1,619 research outputs found
Inhomogeneous Nuclear Spin Flips
We discuss a feedback mechanism between electronic states in a double quantum
dot and the underlying nuclear spin bath. We analyze two pumping cycles for
which this feedback provides a force for the Overhauser fields of the two dots
to either equilibrate or diverge. Which of these effects is favored depends on
the g-factor and Overhauser coupling constant A of the material. The strength
of the effect increases with A/V_x, where V_x is the exchange matrix element,
and also increases as the external magnetic field B_{ext} decreases.Comment: 5 pages, 4 figures (jpg
From Disordered Crystal to Glass: Exact Theory
We calculate thermodynamic properties of a disordered model insulator,
starting from the ideal simple-cubic lattice () and increasing the
disorder parameter to . As in earlier Einstein- and Debye-
approximations, there is a phase transition at . For the
low-T heat-capacity whereas for , . The van
Hove singularities disappear at {\em any finite }. For we discover
novel {\em fixed points} in the self-energy and spectral density of this model
glass.Comment: Submitted to Phys. Rev. Lett., 8 pages, 4 figure
Summing the Instanton Series in N=2 Superconformal Large-N QCD
We consider the multi-instanton collective coordinate integration measure in
N=2 supersymmetric SU(N) gauge theory with N_F fundamental hypermultiplets. In
the large-N limit, at the superconformal point where N_F=2N and all VEVs are
turned off, the k-instanton moduli space collapses to a single copy of
AdS_5*S^1. The resulting k-instanton effective measure is proportional to
N^{1/2} g^4 Z_k^(6), where Z_k^(6) is the partition function of N=(1,0) SYM
theory in six dimensions reduced to zero dimensions. The multi-instanton can in
fact be summed in closed form. As a hint of an AdS/CFT duality, with the usual
relation between the gauge theory and string theory parameters, this precisely
matches the normalization of the charge-k D-instanton measure in type IIB
string theory compactified to six dimensions on K3 with a vanishing two-cycle.Comment: 12 pages, amslate
Relationship between long time scales and the static free-energy in the Hopfield model
The Glauber dynamics of the Hopfield model at low storage level is
considered. We analytically derive the spectrum of relaxation times for large
system sizes. The longest time scales are gathered in families, each family
being in one to one correspondence with a stationary (not necessarily stable)
point of the static mean-field free-energy. Inside a family, the time scales
are given by the reciprocals (of the absolute values) of the eigenvalues of the
free-energy Hessian matrix.Comment: 5 pages RevTex file, accepted for publication in J.Phys.
Anomalous dynamics in two- and three- dimensional Heisenberg-Mattis spin glasses
We investigate the spectral and localization properties of unmagnetized
Heisenberg-Mattis spin glasses, in space dimensionalities and 3, at T=0.
We use numerical transfer-matrix methods combined with finite-size scaling to
calculate Lyapunov exponents, and eigenvalue-counting theorems, coupled with
Gaussian elimination algorithms, to evaluate densities of states. In we
find that all states are localized, with the localization length diverging as
, as energy . Logarithmic corrections to density of
states behave in accordance with theoretical predictions. In the
density-of-states dependence on energy is the same as for spin waves in pure
antiferromagnets, again in agreement with theoretical predictions, though the
corresponding amplitudes differ.Comment: RevTeX4, 9 pages, 9 .eps figure
A solvable model of a one-dimensional quantum gas with pair interaction
We propose a solvable model of a one-dimensional harmonic oscillator quantum
gas of two sorts of particles, fermions or bosons, which allows to describe the
formation of pairs due to a suitable pair interaction. These pairs we call
"pseudo-bosons" since the system can be approximated by an ideal bose gas for
low temperatures. We illustrate this fact by considering the specific heat and
the entropy function for N=8 pairs. The model can also be evaluated in the
thermodynamic limit if the harmonic oscillator potential is suitable scaled
A modified triplet-wave expansion method applied to the alternating Heisenberg chain
An alternative triplet-wave expansion formalism for dimerized spin systems is
presented, a modification of the 'bond operator' formalism of Sachdev and
Bhatt. Projection operators are used to confine the system to the physical
subspace, rather than constraint equations. The method is illustrated for the
case of the alternating Heisenberg chain, and comparisons are made with the
results of dimer series expansions and exact diagonalization. Some discussion
is included of the phenomenon of 'quasiparticle breakdown', as it applies to
the two-triplon bound states in this model.Comment: 16 pages, 12 figure
Application of tridiagonalization to the many-body problem
Journal ArticleThe problem of a single magnetic, Wolff-model impurity in an otherwise ideal metallic host is investigated using the nonperturbative Lanczos method. Convergence is very rapid. The many-body ground-state energy is investigated and comparisons are made with Tomonaga operator theory and other weak-coupling schemes. We believe that this is the first application of tridiagonalization to the many-body problem
Improved Landau-Ginzburg equation near surfaces of solids
Journal ArticleWe study the order parameter near the surface for an Ising model. Applications to the lattice gas, alloy problem, and ferromagnetism are noted. Away from Tc our equations differ from the Landau-Ginzburg results due to an additional nonlinear, term which can substantially affect the order parameter at low T. Our method also provides for a physically meaningful set of boundary conditions
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