Search results for "APPROXIMATION"
showing 10 items of 818 documents
Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"
1998
We consider a system of coupled classical harmonic oscillators with spatially fluctuating nearest-neighbor force constants on a simple cubic lattice. The model is solved both by numerically diagonalizing the Hamiltonian and by applying the single-bond coherent potential approximation. The results for the density of states $g(\omega)$ are in excellent agreement with each other. As the degree of disorder is increased the system becomes unstable due to the presence of negative force constants. If the system is near the borderline of stability a low-frequency peak appears in the reduced density of states $g(\omega)/\omega^2$ as a precursor of the instability. We argue that this peak is the anal…
One dimensional arrays and solitary tunnel junctions in the weak coulomb blockade regime: CBT thermometry
1997
In this article we review the use of the tunnel junction arrays for primary thermometry. In addition to our basic experimental and theoretical results we stress the insensitivity of this method to the fluctuating background charges, to nonidealities in the array and to magnetic field. Important new results of this article are the low temperature corrections to the half width and depth of the measured conductance dip beyond the linear approximation. We also point ou that short arrays, single tunnel junctions in particular, show interesting deviations from the universal behaviour of the long arrays.
Transitions between metastable states in silica clusters
1999
Relaxation phenomena in glasses can be related to jump processes between different minima of the potential energy in the configuration space. These transitions play a key role in the low temperature regime, giving rise to tunneling systems responsible for the anomalous specific heat and thermal conductivity in disordered solids with respect to crystals. By using a recently developed numerical algorithm, we study the potential energy landscape of silica clusters, taking as a starting point the location of first order saddle points. This allows us to find a great number of adjacent minima. We analyze the degree of cooperativity of these transitions and the connection of physical properties wi…
Jahn-Teller deformations of jellium slices
1997
Equilibrium geometries of quasi two-dimensional jellium systems are calculated in the local density approximation, closely following the “Ultimate Jellium Model” of [1]. The background charge is assumed to be fully deformable in a layer between two parallel planes, whereas the wave functions in the direction perpendicular to such a “jellium slice” are confined to their ground state. Like for jellium clusters in three dimensions [1], we find that for various system sizes, a trend towards a breaking of axial and inversion symmetries is observable.
Electronic structure calculations in ordered and disordered solids with spiral magnetic order
2011
A scheme to calculate the electronic structure of systems having a spiral magnetic structure is presented. The approach is based on the Korringa-Kohn-Rostoker Green's function formalism which allows, in combination with the coherent potential approximation alloy theory, dealing with chemically disordered materials. It is applied to the magnetic random alloys Fe${}_{x}$Ni${}_{1\ensuremath{-}x}$, Fe${}_{x}$Co${}_{1\ensuremath{-}x}$, and Fe${}_{x}$Mn${}_{1\ensuremath{-}x}$. For these systems the stability of their magnetic structure was analyzed. For Fe${}_{x}$Ni${}_{1\ensuremath{-}x}$ the spin stiffness for was determined as a function of concentration that was found in satisfying agreement w…
Interference pattern and visibility of a Mott insulator
2005
We analyze theoretically the experiment reported in [F. Gerbier et al, cond-mat/0503452], where the interference pattern produced by an expanding atomic cloud in the Mott insulator regime was observed. This interference pattern, indicative of short-range coherence in the system, could be traced back to the presence of a small amount of particle/hole pairs in the insulating phase for finite lattice depths. In this paper, we analyze the influence of these pairs on the interference pattern using a random phase approximation, and derive the corresponding visibility. We also account for the inhomogeneity inherent to atom traps in a local density approximation. The calculations reproduce the expe…
Electronic structure calculations forZnFe2O4
2011
Local density approximation was applied to scrutinize the electronic structure and magnetic properties of the spinel ferrite ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$. Various cation distributions were established to obtain the ground state for the system. In magnetic crystals, the position of the atoms is not enough for symmetry determination. A structure prediction by decreasing the octahedral point group symmetry ${\mathrm{O}}_{h}$ of Fe to ${\mathrm{D}}_{4h}$, ${\mathrm{C}}_{4v}$, and ${\mathrm{C}}_{3v}$ was carried out. The effect of the exchange and correlation terms on the band structure of ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ was studied by the generalized gradient approximation $+$ th…
Model calculations for vibrational properties of disordered solids and the “boson peak”
1999
Abstract It is demonstrated that a disordered system of coupled classical harmonic oscillators with a continuous distribution of coupling parameters exhibits generally a low-frequency enhancement (“boson peak”) of the density of states, as compared with the Debye law. This phenomenon is most pronounced if the system is close to an instability. This is shown by means of a scalar model on a simple cubic lattice. The force constants are assumed to fluctuate from bond to bond according to a Gaussian distribution which is truncated at its lower end. The model is solved for the density of states and the one-phonon dynamic structure factor S(q, ω) by applying the two-site coherent potential approx…
Dzyaloshinskii-Moriya Interaction and Hall Effects in the Skyrmion Phase ofMn1−xFexGe
2015
We carry out density functional theory calculations which demonstrate that the electron dynamics in the Skyrmion phase of Fe-rich Mn_{1-x}Fe_{x}Ge alloys is governed by Berry phase physics. We observe that the magnitude of the Dzyaloshinskii-Moriya interaction directly related to the mixed space-momentum Berry phases, changes sign and magnitude with concentration x in direct correlation with the data of Shibata et al. [Nat. Nanotechnol. 8, 723 (2013)]. The computed anomalous and topological Hall effects in FeGe are also in good agreement with available experiments. We further develop a simple tight-binding model able to explain these findings. Finally, we show that the adiabatic Berry phase…
The WKB Approximation
2017
In this chapter we shall develop an important semiclassical method which has come back into favor again, particularly in the last few years, since it permits a continuation into field theory. Here, too, one is interested in nonperturbative methods.