3,961 research outputs found
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
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
Aspects of the stochastic Burgers equation and their connection with turbulence
We present results for the 1 dimensional stochastically forced Burgers
equation when the spatial range of the forcing varies. As the range of forcing
moves from small scales to large scales, the system goes from a chaotic,
structureless state to a structured state dominated by shocks. This transition
takes place through an intermediate region where the system exhibits rich
multifractal behavior. This is mainly the region of interest to us. We only
mention in passing the hydrodynamic limit of forcing confined to large scales,
where much work has taken place since that of Polyakov.
In order to make the general framework clear, we give an introduction to
aspects of isotropic, homogeneous turbulence, a description of Kolmogorov
scaling, and, with the help of a simple model, an introduction to the language
of multifractality which is used to discuss intermittency corrections to
scaling.
We continue with a general discussion of the Burgers equation and forcing,
and some aspects of three dimensional turbulence where - because of the
mathematical analogy between equations derived from the Navier-Stokes and
Burgers equations - one can gain insight from the study of the simpler
stochastic Burgers equation. These aspects concern the connection of
dissipation rate intermittency exponents with those characterizing the
structure functions of the velocity field, and the dynamical behavior,
characterized by different time constants, of velocity structure functions. We
also show how the exponents characterizing the multifractal behavior of
velocity structure functions in the above mentioned transition region can
effectively be calculated in the case of the stochastic Burgers equation.Comment: 25 pages, 4 figure
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
Quantum Field Theory and Differential Geometry
We introduce the historical development and physical idea behind topological
Yang-Mills theory and explain how a physical framework describing subatomic
physics can be used as a tool to study differential geometry. Further, we
emphasize that this phenomenon demonstrates that the interrelation between
physics and mathematics have come into a new stage.Comment: 29 pages, enlarged version, some typewritten mistakes have been
corrected, the geometric descrition to BRST symmetry, the chain of descent
equations and its application in TYM as well as an introduction to R-symmetry
have been added, as required by mathematicia
Quantum criticality of U(1) gauge theories with fermionic and bosonic matter in two spatial dimensions
We consider relativistic U(1) gauge theories in 2+1 dimensions, with N_b
species of complex bosons and N_f species of Dirac fermions at finite
temperature. The quantum phase transition between the Higgs and Coulomb phases
is described by a conformal field theory (CFT). At large N_b and N_f, but for
arbitrary values of the ratio N_b/N_f, we present computations of various
critical exponents and universal amplitudes for these CFTs. We make contact
with the different spin-liquids, charge-liquids and deconfined critical points
of quantum magnets that these field theories describe. We compute physical
observables that may be measured in experiments or numerical simulations of
insulating and doped quantum magnets.Comment: 30 pages, 8 figure
Dynamics of Non-Abelian Vortices
The scattering is studied using moduli space metric for well-separated
vortices of non-Abelian vortices in (2+1)-dimensional U(N) gauge theories with
N Higgs fields in the fundamental representation. Unlike vortices in the
Abelian-Higgs model, dynamics of non-Abelian vortices has a lot of new
features; The kinetic energy in real space can be transfered to that of
internal orientational moduli and vice versa, the energy and charge transfer
between two vortices, the scattering angle of collisions with a fixed impact
parameter depends on the internal orientations, and some resonances appear due
to synchronization of the orientations. Scattering of dyonic non-Abelian
vortices in a mass deformed theory is also studied. We find a bound state of
two vortices moving along coils around a circle, like a loop of a phone code.Comment: 45 pages, 13 figure
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