Abstract. We search for anomalous normal and superconductive behavior in the two-band Hubbard model with one narrow band. We analyze the influence of the electron– polaron effect and the Altshuler–Aronov effect on effective mass enhancement and scattering times of heavy and light components in the clean case. We find anomalous behavior of resistivity at high temperatures T >W∗h both in 3D and 2D situations. The SC instability in the model is governed by an enhanced Kohn–Luttinger effect for p-wave pairing of heavy electrons via polarization of light electrons.
We present temperature dependences of the large and the small superconducting gaps measured directly by SnS-Andreev spectroscopy in various Fe-based superconductors and MgB2. The experimental L,S(T ) are wellfitted with a two-gap model based on Moskalenko and Suhl system of equations (supplemented with a BCSintegral renormalization). From the fitting procedure, we estimate the key attribute of superconducting state — relative electron-boson coupling constants and eigen BCSratios for both condensates. Our results evidence for a driving role of a strong intraband coupling in the bands with the large gap, whereas interband coupling is rather weak for all the superconductors under study.
The conferences “Fundamental Problems of High Temperature Superconductivity” (FPS) have become traditional since the first one in 2004. The problem of high-temperature superconductivity remains highly topical: quite regularly, novel HTS materials come on stage (copper oxide high-Tc superconductors in 1986, magnesium diboride in 2001, iron pnictide and iron chalcogenide compounds in 2008, FeSe monolayers in 2012, and sulfur hydrides in 2014–2015). Achieving progressively higher superconducting transition temperatures remains an encouraging motivation for researchers in the field. Up to now, the highest Tc, 203 K, is achieved for H2S(H3S) pressurized at ∼ 2 Mbar. Nevertheless, a commonly accepted approach to the problem of high-temperature superconductivity is still missing.
Whether the apparent metal-insulator transition in two-dimensional (2D) correlated electron system is a true quantum phase transition or is a crossover phenomena—this question is in the core of ongoing debates. I present here a novel scenario of this phenomenon, based on experimental finding of the two-phase state in the correlated 2D system. The transport features in the suggested picture are the finite temperature phenomena and a consequence of the magnetic phase transition; the latter manifests in the sign change of the spin magnetization-per-electron. Physically, the magnetic transition means changing the tendency of the two-phase system to either paramagnetic Fermi liquid state (high density), or to the disordered ferromagnet (low density).
Quantum phase slips (QPS) may produce non-equilibrium voltage fluctuations in current-biased superconducting nanowires. Making use of the Keldysh technique and employing the phase-charge duality arguments we investigate such fluctuations within the four-point measurement scheme and demonstrate that shot noise of the voltage detected in such nanowires may essentially depend on the particular measurement setup. In long wires, the shot noise power decreases with increasing frequency Ω and vanishes beyond a threshold value of Ω at T→0.
In this work, we explore the properties of antiferromagnetic cycloid and the phase transitions between commensurate and incommensurate magnetic states in epitaxial BiFeO3 film. Additional magnetic anisotropy induced by strain effects in the films allocates cycloids with the definite directions of spin rotation. Peculiar feature of the cycloids propagating in the films whose symmetry is different from the single crystals is the orientation of spin rotational plane that does not contain electric polarization in contrast with the bulk materials. We construct a diagram of phase transitions induced by magnetic field applied along normal to the surface and show considerable decrease of the strength of magnetic field destroying cycloid in films compared with the bulk.
Abstract. In the large variety of models such as 3D and 2D Fermi-gas model with hard-core repulsion, 3D and 2D Hubbard model, and the Shubin–Vonsovsky model, we demonstrate the possibility of triplet p-wave pairing at low electron density. We show that the critical temperature of the p-wave pairing can be strongly increased in a spin-polarized case or in a two-band situation already at low density and reach experimentally feasible values of (1–5) K. We also discuss briefly d-wave pairing and high-TC superconductivity with TC ∼ 100 K, which arises in the 2D t-J model in the range of parameters realistic for cuprates.