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Twisted bilayered graphenes at magic angles are systems housing long ranged periodicity of Moir\'e pattern together with short ranged periodicity associated with the individual graphenes. Such materials are a fertile ground for novel states largely driven by electronic correlations. Here we find that the ubiquitous Casimir force can serve as a platform for macroscopic manifestations of the quantum effects stemming from the magic angle bilayered graphenes properties and their phases determined by electronic correlations. By utilizing comprehensive calculations for the electronic and optical response, we find that Casimir torque can probe anisotropy from the Drude conductivities in nematic states, while repulsion in the Casimir force can help identify topologically nontrivial phases in magic angle twisted bilayered graphenes., Comment: 9 pages, 6 figures (main), 7 pages, 7 figures (supplementary); provisionally accepted for publication in 2D Materials

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 recently observed superconductivity in twisted bilayer graphene emerges from insulating states believed to arise from electronic correlations. While there have been many proposals to explain the insulating behaviour, the commensurability at which these states appear suggests that they are Mott insulators. Here we focus on the insulating states with $\pm 2$ electrons or holes with respect to the charge neutrality point. We show that the theoretical expectations for the Mott insulating states are not compatible with the experimentally observed dependence on temperature and magnetic field if, as frequently assumed, only the correlations between electrons on the same site are included. We argue that the inclusion of non-local (inter-site) correlations in the treatment of the Hubbard model can bring the predictions for the magnetic and temperature dependencies of the Mott transition to an agreement with experiments and have consequences for the critical interactions, the size of the gap, and possible pseudogap physics. The importance of the inter-site correlations to explain the experimental observations indicates that the observed insulating gap is not the one between the Hubbard bands and that antiferromagnetic-like correlations play a key role in the Mott transition., 8 pages (including appendix), 5 figures

Unconventional superconductivity is found in correlated materials as a low temperature bridge between phases dominated by high- and low-energy scale of electronic interactions (e.g. Mott physics vs Fermi Liquid regime). The understanding of the nature and strength of correlations is key to unveil the nature of the pairing itself and its role as competitive/cooperative order with other phases. The multiorbital character of the band structure close to the Fermi level complicates the analysis of correlation effects in many unconventional superconductors. In particular in Iron-based materials, contrasting experimental evidences of weak and strong regime of electronic correlation polarized theoretical approaches around low-energy effective models or, on the opposite side, strongly correlated approaches. In this talk I will show that complementary and consistent results concerning the physics of iron-based materials follow from both low- and high- energy approaches once the multiorbital character is taken into account. The orbital selectivity emerges as a main feature at every scale. Our results discloses a scenario in which the key ingredient of the pairing itself comes from a new and unconventional cooperative interplay between low- and high-energy scale of electronic interactions.

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.

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.

Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond angles or bond distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on d or d-p model Hamiltonians only., D. G., H. O. J., and R. V. thank the German Research Foundation (Deutsche Forschungsgemeinschaft) for financial support through grant SPP 1458. I. I. M. was supported by ONR through the NRL basic research program and by the Alexander von Humboldt Foundation. J. K. G. acknowledges the support of the NRC program at NRL. E. B. acknowledges funding from Ministerio de Economía y Competitividad vía Grant No. FIS2014-53219-P and from Fundación Ramón Areces and thanks R. Rurali and X. Cartoixa for early calculations.

[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 properties of nanoscopic superconducting structures fabricated with a scanning tunnelling microscope are reviewed, with emphasis on the effects of high magnetic fields. These systems include the smallest superconducting junctions which can be fabricated, and they are a unique laboratory in which to study superconductivity under extreme conditions. The review covers a variety of recent experimental results on these systems, highlighting their unusual transport properties, and theoretical models developed for their understanding.

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 discuss how to analyze the optical conductivity and Raman spectra of multiorbital systems using the velocity and the Raman vertices in a similar way Raman vertices were used to disentangle nodal and antinodal regions in cuprates. We apply this method to iron superconductors in the magnetic and nonmagnetic states, studied at the mean-field level. We find that the anisotropy in the optical conductivity at low frequencies reflects the difference between the magnetic gaps at the X and Y electron pockets. Both gaps are sampled by Raman spectroscopy. We also show that the Drude weight anisotropy in the magnetic state is sensitive to small changes in the lattice structure. © 2013 American Physical Society., We acknowledge funding from Ministerio de Economía y Competitividad through Grants No. FIS 2008-00124, No. FIS 2009-08744, No. FIS 2011-29689, and from CSIC through Grants No. PIE-200960I033 and No. PIE-200960I180.

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

We theoretically show that the interplay between charging effects and the nonequilibrium spin accumulation has a dramatic effect in the current through an array of nanostructures attached to ferromagnetic electrodes. Large oscillations in the current as a function of bias voltage show up for parallel orientation of the electrodes' magnetizations. These oscillations originate in the inhomogeneity of the spin potentials through the array and correlate with oscillations in the spin accumulation. For antiparallel orientation the spin potential is homogeneous and the oscillations do not show up. This sensitivity results in a huge enhancement of the tunneling magnetoresistance as compared to the single-island case, and opens new routes for improving the spintronic response of nanodevices.

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 examine the magnetic phase diagram of iron pnictides using a five band model. For the intermediate values of the interaction expected to hold in the iron pnictides, we find a metallic low moment state characterized by antiparallel orbital magnetic moments. The anisotropy of the interorbital hopping amplitudes is the key to understanding this low moment state. This state accounts for the small magnetization measured in undoped iron pnictides and leads to the strong exchange anisotropy found in neutron experiments. Orbital ordering is concomitant with magnetism and produces the large zx orbital weight seen at Gamma in photoemission experiments., 4 pages, 4 eps figures. Small changes in the text. Modification of colors in Figs. Final version as accepted in PRL

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.

Nanowires of lead between macroscopic electrodes can be produced experimentally by means of an STM. Magnetic fields higher than the critical field of the bulk may destroy the superconductivity in the electrodes, while the wire remains in the superconducting state[1]. The I-V characteristics can be described in terms of an effective gap and a pair breaking parameter, which depends on the field. The evolution of the order parameter, the density of states and the transport properties, as function of the applied field are experimentally and theoretically analyzed.

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.

We analyze and clarify the transport properties of a one-dimensional metallic nanoparticle array with interaction between charges restricted to charges placed in the same conductor. We study the threshold voltage, the I-V curves and the potential drop through the array and their dependence on the array parameters including the effect of charge and resistance disorder. We show that very close to threshold the current depends linearly on voltage with a slope independent on the array size. At intermediate bias voltages, for which a Coulomb staircase is observed we find that the average potential drop through the array oscillates with position. At higher voltages I-V curves are linear but have a finite offset voltage. We show that the slope is given by the inverse of the resistances added in series and estimate the voltage at which this linear regime is reached. We also calculate the offset voltage and relate it to the potential drop through the array., 16 pages and 8 postscript figures

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.

We study the effect of spin-flip scattering on the density of states in a long diffusive S-N junction with transparent interface. We calculate the critical value of the spin-flip scattering rate at which the minigap closes and give the dependence of the minigap on the spin-flip rate. For the system we consider, the minigap and the critical spin-flip rate have the scale of the Thouless energy, and not of the superconducting gap.

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 induce superconductivity by proximity effect in thin layers of gold and study the number of conduction channels which contribute to the current in one-atom contacts and atomic wires. The atomic contacts and wires are fabricated with a Scanning Tunneling Microscope. The set of transmission probabilities of the conduction channels is obtained from the analysis of the $I(V)$ characteristic curve which is highly non-linear due to multiple Andreev reflections. In agreement with theoretical calculations we find that there is only one channel which is almost completely open., 4 pages, 2 figures. To be published in Phys. Rev. B, Rapid Communications (2003)

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

Metals with many bands at the Fermi level can have different band dependent gaps in the superconducting state. The absence of translational symmetry at an interface can induce interband scattering and modify the superconducting properties. We dicuss the relevance of these effects to recent experiments in ${\mathrm{MgB}}_{2}.$

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

Nanowires of lead between macroscopic electrodes are produced by means of an STM. Magnetic fields may destroy the superconductivity in the electrodes, while the wire remains in the superconducting state. The properties of the resulting microscopic Josephson junctions are investigated., 3 pages,3 eps figures included

In Xavier Bosch's interview “Spain's science minister sees future in telecom” (News Focus, 31 Jan., p. 653), Josep Pique, Spanish Minister of Science and Technology, asserts that “[n]ow there are many more scientists from abroad working in Spain than there are Spanish scientists abroad.”

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.

We investigate the interplay of the pairing state in conventional superconductors (S) and spin-active fields. In conventional S electrons with opposite momenta and spins bind into so-called Cooper pairs. The pair correlations penetrate normal conducting materials (N) on the length scale of the superconducting coherence length, what is known as the proximity effect. The proximity effect gives rise to interesting phase coherent phenomena that are strongly modified in the presence of spin active fields. For example it is strongly suppressed in a ferromagnet (F), which prefers a parallel spins of the electrons and counters the conventional pairing mechanism. A prominent manifestation of the proximity effect is the Josephson effect, where the phase difference between the macroscopic wavefunctions of two spatially separated S leads to a non dissipative current at zero voltage.We consider the Josephson effect in a junctions involving spin splitted S, where the orientation of the exchange field can be controlled individually in both S. In such junctions, when the fields are oriented antiparallely, it is possible to increase the critical current by increasing the magnitude of the exchange fields. This is a counter intuitive result considering the pair breaking nature of the fields. The formation of the Andreev bound states (ABS) has not been investigated so far. We analyze the spectral properties of this junction and show that for collinear orientations of the fields, any deviation from the case of equal fields leads to finite intervals of phases without ABS. In general the spectral composition of the current is found to be a superposition of the contributions from the ABS and the continuous spectrum and strongly depends on the transmissivity of the junction.The suppression of the proximity effect in magnetic heterostructures can be avoided by generating triplet components of the pair correlations with spin projections perpendicular to the field, so-called long range triplet correlations (LRTC). LRTCs can be generated due to the presence of spin-orbit coupling (SOC) and a homogeneous exchange field, what has not been confirmed experimentally yet. We propose favorable junction setups to observe the LRTCs and calculate the Josephson current considering two common types of SOC, that result in spin precession and anisotropic spin relaxation effects. The contributions to the current from the effects depend on the orientation of the exchange field and their competition leads to current reversal scenarios which represent a signature of the LRTCs.We then turn to another equilibrium phenomenon, namely equilibrium spin currents (ESC). We show that in a nanowire with SOC, breaking the time-reversal symmetry by a Zeeman field leads to a bulk equilibrium spin current which manifests itself in a sizable edge spin polarization, transverse to the Zeeman field. This property occurs in both, the normal and superconducting state, independently of the degree of disorder. The transverse edge spin polarization is strongly enhanced in the superconducting state when the Zeeman energy is of the order of the induced superconducting gap. This leads to a unknown transverse magnetic susceptibility that can be much larger than the known longitudinal one.At the end of the thesis, we investigate electronic transport in heterostructures of the recently discovered Weyl semimetals (WSM). This material class exhibits a pseudo-relativistic dispersion around so-called Weyl points in the Brillouin zone, that are characterized by their chirality in the low-energy limit. We discovered a interesting chiral filtering effect when interfacing two distinct WSMs, if the Weyl nodes on each side of the interface are separated in energy and momentum space. We calculate the differential conductance across the interface and identify the regimes where it is possible to achieve transport of one, none, or both chiralities., Nous avons considéré la compétition entre la supraconductivité conventionnelle (S) et les champs agissant sur le spin, comme l'effet Zeeman et le couplage spin-orbite. Dans un supraconducteur conventionnel, les électrons de spins et d'impulsions opposés forment des paires de Cooper. Par effet de proximité, les correlations de paire pénètrent dans des conducteurs dans l'état normal (N) sur une longueur typique. Cet effet de proximité donne lieu à des phénomènes cohérents de phase qui sont très fortement modifiés en présence de correlations ferromagnétiques (F) ou de fort couplage spin-orbite. Par exemple, l'effet de proximité est très fortement réduit dans un matériau ferromagnétique qui favorisera un alignement des spins dans une direction commune, contrairement à l'appariement singulet de spin de la supraconductivité. Un autre effet important est l'effet Josephson qui consiste en un courant non dissipatif de paires de Cooper entre deux supraconducteurs séparés par un lien faible.Nous avons étudié l'effet Josephson dans une jonction entre deux supraconducteurs avec champs d'échange. Nous supposons que l'orientation dun champ d'échange peut être controlé individuellement dans chaque électrode supraconductrice. Quand les champs d'échange sont antiparallèles, nous avons trouvé qu'il est possible d'augmenter le courant Josephson critique en augmentant la norme de ces champs d'échange antiparallèles, ce qui paraissait contre-intuitif au départ. Les états liés d'Andreev (ELA) n'avaient pas été étudiés en détail dans ce type de jonctions. Nous avons donc analysé leurs propriétés spectrales, et démontré que pour des champs d'échange de même sens, on a généralement la suppression des ELA pour certains intervalles de la différence de phase supraconductrice. En général, le courant Josephson est porté en partie par les ELA et en partie par le continuum des états d'énergie au dessus du gap.La suppression de l'effet de proximité supraconducteur dans les hétérojonctions magnétiques peut être évitée en générant des composantes triplet des correlations de paire ayant des projections de spin orthogonales au champ. En théorie, ces corrélations triplet à longue portée (CTLP) peuvent être engendrées par la présence simultanée de couplage spin-orbite (CSO) et d'un champ d'échange uniforme, mais cela n'a pas encore été confirmé expérimentalement. Nous proposons des designs de jonctions favorables pourmettre en évidence ces CTLP, et nous calculons le courant Josephson pour deux types de CSO.Nous nous sommes aussi intéressés aux courants de spin d'équilibre dans des nanofils normaux et supraconducteurs. Dans un nanofil en présence de couplages spin-orbite et Zeeman, nous montrons que des courants de spin à l'équilibre conduisent à des accumulations de spin aux extrémités du fil. Cette propriété existe à la fois dans l'état normal et l'état supraconducteur, indépendamment du degré de désordre. Cette polarisation de spin transverse augmente fortement dans l'état supraconducteur quand le couplage Zeeman est de l'ordre de grandeur du gap supraconducteur, conduisant à une susceptibilité transverse bien supérieure à la susceptibilité longitudinale.Dans le dernier chapitre de la thèse, nous étudions le transport électronique dans des hétérostructures de semimétaux de Weyl. Cette classe de matériaux a une dispersion relativiste autour de points de Weyl caractérisée par une chiralité bien définie à basse énergie. Nous avons découvert un effet de filtre de chiralité à l'interface de deux semimétaux de Weyl, qui apparaît lorsque les points de Weyl de part et d'autre de l'interface sont séparés en énergie et en impulsion. Nous avons calculé la conductance différentielleà travers une telle interface et identifié les régimes de transport laissant passer i) aucune, ii) une ou bien iii) les deux chiralités.