Localization of quasiparticles in a disordered vortex

We study the diffusive motion of low-energy normal quasiparticles along the core of a single vortex in a dirty, type-II, s-wave superconductor. The physics of this system is argued to be described by a one-dimensional supersymmetric non-linear σ model, which differs from the σ models known for disordered metallic wires. For an isolated vortex and quasiparticle energies less than the Thouless energy E_Th, we recover the spectral correlations that are predicted by random matrix theory for the universality class C. We then consider the transport problem of transmission of quasiparticles through a vortex connected to particle reservoirs at both ends. The transmittance at zero energy exhibits a weak localization correction reminiscent of quasi-one-dimensional metallic systems with symmetry index β = 1. Weak localization disappears with increasing energy over a scale set by E_Th This crossover should be observable in measurements of the longitudinal heat conductivity of an ensemble of vortices under mesoscopic conditions. In the regime of strong localization, the localization length is shown to decrease by a factor of 8 as the quasiparticle energy goes to zero.     

Relaxation and collective motion in superconductors: a two-fluid description

A simple, unified discussion of branch imbalance and gap relaxation in superconductors is presented. Both phenomena are treated within the framework of the ordinary Boltzmann equation, supplemented by the BCS gap equation. We show that the physics of the process commonly referred to as quasiparticle branch imbalance relaxation may be understood simply if one introduces a two-fluid model for the charge in the superconductor, and regards the process as one in which charge associated with the normal component is converted into charge associated with the superfluid. We derive in detail the exact solutions of the Boltzmann equation, which are valid near T_c, and allow for the effects of anisotropy. We discuss the comparison between relaxation rates measured in the superfluid and those obtained from normal state measurements and calculations. We then derive a set of two-fluid hydrodynamic equations based on the two-fluid model for the charge, and find that the current of charge associated with the normal component is not in general equal to the usual normal current. On the basis of these equations we derive expressions for the characteristic quasiparticle diffusion length near phase slip centers, and for the frequency of the recently observed collective mode. We compare our result with those of both microscopic and phenomenological calculations.     

D-wave superconductivity in doped Mott insulators

The effect of proximity to a Mott insulating phase on the charge transport properties of a superconductor is determined. An action describing the low energy physics is formulated and different scenarios for the approach to the Mott phase are distinguished by different variation with doping of the parameters in the action. A crucial issue is found to be the doping dependence of the quasiparticle charge which is defined here and which controls the temperature and field dependence of the electromagnetic response functions. Presently available data on high-T_c superconductors are analyzed. The data, while neither complete nor entirely consistent, suggest that neither the quasiparticle velocity nor the quasiparticle charge vanish as the Mott phase is approached, in contradiction to the predictions of several widely studied theories of lightly doped Mott insulators. Implications of the results for the structure of vortices in high-T_c superconductors are determined.     

Interplay among Coulomb interaction, spin-orbit interaction, and multiple electron-boson interactions in Sr2RuO4

Using polarization- and hν-dependent angle-resolved photoemission spectroscopy, we uncovered the fine details of a quasiparticle's dynamics of a typical multiband superconductor, Sr2RuO4. We found strong hybridization between the in-plane and out-of-plane quasiparticles via the Coulomb and spin-orbit interactions. This effect enhances the quasiparticle mass due to the inflow of out-of-plane quasiparticles into the two-dimensional Fermi surface sheet, where the quasiparticles are further subjected to the multiple electron-boson interactions. We suggest that the spin-triplet p-wave superconductivity of Sr2RuO4 is phonon mediated.     

The effect of impurity scattering on the thermally induced charge imbalance in a clean superconductor

We calculate the charge imbalance generated in a current-carrying clean superconductor by a temperature gradient, assuming that non-magnetic or magnetic impurities are the only source of scattering. The charge imbalance is obtained from exact solutions of the quasiparticle Boltzmann equation, valid at any temperature and superfluid velocity.     

Theory of the upper critical fieldH c2 in antiferromagnetic superconductors

The influence of antiferromagnetic order on the upper critical field of superconductors is investigated within the framework of Eliashberg theory. Below the Néel temperature the superconductor is characterized by Cooper pairs of quasiparticle states which are related by time reversal followed by a lattice translation. This leads to a change in the effective attraction between the quasiparticles which we calculate explicitely for the Chevrel phase compounds RE Mo₆S₈ (RE=Gd, Tb, and Dy). The results for the upper critical fields clearly show that our theory yields a consistent explanation of the experimentally observed anomalous behaviour.     

On heat diffusion mode of structural phase transition by two-fluid model

With the help of the two-fluid model developed by Götze and Michel for phonons it is shown for a simple model Hamiltonian that in the low temperature phase the optical soft mode becomes isothermal, the heat diffusion mode is dominant near the transition temperatureT _c and the quasiparticle interaction is of great importance in determining the thermodynamic quantities nearT _c. Green function techniques are applied to describe the two-fluid model functions in a microscopic way. The simplest approximations are discussed for the model equations describing nonequilibrium phenomena of the soft optical phonon mode in the low temperature phase. The quasiparticle interaction operator can be related to the interaction operator between quasiparticles and the condensed mode. This relation enables one to understand the behaviour of the thermodynamic quantities near the transition temperature on a microscopic way. The first order displacive phase transition is also discussed.     

Finite Fermi systems theory and self-consistency relations

The self-consistent theory of the finite Fermi systems is outlined. This approach is based on the same Fermi liquid theory principles as the familiar theory for finite Fermi systems (FFS) by Migdal. We show that the basic Fermi system properties can be evaluated in terms of the quasiparticle Lagrangian L_q which incorporates the energy dependency effects. This Lagrangian is defined so that the corresponding Lagrange equations should coincide with the FFS theory equations of motion of the quasiparticles. The quasiparticle energy E_q defined in the terms of t he quasiparticle Lagrangian L_q according to the usual canonical rules is shown to be equal to the binding energy E_o of the system. For a given Lagrangian L_q the particle densities in nuclei, the nuclear single-particle spectra, the low-lying collective states (LCS) properties, and the amplitude of the interquasiparticle interaction are also evaluated. The suggested approach is compared with the Hartree-Fock theory with effective forces.     

Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.     

Hall effect induced by a spin-polarized current in superconductors

We propose a novel anomalous Hall effect caused by the spin-polarized current in superconductors (SC). The spin-polarized quasiparticles flowing in SC are deflected by spin-orbit scattering to yield a quasiparticle charge imbalance in the transverse direction. Overall charge neutrality gives rise to a compensating change in the number of Cooper pairs. A transverse electric field builds up as opposed to an acceleration of the Cooper pairs, producing the Hall voltage. It is found that the Hall voltages due to the side jump and skew scattering mechanisms have different temperature dependence in the superconducting state. A spin-injection Hall device to generate the ac Josephson effect is proposed.     

Evidence for unconventional s-wave superconductivity in the heavy-fermion compound UPt3

We have discussed the ultrasonic attenuation and specific heat in the superconducting state of the heavy-fermion compound UPt₃ in a quasiparticle picture. Coherence and pair-breaking effect have been taken into account for this system. Our results show that UPt₃ is most probably an unconventional s-wave superconductor.     

Tunneling conductance in topological insulator ferromagnet/p-wave superconductor junctions

The tunneling conductance in topological insulator (TI) ferromagnet/p-wave superconductor (FM/pS) junction is studied based on the Blonder–Tinkham–Klapwijk (BTK) theory. The Fermi energy mismatch between FM and pS as well as the finite quasiparticle lifetime are considered. Three kinds of pairings p_x, p_y, and p_x+ip_y-waves for pS are chosen. It is found that the spectrum strongly depend on the magnetic gap, the gate potential, the quasiparticle lifetime as well as the type of the pair potential symmetry. The pair potential symmetry drastically affects the formation of the zero-energy bound states dependent on the magneto effect or the Fermi energy mismatch effect. The finite quasiparticle lifetime effect can suppress the Andreev resonant scattering process at eV=Δ₀ and smear the dips in the conductance.     

Dynamics of optically excited quasiparticles in YBa2Cu3O7

We present a detailed investigation of the dynamics of laser-excited quasiparticles in YBa₂Cu₃O₇ thin films below the critical temperature. Reflectivity transients at low temperature trace the generation and recombination behavior of quasiparticles. The quasiparticle cascading and recombination rates are determined by comparison with a detailed nonlinear model of the quasiparticle dynamics based on extended Rothwarf-Taylor equations.     

Quasiparticle scattering by quantum phase slips in one-dimensional superfluids

Quantum phase slips (QPS) in narrow superfluid channels generate momentum by unwinding the supercurrent. In a uniform Bose gas, this momentum needs to be absorbed by quasiparticles (phonons). We show that this requirement results in an additional exponential suppression of the QPS rate (compared to the rate of QPS induced by a sharply localized perturbation). In BCS-paired fluids, momentum can be transferred to fermionic quasiparticles, and we find an interesting interplay between quasiparticle scattering on QPS and on disorder.     

Coherent quantum transport in normal-metal/<Emphasis Type="Italic">d</Emphasis>-wave superconductor/normal-metal double tunnel junctions

Taking into account the effects of quantum interference and interface scattering, combining the electron current with hole current contribution to tunnel current, we study the coherent quantum transport in normal-metal/d-wave superconductor/ normal-metal (NM/d-wave SC/NM) double tunnel junctions by using extended Blonder-Tinkham-Klapwijk (BTK) approach. It is shown that all quasiparticle transport coefficients and conductance spectrum exhibit oscillating behavior with the energy, in which periodic vanishing of Andreev reflection (AR) above superconducting gap is found. In tunnel limit for the interface scattering strength taken very large, there are a series of bound states of quasiparticles formed in SC.     

Fermi-arc superconductivity, pseudogap and charge order in Bi2Sr2CaCu2O8+δ

We report STM/STS observations on the 4a×4a charge order in the pseudogap (PG) and superconducting (SC) states of Bi₂Sr₂CaCu₂O_8+δ, and suggest that the charge order is associated with incoherent quasiparticle or pair states around the antinodal fermi surface (FS), which are also responsible for the PG, and it can coexist with the superconductivity caused by the pairing of coherent quasiparticles on the nodal fermi arc. We also suggest that the nanometer-scale gap inhomogeneity in the SC state, reported previously [Pan, et al., Nature 413 (2001) 282; Mommo, et al., J. Phys. Soc. Jpn. 74 (2005) 2400; Hashimoto, et al., Phys. Rev. B74 (2006) 064508] arises from that in the PG state, which occurs around the antinodal FS.     

A contribution from collective rotation to ΔI=±1 transitions in the cranking model

The contribution of the orientational polarization of the core to quasiparticle transition amplitudes in deformed nuclei is derived. In the cranked mean-field model, these contributions arise from the coupling of rotational RPA excitations to the quasiparticles. They reduced to the usual core contribution (g_R) for M1 transitions, and give a collective and possibly oscillatory contribution to E2 transitions with ΔI=±1.     

Superconducting spin filter

Consider two normal leads coupled to a superconductor; the first lead is biased while the second one and the superconductor are grounded. In general, a finite current I₂(V₁, 0) is induced in grounded lead 2; its magnitude depends on the competition between processes of Andreev and normal quasiparticle transmission from lead 1 to lead 2. It is known that, in the tunneling limit, when normal leads are weakly coupled to the superconductor, I₂(V₁, 0)=0 if |V₁|<Δ, and the system is in the clean limit. In other words, Andreev and normal tunneling processes compensate each other. We consider the general case: the voltages are below the gap, the system is either dirty or clean. It is shown that I₂(V₁, 0)=0 for general configuration of the normal leads; if the first lead injects spin-polarized current then I₂=0, but spin current in lead 2 is finite. A XISIN structure, where X is a source of the spin-polarized current, could be applied as a filter separating spin current from charge current. We do an analytical progress calculating I₁(V₁, V₂), I₂(V₁, V₂).     

Microscopic Theory of Pinning of Multiquantum Vortex in Cylindrical Cavity

We have proposed and developed a microscopic model of depinning (escape) of a multiquantum vortex in a superconductor with a cylindrical nonconducting cavity with the transverse size smaller than or on the order of the superconducting coherence length ξ₀ at zero temperature. The spectrum of subgap quasiparticle excitations in two- and three-quantum vortices trapped by a cylindrical cavity has been calculated in the quasiclassical approximation. It is shown that the transformation of the spectrum is accompanied by break of anomalous spectral branches due to normal reflection of quasiparticles from the surface of a defect. A microscopic (spectral) criterion for multiquantum vortex pinning has been proposed; according to this criterion, the multiquantum vortex can be trapped in the cavity during the formation of a minigap in the elementary excitation spectrum near the Fermi level. Self-consistent calculations of density of states N(r, ε) for two- and three-quantum vortices trapped by a cylindrical cavity of radius on the order of ξ₀ have been performed using quasiclassical Eilenberger equations. In the pure limit and for low temperatures T ? T _c , peculiarities observed in the N(r, ε) distribution reflect the presence of M anomalous spectral branches in the M-quantum vortex and confirm the correctness of the spectral criterion of pinning (depinning) of a multiquantum vortex.