14,604 research outputs found
Entropy Function for Heterotic Black Holes
We use the entropy function formalism to study the effect of the Gauss-Bonnet
term on the entropy of spherically symmetric extremal black holes in heterotic
string theory in four dimensions. Surprisingly the resulting entropy and the
near horizon metric, gauge field strengths and the axion-dilaton field are
identical to those obtained by Cardoso et. al. for a supersymmetric version of
the theory that contains Weyl tensor squared term instead of the Gauss-Bonnet
term. We also study the effect of holomorphic anomaly on the entropy using our
formalism. Again the resulting attractor equations for the axion-dilaton field
and the black hole entropy agree with the corresponding equations for the
supersymmetric version of the theory. These results suggest that there might be
a simpler description of supergravity with curvature squared terms in which we
supersymmetrize the Gauss-Bonnet term instead of the Weyl tensor squared term.Comment: LaTeX file, 23 pages; v2: references added; v3: minor addition; v4:
minor change
Black hole entropy functions and attractor equations
The entropy and the attractor equations for static extremal black hole
solutions follow from a variational principle based on an entropy function. In
the general case such an entropy function can be derived from the reduced
action evaluated in a near-horizon geometry. BPS black holes constitute special
solutions of this variational principle, but they can also be derived directly
from a different entropy function based on supersymmetry enhancement at the
horizon. Both functions are consistent with electric/magnetic duality and for
BPS black holes their corresponding OSV-type integrals give identical results
at the semi-classical level. We clarify the relation between the two entropy
functions and the corresponding attractor equations for N=2 supergravity
theories with higher-derivative couplings in four space-time dimensions. We
discuss how non-holomorphic corrections will modify these entropy functions.Comment: 21 pages,LaTeX,minor change
Non-Supersymmetric Attractors in Gravities
We investigate the attractor mechanism for spherically symmetric extremal
black holes in a theory of general gravity in 4-dimensions, coupled to
gauge fields and moduli fields. For the general theory, we look for
solutions which are analytic near the horizon, show that they exist and enjoy
the attractor behavior. The attractor point is determined by extremization of
an effective potential at the horizon. This analysis includes the backreaction
and supports the validity of non-supersymmetric attractors in the presence of
higher derivative interactions. To include a wider class of solutions, we
continue our analysis for the specific case of a Gauss-Bonnet theory which is
non-topological, due to the coupling of Gauss-Bonnet terms to the moduli
fields. We find that the regularity of moduli fields at the horizon is
sufficient for attractor behavior. For the non-analytic sector, this regularity
condition in turns implies the minimality of the effective potential at the
attractor point.Comment: 19 pages, 2 figure
Hybrid GMR Sensor Detecting 950 pT/sqrt(Hz) at 1 Hz and Room Temperature.
Advances in the magnetic sensing technology have been driven by the increasing demand for the capability of measuring ultrasensitive magnetic fields. Among other emerging applications, the detection of magnetic fields in the picotesla range is crucial for biomedical applications. In this work Picosense reports a millimeter-scale, low-power hybrid magnetoresistive-piezoelectric magnetometer with subnanotesla sensitivity at low frequency. Through an innovative noise-cancelation mechanism, the 1/f noise in the MR sensors is surpassed by the mechanical modulation of the external magnetic fields in the high frequency regime. A modulation efficiency of 13% was obtained enabling a final device's sensitivity of ~950 pT/Hz1/2 at 1 Hz. This hybrid device proved to be capable of measuring biomagnetic signals generated in the heart in an unshielded environment. This result paves the way for the development of a portable, contactless, low-cost and low-power magnetocardiography device
Nernst branes from special geometry
We construct new black brane solutions in gauged
supergravity with a general cubic prepotential, which have entropy density
as and thus satisfy the Nernst Law. By using
the real formulation of special geometry, we are able to obtain analytical
solutions in closed form as functions of two parameters, the temperature
and the chemical potential . Our solutions interpolate between
hyperscaling violating Lifshitz geometries with at the
horizon and at infinity. In the zero temperature limit,
where the entropy density goes to zero, we recover the extremal Nernst branes
of Barisch et al, and the parameters of the near horizon geometry change to
.Comment: 37 pages. v2: numerical pre-factors of scalar fields q_A corrected in
Section 3. No changes to conclusions. References adde
Black Hole Microstates and Attractor Without Supersymmetry
Due to the attractor mechanism, the entropy of an extremal black hole does
not vary continuously as we vary the asymptotic values of various moduli
fields. Using this fact we argue that the entropy of an extremal black hole in
string theory, calculated for a range of values of the asymptotic moduli for
which the microscopic theory is strongly coupled, should match the statistical
entropy of the same system calculated for a range of values of the asymptotic
moduli for which the microscopic theory is weakly coupled. This argument does
not rely on supersymmetry and applies equally well to nonsupersymmetric
extremal black holes. We discuss several examples which support this argument
and also several caveats which could invalidate this argument.Comment: 50 pages; references adde
5D Attractors with Higher Derivatives
We analyze higher derivative corrections to attractor geometries in five
dimensions. We find corrected AdS_3xS^2 geometries by solving the equations of
motion coming from a recently constructed four-derivative supergravity action
in five dimensions. The result allows us to explicitly verify a previous
anomaly based derivation of the AdS_3 central charges of this theory. Also, by
dimensional reduction we compare our results with those of the 4D higher
derivative attractor, and find complete agreement.Comment: 18 pages, harvma
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