The Bose-Hubbard model on a triangular lattice with diamond ring-exchange

Ring-exchange interactions have been proposed as a possible mechanism for a Bose-liquid phase at zero temperature, a phase that is compressible with no superfluidity. Using the Stochastic Green Function algorithm (SGF), we study the effect of these interactions for bosons on a two-dimensional triangular lattice. We show that the supersolid phase, that is known to exist in the ground state for a wide range of densities, is rapidly destroyed as the ring-exchange interactions are turned on. We establish the ground-state phase diagram of the system, which is characterized by the absence of the expected Bose-liquid phase.Comment: 6 pages, 10 figure

Using off-diagonal confinement as a cooling method

In a recent letter [Phys. Rev. Lett. 104, 167201 (2010)] we proposed a new confining method for ultracold atoms on optical lattices, based on off-diagonal confinement (ODC). This method was shown to have distinct advantages over the conventional diagonal confinement (DC) that makes use of a trapping potential, including the existence of pure Mott phases and highly populated condensates. In this paper we show that the ODC method can also lead to temperatures that are smaller than with the conventional DC method, depending on the control parameters. We determine these parameters using exact diagonalizations for the hard-core case, then we extend our results to the soft-core case by performing quantum Monte Carlo (QMC) simulations for both DC and ODC systems at fixed temperatures, and analysing the corresponding entropies. We also propose a method for measuring the entropy in QMC simulations.Comment: 6 pages, 6 figure

Re-examining the electronic structure of germanium: A first-principle study

We report results from an efficient, robust, ab-initio method for self-consistent calculations of electronic and structural properties of Ge. Our non-relativistic calculations employed a generalized gradient approximation (GGA) potential and the linear combination of atomic orbitals (LCAO) formalism. The distinctive feature of our computations stem from the use of Bagayoko-Zhao-Williams-Ekuma-Franklin (BZW-EF) method. Our results are in agreement with experimental ones where the latter are available. In particular, our theoretical, indirect band gap of 0.65 eV, at the experimental lattice constant of 5.66 \AA{}, is in excellent agreement with experiment. Our predicted, equilibrium lattice constant is 5.63 \AA{}, with a corresponding indirect band gap of 0.65 eV and a bulk modulus of 80 GPa. We also calculated the effective masses in various directions with respect to the $\Gamma$ point.Comment: 10 Pages, 3 Figures, and 1 tabl

Local Density of the Bose Glass Phase

We study the Bose-Hubbard model in the presence of on-site disorder in the canonical ensemble and conclude that the local density of the Bose glass phase behaves differently at incommensurate filling than it does at commensurate one. Scaling of the superfluid density at incommensurate filling of $\rho=1.1$ and on-site interaction $U=80t$ predicts a superfluid-Bose glass transition at disorder strength of $\Delta_c \approx 30t$. At this filling the local density distribution shows skew behavior with increasing disorder strength. Multifractal analysis also suggests a multifractal behavior resembling that of the Anderson localization. Percolation analysis points to a phase transition of percolating non-integer filled sites around the same value of disorder. Our findings support the scenario of percolating superfluid clusters enhancing Anderson localization near the superfluid-Bose glass transition. On the other hand, the behavior of the commensurate filled system is rather different. Close to the tip of the Mott lobe ($\rho=1, U=22t$) we find a Mott insulator-Bose glass transition at disorder strength of $\Delta_c \approx 16t$. An analysis of the local density distribution shows Gaussian like behavior for a wide range of disorders above and below the transition.Comment: 12 pages, 14 figure

Complex phases in the doped two-species bosonic Hubbard Model

We study a two-dimensional bosonic Hubbard model with two hard-core species away from half filling using Quantum Monte Carlo simulations. The model includes a repulsive interspecies interaction and different nearest-neighbor hopping terms for the two species. By varying the filling we find a total of five distinct phases, including a normal liquid phase at higher temperature, and four different phases at lower temperature. We find an anti-ferromagnetically ordered Mott insulator and a region of coexistent anti-ferromagnetic and superfluid phases near half filling. Further away from half filling the phase diagram displays a superfluid phase and a novel phase inside the superfluid region at even lower temperatures. In this novel phase separated region, the heavy species has a Mott behavior with integer filling, while the lighter species shows phase separated Mott and superfluid behaviors.Comment: 5 pages, 4 figure

PROGRESS ON THE WISCONSIN P INDEX 1/

Phosphorus (P) based nutrient management is a central concept in national nutrient management policy (NRCS, 1999), and several options for developing nutrient management plans using a P strategy are included in the 2002 Wisconsin nutrient management standard (590) (NRCS, 2002). A phosphorus (P) index approach is one of the options (along with soil test P categories) for implementing a P-based nutrient management strategy. The P index considers site-specific P source and transport factors to predict the risk of P loss in runoff from individual fields. Ideally, it can be used to rank or prioritize fields based on their risk of P loss and to identify management options for reducing P loss. Initially, a draft P index was developed for Wisconsin (Murphy, 2000; Bundy, 2001) following the general concepts proposed by Lemunyon and Gilbert (1993) and modified by Gburek et al. (2000). This method assigned index values to specific site characteristics and management practices to arrive at an overall P index value for a production unit. While conceptually sound, this approach lacked the flexibility to include all of the parameters known to influence P losses in runoff. A modeling approach, similar to that used in the Iowa P index (NRCS, 2001; Mallarino et al., 2001) was adopted as a more comprehensive and quantitative method for assessing the risk of P losses in runoff. The current version of the Wisconsin P index, developed largely through the efforts of Dr. Wesley Jarrell, can be viewed at the following web sit

Molecule Microscopy

Contains reports on two research projects.National Institutes of Health (Grant 1 ROI GM23678)Health Sciences Fun

Phase diagram of the Bose-Hubbard model on a ring-shaped lattice with tunable weak links

Motivated by recent experiments on toroidal Bose-Einstein condensates in all-optical traps with tunable weak links, we study the one-dimensional Bose-Hubbard model on a ring-shaped lattice with a small region of weak hopping integrals using quantum Monte Carlo simulations. Besides the usual Mott insulating and superfluid phases, we find a phase which is compressible but non superfluid with a local Mott region. This `local Mott' phase extends in a large region of the phase diagram. These results suggest that the insulating and conducting phases can be tuned by a local parameter which may provide a new insight to the design of atomtronic devices.Comment: 5 pages, 5 figure

Absence of hysteresis at the Mott-Hubbard metal-insulator transition in infinite dimensions

The nature of the Mott-Hubbard metal-insulator transition in the infinite-dimensional Hubbard model is investigated by Quantum Monte Carlo simulations down to temperature T=W/140 (W=bandwidth). Calculating with significantly higher precision than in previous work, we show that the hysteresis below T_{IPT}\simeq 0.022W, reported in earlier studies, disappears. Hence the transition is found to be continuous rather than discontinuous down to at least T=0.325T_{IPT}. We also study the changes in the density of states across the transition, which illustrate that the Fermi liquid breaks down before the gap opens.Comment: 4 pages, 4 eps-figures using epsf.st

Thermoelectric Response Near the Density Driven Mott Transition

We investigate the thermoelectric response of correlated electron systems near the density driven Mott transition using the dynamical mean field theory.Comment: 4 pages, 2 embedded figure