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Featured researches published by Y. L. Wang.


Journal of Applied Physics | 1991

Ground‐state properties for an easy‐plane spin‐one ferromagnet: The coupled‐cluster method

C. F. Lo; Kok-Kwei Pan; Y. L. Wang

In this paper we have investigated the ground state of an easy‐plane spin‐one ferromagnet: H = D∑i(Sxi)2 −(J/2)∑(i, j)Si⋅Sj −h∑iSzi, using the coupled‐cluster method. The mean‐field approximation is commonly used in calculations of thermodynamic quantities for such systems because of the complexities caused by the single‐ion anisotropy term. However, in the mean‐field approximation both quantum and spin fluctuation correlations are neglected. Above the critical point, the zero‐temperature series expansion method has been used to obtain estimates of the critical crystal field potential (Dc) and critical exponent of the susceptibility. Using the coupled‐cluster method we obtained the ground‐state energy, magnetization and critical point for both two‐ and three‐dimensional lattices.


Journal of Applied Physics | 1971

Phase Transition of an Anisotropic Ferromagnet

Y. L. Wang; David X. Yang; M. R. H. Khajehpour

The phase transition of an Ising Ferromagnet with uniaxial single ion anisotropy with spin one has been investigated extensively by Capel. Here we consider a similar but a more general system with anisotropic exchange interaction. The Hamiltonian can be written as H=D  ∑ i (Siz)2−  ∑ ij JijSizSjz−  ∑ ij Kij(SixSjx+SiySjy), where J, K, and D are positive parameters. We show that in addition to the more familiar phase transition from an ordered phase to the paramagnetic phase, as temperature is raised, a first‐order phase transition can occur. Depending upon the relative magnitude of D, J, and K, a phase transition from the ferromagnetic state with spin ordering along (perpendicular to) the z axis to a ferromagnetic state with spin ordering perpendicular to (along) the z axis can take place at a critical temperature lower than the Curie temperature where a transition to the paramagnetic phase occurs. The boundaries of the three phases have been calculated in the molecular field approximation.


Journal of Applied Physics | 1993

The antiferromagnetic Ising model in a magnetic field: Linked‐cluster expansion analysis

C. D. Wentworth; Y. L. Wang

The mean‐field approximation of the critical behavior of the antiferromagnetic Ising model in a magnetic field shows that the critical field increases first at low temperature. However, the exact solution of a decorated square lattice proves that the critical field rapidly decreases with increasing temperature for the two‐dimensional lattice. Three‐dimensional lattices of this model have been studied as well using the high temperature expansion, low temperature expansion, and Monte Carlo simulation. The results are different. In this article we have investigated the critical field of the body centered cubic lattice of this model using the linked‐cluster expansion technique. An eighth‐order perturbation series for the free energy has been calculated. The magnetization and staggered magnetic susceptibility series have been calculated from the free energy. These series have been analyzed using Pade approximants to obtain the phase diagram in the field‐temperature plane. A comparison is made with Monte Carlo ...


Journal of Applied Physics | 1978

Application of the Green's function diagrammatic technique to complex crystal field systems with bilinear and biquadratic exchange

M. J. Sablik; Y. L. Wang

A diagrammatic Green’s function technique is used to obtain the elementary excitations in a rare earth system with cubic crystal field and both bilinear and biquadratic couplings. We find three distinct modes of excitations—longitudinal (L) and transverse (T1 and T2). L modes and T1 modes consist of mixed dipolar and quadrupolar excitations below Tc, but above Tc they are either pure dipolar or pure quadrupolar in character. The T2 modes are pure quadrupolar. Dispersion and temperature dependence of the exciton energies are shown for a particular model system, DySb, but the computer program used is general enough to be used on other systems whose molecular field states are known.


Journal of Applied Physics | 1993

Quantum fluctuations in easy‐plane spin‐one ferromagnet

Kok-Kwei Pan; Y. L. Wang

The magnetic properties of easy‐plane spin‐one Heisenberg ferromagnet, H=D∑i(Six)2 −J∑(i,j)Si⋅Sj−h∑iSiz, are investigated using the linked cluster series expansion. The results of the linked cluster series expansion method cover the whole range of temperatures, both magnetically ordered and disordered phases. The order parameter is included in correlations of spin fluctuation and solved self‐consistently to high orders. The magnetization as a function of temperature for a given value of easy‐plane anisotropy is shown. The observed values of magnetization from the linked cluster series expansion are appreciably depressed from their mean‐field approximation values due to the quantum and thermal fluctuations. The effects of the quantum fluctuation on the magnetic ordering in three‐dimensional (3D) systems are discussed.


Journal of Applied Physics | 1982

General‐temperature series expansion for Ising systems

Felix Lee; B. Westwanski; Y. L. Wang

A linked‐cluster expansion technique for spin systems has been used to obtain the free energy, the magnetization, and the susceptibility series for three Ising systems: (1) The Ising model; (2) The Blume‐Capel model; and (3) The Blume‐Emery‐Griffiths (BEG) model. Each Hamiltonian is first divided into a single‐ion potential and a term describing the interaction of the spin fluctuations. Only the latter is treated as perturbation in the series expansion. In the parametric phase our series reduces to the exact high‐temperature series. However, in the ordered phase the same series can be used to study the low temperature behavior, such as the variation of the magnetization with temperature. The analysis of the eighth‐order series for systems (1) and (2) show results in excellent agreement with the previous ones obtained from the high‐temperature and low‐temperature series. The series analysis for the BEG model shows quite different behavior from that predicted by the mean‐field approximation.


Journal of Applied Physics | 1985

Magnetic and quadrupolar excitations in NdSb

M. J. Sablik; Y. L. Wang

We have extended our previous analysis of magnetic and quadrupolar excitations in rare‐earth compounds to NdSb, which is a type‐I antiferromagnet. The full ten‐level manifold of the Nd+++ crystal field is incorporated into the analysis. Also, a study of excitation amplitudes is included as a function of q across the Brillouin zone. Zone edge effects on these amplitudes are discussed. Comparison of our analysis with the experimental data of Furrer et al. reveals that mixing of quadrupolar excitations with magnetic excitations below the ordering temperature can lead to a flattening of the dispersion if there are competing quadrupolar and exchange couplings.


Journal of Applied Physics | 1979

High‐temperature series expansion for magnetic systems with single‐ion anisotropy

J. W. Johnson; Y. L. Wang

An improved computational technique of the high‐temperature series expansion for magnetic systems with single‐ion anisotropy of arbitrary strength is presented. The method facilitates the calculation of higher‐order terms in the expansion. While applications have been made to obtain susceptibility and specific heat series for spin‐one ferromagnets with general single‐ion anisotroy, only the general method will be shown.


Journal of Applied Physics | 1993

Ground‐state properties for an easy‐plane spin‐one antiferromagnet— The coupled‐cluster method (abstract)

W. H. Wong; C. F. Lo; Y. L. Wang

Using the coupled‐cluster method we investigate the ground‐state properties of an easy‐plane spin‐one antiferromagnet which has a critical point at zero temperature. This spin system represents a nontrivial system with the single‐ion anisotropy. Single‐ion anisotropies have a fundamental influence on the behavior of a magnetic system, and prevail in almost all physical systems with spin greater than one half. Because of the complexities caused by the single‐ion anisotropy term, the mean‐field approximation is commonly used in calculations of the thermodynamic quantities. However, in the mean‐field approximation both quantum and spin fluctuation correlations have been neglected. Here we apply the coupled‐cluster method to obtain the ground‐state energy, staggered magnetization and critical point for both two and three dimensional lattices. In the coupled‐cluster method the correlations of the quantum and spin fluctuations have been systematically included.


Journal of Applied Physics | 1979

Abstract: Magnetic excitons in a type II antiferromagnet with bilinear and biquadratic coupling in a cubic crystal field

M. J. Sablik; Y. L. Wang

We extend the diagrammatic Green’s function technique to a type II antiferromagnet in order to be able to treat a particular system, DySb, in which biquadratic effects are well‐established.(AIP)

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M. J. Sablik

Fairleigh Dickinson University

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J. W. Johnson

Florida State University

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Kok-Kwei Pan

Florida State University

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W. H. Wong

The Chinese University of Hong Kong

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B. Rao

Florida State University

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B. Westwanski

Florida State University

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C. F. Lo

The Chinese University of Hong Kong

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David X. Yang

Florida State University

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