Strongly correlated s-wave superconductivity in the N-type infinite-layer cuprate

Quasiparticle tunneling spectra of the electron-doped ( n-type) infinite-layer cuprate Sr0.9La0.1CuO2 reveal characteristics that counter a number of common phenomena in the hole-doped ( p-type) cuprates. The optimally doped Sr0.9La0.1CuO2 with T(c) = 43 K exhibits a momentum-independent superconducting gap Delta = 13.0+/-1.0 meV that substantially exceeds the BCS value, and the spectral characteristics indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The response to quantum impurities in the Cu sites also differs fundamentally from that of the p-type cuprates with d(x(2)-y(2))-wave pairing symmetry.     

Strongly correlated superconductivity and pseudogap phase near a multiband Mott insulator

Near a Mott transition, strong electron correlations may enhance Cooper pairing. This is demonstrated in the dynamical mean field theory solution of a twofold-orbital degenerate Hubbard model with an inverted on-site Hund rule exchange, favoring local spin-singlet configurations. Close to the Mott insulator (which here is a local version of a valence bond insulator) a pseudogap non-Fermi-liquid metal, a superconductor, and a normal metal appear, in striking similarity with the physics of cuprates. The strongly correlated s-wave superconducting state has a larger Drude weight than the corresponding normal state. The role of the impurity Kondo problem is underscored.     

Strongly correlated quantum spin liquid in herbertsmithite

Strongly correlated Fermi systems are among the most intriguing and fundamental systems in physics. We show that the herbertsmithite ZnCu₃(OH)₆Cl₂ can be regarded as a new type of strongly correlated electrical insulator that possesses properties of heavy-fermion metals with one exception: it resists the flow of electric charge. We demonstrate that herbertsmithite’s low-temperature properties are defined by a strongly correlated quantum spin liquid made with hypothetic particles such as fermionic spinons that carry spin 1/2 and no charge. Our calculations of its thermodynamic and relaxation properties are in good agreement with recent experimental facts and allow us to reveal their scaling behavior, which strongly resembles that observed in heavy-fermion metals. Analysis of the dynamic magnetic susceptibility of strongly correlated Fermi systems suggests that there exist at least two types of its scaling.     

Strongly correlated electron system in the magnetic field

In the range of hole concentrations 0.08<x<0.18$0.08<x<0.18$ the density of states of the two-dimensional t–J model reveals oscillations with changing the magnetic field. Oscillation frequencies correspond to large Fermi surfaces. However, the oscillations are modulated with a frequency which is smaller by an order of magnitude. The modulation is related to van Hove singularities in the Landau subbands, which traverse the Fermi level with changing the field. The singularities are connected with bending the subbands due to strong electron correlations. The frequency of the modulation is of the same order of magnitude as quantum oscillation frequencies in underdoped cuprates.     

Strongly correlated fermions after a quantum quench

Using the adaptive time-dependent density-matrix renormalization group method, we study the time evolution of strongly correlated spinless fermions on a one-dimensional lattice after a sudden change of the interaction strength. For certain parameter values, two different initial states (e.g., metallic and insulating) lead to observables which become indistinguishable after relaxation. We find that the resulting quasistationary state is nonthermal. This result holds for both integrable and nonintegrable variants of the system.     

Strongly correlated fractional quantum Hall line junctions

We have studied a clean finite-length line junction between interacting counterpropagating single-branch fractional quantum Hall edge channels. Exact solutions for low-lying excitations and transport properties are obtained when the two edges belong to quantum Hall systems with different filling factors and interact via the long-range Coulomb interaction. Charging effects due to the coupling to external edge-channel leads are fully taken into account. Conductances and power laws in the current-voltage characteristics of tunneling are strongly affected by interedge correlations.     

Anisotropic singlet superconductivity for systems with strongly correlated electrons

The periodic Anderson model extended by pairing interactions amongf- and conduction electrons is investigated in the context of the singlet-like superconductivity. We evaluate the transition temperatureT _c and the ratios of superconducting order parameters atT _c as functions of an effective parameter which describes the correlation off-electrons. The relative stability of thes- andd -superconductivity depends on the values of the Fermi wave vector and on the magnitude of . Due to the absence of on-site pairing forf-electrons, the increase of Coulomb correlations makes thes-like superconductivity more favourable than thed -superconductivity. This behaviour is accompanied by crossover effects: with the increase of Coulomb correlations amongf-electrons the superconductivity is taken over by conduction electrons.Supported by PAN, CPBP 01.12     

A new aspect of superconductivity in A-15 compounds

We present a new aspect of superconductivity in A-15 compounds which is able to explain their exceptional role among the highT _c superconductors. The basic idea is that a strong energy dependence of the electronic density of states near the Fermi level may greatly reduce the repulsive part of the frequency dependent electron-phonon interaction. This leads to a large enhancement ofT _c which is a maximum when the Fermi energy is comparable to a typical phonon energy. Our findings are based on numerical solutions of the Eliashberg equations where both the retardation of the electron-phonon coupling and the energy dependence of the electronic density of states have been included. For the electronic density of states we use the models of Labbé und Friedel and of Cohen et al., while the shape of the Eliashberg function ² F() is taken from the tunneling results of Shen. We compare our theory to experimental results for ternary A-15 compounds.     

Strongly correlated Fermi systems as a new state of matter

The aim of this review paper is to expose a new state of matter exhibited by strongly correlated Fermi systems represented by various heavy-fermion (HF) metals, two-dimensional liquids like ³He, compounds with quantum spin liquids, quasicrystals, and systems with one-dimensional quantum spin liquid. We name these various systems HF compounds, since they exhibit the behavior typical of HF metals. In HF compounds at zero temperature the unique phase transition, dubbed throughout as the fermion condensation quantum phase transition (FCQPT) can occur; this FCQPT creates flat bands which in turn lead to the specific state, known as the fermion condensate. Unlimited increase of the effective mass of quasiparticles signifies FCQPT; these quasiparticles determine the thermodynamic, transport and relaxation properties of HF compounds. Our discussion of numerous salient experimental data within the framework of FCQPT resolves the mystery of the new state of matter. Thus, FCQPT and the fermion condensation can be considered as the universal reason for the non-Fermi liquid behavior observed in various HF compounds. We show analytically and using arguments based completely on the experimental grounds that these systems exhibit universal scaling behavior of their thermodynamic, transport and relaxation properties. Therefore, the quantum physics of different HF compounds is universal, and emerges regardless of the microscopic structure of the compounds. This uniform behavior allows us to view it as the main characteristic of a new state of matter exhibited by HF compounds.     

Theory of superconductivity in strongly correlated materials: Application to cuprate superconductors

A microscopic theory of superconductivity in systems with strong electron correlations is considered within the Hubbard model. The Dyson equation for the matrix Green function in terms of the Hubbard operators is derived and solved in the noncrossing approximation for the self-energy. Two channels of superconducting pairing are revealed: mediated by antiferromagnetic (AFM) exchange and spin-fluctuations. It is proved that AFM exchange interaction results in pairing of all electrons in the conduction band and high T _c proportional to the Fermi energy. T _c dependence on lattice constants (or pressure) and an oxygen isotope shift of T _c are explained. The text was submitted by the author in English.     

Coexistence of superconductivity and density wave orders in a one-dimensional correlated system

By the field theory approach, we investigate a one-dimensional correlated electronic system modelled by the extended Hubbard Hamiltonian including a nearest-neighbor and a next-nearest-neighbor spin-exchange interactions with easy-axis anisotropy. At half filling, we obtain weak-coupling phase diagram. In addition to two insulating phases with transverse and longitudinal spin-density-wave orders, two metallic phases, characterized by the coexistence of singlet superconductivity and charge-density-wave orders and by the coexistence of triplet superconductivity and spin-density-wave orders, are realized in the ground state. Away from half filling, the degeneracy is split and the superconducting orders are favored.     

Instability of layered quantum liquids: 6. Strongly correlated bosons

We calculate the local-field corrections G(q, q _z) for charged bosons at zero temperature in a superlattice with interlayer distance d. An analytical expression for the local-field correction is described. The sum-rule version of the self-consistent approach for the local-field correction is used to discuss the effects of correlation. The RPAparameter r _s and the ratio d/a* determine correlation effects. a* is the effective Bohr radius. The stability region for the Bose condensate r _s < r _sc as a function of d/a* is determined: r _sc ≈ (d/a*)^3/4. The ground-state energy of the layered Bose condensate is calculated and optical and acoustical plasmons are discussed. We predict a roton structure for optical plasmons for r _s > r _sr with r _sr ≈ 0.5 (d/a*)^3/4.     

Strongly correlated two-photon transport in a one-dimensional waveguide coupled to a two-level system

We show that two-photon transport is strongly correlated in one-dimensional waveguide coupled to a two-level system. The exact S matrix is constructed using a generalized Bethe-ansatz technique. We show that the scattering eigenstates of this system include a two-photon bound state that passes through the two-level system as a composite single particle. Also, the two-level system can induce effective attractive or repulsive interactions in space for photons. This general procedure can be applied to the Anderson model as well.