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ABSTRACT: We review a novel approach to the superconductive proximity effect in disordered normal-superconducting (N-S) structures.
The method is based on the multicharge Keldysh action and is suitable for the treatment of interaction and fluctuation effects.
As an application of the formalism, we study the subgap conductance and noise in two-dimensional N-S systems in the presence
of the electron-electron interaction in the Cooper channel. It is shown that singular nature of the interaction correction
at large scales leads to a nonmonotonuos temperature, voltage and magnetic field dependence of the Andreev conductance.
Pramana 04/2012; 64(6):1039-1049. · 0.57 Impact Factor
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ABSTRACT: Local vortex dynamics in Bi2Sr2CaCu2O8 single crystals was studied using novel microscopic GaAs/AlGaAs Hall-sensor arrays. The irreversibility line (IL) is found to exist in the absence of bulk pinning. At high temperatures the IL is due to geometrical barriers whereas at intermediate temperatures the irreversible behavior is determined by surface barriers. Bulk pinning governs the IL only at T < 22 K.
EPL (Europhysics Letters) 07/2007; 30(6):367. · 2.17 Impact Factor
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ABSTRACT: The temperature dependence of electron dephasing time tau(phi)(T) is calculated for a disordered metal with a 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 tau(-1)(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.
Physical Review Letters 07/2004; 92(24):247002. · 7.37 Impact Factor
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ABSTRACT: Temperature dependence of electron dephasing time τϕ(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 τϕ−1(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.
12/2003: pages 33-47;
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ABSTRACT: What quantity controls the Coulomb blockade oscillations if the dot-lead conductance is essentially frequency dependent? We argue that it is the conductance at the imaginary frequency given by the effective charging energy. The latter may be very different from the bare charging energy due to the interface-induced capacitance (or inductance). These observations are supported by a number of examples, considered from the weak and strong coupling (perturbation theory versus instanton calculus) perspectives.
Physical Review Letters 01/2003; 89(23):236801. · 7.37 Impact Factor
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ABSTRACT: We consider Coulomb blockade on a superconductive quantum dot strongly coupled to a lead through a tunneling barrier and/or normal diffusive metal. Andreev transport of the correlated pairs leads to quantum fluctuations of the charge on the dot. These fluctuations result in exponential renormalization of the effective charging energy. We employ two complimentary ways to approach the problem: the instanton and the functional renormalization group treatment of the nonlinear σ model. We show that these two different methods produce identical results. We also derive the charging energy renormalization in terms of the arbitrary transmission matrix of the multichannel interface.
Phys. Rev. B. 07/2002; 66(5).
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ABSTRACT: We consider the Coulomb blockade on a superconductive quantum dot strongly coupled to a lead through a tunnelling barrier and/or normal diffusive metal. Andreev transport of the correlated pairs leads to quantum fluctuations of the charge on the dot. These fluctuations result in exponential renormalization of the effective charging energy. We employ two complimentary ways to approach the problem, leading to the coinciding results: the instanton and the functional RG treatment of the non-linear sigma model. We also derive the charging energy renormalization in terms of arbitrary transmission matrix of the multi-channel interface. Comment: 21 pages, 4 eps figures, RevTeX
03/2002;
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ABSTRACT: A theory of the zero-temperature superconductor-metal transition is developed for an array of superconductive islands (of size d) coupled via a disordered two-dimensional conductor with the dimensionless conductance g = Planck's over 2 pi/e(2)R(square)>1. At T = 0 the macroscopically superconductive state of the array with lattice spacing b>d is destroyed at g<g(c) approximately 0.1ln(2)(b/d). At high temperatures the normal-state resistance between neighboring islands at b = b(c) is much smaller than R(Q) = h/4e(2).
Physical Review Letters 02/2001; 86(9):1869-72. · 7.37 Impact Factor
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ABSTRACT: We review a novel approach to the superconductive proximity effect in disordered normal-superconducting (N-S) structures. The method is based on the multicharge Keldysh action and is suitable for the treatment of interaction and fluctuation effects. As an application of the formalism, we study the subgap conductance and noise in two-dimensional N-S systems in the presence of the electron-electron interaction in the Cooper channel. It is shown that singular nature of the interaction correction at large scales leads to a nonmonotonuos temperature, voltage and magnetic field dependence of the Andreev conductance. Comment: RevTeX, 6 pages, 5 eps figures. This is a concise review of cond-mat/0008463; to be published in the Proceedings of the conference "Mesoscopic and strongly correlated electron systems" (Chernogolovka, Russia, July 2000)
11/2000;
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ABSTRACT: We present a general theory of the superconductive proximity effect in disordered normal--superconducting (N-S) structures, based on the recently developed Keldysh action approach. In the case of the absence of interaction in the normal conductor we reproduce known results for the Andreev conductance G_A at arbitrary relation between the interface resistance R_T and the diffusive resistance R_D. In two-dimensional N-S systems, electron-electron interaction in the Cooper channel of normal conductor is shown to strongly affect the value of G_A as well as its dependence on temperature, voltage and magnetic field. In particular, an unusual maximum of G_A as a function of temperature and/or magnetic field is predicted for some range of parameters R_D and R_T. The Keldysh action approach makes it possible to calculate the full statistics of charge transfer in such structures. As an application of this method, we calculate the noise power of an N-S contact as a function of voltage, temperature, magnetic field and frequency for arbitrary Cooper repulsion in the normal metal and arbitrary values of the ratio R_D/R_T. Comment: RevTeX, 28 pages, 18 PostScript figures; added and updated references
08/2000;
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ABSTRACT: The effect of Coulomb interaction upon superconductive proximity effect in disordered metals is studied, employing newly developed Keldysh functional approach (cond-mat/9907358). We have calculated subgap Andreev conductance between superconductor and 2D dirty film, as well as Josephson coupling via such a film. Both two qualitatively different Coulomb effects - suppression of the tunneling density of states and disorder-enchanced repulsion in the Cooper channel - are shown to be important at sufficiently low temperatures.
09/1999;
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ABSTRACT: Keldysh representation of the functional integral for the interacting electron system with disorder is used to derive microscopically an effective action for dirty superconductors. In the most general case this action is a functional of the 8 x 8 matrix Q(t,t') which depends on two time variables, and on the fluctuating order parameter field and electric potential. We show that this approach reproduces, without the use of the replica trick, the well-known result for the Coulomb-induced renormalization of the electron-electron coupling constant in the Cooper channel. Turning to the new results, we calculate the effects of the Coulomb interaction upon: i) the subgap Andreev conductance between superconductor and 2D dirty normal metal, and ii) the Josephson proximity coupling between superconductive islands via such a metal. These quantities are shown to be strongly suppressed by the Coulomb interaction at sufficiently low temperatures due to both zero-bias anomaly in the density of states and disorder-enhanced repulsion in the Cooper channel. Comment: RevTeX; 39 pages + 10 EPS figures
07/1999;
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ABSTRACT: The effect of randomly distributed impurities on vortex lattices in isotropic type-II superconductors is studied within the framework of weak collective pinning theory. Using a renormalization-group (RG) approach, we calculate the size Rc of collectively pinned vortex bundles in the dispersive regime, a0<Rc<λ (small bundles), to two-loop accuracy. We assume impurity disorder to be weak and short-range correlated, and neglect thermal effects. Our findings quantitatively refine the lowest-order perturbation result due to A. I. Larkin and Yu. N. Ovchinnikov [J. Low Temp. Phys. 34, 409 (1979)]. In particular, we determine the numerical constant in the exponential function and find the algebraic prefactor in Rc∝B-1/2δpα2exp(α1/δp), where the (dimensionless) parameter δp∝B-3/2 is a measure of the effective disorder strength, B is the magnetic induction, α1=16/(9√π)≈1, and α2=(7-5ln4/3)/27≈0.2. These refinements lead to an improved description of the activated dynamics of the vortex lattice (creep) and provide us with a more accurate functional dependence of the critical current jc on the magnetic field jc∝B1+3α2exp(-constB3/2).
Phys. Rev. B. 04/1999; 59(17).
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ABSTRACT: We construct a theory of quantum fluctuatons in a regular array of small superconductive islands connected via low-resistance tunnel contacts to a dirty thin metal film. Electron-electron interaction in the film is assumed to be repulsive. The system is macroscopically superconductive when the distance between neighbouring islands is short enough. The zero-temperature phase transition from superconductive to normal-conductive state is shown to occur with the increase of distance between superconductive islands; the logarithm of the critical distance is proportional to the low-frequency zero-voltage Andreev conductance between the SC island and the film. This critical distance is always much less the than the two-dimensional localization length, so the considered effect develops when weak-localization corrections are still small. The dependence of the critical temperature on the film conductance and inter-island distance is found. Comment: RevTex, 8 pages, to be published in a special issue of "Chemical Physics" in memory of the late Professor V. I. Mel'nikov
03/1998;
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ABSTRACT: 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). Comment: 18 pages, 2 figures, to appear in Phys. Rev. B
01/1998;
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ABSTRACT: We present fermi liquid expressions for the low temperature behavior of the superfluid stiffness, explain why they differ from those suggested recently by Lee and Wen, and discuss their applicability to data on high-$T_c$ superconductors. We find that a consistent description requires a strong, doping dependent anisotropy, which affects states near the zone corners much more strongly than those near the zone diagonals.
09/1997;
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ABSTRACT: We discuss the phenomenology of superconductivity resulting from the Bose condensation of preformed pairs coexisting with unpaired fermions. We show that this transition is more mean-field-like than the 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 Tc, and show that in this situation it is much less than in the transition driven entirely by Bose condensation but much larger than in usual superconductivity. We analyze the available Hall data and conclude that this phenomenology describes reasonably well the data in the underdoped materials of Y-Ba-Cu-O family but is not an appropriate description of optimally doped materials or underdoped La-Sr-Cu-O.
Phys. Rev. B. 09/1996; 55(5).
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ABSTRACT: The effect of impurity disorder on vortex lattices in type-II superconductors is studied within the framework of the weak
collective pinning theory. Using a renormalization group approach, we calculate the sizeR
c
of collectively pinned vortex bundles in the regimea
o
<R
c
<λ (small bundles) for isotropic superconductors. We assume weak disorder and neglect thermal effects. Our findings quantitatively
refine the lowest-order perturbation result,R
c
∞ÖB\sqrt B , obtained by Larkin and Ovchinnikov in 1979 (B is the magnetic induction). In particular, we find a different numerical constant in the exponential as well as a new algebraic
prefactor. These improvements provide us with a more accurate functional dependence of the critical current on the magnetic
field.
Czechoslovak Journal of Physics 02/1996; 46:1815-1816. · 0.42 Impact Factor
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ABSTRACT: A novel mechanism for the sign change of the Hall effect in the flux flow region is proposed. The difference $\delta n $ 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 $\delta\sigma_{xy}=-\delta n ec/B$. This contribution can be larger than the conventional one in the dirty case $\Delta(T)\tau <1$. If the carrier density inside the core exceeds that far outside, a double sign change may occur as a function of temperature.
04/1995;
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ABSTRACT: A sharp maximum in the critical current $J_c$ as a function of temperature just below the melting point of the Abrikosov flux lattice has recently been observed in both low and high temperature superconductors. This peak effect is strongest in twinned crystals for fields aligned with the twin planes. We propose that this peak signals the breakdown of the collective pinning regime and the crossover to strong pinning of single vortices on the twin boundaries. This crossover is very sharp and can account for the steep drop of the differential resistivity observed in experiments. Comment: 4 pages, revtex 3.0, no figures
04/1995;
