1,360,199 research outputs found
Landau versus Spin Superfluidity in Spinor Bose-Einstein Condensates
We consider a spin-1/2 Bose-Einstein condensate prepared initially in a
single spin projection. The two channels of excitations existing in such a
system (namely density and spin waves) are discussed and we show how pure spin
waves can be excited in the presence of local magnetic defects. We analyze the
role played by spin excitations on the Landau superfluidity criterion and
demonstrate the absence of absolute superfluidity for the antiferromagnetic
condensate. In the ferromagnetic case, we identify two critical velocities for
the breakdown of superfluidity.Comment: 5 pages, 3 figure
Non-Abelian Gauge Fields in Photonic Cavities and Photonic Superfluids
We show that the interplay between the structure anisotropy and the energy
splitting between the TE and TM modes of a microcavity leads to the appearance
of a gauge field for a propagating polariton condensate. This field is analog
with a spin-orbit coupling and the field texture can be tuned by rotating the
sample and ranges continuously from a Rashba to a monopolar field. In the
linear regime, the monopolar field leads to a remarkable focusing effect. In
the interacting regime, the effective spin-orbit coupling induces a breakdown
of superfluidity. The spatially homogeneous flows become unstable and
dynamically evolve into textured ground states such as stripes and domain
walls.Comment: 5 pages, 5 figures (Phys. Rev. Lett., in press
Gauge Transformations in Quantum Mechanics and the Unification of Nonlinear Schr\"odinger Equations
Beginning with ordinary quantum mechanics for spinless particles, together
with the hypothesis that all experimental measurements consist of positional
measurements at different times, we characterize directly a class of nonlinear
quantum theories physically equivalent to linear quantum mechanics through
nonlinear gauge transformations. We show that under two physically-motivated
assumptions, these transformations are uniquely determined: they are exactly
the group of time-dependent, nonlinear gauge transformations introduced
previously for a family of nonlinear Schr\"odinger equations. The general
equation in this family, including terms considered by Kostin, by
Bialynicki-Birula and Mycielski, and by Doebner and Goldin, with time-dependent
coefficients, can be obtained from the linear Schr\"odinger equation through
gauge transformation and a subsequent process we call gauge generalization. We
thus unify, on fundamental grounds, a rather diverse set of nonlinear
time-evolutions in quantum mechanics.Comment: RevTeX, 20 pages, no figures. also available on
http://www.pt.tu-clausthal.de/preprints/asi-tpa/021-96.htm
On the Reconstructed Fermi Surface in the Underdoped Cuprates
The Fermi surface topologies of underdoped samples the high-Tc superconductor
Bi2212 have been measured with angle resolved photoemission. By examining
thermally excited states above the Fermi level, we show that the Fermi surfaces
in the pseudogap phase of underdoped samples are actually composed of fully
enclosed hole pockets. The spectral weight of these pockets is vanishingly
small at the anti-ferromagnetic zone boundary, which creates the illusion of
Fermi "arcs" in standard photoemission measurements. The area of the pockets as
measured in this study is consistent with the doping level, and hence carrier
density, of the samples measured. Furthermore, the shape and area of the
pockets is well reproduced by a phenomenological model of the pseudogap phase
as a spin liquid.Comment: 4 pages, 4 figures. Submitted to Physics Review Letter
The Faraday effect revisited: Thermodynamic limit
This paper is the second in a series revisiting the (effect of) Faraday
rotation. We formulate and prove the thermodynamic limit for the transverse
electric conductivity of Bloch electrons, as well as for the Verdet constant.
The main mathematical tool is a regularized magnetic and geometric
perturbation theory combined with elliptic regularity and Agmon-Combes-Thomas
uniform exponential decay estimates.Comment: 35 pages, accepted for publication in Journal of Functional Analysi
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