Search results for "model [interaction]"
showing 10 items of 1495 documents
Antiferromagnetic Heisenberg chains with bond alternation and quenched disorder
2004
We consider S=1/2 antiferromagnetic Heisenberg chains with alternating bonds and quenched disorder, which represents a theoretical model of the compound CuCl_{2x}Br_{2(1-x)}(\gamma-{pic})_2. Using a numerical implementation of the strong disorder renormalization group method we study the low-energy properties of the system as a function of the concentration, x, and the type of correlations in the disorder. For perfect correlation of disorder the system is in the random dimer (Griffiths) phase having a concentration dependent dynamical exponent. For weak or vanishing disorder correlations the system is in the random singlet phase, in which the dynamical exponent is formally infinity. We disc…
Canard Cycles with Three Breaking Mechanisms
2016
This article deals with relaxation oscillations from a generic balanced canard cycle \(\Gamma\) subject to three breaking parameters of Hopf or jump type. We prove that in a rescaled layer of \(\Gamma\) there bifurcate at most five relaxation oscillations.
Electronic and acoustic-phonon inter-Landau-level Raman scattering in GaAs/AlxGa1−xAs multiple quantum wells
1995
We present an experimental study of inter-Landau-level excitations in undoped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple quantum wells in high magnetic fields by means of Raman scattering. The experiments were performed in Faraday backscattering geometry with the field along the growth axis, using circularly polarized light for resonant excitation of low-index magneto-optical transitions between Landau levels. We observe two types of peaks. One of them, present in both Stokes and anti-Stokes regions at a constant Raman shift, corresponds to the electron cyclotron energy. We attribute it to electronic Raman scattering from a quasistationa…
Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules
1997
Using molecular dynamics computer simulations, we investigate the dynamics of the rotational degrees of freedom in a supercooled system composed of rigid, diatomic molecules. The interaction between the molecules is given by the sum of interaction-site potentials of the Lennard-Jones type. In agreement with mode-coupling theory (MCT), we find that the relaxation times of the orientational time correlation functions C_1^(s), C_2^(s) and C_1 show at low temperatures a power-law with the same critical temperature T_c, and which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT we find, however, that for these correlators the time-tempera…
Cooperative motion and growing length scales in supercooled confined liquids
2002
Using molecular dynamics simulations we investigate the relaxation dynamics of a supercooled liquid close to a rough as well as close to a smooth wall. For the former situation the relaxation times increase strongly with decreasing distance from the wall whereas in the second case they strongly decrease. We use this dependence to extract various dynamical length scales and show that they grow with decreasing temperature. By calculating the frequency dependent average susceptibility of such confined systems we show that the experimental interpretation of such data is very difficult.
Understanding and controlling N-dimensional quantum walks via dispersion relations: application to the two-dimensional and three-dimensional Grover w…
2013
The discrete quantum walk in N dimensions is analyzed from the perspective of its dispersion relations. This allows understanding known properties, as well as designing new ones when spatially extended initial conditions are considered. This is done by deriving wave equations in the continuum, which are generically of the Schrodinger type, and allows devising interesting behavior, such as ballistic propagation without deformation, or the generation of almost flat probability distributions, which is corroborated numerically. There are however special points where the energy surfaces display intersections and, near them, the dynamics is entirely different. Applications to the two- and three-d…
Reply to “Comment on ‘Elastic constants from microscopic strain fluctuations’ ”
2010
We agree with Coupier et al. [Phys. Rev. E 81, 013101 (2010)] that their technique for extracting elastic constants from microscopic strain fluctuations improves upon ours because of a more accurate computation of the integral of the elastic correlation function over sub-blocks. However, we believe that their interpretation of the physical relevance of the elastic correlation length extracted from the fits is misleading.
Calculations of the ground and excited states ofF-type centers in corundum crystals
1994
The semiempirical intermediate neglect of differential overlap method was used for calculating optical properties of ${\mathit{F}}^{+}$ and F centers (oxygen vacancy trapped one and two electrons, respectively) embedded into large quantum clusters, ${\mathrm{Al}}_{26}$${\mathrm{O}}_{39}$. The geometry was optimized for both the ground and excited states of defects. Calculated absorption and luminescence energies obtained for ${\mathit{F}}^{+}$ and F centers are in good agreement with experimental data. Their energy levels lie in the gap between the upper valence band and conduction band, like for similar centers in MgO and alkali halides. It is shown that the oxygen vacancy in corundum crys…
Spin-restricted coupled-cluster theory with triple excitations
2002
Working equations for a spin-restricted coupled-cluster (SR-CC) ansatz with full inclusion of triple excitations are presented. The equations have been derived using a new formulation of the SR-CC theory that is equivalent to the original one but much easier processed and also provides a new interpretation of the underlying concepts of the SR-CC approach. Test calculations with a preliminary SR-CC singles, doubles, triples (SR-CCSDT) implementation indicate that spin-restriction has a rather small effect on the computed energies and that the effects are—as expected—less pronounced than in the case of the CC singles, doubles approximation. The corresponding partially spin-adapted scheme turn…
Ising Spins on 3D Random Lattices
1999
We perform single-cluster Monte Carlo simulations of the Ising model on three-dimensional Poissonian random lattices of Voronoi/Delaunay type with up to 128 000 sites. For each lattice size quenched averages are computed over 96 realizations. From a finite-size scaling analysis we obtain strong evidence that the critical exponents coincide with those on regular cubic lattices.