VTechWorks Repository :: Browsing by Author "Lee, T. K."

Browsing by Author "Lee, T. K."

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1/N expansion for the degenerate Anderson model in the mixed-valence regime
Zhang, F. C.; Lee, T. K. (American Physical Society, 1983-07)
The 1N expansion method for the degenerate Anderson model is formulated. N is the degeneracy factor of one of the f-electron configurations. Various ground-state properties are calculated. Excellent agreement with the result of Bethe ansatz for N=6 is shown. The rate of convergence of the series is analyzed. The merit and inadequacy of the method are discussed. At zero temperature the ratio of the magnetic susceptibility and the specific-heat linear coefficient is shown to lie within a range of 1 and 1+(N-1)-1.
Dielectric function of polyacetylene
Lee, T. K.; Kivelson, S. (American Physical Society, 1984-06)
The optical dielectric function for polyacetylene has been calculated in the random-phase approximation. Local-field corrections are included. The optical-absorption coefficient obtained is in good agreement with experimental data. The contributions of interchain coupling and exchange interaction are discussed.
Dispersion of a single hole in the t-J model
Lee, T. K.; Shih, C. T. (American Physical Society, 1997-03-01)
The dispersion of a single hole in the t-J model obtained by the exact result of 32 sites and the results obtained by self-consistent Born approximation and the Green function Monte Carlo method can be simply derived by a mean-field theory with d-wave resonating-valence-bond (d-RVB) and antiferromagnetic order parameters. In addition, it offers a simple explanation for the difference observed between those results. The presence of the extended van Hove region at (pi,0) is a consequence of the (d-RVB pairing instead of the antiferromagnetic order. Results including t' and t '' are also presented and explained consistently in a similar way.
Energies of the staggered flux phase: A numerical study
Lee, T. K.; Chang, Lay Nam (American Physical Society, 1990-11)
Kinetic and magnetic energies of the staggered flux phase, with a fictitious flux of equal magnitude but opposite direction in adjacent square plaquettes, are calculated for the two-dimensional t-J model using the variational Monte Carlo method. They are compared to the energies of the resonating-valence-bond state, the flux phase with half a quantum per plaquette, and the projected Fermi-liquid state. For about 10% hole concentration the staggered flux phase has the lowest energy of the nonsuperconducting states, but its energy is still higher than that of the superconducting d-wave state.
Extended and localized states in the periodic Anderson model
Lee, T. K.; Zhang, F. C. (American Physical Society, 1986-12)
The renormalized quasiparticle states are derived for a periodic Anderson model with a general hybridization matrix element between conduction electrons with two degrees of freedom and f electrons with N degrees of freedom. Only two out of N local f states form the extended quasi-particle bands while N−2 localized states remain. As an illustration we show that the self-energy due to the Kondo effect produces quasiparticle bands and a gap as obtained in the Kondo-boson approach.
Hard-core repulsive interactions in even-parity electron pairings for heavy-fermion systems
Zhang, F. C.; Lee, T. K. (American Physical Society, 1987-03)
By studying the Anderson lattice Hamiltonian with spin-orbit coupling using an auxiliary boson method, we have examined the hard-core repulsive interactions in heavy-fermion materials. As a consequence of the anisotropy of the repulsive interaction, all lower-order partial-wave Cooper pairings in the even-parity channel are strongly impeded.
Hole dynamics and effective hole-hole interaction in a quantum antiferromagnet
Xu, J. H.; Ting, C. S.; Lee, T. K. (American Physical Society, 1991-04)
By the use of an effective Hamiltonian which takes into account the constraint on the motion of holes in a quantum antiferromagnet, the dynamics of holes is investigated. It is found that there is a quasiparticle band whose width is of order J at the bottom of the hole spectrum. The effective hole-hole interaction mediated by spin fluctuations is derived and the attraction between holes in the d-wave channel is obtained. The results suggest that a d-wave-pairing condensation in a quantum antiferromagnet is very possible.
Impulse electrical breakdown of high-purity water
Gehman, Victor H. (Virginia Tech, 1995-05-05)
Experiments have been conducted on the electrical breakdown of high-purity water and water mixtures. The electrical regime of interest has been carefully defined and documented to consist of electrical impulses with approximately microsecond rise time and fall time greater than 65 microseconds, on approximately 81-square-centimeter-area planar electrodes with a dielectric gap of approximately one centimeter. The results of over 25,000 shots by a Marx generator have been distilled into database form in an Excel spreadsheet and analysis performed to try to find patterns or indirect evidence into the nature of the breakdown-initiation process. An extensive review of all the experiments, which had been conducted over eight years by the Naval Surface Warfare Center and which had been designed to find the largest water-breakdown fields, was conducted with the intention of delineating the physical factors that led to breakdown. A variety of theoretical models of breakdown initiation were compared to the data, until it became clear that many of the breakdowns were dominated by impurities of various sorts. An extensive study of old and new experiments led to a more detailed understanding of the phenomenology of impurity-dominated water breakdown (such as the process of "conditioning" the electrodes and hysteresis) and the proposal of a number of new experiments to further characterize the intrinsic role of electrode materials on determining high-electric-field dielectric breakdown in water.
Mobile spin bags and their interaction in the spin-density-wave background
Weng, Z. Y.; Ting, C. S.; Lee, T. K. (American Physical Society, 1990-02)
The motion of one doped hole in the spin-density-wave background has been studied by the variational approach. A spin-bag solution for the doped hole has been obtained that applies in the weak- and intermediate-coupling regime. The effective attraction between two spin bags with antiparallel spins has been shown to be p- and d-wave like. The low-lying spin-flip excitations are found to be important both in the spin-bag effect of the doped holes and the effective attraction between them, which, in the latter case, are combined with the exchange of the simple-amplitude fluctuations to give a dominant contribution. On the other hand, the total attractive interaction is reduced by the Coulomb repulsion, which is restored in the presence of the "bag" effect.
New phase in the one-dimensional t-J model
Chen, Y. C.; Lee, T. K. (American Physical Society, 1993-05)
A new phase of a gas of pairs of electrons bounded in a singlet state is found in the one-dimensional t -J model for J > 2t and the density of electrons less than 0.2. This phase was conjectured in the study of the diagonalization of small lattices [Phys. Rev. Lett. 66, 2388 (1991)]. The existence of this new phase for much larger lattice sizes is demonstrated by a combination of two numerical methods, the variational Monte Carlo and the power method. A trial wave function for this phase is proposed and shown to be in good agreement with the ground state obtained by the power method.
Path-integral approach to the Hubbard model
Weng, Z. Y.; Ting, C. S.; Lee, T. K. (American Physical Society, 1991-02)
A path-integral approach to the Hubbard model is developed for the whole range of the coupling strength U. At half filling, the strong-coupling results are readily reproduced within the simple Gaussian fluctuations. The low-lying spin wave is shown to be described by the nonlinear sigma-model. The effective coupling of the doped hole with the background fluctuations also agrees with that obtained from the t-J model in the small-doping limit. At finite doping, such a formalism may provide a starting point for investigating the short-range spin-liquid state.
Resonating-valence-bond wave function for the two-dimensional Heisenberg model on a triangular lattice
Lee, T. K.; Feng, S. (American Physical Society, 1990-06)
A resonating-valence-bond (RVB) wave function for the triangular lattice is constructed by following the renormalized Hamiltonian approach (or Gutzwiller approximation) successfully developed for the square lattice. This wave function is then used to calculate the ground-state energy by using the variational Monte Carlo method. Comparison with results of other trial wave functions is discussed. Energy of this paramagnetic RVB state is not as low as other antiferromagnetically ordered wave functions. A flux-phase state similar to the RVB state is also presented. This state with one-quarter flux quantum per plaquette breaks time-reversal symmetry.
A revised diagrammatic technique for the degenerate Anderson model
Lee, T. K.; Zhang, F. C. (American Institute of Physics, 1984)
The Goldstone diagrammatic technique developed by Keiter and Kimball for single impurity Anderson model is reformulated. Instead of having the self_energy functions defined on the real axis as the Brillouin_Wigner theory, we have defined the functions on the complex plane. This avoids the complicated and cumbersome regularization procedure required in the Keiter and Kimball formulation. Most important of all it makes the numerical calculations possible. The exact partition function may be written down in terms of irreducible self_energy diagrams. The Green function and spectral function are derived.
Spectral density and magnetic susceptibility for the asymmetric degenerate Anderson model
Zhang, F. C.; Lee, T. K. (American Physical Society, 1984-08)
With the use of a new diagrammatic formulation, two coupled integral equations for the self-energy functions of the f hole and f particle in the asymmetric degenerate Anderson model are solved numerically. All the diagrams are included in the equations except the cross terms (or the vertex correction). The results for the spectral density function and the magnetic susceptibility show the scaling property described by the renormalization-group theory.
Spin-charge separation in the two-dimensional Hubbard and t-J models at low electronic density
Chen, Y. C.; Moreo, A.; Ortolani, F.; Dagotto, E.; Lee, T. K. (American Physical Society, 1994-07)
The spin- and density-correlation functions of the two-dimensional Hubbard model at low electronic density [n] are calculated in the ground state by using the power method, and at finite temperatures by using the quantum Monte Carlo technique. Both approaches produce similar results, which are in close agreement with numerical and high-temperature-expansion results for the two-dimensional t-J model. Using perturbative approximations, we show that the examination of the density-correlation function alone is not enough to support recent claims in the literature that suggested spin and charge separation in the low electronic density regime of the t-J model.
Systematic scaling in the low-energy excitations of the t-J model in one and two dimensions
Eder, R.; Chen, Y. C.; Lin, H. Q.; Ohta, Y.; Shih, C. T.; Lee, T. K. (American Physical Society, 1997-05-01)
We present an exact diagonalization study of the low-energy singlet and triplet states for both the one-dimensional (1D) and 2D t-J models. A scan of the parameter ratio J/t shows that for most low-energy states in both 1D and 2D the excitation energy takes the form E(t,J)=a . t+b . J. In 1D this is the natural conse quence of the factorization of the low-energy wave functions, i.e., spin-charge separation. Examination of the low-energy eigenstates in 2D shows that most of these are collective modes, which for larger J correspond to a periodic modulation of the hole density. The modulation is well reproduced by treating holes as hard-core bosons with an attractive interaction.
t-J model studied by the power Lanczos method
Chen, Y. C.; Lee, T. K. (American Physical Society, 1995-03)
The initial trial wave function used in a simple ground-state projection method, the power method, is systematically improved by using Lanczos algorithm. Much faster convergence to the ground state achieved by using these wave functions significantly reduces the effect of the fermion sign problem. The results for the ground state of the two-dimensional t-J model are presented. The density correlation function for the t-J model at small J shows a surprisingly good agreement with that of a system of noninteracting hard-core bosons.
Theoretical study of effective magnetic interactions in high-Tc Cu oxides
Shen, J. L.; Xu, J. H.; Ting, C. S.; Lee, T. K. (American Physical Society, 1990-11)
Starting from a two-band Hubbard model for cupric oxides and using perturbation theory, we study the effect of a small O-O hopping term tp on the parameters of the single-band t-J model. It is found that as long as the wave function on the oxygen site is extended and Bloch-like, the antiferromagnetic coupling between the nearest-neighbor copper spins decreases rapidly with increasing hole doping. This behavior seems to be insensitive to the value of tp and dispersion of the oxygen band, and is consistent with the results of recent experiments. We also show that between next-nearest-neighbor copper spins, there exists a non-negligible antiferromagnetic coupling even in the zero-doping limit.
Two competing interactions in the Anderson lattice model
Zhang, F. C.; Lee, T. K.; Su, Z. B. (American Physical Society, 1987-04)
The vertex function of the SU(N) Anderson lattice model is calculated by treating the intersite coupling perturbatively. There are two different scattering processes that contribute to the effective interactions. In one process the low-frequency Kondo resonance dominates and the effective interaction between quasiparticles is favorable for p-wave Cooper pairing at small values of kFR. In the other process all frequencies contribute and the effective interaction is against p-wave pairing for small kFR. This latter interaction is antiferromagnetic in nature.
Variational study of the spin-gap phase of the one-dimensional t-J model
Chen, Y. C.; Lee, T. K. (American Physical Society, 1996-10-01)
We propose a correlated spin-singlet-pair wave function to describe the spin-gap phase of the one-dimensional t-J model at low density and large J/t. In addition to having singlet pairs, this wave function has a Jastrow factor with a variational parameter nu. Several correlation functions are calculated by using the variational Monte Carlo method. The result shows the expected long-range behavior of the Luther-Emery phase with the Luttinger exponent K-rho, related to nu, K-rho=1/2 nu.

Browsing by Author "Lee, T. K."

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