8,634 research outputs found
Phase transition between non-extremal and extremal Reissner-Nordstr\"om black holes
We discuss the phase transition between non-extremal and extremal
Reissner-Nordstr\"om black holes. This transition is considered as the
limit of the transition between the non-extremal and near-extremal
black holes.
We show that an evaporating process from non-extremal black hole to extremal
one is possible to occur, but its reverse process is not possible to occur
because of the presence of the maximum temperature.
Furthermore, it is shown that the Hawking-Page phase transition between small
and large black holes unlikely occurs in the AdS Reissner-Nordstr\"om black
holes.Comment: 12 pages, 6 figures, version to appear in MPL
Thermodynamic Geometry of the Born-Infeld-anti-de Sitter black holes
Thermodynamic geometry is applied to the Born-Infeld-anti-de Sitter black
hole (BIAdS) in the four dimensions, which is a nonlinear generalization of the
Reissner-Norstr\"Aom-AdS black hole (RNAdS). We compute the Weinhold as well as
the Ruppeiner scalar curvature and find that the singular points are not the
same with the ones obtained using the heat capacity. Legendre-invariant metric
proposed by Quevedo and the metric obtained by using the free energy as the
thermodynamic potential are obtained and the corresponding scalar curvatures
diverge at the Davies points.Comment: Latex,19 pages,14 figure
Enhanced grain surface effect on magnetic properties of nanometric La0.7Ca0.3MnO3 manganite : Evidence of surface spin freezing of manganite nanoparticles
We have investigated the effect of nanometric grain size on magnetic
properties of single phase, nanocrystalline, granular La0.7Ca0.3MnO3 (LCMO)
sample. We have considered core-shell structure of our LCMO nanoparticles,
which can explain its magnetic properties. From the temperature dependence of
field cooled (FC) and zero-field cooled (ZFC) dc magnetization (DCM), the
magnetic properties could be distinguished into two regimes: a relatively high
temperature regime T > 40 K where the broad maximum of ZFC curve (at T = Tmax)
is associated with the blocking of core particle moments, whereas the sharp
maximum (at T = TS) is related to the freezing of surface (shell) spins. The
unusual shape of M (H) loop at T = 1.5 K, temperature dependent feature of
coercive field and remanent magnetization give a strong support of surface spin
freezing that are occurring at lower temperature regime (T < 40 K) in this LCMO
nanoparticles. Additionally, waiting time (tw) dependence of ZFC relaxation
measurements at T = 50 K show weak dependence of relaxation rate [S(t)] on tw
and dM/dln(t) following a logarithmic variation on time. Both of these features
strongly support the high temperature regime to be associated with the blocking
of core moments. At T = 20 K, ZFC relaxation measurements indicates the
existence of two different types of relaxation processes in the sample with
S(t) attaining a maximum at the elapsed time very close to the wait time tw =
1000 sec, which is an unequivocal sign of glassy behavior. This age-dependent
effect convincingly establish the surface spin freezing of our LCMO
nanoparticles associated with a background of superparamagnetic (SPM) phase of
core moments.Comment: 41 pages, 10 figure
Born-Infeld black holes coupled to a massive scalar field
Born-Infeld black holes in the Scalar-Tensor Theories of Gravity, in the case
of massless scalar field, have been recently obtained. The aim of the current
paper is to study the effect from the inclusion of a potential for the scalar
field in the theory, through a combination of analytical techniques and
numerical methods. The black holes coupled to a massive scalar field have
richer causal structure in comparison to the massless scalar field case. In the
latter case, the black holes may have a second, inner horizon. The presence of
potential for the scalar field allows the existence of extremal black holes for
certain values of the mass of the scalar field and the magnetic (electric)
charge of the black hole. The linear stability against spherically symmetric
perturbations is studied. Arguments in favor of the general stability of the
solutions coming from the application of the "turning point" method are also
presented.Comment: 26 pages, 16 figure
A note on matrix model with IR cutoff and AdS/CFT
We propose an effective model of strongly coupled gauge theory at finite
temperature on in the presence of an infrared cutoff. It is constructed
by considering the theory on with an infrared cutoff and then taking the
size of the to infinity while keeping the cutoff fixed. This model
reproduces various qualitative features expected from its gravity dual.Comment: 27 pages, 9 figures, an appendix added, other minor changes, journal
versio
Giant magneto-impedance in Ag-doped La0.7Sr0.3MnO3
The resistive and reactive parts of the magneto-impedance of sintered
ferromagnetic samples of La0.7Sr0.3-xAgxMnO3 (x = 0.05, 0.25) have been
measured at room temperature (<Tc) over frequency interval 1KHz to 15MHz and in
presence of magnetic field up to 4KOe. The field dependence of relative change
in resistance is small in KHz region but increases strongly for higher
frequency of excitation. The maximum value of relative change in resistance at
H =4KOe was found to be around 70% at 15MHz frequency.On the contrary the
corresponding change in reactance has less frequency sensitivity and the
maximum occurs at 1MHz frequency. The magneto-impedance is negative for all
frequencies. The 'normalized magneto-impedance' as defined by
[Z(H)-Z(0)]/[Z(0)-Z(4K)] when plotted against scaled field H/H1/2 is found to
be frequency independent ; H1/2 is the field where 'normalized
magneto-impedance' is reduced to half its maximum. A phenomenological formula
for magneto-impedance Z (H) in ferromagnetic material is proposed based on Pade
approximant. The formula for Z (H) predicts the scaled behavior of 'normalized
magneto-impedance'.Comment: 26 pages,9 figure
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