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arXiv2009

Andreev reflections at large ferromagnet/high-T_C superconductor area junctions with rough interface

Sub-gap conductance at a large area junction with a rough interface of a ferromagnet and a high-T$_{C}$ superconductor is superimposed by multiple peaks which is not expected from an ideal point contact Andreev reflection process. We demonstrate this phenomenon by measuring resistance as a function of bias voltage of a Co/Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ junction with contact area 50 x 70 $μ$ $m^{2}$ at various temperatures. In order to analyze such Andreev reflection data, the interface is assumed to have random potentials which can create local electric fields. The Blonder-Tinkham-Klapwijk theory is modified with the inclusion of a broadening parameter due to finite life time effects of quasi particles. An additional voltage drop due to local electric fields at the rough interface has been included in terms of an extra energy shift which may be related to the asymmetry of normalized resistance data. Spin polarization has been introduced for the ferromagnet. The presented model explains the multi-peak nature and asymmetry of Andreev reflection data experimentally observed at large area junctions. Extension of the model also interprets the experimentally observed anomalous enhancement of resistance peaks in the sub-gap region which may result from crossing the critical current limit across the junction.

cond-mat.supr-con↗
arXiv2007

Resistivity of Inhomogeneous Superconducting Wires

We study the contribution of quantum phase fluctuations in the superconducting order parameter to the low--temperature resistivity $ρ(T)$ of a dirty and inhomogeneous superconducting wire. In particular, we account for random spatial fluctuations of arbitrary size in the wire thickness. For a typical wire thickness above the critical value for superconductor--insulator transition, phase--slips processes can be treated perturbatively. We use a memory formalism approach, which underlines the role played by weak violation of conservation laws in the mechanism for generating finite resistivity. Our calculations yield an expression for $ρ(T)$ which exhibits a smooth crossover from a homogeneous to a ``granular'' limit upon increase of $T$, controlled by a ``granularity parameter'' $D$ characterizing the size of thickness fluctuations. For extremely small $D$, we recover the power--law dependence $ρ(T)\sim T^α$ obtained by unbinding of quantum phase--slips. However in the strongly inhomogeneous limit, the exponent $α$ is modified and the prefactor is {\em exponentially enhanced}. We examine the dependence of the exponent $α$ on an external magnetic field applied parallel to the wire. Finally, we show that the power--law dependence at low $T$ is consistent with a series of experimental data obtained in a variety of long and narrow samples. The values of $α$ extracted from the data, and the corresponding field dependence, are consistent with known parameters of the corresponding samples.

cond-mat.supr-con↗
arXiv2008

Density of state and non-magnetic impurity effects in electron-doped cuprates

We analyze the density of state (DOS) and a non-magnetic impurity effect in electron-doped cuprates starting from two different scenarios: the $d_{x^{2}-y^{2}}$-wave superconductivity coexisting with antiferromagnetic spin density wave (SDW) order versus $d_{x^{2}-y^{2}}$-wave superconductivity with a higher harmonic. We find that in both cases the local density of state (LDOS) exhibits two impurity-induced resonance states at low energies. We also find that for the intermediate value of the SDW gap, the DOS looks similar to that obtained from the scenario of the $d_{x^{2}-y^{2}}$-wave gap with a higher harmonic, suggesting the presence of a non-monotonic $d_{x^{2}-y^{2}}$-wave gap. However, if the SDW gap is sufficiently large the DOS looks more conventional s-wave like. This obvious difference from the DOS resulted from the $d_{x^{2}-y^{2}}$-wave gap with a higher harmonic model, could differentiate the two above scenarios and is needed to be proved in the further doping dependence of tunneling spectrum measurement.

cond-mat.str-el↗
arXiv2009

Effects of geometrical fluctuations on the transition temperature of disordered quasi-one-dimensional superconductors

We study the superconducting transition temperature, T_c, of an array of fluctuating, maximally gapped interacting ladders, embedded in a disordered plane. Renormalization group analysis indicates that geometrical fluctuations mitigate the suppression of the intra-ladder pairing correlations by the disorder. In addition, the fluctuations enhance the Josephson tunneling between the ladders. Both effects lead to an increase of T_c in the Josephson coupled array. These findings may be relevant to the understanding of the 1/8 anomaly in the lanthanum based high-temperature superconductors.

cond-mat.supr-con↗
arXiv2008

Fabrication and Characterization of Short Josephson Junctions with Stepped Ferromagnetic Barrier

We present novel low-T_c superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions with planar and stepped ferromagnetic interlayer. We optimized the fabrication process to set a step in the ferromagnetic layer thickness. Depending on the thickness of the ferromagnetic layer the ground state of the SIFS junction has a phase drop of either 0 or pi. So-called 0-pi Josephson junctions, in which 0 and pi ground states compete with each other, were obtained. These stepped junctions may have a double degenerate ground state, corresponding to a vortex of supercurrent circulating clock- or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum Φ_0. Here, we limit the presentation to static properties of short junctions.

cond-mat.supr-con↗
arXiv2008

In-situ frequency tuning of photons stored in a high Q microwave cavity

Photons are fundamental excitations of the electromagnetic field and can be captured in cavities. For a given cavity with a certain size, the fundamental mode has a fixed frequency f which gives the photons a specific "colour". The cavity also has a typical lifetime tau, which results in a finite linewidth delta f}. If the size of the cavity is changed fast compared to tau, and so that the frequency change Delta f >> delta f, then it is possible to change the "colour" of the captured photons. Here we demonstrate superconducting microwave cavities, with tunable effective lengths. The tuning is obtained by varying a Josephson inductance at one end of the cavity. We show tuning by several hundred linewidths in a time Delta t << tau. Working in the few photon limit, we show that photons stored in the cavity at one frequency will leak out from the cavity with the new frequency after the detuning. The characteristics of the measured devices make them suitable for dynamic coupling of qubits.

cond-mat.supr-con↗
arXiv2008

Crossover from Thermal Activation to Quantum Interlayer Transport at the Superconducting Transition Temperature of Bi-2212

We perform a detailed study of temperature, bias and doping dependance of interlayer transport in the layered high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We observe that the shape of interlayer characteristics in underdoped crystals exhibit a remarkable crossover at the superconducting transition temperature: from thermal activation-type at $T>T_c$, to almost $T-$independent quantum tunneling-type, at $T<T_c$. Our data indicates that the interlayer transport mechanism may change with doping: from the conventional single quasiparticle tunneling in overdoped, to a progressively increasing Cooper pair contribution in underdoped crystals.

cond-mat.supr-con↗
arXiv2008

A vortex dynamics approach to the Nernst effect in fluctuating superconductors

We present a new method to study the Nernst effect and diamagetism of an extreme type-II superconductor dominated by phase fluctuations. We work directly with vortex variables and our method allows us to tune vortex parameters (e.g., core energy and number of vortex species). We find that diamagnetic response and transverse thermoelectric conductivity ($α_{xy}$) persist well above the Kosterlitz-Thouless transition temperature, and become more pronounced as the vortex core energy is increased. However, they \textit{weaken} as the number of internal vortex states are increased. We find that $α_{xy}$ closely tracks the magnetization $(-M/T)$ over a wide range of parameters.

cond-mat.supr-con↗
arXiv2008

Pairing Glue in the Hubbard and t-J Models

The question of whether one should speak of a "pairing glue" in the Hubbard and t-J models is basically a question about the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies correspond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case, while one might speak of an "instantaneous glue", this interaction differs from the traditional picture of a retarded pairing interaction. However, if the energies correspond to the spectrum seen in the dynamic spin susceptibility, then the interaction is retarded and one speaks of a spin-fluctuation glue which mediates the d-wave pairing. Here we present results from numerical studies which provide insight into this question.

cond-mat.supr-con↗
arXiv2008

Spontaneous vortex phases in ferromagnet-superconductor nanocomposites

The interplay between superconductivity and magnetism gives rise to many intriguing and exciting phenomena. In this Letter we report about a novel manifestation of this interplay: a temperature induced phase transition between different spontaneous vortex phases in lead superconducting films with embedded magnetic nanoparticles. Unlike common vortices in superconductors the vortex phase appears without any applied magnetic field. The vortices nucleate exclusively due to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field and simultaneously acting as pinning centers. As in usual superconductors, one can move the spontaneous vortices with an applied electric current. Transport measurements reveal dynamical phase transitions that depend on temperature (T) and applied field (H) and support the obtained (H-T) phase diagram. In particular, we used a scaling analysis to characterize a transition from a liquid to a novel disordered solid resembling a vortex glass.

cond-mat.supr-con↗
arXiv2008

Breakdown of Fermi Liquid Theory in Doped Mott Insulators by Dynamical Spectral Weight Transfer

We show that doped Mott insulators exhibit a collective degree of freedom, not made out of the elemental excitations, because the number of single-particle addition states at low energy per electron per spin is greater than one. The presence of such a collective degree of freedom which is not a consequence of proximity to a phase transition is a consequence of dynamical spectral weight transfer from high to low energies. This physics is captured by the charge $2e$ boson that emerges by explicitly integrating out the high-energy scale in the Hubbard model. The charge $2e$ boson binds to a hole, thereby mediating new charge $e$ states at low energy. It is the presence of such charge $e$ states which have no counterpart in the non-interacting system that provides the general mechanism for the breakdown of Fermi liquid theory in doped Mott insulators. The relationship between the charge $2e$ boson formulation and the standard perturbative treatment is explained.

cond-mat.str-el↗
arXiv2008

High fidelity universal set of quantum gates using non-adiabatic rapid passage

Numerical simulation results are presented which suggest that a class of non-adiabatic rapid passage sweeps first realized experimentally in 1991 should be capable of implementing a universal set of quantum gates G_{u} that operate with high fidelity. The gates constituting G_{u} are the Hadamard and NOT gates, together with variants of the phase, π/8, and controlled-phase gates. The universality of G_{u} is established by showing that it can construct the universal set consisting of Hadamard, phase, π/8, and controlled-NOT gates. Sweep parameter values are provided which simulations indicate will produce the different gates in G_{u}, and for which the gate error probability P_{e} satisfies: (i) P_{e} < 10^{-4} for the one-qubit gates; and (ii) P_{e} < 1.27x 10^{-3} for the modified controlled-phase gate. The sweeps in this class are non-composite and generate controllable quantum interference effects that allow the gates in G_{u} to operate non-adiabatically while maintaining high fidelity. These interference effects have been observed using NMR, and it has previously been shown how these rapid passage sweeps can be applied to atomic systems using electric fields. Here we show how these sweeps can be applied to both superconducting charge and flux qubit systems. The simulations suggest that the universal set of gates G_{u} produced by these rapid passage sweeps shows promise as possible elements of a fault-tolerant scheme for quantum computing.

quant-ph↗
arXiv2008

Thermoelectric transport through a quantum dot coupled to a normal metal and BCS superconductor

I discuss thermoelectric properties of a quantum dot coupled to one normal and one superconducting lead in the presence of Kondo effect and Andreev scattering. I will focus on conductance, thermal conductance, thermopower and related quantities like thermoelectric figure of merit which is a direct measure of the usefulness of the system for applications and Wiedemann-Franz ratio which indicates if the system is in the Fermi liquid state. I will show that the superconductivity strongly modifies the thermal properties of the system. In particular, the thermopower is strongly enhanced near the superconducting transition temperature. Moreover, the Andreev reflections are suppressed due to strong on-dot Coulomb repulsion. The suppression of the Andreev reflections leads to a violation of the Wiedemann-Franz law and to a non-Fermi liquid ground state.

cond-mat.mes-hall↗
arXiv2008

A gap in the quarkyonic matter

It has recently been suggested that at a reasonably large chemical potential a confining quarkyonic matter is formed that consists of the quark Fermi sea and confined hadrons on top of this Fermi sea. We study some properties of this matter. It is demonstrated that below the chiral restoration point there are gapless excitations of this matter through excitations of the Goldstone bosons. Above the chiral restoration point the single quarks are still removed from the spectrum of excitations and the only possible excitations are confined color-singlet hadrons with finite mass. Hence there appears a gap in the excitation spectrum of the quarkyonic matter that should be crucially important for its properties above the chiral restoration point. This gap is of a new type and is not related with the condensation of the fermionic system into a quasibosonic system. It is only due to such properties as confinement and manifest chiral symmetry at the same time.

hep-ph↗
arXiv2008

Pseudogap-driven Hall effect sign reversal

We present a calculation of the Hall coefficient in 2H-TaSe2 and 2H-Cu0.2NbS2 relied on the photoemission data and compare the results to transport observations. The approach, based on the solution of the semiclassical Boltzmann equation in the isotropic tau-approximation yields high-temperature Hall coefficient consistent with the one measured directly. Taking into account the opening of the pseudogap and redistribution of the spectral weight, recently observed in angle resolved photoemission spectra of 2H-TaSe2, allows us to reproduce the temperature dependence of the Hall coefficient including prominent sign change with no adjustable parameters.

cond-mat.supr-con↗
arXiv2008

Mobility and its temperature dependence in LSCO: viscous motion?

We argue that charges in underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ move in a dissipative environment of strong spatial and temporal fluctuations. The unusual temperature dependence of the Hall angle known as "the separation of lifetimes" is reinterpreted and attributed to the appearance of the thermally activated component in the effective number of carriers with the temperature increase. We consider the temperature interval above $T_{c}$ where localization effects can be neglected.

cond-mat.supr-con↗
arXiv2008

Superconductivity in $β$-pyrochlore superconductor KOs$_{2}$O$_{6}$: treatment within strong-coupling Eliashberg theory

We study the influence of the rattling phonons on superconductivity in $β$-pyrochlore KOs$_{2}$O$_{6}$ compound based on the strong-coupling Eliashberg approach. In particular, analyzing the specific heat data we find that the rattling phonon frequency changes discontinuously at the critical temperature of the first order phase transition. Solving the strong-coupling Eliashberg equations with temperature dependent $α^{2}F(ω)$, we investigate the consequence of this first order phase transition for the anomalous temperature dependence of the superconducting gap. We discuss our results in context of the recent experimental data.

cond-mat.supr-con↗