47,865 research outputs found
Uniform random generation of large acyclic digraphs
Directed acyclic graphs are the basic representation of the structure
underlying Bayesian networks, which represent multivariate probability
distributions. In many practical applications, such as the reverse engineering
of gene regulatory networks, not only the estimation of model parameters but
the reconstruction of the structure itself is of great interest. As well as for
the assessment of different structure learning algorithms in simulation
studies, a uniform sample from the space of directed acyclic graphs is required
to evaluate the prevalence of certain structural features. Here we analyse how
to sample acyclic digraphs uniformly at random through recursive enumeration,
an approach previously thought too computationally involved. Based on
complexity considerations, we discuss in particular how the enumeration
directly provides an exact method, which avoids the convergence issues of the
alternative Markov chain methods and is actually computationally much faster.
The limiting behaviour of the distribution of acyclic digraphs then allows us
to sample arbitrarily large graphs. Building on the ideas of recursive
enumeration based sampling we also introduce a novel hybrid Markov chain with
much faster convergence than current alternatives while still being easy to
adapt to various restrictions. Finally we discuss how to include such
restrictions in the combinatorial enumeration and the new hybrid Markov chain
method for efficient uniform sampling of the corresponding graphs.Comment: 15 pages, 2 figures. To appear in Statistics and Computin
Anomalous spin Hall effects in Dresselhaus (110) quantum wells
Anomalous spin Hall effects that belong to the intrinsic type in Dresselhaus
(110) quantum wells are discussed. For the out-of-plane spin component,
antisymmetric current-induced spin polarization induces opposite spin Hall
accumulation, even though there is no spin-orbit force due to Dresselhaus (110)
coupling. A surprising feature of this spin Hall induction is that the spin
accumulation sign does not change upon bias reversal. Contribution to the spin
Hall accumulation from the spin Hall induction and the spin deviation due to
intrinsic spin-orbit force as well as extrinsic spin scattering, can be
straightforwardly distinguished simply by reversing the bias. For the inplane
component, inclusion of a weak Rashba coupling leads to a new type of
intrinsic spin Hall effect solely due to spin-orbit-force-driven spin
separation.Comment: 6 pages, 5 figure
Universality of the Kondo effect in quantum dots with ferromagnetic leads
We investigate quantum dots in clean single-wall carbon nanotubes with
ferromagnetic PdNi-leads in the Kondo regime. In most odd Coulomb valleys the
Kondo resonance exhibits a pronounced splitting, which depends on the tunnel
coupling to the leads and an external magnetic field , and only weakly on
gate voltage. Using numerical renormalization group calculations, we
demonstrate that all salient features of the data can be understood using a
simple model for the magnetic properties of the leads. The magnetoconductance
at zero bias and low temperature depends in a universal way on , where is the Kondo temperature and the external field
compensating the splitting.Comment: 4 pages, 4 figure
Dielectric function of the semiconductor hole liquid: Full frequency and wave vector dependence
We study the dielectric function of the homogeneous semiconductor hole liquid
of p-doped bulk III-V zinc-blende semiconductors within random phase
approximation. The single-particle physics of the hole system is modeled by
Luttinger's four-band Hamiltonian in its spherical approximation. Regarding the
Coulomb-interacting hole liquid, the full dependence of the zero-temperature
dielectric function on wave vector and frequency is explored. The imaginary
part of the dielectric function is analytically obtained in terms of
complicated but fully elementary expressions, while in the result for the real
part nonelementary one-dimensional integrations remain to be performed. The
correctness of these two independent calculations is checked via Kramers-Kronig
relations.
The mass difference between heavy and light holes, along with variations in
the background dielectric constant, leads to dramatic alternations in the
plasmon excitation pattern, and generically, two plasmon branches can be
identified. These findings are the result of the evaluation of the full
dielectric function and are not accessible via a high-frequency expansion. In
the static limit a beating of Friedel oscillations between the Fermi wave
numbers of heavy and light holes occurs.Comment: 16 pages, 11 figures included. Update: Minor additions and
adjustments, published versio
Ancilla-assisted sequential approximation of nonlocal unitary operations
We consider the recently proposed "no-go" theorem of Lamata et al [Phys. Rev.
Lett. 101, 180506 (2008)] on the impossibility of sequential implementation of
global unitary operations with the aid of an itinerant ancillary system and
view the claim within the language of Kraus representation. By virtue of an
extremely useful tool for analyzing entanglement properties of quantum
operations, namely, operator-Schmidt decomposition, we provide alternative
proof to the "no-go" theorem and also study the role of initial correlations
between the qubits and ancilla in sequential preparation of unitary entanglers.
Despite the negative response from the "no-go" theorem, we demonstrate
explicitly how the matrix-product operator(MPO) formalism provides a flexible
structure to develop protocols for sequential implementation of such entanglers
with an optimal fidelity. The proposed numerical technique, that we call
variational matrix-product operator (VMPO), offers a computationally efficient
tool for characterizing the "globalness" and entangling capabilities of
nonlocal unitary operations.Comment: Slightly improved version as published in Phys. Rev.
Statistical description of eigenfunctions in chaotic and weakly disordered systems beyond universality
We present a semiclassical approach to eigenfunction statistics in chaotic
and weakly disordered quantum systems which goes beyond Random Matrix Theory,
supersymmetry techniques, and existing semiclassical methods. The approach is
based on a generalization of Berry's Random Wave Model, combined with a
consistent semiclassical representation of spatial two-point correlations. We
derive closed expressions for arbitrary wavefunction averages in terms of
universal coefficients and sums over classical paths, which contain, besides
the supersymmetry results, novel oscillatory contributions. Their physical
relevance is demonstrated in the context of Coulomb blockade physics
Semiclassical theory of spin-orbit interactions using spin coherent states
We formulate a semiclassical theory for systems with spin-orbit interactions.
Using spin coherent states, we start from the path integral in an extended
phase space, formulate the classical dynamics of the coupled orbital and spin
degrees of freedom, and calculate the ingredients of Gutzwiller's trace formula
for the density of states. For a two-dimensional quantum dot with a spin-orbit
interaction of Rashba type, we obtain satisfactory agreement with fully
quantum-mechanical calculations. The mode-conversion problem, which arose in an
earlier semiclassical approach, has hereby been overcome.Comment: LaTeX (RevTeX), 4 pages, 2 figures, accepted for Physical Review
Letters; final version (v2) for publication with minor editorial change
Polarization-sensitive absorption of THz radiation by interacting electrons in chirally stacked multilayer graphene
We show that opacity of a clean multilayer graphene flake depends on the
helicity of the circular polarized electromagnetic radiation. The effect can be
understood in terms of the pseudospin selection rules for the interband optical
transitions in the presence of exchange electron-electron interactions which
alter the pseudospin texture in momentum space. The interactions described
within a semi-analytical Hartree--Fock approach lead to the formation of the
topologically different broken--symmetry states characterized by Chern numbers
and zero-field anomalous Hall conductivities.Comment: 6 pages, final versio
Parallelization of the exact diagonalization of the t-t'-Hubbard model
We present a new parallel algorithm for the exact diagonalization of the
-Hubbard model with the Lanczos-method. By invoking a new scheme of
labeling the states we were able to obtain a speedup of up to four on 16 nodes
of an IBM SP2 for the calculation of the ground state energy and an almost
linear speedup for the calculation of the correlation functions. Using this
algorithm we performed an extensive study of the influence of the next-nearest
hopping parameter in the -Hubbard model on ground state energy and
the superconducting correlation functions for both attractive and repulsive
interaction.Comment: 18 Pages, 1 table, 8 figures, Latex uses revtex, submitted to Comp.
Phys. Com
Vortices in Quantum Spin Systems
We examine spin vortices in ferromagnetic quantum Heisenberg models with
planar anisotropy on two-dimensional lattices. The symmetry properties and the
time evolution of vortices built up from spin-coherent states are studied in
detail. Although these states show a dispersion typical for wave packets,
important features of classical vortices are conserved. Moreover, the results
on symmetry properties provide a construction scheme for vortex-like
excitations from exact eigenstates, which have a well-controlled time
evolution. Our approach works for arbitrary spin length both on triangular and
square lattices.Comment: Remarks added and conclusions enlarged, version to be published in
European Physical Journal
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