Quasiparticle Transport Research Articles

Effect of the insulating layer on the differential conductance in the normal metal/insulator/s-wave superconductor tunnel junctions

Making use of the square-potential barrier to describe the influence of the insulating layer on the quasi-particle transport in normal metal/insulator/superconductor (NIS) tunnel junctions, we calculate the quasi-particle transport coeffic ients and the differential conductance using the Bogoliubov-de Gennes（BdG）equa tion and Blonder-Tinkham-Klapwijk(BTK) theory. It is found that the change of th e differential conductance with insulating layer thickness takes the form of the oscillation and attenuation, the period of oscillation and the speed of attenua tion depend strongly on the potential magnitude of the insulating layer and the bias value, the height of the conductance peaks and their positions are related to the insulating layer thickness intimately. It is shown that the tunneling spe ctra are different from those described by the δ-potential.

Dynamics and energy distribution of nonequilibrium quasiparticles in superconducting tunnel junctions

We present a full theoretical and experimental study of the dynamics and energy distribution of nonequilibrium quasiparticles in superconducting tunnel junctions (STJ's). STJ's are often used for single-photon spectrometers, where the numbers of quasiparticles excited by a photon provide a measure of the photon energy. The magnitude and fluctuations of the signal current in STJ detectors are in large part determined by the quasiparticle dynamics and energy distribution during the detection process. We use this as motivation to study the transport and energy distribution of nonequilibrium quasiparticles excited by x-ray photons in a lateral, imaging junction configuration. We present a full numerical model for the tunneling current of the major physical processes which determine the signal. We find that a diffusion framework models the quasiparticle dynamics well and that excited quasiparticles do not equilibrate to the lattice temperature during the time scales for tunneling. We extract physical time scales from the measured data, make comparisons with existing theories, and comment on implications for superconducting mesoscopic systems and single-photon detectors.

Nernst effect, quasiparticles, andd-density waves in cuprates

We examine the possibility that the large Nernst signal observed in the pseudogap regime of hole-doped cuprates originates from quasiparticle transport in a state with d-density wave (DDW) order, proposed by S. Chakravarty et al. [Phys. Rev. B 63, 094503 (2001)]. We find that the Nernst coefficient can be moderately enhanced in magnitude by DDW order, and is generally of negative sign. Thus, the quasiparticles of the DDW state cannot account for the large and positive Nernst signal observed in the pseudogap phase of the cuprates. However, the general considerations outlined in this paper may be of broader relevance, in particular to the recent measurements of Bel et al. in NbSe_2 and CeCoIn_5 [Phys. Rev. Lett. 91, 066602 (2003); ibid. 92, 217002 (2004)].

Interedge strong-to-weak scattering evolution at a constriction in the fractional quantum Hall regime.

Gate-voltage control of interedge tunneling at a split-gate constriction in the fractional quantum Hall regime is reported. Quantitative agreement with the behavior predicted for out-of-equilibrium quasiparticle transport between chiral Luttinger liquids is shown at low temperatures at specific values of the backscattering strength. When the latter is lowered by changing the gate voltage, the zero-bias peak of the tunneling conductance evolves into a minimum, and a nonlinear quasiholelike characteristic emerges. Our analysis emphasizes the role of the local filling factor in the split-gate constriction region.

Coherent quantum transport in normal-metal/d-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.

TERAHERTZ TRANSPORT IN SEMICONDUCTOR QUANTUM STRUCTURES

Photon assisted transport, dynamic localization and absolute negative conductance appear in the terahertz photoconductivity in semiconductor quantum structures and are close analogs of quasi-particle transport in microwave irradiated superconducting junctions. By embedding superlattice devices in quasi-optical arrays and integrating them into terahertz cavities, the dynamical conductance of electrically biased superlattices can be measured. Models including the complications of electric field domains can account for the results in a semi quantitative manner. Uniform electrically biased superlattices appear to be potentially important as a terahertz gain medium.

Quasi-particle transport in narrow superconducting wires with superconductor/insulator/normal tunnel junctions

We have studied the low temperature quasi-particle charge transport in a narrow superconducting metal wire by generating an imbalance of charge density in the quasi-particle system and detecting the transferred imbalance from an excess current in small S/I/N junctions. From the measured spatial distribution of the charge imbalance, it is shown that the charge imbalance is transferred in the wire by diffusion.

Phase modulated thermal conductance of Josephson weak links.

We present a theory for quasiparticle heat transport through superconducting weak links. The thermal conductance depends on the phase difference (phi) of the superconducting leads. Branch-conversion processes, low-energy Andreev bound states near the contact, and the suppression of the local density of states near the gap edge are related to phase-sensitive transport processes. Theoretical results for the influence of junction transparency, temperature, and disorder, on the conductance, are reported. For high-transmission weak links, D-->1, the formation of an Andreev bound state leads to suppression of the density of states for the continuum excitations, and thus, to a reduction in the conductance for phi approximately pi. For low-transmission (D<<1) barriers resonant scattering leads to an increase in the thermal conductance as T drops below T(c) (for phase differences near phi=pi).

Memory effects and nonequilibrium transport in open many-particle quantum systems

Full understanding of the relaxation mechanisms and far-from-equilibrium transport in modern mesoscopic structures requires that such systems be treated as open. We therefore generalize some of the core elements of the Kadanoff-Baym-Keldysh nonequilibrium Green's function formalism, inherently formulated for closed systems, to treatment of an open system, coupled with its environment. We define the two-time correlation functions and analyze the influence of the memory effects on the open-system transport. In the transient regime, the two-time correlation functions clearly show four distinct terms: a closed-system-like term, an entanglement term, and two memory terms that depend explicitly on the initial state of the environment. We show that it is not possible to completely eliminate the influence of the environment by a fortunate choice of the initial state, and approximating the system as closed is valid only in the limit of negligible system-environment coupling, which is never the case in the transient regime. We derive the transport equations for transients that properly account for the system-environment coupling. On the other hand, we address the important issue of transport in a far-from-equilibrium steady state. We show that, once a steady state is reached, the balance between the driving and relaxation forces implies that the two-time correlation functions regain a closed-system-like form, but with an effective, modified system Hamiltonian, and with the system statistical operator unrelated to that of the initial state. We emphasize that the difference between the transient and the far-from-equilibrium steady-state regimes, crucial for theoretical investigation of nonequilibrium quasiparticle transport, effectively lies within the different relative magnitude of the combined entanglement and memory terms with respect to the closed-system-like term in two-time correlation functions.

Anisotropy of microwave conductivity in the superconducting and normal states of YBa2Cu3O7−x: 3D-2D crossover

The imaginary parts of microwave conductivity σ″(T<T c) and resistivity ρ (T)=1/σ(T>T c) along (σ ab ″ and ρab) and across and (σ c ″ and ρc the cuprate ab planes of a YBa2Cu3O7−x crystal with the oxygen doping level x varying from 0.07 to 0.47 were measured in the temperature range 5≤ T≤200 K. In the superconducting state, the σ ab ″ (T)/σ ab ″ (0) and σ c ″ (T/σ c ″ (0) curves coincide for an optimally doped (x=0.07) crystal, but, with an increase in x, the slopes of the σ c ″ (T)/σ c ″ (0) curves decrease noticeably at T<T c/3, on the background of small changes happening to the σ ab ″ (T/σ ab ″ (0) curves. The two-dimensional (2D) transport along the ab planes in the normal state of YBa2Cu3O7−x is always metallic, but there is a crossover (at x=0.07) from the Drude to hopping (at x>0.07) conductivity along the c axis. This is confirmed both by the estimates of the lowest metallic and the highest tunneling conductivities along the c axis and by quantitative comparison of the measured ρc(T) curves with the curves calculated in the polaron model of quasiparticle transport along the c axis.

Berry phase and spin quantum Hall effect in the vortex state of superfluid3Hein two dimensions

We show that the spin quantum Hall effect in the vortex state of two-dimensional rotating superfluid He3 can be described as an adiabatic spin transport of Bloch quasiparticles. We show that the spin Hall conductivity is written by the Berry phase as well as the Chern number. The results have similarity to the adiabatic pumping of Bloch electrons and the spontaneous polarization in crystalline dielectrics.

Thermal conductivity tensor inYBa2Cu3O7−x: Effects of a planar magnetic field

We have measured the thermal conductivity tensor of a twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ single crystal as a function of angle $\ensuremath{\theta}$ between the magnetic field applied parallel to the ${\mathrm{CuO}}_{2}$ planes and the heat current direction, at different magnetic fields and at $T=13.8 \mathrm{K}.$ Clear fourfold and twofold variations in the field-angle dependence of ${\ensuremath{\kappa}}_{\mathrm{xx}}$ and ${\ensuremath{\kappa}}_{\mathrm{xy}}$ were, respectively, recorded in accordance with the d-wave pairing symmetry of the order parameter. The oscillation amplitude of the transverse thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{xy}}^{0}$ was found to be larger than the longitudinal one, ${\ensuremath{\kappa}}_{\mathrm{xx}}^{0},$ in the range of magnetic field studied here $(0 \mathrm{T}&lt;~B&lt;~9 \mathrm{T}).$ From our data we obtain quantities that are free from nonelectronic contributions, and they allow us a comparison of the experimental results with current models for the quasiparticle transport in the mixed state.

Spin-polarized quasiparticle transport in cuprate superconductors

The effects of spin-polarized quasiparticle transport in superconducting ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite nonmagnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density ${(J}_{c})$ as a function of the injection current density ${(J}_{\mathrm{inj}}),$ temperature $(T),$ and the thickness (d) of the superconductor reveal drastic differences between the F-I-S and N-I-S heterostructures, with strong suppression of ${J}_{c}$ and a rapidly increasing characteristic transport length near the superconducting transition temperature ${T}_{c}$ only in the F-I-S samples. The temperature dependence of the efficiency $(\ensuremath{\eta}\ensuremath{\equiv}\ensuremath{\Delta}{J}_{c}{/J}_{\mathrm{inj}};$ $\ensuremath{\Delta}{J}_{c}:$ the suppression of critical current due to finite ${J}_{\mathrm{inj}})$ in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveal that a substantial chemical potential shift ${\ensuremath{\mu}}^{*}$ in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the ${\mathrm{CuO}}_{2}$ planes. Several alternative scenarios attempted to account for the suppression of critical currents in F-I-S samples are also critically examined, and are found to be neither compatible with experimental data nor with the established theory of nonequilibrium superconductivity.

Channel conduction mechanisms in Y–Ba–Cu–O grain boundary Josephson junctions

Mechanisms of charge transport in high critical temperature Y–Ba–Cu–O grain boundary junctions have been analyzed in the framework of existing models. In particular, transport properties have been described by a two-channel model. Theoretical behaviors have been compared with experimental data. The scaling of the grain boundary I c GB R n GB products with the critical current density have been successfully accounted for. The performed study supports a description of grain boundaries as an array of superconductive channels, separated by non-superconductive regions, which allow the transport of quasiparticles only.

Low- and zero-energy quasiparticle heat transport in high- Tc superconductors

A review of our recent measurements on subkelvin thermal conductivity in single crystals of high- T c cuprates is presented. At optimal doping, both Bi 2Sr 2CaCu 2O 8 and YBa 2Cu 3O 7 exhibit a finite linear term in the temperature-dependence of their thermal conductivity. Moreover, in the former compound the size of the linear term does not change after the introduction of disorder by electron irradiation. Both these observations are in agreement with the standard theory of heat transport by nodal quasiparticles in d-wave superconductors. Surprisingly in the two underdoped systems studied here, the linear term is either too small (La 1− x SR x CuO 4, x=0.8) or absent (YBa 2Cu 4O 8). The results point out the limits of our current theoretical understanding of transport by low-energy quasiparticles in the underdoped regime.

Quasiparticle conductivities in disordered d-wave superconductors.

We study the quasiparticle transport coefficients in disordered d-wave superconductors. We find that spin and charge excitations are generally localized unless magnetic impurities are present. If the system is close to a nesting point in the impurity-scattering unitary limit, the tendency towards localization is reduced while the quasiparticle density of states gets enhanced by disorder. We also show that the residual repulsive interaction among quasiparticles has a delocalizing effect and increases the density of states.

Electron-quasiparticle interaction and electrical resistivity of cubic HoCu 5

Point-contact (PC) spectroscopy and temperature dependence of the electrical resistivity ρ( T) have been combined to study the electron–quasiparticle interaction function and transport properties of the cubic (AuBe 5-structure type) HoCu 5 prepared by melt spinning. The ρ( T) dependence revealed two anomalies at ∼3 and ∼8 K. The peaks observed in the low energy part of PC spectrum (∼1.2, ∼4.3 meV) have the crystalline electric field origin. The broad maximum at about 19 meV is connected with phonons of HoCu 5.

Retarded versus Time-Nonlocal Quantum Kinetic Equations

The finite duration of the collisions in Fermionic systems as expressed by the retardation time in non-Markovian Levinson-type kinetic equations is discussed in the quasiclassical limit. We separate individual contributions included in the memory effect resulting in (i) off-shell tails of the Wigner distribution, (ii) renormalization of scattering rates, (iii) renormalization of the single-particle energy, (iv) collision delay, and (v) related nonlocal corrections to the scattering integral. In this way we transform the Levinson equation into the Landau–Silin equation extended by the nonlocal corrections known from the theory of dense gases. The derived nonlocal kinetic equation unifies the Landau theory of quasiparticle transport with the classical kinetic theory of dense gases. The space-time symmetry is discussed versus particle-hole symmetry and a solution is proposed which transforms these two exclusive pictures into each other.

Effect of magnetic field on impurity bound states in high-Tc superconductors

The influence of magnetic field H on the quasiparticle bound states near strong impurities in d-wave superconductors is considered. At large but finite impurity strength, there are corrections to the energy and width of the impurity-induced resonance, proportional to H2. In the limit of very strong impurity potential (unitary limit), the bound state is destroyed and acquires a finite width which depends non-analytically on H. There are also considerable changes in the asymptotic behaviour of the bound state wave functions, which might affect the quasiparticle transport in HTSC.

Electron localization in narrow surface-corrugated conducting channels: Manifestation of competing scattering mechanisms

Transport properties of narrow two-dimensional conducting wires in which the electron scattering is caused by side edges' roughness have been studied. The method for calculating dynamic characteristics of such conductors is proposed which is based on the two-scale representation of the mode wave functions at weak scattering. With this method, fundamentally different {\it by-height} and {\it by-slope} scattering mechanisms associated with edge roughness are discriminated. The results for single-mode systems, previously obtained by conventional methods, are proven to correspond to the former mechanism only. Yet the commonly ignored by-slope scattering is more likely dominant. The electron extinction lengths relevant to this scattering differ substantially in functional structure from those pertinent to the by-height scattering. The transmittance of ultra-quantum wires is calculated over all range of scattering parameters, from ballistic to localized transport of quasi-particles. The obtained dependence of scattering lengths on the disorder parameters is valid qualitatively for arbitrary inter-correlation of the boundaries' defects.