5,141 research outputs found
Is the Mott transition relevant to f-electron metals ?
We study how a finite hybridization between a narrow correlated band and a
wide conduction band affects the Mott transition. At zero temperature, the
hybridization is found to be a relevant perturbation, so that the Mott
transition is suppressed by Kondo screening. In contrast, a first-order
transition remains at finite temperature, separating a local moment phase and a
Kondo- screened phase. The first-order transition line terminates in two
critical endpoints. Implications for experiments on f-electron materials such
as the Cerium alloy CeLaTh are discussed.Comment: 5 pages, 3 figure
Importance of interorbital charge transfers for the metal-to-insulator transition of BaVS
The underlying mechanism of the metal-to-insulator transition (MIT) in
BaVS is investigated, using dynamical mean-field theory in combination with
density functional theory. It is shown that correlation effects are responsible
for a strong charge redistribution, which lowers the occupancy of the broader
\a1g band in favor of the narrower bands. This resolves several
discrepancies between band theory and the experimental findings, such as the
observed value of the charge-density wave ordering vector associated with the
MIT, and the presence of local moments in the metallic phase.Comment: improved discussion, new figure, added reference
Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory
We examine whether the Mott transition of a half-filled, two-orbital Hubbard
model with unequal bandwidths occurs simultaneously for both bands or whether
it is a two-stage process in which the orbital with narrower bandwith localizes
first (giving rise to an intermediate `orbital-selective' Mott phase). This
question is addressed using both dynamical mean-field theory, and a
representation of fermion operators in terms of slave quantum spins, followed
by a mean-field approximation (similar in spirit to a Gutzwiller
approximation). In the latter approach, the Mott transition is found to be
orbital-selective for all values of the Coulomb exchange (Hund) coupling J when
the bandwidth ratio is small, and only beyond a critical value of J when the
bandwidth ratio is larger. Dynamical mean-field theory partially confirms these
findings, but the intermediate phase at J=0 is found to differ from a
conventional Mott insulator, with spectral weight extending down to arbitrary
low energy. Finally, the orbital-selective Mott phase is found, at
zero-temperature, to be unstable with respect to an inter-orbital
hybridization, and replaced by a state with a large effective mass (and a low
quasiparticle coherence scale) for the narrower band.Comment: Discussion on the effect of hybridization on the OSMT has been
extende
The alpha-gamma transition of Cerium is entropy-driven
We emphasize, on the basis of experimental data and theoretical calculations,
that the entropic stabilization of the gamma-phase is the main driving force of
the alpha-gamma transition of cerium in a wide temperature range below the
critical point. Using a formulation of the total energy as a functional of the
local density and of the f-orbital local Green's functions, we perform
dynamical mean-field theory calculations within a new implementation based on
the multiple LMTO method, which allows to include semi-core states. Our results
are consistent with the experimental energy differences and with the
qualitative picture of an entropy-driven transition, while also confirming the
appearance of a stabilization energy of the alpha phase as the quasiparticle
Kondo resonance develops.Comment: 5 pages, 6 figure
Stable fractal sums of pulses: the cylindrical case
A class of α-stable, 0\textlessα\textless2, processes is obtained as a sum of ’up-and-down’ pulses determined by an appropriate Poisson random measure. Processes are H-self-affine (also frequently called ’self-similar’) with H\textless1/α and have stationary increments. Their two-dimensional dependence structure resembles that of the fractional Brownian motion (for H\textless1/2), but their sample paths are highly irregular (nowhere bounded with probability 1). Generalizations using different shapes of pulses are also discussed
Dynamical solutions of a quantum Heisenberg spin glass model
We consider quantum-dynamical phenomena in the ,
infinite-range quantum Heisenberg spin glass. For a fermionic generalization of
the model we formulate generic dynamical self-consistency equations. Using the
Popov-Fedotov trick to eliminate contributions of the non-magnetic fermionic
states we study in particular the isotropic model variant on the spin space.
Two complementary approximation schemes are applied: one restricts the quantum
spin dynamics to a manageable number of Matsubara frequencies while the other
employs an expansion in terms of the dynamical local spin susceptibility. We
accurately determine the critical temperature of the spin glass to
paramagnet transition. We find that the dynamical correlations cause an
increase of by 2% compared to the result obtained in the spin-static
approximation. The specific heat exhibits a pronounced cusp at .
Contradictory to other reports we do not observe a maximum in the -curve
above .Comment: 8 pages, 7 figure
Robust self-trapping of vortex beams in a saturable optical medium
We report the first observation of robust self-trapping of vortex beams
propagating in a uniform condensed medium featuring local saturable
self-focusing nonlinearity. Optical vortices with topological charge m=1, that
remain self-trapped over ~ 5 Rayleigh lengths, are excited in carbon disulfide
using a helical light beam at 532 nm and intensities from 8 to 10 GW/cm^2. At
larger intensities, the vortex beams lose their stability, spontaneously
breaking into two fragments. Numerical simulations based on the nonlinear
Schr\"odinger equation including the three-photon absorption and nonpolynomial
saturation of the refractive nonlinearity demonstrate close agreement with the
experimental findings.Comment: 27 pages, 7 figures,to be published in Phys. Rev. A (2016
The effect of load in a contact with boundary lubrication
The effect of the transition load on the wear in a contact with boundary lubrication was investigated. An experimental method was developed for this purpose, and parameters affecting the boundary lubrication under industrial operating conditions were identified. These parameters are the adsorbed boundary film, the contact microgeometry (surface roughness), macrogeometry, and hardness of materials used. It was found that the curve of the tops of the surface protrustion affect the transition load, and thus the boundary lubrication. The transition load also depends on the chemical nature of the contact and its geometrical and mechanical aspects
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