582 research outputs found
Microscopic theory of the Andreev gap
We present a microscopic theory of the Andreev gap, i.e. the phenomenon that
the density of states (DoS) of normal chaotic cavities attached to
superconductors displays a hard gap centered around the Fermi energy. Our
approach is based on a solution of the quantum Eilenberger equation in the
regime , where and are the classical dwell time and
Ehrenfest-time, respectively. We show how quantum fluctuations eradicate the
DoS at low energies and compute the profile of the gap to leading order in the
parameter .Comment: 4 pages, 3 figures; revised version, more details, extra figure, new
titl
What is the Thouless Energy for Ballistic Systems?
The Thouless energy, \Ec characterizes numerous quantities associated with
sensitivity to boundary conditions in diffusive mesoscopic conductors. What
happens to these quantities if the disorder strength is decreased and a
transition to the ballistic regime takes place? In the present analysis we
refute the intuitively plausible assumption that \Ec loses its meaning as an
inverse diffusion time through the system at hand, and generally disorder
independent scales take over. Instead we find that a variety of (thermodynamic)
observables are still characterized by the Thouless energy.Comment: 4 pages REVTEX, uuencoded file. To appear in Physical Review Letter
Crossovers between superconducting symmetry classes
We study the average density of states in a small metallic grain coupled to
two superconductors with the phase difference , in a magnetic field. The
spectrum of the low-energy excitations in the grain is described by the random
matrix theory whose symmetry depends on the magnetic field strength and
coupling to the superconductors. In the limiting cases, a pure superconducting
symmetry class is realized. For intermediate magnetic fields or couplings to
the superconductors, the system experiences a crossover between different
symmetry classes. With the help of the supersymmetric sigma-model we derive the
exact expressions for the average density of states in the crossovers between
the symmetry classes A-C and CI-C.Comment: 6 page
Phonons in Random Elastic Media and the Boson Peak
We show that the density of states of random wave equations, normalized by
the square of the frequency, has a peak - sometimes narrow and sometimes broad
- in the range of wave vectors between the disorder correlation length and the
interatomic spacing. The results of this letter may be relevant for
understanding vibrational spectra and light propagation in disordered solids
A Classification of Non-Hermitian Random Matrices
We present a classification of non-hermitian random matrices based on
implementing commuting discrete symmetries. It contains 38 classes. This
generalizes the classification of hermitian random matrices due to
Altland-Zirnbauer and it also extends the Ginibre ensembles of non-hermitian
matrices.Comment: 8 pages, contribution to the proceedings of the NATO Advanced
Research Workshop on Statistical Field Theories, Como (Italy), 18-23 June
2001. Compared to our 2001 version, we corrected two misprints in one table
that in the previous version led us to miscount the number of classes as 43
whereas it should have been 38. Explicit details of the classification are
unchange
The supersymmetric technique for random-matrix ensembles with zero eigenvalues
The supersymmetric technique is applied to computing the average spectral
density near zero energy in the large-N limit of the random-matrix ensembles
with zero eigenvalues: B, DIII-odd, and the chiral ensembles (classes AIII,
BDI, and CII). The supersymmetric calculations reproduce the existing results
obtained by other methods. The effect of zero eigenvalues may be interpreted as
reducing the symmetry of the zero-energy supersymmetric action by breaking a
certain abelian symmetry.Comment: 22 pages, introduction modified, one reference adde
Quantum interference and the formation of the proximity effect in chaotic normal-metal/superconducting structures
We discuss a number of basic physical mechanisms relevant to the formation of
the proximity effect in superconductor/normal metal (SN) systems. Specifically,
we review why the proximity effect sharply discriminates between systems with
integrable and chaotic dynamics, respectively, and how this feature can be
incorporated into theories of SN systems. Turning to less well investigated
terrain, we discuss the impact of quantum diffractive scattering on the
structure of the density of states in the normal region. We consider ballistic
systems weakly disordered by pointlike impurities as a test case and
demonstrate that diffractive processes akin to normal metal weak localization
lead to the formation of a hard spectral gap -- a hallmark of SN systems with
chaotic dynamics. Turning to the more difficult case of clean systems with
chaotic boundary scattering, we argue that semiclassical approaches, based on
classifications in terms of classical trajectories, cannot explain the gap
phenomenon. Employing an alternative formalism based on elements of
quasiclassics and the ballistic -model, we demonstrate that the inverse
of the so-called Ehrenfest time is the relevant energy scale in this context.
We discuss some fundamental difficulties related to the formulation of low
energy theories of mesoscopic chaotic systems in general and how they prevent
us from analysing the gap structure in a rigorous manner. Given these
difficulties, we argue that the proximity effect represents a basic and
challenging test phenomenon for theories of quantum chaotic systems.Comment: 21 pages (two-column), 6 figures; references adde
Distribution of the local density of states, reflection coefficient and Wigner delay time in absorbing ergodic systems at the point of chiral symmetry
Employing the chiral Unitary Ensemble of random matrices we calculate the
probability distribution of the local density of states for zero-dimensional
("quantum chaotic") two-sublattice systems at the point of chiral symmetry E=0
and in the presence of uniform absorption. The obtained result can be used to
find the distributions of the reflection coefficent and of the Wigner time
delay for such systems.Comment: 4 pages, 3 figure
Exploring corner transfer matrices and corner tensors for the classical simulation of quantum lattice systems
In this paper we explore the practical use of the corner transfer matrix and
its higher-dimensional generalization, the corner tensor, to develop tensor
network algorithms for the classical simulation of quantum lattice systems of
infinite size. This exploration is done mainly in one and two spatial
dimensions (1d and 2d). We describe a number of numerical algorithms based on
corner matri- ces and tensors to approximate different ground state properties
of these systems. The proposed methods make also use of matrix product
operators and projected entangled pair operators, and naturally preserve
spatial symmetries of the system such as translation invariance. In order to
assess the validity of our algorithms, we provide preliminary benchmarking
calculations for the spin-1/2 quantum Ising model in a transverse field in both
1d and 2d. Our methods are a plausible alternative to other well-established
tensor network approaches such as iDMRG and iTEBD in 1d, and iPEPS and TERG in
2d. The computational complexity of the proposed algorithms is also considered
and, in 2d, important differences are found depending on the chosen simulation
scheme. We also discuss further possibilities, such as 3d quantum lattice
systems, periodic boundary conditions, and real time evolution. This discussion
leads us to reinterpret the standard iTEBD and iPEPS algorithms in terms of
corner transfer matrices and corner tensors. Our paper also offers a
perspective on many properties of the corner transfer matrix and its
higher-dimensional generalizations in the light of novel tensor network
methods.Comment: 25 pages, 32 figures, 2 tables. Revised version. Technical details on
some of the algorithms have been moved to appendices. To appear in PR
Topological dualities via tensor networks
The ground state of the toric code, that of the two-dimensional class D
superconductor, and the partition sum of the two-dimensional Ising model are
dual to each other. This duality is remarkable inasmuch as it connects systems
commonly associated to different areas of physics -- that of long range
entangled topological order, (topological) band insulators, and classical
statistical mechanics, respectively. Connecting fermionic and bosonic systems,
the duality construction is intrinsically non-local, a complication that has
been addressed in a plethora of different approaches, including dimensional
reduction to one dimension, conformal field theory methods, and operator
algebra. In this work, we propose a unified approach to this duality, whose
main protagonist is a tensor network (TN) assuming the role of an intermediate
translator. Introducing a fourth node into the net of dualities offers several
advantages: the formulation is integrative in that all links of the duality are
treated on an equal footing, (unlike in field theoretical approaches) it is
formulated with lattice precision, a feature that becomes key in the mapping of
correlation functions, and their possible numerical implementation. Finally,
the passage from bosons to fermions is formulated entirely within the
two-dimensional TN framework where it assumes an intuitive and technically
convenient form. We illustrate the predictive potential of the formalism by
exploring the fate of phase transitions, point and line defects, topological
boundary modes, and other structures under the mapping between system classes.
Having condensed matter readerships in mind, we introduce the construction
pedagogically in a manner assuming only minimal familiarity with the concept of
TNs.Comment: 19 pages, 19 figure
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