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Remarkable feature of the phase diagram of sulfur hydrides is a sharp increase in critical temperature from 120K up to200K. This increase is a signature of the structural transition. The study described below is concerned with the nature of this phase transition. One can demonstrate that the symmetry analysis along with an analysis of the impact of lattice deformations lead to the conclusion that we are dealing with the first order transition. It leads to an abrupt appearance of small pockets on the Fermi surface and to the two-gap energy spectrum., Comment: 21 pages,3 figures

The rapid variation of the superconductivity Tc in hydride sulfur (H3S) under high pressure [A. Drozdov et al, Nature 525, 73 (2015); M. Einaga et al, arXiv: 1509.03156] in a vicinity Pcr=123GPa is interpreted in terms of the 1st order transition, possibly related to a CDW-instability with a non-zero structural vector. The superconductivity mechanisms in high-Tc phase be discussed, ordinary methods of calculating Tc are shown not applicable in H3S because, beside the acoustic branches, its phonon spectrum contains hydrogen modes with much higher frequency. A modified approach provides realistic Tc values. The isotope effect (change of Tc at the substitution of deuterium for hydrogen) owes its origin to the high frequency of phonons and is different in different phases. The decrease of Tc after reaching a maximum in high-T phase is due to the interaction with the second gap arising on a hole-like pocket. The phonon-mediated pairing on the latter is in a non-adiabatic regime of the frequency omega of hydrogen modes comparable to the Fermi energy., 11 pages + Supplemental Materials

Observation of replica bands in the ARPES spectra of the single-layer FeSe on strontium titanate substrate revealed a phonon component among mechanisms behind high Tc superconductivity there. We study the interaction of the in-layer FeSe electrons with the electric potential of the longitudinal (LO) modes at surface of bulk SrTiO3. A two-dimensional system of charges at the FeSe/SrTiO3 interface includes both the itinerant and immobile electrons. The latter significantly change the interface characteristics, increasing screening at the substrate surface and reducing thereby the strength of the electron-LO phonon interactions. In that follows the dielectric constant serves as a free parameter and is found from the ARPES data for the replicas. The two-dimensional Coulomb screening is accounted for in the random phase approximation. It is shown that the model is applicable within the whole range of the parameters typical for current experiments. The estimates allow concluding that the LO-phonon mediated pairing alone cannot account for the temperatures of the superconducting transition Tc in the single-layer FeSe/SrTiO3 reported in these experiments. This does not exclude that the LO-phonons mechanism can become more significant in the differently and better prepared single layer FeSe films. Available experiments are briefly discussed. Thus far no data exist on the dependence of Tc on the concentration of electrons doped into the in-layer FeSe band.

We analyze the mathematical structure of equations for temperature Tc of the superconductivity transition in a gas of interacting Fermi particles or at the phonon-mediated pairing in a metal in case of non-adiabatic conditions, i. e., when the characteristic phonon frequency is comparable or larger than the Fermi energy . The integral equations for Tc are derived in the logarithmical approximation. The equations contain no divergent terms in the anti-adiabatic limit.

We study properties of electrons on illuminated surfaces of SrTiO3 with titanium dxz_dyz and dxy bands for their spectrum. Recently A. F. Santander-Syro et al [Nature Materials, 13, 1085 (2014)] found that the dxy bands actually comprise two chiral branches with the Kramers degeneracy at the zone center lifted in absence of a magnetic moment. From symmetry analysis of instabilities possible in the Fermi liquid with exchange interactions we identified the metallic in-layer state with the concrete antiferromagnetic phase and discuss if the same state materializes at conducting LaAlO3_SrTiO3 interfaces., 8 pages, 1 figure

N. Doiron-Leyraud et al. in [1] report on the experimental observation of small holes pockets in the energy spectrum of YBa2Cu3Oy (YBCO) coming below the transition into the charge ordered (CO) phase as a result of a Fermi surface (FS) reconstruction. We comment that their results seem to be more consistent with a unidirectional charge density wave (CDW) vector direction of which alternates along the c-axis between neighboring CuO2 -planes., Comment: 8 pages, 1 figure, some formulas are corrected, the reference list and the figure are updated

Angle resolved photoemission (ARPES) experiments reveal the coherent electronic excitation in the energy spectrum of under- and optimally doped cuprates at temperatures above their temperature of transition into superconducting state only near nodal points on the parts of the "bare" Fermi surface known as the Fermi arcs. That raises questions about the meaning and possible interpretations of the experimental data. Below we derive expressions for the Hall coefficient for the Fermi arc model both in weak and strong magnetic fields., 4 pages

We address recent experiments shedding light on the energy spectrum of under- and optimally doped cuprates at temperatures above superconducting transition. Angle resolved photoemission reveals coherent excitation only near nodal points on parts of the "bare" Fermi surface known as the Fermi arcs. The question debated in the literature is whether the small normal pocket, seen via quantum oscillations exists at higher temperatures or forms below a charge order transition in strong magnetic fields. Assuming the former case as possibility, expressions derived for the resistivity and the Hall coefficient (in weak and strong magnetic fields) with both types of carriers participating in transport. There are two regimes. At higher temperatures (at a fixed field) electrons are dragged by the Fermi arcs' holes. The pocket being small its contribution into conductivity and the Hall coefficient is negligible. At lower temperatures electrons decouple from holes behaving as a Fermi gas in the magnetic field. Mobility of holes on the arcs decreasing in strong fields with decrease of temperature, below a crossover point the pocket electrons prevail changing sign of the Hall coefficient in the low temperature limit. Such behavior finds its confirmation in recent high-field experiments., Comment: 10 pages. arXiv admin note: substantial text overlap with arXiv:1309.0778

In the stoichiometric Fe-pnictides the same d-electrons form the local moments and participate in the metallic conductivity. In the manuscript the emergence of the magnetic moments on the Fe-sites, their interaction with the itinerant electrons are studied in the paramagnetic phase at the different relations between the onsite Coulomb repulsion and the intraatomic Hund's exchange. The energy of the local center that is due to its interaction with the conduction electrons is calculated exactly. It is found that the spin density wave transition is controlled by the RKKY interactions between the centers rather than by the nesting mechanism. In the paramagnetic phase the electrons scatter on the randomly oriented moments. The calculated inverse mean free time provides the interpretation for few available experimental data on the conductivity. The multi-band structure of the energy spectrum in iron pnictides is taken into account., 16 pages

We argue that the low- frequency quantum oscillations observed recently in the vortex state of underdoped ortho II-YBCO have the same origin as in other strongly correlated electronic systems. Superconductivity driven by strong interactions on several Fermi surfaces creates proximity gaps on the other. The gap transferred on a small-sized pocket from larger Fermi surfaces is small. In case of unconventional pairing symmetry the induced gap is proportional to ratio of the oscillations' frequencies for small and large Fermi surfaces. The gap is in inverse proportion to mass enhancement on the latter. We share the view that small pockets themselves are certain feature of topology of the band Fermi surfaces.

We consider single band of conduction electrons interacting with displacements of the transitional ions.In the classical regime strong coupling transforms the harmonic elastic energy for an ion to the one of the well with two deep minima,so that the system is described in terms of Ising spins. Inter-site interactions via the exchange by electrons order spins at lower temperatures. Extention to the quantum regime is discussed. Below the CDW-transition the energy spectrum of electrons remains metallic because the structural vector Q and the FS sizes are not related.Large values of the CDW gaps seen in the tunneling experiments find their natural explanation as due to the deep energy minima in the bound two-well electron-ion complex., 6 pages

The heterogeneous coexistence of antiferromagnetism (SDW) and superconductivity on a mesoscopic scale was observed in iron-pnictides in many recent experiments. We suggest and discuss the scenario in which the heterogeneity is caused by formation of domain walls inherent to the SDW state of pnictides at a proper doping or under applied pressure. Superconductivity would emerge from the modulated SDW structure. The phenomenon is akin to the FFLO-phase in superconductors., 5 pages, 1 figure; the title is changed, the estimations of energy cost of one soliton and DOS imbedded into it are added

We argue that charges in underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ move in a dissipative environment of strong spatial and temporal fluctuations. The unusual temperature dependence of the Hall angle known as "the separation of lifetimes" is reinterpreted and attributed to the appearance of the thermally activated component in the effective number of carriers with the temperature increase. We consider the temperature interval above $T_{c}$ where localization effects can be neglected., Comment: 4 pages, 3 figures

The Hall coefficient data for cuprates show that number of carriers exceeds external doping $x$ at higher $x$ and varies with temperature. Hence, spins on the Cu-sites are not conserved. Activation energy for thermally excited carriers equals the energy between the Fermi surface "arc" and the band bottom near the van Hove singularities. Crossover from marginal Fermi liquid- to pseudogap- regime happens at temperatures at which number of activated carriers gets comparable with the number of externally doped holes. Implications for the $(T,x)$-phase diagram of cuprates are discussed., Comment: 5 pages, 3 figures

We critically investigate the model of a doped excitonic insulator, which recently has been invoked to explain some experimental properties of the ferromagnetic state in Ca_{1-x}La_xB_6. We demonstrate that the ground state of this model is intrinsically unstable towards the appearance of a superstructure. In addition, the model would lead to a phase separation regime and the domain structure which may be prevented by the Coulomb forces only. Recent experiments indicate that a superstructure may indeed show up in this material., 4 pages, 2 figures, submitted to PRL

A phenomenological model with itinerant bands and local states trapped by the lattice on the Cu-sites, is discussed to describe global features of cuprates. Relative energy positions of localized and itinerant states being tuned (thermodynamically or by doping), the system must undergo 1st order Mott metal-insulator transition. Decreasing the local level (from the metallic end of a stoichiometric compound), charge separation instability occurs first before the Mott transition. Crossing and hybridization between local (flat) and itinerant bands introduce a structure in density of states which may account for ``pseudogap'' features in cuprates. Model results in polaronic lattice effects and is rich enough to serve as a phenomenology of cuprates.