20,315 research outputs found

### Characteristic length of an AdS/CFT superconductor

We investigate in more detail the holographic model of a superconductor
recently found by Hartnoll, Herzog, and Horowitz [Phys. Rev. Lett. 101,
031601], which is constructed from a condensate of a charged scalar field in
AdS_4-Schwarzschild background. By analytically studying the perturbation of
the gravitational system near the critical temperature T_c, we obtain the
superconducting coherence length proportional to 1/\sqrt{1-T/T_c} via AdS/CFT
correspondence. By adding a small external homogeneous magnetic field to the
system, we find that a stationary diamagnetic current proportional to the
square of the order parameter is induced by the magnetic field. These results
agree with Ginzburg-Landau theory and strongly support the idea that a
superconductor can be described by a charged scalar field on a black hole via
AdS/CFT duality.Comment: 9 pages, no figure; v2: typos corrected; v3: version to appear in
PRD, an early discussion based on convensional superconductor with dynamical
photon removed and an argument about the type of the holographic
superconductor adde

### Mapping the current flow in sacral nerve stimulation using computational modelling

© The Institution of Engineering and Technology 2019Sacral nerve stimulation (SNS) is an established treatment for faecal incontinence involving the implantation of a quadripolar electrode into a sacral foramen, through which an electrical stimulus is applied. Little is known about the induced spread of electric current around the SNS electrode and its effect on adjacent tissues, which limits optimisation of this treatment. The authors constructed a 3-dimensional imaging based finite element model in order to calculate and visualise the stimulation induced current and coupled this to biophysical models of nerve fibres. They investigated the impact of tissue inhomogeneity, electrode model choice and contact configuration and found a number of effects. (i) The presence of anatomical detail changes the estimate of stimulation effects in size and shape. (ii) The difference between the two models of electrodes is minimal for electrode contacts of the same length. (iii) Surprisingly, in this arrangement of electrode and neural fibre, monopolar and bipolar stimulation induce a similar effect. (iv) Interestingly when the active contact is larger, the volume of tissue activated reduces. This work establishes a protocol to better understand both therapeutic and adverse stimulation effects and in the future will enable patient-specific adjustments of stimulation parameters.Peer reviewe

### Origin of matter out of pure curvature

We propose a mechanism for origin of matter in the universe in the framework
of Einstein-Gauss-Bonnet gravity in higher dimensions. The recently discovered
new static black hole solution by the authors \cite{md2006} with the
Kaluza-Klein split up of spacetime as a product of the usual {\ma M}^4 with a
space of negative constant curvature is indeed a pure gravitational creation of
a black hole which is also endowed with a Maxwell-like {\it gravitational
charge} in four-dimensional vacuum spacetime. Further it could be envisioned as
being formed from anti-de Sitter spacetime by collapse of radially inflowing
charged null dust. It thus establishes the remarkable reciprocity between
matter and gravity - as matter produces gravity (curvature), gravity too
produces matter.Comment: 8 pages, 1 Fig, Received Honorable Mention in 2007 GRF Essay
Competition, Summary of the talk given at Himalayan Relativity Dialogue at
Mirik, April 18-20, 200

### Kaluza-Klein black hole with negatively curved extra dimensions in string generated gravity models

We obtain a new exact black-hole solution in Einstein-Gauss-Bonnet gravity
with a cosmological constant which bears a specific relation to the
Gauss-Bonnet coupling constant. The spacetime is a product of the usual
4-dimensional manifold with a $(n-4)$-dimensional space of constant negative
curvature, i.e., its topology is locally {\ma M}^n \approx {\ma M}^4 \times
{\ma H}^{n-4}. The solution has two parameters and asymptotically approximates
to the field of a charged black hole in anti-de Sitter spacetime. The most
interesting and remarkable feature is that the Gauss-Bonnet term acts like a
Maxwell source for large $r$ while at the other end it regularizes the metric
and weakens the central singularity.Comment: 4 pages, 2 figures, final version to appear in Physical Review D as a
rapid communicatio

### Mirror effect induced by the dilaton field on the Hawking radiation

We discuss the string creation in the near-extremal NS1 black string
solution. The string creation is described by an effective field equation
derived from a fundamental string action coupled to the dilaton field in a
conformally invariant manner. In the non-critical string model the dilaton
field causes a timelike mirror surface outside the horizon when the size of the
black string is comparable to the Planck scale. Since the fundamental strings
are reflected by the mirror surface, the negative energy flux does not
propagate across the surface. This means that the evaporation stops just before
the naked singularity of the extremal black string appears even though the
surface gravity is non-zero in the extremal limit.Comment: 15 page

### Nebular Spectra and Explosion Asymmetry of Type Ia Supernovae

The spectral signatures of asymmetry in Type Ia Supernova (SN Ia) explosions
are investigated, using a sample of late-time nebular spectra. First, a
kinematical model is constructed for SN Ia 2003hv, which can account for the
main features in its optical, Near-Infrared (NIR), and Mid-Infrared (Mid-IR)
late-time spectra. It is found that an asymmetric off-center model can explain
the observed characteristics of SN 2003hv. This model includes a relatively
high density, Fe-rich region which displays a large velocity off-set, and a
relatively low density, extended 56Ni-rich region which is more spherically
distributed. The high density region consists of the inner stable Fe-Ni region
and outer 56Ni-rich region. Such a distribution may be the result of a
delayed-detonation explosion, in which the first deflagration produces the
global asymmetry in the innermost ejecta, while the subsequent detonation can
lead to the bulk spherical symmetry. This configuration, if viewed from the
direction of the off-set, can consistently explain the blueshift in some of the
emission lines and virtually no observed shift in other lines in SN 2003hv. For
this model, we then explore the effects of different viewing angles and the
implications for SNe Ia in general. The model predicts that a variation of the
central wavelength, depending on the viewing angle, should be seen in some
lines (e.g., [Ni II]7378), while the strongest lines (e.g., [Fe III] blend at
4700A) will not show this effect. By examining optical nebular spectra of 12
SNe Ia, we have found that such a variation indeed exists. We suggest that the
global asymmetry in the innermost ejecta, as likely imprint of the deflagration
flame propagation, is a generic feature of SNe Ia (abridged).Comment: 14 pages, 11 figures, 4 tables. Accepted for publication in the
Astrophysical Journal. Minor correction

### Domain Wall Dynamics in Brane World and Non-singular Cosmological Models

We study brane cosmology as 4D (4-dimensional) domain wall dynamics in 5D
bulk spacetime. For a generic 5D bulk with 3D maximal symmetry, we derive the
equation of motion of a domain wall and find that it depends on mass function
of the bulk spacetime and the energy-momentum conservation in a domain wall is
affected by a lapse function in the bulk. Especially, for a bulk spacetime with
non-trivial lapse function, energy of matter field on the domain wall goes out
or comes in from the bulk spacetime. Applying our result to the case with SU(2)
gauge bulk field, we obtain a singularity-free universe in brane world
scenario, that is, not only a big bang initial singularity of the brane is
avoided but also a singularity in a 5D bulk does not exist.Comment: 12 pages, 11 figures, submitted to PRD. One reference is added. (v2

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