11,498 research outputs found
Theoretical study of the charge transport through C60-based single-molecule junctions
We present a theoretical study of the conductance and thermopower of
single-molecule junctions based on C60 and C60-terminated molecules. We first
analyze the transport properties of gold-C60-gold junctions and show that these
junctions can be highly conductive (with conductances above 0.1G0, where G0 is
the quantum of conductance). Moreover, we find that the thermopower in these
junctions is negative due to the fact that the LUMO dominates the charge
transport, and its magnitude can reach several tens of micro-V/K, depending on
the contact geometry. On the other hand, we study the suitability of C60 as an
anchoring group in single-molecule junctions. For this purpose, we analyze the
transport through several dumbbell derivatives using C60 as anchors, and we
compare the results with those obtained with thiol and amine groups. Our
results show that the conductance of C60-terminated molecules is rather
sensitive to the binding geometry. Moreover, the conductance of the molecules
is typically reduced by the presence of the C60 anchors, which in turn makes
the junctions more sensitive to the functionalization of the molecular core
with appropriate side groups.Comment: 9 pages, 7 figure
Measuring dopant concentrations in compensated p-type crystalline silicon via iron-acceptor pairing
We present a method for measuring the concentrations of ionized acceptors and donors in compensated p-type silicon at room temperature.Carrier lifetimemeasurements on silicon wafers that contain minute traces of iron allow the iron-acceptor pair formation rate to be determined, which in turn allows the acceptor concentration to be calculated. Coupled with an independent measurement of the resistivity and a mobility model that accounts for majority and minority impurity scatterings of charge carriers, it is then possible to also estimate the total concentration of ionized donors. The method is valid for combinations of different acceptor and donor species.D.M. is supported by an Australian Research Council
fellowship. L.J.G. would like to acknowledge SenterNovem
for support
Anderson transition in systems with chiral symmetry
Anderson localization is a universal quantum feature caused by destructive
interference. On the other hand chiral symmetry is a key ingredient in
different problems of theoretical physics: from nonperturbative QCD to highly
doped semiconductors. We investigate the interplay of these two phenomena in
the context of a three-dimensional disordered system. We show that chiral
symmetry induces an Anderson transition (AT) in the region close to the band
center. Typical properties at the AT such as multifractality and critical
statistics are quantitatively affected by this additional symmetry. The origin
of the AT has been traced back to the power-law decay of the eigenstates; this
feature may also be relevant in systems without chiral symmetry.Comment: RevTex4, 4 two-column pages, 3 .eps figures, updated references,
final version as published in Phys. Rev.
On the use of reproducing kernel Hilbert spaces in functional classification
The H\'ajek-Feldman dichotomy establishes that two Gaussian measures are
either mutually absolutely continuous with respect to each other (and hence
there is a Radon-Nikodym density for each measure with respect to the other
one) or mutually singular. Unlike the case of finite dimensional Gaussian
measures, there are non-trivial examples of both situations when dealing with
Gaussian stochastic processes. This paper provides:
(a) Explicit expressions for the optimal (Bayes) rule and the minimal
classification error probability in several relevant problems of supervised
binary classification of mutually absolutely continuous Gaussian processes. The
approach relies on some classical results in the theory of Reproducing Kernel
Hilbert Spaces (RKHS).
(b) An interpretation, in terms of mutual singularity, for the "near perfect
classification" phenomenon described by Delaigle and Hall (2012). We show that
the asymptotically optimal rule proposed by these authors can be identified
with the sequence of optimal rules for an approximating sequence of
classification problems in the absolutely continuous case.
(c) A new model-based method for variable selection in binary classification
problems, which arises in a very natural way from the explicit knowledge of the
RN-derivatives and the underlying RKHS structure. Different classifiers might
be used from the selected variables. In particular, the classical, linear
finite-dimensional Fisher rule turns out to be consistent under some standard
conditions on the underlying functional model
Non-ergodic phases in strongly disordered random regular graphs
We combine numerical diagonalization with a semi-analytical calculations to
prove the existence of the intermediate non-ergodic but delocalized phase in
the Anderson model on disordered hierarchical lattices. We suggest a new
generalized population dynamics that is able to detect the violation of
ergodicity of the delocalized states within the Abou-Chakra, Anderson and
Thouless recursive scheme. This result is supplemented by statistics of random
wave functions extracted from exact diagonalization of the Anderson model on
ensemble of disordered Random Regular Graphs (RRG) of N sites with the
connectivity K=2. By extrapolation of the results of both approaches to
N->infinity we obtain the fractal dimensions D_{1}(W) and D_{2}(W) as well as
the population dynamic exponent D(W) with the accuracy sufficient to claim that
they are non-trivial in the broad interval of disorder strength W_{E}<W<W_{c}.
The thorough analysis of the exact diagonalization results for RRG with
N>10^{5} reveals a singularity in D_{1,2}(W)-dependencies which provides a
clear evidence for the first order transition between the two delocalized
phases on RRG at W_{E}\approx 10.0. We discuss the implications of these
results for quantum and classical non-integrable and many-body systems.Comment: 4 pages paper with 5 figures + Supplementary Material with 5 figure
Two-dimensional discrete solitons in rotating lattices
We introduce a two-dimensional (2D) discrete nonlinear Schr\"{o}dinger (DNLS)
equation with self-attractive cubic nonlinearity in a rotating reference frame.
The model applies to a Bose-Einstein condensate stirred by a rotating strong
optical lattice, or light propagation in a twisted bundle of nonlinear fibers.
Two species of localized states are constructed: off-axis fundamental solitons
(FSs), placed at distance from the rotation pivot, and on-axis (R=0) vortex
solitons (VSs), with vorticities and 2. At a fixed value of rotation
frequency , a stability interval for the FSs is found in terms of the
lattice coupling constant , , with monotonically
decreasing . VSs with S=1 have a stability interval,
\tilde{C}_{\mathrm{cr}%}^{(S=1)}(\Omega),
which exists for below a certain critical value,
. This implies that the VSs with S=1 are
\emph{destabilized} in the weak-coupling limit by the rotation. On the
contrary, VSs with S=2, that are known to be unstable in the standard DNLS
equation, with , are \emph{stabilized} by the rotation in region
%, with growing as a
function of . Quadrupole and octupole on-axis solitons are considered
too, their stability regions being weakly affected by .Comment: To be published in Physical Review
Impulse-induced localized nonlinear modes in an electrical lattice
Intrinsic localized modes, also called discrete breathers, can exist under
certain conditions in one-dimensional nonlinear electrical lattices driven by
external harmonic excitations. In this work, we have studied experimentally the
efectiveness of generic periodic excitations of variable waveform at generating
discrete breathers in such lattices. We have found that this generation
phenomenon is optimally controlled by the impulse transmitted by the external
excitation (time integral over two consecutive zerosComment: 5 pages, 8 figure
Interplay Between Yu-Shiba-Rusinov States and Multiple Andreev Reflections
Motivated by recent scanning tunneling microscopy experiments on single
magnetic impurities on superconducting surfaces, we present here a
comprehensive theoretical study of the interplay between Yu-Shiba-Rusinov bound
states and (multiple) Andreev reflections. Our theory is based on a combination
of an Anderson model with broken spin degeneracy and nonequilibrium Green's
function techniques that allows us to describe the electronic transport through
a magnetic impurity coupled to superconducting leads for arbitrary junction
transparency. Using this combination we are able to elucidate the different
tunneling processes that give a significant contribution to the subgap
transport. In particular, we predict the occurrence of a large variety of
Andreev reflections mediated by Yu-Shiba-Rusinov bound states that clearly
differ from the standard Andreev processes in non-magnetic systems. Moreover,
we provide concrete guidelines on how to experimentally identify the subgap
features originating from these tunneling events. Overall, our work provides
new insight into the role of the spin degree of freedom in Andreev transport
physics.Comment: 15 pages, 10 figure
Critical generalized inverse participation ratio distributions
The system size dependence of the fluctuations in generalized inverse
participation ratios (IPR's) at criticality is investigated
numerically. The variances of the IPR logarithms are found to be
scale-invariant at the macroscopic limit. The finite size corrections to the
variances decay algebraically with nontrivial exponents, which depend on the
Hamiltonian symmetry and the dimensionality. The large- dependence of the
asymptotic values of the variances behaves as according to theoretical
estimates. These results ensure the self-averaging of the corresponding
generalized dimensions.Comment: RevTex4, 5 pages, 4 .eps figures, to be published in Phys. Rev.
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