arXiv subjects

V. B. Geshkenbein

Publications and source records attributed to V. B. Geshkenbein.

32 records · Page 2Linked to original sources

arXiv2001

Superfluidity versus Bloch oscillations in confined atomic gases

We study the superfluid properties of (quasi) one-dimensional bosonic atom gases/liquids in traps with finite geometries in the presence of strong quantum fluctuations. Driving the condensate with a moving defect we find the nucleation rate for phase slips using instanton techniques. While phase slips are quenched in a ring resulting in a superfluid response, they proliferate in a tube geometry where we find Bloch oscillations in the chemical potential. These Bloch oscillations describe the individual tunneling of atoms through the defect and thus are a consequence of particle quantization.

cond-mat.soft↗

arXiv2001

Interference effects in isolated Josephson junction arrays with geometric symmetries

As the size of a Josephson junction is reduced, charging effects become important and the superconducting phase across the link turns into a periodic quantum variable. Isolated Josephson junction arrays are described in terms of such periodic quantum variables and thus exhibit pronounced quantum interference effects arising from paths with different winding numbers (Aharonov-Casher effects). These interference effects have strong implications for the excitation spectrum of the array which are relevant in applications of superconducting junction arrays for quantum computing. The interference effects are most pronounced in arrays composed of identical junctions and possessing geometric symmetries; they may be controlled by either external gate potentials or by adding/removing charge to/from the array. Here we consider a loop of N identical junctions encircling one half superconducting quantum of magnetic flux. In this system, the ground state is found to be non-degenerate if the total number of Cooper pairs on the array is divisible by N, and doubly degenerate otherwise (after the stray charges are compensated by the gate voltages).

cond-mat.supr-con↗

arXiv2000

Self-consistent theory of the thermal softening and instability of simple crystals

We consider the thermal softening of crystals due to anharmonicity. Self-consistent methods find a maximum temperature for a stable crystal, which gives an upper bound to the melting temperature. Previous workers have shown that the self-consistent harmonic approximation (SCHA) gives misleading results for the thermal stability of crystals. The reason is that the most important diagrams in the perturbation expansion around harmonic theory are not included in the SCHA. An alternative approach is to solve a self-consistent Dyson equation (SCA) for a selection of diagrams, the simplest being a (3+4)-SCA. However, this gives an unsatisfactory comparison to numerical results on the thermal and quantum melting of two-dimensional (2D) Coulomb-interacting particles (equivalent to vortex-lattice melting in two and three dimensions). We derive an improved self-consistent method, the two-vertex-SCHA, which gives much better agreement to the simulations. Our method allows for accurate calculation of the thermal softening of the shear modulus for 2D crystals and for the lattice of vortex-lines in type-II superconductors.

cond-mat.stat-mech↗

arXiv1999

Engineering Superconducting Phase Qubits

The superconducting phase qubit combines Josephson junctions into superconducting loops and defines one of the promising solid state device implementations for quantum computing. While conventional designs are based on magnetically frustrated superconducting loops, here we discuss the advantages offered by $π$-junctions in obtaining naturally degenerate two-level systems. Starting from a basic five-junction loop, we show how to construct degenerate two-level junctions and superconducting phase switches. These elements are then effectively engineered into a superconducting phase qubit which operates exclusively with switches, thus avoiding permanent contact with the environment through external biasing. The resulting superconducting phase qubits can be understood as the macroscopic analogue of the `quiet' s-wave-d-wave-s-wave Josephson junction qubits introduced by Ioffe {\it et al.} [Nature {\bf 398}, 679 (1999)].

cond-mat.mes-hall↗

arXiv1999

Evaporation of the pancake-vortex lattice in weakly-coupled layered superconductors

We calculate the melting line of the pancake-vortex system in a layered superconductor, interpolating between two-dimensional (2D) melting at high fields and the zero-field limit of single-stack evaporation. Long-range interactions between pancake vortices in different layers permit a mean-field approach, the ``substrate model'', where each 2D crystal fluctuates in a substrate potential due to the vortices in other layers. We find the thermal stability limit of the 3D solid, and compare the free energy to a 2D liquid to determine the first-order melting transition and its jump in entropy.

cond-mat.supr-con↗

arXiv1999

Transport in a One-Dimensional Superfluid: Quantum Nucleation of Phase Slips

We present an analytical derivation for the quantum decay rate of the superflow through a weak link in a one-dimensional Bose-Einstein-condensate. The effective action for the phase difference across the link reduces to that of a massive particle with damping subject to a periodic potential. We find an algebraic flow-pressure relation, characteristic for quantum nucleation of phase slips in the link and show how short-wave length fluctuations renormalizing the interaction between the Bosons remove the quantum phase transition expected in this class of systems.

cond-mat.mes-hall↗

arXiv1999

Andreev Spectroscopy for Superconductive Phase Qubit

New method to probe quantum dynamics of the order parameter phase of a small superconductive island is proposed. The idea is to measure Andreev subgap conductance between the island and a dirty normal-metal wire connected with the island by very high-resistance tunnel barrier. The phase on the island is supposed to stay primarly in one of the two nearly degenerate classical minima i.e. the island is an active element of the Superconductive Phase Qubit. Voltage-dependence of the differential subgap conductance measured at low temperatures are predicted to demonstrate crucially different behavior depending on the type of the internal dynamics of phase (coherent tunnelling of phase, non-coherent tunnelling, or no tunneling). Thus the measurements of nonliner Andreev conductance is a tool to probe the type of quantum behavior of the artificial two-level system.

cond-mat↗

arXiv1999

Quiet SDS Josephson Junctions for Quantum Computing

Unconventional superconductors exhibit an order parameter symmetry lower than the symmetry of the underlying crystal lattice. Recent phase sensitive experiments on YBCO single crystals have established the d-wave nature of the cuprate materials, thus identifying unambiguously the first unconventional superconductor. The sign change in the order parameter can be exploited to construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting a degenerate ground state and a double-periodic current-phase characteristic. Here we discuss how to make use of these special junction characteristics in the construction of a quantum computer. Combining such junctions together with a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit --- a solid state implementation of a quantum bit which remains optimally isolated from its environment.

cond-mat.mes-hall↗

arXiv1998

Theory of the Resistive Transition in Overdoped $Tl_2Ba_2CuO_{6+x}$: Implications for the angular dependence of the quasiparticle scattering rate in High-$T_c$ superconductors

We show that recent measurements of the magnetic field dependence of the magnetization, specific heat and resistivity of overdoped $T_c \sim 17K$ $Tl_{2}Ba_{2}CuO_{6+δ}$ in the vicinity of the superconducting $H_{c2}$ imply that the vortex viscosity is anomalously small and that the material studied is inhomogeneous with small, a few hundred $Å$, regions in which the local $T_{c}$ is much higher than the bulk $T_{c}$. The anomalously small vortex viscosity can be derived from a microscopic model in which the quasiparticle lifetime varies dramatically around the Fermi surface, being small everywhere except along the zone diagonal (``cold spot''). We propose experimental tests of our results.

cond-mat.supr-con↗

arXiv1998

(4+N)-Dimensional Elastic Manifolds in Random Media: a Renormalization-Group Analysis

Motivated by the problem of weak collective pinning of vortex lattices in high-temperature superconductors, we study the model system of a four-dimensional elastic manifold with N transverse degrees of freedom (4+N-model) in a quenched disorder environment. We assume the disorder to be weak and short-range correlated, and neglect thermal effects. Using a real-space functional renormalization group (FRG) approach, we derive a RG equation for the pinning-energy correlator up to two-loop correction. The solution of this equation allows us to calculate the size R_c of collectively pinned elastic domains as well as the critical force F_c, i.e., the smallest external force needed to drive these domains. We find R_c prop. to delta_p^alpha_2 exp(alpha_1/delta_p) and F_c prop. to delta_p^(-2 alpha_2) exp(-2 alpha_1/delta_p), where delta_p <<1 parametrizes the disorder strength, alpha_1=(2/pi)^(N/2) 8 pi^2/(N+8), and alpha_2=2(5N+22)/(N+8)^2. In contrast to lowest-order perturbation calculations which we briefly review, we thus arrive at determining both alpha_1 (one-loop) and alpha_2 (two-loop).

cond-mat.supr-con↗

arXiv1997

Characteristics of First-Order Vortex Lattice Melting: Jumps in Entropy and Magnetization

We derive expressions for the jumps in entropy and magnetization characterizing the first-order melting transition of a flux line lattice. In our analysis we account for the temperature dependence of the Landau parameters and make use of the proper shape of the melting line as determined by the relative importance of electromagnetic and Josephson interactions. The results agree well with experiments on anisotropic Y$_1$Ba$_2$Cu$_3$O$_{7-δ}$ and layered Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_8$ materials and reaffirm the validity of the London model.

cond-mat.supr-con↗

arXiv1996

On the superconductivity in the system with preformed pairs

We discuss the phenomenology of the superconductivity resulting from the bose condensation of the preformed pairs coexisting with unpaired fermions. We show that this transition is more mean field like than usual bose condensation, i.e. it is characterized by a relatively small value of the Ginzburg parameter. We consider the Hall effect in the vortex flow regime and in the fluctuational regime above $T_c$ and show that in this situation it is much less than in the transition driven entirely by bose condesation but much larger than in a usual superconductivity. We analyse the available Hall data and conclude that this phenomenology describes reasonably well the data in the underdoped materials of $YBaCuO$ family but is not an appropriate description of optimally doped materials or underdoped $LaSrCuO$.

cond-mat↗

arXiv1996

Plastic vortex-creep in YBa$_2$Cu$_3$O$_{7-x}$ crystals

Local magnetic relaxation measurements in YBa$_2$Cu$_3$O$_{7-x}$ crystals show evidence for plastic vortex-creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the ''fishtail'' magnetization. In this range the activation energy $U_{pl}$, which decreases with field, drops below the elastic (collective) creep activation energy, $U_{el}$, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa$_2$Cu$_3$O$_{7-x}$.

supr-con↗

arXiv1995

Sign Change of the Flux Flow Hall Effect in HTSC

A novel mechanism for the sign change of the Hall effect in the flux flow region is proposed. The difference $δn $ between the electron density at the center of the vortex core and that far outside the vortex causes the additional contribution to the Hall conductivity $δσ_{xy}=-δn ec/B$. This contribution can be larger than the conventional one in the dirty case $Δ(T)τ<1$. If the carrier density inside the core exceeds that far outside, a double sign change may occur as a function of temperature.

cond-mat↗