Browsing by Author "Ma, WJ"

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Characteristics of instantaneous resonant modes in simple dense fluids with short-ranged repulsive interactions
(2000-07-01)
Wu, TM
;
Ma, WJ
;
Chang, SL
;
物理研究所
;
Institute of Physics
We manifest the characteristics of the low-frequency, quasilocalized instantaneous normal modes, named as the instantaneous resonant modes (IRMs), in simple dense fluids with short-ranged repulsive interactions. The analyses include the potential energy profiles of the IRMs, and the local geometric structures and the number of the interacting neighbors of the particles at which the centers of the quasilocalization are located. We conclude that an IRM is created due to fluctuations in the local density, and has a barely-isolated center, which slightly interacts with one or two nearest neighbors, and the potential energy profile of an IRM is basically single-well with strong anharmonicity. The differences in character between the IRMs and the high-frequency localized instantaneous normal modes are also examined. Based on the barely isolated center picture, a necessary criterion for the occurrence of the IRMs is proposed. While only the imaginary-frequency IRMs are found in dense fluids with purely repulsive interactions satisfying the criterion, a tiny attractive well in the pair potential allows the occurrence of the real-frequency IRMs. The physical systems to detect the presence of the IRMs are discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)50225-6].
Conservation constraints on random matrices
(2003-02-07)
Ma, WJ
;
Wu, TM
;
Hsieh, J
;
物理研究所
;
Institute of Physics
We study the random matrices constrained by the summation rules that are present in the Hessian of the potential energy surface in the instantaneous normal mode calculations, as a result of momentum conservation. In this paper, we analyse the properties related to such conservation constraints in two classes of real symmetric matrices: one with purely row-wise summation rules and the other with the constraints on the blocks of each matrix, which underscores partially the spatial dimension. We show explicitly that the constraints are removable by separating the degrees of freedom of the zero-eigenvalue modes. The non-spectral degrees of freedom under the constraints can be realized in terms of the ordinary constraint-free orthogonal symmetry but with the rank deducted by the block dimension. We propose that the ensemble of real symmetric matrices with full randomness, constrained by the summation rules, is equivalent to the Gaussian orthogonal ensemble (GOE) with lowered rank. Independent of the joint probability distribution, the Jacobian contributed by the transformation from the matrix coordinates to the spectral coordinates, possesses the same spectral-variable dependence in these two cases. We verify numerically that the small-separation asymptotic in the nearest-neighbour level spacing distribution is indeed governed by the same symmetry power factor, beta = 1, as that of the GOE, under conservation constraints and other underlying spatial correlations.
Evidence for instantaneous resonant modes in dense fluids with repulsive Lennard-Jones force
(1999-01-01)
Wu, TM
;
Ma, WJ
;
物理研究所
;
Institute of Physics
In terms of instantaneous-normal-mode (INM) analysis and a newly defined measure for quasilocalization, we present the evidence for the resonant modes in a model fluid, in which the pair interaction is merely the repulsive portion of the Lennard-Jones potential. We name such a quasilocalized INM as an instantaneous resonant mode (IRM). By examining the potential energy profile beyond the INM approximation, we conclude that the IRMs occur in single-well potentials with strong enough anharmonicity. (C) 1999 American Institute of Physics. [S0021-9606(99)50301-2].
Evidence for instantaneous resonant modes in dense fluids with repulsive Lennard-Jones force
(1999-01-01)
Wu, TM
;
Ma, WJ
;
物理研究所
;
Institute of Physics
In terms of instantaneous-normal-mode (INM) analysis and a newly defined measure for quasilocalization, we present the evidence for the resonant modes in a model fluid, in which the pair interaction is merely the repulsive portion of the Lennard-Jones potential. We name such a quasilocalized INM as an instantaneous resonant mode (IRM). By examining the potential energy profile beyond the INM approximation, we conclude that the IRMs occur in single-well potentials with strong enough anharmonicity. (C) 1999 American Institute of Physics. [S0021-9606(99)50301-2].
Instantaneous resonant modes in high-temperature gallium liquids
(2001-08-01)
Wu, TM
;
Tsay, SF
;
Chang, SL
;
Ma, WJ
;
物理研究所
We have studied the low-frequency, quasilocalized vibrational excitations in Ga liquids in terms of instantaneous-normal-mode analysis. We present strong numerical evidence for the existence of the instantaneous resonant modes (IRM's) in high-temperature Ga liquids. It is the exotic repulsive-core interaction between Ga particles that gives rise to the IRM's, which are associated with special local structures created by density fluctuation. In general, the IRM density of states is found to increase with temperature.
Level statistics of Hessian matrices: random matrices with conservation constraints
(2003-04-01)
Ma, WJ
;
Wu, TM
;
Hsieh, J
;
Chang, SL
;
物理研究所
We consider the Hessian matrices of simple liquid systems as a new type of random matrices. By numerically comparing the distribution of the nearest-neighbor level spacing of the eigenvalues with the Wigner's surmise, we round that the level statistics is akin to the generic Gaussian Orthogonal Ensemble (GOE), in spite of the constraints due to the summation rules and the presence of the correlation among the components inherited with the underlying spatial configuration. The distribution is in good agreement with the Wigner's prediction if only the extended eigenstates are considered. Indeed, our theoretical analysis shows that the ensemble of real symmetric matrices with full randomness. but constrained by the summation rules, is equivalent to the GOE with matrices of the rank lowered by the spatial dimension. (C) 2002 Published by Elsevier Science B.V.
Local geometric structures of instantaneous resonant modes in Ga liquids
(2002-05-01)
Wu, TM
;
Ma, WJ
;
Chang, SL
;
Tsay, SF
;
物理研究所
Recently, we have numerically found the instantaneous resonant modes (IRMs) in high-temperature Ga liquids. The occurrence of these IRMs is attributed to the cooperation of the exotic repulsive core of the Ga pseudopotential and the local volume expansion. In this paper, the local structures around the quasilocalized centers of these IRMs are analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.
Microscopic morphology and energy surface landscape in a supercooled soft-sphere system
(2000-06-15)
Ma, WJ
;
Wu, TM
;
物理研究所
;
Institute of Physics
We carry out a systematic study on the microscopic morphology and its relationship with the signatures of the traces swept over the potential energy surface in a soft-sphere fluid system interacting via a truncated Lennard-Jones potential. The state of the model system is tuned by changing its temperature obtained in a molecular dynamics simulation, ranging from the equilibrium fluid phase to the deeply quenched supercooled states. The density and the range of interaction are chosen so that the instantaneously barely isolated centers are present. We analyze the microscopic structural origin of the features in the instantaneous-normal-mode (INM) spectrum and search for evidence of state transformation. It is found that the presence of drastic changes in the INM spectra upon entering the deep supercooled regime is accompanied by the reduction in the spatial connectivity. We develop a method to characterize the spatial configuration ration in an attempt to address the origin of the temperature-dependent changes in the spectra. (C) 2000 Elsevier Science B.V. All rights reserved.