Search results for "Fermi"
showing 10 items of 1388 documents
Flat Bands as a Route to High-Temperature Superconductivity in Graphite
2016
Superconductivity is traditionally viewed as a low-temperature phenomenon. Within the BCS theory this is understood to result from the fact that the pairing of electrons takes place only close to the usually two-dimensional Fermi surface residing at a finite chemical potential. Because of this, the critical temperature is exponentially suppressed compared to the microscopic energy scales. On the other hand, pairing electrons around a dispersionless (flat) energy band leads to very strong superconductivity, with a mean-field critical temperature linearly proportional to the microscopic coupling constant. The prize to be paid is that flat bands can probably be generated only on surfaces and i…
Magnetic Lifshitz transition and its consequences in multi-band iron-based superconductors
2017
In this paper we address Lifshitz transition induced by applied external magnetic field in a case of iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximately constant magnetic field. The chosen sets of the model parameters can describe characteristic ban…
Robust semi-Dirac points and unconventional topological phase transitions in doped superconducting Sr2IrO4 tunnel coupled to t2g electron systems
2017
Semi-Dirac fermions are known to exist at the critical points of topological phase transitions requiring fine-tuning of the parameters. We show that robust semi-Dirac points can appear in a heterostructure consisting of superconducting Sr2IrO4 and a t2g electron system (t2g-ES) without fine-tuning. They are topologically stable in the presence of the symmetries of the model, metallic t2g-ES and a single active band in Sr2IrO4. If the t2g metal is coupled to two different layers of Sr2IrO4 (effectively a multiband superconductor) in a three-layer-structure the semi-Dirac points can split into two stable Dirac points with opposite chiralities. A similar transition can be achieved if the t2g-E…
Instabilities in heavy-fermion systems
1992
Abstract We review (i) an itinerant antiferromagnetic phase transition below 4 K in Ni-rich Ce(Cu 1− x Ni x ) 2 Ge 2 systems, (ii) the coincidence at T = 0.63 K of both a structural lattice instability in “as-grown” (non-superconducting) CeCu 2 Si 2 single crystals and bulk superconductivity in annealed ones as well as (iii) antiferromagnetic and superconducting transitions at T N = 4.6 K and T c = 1 K, respectively, in the heavy-fermion compound UNi 2 Al 3 .
Enhanced superconductivity upon weakening of charge density wave transport in 2H-TaS2 in the two-dimensional limit
2018
Layered transition-metal dichalcogenides that host coexisting charge-density wave (CDW) and superconducting orders provide ideal systems for exploring the effects of dimensionality on correlated electronic phases. Dimensionality has a profound effect on both superconductivity and CDW instabilities. Here we report a substantial enhancement of the superconducting ${T}_{c}$ to 3.4 K for $2H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ in the monolayer limit, compared to 0.8 K in the bulk. In addition, the transport signature of a CDW phase transition vanishes in the two-dimensional limit. In our analysis of electronic and vibrational properties of this material, we show that a reduction of the CDW …
Quantum Criticality of Heavy-Fermion Compounds
2014
Chapter 17 is devoted to the quantum criticality of quantum spin liquids. In this chapter we continue to consider the nature of quantum criticality in HF compounds. The quantum criticality induced by the fermion condensation quantum phase transition extends over a wide range in the \(T-B\) phase diagram. As we shall see, the quantum criticality in all such different HF compounds, as high-\(T_c\) superconductors, HF metals, compounds with quantum spin liquids, quasicrystals, and 2D quantum liquids, is of the same nature. This challenging similarity between different HF compounds expresses universal physics that transcends the microscopic details of the compounds. This uniform behavior, induc…
Magnetoresistance in the HF Metal at Zero Temperature
2014
In this chapter we consider the paradoxical behavior of the residual resistivity \(\rho _0\) of HF metals in magnetic fields and under pressure. Our consideration is based on the idea of flattening of the single-particle spectrum \(\varepsilon (\mathbf{p})\) that profoundly effects on the specific heat \(C\), thermal expansion coefficient \(\alpha \) and magnetic susceptibility \(\chi \) in the normal state, the jump of \(C\) at the point of superconducting phase transition etc. We show that FC associated with flat bands contributes to the residual resistivity \(\rho _0\), while the application of the magnetic field or pressure to the system with a flat band removes the flat band and leads …
Heavy-fermion superconductivity induced by antiferromagnetic spin fluctuations
2007
Superconductivity is caused by an attractive interaction between electrons at the Fermi level that induces the pairing of time-reversed electron states to Cooper pairs. Conventionally this attractive interaction is mediated by phonons. Theoretically, non-phonon mediated coupling seems to be likely for heavy-fermion superconductors whose low-temperature dynamics is dominated by antiferromagnetic spin correlations. However, evidence for spin-fluctuation coupling has not yet been experimentally observed. One of the most direct methods of investigation of the superconducting state is tunneling spectroscopy. We prepared cross-type tunneling junctions composed of the heavy-fermion superconductor …
Intensity of theB1gphonon Raman scattering inYBa2Cu3O7: Comparison of normal and superconducting states
1995
We compare theoretically the intensity of the ${\mathit{B}}_{1\mathit{g}}$ phonon Raman scattering in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ above and below the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. Our analysis shows that a considerable enhancement of the scattering intensity in the superconducting state that is observed experimentally can be caused by an extension of the number of intermediate electronic states near the Fermi surface that participate in the Raman process.
Collective amplitude mode fluctuations in a flat band superconductor formed at a semimetal surface
2016
We study the fluctuations of the amplitude (i.e., the Higgs-Anderson) mode in a superconducting system of coupled Dirac particles proposed as a model for possible surface or interface superconductivity in rhombohedral graphite. This system also serves as a generic model of a topological semimetal with an interaction-driven transition on its surface. We show that the absence of Fermi energy and vanishing of the excitation gap of the collective amplitude mode in the model leads to a large fluctuation contribution to thermodynamic quantities, such as the heat capacity. As a consequence, the mean-field theory becomes inaccurate, indicating that the interactions lead to a strongly correlated sta…