of nonequilibrium superconductivity by considering the magnetic field dependence of the quasiparticle charge imbalance length ΛQ * through the pair breaking effect. At intermediate fields, our curves can be interpreted as an indication for the nucleation of phase slip centers within the bridge (of dimensions orders of magnitude smaller than usual phase slip wires), and at high magnetic fields the observed anomaly should be related to the contact resistance of the N -s interface formed by the destruction of superconductivity in the bulk. The different magnetic field dependence of ΛQ * and of the length of the superconducting part L s makes the appearence of two regimes possible.

Recent inelastic neutron scattering measurements found that the spin susceptibility of detwinned and highly ordered ortho-II YBa_2Cu_3O_{6.5} exhibits, in both the normal and superconducting states, one-dimensional incommensurate modulations at low energies which were interpreted as a signature of dynamic stripes. We propose an alternative model based on quasiparticle transitions between the arcs of a truncated Fermi surface. Such transitions are resonantly enhanced by scattering to the triplet spin resonance. We show that the anisotropy in the experimental spin response is consistent with this model if the gap at the saddle points is anisotropic.

The ortho-II phase of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ is characterized by a periodic alternation of empty Cu and filled $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ $b$-axis chains doubling the unit cell in the $a$ direction. The extra oxygen in the full chains gives rise to an attractive potential for the holes in the planes. The planar bands split in two with a gap opening at the new Brillouin zone boundary ${k}_{x}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2$, which we estimate from local-density approximation calculations. Using a planar model which treats the $d$-wave superconductivity in a mean field approximation, we show that interband transitions produce a strongly anisotropic feature in the optical conductivity controlled by a region in $\stackrel{P\vec}{k}$-space close to $(\ensuremath{\pi}∕2,\ensuremath{\pi}∕2)$. The edge position of this feature gives information on the temperature dependence of quasiparticle spectrum in this region. Bilayer splitting would show up as a double edge shape.

We analyze the influence of the structure within a SNS junction on the multiple Andreev resonances in the subgap I-V characteristics. Coherent interference processes and incoherent propagation in the normal region are considered. The detailed geometry of the normal region where the voltage drops in superconducting contacts can lead to observable effects in the conductance at low voltages., 11 pages, including 7 postscript files

Nesting in a semimetal can lead to an excitonic insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. In this paper we present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist., Comment: 5 pages, 3 postscript files

Transport through quantum dots is usually analyzed in terms of the constant interaction (CI) model [1], which assumes that the addition of an electron to the dot only shifts the level spectrum by a fixed amount of energy E c independently of the number of electrons in the dot. The dot is characterized by a geometrical capacitance C, in terms of which one has E C = e 2/2C However, recent experiments have shown important deviations from the CI model [2]. The addition of electrons to the quantum dot scrambles the energy spectrum and causes charge rearrangements in the dot.

We present high resolution tunneling spectroscopy data at very low temperatures on superconducting nanostructures of lead built with an STM. By applying magnetic fields, superconductivity is restricted to length scales of the order of the coherence length. We measure the tunneling conductance and analyze the phonon structure and the low energy DOS. We demonstrate the influence of the geometry of the system on the magnetic field dependence of the tunneling density of states, which is gapless in a large range of fields. The behavior of the features in the tunneling conductance associated to phonon modes are explained within current models., Comment: 4 figures, 4 pages

Recent experiments on atomic-scale metallic contacts have shown that the quantization of the conductance appears clearly only after the average of the experimental results. Motivated by these results we have analyzed a simplified model system in which a narrow neck is randomly coupled to wide ideal leads, both in absence and presence of time reversal invariance. Based on random matrix theory we study analytically the probability distribution for the conductance of such system. As the width of the leads increases the distribution becomes sharply peaked close to an integer multiple of the quantum of conductance. Our results suggest a possible statistical origin of conductance quantization in atomic-scale metallic contacts.

A theoretical analysis of wave scattering from perfectly reflective random media is developed from general postulates of invariance. Based on only the assumptions of flux conservation and time-reversal invariance, we use random matrix theory to calculate the average reflection coefficients as well as the correlation function between them. As a consequence, enhanced coherent backscattering is obtained. The predicted behavior of the correlations differs substantially from that obtained for the case of semiballistic or diffusive transport.