Comment on ``Ground State Phase Diagram of a Half-Filled One-Dimensional Extended Hubbard Model''

In Phys. Rev. Lett. 89, 236401 (2002), Jeckelmann argued that the recently discovered bond-order-wave (BOW) phase of the 1D extended Hubbard model does not have a finite extent in the (U,V) plane, but exists only on a segment of a first-order SDW-CDW phase boundary. We here present quantum Monte Carlo result of higher precision and for larger system sizes than previously and reconfirm that the BOW phase does exist a finite distance away from the phase boundary, which hence is a BOW-CDW transition curve.Comment: 1 page, 1 figure, v2: final published versio

The Impact of Insurance on the Law of Torts

An experimental study of "Model-on-Demand" (MoD) identification is made on a pilot-scale brine-water mixing tank. MoD estimation is compared against semi-physical modeling techniques using identification data generated from a systematically designed m-level Pseudo Random Sequence (PRS) input. The estimated models are the basis for evaluating the usefulness of MoD-based Model Predictive Control (MPC). For this application, MoD-MPC is shown to provide better performance at high bandwidths compared to a linear MPC controller

"Things Are Different Now But": Older LGBT Adults' Experiences and Unmet Needs in Health Care.

Background: Approximately 2.7 million U.S. older adults self-identify as lesbian, gay, bisexual, and transgender (LGBT). Many felt unsafe revealing their sexual orientation until relatively recently, and may still not be "out" to medical providers. The aim of this study was to increase understanding of the experiences and needs of older LGBT adults when accessing care. Method: Individual semistructured interviews were conducted with 10 individuals aged 65 years or older from a local LGBT community. Interviews were audio taped and transcribed verbatim. Transcripts were analyzed via thematic analysis. Results: Major themes were "Outness," "Things are Different Now," and "Additional Resources." These describe participant comfort with being "out"; how treatment they received changed over time, and needed services or other options from the community. Conclusion: While many older LGBT adults are accustomed to navigating social mores to avoid negative experiences, nurses as well as other health care providers must be prepared to create trusting relationships with these individuals to provide truly comprehensive care

Magnetic ordering in a doped frustrated spin-Peierls system

Based on a model of a quasi-one dimensional spin-Peierls system doped with non-magnetic impurities, an effective two-dimensional Hamiltonian of randomly distributed S=1/2 spins interacting via long-range pair-wise interaction is studied using a stochastic series expansion quantum Monte Carlo method. The susceptibility shows Curie-like behavior at the lowest temperatures reached although the staggered magnetisation is found to be finite for $T\to 0$. The doping dependance of the corresponding three-dimensional Neel temperature is also computed.Comment: Published version, 4 pages, 5 figure

Thermal effects on slow-roll dynamics

A description of the transition from the inflationary epoch to radiation domination requires the understanding of quantum fields out of thermal equilibrium, particle creation and thermalisation. This can be studied from first principles by solving a set of truncated real-time Schwinger-Dyson equations, written in terms of the mean field (inflaton) and the field propagators, derived from the two-particle irreducible effective action. We investigate some aspects of this problem by considering the dynamics of a slow-rolling mean field coupled to a second quantum field, using a \phi^2\chi^2 interaction. We focus on thermal effects. It is found that interactions lead to an earlier end of slow-roll and that the evolution afterwards depends on details of the heatbath.Comment: 25 pages, 11 eps figures. v2: paper reorganized, title changed, conclusions unchanged, to appear in PR

Site dilution of quantum spins in the honeycomb lattice

We discuss the effect of site dilution on both the magnetization and the density of states of quantum spins in the honeycomb lattice, described by the antiferromagnetic Heisenberg spin-S model. For this purpose a real-space Bogoliubov-Valatin transformation is used. In this work we show that for the S>1/2 the system can be analyzed in terms of linear spin wave theory. For spin S=1/2, however, the linear spin wave approximation breaks down. In this case, we have studied the effect of dilution on the staggered magnetization using the Stochastic Series Expansion Monte Carlo method. Two main results are to be stressed from the Monte Carlo method: (i) a better value for the staggered magnetization of the undiluted system, m=0.2677(6); (ii) a finite value of the staggered magnetization of the percolating cluster at the classical percolation threshold, showing that there is no quantum critical transition driven by dilution in the Heisenberg model. In the solution of the problem using linear the spin wave method we pay special attention to the presence of zero energy modes. Using a combination of linear spin wave analysis and the recursion method we were able to obtain the thermodynamic limit behavior of the density of states for both the square and the honeycomb lattices. We have used both the staggered magnetization and the density of states to analyze neutron scattering experiments and Neel temperature measurements on quasi-two- -dimensional honeycomb systems. Our results are in quantitative agreement with experimental results on Mn_pZn_{1-p}PS_3 and on the Ba(Ni_pMg_{1-p})_2V_2O_8.Comment: 21 pages (REVTEX), 16 figure

Susceptibility of the 2D S=1/2 Heisenberg antiferromagnet with an impurity

We use a quantum Monte Carlo method (stochastic series expansion) to study the effects of a magnetic or nonmagnetic impurity on the magnetic susceptibility of the two-dimensional Heisenberg antiferromagnet. At low temperatures, we find a log-divergent contribution to the transverse susceptibility. We also introduce an effective few-spin model that can quantitatively capture the differences between magnetic and nonmagnetic impurities at high and intermediate temperatures.Comment: 5 pages, 4 figures, v2: Updated data in figures, minor changes in text, v3: Final version, cosmetic change