Theses and Dissertations

Issuing Body

Mississippi State University


Clay, R. Torsten

Committee Member

Koshka, Yaroslav

Committee Member

Kim, Seong-Gon

Committee Member

Ye, Jinwu

Committee Member

Dunne, James A.

Date of Degree


Original embargo terms

MSU Only Indefinitely

Document Type

Dissertation - Campus Access Only


Engineering (Applied Physics)

Degree Name

Doctor of Philosophy


James Worth Bagley College of Engineering


Applied Physics Program


The first project of this dissertation focuses on an extension of the Path Integral Renormalization Group (PIRG) method to the extended Hubbard model (EHM) including on-site U and a nearest-neighbor interaction V. The PIRG method is an efficient numerical algorithm for studying ground state properties of strongly correlated electron systems. A major advantage of the PIRG is that it is free from the Fermion sign problem. Many observables can be calculated using Wick’s theorem. The EHM is particularly important in models of charge-transfer solids (CTS) and at 1/4illing the V interaction drives a charge-ordered state. We test the method with comparisons to small two-dimensional (2D) clusters and long one-dimensional (1D) chains. The second project of this dissertation focuses on the Coulomb enhancement of superconducting pair-pair correlations in frustrated quarterilled band lattice systems. A necessary condition for superconductivity (SC) driven by electron correlation is that electronelectron (e-e) interactions enhance long range superconducting pair-pair correlations relative to the noninteracting limit. We present high-precision numerical calculations within the 2D Hubbard model on up to 100 sites showing that long range superconducting pair correlations are enhanced only for electron density 0.5. At all other fillings e-e interactions suppress pair correlations. We argue that the enhancement of pairing is due to a tendency to form local spin singlets at density 0.5. Our work provides a key ingredient to the mechanism of SC in the 2D organic-CTS superconductors, as well as in many other unconventional superconductors with frustrated crystal lattices and density 0.5. In the third project we apply our proposed concept to a real material, kappa-(BEDTTTF)2X. We present numerical results for 32 and 64 site lattices using the Constrained Path Monte Carlo and PIRG methods over a wide range of carrier density. We show that superconducting pair-pair correlations in this model are enhanced by e-e interactions for d-wave pairing symmetry uniquely for a hole density close to quarterilling. Our results indicate that this enhancement of superconductivity is not related to the presence of antiferromagnetic order, but to the strong tendency to spin-singlet formation in the quarterilled band.