92 research outputs found
Atomistic simulations of magnetoelastic effects on sound velocity
In this work, we leverage atomistic spin-lattice simulations to examine how
magnetic interactions impact the propagation of sound waves through a
ferromagnetic material. To achieve this, we characterize the sound wave
velocity in BCC iron, a prototypical ferromagnetic material, using three
different approaches that are based on the oscillations of kinetic energy,
finite-displacement derived forces, and corrections to the elastic constants,
respectively. Successfully applying these methods within the spin-lattice
framework, we find good agreement with the Simon effect including high order
terms. In analogy to experiments, morphic coefficients associated with the
transverse and longitudinal waves propagating along the [001] direction are
extracted from fits to the fractional change in velocity data. The present
efforts represent an advancement in magnetoelastic modelling capabilities which
can expedite the design of future magneto-acoustic devices
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
M\"ossbauer studies of spin- and charge-modulations in BaFe2(As1-xPx)2
The BaFe2(As1-xPx)2 compounds with x = 0 (parent), x = 0.10 (under-doped), x
= 0.31, 0.33, 0.53 (superconductors with Tc = 27.3 K, 27.6 K, 13.9 K,
respectively) and x = 0.70, 0.77 (over-doped) have been investigated versus
temperature using 57Fe M\"ossbauer spectroscopy. Special attention was paid to
regions of the spin-density-wave (SDW) antiferromagnetic order, spin-nematic
phase, and superconducting transition. The BaFe2(As0.90P0.10)2 compound
exhibits a reduced amplitude of SDW as compared to the parent compound and
preserved universality class of two-dimensional magnetic planes with
one-dimensional spins. The spin-nematic phase region for x = 0.10 is
characterized by an incoherent magnetic order. BaFe2(As0.69P0.31)2 shows
coexistence of a weak magnetic order and superconductivity due to the vicinity
of the quantum critical point. The charge density modulations in the
BaFe2(As0.67P0.33)2 and BaFe2(As0.47P0.53)2 superconductors are perturbed near
Tc. Pronounced hump of the average quadrupole splitting across superconducting
transition is observed for the system with x = 0.33. The phosphorus
substitution increases the Debye temperature of the BaFe2(As1-xPx)2 compound.
Moreover, experimental electron charge densities at Fe nuclei in this material
conclusively show that it should be recognized as a hole-doped system. The
measured M\"ossbauer spectral shift and spectral area are not affected by
transition to the superconducting state. This indicates that neither the
average electron density at Fe nuclei nor the dynamical properties of the
Fe-sublattice in BaFe2(As1-xPx)2 are sensitive to the superconducting
transition. Theoretical calculations of hyperfine parameters determining the
patterns of M\"ossbauer spectra of BaFe2(As1-xPx)2 with x = 0, 0.31, 0.5, and
1.0 are performed within the framework of the density functional theory
Stability and strength of transition-metal tetraborides and triborides
Using density functional theory, we show that the long-believed transition-metal tetraborides (TB4) of tungsten and molybdenum are in fact triborides (TB3). This finding is supported by thermodynamic, mechanical, and phonon instabilities of TB4, and it challenges the previously proposed origin of superhardness of these compounds and the predictability of the generally used hardness model. Theoretical calculations for the newly identified stable TB3 structure correctly reproduce their structural and mechanical properties, as well as the experimental x-ray diffraction pattern. However, the relatively low shear moduli and strengths suggest that TB3 cannot be intrinsically stronger than c-BN. The origin of the lattice instability of TB3 under large shear strain that occurs at the atomic level during plastic deformation can be attributed to valence charge depletion between boron and metal atoms, which enables easy sliding of boron layers between the metal ones.Web of Science10826art. no. 25550
Extraordinary two-dimensionality in the S=1/2 spatially anisotropic triangular quantum magnet Cu(1,3-diaminopropane)Cl2 with modulated structure
Elastic constants and volume changes associated with two high-pressure rhombohedral phase transformations in vanadium
We present results from ab initio calculations of the mechanical properties
of the rhombohedral phase (beta) of vanadium metal reported in recent
experiments, and other predicted high-pressure phases (gamma and bcc), focusing
on properties relevant to dynamic experiments. We find that the volume change
associated with these transitions is small: no more than 0.15% (for beta -
gamma). Calculations of the single crystal and polycrystal elastic moduli
(stress-strain coefficients) reveal a remarkably small discontinuity in the
shear modulus and other elastic properties across the phase transitions even at
zero temperature where the transitions are first order.Comment: 6 pages, 3 figure
Dual molecular mechanisms govern escape at immunodominant HLA A2-restricted HIV epitope
Serial accumulation of mutations to fixation in the SLYNTVATL (SL9) immunodominant, HIV p17 Gag-derived, HLA A2-restricted CTL epitope produce the SLFNTIAVL triple mutant ‘ultimate’ escape variant. These mutations in solvent-exposed residues are believed to interfere with TCR recognition, although confirmation has awaited structural verification. Here, we solved a TCR co-complex structure with SL9 and the triple escape mutant to determine the mechanism of immune escape in this eminent system. We show that, in contrast to prevailing hypotheses, the main TCR contact residue is 4N and the dominant mechanism of escape is not via lack of TCR engagement. Instead, mutation of solvent exposed residues in the peptide destabilize the peptide-HLA and reduce peptide density at the cell surface. These results highlight the extraordinary lengths that HIV employs to evade detection by high-affinity TCRs with a broad peptide-binding footprint and necessitate reevaluation of this exemplar model of HIV TCR escape
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