11,024 research outputs found
Effective theory for the propagation of a wave-packet in a disordered and nonlinear medium
The propagation of a wave-packet in a nonlinear disordered medium exhibits
interesting dynamics. Here, we present an analysis based on the nonlinear
Schr\"odinger equation (Gross-Pitaevskii equation). This problem is directly
connected to experiments on expanding Bose gases and to studies of transverse
localization in nonlinear optical media. In a nonlinear medium the energy of
the wave-packet is stored both in the kinetic and potential parts, and details
of its propagation are to a large extent determined by the transfer from one
form of energy to the other. A theory describing the evolution of the
wave-packet has been formulated in [G. Schwiete and A. Finkelstein, Phys. Rev.
Lett. 104, 103904 (2010)] in terms of a nonlinear kinetic equation. In this
paper, we present details of the derivation of the kinetic equation and of its
analysis. As an important new ingredient we study interparticle-collisions
induced by the nonlinearity and derive the corresponding collision integral. We
restrict ourselves to the weakly nonlinear limit, for which disorder scattering
is the dominant scattering mechanism. We find that in the special case of a
white noise impurity potential the mean squared radius in a two-dimensional
system scales linearly with t. This result has previously been obtained in the
collisionless limit, but it also holds in the presence of collisions. Finally,
we mention different mechanisms through which the nonlinearity may influence
localization of the expanding wave-packet.Comment: 21 pages, 10 figure
Ferromagnetism of Weakly-Interacting Electrons in Disordered Systems
It was realized two decades ago that the two-dimensional diffusive Fermi
liquid phase is unstable against arbitrarily weak electron-electron
interactions. Recently, using the nonlinear sigma model developed by
Finkelstein, several authors have shown that the instability leads to a
ferromagnetic state. In this paper, we consider diffusing electrons interacting
through a ferromagnetic exchange interaction. Using the Hartree-Fock
approximation to directly calculate the electron self energy, we find that the
total energy is minimized by a finite ferromagnetic moment for arbitrarily weak
interactions in two dimensions and for interaction strengths exceeding a
critical proportional to the conductivity in three dimensions. We discuss the
relation between our results and previous ones
Consistency analysis of Kaluza-Klein geometric sigma models
Geometric sigma models are purely geometric theories of scalar fields coupled
to gravity. Geometrically, these scalars represent the very coordinates of
space-time, and, as such, can be gauged away. A particular theory is built over
a given metric field configuration which becomes the vacuum of the theory.
Kaluza-Klein theories of the kind have been shown to be free of the classical
cosmological constant problem, and to give massless gauge fields after
dimensional reduction. In this paper, the consistency of dimensional reduction,
as well as the stability of the internal excitations, are analyzed. Choosing
the internal space in the form of a group manifold, one meets no
inconsistencies in the dimensional reduction procedure. As an example, the
SO(n) groups are analyzed, with the result that the mass matrix of the internal
excitations necessarily possesses negative modes. In the case of coset spaces,
the consistency of dimensional reduction rules out all but the stable mode,
although the full vacuum stability remains an open problem.Comment: 13 pages, RevTe
Light environment - A. Visible light. B. Ultraviolet light
Visible and ultraviolet light environment as related to human performance and safety during space mission
Are Bosonic Replicas Faulty?
Motivated by the ongoing discussion about a seeming asymmetry in the
performance of fermionic and bosonic replicas, we present an exact,
nonperturbative approach to zero-dimensional replica field theories belonging
to the broadly interpreted "beta=2" Dyson symmetry class. We then utilise the
formalism developed to demonstrate that the bosonic replicas do correctly
reproduce the microscopic spectral density in the QCD inspired chiral Gaussian
unitary ensemble. This disproves the myth that the bosonic replica field
theories are intrinsically faulty.Comment: 4.3 pages; final version to appear in PR
Universal Description of Granular Metals at Low Temperatures: Granular Fermi Liquid
We present a unified description of the low temperature phase of granular
metals that reveals a striking generality of the low temperature behaviors. Our
model explains the universality of the low-temperature conductivity that
coincides exactly with that of the homogeneously disordered systems and enables
a straightforward derivation of low temperature characteristics of disordered
conductors.Comment: 4 pages, 1 figur
Renormalization of hole-hole interaction at decreasing Drude conductivity
The diffusion contribution of the hole-hole interaction to the conductivity
is analyzed in gated GaAs/InGaAs/GaAs heterostructures. We show
that the change of the interaction correction to the conductivity with the
decreasing Drude conductivity results both from the compensation of the singlet
and triplet channels and from the arising prefactor in the
conventional expression for the interaction correction.Comment: 6 pages, 5 figure
Development and Validation of the Colorado Learning Attitudes about Science Survey for Experimental Physics
As part of a comprehensive effort to transform our undergraduate physics
laboratories and evaluate the impacts of these efforts, we have developed the
Colorado Learning Attitudes about Science Survey for Experimental Physics
(E-CLASS). The E-CLASS assesses the changes in students' attitudes about a
variety of scientific laboratory practices before and after a lab course and
compares attitudes with perceptions of the course grading requirements and
laboratory practices. The E-CLASS is designed to give researchers insight into
students' attitudes and also to provide actionable evidence to instructors
looking for feedback on their courses. We present the development, validation,
and preliminary results from the initial implementation of the survey in three
undergraduate physics lab courses.Comment: 8 pages, 4 figures, 1 table, submitted to 2012 PERC Proceeding
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