9,449 research outputs found
All order covariant tubular expansion
We consider tubular neighborhood of an arbitrary submanifold embedded in a
(pseudo-)Riemannian manifold. This can be described by Fermi normal coordinates
(FNC) satisfying certain conditions as described by Florides and Synge in
\cite{FS}. By generalizing the work of Muller {\it et al} in \cite{muller} on
Riemann normal coordinate expansion, we derive all order FNC expansion of
vielbein in this neighborhood with closed form expressions for the curvature
expansion coefficients. Our result is shown to be consistent with certain
integral theorem for the metric proved in \cite{FS}.Comment: 27 pages. Corrected an error in a class of coefficients resulting
from a typo. Integral theorem and all other results remain unchange
Superstrings in type IIB R-R plane-wave in semi-light-cone gauge and conformal invariance
We reconsider the analysis done by Kazama and Yokoi in arXiv:0801.1561
(hep-th). We find that although the right vacuum of the theory is the one
associated to massless normal ordering (MNO), phase space normal ordering (PNO)
plays crucial role in the analysis in the following way. While defining the
quantum energy-momentum (EM) tensor one needs to take into account the field
redefinition relating the space-time field and the corresponding world-sheet
coupling. We argue that for a simple off-shell ansatz for the background this
field redefinition can be taken to be identity if the interaction term is
ordered according to PNO. This definition reproduces the correct physical
spectrum when the background is on-shell. We further show that the right way to
extract the effective equation of motion from the Virasoro anomaly is to first
order the anomaly terms according to PNO at a finite regularization parameter
\eps and then take the \eps \to 0 limit. This prescription fixes an
ambiguity in taking the limit for certain bosonic and fermionic contributions
to the Virasoro anomaly and is the natural one to consider given the above
definition of the EM tensor.Comment: 22 page
DeWitt-Virasoro construction
We study a particular approach for analyzing worldsheet conformal invariance
for bosonic string propagating in a curved background using hamiltonian
formalism. We work in the Schrodinger picture of a single particle description
of the problem where the particle moves in an infinite-dimensional space.
Background independence is maintained in this approach by adopting DeWitt's
(Phys.Rev.85:653-661,1952) coordinate independent formulation of quantum
mechanics. This enables us to construct certain background independent notion
of Virasoro generators, called DeWitt-Virasoro (DWV) generators, and invariant
matrix elements of an arbitrary operator constructed out of them in spin-zero
representation. We show that the DWV algebra is given by the Witt algebra with
additional anomalous terms that vanish for Ricci-flat backgrounds. The actual
quantum Virasoro generators should be obtained by first introducing the vacuum
state and then normal ordering the DWV generators with respect to that. We
demonstrate the procedure in the simple cases of flat and pp-wave backgrounds.
This is a shorter version of arXiv:0912.3987 [hep-th] with many technical
derivations omitted.Comment: 18 pages, shorter version of arXiv:0912.3987 [hep-th] accepted for
publication in Pramana - Journal of Physic
The role of critical current on point contact Andreev Reflection spectrum between a normal metal and a superconductor
The point contact spectrum between a normal metal and a superconductor often
shows unexpected sharp dips in the conductance at voltage values larger than
the superconducting energy gap. These dips are not predicted in the
Blonder-Tinkham-Klapwizk (BTK) theory, commonly used to analyse these contacts.
We present here a systematic study of these dips in a variety of contacts
between different combinations of a superconductor and a normal metal. From the
correlation between the characteristics of these dips with the contact area, we
can surmise that such dips are caused by the contact not being in the ballistic
limit. An analysis of the possible errors introduced while analysing such a
spectrum with the standard BTK model is also presented.Comment: 16 pages, PS, figure include
Magnetism in Transition metal doped Cubic SiC
We report here our study on SiC doped with transition metals using first
principle density functional theory calculations. We have considered cubic SiC
with 3d transition metals as substitutional impurities for Si and C site
separately. Cubic SiC doped with Cr, Mn, show ferromagnetism whereas with Sc,
Ti, V and Co show site dependency of magnetic properties. Rests of the
impurities are found to be non-magnetic.Comment: Presented in the 55th DAE-Solid State Physics Symposium, 26th to 30th
December, 2010, Manipal University, Manipal, India; AIP Conf. Proc. 1349,
1087-1088 (2011
Dynamic elastic properties and magnetic susceptibility across the austenite-martensite transformation in site-disordered ferromagnetic Ni-Fe-Al alloy
Besides permitting an accurate determination of the
ferromagnetic-to-paramagnetic phase transition temperature and the
characteristic temperatures for the beginning and end of the growth of
martensite (austenite) phase at the expense of austenite (martensite) phase
while cooling (heating), the results of an extensive ac susceptibility, sound
velocity and internal friction investigation of the thermoelastic martensitic
transformation in melt-quenched (site-disordered) Ni55Fe20Al25 alloy provide a
clear experimental evidence for the following. Irreversible thermoelastic
changes (thermal hysteresis) occur in the austenite phase in the premartensitic
regime. In the heating cycle, the system retains the "memory" of the initiation
and subsequent growth of the martensitic phase (at the expense of the parent
austenite phase) that had taken place during the cooling cycle in the
austenite-martensite phase coexistence region. We report and discuss these
novel findings in this communication.Comment: 5 figure
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