共查询到20条相似文献,搜索用时 31 毫秒
1.
P. Fröbrich P.J. Jensen P.J. Kuntz 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):477-489
The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function
theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire
temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus
a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation:
RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation
of the magnetization is determined by the spin components (), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient.
Results for the Green's function theory are compared with those obtained with mean-field theory (MFT). We find significant
differences between these approaches.
Received 6 April 1999 and Received in final form 9 July 1999 相似文献
2.
P. Fröbrich P.J. Jensen P.J. Kuntz A. Ecker 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):579-594
The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's
function theory by considering all components of the magnetization. We present a new method for the calculation of expectation
values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green's functions. This
formulation allows a straightforward extension of the monolayer case to thin films with many layers and for arbitrary spin
and moreover provides a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term,
an external magnetic field, a second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole coupling. We utilize
the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotropy terms.
The dipole coupling is treated in the mean-field approximation, a procedure which we demonstrate to be a sufficiently good
approximation for realistic coupling strengths. We apply the new method to monolayers with spin and to multilayer systems with S=1. We compare some of our results to those where mean-field theory (MFT) is applied to all interactions, pointing out some
significant differences.
Received 19 June 2000 and Received in final form 2 August 2000 相似文献
3.
JIANG Li-Xia ZHAO Xue-An LAI Huan-Wen LU Ding-Hui 《理论物理通讯》2006,46(5):938-944
The spin-dependent transport properties in the non-collinear pattern of series of δ-magnetic barriers are studied by using scattering theory and Green's function methods. The Green's function is obtained by using distorted wave approach and the scattering matrix is related by Fisher-Lee relationship. In addition to reproducing the results of Papp's and Xu's in parallel and antiparallel configurations, we also obtain further results, where arbitrary orientations of the magnetic barriers and arbitrary number of barriers are included. The main finding of our results is that the signs of polarizations can be switched around some "geometric unpolarized windows". The well-known antiparallel configuration has no such characteristics. Furthermore, we discuss spin-related partial densities of states in both polarized and unpolarized structures. 相似文献
4.
T. Kamppeter F.G. Mertens A. Sánchez A.R. Bishop F. Domínguez-Adame N. Grønbech-Jensen 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(4):607-618
We study the effects of finite temperature on the dynamics of non-planar vortices in the classical, two-dimensional anisotropic
Heisenberg model with XY- or easy-plane symmetry. To this end, we analyze a generalized Landau-Lifshitz equation including additive white noise and
Gilbert damping. Using a collective variable theory with no adjustable parameters we derive an equation of motion for the
vortices with stochastic forces which are shown to represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a Green's function formalism and obtain the mean
vortex trajectory and its variance. We find a non-standard time dependence for the variance of the components perpendicular
to the driving force. We compare the analytical results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition temperature. Finally, we discuss the reasons why our
approach is not appropriate for higher temperatures as well as the discreteness effects observed in the numerical simulations.
Received: 27 April 1998 / Revised: 2 September 1998 / Accepted: 10 September 1998 相似文献
5.
T. Saikawa A. Ferraz 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(1):17-22
Dynamical properties of the spin and charge response functions in the doped two-dimensional Hubbard model are calculated by
taking into account the drastic separation of the single-particle spectral function into the low-energy coherent and high-energy
incoherent parts due to the strong Coulomb interaction. We show that this evolution of the electronic states is the origin
of the broad and structureless feature in the charge response function. In the weak coupling regime the low-energy enhancement
of the spin excitation is produced which can be explained within the random phase approximation. However, for the larger interaction
close to the antiferromagnetic Stoner condition, the low-energy intensity of the spin excitation is suppressed.
Received: 25 September 1997 / Revised: 19 December 1997 / Accepted: 9 January 1998 相似文献
6.
The spin-wave excitations in anisotropic Kondo lattice model are studied using the spin Green's function. Both the ferromagnetic and antiferromagnetic cases are considered. The equations to determine the spectrum of low-energy excitations are given. The anisotropy gaps are obtained, and the long-wavelength and strong-coupling limits are analyzed. 相似文献
7.
This paper describes the magneto-optical effects of metallic multilayers under the condition of total internal reflection.
In the framework of the Green's dyadic technique, we present numerical simulations which account for the variation of the
magneto-optical signal with the angle of incidence. The Attenuated Total Reflection (ATR) has become a new technique of characterization
for thin films. We show, in this paper, optical effects due to a slight variation of the indice of refraction for thin dielectric
films in reflection by the reflectivity and the Kerr rotation spectra of an optimized system. In transmission, this variation
is brought to the fore by the near-field intensity spectra.
Received 29 March 2000 相似文献
8.
Investigation of ultra—thin ferromagnetic films with a simple cubic lattice 总被引:1,自引:0,他引:1
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Using Green‘s function method,we investigate ferromagnetic films with a simple cubic lattice containing up to ten monolayers.The Hamiltonian includes the Heisenberg exchange term,surface anisotropy (SA) and dipole interaction (DI).We calculate the magnetization as a function of temperature and film thickness,and we analyse the behaviour of spin canting.The result is in agreement with experiments.We calculate phase diagrams of SA versus DI to show the conditions under which spontaneous magnetization can occur.As a special case,we discuss the Heisenberg model without SA and DI. 相似文献
9.
J. Dietel 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):307-318
We consider the asymptotic behaviour of the Chern-Simons Green's function of the ν = 1/ system for an infinite area in position-time representation. We calculate explicitly the asymptotic form of the Green's function
of the interaction free Chern-Simons system for small times. The calculated Green's function vanishes exponentially with the
logarithm of the area. Furthermore, we discuss the form of the divergence for all τ and also for the Coulomb interacting Chern-Simons
system. We compare the asymptotics of the exact Chern-Simons Green's function with the asymptotics of the Green's function
in the Hartree-Fock as well as the random-phase approximation (RPA). The asymptotics of the Hartree-Fock Green's function
correspondence well with the exact Green's function. In the case of the RPA Green's function we do not get the correct asymptotics.
At last, we calculate the self consistent Hartree-Fock Green's function.
Received 5 July 2001 and Received in final form 30 November 2001 相似文献
10.
T.J. Smith H. Capellmann R.K. Kremer K-U. Neumann K.R.A. Ziebeck 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):341-347
A polarised neutron scattering investigation has been carried out on a powder sample of CuGeO3 within the temperature range of 1.5 K to 600 K. The magnetic scattering has been separated from all other contributions by
using polarised neutrons and polarisation analysis and placed onto an absolute scale. At low temperatures the long wavelength
components of the paramagnetic response are suppressed consistent with the formation of Cu dimers in which the magnetic moments
are correlated antiferromagnetically. This form of the scattering persists to temperatures well above the dimerisation temperature
T
sp
∼ 14 K. However as the temperature is raised the intensity of the long wavelength spin fluctuations increases and above ∼150 K they are the dominant feature in the wave vector dependence of the response. At all temperatures the observed scattering
extrapolates smoothly to the Q
= 0 value given by the uniform susceptibility. Consequently the thermal variation of the uniform susceptibility arises from the
evolution of the long wavelength magnetic fluctuations. At large wave vectors the energy dependence of the scattering revealed
that the response occurs below 16 meV in agreement with the reported maximum magnetic excitation energy at the zone boundary
in the ground state. However the total magnetic scattering is significantly less than that expected for a local moment system
suggesting that the spectrum of thermal and quantum fluctuations overlap.
Received 30 May 2000 and Received in final form 22 March 2001 相似文献
11.
T. Domański 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(1):41-45
Using the equation of motion technique for Green's functions we derive the exact solution of the boson fermion model in the
atomic limit. Both (fermion and boson) subsystems are characterised by the effective three level excitation spectra. We compute
the spectral weights of these states and analyse them in detail with respect to all possible parameters. Although in the atomic
limit there is no true phase transition, we notice that upon decreasing temperature some pairing correlations start to appear.
Their intensity is found to be proportional to the depleted amount of the fermion nonbonding state. We notice that pairing
correlations behave in a fashion observed for the optimally doped and underdoped high Tc superconductors. We try to identify which parameter of the boson fermion model can possibly correspond to the actual doping
level. This study clarifies the origin of pairing correlations within the boson fermion model and may elucidate how to apply
it for interpretation of experimental data.
Received 31 January 2003 / Received in final form 18 March 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: doman@kft.umcs.lublin.pl 相似文献
12.
C. Djurberg K. Jonason P. Nordblad 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(1):15-21
Experiments on the temperature and time dependence of the response function and the field cooled magnetisation of a Cu(Mn)
spin glass at temperatures below the zero field spin glass temperature are used to explore the non-equilibrium nature of the
underlying spin configuration. The results imply that a certain spin configuration is imprinted on the system as the temperature
is decreased at a constant cooling rate. The cooling rate governs the magnitude of the FC magnetisation ((H,T)). Any intermittent halt at a constant temperature, , imprints an extended spin configuration, a process that is reflected e.g. in a downward relaxation of . On continued cooling at the same rate, the magnitude of (T) remains at a lower level than that of a continuous cooling curve. These results are put into the context of the corresponding
behaviour of the response function as observed in measurements of the relaxation of the zero field cooled magnetisation.
Received 27 October 1998 and Received in final form 30 November 1998 相似文献
13.
Using Green's function method, we investigate the spin transport properties of armchair graphene nanoribbons (AG- NRs) under magnetic field and uniaxial strain. Our results show that it is very difficult to transform narrow AGNRs directly from semiconductor to spin gapless semiconductors (SGS) by applying magnetic fields. However, as a uniaxial strain is exerted on the nanoribbons, the AGNRs can transform to SGS by a small magnetic field. The combination mode be- tween magnetic field and uniaxial strain displays a nonmonotonic arch-pattern relationship. In addition, we find that the combination mode is associated with the widths of nanoribbons, which exhibits group behaviors. 相似文献
14.
I.I. Satija 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(2):345-348
Disordered systems exhibiting exponential localization are mapped to anisotropic spin chains with localization length being
related to the anisotropy of the spin model. This relates localization phenomenon in fermions to the rotational symmetry breaking
in the critical spin chains. One of the intriguing consequence is that the statement of Onsager universality in spin chains
implies universality of the localized fermions where the fluctuations in localized wave functions are universal. We further
show that the fluctuations about localized nonrelativistic fermions describe relativistic fermions. This provides a new approach
to understand the absence of localization in disordered Dirac fermions. We investigate how disorder affects well known universality
of the spin chains by examining the multifractal exponents. Finally, we examine the effects of correlations on the localization
characteristics of relativistic fermions.
Received 28 September 2001 / Received in final form 30 November 2001 Published online 2 October 2002
RID="a"
ID="a"e-mail: isatija@nickel.nist.gov 相似文献
15.
P. Fröbrich P.J. Kuntz 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(4):445-455
The many-body Green's function theory developed in our previous work for treating the reorientation of the magnetization of
thin ferromagnetic films is extended to include the exchange anisotropy. This leads to additional momentum dependencies which
require some non-trivial changes in the formalism. The theory is developed for arbitrary spin values S and for multilayers. The effects of the exchange anisotropy and the single-ion anisotropy, which was treated in our earlier
work, on the magnetic properties of thin ferromagnetic films are compared.
Received 31 October 2002 Published online 7 May 2003 相似文献
16.
对单边噪声源环境下空间中存在散射体时的格林函数到达时间结构提取展开了实验性研究。通过对位于沙滩上的传声器上记录的海浪噪声进行互相关处理,成功提取出两传声器间的格林函数到达时间结构。到达时间结构中观察到伪散射路径,并且伪散射路径在幅值上远高于散射路径。针对上述现象给出了理论及仿真解释:当传声器对受噪声源单边照射时,广义光学定理失效,伪散射路径将会出现;同时,由于伪散射路径由所有方向噪声源贡献,散射路径仅有稳相点附近声源贡献,因此伪散射路径在幅值上可能高于散射路径。结果表明,在单边噪声源环境下,基于互相关技术可以提取伪散射路径。伪散射路径的短时稳定提取可以为散射体无源成像提供更多信息,在海洋声学、地震学等领域具有潜在应用价值。 相似文献
17.
F. Ricoul M. Dubois T. Zemb D. Plusquellec 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(3):333-340
By use of a mean-field approach the spin-wave dispersion of the Cu degrees of freedom in the undoped high-T C material Nd2CuO4is investigated. The experimentally observed sharp decrease of the Cu spin-wave gap with increasing temperature in the range
is explained by a paramagnetic-like susceptibility of the Nd spins which couple to the Cu subsystem. The degeneracy of the
“in-plane” and “out-of-plane” polarized Cu spin-wave branches is shown to be lifted by the uniaxial anisotropy of the Cu-Cu
nearest-neighbor interaction.
Received: 13 August 1997 / Revised: 4 December 1997 / Accepted: 11 December 1997 相似文献
18.
A theoretical study is made for the role of an impurity layer embedded within a semi-infinite ferromagnet in determining the spectra of (0 0 1) surface spin waves and the layer magnetization for the surface and impurity layer. The calculations are described using simultaneously a closed form of the spin-wave Green's function and the matching procedure in the random-phase approximation. Analytic expressions for the Green's functions are also derived in a low-temperature spin-wave approximation. The theoretical approach determines the bulk and evanescent spin fluctuation fields in the two-dimensional plane normal to the surface. The results are used to calculate the energies of localized modes associated with the impurity layer as well as with the surface. Numerical examples of the modes are given and they are found to exhibit various effects due to the interplay between the impurity layer and surface modes. The results derived from the dynamic correlation functions between a pair of spin operators at any two sites are employed to evaluate the spin deviation in the ferromagnet due to the localized modes associated with the surface and with the impurity layer obtained by means of the matching procedure. The correlation functions and the layer magnetization are then illustrated as function of the impurity layer distance from the surface for a given temperature. 相似文献
19.
J. H. Asad A. Sakaji R. S. Hijjawi J. M. Khalifeh 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(3):365-370
A review of the theoretical approach for calculating the resistance between
two arbitrary lattice points in an infinite square lattice (perfect and
perturbed cases) is carried out using the lattice Green's function. We show
how to calculate the resistance between the origin and any other site using
the lattice Green's function at the origin, Go (0, 0), and its
derivatives. Experimental results are obtained for a finite square network
consisting of 30 ×30 identical resistors, and a comparison with those
obtained theoretically is presented. 相似文献