886 research outputs found
Entropy of Killing horizons from Virasoro algebra in D-dimensional extended Gauss-Bonnet gravity
We treat D-dimensional black holes with Killing horizon for extended
Gauss-Bonnet gravity. We use Carlip method and impose boundary conditions on
horizon what enables us to identify Virasoro algebra and evaluate its central
charge and Hamiltonian eigenvalue. The Cardy formula allows then to calculate
the number of states and thus provides for microscopic interpretation of
entropy.Comment: 15 page
Experimental evidence of a fractal dissipative regime in high-T_c superconductors
We report on our experimental evidence of a substantial geometrical
ingredient characterizing the problem of incipient dissipation in high-T_c
superconductors(HTS): high-resolution studies of differential
resistance-current characteristics in absence of magnetic field enabled us to
identify and quantify the fractal dissipative regime inside which the actual
current-carrying medium is an object of fractal geometry. The discovery of a
fractal regime proves the reality and consistency of critical-phenomena
scenario as a model for dissipation in inhomogeneous and disordered HTS, gives
the experimentally-based value of the relevant finite-size scaling exponent and
offers some interesting new guidelines to the problem of pairing mechanisms in
HTS.Comment: 5 pages, 3 figures, RevTex; Accepted for publication in Physical
Review B; (figures enlarged
Axial gravity: a non-perturbative approach to split anomalies
In a theory of a Dirac fermion field coupled to a metric-axial-tensor (MAT)
background, using a Schwinger-DeWitt heat kernel technique, we compute
non-perturbatively the two (odd parity) trace anomalies. A suitable collapsing
limit of this model corresponds to a theory of chiral fermions coupled to
(ordinary) gravity. Taking this limit on the two computed trace anomalies we
verify that they tend to the same expression, which coincides with the already
found odd parity trace anomaly, with the identical coefficient. This confirms
our previous results on this issue.Comment: 43 pages, some additions in section 6.3 and 6.5 plus minor
correction
Slow magnetic dynamics and hysteresis loops of a bulk ferromagnet
Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound
Co7(TeO3)4Br6, was studied by ac susceptibility and the related techniques.
Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt
time (2x10^(-4)s) span the full spectrum of magnetic dynamics inside a
convenient frequency window, offering a rare opportunity for general studies of
magnetic dynamics. Within the experimental window the ac susceptibility data
build almost ideally semicircular Cole-Cole plots. Comprehensive study of
experimental dynamic hysteresis loops of the compound is presented and
interpreted within a simple thermal-activation-assisted spin lattice relaxation
model for spin reversal. Quantitative agreement between the experimental
results and the model's prediction for dynamic coercive field is achieved by
assuming the central physical quantity, the Debye relaxation rate, to depend on
frequency, as well as on the applied field strength and sample temperature.
Cross-over between minor- to major hysteresis loops is carefully analyzed.
Low-frequency limitations of the model, relying on domain wall pinning effects,
are experimentally detected and appropriately discussed.Comment: A paragraph on dynamical-hysteresis assymetry added, text partially
revised; Accepted in Physical Review
Axial gravity, massless fermions and trace anomalies
This article deals with two main topics. One is odd parity trace anomalies in
Weyl fermion theories in a 4d curved background, the second is the introduction
of axial gravity. The motivation for reconsidering the former is to clarify the
theoretical background underlying the approach and complete the calculation of
the anomaly. The reference is in particular to the difference between Weyl and
massless Majorana fermions and to the possible contributions from tadpole and
seagull terms in the Feynman diagram approach. A first, basic, result of this
paper is that a more thorough treatment, taking account of such additional
terms { and using dimensional regularization}, confirms the earlier result. The
introduction of an axial symmetric tensor besides the usual gravitational
metric is instrumental to a different derivation of the same result using Dirac
fermions, which are coupled not only to the usual metric but also to the
additional axial tensor. The action of Majorana and Weyl fermions can be
obtained in two different limits of such a general configuration. The results
obtained in this way confirm the previously obtained ones.Comment: 55 pages, comments added in section 2 and 5. Sections 6.4, 6.6, 7,
7.1, 7.2 and Appendices 5.3, 5.5 partially modifie
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