4,134 research outputs found
Dimensionless Coupling of Superstrings to Supersymmetric Gauge Theories and Scale Invariant Superstring Actions
We construct new superstring actions which are distinguished from standard
superstrings by being space-time scale invariant. Like standard superstrings,
they are also reparametrization invariant, space-time supersymmetric, and
invariant under local scale transformations of the world sheet. We discuss
scenarios in which these actions could play a significant role, in particular
one which involves their coupling to supersymmetric gauge theories.Comment: 9 pages, LaTe
Nonadiabatic scattering of a quantum particle in an inhomogenous magnetic field
We investigate the quantum effects, in particular the Landau-level
quantization, in the scattering of a particle the nonadiabatic classical
dynamics of which is governed by an adiabatic invariant. As a relevant example,
we study the scattering of a drifting particle on a magnetic barrier in the
quantum limit where the cyclotron energy is much larger than a broadening of
the Landau levels induced by the nonadiabatic transitions. We find that,
despite the level quantization, the exponential suppression (barrier width , orbital shift per cyclotron revolution )
of the root-mean-square transverse displacement experienced by the particle
after the scattering is the same in the quantum and the classical regime.Comment: 4 page
On the validity of ADM formulation in 2D quantum gravity
We investigate 2d gravity quantized in the ADM formulation, where only the
loop length is retained as a dynamical variable of the gravitation, in
order to get an intuitive physical insight of the theory. The effective action
of is calculated by adding scalar fields of conformal coupling, and the
problems of the critical dimension and the time development of are
addressed.Comment: 12 page
Effects of Strain coupling and Marginal dimensionality in the nature of phase transition in Quantum paraelectrics
Here a recently observed weak first order transition in doped SrTiO3 is
argued to be a consequence of the coupling between strain and order parameter
fluctuations. Starting with a semi-microscopic action, and using
renormalization group equations for vertices, we write the free energy of such
a system. This fluctuation renormalized free energy is then used to discuss the
possibility of first order transition at zero temperature as well as at finite
temperature. An asymptotic analysis predicts small but a finite discontinuity
in the order parameter near a mean field quantum critical point at zero
temperature. In case of finite temperature transition, near quantum critical
point such a possibility is found to be extremely weak. Results are in accord
with some experimental findings on quantum paraelectrics such as SrTiO3 and
KTaO3.Comment: Revised versio
Zero-frequency anomaly in quasiclassical ac transport: Memory effects in a two-dimensional metal with a long-range random potential or random magnetic field
We study the low-frequency behavior of the {\it ac} conductivity
of a two-dimensional fermion gas subject to a smooth random
potential (RP) or random magnetic field (RMF). We find a non-analytic
correction to , which corresponds to a
long-time tail in the velocity correlation function. This contribution
is induced by return processes neglected in Boltzmann transport theory. The
prefactor of this -term is positive and proportional to for
RP, while it is of opposite sign and proportional to in the weak RMF
case, where is the mean free path and the disorder correlation length.
This non-analytic correction also exists in the strong RMF regime, when the
transport is of a percolating nature. The analytical results are supported and
complemented by numerical simulations.Comment: 12 pages, RevTeX, 7 figure
Stability of the U(1) spin liquid with spinon Fermi surface in 2+1 dimensions
We study the stability of the 2+1 dimensional U(1) spin liquid state against
proliferation of instantons in the presence of spinon Fermi surface. By mapping
the spinon Fermi surface into an infinite set of 1+1 dimensional chiral
fermions, it is argued that an instanton has an infinite scaling dimension for
any nonzero number of spinon flavors. Therefore, the spin liquid phase is
stable against instantons and the non-compact U(1) gauge theory is a good low
energy description.Comment: 14 pages, 7 figures, v3) minor corrections, to appear in PR
Strong magnetoresistance induced by long-range disorder
We calculate the semiclassical magnetoresistivity of
non-interacting fermions in two dimensions moving in a weak and smoothly
varying random potential or random magnetic field. We demonstrate that in a
broad range of magnetic fields the non-Markovian character of the transport
leads to a strong positive magnetoresistance. The effect is especially
pronounced in the case of a random magnetic field where becomes
parametrically much larger than its B=0 value.Comment: REVTEX, 4 pages, 2 eps figure
Laminar boundary layer in conditions of natural transition to turbulent flow
Results of experimental study of regularities of a natural transition of a laminar boundary layer to a turbulent layer at low subsonic air flow velocities are presented, analyzed and compared with theory and model experiments
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