80 research outputs found
Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets
We report a computational theoretical investigation of electron spin-flip
scattering induced by the electron-phonon interaction in the transition-metal
ferromagnets bcc Fe, fcc Co and fcc Ni. The Elliott-Yafet electron-phonon
spin-flip scattering is computed from first-principles, employing a generalized
spin-flip Eliashberg function as well as ab initio computed phonon dispersions.
Aiming at investigating the amount of electron-phonon mediated demagnetization
in femtosecond laser-excited ferromagnets, the formalism is extended to treat
laser-created thermalized as well as nonequilibrium, nonthermal hot electron
distributions. Using the developed formalism we compute the phonon-induced spin
lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated
demagnetization rate is evaluated for laser-created thermalized and
nonequilibrium electron distributions. Nonthermal distributions are found to
lead to a stronger demagnetization rate than hot, thermalized distributions,
yet their demagnetizing effect is not enough to explain the experimentally
observed demagnetization occurring in the subpicosecond regime.Comment: 14 pages, 8 figures, to appear in PR
Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni
In pump-probe experiments an intensive laser pulse creates non-equilibrium
excited electron distributions in the first few hundred femtoseconds after the
pulse. The influence of non-equilibrium electron distributions caused by a pump
laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is
investigated theoretically here for the first time, considering electron
distributions immediately after the pulse as well as thermalized ones, that are
not in equilibrium with the lattice or spin systems. The XMCD signal is shown
not to be simply proportional to the spin momentum in these situations. The
computed spectra are compared to recent pump-probe XMCD experiments on Ni. We
find that the majority of experimentally observed features considered to be a
proof of ultrafast spin momentum transfer to the lattice can alternatively be
attributed to non-equilibrium electron distributions. Furthermore, we find the
XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid,
even for the laser induced non-equilibrium electron distributions.Comment: 6 pages, 3 figure
Ab-initio investigation of phonon dispersion and anomalies in palladium
In recent years, palladium has proven to be a crucial component for devices
ranging from nanotube field effect transistors to advanced hydrogen storage
devices. In this work, I examine the phonon dispersion of fcc Pd using first
principle calculations based on density functional perturbation theory. While
several groups in the past have studied the acoustic properties of palladium,
this is the first study to reproduce the phonon dispersion and associated
anomaly with high accuracy and no adjustable parameters. In particular, I focus
on the Kohn anomaly in the [110] direction.Comment: 19 pages, preprint format, 7 figures, added new figures and
discussio
Finding maxmin allocations in cooperative and competitive fair division
We consider upper and lower bounds for maxmin allocations of a completely
divisible good in both competitive and cooperative strategic contexts. We then
derive a subgradient algorithm to compute the exact value up to any fixed
degree of precision.Comment: 20 pages, 3 figures. This third version improves the overll
presentation; Optimization and Control (math.OC), Computer Science and Game
Theory (cs.GT), Probability (math.PR
Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite
Symmetry breaking across phase transitions often causes changes in selection
rules and emergence of optical modes which can be detected via spectroscopic
techniques or generated coherently in pump-probe experiments. In second-order
or weakly first-order transitions, fluctuations of the order parameter are
present above the ordering temperature, giving rise to intriguing precursor
phenomena, such as critical opalescence. Here, we demonstrate that in magnetite
(FeO) light excitation couples to the critical fluctuations of the
charge order and coherently generates structural modes of the ordered phase
above the critical temperature of the Verwey transition. Our findings are
obtained by detecting coherent oscillations of the optical constants through
ultrafast broadband spectroscopy and analyzing their dependence on temperature.
To unveil the coupling between the structural modes and the electronic
excitations, at the origin of the Verwey transition, we combine our results
from pump-probe experiments with spontaneous Raman scattering data and
theoretical calculations of both the phonon dispersion curves and the optical
constants. Our methodology represents an effective tool to study the real-time
dynamics of critical fluctuations across phase transitions
Optimized peptide-MHC multimer protocols for detection and isolation of autoimmune T-cells
<p>Peptide–MHC (pMHC) multimers have become the “gold standard” for the detection and isolation of antigen-specific T-cells but recent evidence shows that normal use of these reagents can miss fully functional T-cells that bear T-cell receptors (TCRs) with low affinity for cognate antigen. This issue is particularly pronounced for anticancer and autoimmune T-cells as self-reactive T-cell populations are enriched for low-affinity TCRs due to the removal of cells with higher affinity receptors by immune tolerance mechanisms. Here, we stained a wide variety of self-reactive human T-cells using regular pMHC staining and an optimized technique that included: (i) protein kinase inhibitor (PKI), to prevent TCR triggering and internalization, and (ii) anti-fluorochrome antibody, to reduce reagent dissociation during washing steps. Lymphocytes derived from the peripheral blood of type 1 diabetes patients were stained with pMHC multimers made with epitopes from preproinsulin (PPI), insulin-β chain, glutamic acid decarboxylase 65 (GAD65), or glucose-6-phospate catalytic subunit-related protein (IGRP) presented by disease-risk allelles HLA A*02:01 or HLA*24:02. Samples from ankylosing spondylitis patients were stained with a multimerized epitope from vasoactive intestinal polypeptide receptor 1 (VIPR1) presented by HLA B*27:05. Optimized procedures stained an average of 40.5-fold (p = 0.01, range between 1.4 and 198) more cells than could be detected without the inclusion of PKI and cross-linking anti-fluorochrome antibody. Higher order pMHC dextramers recovered more cells than pMHC tetramers in parallel assays, and standard staining protocols with pMHC tetramers routinely recovered less cells than functional assays. HLA A*02:01-restricted PPI-specific and HLA B*27:05-restricted VIPR1-specific T-cell clones generated using the optimized procedure could not be stained by standard pMHC tetramer staining. However, these clones responded well to exogenously supplied peptide and endogenously processed and presented epitopes. We also showed that anti-fluorochrome antibody-conjugated magnetic beads enhanced staining of self-reactive T-cells that could not be stained using standard protocols, thus enabling rapid ex vivo isolation of autoimmune T-cells. We, therefore, conclude that regular pMHC tetramer staining is generally unsuitable for recovering self-reactive T-cells from clinical samples and recommend the use of the optimized protocols described herein.</p
Lattice dynamics of the heavy fermion compound URuSi
We report a comprehensive investigation of the lattice dynamics of
URuSi as a function of temperature using Raman scattering, optical
conductivity and inelastic neutron scattering measurements as well as
theoretical {\it ab initio} calculations. The main effects on the optical
phonon modes are related to Kondo physics. The B ( symmetry)
phonon mode slightly softens below 100~K, in connection with the
previously reported softening of the elastic constant, , of the
same symmetry, both observations suggesting a B symmetry-breaking
instability in the Kondo regime. Through optical conductivity, we detect clear
signatures of strong electron-phonon coupling, with temperature dependent
spectral weight and Fano line shape of some phonon modes. Surprisingly, the
line shapes of two phonon modes, E(1) and A(2), show opposite
temperature dependencies. The A(2) mode loses its Fano shape below 150
K, whereas the E(1) mode acquires it below 100~K, in the Kondo cross-over
regime. This may point out to momentum-dependent Kondo physics. By inelastic
neutron scattering measurements, we have drawn the full dispersion of the
phonon modes between 300~K and 2~K. No remarkable temperature dependence has
been obtained including through the hidden order transition. {\it Ab initio}
calculations with the spin-orbit coupling are in good agreement with the data
except for a few low energy branches with propagation in the (a,b) plane
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