9,510 research outputs found
Dielectric response of Anderson and pseudogapped insulators
Using a combination of analytic and numerical methods, we study the
polarizability of a (non-interacting) Anderson insulator in one, two, and three
dimensions and demonstrate that, in a wide range of parameters, it scales
proportionally to the square of the localization length, contrary to earlier
claims based on the effective-medium approximation. We further analyze the
effect of electron-electron interactions on the dielectric constant in
quasi-1D, quasi-2D and 3D materials with large localization length, including
both Coulomb repulsion and phonon-mediated attraction. The phonon-mediated
attraction (in the pseudogapped state on the insulating side of the
Superconductor-Insulator Transition) produces a correction to the dielectric
constant, which may be detected from a linear response of a dielectric constant
to an external magnetic field.Comment: 9 page
Conduction Channels of One-Atom Zinc Contacts
We have determined the transmission coefficients of atomic-sized Zn contacts
using a new type of breakjunction which contains a whisker as a central bridge.
We find that in the last conductance plateau the transport is unexpectedly
dominated by a well-transmitting single conduction channel. We explain the
experimental findings with the help of a tight-binding model which shows that
in an one-atom Zn contact the current proceeds through the 4s and 4p orbitals
of the central atom.Comment: revtex4, 5 pages, 5 figure
Anomalously large critical regions in power-law random matrix ensembles
We investigate numerically the power-law random matrix ensembles.
Wavefunctions are fractal up to a characteristic length whose logarithm
diverges asymmetrically with different exponents, 1 in the localized phase and
0.5 in the extended phase. The characteristic length is so anomalously large
that for macroscopic samples there exists a finite critical region, in which
this length is larger than the system size. The Green's functions decrease with
distance as a power law with an exponent related to the correlation dimension.Comment: RevTex, 4 pages, 4 eps figures. Final version to be published in
Phys. Rev. Let
Single-channel transmission in gold one-atom contacts and chains
We induce superconductivity by proximity effect in thin layers of gold and
study the number of conduction channels which contribute to the current in
one-atom contacts and atomic wires. The atomic contacts and wires are
fabricated with a Scanning Tunneling Microscope. The set of transmission
probabilities of the conduction channels is obtained from the analysis of the
characteristic curve which is highly non-linear due to multiple Andreev
reflections. In agreement with theoretical calculations we find that there is
only one channel which is almost completely open.Comment: 4 pages, 2 figures. To be published in Phys. Rev. B, Rapid
Communications (2003
Not Hydro: Using Neural Networks to estimate galaxy properties on a Dark-Matter-Only simulation
Using data from TNG300-2, we train a neural network (NN) to recreate the
stellar mass () and star formation rate (SFR) of central galaxies in a
dark-matter-only simulation. We consider 12 input properties from the halo and
sub-halo hosting the galaxy and the near environment. predictions are
robust, but the machine does not fully reproduce its scatter. The same happens
for SFR, but the predictions are not as good as for . We chained neural
networks, improving the predictions on SFR to some extent. For SFR, we
time-averaged this value between and , which improved results for
. Predictions of both variables have trouble reproducing values at lower
and higher ends. We also study the impact of each input variable in the
performance of the predictions using a leave-one-covariate-out approach, which
led to insights about the physical and statistical relation between input
variables. In terms of metrics, our machine outperforms similar studies, but
the main discoveries in this work are not linked with the quality of the
predictions themselves, but to how the predictions relate to the input
variables. We find that previously studied relations between physical variables
are meaningful to the machine. We also find that some merger tree properties
strongly impact the performance of the machine. %We highlight the value of
machine learning (ML) methods in helping understand the information contained
in different variables, since with its help we were able to obtain useful
insights resulting from studying the impact of input variables on the resulting
behaviour of galaxy properties. We conclude that ML models are useful tools to
understand the significance of physical different properties and their impact
on target characteristics, as well as strong candidates for potential
simulation methods.Comment: 17 pages, 16 figures, to be published in MNRA
Mean Free Path and Energy Fluctuations in Quantum Chaotic Billiards
The elastic mean free path of carriers in a recently introduced model of
quantum chaotic billiards in two and three dimensions is calculated. The model
incorporates surface roughness at a microscopic scale by randomly choosing the
atomic levels at the surface sites between -W/2 and W/2. Surface roughness
yields a mean free path l that decreases as L/W^2 as W increases, L being the
linear size of the system. But this diminution ceases when the surface layer
begins to decouple from the bulk for large enough values of W, leaving more or
less unperturbed states on the bulk. Consequently, the mean free path shows a
minimum of about L/2 for W of the order of the band width. Energy fluctuations
reflect the behavior of the mean free path. At small energy scales, strong
level correlations manifest themselves by small values of the number of levels
variance Sigma^2(E) that are close to Random Matrix Theory (RMT) in all cases.
At larger energy scales, fluctuations are below the logarithmic behavior of RMT
for l > L, and above RMT value when l < L.Comment: 8 twocolumn pages, seven figures, revtex and epsf macros. To be
published in Physical Review B
Computation of the Euler Number of a Binary Image Composed of Hexagonal Cells
ABSTRACTMost of the proposals to compute the Euler number of a binary image have been designed to work with imagescomposed of squared cells. Only a few of these methods (in the case of images composed of hexagonal cells) havebeen reported in literature, although it is known that images composed of hexagonal cells do not suffer from theproblems of connectivity frequently found in the case of images composed of squared cells. In this paper, a new wayto compute the Euler number (E) of a binary image composed of hexagonal cells is presented. For this, the perimeterP of the isolated regions in the image, their contact perimeter c P and the type T of a cell are used to obtain thisimportant invariant. The proposal can be used alone or in combination with other features to describe any binaryplanar shape composed of hexagonal pixels for its further recognition
Critical spectral statistics in two-dimensional interacting disordered systems
The effect of Coulomb and short-range interactions on the spectral properties
of two-dimensional disordered systems with two spinless fermions is
investigated by numerical scaling techniques. The size independent universality
of the critical nearest level-spacing distribution allows one to find a
delocalization transition at a critical disorder for any non-zero
value of the interaction strength. At the critical point the spacings
distribution has a small- behavior , and a Poisson-like
decay at large spacings.Comment: 4 two-column pages, 3 eps figures, RevTeX, new results adde
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