679 research outputs found
Theta-terms in nonlinear sigma-models
We trace the origin of theta-terms in non-linear sigma-models as a
nonperturbative anomaly of current algebras. The non-linear sigma-models emerge
as a low energy limit of fermionic sigma-models. The latter describe Dirac
fermions coupled to chiral bosonic fields. We discuss the geometric phases in
three hierarchies of fermionic sigma-models in spacetime dimension (d+1) with
chiral bosonic fields taking values on d-, d+1-, and d+2-dimensional spheres.
The geometric phases in the first two hierarchies are theta-terms. We emphasize
a relation between theta-terms and quantum numbers of solitons.Comment: 10 pages, no figures, revtex, typos correcte
Dynamics of barrier penetration in thermal medium: exact result for inverted harmonic oscillator
Time evolution of quantum tunneling is studied when the tunneling system is
immersed in thermal medium. We analyze in detail the behavior of the system
after integrating out the environment. Exact result for the inverted harmonic
oscillator of the tunneling potential is derived and the barrier penetration
factor is explicitly worked out as a function of time. Quantum mechanical
formula without environment is modifed both by the potential renormalization
effect and by a dynamical factor which may appreciably differ from the
previously obtained one in the time range of 1/(curvature at the top of
potential barrier).Comment: 30 pages, LATEX file with 11 PS figure
Dynamics of viscoelastic membranes
We determine both the in-plane and out-of-plane dynamics of viscoelastic
membranes separating two viscous fluids in order to understand microrheological
studies of such membranes. We demonstrate the general viscoelastic signatures
in the dynamics of shear, bending, and compression modes. We also find a
screening of the otherwise two-dimensional character of the response to point
forces due to the presence of solvent. Finally, we show that there is a linear,
hydrodynamic coupling between the in-plane compression modes of the membrane
and the out-of-plane bending modes in the case where the membrane separates two
different fluids or environments
Quantum phase transitions from topology in momentum space
Many quantum condensed matter systems are strongly correlated and strongly
interacting fermionic systems, which cannot be treated perturbatively. However,
physics which emerges in the low-energy corner does not depend on the
complicated details of the system and is relatively simple. It is determined by
the nodes in the fermionic spectrum, which are protected by topology in
momentum space (in some cases, in combination with the vacuum symmetry). Close
to the nodes the behavior of the system becomes universal; and the universality
classes are determined by the toplogical invariants in momentum space. When one
changes the parameters of the system, the transitions are expected to occur
between the vacua with the same symmetry but which belong to different
universality classes. Different types of quantum phase transitions governed by
topology in momentum space are discussed in this Chapter. They involve Fermi
surfaces, Fermi points, Fermi lines, and also the topological transitions
between the fully gapped states. The consideration based on the momentum space
topology of the Green's function is general and is applicable to the vacua of
relativistic quantum fields. This is illustrated by the possible quantum phase
transition governed by topology of nodes in the spectrum of elementary
particles of Standard Model.Comment: 45 pages, 17 figures, 83 references, Chapter for the book "Quantum
Simulations via Analogues: From Phase Transitions to Black Holes", to appear
in Springer lecture notes in physics (LNP
Haldane's Fractional Exclusion Statistics for Multicomponent Systems
The idea of fractional exclusion statistics proposed by Haldane is applied to
systems with internal degrees of freedom, and its thermodynamics is examined.
In case of one dimension, various bulk quantities calculated show that the
critical behavior of such systems can be described by conformal field
theories and conformal weights are completely characterized by statistical
interactions. It is also found that statistical interactions have intimate
relationship with a topological order matrix in Chern-Simons theory for the
fractional quantum Hall effect.Comment: 12 pages, Revtex, preprint YITP/K-107
Graphene based superconducting quantum point contacts
We investigate the Josephson effect in the graphene nanoribbons of length
smaller than the superconducting coherence length and an arbitrary width .
We find that in contrast to an ordinary superconducting quantum point contact
(SQPC) the critical supercurrent is not quantized for the nanoribbons
with smooth and armchair edges. For a low concentration of the carriers
decreases monotonically with lowering and tends to a constant minimum for
a narrow nanoribbon with . The minimum is zero for the
smooth edges but for the armchair edges. At higher
concentrations of the carriers this monotonic variation acquires a series of
peaks. Further analysis of the current-phase relation and the Josephson
coupling strength in terms of and the concentration of carriers
revels significant differences with those of an ordinary SQPC. On the other
hand for a zigzag nanoribbon we find that, similar to an ordinary SQPC,
is quantized but to the half-integer values .Comment: 8 pages, 5 figure
Magnetotunneling spectroscopy of mesoscopic correlations in two-dimensional electron systems
An approach to experimentally exploring electronic correlation functions in
mesoscopic regimes is proposed. The idea is to monitor the mesoscopic
fluctuations of a tunneling current flowing between the two layers of a
semiconductor double-quantum-well structure. From the dependence of these
fluctuations on external parameters, such as in-plane or perpendicular magnetic
fields, external bias voltages, etc., the temporal and spatial dependence of
various prominent correlation functions of mesoscopic physics can be
determined. Due to the absence of spatially localized external probes, the
method provides a way to explore the interplay of interaction and localization
effects in two-dimensional systems within a relatively unperturbed environment.
We describe the theoretical background of the approach and quantitatively
discuss the behavior of the current fluctuations in diffusive and ergodic
regimes. The influence of both various interaction mechanisms and localization
effects on the current is discussed. Finally a proposal is made on how, at
least in principle, the method may be used to experimentally determine the
relevant critical exponents of localization-delocalization transitions.Comment: 15 pages, 3 figures include
Conductance of Mesoscopic Systems with Magnetic Impurities
We investigate the combined effects of magnetic impurities and applied
magnetic field on the interference contribution to the conductance of
disordered metals. We show that in a metal with weak spin-orbit interaction,
the polarization of impurity spins reduces the rate of electron phase
relaxation, thus enhancing the weak localization correction to conductivity.
Magnetic field also suppresses thermal fluctuations of magnetic impurities,
leading to a recovery of the conductance fluctuations. This recovery occurs
regardless the strength of the spin-orbit interaction. We calculate the
magnitudes of the weak localization correction and of the mesoscopic
conductance fluctuations at an arbitrary level of the spin polarization induced
by a magnetic field. Our analytical results for the ``'' Aharonov-Bohm
conductance oscillations in metal rings can be used to extract spin and
gyromagnetic factor of magnetic impurities from existing experimental data.Comment: 18 pages, 8 figure
Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors
It is predicted that certain atomically ordered interfaces between some
ferromagnetic metals (F) and semiconductors (S) should act as ideal spin
filters that transmit electrons only from the majority spin bands or only from
the minority spin bands of the F to the S at the Fermi energy, even for F with
both majority and minority bands at the Fermi level. Criteria for determining
which combinations of F, S and interface should be ideal spin filters are
formulated. The criteria depend only on the bulk band structures of the S and F
and on the translational symmetries of the S, F and interface. Several examples
of systems that meet these criteria to a high degree of precision are
identified. Disordered interfaces between F and S are also studied and it is
found that intermixing between the S and F can result in interfaces with spin
anti-filtering properties, the transmitted electrons being much less spin
polarized than those in the ferromagnetic metal at the Fermi energy. A patent
application based on this work has been commenced by Simon Fraser University.Comment: RevTeX, 12 pages, 5 figure
Computational Physics on Graphics Processing Units
The use of graphics processing units for scientific computations is an
emerging strategy that can significantly speed up various different algorithms.
In this review, we discuss advances made in the field of computational physics,
focusing on classical molecular dynamics, and on quantum simulations for
electronic structure calculations using the density functional theory, wave
function techniques, and quantum field theory.Comment: Proceedings of the 11th International Conference, PARA 2012,
Helsinki, Finland, June 10-13, 201
- âŠ