2,685 research outputs found
Ferromagnetic order of nuclear spins coupled to conduction electrons: a combined effect of the electron-electron and spin-orbit interactions
We analyze the ordered state of nuclear spins embedded in an interacting
two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI).
Stability of the ferromagnetic nuclear-spin phase is governed by nonanalytic
dependences of the electron spin susceptibility on the momentum
() and on the SOI coupling constant (). The uniform
(\tq=0) spin susceptibility is anisotropic (with the out-of-plane component,
, being larger than the in-plane one, , by a term
proportional to , where is the electron-electron
interaction). For \tq \leq 2m^*|\alpha|, corrections to the leading,
, term scale linearly with \tq for and are
absent for . This anisotropy has important consequences for the
ferromagnetic nuclear-spin phase: the ordered state--if achieved--is of
an Ising type and the spin-wave dispersion is gapped at \tq=0. To
second order in , the dispersion a decreasing function of \tq, and
anisotropy is not sufficient to stabilize long-range order. However,
renormalization in the Cooper channel for \tq\ll2m^*|\alpha| is capable of
reversing the sign of the \tq-dependence of and thus stabilizing
the ordered state. We also show that a combination of the electron-electron and
SO interactions leads to a new effect: long-wavelength Friedel oscillations in
the spin (but not charge) electron density induced by local magnetic moments.
The period of these oscillations is given by the SO length .Comment: 22 pages, 15 figure
Spin susceptibility of interacting two-dimensional electrons in the presence of spin-orbit coupling
A long-range interaction via virtual particle-hole pairs between Fermi-liquid
quasiparticles leads to a nonanalytic behavior of the spin susceptibility
as a function of the temperature (), magnetic field (),
and wavenumber. In this paper, we study the effect of the Rashba spin-orbit
interaction (SOI) on the nonanalytic behavior of for a two-dimensional
electron liquid. Although the SOI breaks the SU(2) symmetry, it does not
eliminate nonanalyticity but rather makes it anisotropic: while the linear
scaling of with and saturates at the energy
scale set by the SOI, that of () continues through this
energy scale, until renormalization of the electron-electron interaction in the
Cooper channel becomes important. We show that the Renormalization Group flow
in the Cooper channel has a non-trivial fixed point, and study the consequences
of this fixed point for the nonanalytic behavior of . An immediate
consequence of SOI-induced anisotropy in the nonanalytic behavior of is
a possible instability of a second-order ferromagnetic quantum phase transition
with respect to a first-order transition to an XY ferromagnetic state.Comment: 34 pages, 12 figure
Pairs of Bloch electrons and magnetic translation groups
A product of irreducible representations of magnetic translation group is
considered. It leads to irreducible representations which were previously
rejected as nonphysical. A very simple example indicates a possible application
of these representations. In particular, they are important in descriptions of
pairs of electrons in a magnetic field and a periodic potential. The
periodicity of some properties with respect to the charge of a particle is
briefly discussed.Comment: 4 pages, RevTex. Latex2.09, amsfont
Berry phases in superconducting transitions
I generalize the concept of Berry's geometrical phase for quasicyclic
Hamiltonians to the case in which the ground state evolves adiabatically to an
excited state after one cycle, but returns to the ground state after an integer
number of cycles. This allows to extend the charge Berry phase gamma_c related
to the macroscopic polarization, to many-body systems with fractional number of
particles per site. Under certain conditions, gamma_c and the spin Berry phase
gamma_s jump in pi at the boundary of superconducting phases. In the extended
Hubbard chain with on-site attraction U and nearest-neighbor interaction V at
quarter filling, the transitions detected agree very well with exact results in
two limits solved by the Bethe ansatz, and with previous numerical studies. In
chains with spin SU(2) symmetry, gamma_s jumps when a spin gap opens.Comment: 5 pages, 3 figures, accepted in Europhys. Let
Performance evaluation of a six-axis generalized force-reflecting teleoperator
Work in real-time distributed computation and control has culminated in a prototype force-reflecting telemanipulation system having a dissimilar master (cable-driven, force-reflecting hand controller) and a slave (PUMA 560 robot with custom controller), an extremely high sampling rate (1000 Hz), and a low loop computation delay (5 msec). In a series of experiments with this system and five trained test operators covering over 100 hours of teleoperation, performance was measured in a series of generic and application-driven tasks with and without force feedback, and with control shared between teleoperation and local sensor referenced control. Measurements defining task performance included 100-Hz recording of six-axis force/torque information from the slave manipulator wrist, task completion time, and visual observation of predefined task errors. The task consisted of high precision peg-in-hole insertion, electrical connectors, velcro attach-de-attach, and a twist-lock multi-pin connector. Each task was repeated three times under several operating conditions: normal bilateral telemanipulation, forward position control without force feedback, and shared control. In shared control, orientation was locally servo controlled to comply with applied torques, while translation was under operator control. All performance measures improved as capability was added along a spectrum of capabilities ranging from pure position control through force-reflecting teleoperation and shared control. Performance was optimal for the bare-handed operator
Magneto-optical properties of Co|Pt multilayer systems
We are reporting, for the first time in the literature, theoretical Kerr
spectra of Co|Pt multilayer systems as obtained on a first principles basis
including multiple reflections and interferences from all the boundaries
in-between the layers.Comment: 4 pages (LaTeX), 1 (a,b) figures (Encapsulated PostScript), J. Appl.
Physics, in pres
Molecular dynamics simulations of mixed DOPCâÎČ-sitosterol bilayers and their interactions with DMSO
ell membrane phospholipid bilayers can be damaged by the large amounts of dimethyl sulphoxide (DMSO) commonly used in cryopreservation. The interaction of DMSO with model bilayers consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and Ă-sitosterol has been studied using molecular dynamics simulations. Initially the effect of sterol concentration and temperature upon bilayers solvated in pure water was determined, and membranes containing Ă-sitosterol were compared with membranes containing cholesterol. These simulations showed that the presence of sterols has a condensing effect on the phospholipids, causing a reduction in the area per lipid as the sterol concentration increases, up to a phospholipidâsterol ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. The incorporation of sterols into the bilayer also increased the thickness and order of the phospholipid acyl tails. DOPCâĂ-sitosterol bilayers at different relative concentrations were simulated in solutions of 2.5 and 25.0 mol% DMSO. The interaction of DMSO with the bilayers caused the bilayers to expand laterally, while thinning normal to the plane of the bilayer expansion. The same qualitative behaviour has been shown to occur in pure phosphocholine bilayers. However, the presence of sterols made the membranes more resistant to the effects of DMSO, to the extent that the membranes where able to maintain their integrity in 25.0 mol% DMSO, a concentration that would otherwise cause the destruction of a pure DOPC bilayer. Increasing the concentration of Ă-sitosterol within the bilayers reduced the rate of DMSO diffusion across the bilayer and, if the concentration was large enough, caused the diffusion mechanism to change. DMSO was observed to disorder the membranes enough to cause an increase in the number of sterol âflipâflopsâ. The findings of this work provide a more realistic description of how DMSO interacts with cell membranes and the role of the composition of the membrane
A room temperature CO line list with ab initio computed intensities
Atmospheric carbon dioxide concentrations are being closely monitored by
remote sensing experiments which rely on knowing line intensities with an
uncertainty of 0.5% or better. We report a theoretical study providing
rotation-vibration line intensities substantially within the required accuracy
based on the use of a highly accurate {\it ab initio} dipole moment surface
(DMS). The theoretical model developed is used to compute CO intensities
with uncertainty estimates informed by cross comparing line lists calculated
using pairs of potential energy surfaces (PES) and DMS's of similar high
quality. This yields lines sensitivities which are utilized in reliability
analysis of our results. The final outcome is compared to recent accurate
measurements as well as the HITRAN2012 database. Transition frequencies are
obtained from effective Hamiltonian calculations to produce a comprehensive
line list covering all CO transitions below 8000 cm
and stronger than 10 cm / molecule at ~
Fundamental Studies Relating to the Mechanical Behavior of Solid Propellants, Rocket Grains and Rocket Motors
During the past three years, the mechanical testing of solid
propellants, solid rocket grains, and solid rocket motors under idealized conditions has been receiving increased attention. Today it is not uncommon to see a multitude of new techniques and analyses being investigated. One may expect to see dummy propellant prepared with
glass bead filler to observe its dilatation to rupture; to ink circles, rectangular g rids at various critical areas on a grain surface, and to observe the distortion of these grids as a result of thermal cycling and/or slump; to subj e ct rectangular parallel-opipedal-shaped specimens
to both torsion and biaxial tension as well as hydrostatic compression and parallel-plate tension; to apply theories of large elastic strain, and non-linear viscoelasticity; to search for an isotropic failure criterion
as well as a crack propagation criterion. In short the mechanics of propellant behavior from small deformation all the way to fracture initiation and propagation has become quite sophisticated. Gradually the results of this testing and their thinking are being integrated in a
logical scheme of analysis which is being passed along to the engineer and being used in predicting performance of rocket motors.
This particular program will pertain to four areas:
1) The characterization of polyurethane propellant behavior
out to fracture initiation in terms of large strain theory.
2) The development of a failure criterion and crack propagation criteria for said materials.
3) The generation, where possible, of macroscopic mechanical
parameters in terms of molecular parameters.
4} The solution of certain stress problems, in both linear and non-linear theory, which are prerequisite to engineering
applications.
As such it is part of a continuing research study of structural integrity problems in solid propellant rocket motors being conducted under the general direction of Dr. M. L. Williams in the Guggenheim Aeronautical
Laboratory.
This preliminary report is intended as an interim working document to be circulated for the purpose of stimulating discussion
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