arXiv subjects

M. V. Feigel'man

Publications and source records attributed to M. V. Feigel'man.

At least 55 records · Page 3Linked to original sources

arXiv2005

Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field

We show that the superconducting transition temperature T_c(H) of a very thin highly disordered film with strong spin-orbital scattering can be increased by parallel magnetic field H. This effect is due to polarization of magnetic impurity spins which reduces the full exchange scattering rate of electrons; the largest effect is predicted for spin-1/2 impurities. Moreover, for some range of magnetic impurity concentrations the phenomenon of {\it superconductivity induced by magnetic field} is predicted: superconducting transition temperature T_c(H) is found to be nonzero in the range of magnetic fields $0 < H^* <= H <= H_c$.

cond-mat.supr-con↗

arXiv2005

Superconductivity in disordered thin films: giant mesoscopic fluctuations

We discuss intrinsic inhomogeneities of superconductive properties of uniformly disordered thin films with large dimensionless conductance g. It is shown that mesoscopic fluctuations, which usually contain a small factor 1/g, are crucially enhanced near the critical conductance g_c >> 1 where superconductivity is destroyed at T=0 due to Coulomb suppression of the Cooper attraction. This leads to strong spatial fluctuations of the local transition temperature and thus to percolative nature of the thermal superconductive transition.

cond-mat.supr-con↗

arXiv2005

Variable range cotunneling and conductivity of a granular metal

The Efros-Shklovskii law for the conductivity of granular metals is interpreted as a result of a variable range cotunneling process. The cotunneling between distant resonant grains is predominantly elastic at low T << T_c, while it is inelastic (i.e., accompanied by creation of electron-hole pairs on a string of intermediate non-resonant grains) at T > T_c. The corresponding E-S temperature T_ES in the latter case is slightly (logarithmically) T-dependent. The magnetoresistance in the two cases is different: it may be relatively strong and negative at T much below T_c, while at T>T_c it is suppressed due to inelastic processes which destroy the interference.

cond-mat.mes-hall↗

arXiv2004

Superconducting tetrahedral quantum bits

We propose a new design for a quantum bit with four superconducting islands in the topology of a symmetric tetrahedron, uniformly frustrated with one-half flux-quantum per loop and one-half Cooper-pair per island. This structure emulates a noise-resistant spin-1/2 system in a vanishing magnetic field. The tetrahedral quantum bit combines a number of advances such as a doubly degeneracy ground state minimizing decoherence via phonon radiation, a weak quadratic sensitivity to electric and magnetic noise, relieved constraints on the junction fabrication, a large freedom in manipulation, and attractive measurement schemes. The simultaneous appearance of a degenerate ground state and a weak noise sensitivity are consequences of the tetrahedral symmetry, while enhanced quantum fluctuations derive from the special magnetic frustration. We determine the spectral properties of the tetrahedral structure within a semiclassical analysis and confirm the results numerically. We show how proper tuning of the charge-frustration selects a doubly degenerate ground state and discuss the qubit's manipulation through capacitive and inductive coupling to external bias sources. The complete readout of all the spin-components is achieved through coupling of the internal qubit currents to external junctions driven close to criticality during the measurement.

cond-mat.supr-con↗

arXiv2004

Phase diagram of a surface superconductor in parallel magnetic field

Detailed theory of phase diagram of clean 2D surface superconductor in a parallel magnetic field is presented. Regular spin-orbital interaction of the Rashba type is known to produce inhomogeneous superconductive state similar to the Larkin-Ovchinnikov-Fulde-Ferrel (LOFF) state with $Δ({\bf r})\propto \cos({\bf Qr})$ at high magnetic fields, with $Q \sim gμ_B h/v_F$. We consider the case of relatively strong Rashba interaction and show that at low temperatures $T\leq 0.4 T_{c0}$ the LOFF-type state is separated from the usual homogeneous state by the first-order phase transition line. At higher temperatures new "helical" state with $Δ({\bf r}) \propto \exp(i{\bf Qr})$ intervene between uniform BCS state and LOFF-like state. One component of superfluid density tensor $n_s$ vanishes on the second-order transition line between BCS state and helical state. Nonmagnetic impurities suppresses both inhomogeneous states, and eliminate them completely at $T_{c0}τ\leq 0.11$.

cond-mat.supr-con↗

arXiv2004

Coulomb Blockade of Proximity Effect at Large Conductance

We consider the proximity effect in a normal dot coupled to a bulk superconducting reservoir by the tunnel contact with large normal conductance. Coulomb interaction in the dot suppresses the proximity minigap induced in the normal part of the system. We find exact expressions for the thermodynamic and tunneling minigaps as functions of the junction's capacitance. The tunneling minigap interpolates between its proximity-induced value in the regime of weak Coulomb interaction to the Coulomb gap in the regime of strong interaction. In the intermediate case a non-universal two-step structure of the tunneling density of states is predicted. The charge quantization in the dot is also studied.

cond-mat.mes-hall↗

arXiv2004

Quantum percolation in granular metals

Theory of quantum corrections to conductivity of granular metal films is developed for the realistic case of large randomly distributed tunnel conductances. Quantum fluctuations of intergrain voltages (at energies E much below bare charging energy scale E_C) suppress the mean conductance \bar{g}(E) much stronger than its standard deviation σ(E). At sufficiently low energies E_* any distribution becomes broad, with σ(E_*) ~ \bar{g}(E_*), leading to strong local fluctuations of the tunneling density of states. Percolative nature of metal-insulator transition is established by combination of analytic and numerical analysis of the matrix renormalization group equations.

cond-mat.mes-hall↗

arXiv2004

Coulomb Effects in Nanoscale SINIS Junction

We study the system of two superconductors connected by a small normal grain. We consider the modification of the Josephson effect by the Coulomb interaction on the grain. Coherent charge transport through the junction is suppressed by Coulomb repulsion. An optional gate electrode may relax the charge blocking and enhance the current leading to the single Cooper pair transistor effect. Temperature dependences of critical current and of the minigap induced in the normal grain by the proximity to superconductor are studied. Both temperature and Coulomb interaction suppress critical current and minigap but their interplay may lead to the nonmonotonous and even reentrant temperature dependence.

cond-mat.mes-hall↗

arXiv2004

Protected Qubits and Chern Simons theories in Josephson Junction Arrays

We present general symmetry arguments that show the appearance of doubly denerate states protected from external perturbations in a wide class of Hamiltonians. We construct the simplest spin Hamiltonian belonging to this class and study its properties both analytically and numerically. We find that this model generally has a number of low energy modes which might destroy the protection in the thermodynamic limit. These modes are qualitatively different from the usual gapless excitations as their number scales as the linear size (instead of volume) of the system. We show that the Hamiltonians with this symmetry can be physically implemented in Josephson junction arrays and that in these arrays one can eliminate the low energy modes with a proper boundary condition. We argue that these arrays provide fault tolerant quantum bits. Further we show that the simplest spin model with this symmetry can be mapped to a very special Z_2 Chern-Simons model on the square lattice. We argue that appearance of the low energy modes and the protected degeneracy is a natural property of lattice Chern-Simons theories. Finally, we discuss a general formalism for the construction of discrete Chern-Simons theories on a lattice.

cond-mat.supr-con↗

arXiv2003

Landau-Zener interferometry for qubits

One may probe intrinsic coherence of a qubit by periodically sweeping its control parameter. The qubit is then excited by Landau-Zener (LZ) mechanism. The interference between multiple LZ transition leads to the unusual oscillatory dependence of the energy absorption rate on sweeping amplitude and period. This interference pattern allows to determine the decoherence time of the qubit. We introduce a simple phenomenological model describing this "interferometer", and find the form of the interference pattern.

cond-mat↗

arXiv2003

Dephasing in disordered metals with superconductive grains

Temperature dependence of electron dephasing time $τ_ϕ(T)$ is calculated for a disordered metal with small concentration of superconductive grains. Above the macroscopic superconducting transition line, when electrons in the metal are normal, Andreev reflection from the grains leads to a nearly temperature-independent contribution to the dephasing rate. In a broad temperature range $1/τ_phi(T)$ strongly exceeds the prediction of the classical theory of dephasing in normal disordered conductors, whereas magnetoresistance is dominated (in two dimensions) by the Maki-Tompson correction and is positive.

cond-mat.mes-hall↗

arXiv2003

Comment on ""Proximity effect in ultrafin Pb/Ag multilayers within the Cooper limit"

I present here critical comment to the recent e-print "Proximity effect in ultrafin Pb/Ag multilayers within the Cooper limit" by by O.Bourgeois, A.Frydman and R.C.Dynes, demonstrating that the interpretaion of the data presented in cond-mat/0302251 does not seem to be reliable, and, simulteneously, large amount of previous work was ignored.

cond-mat.supr-con↗

arXiv2002

Topological order in the insulating Josephson junction array

We propose a Josephson junction array which can be tuned into an unconventional insulating state by varying external magnetic field. This insulating state retains a gap to half vortices; as a consequence, such array with non-trivial global geometry exhibits a ground state degeneracy. This degeneracy is protected from the effects of external noise. We compute the gaps separating higher energy states from the degenerate ground state and we discuss experiments probing the unusual properties of this insulator.

cond-mat.str-el↗

arXiv2002

Possible realization of an ideal quantum computer in Josephson junction array

We introduce a new class of Josephson arrays which have non-trivial topology and exhibit a novel state at low temperatures. This state is characterized by long range order in a two Cooper pair condensate and by a discrete topological order parameter. These arrays have degenerate ground states with this degeneracy 'protected' from the external perturbations (and noise) by the topological order parameter. We show that in ideal conditions the low order effect of the external perturbations on this degeneracy is exactly zero and that deviations from ideality lead to only exponentially small effects of perturbations. We argue that this system provides a physical implementation of an ideal quantum computer with a built in error correction and show that even a small array exhibits interesting physical properties such as superconductivity with double charge, 4e, and extremely long decoherence times.

cond-mat.mes-hall↗

arXiv2002

Density of States in a Mesoscopic SNS Junction

Semiclassical theory of proximity effect predicts a gap E_g \sim hD/L^2 in the excitation spectrum of a long diffusive SNS junction. Mesoscopic fluctuations lead to anomalously localized states in the normal part of the junction. As a result, a non-zero, yet exponentially small, density of states appears at energies below E_g. In the framework of the supermatrix nonlinear sigma-model these prelocalized states are due to instanton configurations with broken supersymmetry. The exact result for the DOS near the semiclassical threshold is found provided the dimensionless conductance of the normal part is large. The case of poorly transparent interfaces between the normal and superconductive regions is also considered. In this limit the total number of the subgap states may be large.

cond-mat.mes-hall↗

arXiv2001

Topologically protected quantum bits from Josephson junction arrays

All physical implementations of quantum bits (qubits), carrying the information and computation in a putative quantum computer, have to meet the conflicting requirements of environmental decoupling while remaining manipulable through designed external signals. Proposals based on quantum optics naturally emphasize the aspect of optimal isolation, while those following the solid state route exploit the variability and scalability of modern nanoscale fabrication techniques. Recently, various designs using superconducting structures have been successfully tested for quantum coherent operation, however, the ultimate goal of reaching coherent evolution over thousands of elementary operations remains a formidable task. Protecting qubits from decoherence by exploiting topological stability, a qualitatively new proposal due to Kitaev, holds the promise for long decoherence times, but its practical physical implementation has remained unclear so far. Here, we show how strongly correlated systems developing an isolated two-fold degenerate quantum dimer liquid groundstate can be used in the construction of topologically stable qubits and discuss their implementation using Josephson junction arrays.

cond-mat↗

arXiv2001

Density of States below the Thouless Gap in a Mesoscopic SNS Junction

Quasiclassical theory predicts an existence of a sharp energy gap E_g \sim D/L^2 in the excitation spectrum of a long diffusive superconductor--normal metal--superconductor (SNS) junction. We show that mesoscopic fluctuations remove the sharp edge of the spectrum, leading to a nonzero DoS for all energies. Physically, this effect originates from the quasi-localized states in the normal metal. Technically, we use an extension of Efetov's supermatrix sigma-model for mixed NS systems. We demonstrate the equivalence of the supersymmetric saddle-point solution to the standard quasiclassical Usadel solution, and show that the instanton solution with broken supersymmetry provides a non-zero DoS at energies E < E_g.

cond-mat.mes-hall↗

arXiv2000

Superconductive properties of thin dirty SN bilayers

The theory of superconductivity in thin SN sandwiches (bilayers) in the diffusive limit is developed within the standard Usadel equation method, with particular emphasis on the case of very thin superconductive layers, d_S << d_N. The proximity effect in the system is governed by the interlayer interface resistance (per channel) ρ_{int}. The case of relatively low resistance (which can still have large absolute values) can be completely studied analytically. The theory describing the bilayer in this limit is of BCS type but with the minigap (in the single-particle density of states) E_g << Δsubstituting the order parameter Δin the standard BCS relations; the original relations are thus severely violated. In the opposite limit of an opaque interface, the behavior of the system is in many respects close to the BCS predictions. Over the entire range of ρ_{int}, the properties of the bilayer are found numerically. Finally, it is shown that the results obtained for the bilayer also apply to more complicated structures such as SNS and NSN trilayers, SNINS and NSISN systems, and SN superlattices.

cond-mat.supr-con↗