2,684 research outputs found
Reentrant Kondo effect in Landau quantized graphene
We have studied the interplay of an Anderson impurity in Landau quantized
graphene, with special emphasis on the influence of the chemical potential.
Within the slave-boson mean-field theory, we found reentrant Kondo behaviour by
varying the chemical potential or gate voltage. Between Landau levels, the
density of states is suppressed, and by changing the graphene's Fermi energy,
we cross from metallic to semiconducting regions. Hence, the corresponding
Kondo behaviour is also influenced. The f-level spectral function reveals both
the presence of Landau levels in the conduction band and the Kondo resonance.Comment: 8 pages, 6 figure
Anisotropy of the sky distribution of gamma-ray bursts
The isotropy of gamma-ray bursts collected in current BATSE catalog is studied. It is shown that the quadrupole term being proportional to \sim sin 2b sin l is non-zero with a probability of 99.9%. The occurrence of this anisotropy term is then confirmed by the binomial test even with the probability of 99.97 %. Hence, the sky distribution of all known gamma-ray bursts is anisotropic. It is also argued that this anisotropy cannot be caused exclusively by instrumental effects due to the nonuniform sky exposure of BATSE instrument. Separating the GRBs into short and long subclasses, it is shown that the short ones are distributed anisotropically, but the long ones seem to be distributed still isotropically. The character of anisotropy suggests that the cosmological origin of short GRBs further holds, and there is no evidence for their Galactical origin
The three-dimensional carrier-envelope-phase map of focused few-cycle pulsed Gaussian beams
We derive an analytical expression that describes the complete
three-dimensional carrier-envelope phase (CEP) distribution of in the focal
volume of ultrashort pulsed Gaussian beams focused by spherical mirrors or
lenses. The focal CEP map depends on the so-called factor specifying the
frequency-dependence of the beam width of the source few-cycle pulse, on its
chirp and on the small chromatic aberration introduced by a lens without
appreciably distorting or broadening the few-cycle pulse. We show how to tailor
the CEP map of mirror-focused and lens-focused few-cycle pulses in order to
produce negligible transversal and axial CEP variations in specific regions of
the focal volume for phase-sensitive interactions of light with matter taking
place in a volume or on a surface. We propose a quasi-achromatic doublet lens
that can implement in practice these tailored CEP distributions.Comment: 9 pages, 6 figure
An intrinsic anisotropy in the angular distribution of gamma-ray bursts
The anisotropy of the sky distribution of 2025 gamma-ray bursts (GRBs) collected in Current BATSE catalog is confirmed. It is shown that the quadrupole term being proportional to similar to sin 2b sin I is non-zero with a probability 99.9%. The occurrence of this anisotropy term is then supported by the binomial test even with the probability 99.97%. It is also argued that this anisotropy cannot be caused exclusively by instrumental effects due to the non-uniform sky exposure of BATSE instrument; there should exist also some intrinsic anisotropy in the angular distribution of GRBs. Separating GRBs into short and long subclasses, it is shown that the 251 short ones are distributed anisotropically, but the 681 long ones seem to be distributed still isotropically. The 2-sample Kolmogorov Smirnov test shows that they are distributed differently with a 98.7% probability. The character of anisotropy suggests that the cosmological origin of short GRBs further holds, and there is no evidence for their Galactical origin. The work in essence contains the key ideas and results of a recently published paper (Balazs et al. 1998), to which the new result following from the 2-sample Kolmogorov-Smirnov test is added, too
Testing the randomness in the sky-distribution of gamma-ray bursts
We have studied the complete randomness of the angular distribution of gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment (BATSE). Because GRBs seem to be a mixture of objects of different physical nature, we divided the BATSE sample into five subsamples (short1, short2, intermediate, long1, long2) based on their durations and peak fluxes, and we studied the angular distributions separately. We used three methods, Voronoi tesselation, minimal spanning tree and multifractal spectra, to search for non-randomness in the subsamples. To investigate the eventual non-randomness in the subsamples, we defined 13 test variables (nine from the Voronoi tesselation, three from the minimal spanning tree and one from the multifractal spectrum). Assuming that the point patterns obtained from the BATSE subsamples are fully random, we made Monte Carlo simulations taking into account the BATSE's sky-exposure function. The Monte Carlo simulations enabled us to test the null hypothesis (i.e. that the angular distributions are fully random). We tested the randomness using a binomial test and by introducing squared Euclidean distances in the parameter space of the test variables. We concluded that the short1 and short2 groups deviate significantly (99.90 and 99.98 per cent, respectively) from the full randomness in the distribution of the squared Euclidean distances; however, this is not the case for the long samples. For the intermediate group, the squared Euclidean distances also give a significant deviation (98.51 per cent)
Unusual hyperfine interaction of Dirac electrons and NMR spectroscopy in graphene
Theory of nuclear magnetic resonance (NMR) in graphene is presented. The
canonical form of the electron-nucleus hyperfine interaction is strongly
modified by the linear electronic dispersion. The NMR shift and spin-lattice
relaxation time are calculated as function of temperature, chemical potential,
and magnetic field and three distinct regimes are identified: Fermi-,
Dirac-gas, and extreme quantum limit behaviors. A critical spectrometer
assessment shows that NMR is within reach for fully 13C enriched graphene of
reasonable size.Comment: 5 pages, 3 figure
The Fermi edge singularity in the SU(N) Wolff model
The low temperature properties of the SU(N) Wolff impurity model are studied
via Abelian bosonization. The path integral treatment of the problem allows for
an exact evaluation of low temperature properties of the model. The single
particle Green's function enhances due to the presence of local correlation.
The basic correlation function such as the charge or spin correlator are also
influenced by the presence of impurity, and show local Fermi liquid behaviour.
The X-ray absorption is affected by the presence of local Hubbard interaction.
The exponent is decreased (increased) for repulsive (attractive) interactions.Comment: 7 pages, 4 figure
The Wiedemann-Franz law in the SU(N) Wolff model
We study the electrical and thermal transport through the SU(N) Wolff model
with the use of bosonization. The Wilson ratio reaches unity as N grows to
infinity. The electric conductance is dominated by the charge channel, and
decreases monotonically with increasing interaction. The thermal conductivity
enhances in the presence of local Hubbard U. The Wiedemann-Franz law is
violated, the Lorentz number depends strongly on the interaction parameter,
which can be regarded as a manifestation of spin-charge separation.Comment: 4 pages, 3 figure
An Observational Evidence for the Difference Between the Short and Long Gamma-Ray Bursts
The intrinsic fluence and duration distributions of gamma-ray bursts are well represented by log-normal
distributions. This allows a bivariate log-normal distribution fit to be made to the BATSE short and long bursts
separately. A statistically significant difference between the long and short groups is found. We argue that the
effect is probably real. Applying the CramĂŠrâs theorem these results lead to some predictions for models of long
and short bursts
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