Continuum Theory of Edge States of Topological Insulators: Variational Principle and Boundary Conditions

We develop a continuum theory to model low energy excitations of a generic four-band time reversal invariant electronic system with boundaries. We propose a variational energy functional for the wavefunctions which allows us derive natural boundary conditions valid for such systems. Our formulation is particularly suited to develop a continuum theory of the protected edge/surface excitations of topological insulators both in two and three dimensions. By a detailed comparison of our analytical formulation with tight binding calculations of ribbons of topological insulators modeled by the Bernevig-Hughes-Zhang (BHZ) hamiltonian, we show that the continuum theory with the natural boundary condition provides an appropriate description of the low energy physics. As a spin-off, we find that in a certain parameter regime, the gap that arises in topological insulator ribbons of finite width due to the hybridization of edges states from opposite edges, depends non-monotonically on the ribbon width and can nearly vanish at certain "magic widths".Comment: 8 pages, 5 figure

Effects of ownership, subsidization and teaching activities on hospital costs in Switzerland

This paper explores the cost structure of Swiss hospitals, focusing on differences due to teaching activities and those across different ownership and subsidization types. A stochastic total cost frontier with a Cobb-Douglas functional form has been estimated for a panel of 150 general hospitals over the six-year period from 1998 and 2003. Inpatient cases adjusted by DRG cost weights and ambulatory revenues are considered as two separate outputs. The adopted econometric specification allows for unobserved heterogeneity across hospitals. The results indicate that the time-invariant unobserved factors could account for considerable cost differences that could be only partly due to inefficiency. The results suggest that teaching activities are an important cost driving factor and hospitals that have a broader range of specialization are relatively more costly. The excess costs of university hospitals can be explained by more extensive teaching activities as well as the relatively high quality of medical units. However, even after controlling for such differences university hospitals have shown a relatively low cost-efficiency especially in the first two or three years of the sample period. The analysis does not provide any evidence of significant efficiency differences across ownership and subsidization categories.general hospitals, teaching hospitals, stochastic frontier, cost efficiency

Fermionic Superfluid from a Bilayer Band Insulator in an Optical Lattice

We propose a model to realize a fermionic superfluid state in an optical lattice circumventing the cooling problem. Our proposal exploits the idea of tuning the interaction in a characteristically low entropy state, a band-insulator in an optical bilayer system, to obtain a superfluid. By performing a detailed analysis of the model including fluctuations and augmented by a variational quantum Monte Carlo calculations of the ground state, we show that the superfluid state obtained has high transition temperature of the order of the hopping energy. Our system is designed to suppress other competing orders such as a charge density wave. We suggest a laboratory realization of this model via an orthogonally shaken optical lattice bilayer.Comment: 5 pages, 7 figures, typos fixed, figures modifie

Economies of scale and efficiency measurement in Switzerland's Nursing homes

This paper examines the cost efficiency in the nursing home industry, an issue of concern to Swiss policy makers because of the explosive growth of national expenditure on elderly care and the aging of the population. A stochastic cost frontier model with a translog function has been applied to a balanced panel data of 1780 observations from 356 nursing homes operating over five years (1998-2002) in Switzerland. We compare the estimation results from different panel data econometric techniques focusing on the various methods of specification of unobserved heterogeneity across firms. In particular, the potential effects of such unobserved factors on the estimation results and their interpretation have been discussed. The paper eventually addresses three empirical issues: (1) the measurement of economies of scale in the nursing home sector, (2) the assessment of the economic performance of the firms by estimating their cost efficiency scores, and (3) the role of unobserved heterogeneity in the estimation process. The findings suggest that the economies of scale are an important potential source of cost reduction in a majority of Swiss nursing homes. Taking the size as given the efficiency performance of most individual units is practically very close to the estimated best practice. Nevertheless, the efficiency estimates suggest that some of the nursing homes can significantly reduce their costs by improving their operations.COST EFFICIENCY, ECONOMIES OF SCALE, NURSING HOMES, STOCHASTIC FRONTIER, PANEL DATA

Synchronous and Asynchronous Mott Transitions in Topological Insulator Ribbons

We address how the nature of linearly dispersing edge states of two dimensional (2D) topological insulators evolves with increasing electron-electron correlation engendered by a Hubbard like on-site repulsion $U$ in finite ribbons of two models of topological band insulators. Using an inhomogeneous cluster slave rotor mean-field method developed here, we show that electronic correlations drive the topologically nontrivial phase into a Mott insulating phase via two different routes. In a synchronous transition, the entire ribbon attains a Mott insulating state at one critical $U$ that depends weakly on the width of the ribbon. In the second, asynchronous route, Mott localization first occurs on the edge layers at a smaller critical value of electronic interaction which then propagates into the bulk as $U$ is further increased until all layers of the ribbon become Mott localized. We show that the kind of Mott transition that takes place is determined by certain properties of the linearly dispersing edge states which characterize the topological resilience to Mott localization.Comment: 4+ pages, 5 figure

A comparative study of local cortisone administrations to that of systemic in buccal mucosa carcinogenesis

Thesis (M.Sc.D.)--Boston University School of Graduate Dentistry, 1972. (Oral surgery)Bibliography included.Sixty male golden Syrian hamsters (Lakeview Hamster Colony, Newfield, New Jersey), three months of age, were divided into five equal experimental groups and fed a standard laboratory diet. The animals of Group I (Control) received triweekly application of heavy mineral oil (USP) into the right buccal pouch with a No. 4 camel's hair brush for 16 weeks. The animals of Groups II, III, IV, and V received triweekly topical application of 7-12-dimethylbenzanthracene (DMBA) at a concentration of 0.5% w/v in heavy mineral oil (USP), according to the technic of Salley (J. Dent. Res., 33:253-262, 1954). In this manner about 0.2 ml. of solution was deposited in the pouch at each painting, containing 1 mg. of DMBA. Animals in Group III, in addition to the application of DMBA, received a triweekly S.C. , injection into the groin of 2.5 mg. of cortisone acetate (Premo Pharmaceutical Laboratory, Inc., Hackensack, New Jersey). [TRUNCATED