44,524 research outputs found
Smooth polytopes with negative Ehrhart coefficients
We present examples of smooth lattice polytopes in dimensions 3 and higher
where each coefficient of their Ehrhart polynomials that can potentially be
negative is indeed negative. This answers a question by Bruns. We also discuss
Berline-Vergne valuations as a useful tool in proving Ehrhart positivity
results.Comment: 15 pages, 1 figure, 1 table, accepted by Journal of Combinatorial
Theory Ser.
Variational matrix product state approach to quantum impurity models
We present a unified framework for renormalization group methods, including
Wilson's numerical renormalization group (NRG) and White's density-matrix
renormalization group (DMRG), within the language of matrix product states.
This allows improvements over Wilson's NRG for quantum impurity models, as we
illustrate for the one-channel Kondo model. Moreover, we use a variational
method for evaluating Green's functions. The proposed method is more flexible
in its description of spectral properties at finite frequencies, opening the
way to time-dependent, out-of-equilibrium impurity problems. It also
substantially improves computational efficiency for one-channel impurity
problems, suggesting potentially \emph{linear} scaling of complexity for
-channel problems.Comment: revised version with application to Kondo model at large magnetic
field (5 pages, 2 figures
Thermodynamics and kinetics of the Mg65Cu25Y10 bulk metallic glass forming liquid
The thermodynamics and kinetics of the bulk metallic glass forming Mg65Cu25Y10 liquid were investigated using differential scanning calorimetry and three-point beam bending. The experiments lead to the determination of the thermodynamic functions as well as the viscosity of the supercooled liquid. The viscosity shows a temperature dependence, which is consistent with that of a strong glass similar to Zr–Ti–Cu–Ni–Be bulk metallic glasses or sodium silicate glasses. This contrasts with more fragile conventional metallic glass formers or pure metals. The relatively weak temperature dependence of the thermodynamic functions of the supercooled liquid is related to these sluggish kinetics in the supercooled liquid. Entropy, viscosity, and kinetic glass transition are compared in the frameworks of the fragility concept and the Adam–Gibbs theory. Strong liquid behavior retards the formation of crystals kinetically and thermodynamically
Magnetic enhancement of CoZnFeO spinel oxide by mechanical milling
We report the magnetic properties of mechanically milled
CoZnFeO spinel oxide. After 24 hours milling of the
bulk sample, the XRD spectra show nanostructure with average particle size
20 nm. The as milled sample shows an enhancement in magnetization and
ordering temperature compared to the bulk sample. If the as milled sample is
annealed at different temperatures for the same duration, recrystallization
process occurs and approaches to the bulk structure on increasing the annealing
temperatures. The magnetization of the annealed samples first increases and
then decreases. At higher annealing temperature ( 1000C) the system
shows two coexisting magnetic phases {\it i.e.}, spin glass state and
ferrimagnetic state, similar to the as prepared bulk sample. The room
temperature M\"{o}ssbauer spectra of the as milled sample, annealed at
300C for different durations (upto 575 hours), suggest that the observed
change in magnetic behaviour is strongly related with cations redistribution
between tetrahedral (A) and octahedral (O) sites in the spinel structure. Apart
from the cation redistribution, we suggest that the enhancement of
magnetization and ordering temperature is related with the reduction of B site
spin canting and increase of strain induced anisotropic energy during
mechanical milling.Comment: 14 pages LaTeX, 10 ps figure
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