19,400 research outputs found
Coarse-graining of non-reversible stochastic differential equations: quantitative results and connections to averaging
This work is concerned with model reduction of stochastic differential equations and builds on the idea of replacing drift and noise coefficients of preselected relevant, e.g. slow variables by their conditional expectations. We extend recent results by Legoll & Lelièvre [Nonlinearity 23, 2131, 2010] and Duong et al. [Nonlinearity 31, 4517, 2018] on effective reversible dynamics by conditional expectations to the setting of general non-reversible processes with non-constant diffusion coefficient. We prove relative entropy and Wasserstein error estimates for the difference between the time marginals of the effective and original dynamics as well as an entropy error bound for the corresponding path space measures. A comparison with the averaging principle for systems with time-scale separation reveals that, unlike in the reversible setting, the effective dynamics for a non-reversible system need not agree with the averaged equations. We present a thorough comparison for the Ornstein-Uhlenbeck process and make a conjecture about necessary and sufficient conditions for when averaged and effective dynamics agree for nonlinear non-reversible processes. The theoretical results are illustrated with suitable numerical examples
Non-collinear spin-spiral phase for the uniform electron gas within Reduced-Density-Matrix-Functional Theory
The non-collinear spin-spiral density wave of the uniform electron gas is
studied in the framework of Reduced-Density-Matrix-Functional Theory. For the
Hartree-Fock approximation, which can be obtained as a limiting case of
Reduced-Density-Matrix-Functional Theory, Overhauser showed a long time ago
that the paramagnetic state of the electron gas is unstable with respect to the
formation of charge or spin density waves. Here we not only present a detailed
numerical investigation of the spin-spiral density wave in the Hartree-Fock
approximation but also investigate the effects of correlations on the
spin-spiral density wave instability by means of a recently proposed
density-matrix functional.Comment: 9 pages, 10 figure
Large magnetocrystalline anisotropy in tetragonally distorted Heuslers: a systematic study
With a view to the design of hard magnets without rare earths we explore the
possibility of large magnetocrystalline anisotropy energies in Heusler
compounds that are unstable with respect to a tetragonal distortion. We
consider the Heusler compounds FeYZ with Y = (Ni, Co, Pt), and CoYZ
with Y = (Ni, Fe, Pt) where, in both cases, Z = (Al, Ga, Ge, In, Sn). We find
that for the CoNiZ, CoPtZ, and FePtZ families the cubic phase is
always, at , unstable with respect to a tetragonal distortion, while, in
contrast, for the FeNiZ and FeCoZ families this is the case for only 2
compounds -- FeCoGe and FeCoSn. For all compounds in which a tetragonal
distortion occurs we calculate the MAE finding remarkably large values for the
Pt containing Heuslers, but also large values for a number of the other
compounds (e.g. CoNiGa has an MAE of -2.11~MJ/m). The tendency to a
tetragonal distortion we find to be strongly correlated with a high density of
states at the Fermi level in the cubic phase. As a corollary to this fact we
observe that upon doping compounds for which the cubic structure is stable such
that the Fermi level enters a region of high DOS, a tetragonal distortion is
induced and a correspondingly large value of the MAE is then observed.Comment: 8 pages, 5 figure
Semiclassical ordering in the large-N pyrochlore antiferromagnet
We study the semiclassical limit of the generalization of the
pyrochlore lattice Heisenberg antiferromagnet by expanding about the saddlepoint in powers of a generalized inverse spin. To leading order,
we write down an effective Hamiltonian as a series in loops on the lattice.
Using this as a formula for calculating the energy of any classical ground
state, we perform Monte-Carlo simulations and find a unique collinear ground
state. This state is not a ground state of linear spin-wave theory, and can
therefore not be a physical (N=1) semiclassical ground state.Comment: 4 pages, 4 eps figures; published versio
Study of the characteristics of GEM detectors for the future FAIR experiment CBM
Characteristics of triple GEM detector have been studied systematically. The
variation of the effective gain and energy resolution of GEM with variation of
the applied voltage has been measured with Fe55 X-ray source for different gas
mixtures and with different gas flow rates. Long-term test of the GEM has also
been performed.Comment: 2 Pages, 6 figure
Static and Dynamical Susceptibility of LaO1-xFxFeAs
The mechanism of superconductivity and magnetism and their possible interplay
have recently been under debate in pnictides. A likely pairing mechanism
includes an important role of spin fluctuations and can be expressed in terms
of the magnetic susceptibility chi. The latter is therefore a key quantity in
the determination of both the magnetic properties of the system in the normal
state, and of the contribution of spin fluctuations to the pairing potential. A
basic ingredient to obtain chi is the independent-electron susceptibility chi0.
Using LaO1-xFxFeAs as a prototype material, in this report we present a
detailed ab-initio study of chi0(q,omega), as a function of doping and of the
internal atomic positions. The resulting static chi0(q,0) is consistent with
both the observed M-point related magnetic stripe phase in the parent compound,
and with the existence of incommensurate magnetic structures predicted by
ab-initio calculations upon doping.Comment: 15 pages, 8 figure
Superheavy nuclei in relativistic effective Lagrangian model
Isotopic and isotonic chains of superheavy nuclei are analyzed to search for
spherical double shell closures beyond Z=82 and N=126 within the new effective
field theory model of Furnstahl, Serot, and Tang for the relativistic nuclear
many-body problem. We take into account several indicators to identify the
occurrence of possible shell closures, such as two-nucleon separation energies,
two-nucleon shell gaps, average pairing gaps, and the shell correction energy.
The effective Lagrangian model predicts N=172 and Z=120 and N=258 and Z=120 as
spherical doubly magic superheavy nuclei, whereas N=184 and Z=114 show some
magic character depending on the parameter set. The magicity of a particular
neutron (proton) number in the analyzed mass region is found to depend on the
number of protons (neutrons) present in the nucleus.Comment: 26 pages, REVTeX, 10 ps figures; changed conten
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