共查询到20条相似文献,搜索用时 15 毫秒
1.
We consider the spin-1/2 model on the honeycomb lattice in the presence of a weak magnetic field hα < 1. Such a perturbation destroys the exact integrability of the model in terms of gapless fermions and static Z2 fluxes. We show that it results in the appearance of a long-range tail in the irreducible dynamic spin correlation function: ∝ h(z)(2)f(t,r), where f(t,r) ∝ [max(t,r)]-4 is proportional to the density polarization function of fermions. 相似文献
2.
Sakir Ayik 《Zeitschrift für Physik A Hadrons and Nuclei》1994,350(1):45-49
The average phase-space density described by the Boltzmann-Langevin model can largely deviate from the one provided by the Boltzmann-Uhling-Uhlenbeck model, due to the non-linear evolution of density fluctuations. This aspect is investigated for long-wavelength, small density fluctuations in the framework of a memory incorporated Boltzmann-Langevin model. It is shown that the correlations associated with density fluctuations yield a collision term describing coupling between the collective vibrations and the single-particle degrees of freedom, which may play an important role in damping of collective motion in both the stable and unstable regions. 相似文献
3.
We consider the nonstationary quantum relaxation of the Ising spin chain in a transverse field of strength h. Starting from a homogeneously magnetized initial state the system approaches a stationary state by a process possessing quasi-long-range correlations in time and space, independent of the value of h. In particular, the system exhibits aging (or lack of time-translational invariance on intermediate time scales) although no indications of coarsening are present. 相似文献
4.
H. van Beijeren 《Journal of statistical physics》1990,60(5-6):845-849
A simple anisotropic diffusion model, according to semiphenomenological arguments, exhibits long-ranged spatial correlations in uniform stationary states. 相似文献
5.
M. Dudzinski J. Sznajd J. Zittartz 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(4):575-579
The zero temperature phase diagram of a one-dimensional ferromagnet with cubic single ion anisotropy in an external magnetic
field is studied. The mean-field approximation and the density-matrix renormalization group method are applied. Two phases
at finite magnetic fields are identified: a canted phase with spontaneously broken symmetry and a phase with magnetization
along the magnetic field. Both methods predict that the canted phase exists even for the single-ion anisotropy strong enough
to destroy the magnetic order at zero magnetic field. In contrast to the mean-field theory, the density-matrix renormalization
group predicts a reentrant behavior for the model. The character of the phase transition at finite magnetic field has also
been considered and the critical index has been found.
Received 9 May 2000 and Received in final form 5 July 2000 相似文献
6.
A generalization of the ABC model, a one-dimensional model of a driven system of three particle species with local dynamics, is introduced, in which the model evolves under either (i) density-conserving or (ii) nonconserving dynamics. For equal average densities of the three species, both dynamical models are demonstrated to exhibit detailed balance with respect to a Hamiltonian with long-range interactions. The model is found to exhibit two distinct phase diagrams, corresponding to the canonical (density-conserving) and grand canonical (density nonconserving) ensembles, as expected in long-range interacting systems. The implications of this result to nonequilibrium steady states, such as those of the ABC model with unequal average densities, are briefly discussed. 相似文献
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8.
《Physics letters. A》2020,384(24):126454
Previous theoretical studies show that the spin current in spin-orbit coupled systems can be effectively conserved. In this study, we show that in the presence of an external magnetic field B perpendicular to the surface without causing Landau levels, the spin-Hall conductivity, including the conventional spin and spin-torque Hall currents exhibit an interesting symmetry, valid for k-linear and k-cubic Rashba systems. The phenomenon where the electric field generated spin z component is unaltered under is attributed to the fact that the spin precession is locked in spin-orbit coupled systems. The perpendicular magnetic field generates spin x and y components, which are linear to B, and thus, there is no time-reversal symmetry. This result provides evidence for the detection of the bulk spin-Hall current. Furthermore, the applied magnetic field breaks the degenerate point of the two-band model, and the resulting spin-Hall conductivity does not vanish even for systems with linear momentum, which implies that the Berry phase is not the principal mechanism in k-linear systems. The non-zero charge-Hall conductivity generated by the perpendicular magnetic field is discussed here. 相似文献
9.
We study a toy square-lattice model under a uniform magnetic field. Using the Landauer-Bttiker formula, we calculate the transport properties of the system on a two-terminal, a four-terminal and a six-terminal device. We find that the quantum spin Hall (QSH) effect appears in energy ranges where the spin-up and spin-down subsystems have different filling factors. We also study the robustness of the resulting QSH effect and find that it is robust when the Fermi levels of both spin subsystems are far away from the energy plateaus but is fragile when the Fermi level of any spin subsystem is near the energy plateaus. These results provide an example of the QSH effect with a physical origin other than time-reversal (TR) preserving spin-orbit coupling (SOC). 相似文献
10.
Making the assumption that high-energy fermions exist in the two dimensional spin- 1/2 Heisenberg antiferromagnet, we present predictions based on the pi-flux ansatz for the dynamic structure factor when the antiferromagnet is subject to a uniform magnetic field. The main result is the presence of gapped excitations in a momentum region near (pi,pi) with energy lower than that at (pi,pi). This is qualitatively different from spin-wave theory predictions and may be tested by experiments or by quantum Monte Carlo. 相似文献
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We introduce a completely different method to calculate the evolution of a spin interacting with a sufficient large spin bath,especially suitable for treating the central spin model in a quantum dot(QD).With only an approximation on the envelope of central spin,the symmetry can be exploited to reduce a huge Hilbert space which cannot be calculated with computers to many small ones which can be solved exactly.This method can be used to calculate spin-bath evolution for a spin bath containing many(say,1000)spins,without a perturbative limit such as strong magnetic field condition,and works for long-time regime with sufficient accuracy.As the spin-bath evolution can be calculated for a wide range of time and magnetic field,an optimal dynamic of spin flip-flop can be found,and more sophisticated approaches to achieve extremely high polarization of nuclear spins in a QD could be developed. 相似文献
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14.
H. Frahm C. Rödenbeck 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):409-414
We study the phase diagram of coupled spin-1/2 chains with bilinear and (chiral) three-spin exchange interactions in a magnetic
field. The model is soluble on a one-parametric line in the space of coupling constants connecting the limiting cases of a
single and two decoupled Heisenberg chains with nearest neighbour exchange only. We give a complete classification of the
low-energy properties of the integrable system and introduce a numerical method which allows to study the possible phases
of spin ladder systems away from the soluble line in a magnetic field.
Received 17 November 1998 and Received in final form 22
January 1999 相似文献
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We study the existence of a spin-glass phase in a field using Monte Carlo simulations performed along a nontrivial path in the field-temperature plane that must cross any putative de Almeida-Thouless instability line. The method is first tested on the Ising spin glass on a Bethe lattice where the instability line separating the spin glass from the paramagnetic state is also computed analytically. While the instability line is reproduced by our simulations on the mean-field Bethe lattice, no such instability line can be found numerically for the short-range three-dimensional model. 相似文献
18.
We study the superconductor to insulator transition at zero temperature in aJosephson-junction array model on a honeycomb lattice with f flux quantum perplaquette. The path integral representation of the model corresponds to a (2 + 1)-dimensional classical model, which isused to investigate the critical behavior by extensive Monte Carlo simulations on largesystem sizes. In contrast to the model on a square lattice, the transition is found to befirst order for f = 1 /3 and continuous for f = 1 / 2 but in a different universality class.The correlation-length critical exponent is estimated from finite-size scaling of vortexcorrelations. The estimated universal conductivity at the transition is approximately fourtimes its value for f =0. The results are compared with experimental observations on ultrathinsuperconducting films with a triangular lattice of nanoholes in a transverse magneticfield. 相似文献
19.
Nuclear spin-spin relaxation of60Co and56Co in iron single crystals has been studied, using the three-pulse NMRON spin echo. A previously reported rapidT
2 in60CoFe is shown to have arisen from a modulation of the echo amplitude, caused by variations in the phase of the Larmor precession
relative to the applied rf field. A lower limit ofT
2∼0.2s is found in56Co56
Fe. Extension of this result to other CoFe samples is discussed. 相似文献
20.
The behavior of a neutral particle with an intrinsic magnetic moment in an axisymmetric constant magnetic field is considered. The trajectory of the particle is found and the dynamics of the spin polarization vector in this field is found.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 70–72, July, 1982.The authors thank V. G. Bagrov and V. A. Bordovitsyn for discussion of the results and useful remarks. 相似文献