14,499 research outputs found
Selective excitation of homogeneous spectral lines
It is possible, for homogeneously broadened lines, to excite selectively the
response signals, which are orders of magnitude narrower than the original
lines. The new type of echo, which allows detecting such signals, and the
formalism, useful for understanding the phenomenon, as well as the experimental
examples from NMR spectroscopy are presented.Comment: 19 pages, 8 figure
Hot Electron Effects in the 2D Superconductor-Insulator Transition
The parallel magnetic field tuned two-dimensional superconductor-insulator
transition has been investigated in ultrathin films of amorphous Bi. The
resistance is found to be independent of temperature on both sides of the
transition below approximately 120 mK. Several observations suggest that this
regime is not intrinsically "metallic" but results from the failure of the
films' electrons to cool. The onset of this temperature-independent regime can
be moved to higher temperatures by either increasing the measuring current or
the level of electromagnetic noise. Temperature scaling is successful above 120
mK. Electric field scaling can be mapped onto temperature scaling by relating
the electric fields to elevated electron temperatures. These results cast doubt
on the existence of an intrinsic metallic regime and on the independent
determination of the correlation length and dynamical critical exponents
obtained by combining the results of electric field and temperature scaling.Comment: 4 pages, 4 figure
Electrostatic Tuning of the Superconductor-Insulator Transition in Two Dimensions
Superconductivity has been induced in insulating ultra-thin films of
amorphous bismuth using the electric field effect. The screening of
electron-electron interaction was found to increase with electron concentration
in a manner correlated with the tendency towards superconductivity. This does
not preclude an increase in the density of states being important in the
development of superconductivity. The superconductor-insulator transition
appears to belong to the universality class of the three dimensional XY model.Comment: Four pages, three figures. Revised slightly to reflect referees'
comment
Supporting Pluralism by Artificial Intelligence: Conceptualizing Epistemic Disagreements as Digital Artifacts
A crucial concept in philosophy and social sciences, epistemic disagreement, has not yet been adequately reflected in the Web. In this paper, we call for development of intelligent tools dealing with epistemic disagreements on the Web to support pluralism. As a first step, we present Polyphony, an ontology for representing and annotating epistemic disagreements
Effective One-Dimensional Coupling in the Highly-Frustrated Square-Lattice Itinerant Magnet CaCoAs
Inelastic neutron scattering measurements on the itinerant antiferromagnet
(AFM) CaCoAs at a temperature of 8 K reveal two
orthogonal planes of scattering perpendicular to the Co square lattice in
reciprocal space, demonstrating the presence of effective one-dimensional spin
interactions. These results are shown to arise from near-perfect bond
frustration within the - Heisenberg model on a square lattice with
ferromagnetic , and hence indicate that the extensive previous
experimental and theoretical study of the - Heisenberg model on
local-moment square spin lattices should be expanded to include itinerant spin
systems
Radiative polarization of electrons in a strong laser wave
We reanalyze the problem of radiative polarization of electrons brought into
collision with a circularly polarized strong plane wave. We present an
independent analytical verification of formulae for the cross section given by
D.\,Yu. Ivanov et al [Eur.\ Phys.\ J. C \textbf{36}, 127 (2004)]. By choosing
the exact electron's helicity as the spin quantum number we show that the
self-polarization effect exists only for the moderately relativistic electrons
with energy and only for a non-head-on collision
geometry. In these conditions polarization degree may achieve the values up to
65%, but the effective polarization time is found to be larger than 1\,s even
for a high power optical or infrared laser with intensity parameter (). This
makes such a polarization practically unrealizable. We also compare these
results with the ones of some papers where the high degree of polarization was
predicted for ultrarelativistic case. We argue that this apparent contradiction
arises due to the different choice of the spin quantum numbers. In particular,
the quantum numbers which provide the high polarization degree represent
neither helicity nor transverse polarization, that makes the use of them
inconvenient in practice.Comment: minor changes compared to v3; to appear in PR
Engineering Time-Reversal Invariant Topological Insulators With Ultra-Cold Atoms
Topological insulators are a broad class of unconventional materials that are
insulating in the interior but conduct along the edges. This edge transport is
topologically protected and dissipationless. Until recently, all existing
topological insulators, known as quantum Hall states, violated time-reversal
symmetry. However, the discovery of the quantum spin Hall effect demonstrated
the existence of novel topological states not rooted in time-reversal
violations. Here, we lay out an experiment to realize time-reversal topological
insulators in ultra-cold atomic gases subjected to synthetic gauge fields in
the near-field of an atom-chip. In particular, we introduce a feasible scheme
to engineer sharp boundaries where the "edge states" are localized. Besides,
this multi-band system has a large parameter space exhibiting a variety of
quantum phase transitions between topological and normal insulating phases. Due
to their unprecedented controllability, cold-atom systems are ideally suited to
realize topological states of matter and drive the development of topological
quantum computing.Comment: 11 pages, 6 figure
- …