69 research outputs found
Exact Eigenfunctions of a Chaotic System
The interest in the properties of quantum systems, whose classical dynamics
are chaotic, derives from their abundance in nature. The spectrum of such
systems can be related, in the semiclassical approximation (SCA), to the
unstable classical periodic orbits, through Gutzwiller's trace formula. The
class of systems studied in this work, tiling billiards on the pseudo-sphere,
is special in this correspondence being exact, via Selberg's trace formula. In
this work, an exact expression for Green's function (GF) and the eigenfunctions
(EF) of tiling billiards on the pseudo-sphere, whose classical dynamics are
chaotic, is derived. GF is shown to be equal to the quotient of two infinite
sums over periodic orbits, where the denominator is the spectral determinant.
Such a result is known to be true for typical chaotic systems, in the leading
SCA. From the exact expression for GF, individual EF can be identified. In
order to obtain a SCA by finite series for the infinite sums encountered,
resummation by analytic continuation in was performed. The result is
similar to known results for EF of typical chaotic systems. The lowest EF of
the Hamiltonian were calculated with the help of the resulting formulae, and
compared with exact numerical results. A search for scars with the help of
analytical and numerical methods failed to find evidence for their existence.Comment: 53 pages LaTeX, 10 Postscript figure
Viscous Decoupling Transitions for Individually Dragged Particles in Systems with Quenched Disorder
We show that when an individual particle is dragged through an assembly of
other particles in the presence of quenched disorder, a viscous decoupling
transition occurs between the dragged particle and the surrounding particles
which is controlled by the quenched disorder. A counterintuitive consequence of
this transition is that the velocity of the dragged particle can be increased
by increasing the strength or density of the quenched disorder. The decoupling
transition can also occur when the external drive on the dragged particle is
increased, and is observable as a clear signature in the velocity-force
response.Comment: 4 pages, 3 postscript figure
Controlled Manipulation of Individual Vortices in a Superconductor
We report controlled local manipulation of single vortices by low temperature
magnetic force microscope (MFM) in a thin film of superconducting Nb. We are
able to position the vortices in arbitrary configurations and to measure the
distribution of local depinning forces. This technique opens up new
possibilities for the characterization and use of vortices in superconductors
Four-point measurements of n- and p-type two-dimensional systems fabricated with cleaved-edge overgrowth
We demonstrate a contact design that allows four-terminal magnetotransport
measurements of cleaved-edge overgrown two-dimensional electron and hole
systems. By lithographically patterning and etching a bulk-doped surface layer,
finger-shaped leads are fabricated, which contact the two-dimensional systems
on the cleave facet. Both n- and p-type two-dimensional systems are
demonstrated at the cleaved edge, using Si as either donor or acceptor,
dependent on the growth conditions. Four-point measurements of both gated and
modulation-doped samples yield fractional quantum Hall features for both n- and
p-type, with several higher-order fractions evident in n-type modulation-doped
samples.Comment: 3 pages, 3 figure
Stray field signatures of N\'eel textured skyrmions in Ir/Fe/Co/Pt multilayer films
Skyrmions are nanoscale spin configurations with topological properties that
hold great promise for spintronic devices. Here, we establish their N\'eel
texture, helicity, and size in Ir/Fe/Co/Pt multilayer films by constructing a
multipole expansion to model their stray field signatures and applying it to
magnetic force microscopy (MFM) images. Furthermore, the demonstrated
sensitivity to inhomogeneity in skyrmion properties, coupled with a unique
capability to estimate the pinning force governing dynamics, portends broad
applicability in the burgeoning field of topological spin textures.Comment: 6 pages, 4 figures, significantly revised and upgraded. For the
updated supplementary material please contact one of the corresponding
author
Intersubband Electron Interaction in 1D-2D Junctions
We have shown that the electron transport through junctions of
one-dimensional and two-dimensional systems, as well as through quantum point
contacts, is considerably affected by the interaction of electrons of different
subbands. The interaction mechanism is caused by Friedel oscillations, which
are produced by electrons of the closed subbands even in smooth junctions.
Because of the interaction with these oscillations, electrons of the open
subbands experience a backscattering. The electron reflection coefficient,
which describes the backscattering, has a sharp peak at the energy equal to the
Fermi energy and may be as high as about 0.1. This result allows one to explain
a number of available experimental facts.Comment: 5 pages, 3 figure
Chiral magnetic textures in Ir/Fe/Co/Pt multilayers: Evolution and topological Hall signature
Skyrmions are topologically protected, two-dimensional, localized hedgehogs
and whorls of spin. Originally invented as a concept in field theory for
nuclear interactions, skyrmions are central to a wide range of phenomena in
condensed matter. Their realization at room temperature (RT) in magnetic
multilayers has generated considerable interest, fueled by technological
prospects and the access granted to fundamental questions. The interaction of
skyrmions with charge carriers gives rise to exotic electrodynamics, such as
the topological Hall effect (THE), the Hall response to an emergent magnetic
field, a manifestation of the skyrmion Berry-phase. The proposal that THE can
be used to detect skyrmions needs to be tested quantitatively. For that it is
imperative to develop comprehensive understanding of skyrmions and other chiral
textures, and their electrical fingerprint. Here, using Hall transport and
magnetic imaging, we track the evolution of magnetic textures and their THE
signature in a technologically viable multilayer film as a function of
temperature () and out-of-plane applied magnetic field (). We show that
topological Hall resistivity () scales with the density of
isolated skyrmions () over a wide range of , confirming the
impact of the skyrmion Berry-phase on electronic transport. We find that at
higher skyrmions cluster into worms which carry considerable
topological charge, unlike topologically-trivial spin spirals. While we
establish a qualitative agreement between and areal
density of topological charge , our detailed quantitative
analysis shows a much larger than the prevailing theory
predicts for observed .Comment: Major revision of the original version. The extensive Supplementary
Information is available upon reques
Spin-charge separation and localization in one-dimension
We report on measurements of quantum many-body modes in ballistic wires and
their dependence on Coulomb interactions, obtained from tunneling between two
parallel wires in a GaAs/AlGaAs heterostructure while varying electron density.
We observe two spin modes and one charge mode of the coupled wires, and map the
dispersion velocities of the modes down to a critical density, at which
spontaneous localization is observed. Theoretical calculations of the charge
velocity agree well with the data, although they also predict an additional
charge mode that is not observed. The measured spin velocity is found to be
smaller than theoretically predicted.Comment: There are minor textual differences between this version and the
version that has been published in Science (follow the DOI link below to
obtain it). In addition, here we have had to reduce figure quality to save
space on the serve
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