141 research outputs found
Exactly solvable random matrix models with additional two-body interactions
It has been argued that despite remarkable success, existing random matrix
theories are not adequate to describe disordered conductors in the metallic
regime, due to the presence of certain two-body interactions in the effective
Hamiltonian for the eigenvalues, in addition to the standard logarithmic
interaction that arises entirely from symmetry considerations. We present a new
method that allows exact solution of random matrix models with such additional
two-body interactions. This should broaden the scope of random matrix models in
general.Comment: 12 pages, late
Distribution function for shotnoise in disordered multi-channel quantum conductors
We obtain the shot noise generating function for a multi-channel quantum
conductor in the diffusive limit.Comment: 8, late
Universality of phonon transport in surface-roughness dominated nanowires
We analyze, both theoretically and numerically, the temperature dependent
thermal conductivity \k{appa} of two-dimensional nanowires with surface
roughness. Although each sample is characterized by three independent
parameters - the diameter (width) of the wire, the correlation length and
strength of the surface corrugation - our theory predicts that there exists a
universal regime where \k{appa} is a function of a single combination of all
three model parameters. Numerical simulations of propagation of acoustic
phonons across thin wires confirm this universality and predict a d 1/2
dependence of \k{appa} on the diameter d
Distribution of conductance for Anderson Insulators: A theory with a single parameter
We obtain an analytic expression for the full distribution of conductance for
a strongly disordered three dimensional conductor within a perturbative
approach based on the transfer-matrix formulation. Our results confirm
numerical evidence that the log-normal limit of the distribution is not reached
even in the deeply insulating regime. We show that the variance of the
logarithm of the conductance scales as a fractional power of the mean, while
the skewness changes sign as one approaches the Anderson metal-insulator
transition from the deeply insulating limit, all described as a function of a
single parameter. The approach suggests a possible single parameter description
of the Anderson transition that takes into account the full nontrivial
distribution of conductance.Comment: 4 pages, 4 figure
Suppressing phonon transport in nanowires: a simple model for phonon-surface roughness interaction
Suppressing phonon propagation in nanowires is an essential goal towards
achieving efficient thermoelectric devices. Recent experiments have shown
unambiguously that surface roughness is a key factor that can reduce the
thermal conductivity well below the Casimir limit in thin crystalline silicon
nanowires. We use insights gained from the experimental studies to construct a
simple analytically tractable model of the phonon-surface roughness interaction
that provides a better theoretical understanding of the effects of surface
roughness on the thermal conductivity, which could potentially help in
designing better thermoelectric devices.Comment: 7 pages, 3 figures; accepted for publication in Phys. Rev.
Phonon localization in surface-roughness dominated nanowires
Studies of possible localization of phonons in nanomaterials have gained
importance in recent years in the context of thermoelectricity where
phonon-localization can reduce thermal conductivity, thereby improving the
efficiency of thermoelectric devices. However, despite significant efforts,
phonon-localization has not yet been observed experimentally in real materials.
Here we propose that surface-roughness dominated nanowires are ideal candidates
to observe localization of phonons, and show numerically that the space and
time evolution of the energy generated by a heat-pulse injected at a given
point shows clear signatures of phonon localization. We suggest that the same
configuration might allow experimental observation of localization of phonons.
Our results confirm the universality in the surface-roughness dominated regime
proposed earlier, which allows us to characterize the strength of disorder by a
single parameter combining the width of the wire as well as the mean height of
the corrugation and its correlation length
Conductance distribution in quasi-one-dimensional disordered quantum wires
We develop a simple systematic method, valid for all strengths of disorder,
to obtain analytically the full distribution of conductances P(g) for a quasi
one dimensional wire within the model of non-interacting fermions. The method
has been used in [1-3] to predict sharp features in P(g) near g=1 and the
existence of non-analyticity in the conductance distribution in the insulating
and crossover regimes, as well as to show how P(g) changes from Gaussian to
log-normal behavior as the disorder strength is increased. Here we provide many
details of the method, including intermediate results that offer much insight
into the nature of the solutions. In addition, we show within the same
framework that while for metals P(g) is a Gaussian around g >>1, there exists a
log-normal tail for g << 1, consistent with earlier field theory calculations.
We also obtain several other results that compare very well with available
exact results in the metallic and insulating regimes.Comment: 9 figures, 50 pages (figures included). To appear in Annals of
Physic
Conductance distribution in strongly disordered mesoscopic systems in three dimensions
Recent numerical simulations have shown that the distribution of conductance
P(g) in 3D strongly localized regiem differs significally from the expected log
normal distribution. To understand the origin of this difference analytically,
we used a generalized DMPK equation for the joint probablity distribution of
the transmission eigenvalues which includes a phenomenological (disorder and
dimensionality dependent) matrix K containing certain correlations of the
transfer matrices. We first of all examine the assumptions made in the
derivation if the generalized DMPK and find that to a good approximation they
remain valid in 3D. We then evaluate the matrix K numerically for various
strength of the disorder and various system sizes. In the strong disorder limit
we find that K can be described by a simple model which, for a cubic system,
depends on a single parameter. We use this phenomenological model to
analytically evaluate the full distribution P(g) for Anderson insulators in 3D.
The analytic results allow us to develop an intuitive understanding of the
entire distribution, which differs qualitatively from the log-normal
distribution of a Q1D wire. We also show that out method could be applicable in
the critical regime of the Anderson transition
Non-linear thermoelectric transport: A class of nano-devices for high efficiency and large power output
Molecular junctions and similar devices described by an energy dependent
transmission coefficient can have a high linear response thermoelectric figure
of merit. Since such devices are inherently non-linear, the full thermodynamic
efficiency valid for any temperature and chemical potential difference across
the leads is calculated. The general features in the energy dependence of the
tranmission function that lead to high efficiency and also high power output
are determined. It is shown that the device with the highest efficiency does
not necessarily lead to large power output. To illustrate this, we use a model
called the t-stub model representing tunneling through an energy level
connected to another energy level. Within this model both high efficiency and
high power output are achievable. Futhermore, by connecting many nanodevices it
is shown to be possible to scale up the power output without compromising
efficiency in an (exactly solvable) n-channel model even with tunneling between
the devices.Comment: References added, minor corrections; published versio
Electric field induced memory and aging effects in pure solid N_2
We report combined high sensitivity dielectric constant and heat capacity
measurements of pure solid N_2 in the presence of a small external ac electric
field in the audio frequency range. We have observed strong field induced aging
and memory effects which show that field cooled samples may be prepared in a
variety of metastable states leading to a free energy landscape with
experimentally ``tunable'' barriers, and tunneling between these states may
occur within laboratory time scales.Comment: 4 pages, 3 figure
- …