Jamshid Moradi Kurdestany

The inhomogeneous extended Bose-Hubbard model: A mean-field theory

We develop an inhomogeneous mean-field theory for the extended Bose-Hubbard model with a quadratic, confining potential. In the absence of this potential, our mean-field theory yields the phase diagram of the homogeneous extended Bose-Hubbard model. This phase diagram shows a superfluid (SF) phase and lobes of Mott-insulator (MI), density-wave (DW), and supersolid (SS) phases in the plane of the chemical potential mu and on-site repulsion U; we present phase diagrams for representative values of V, the repulsive energy for bosons on nearest-neighbor sites. We demonstrate that, when the confining potential is present, superfluid and density-wave order parameters are nonuniform; in particular, we obtain, for a few representative values of parameters, spherical shells of SF, MI, DW, and SS phases. We explore the implications of our study for experiments on cold-atom dipolar condensates in optical lattices in a confining potential.

Mott metal-insulator transition in the doped Hubbard-Holstein model

Motivated by the current interest in the understanding of the Mott insulators away from half filling, observed in many perovskite oxides, we study the Mott metal-insulator transition (MIT) in the doped Hubbard-Holstein model using the Hatree-Fock mean field theory. The Hubbard-Holstein model is the simplest model containing both the Coulomb and the electron-lattice interactions, which are important ingredients in the physics of the perovskite oxides. In contrast to the half-filled Hubbard model, which always results in a single phase (either metallic or insulating), our results show that away from half-filling, a mixed phase of metallic and insulating regions occur. As the dopant concentration is increased, the metallic part progressively grows in volume, until it exceeds the percolation threshold, leading to percolative conduction. This happens above a critical dopant concentration

Plasmons in spatially separated rolled-up electron-hole double-layer systems

\delta_c

Bose-Hubbard models in confining potentials: Inhomogeneous mean-field theory

, which, depending on the strength of the electron-lattice interaction, can be a significant fraction of unity. This means that the material could be insulating even for a substantial amount of doping, in contrast with the Nagaoka theorem, where a single hole destroys the insulating behavior of the half-filled Hubbard model. Our theory provides a framework for the understanding of the density-driven metal-insulator transition observed in many complex oxides.

Carbon dioxide detection by boron nitride nanotubes

Using the two-component random phase approximation, we report the collective mode spectrum of a quasi-one-dimensional spatially separated electron-hole double-layer system characterized by rolled-up type-II band aligned quantum wells. We find two intra-subband collective excitations, which can be classified into optic and acoustic plasmon branches, and several inter-subband plasmon modes. At the long wavelength limit and up to a given wave vector, our model predicts and admits an undamped acoustic branch, which always lies in the gap between the intra-subband electron and hole continua, and an undamped optic branch residing within the gap between the inter-subband electron and hole continua, for all values of the electron-hole charge separations. This theoretical investigation suggests that the low-energy and Landau-undamped plasmon modes might exist based on quasi-one-dimensional, two-component spatially separated electron-hole plasmas, and their possibility could be experimentally examined.

Electronic structure and anisotropic Rashba spin-orbit coupling in monolayer black phosphorus

We present an extensive study of Mott insulator (MI) and superfluid (SF) shells in Bose-Hubbard (BH) models for bosons in optical lattices with harmonic traps. For this we develop an inhomogeneous mean-field theory. Our results for the BH model with one type of spinless bosons agrees quantitatively with quantum Monte Carlo (QMC) simulations. Our approach is numerically less intensive than such simulations, so we are able to perform calculation on experimentally realistic, large 3D systems, explore a wide range of parameter values, and make direct contact with a variety of experimental measurements. We also generalize our inhomogeneous mean-field theory to study BH models with harmonic traps and (a) two species of bosons or (b) spin-1 bosons. With two species of bosons we obtain rich phase diagrams with a variety of SF and MI phases and associated shells, when we include a quadratic confining potential. For the spin-1 BH model we show, in a representative case, that the system can display alternating shells of polar SF and MI phases; and we make interesting predictions for experiments in such systems.