742 research outputs found
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
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
Perturbative spectrum of the dressed sliver
We analyze the fluctuations of the dressed sliver solution found in a previous paper, hep-th/0311198, in the operator formulation of Vacuum String Field Theory. We derive the tachyon wave function and then analyze the higher level fluctuations. We show that the dressing is responsible for implementing the transversality condition on the massless vector. In order to consistently deal with the singular k = 0 mode we introduce a string midpoint regulator and we show that it is possible to accommodate all the open string states among the solutions to the linearized equations of motion. We finally show how the dressing can give rise to the correct ratio between the energy density of the dressed sliver and the brane tension computed via the three-tachyons-coupling
Gravitational Chern-Simons Lagrangians and black hole entropy
We analyze the problem of defining the black hole entropy when Chern-Simons
terms are present in the action. Extending previous works, we define a general
procedure, valid in any odd dimensions both for purely gravitational CS terms
and for mixed gauge-gravitational ones. The final formula is very similar to
Wald's original formula valid for covariant actions, with a significant
modification. Notwithstanding an apparent violation of covariance we argue that
the entropy formula is indeed covariant.Comment: 39 page
alpha'-exact entropies for BPS and non-BPS extremal dyonic black holes in heterotic string theory from ten-dimensional supersymmetry
We calculate near-horizon solutions for four-dimensional 4-charge and
five-dimensional 3-charge black holes in heterotic string theory from the part
of the ten-dimensional tree-level effective action which is connected to
gravitational Chern-Simons term by supersymmetry. We obtain that the entropies
of large black holes exactly match the alpha'-exact statistical entropies
obtained from microstate counting (D=4) and AdS/CFT correspondence (D=5).
Especially interesting is that we obtain agreement for both BPS and non-BPS
black holes, contrary to the case of R^2-truncated (four-derivative) actions
(D-dimensional N=2 off-shell supersymmetric or Gauss-Bonnet) were used, which
give the entropies agreeing (at best) just for BPS black holes. The key
property of the solutions, which enabled us to tackle the action containing
infinite number of terms, is vanishing of the Riemann tensor \bar{R}_{MNPQ}
obtained from torsional connection defined with \bar{\Gamma} = \Gamma - H/2.
Morover, if every monomial of the remaining part of the effective action would
contain at least two Riemanns \bar{R}_{MNPQ}, it would trivially follow that
our solutions are exact solutions of the full heterotic effective action in
D=10. The above conjecture, which appeared (in this or stronger form) from time
to time in the literature, has controversial status, but is supported by the
most recent calculations of Richards (arXiv:0807.3453 [hep-th]). Agreement of
our results for the entropies with the microscopic ones supports the
conjecture. As for small black holes, our solutions in D=5 still have singular
horizons.Comment: 28 pages; v2: minor changes, references added; v3: extended
discussion on small black holes in sec. 5.4, more references added, accepted
in JHE
Massive fermion model in 3d and higher spin currents
We analyze the 3d free massive fermion theory coupled to external sources. The presence of a mass explicitly breaks parity invariance. We calculate two- and three-point functions of a gauge current and the energy momentum tensor and, for instance, obtain the well-known result that in the IR limit (but also in the UV one) we reconstruct the relevant CS action. We then couple the model to higher spin currents and explicitly work out the spin 3 case. In the UV limit we obtain an effective action which was proposed many years ago as a possible generalization of spin 3 CS action. In the IR limit we derive a different higher spin action. This analysis can evidently be generalized to higher spins. We also discuss the conservation and properties of the correlators we obtain in the intermediate steps of our derivation
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