共查询到20条相似文献,搜索用时 31 毫秒
1.
L. S. Lima A. S.T. Pires 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,70(3):335-342
In this paper we study the dynamics of the two-dimensional XY model with single-ion anisotropy, and spin S = 1, in the large
D phase, and low temperatures, using the bond operator formalism. The in-plane structure factor is a delta function. The out
of plane shows a three peak structure, which merges in a single peak at the Brillouin zone boundary. We analyze also spin
currents generated by a magnetic field gradient. The spin conductivity is calculated, at finite temperature, using the Kubo
formula. The model shows unconventional ballistic spin transport at finite temperature. The computed spin conductivity exhibits
a nonzero Drude weight at finite temperature. For ω< 2m, where m is the energy gap, the spin conductivity is described solely
by the Drude weight. There is a regular contribution to the spin conductivity for ω> 2m, which persist in the zero temperature
limit. The conductivity at the critical point, and for small frequencies, is (gμB)2/ħ times a universal scaling function of ħω/kB T. 相似文献
2.
Yuri P. Kalmykov William T. Coffey Serguey V. Titov 《Journal of statistical physics》2008,131(5):969-987
A master equation, for the time evolution of the quasi-probability density function of spin orientations in the phase space
representation of the polar and azimuthal angles is derived for a uniaxial spin system subject to a magnetic field parallel
to the axis of symmetry. This equation is obtained from the reduced density matrix evolution equation (assuming that the spin-bath
coupling is weak and that the correlation time of the bath is so short that the stochastic process resulting from it is Markovian)
by expressing it in terms of the inverse Wigner-Stratonovich transformation and evaluating the various commutators via the
properties of polarization operators and spherical harmonics. The properties of this phase space master equation, resembling
the Fokker-Planck equation, are investigated, leading to a finite series (in terms of the spherical harmonics) for its stationary
solution, which is the equilibrium quasi-probability density function of spin “orientations” corresponding to the canonical
density matrix and which may be expressed in closed form for a given spin number. Moreover, in the large spin limit, the master
equation transforms to the classical Fokker-Planck equation describing the magnetization dynamics of a uniaxial paramagnet. 相似文献
3.
Yuri P. Kalmykov Bernard P. J. Mulligan Serguey V. Titov William T. Coffey 《Journal of statistical physics》2010,141(3):589-606
The time evolution equation for the probability density function of spin orientations in the phase space representation of
the polar and azimuthal angles is derived for the nonaxially symmetric problem of a quantum paramagnet subjected to a uniform
magnetic field of arbitrary direction. This is accomplished by first rotating the coordinate system into one in which the polar axis is collinear with
the field vector, then writing the reduced density matrix equation in the new coordinate system as an explicit inverse Wigner-Stratonovich
transformation so that the phase space master equation may be derived just as in the axially symmetric case [Yu.P. Kalmykov
et al., J. Stat. Phys. 131:969, 2008]. The properties of this equation, resembling the corresponding Fokker-Planck equation, are investigated. In particular,
in the large spin limit, S→∞, the master equation becomes the classical Fokker-Planck equation describing the magnetization dynamics of a classical
paramagnet in an arbitrarily directed uniform external field. 相似文献
4.
Virgiliu Craciun Ghiorghe Calugaru Vasile Badescu 《Czechoslovak Journal of Physics》2002,52(5):725-728
We have created a fast algorithm for calculating the temperature profile in a living tissue treated by magnetic fluid hyperthermia.
Our algorithm solves an equation by the finite difference method. This equation includes the “heat sink” and “K-effective”
effects. The algorithm need not an initial solution and it has the peculiarity that it makes a recursive division of the network,
reducing thus the calculation period 5–9 times. 相似文献
5.
Enrico Lipparini Leonardo Colletti Giusi Orlandini Liorenç Serra 《Czechoslovak Journal of Physics》1998,48(6-7):725-731
We study spin longitudinal and transverse linear response of the 3-dimensional electron gas, metal clusters and quantum dots.
When the systems are spin unpolarized in the ground state, a low energy collective state emerges in finite size systems due
to the discrete shell structure, whereas it is absent in the bulk due to the Landau damping. In the case of spin polarization
of the ground state a collective state is present also in the bulk and a family of new collective states appears in finite
size systems.
Presented by E. Lipparini at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5,
1997. 相似文献
6.
The magnetic relaxation of a system of single-domain ferromagnetic particles in the presence of a strong static magnetic field
directed at an arbitrary angle relative to the particle anisotropy axis is investigated. A system of linear difference-differential
equations for the moments (averaged spherical harmonics) is derived without recourse to the Fokker-Planck equation by averaging
Gilbert’s equations with a fluctuating field. An exact solution (in terms of matrix continuous fractions) is found for this
system. The relaxation times and spectra of the complex magnetic susceptibility are calculated.
Fiz. Tverd. Tela (St. Petersburg) 40, 1642–1649 (September 1998) 相似文献
7.
A. Bhattacharyya S. Chatterjee S. Giri S. Majumdar 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,70(3):347-351
The magnetotransport and magnetic properties of the binary intermetallic compound Ho2In have been investigated. Clear signature of long range ferromagnetic order in the resistivity and the magnetization data
at TC = 85 K is observed. A further spin reorientation type transition is also apparent in our measured data at around Tt = 32 K. The sample exhibits negative magnetoresistance (peak value of –14% at 5 T) over a wide temperature range that extends
well above TC. Substantially large magneto-caloric effect is also observed in the sample (maximum value of –8.5 J kg-1K-1 for 0 → 5 T), which peaks around the TC of the sample. The observed magnetoresistance and magnetocaloric effect are related to the suppression of spin disorder by
an external magnetic field. Ho2In can be an interesting addition to the list of rare-earth based magnetic refrigerant materials showing magneto-caloric effect
across a second order phase transition. 相似文献
8.
L. S. Ewe I. Hamadneh H. Salama N. A. Hamid S. A. Halim R. Abd-Shukor 《Applied Physics A: Materials Science & Processing》2009,95(2):457-463
The magnetotransport and magnetoresistive (MR) properties of manganese-based La0.67Ca0.33MnO3 perovskite with different grain sizes are reported. The electrical resistivity was measured as a function of temperature
in magnetic fields of 0.5 and 1 T. The insulator–metal transition temperature, T
IM, shifted to a higher temperature with the application of the magnetic field. In zero field, T
IM is almost constant (∼271 K) for all samples except for the sample with the largest grain size, where T
IM=265 K. The temperature dependence of resistivity was fitted with several equations in the metallic (ferromagnetic) region
and the insulating (paramagnetic) region. The density of states at the Fermi level, N(E
F), and the activation energy of electron hopping were estimated by fitting the resistivity versus temperature curves. The
ρ–T
2 curves are nearly linear in the metallic regime, but the ρ–T
2.5 curves exhibit a deviation from linearity. The variable range hopping model and small polaron hopping model fit the data
well in the high-temperature region, indicating the existence of the Jahn–Teller distortion that localizes the charge carriers.
MR was found to increase with an increase in the magnetic field, an effect which is attributed to the intergrain spin tunneling
effect. 相似文献
9.
H. Kuratsuji R. Botet 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,49(1):111-116
We investigate a novel aspect of rotational tunneling of the macroscopic spin for multicomponent spinor Bose-Einstein condensate (BEC). The Lagrangian is deduced from
the multi-component BEC system formalism, and is written in terms of spin coherent states. From the effective Hamiltonian
for the collective spin, the tunneling rate is obtained through a functional integral of the spin variable. It is pointed
out that the cooperative effect between the Zeeman energy and the anisotropic nature of the spin-dependent inter-atomic interaction
plays a key role for occurrence of collective spin tunneling. 相似文献
10.
We study synchronization as a means of control of collective behavior of an ensemble of coupled stochastic units in which oscillations are induced merely by external noise. For a large number of one-dimensional continuous stochastic elements coupled non-homogeneously through the mean field with delay we developed an approach to find a boundary of synchronization domain and the frequency of the mean-field oscillations on it. Namely, the exact location of the synchronization threshold is shown to be a solution of the boundary value problem (BVP) which was derived from the linearized Fokker-Planck equation. Here the synchronization threshold is found by solving this BVP numerically. Approximate analytics is obtained by expanding the solution of the linearized Fokker-Planck equation into a series of eigenfunctions of the stationary Fokker-Planck operator. Bistable systems with a polynomial and piece-wise linear potential are considered as examples. Multistability and hysteresis in the mean-field behavior are observed in the stochastic network at finite noise intensities. In the limit of small noise intensities the critical coupling strength is shown to remain finite, provided that the delay in the coupling function is not infinitely small. Delay in the coupling term can be used as a control parameter that manipulates the location of the synchronization threshold. 相似文献
11.
F. Mancini F. P. Mancini 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,68(3):341-351
Within the Green’s function and equations of motion formalism it is possible to exactly solve a large class of models useful
for the study of strongly correlated systems. Here, we present the exact solution of the one-dimensional extended Hubbard
model with on-site U and first nearest neighbor repulsive V interactions in the presence of an external magnetic field h,
in the narrow band limit. At zero temperature our results establish the existence of four phases in the three-dimensional
space (U, n, h) – n is the filling – with relative phase transitions, as well as different types of charge ordering. The magnetic
field may dramatically affect the behavior of thermodynamic quantities, inducing, for instance, magnetization plateaus in
the magnetization curves, and a change from a single to a double-peak tructure in the specific heat. According to the value
of the particle density, we find one or two critical fields, marking the beginning of full or partial polarization. A detailed
study of several thermodynamic quantities is also presented at finite temperature. 相似文献
12.
13.
A. G. Isavnin 《Russian Physics Journal》2007,50(5):471-476
An analytical expression for the superparamagnetic relaxation rate of uniaxial particles modulated by a radio-frequency field
is chosen that describes adequately the magnetic moment dynamics under conditions of a stochastic resonance in the approximation
of discrete orientations. The complex magnetic susceptibility components are calculated for a weak external variable field.
A comparison is made with the results of numerical modeling based on a solution of the Fokker-Planck equation with a periodic
drift term.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 54–58, May, 2007. 相似文献
14.
A. Ya. Silenko 《Journal of Experimental and Theoretical Physics》1998,87(2):246-250
The Dirac-Pauli equation is used to obtain the exact equation of spin motion for spin-1/2 particles with an anomalous magnetic
moment in a constant and uniform magnetic field. Exact formulas are established for the angular velocity of the revolution
of such particles along circular orbits and the rotation of the particle spin with respect to momentum. Finally, a quantum
mechanical equation for the motion of the particles in a strong magnetic field is derived.
Zh. éksp. Teor. Fiz. 114, 448–457 (August 1998) 相似文献
15.
16.
It is found that at a critical value of the magnetic field in which a system of composite fermions becomes completely spin-polarized,
the temperature dependence of the electronic spin polarization is a linear function at low temperatures. It is shown that
the slope of this dependence is determined by the Fermi energy of the composite fermions. This made it possible to measure
the Fermi energy and the Zeeman splitting of the composite fermions. A large amplification of the spin splitting of composite
fermions for complete spin polarization of the system is found. This makes it possible to measure the strength of the interaction
between composite fermions.
Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 722–726 (10 December 1999) 相似文献
17.
Chengfu Mu Pengfei Zhuang 《The European Physical Journal C - Particles and Fields》2008,58(2):271-279
We investigate the effect of the restoration of chiral symmetry on the quark potential in a quark–meson plasma by considering
meson exchanges in the two flavor Nambu–Jona-Lasinio model at finite temperature and density. There are two possible oscillations
in the chiral restoration phase; one is the Friedel oscillation due to the sharp quark Fermi surface at high density, and
the other is the Yukawa oscillation driven by the complex meson poles at high temperature. The quark–meson plasma is strongly
coupled in the temperature region 1≤T/T
c≤3, with T
c being the critical temperature of the chiral phase transition. The maximum coupling in this region is located at the phase
transition point. 相似文献
18.
V. E. Bisti 《JETP Letters》1999,69(8):584-588
The spectrum of intersubband collective excitations of spin and charge density in a system of quasi-two-dimensional electrons
is calculated in the strong magnetic field limit for filling factors υ≤4. For υ≤2 two new closely spaced modes of collective excitations are obtained. The modes obtained make it possible to give a new
interpretation of the experimentally observed line, which is usually interpreted as being due to single-particle excitations.
Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 543–547 (25 April 1999) 相似文献
19.
A.A. Zvyagin D.M. Apal'kov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):565-569
We present a Bethe Ansatz based investigation of a one-dimensional (1D) Heisenberg spin chain in a real 3D crystal lattice.
We have shown that due to an influence of the lattice distortion on a crystalline field of ligands of magnetic ions, a Heisenberg
antiferromagnetic spin chain is unstable under the appearance of a magnetic anisotropy of the “easy-plane” type. The effects of an external magnetic
field and nonzero temperature onto such a phase transition are studied.
Received: 19 January 1998 / Revised: 16 March 1998 / Accepted: 17 March 1998 相似文献
20.
F. H. M. Cavalcante A. W. Carbonari R. N. Saxena J. Mestnik-Filho 《Hyperfine Interactions》2004,158(1-4):125-129
Perturbed gamma–gamma angular correlation technique was used to measure the magnetic hyperfine field at Gd sites in the intermetallic
compound GdAg using the 140La→140Ce nuclear probe. A major and well-defined magnetic interaction is observed at 140Ce substituting Gd sites in GdAg below 130 K, corresponding to a ferromagnetic ordering of Gd moments. The temperature dependence
of magnetic hyperfine field, however, shows a sharp deviation from an expected Brillouin-like behavior for temperatures below
75 K. This additional magnetic interaction is believed to result from the polarization of Ce spin moments induced by the magnetic
field from Gd atoms. 相似文献