Quantum Hall Bilayer as Pseudospin Magnet

We revisit the physics of electron gas bilayers in the quantum Hall regime [Nature, 432 (2004) 691; Science, 305 (2004) 950], where transport and tunneling measurements provided evidence of a superfluid phase being present in the system. Previously, this behavior was explained by the possible formation of a BEC of excitons in the half-filled electron bilayers, where empty states play the role of holes. We discuss the fundamental difficulties with this scenario, and propose an alternative approach based on a treatment of the system as a pseudospin magnet. We show that the experimentally observed tunneling peak can be linked to the XY ferromagnet (FM) to Ising antiferromagnet (AFM) phase transition of the S=1/2 XXZ pseudospin model, driven by the change in total electron density. This transition is accompanied by a qualitative change in the nature of the low energy spin wave dispersion from a gapless linear mode in the XY-FM phase to a gapped, quadratic mode in the Ising-AFM phase.Comment: 5 pages, 4 figures; corrected and close to printed versio

Resonant Raman of OH/OD vibrations and photoluminescence studies in LiTaO3 thin film

Resonant Raman spectra (RRS) of O-H and O-D vibration and libration modes, their combinations and higher harmonics have been observed in LiTaO3 polycrystalline thin films. RRS peaks are superimposed on photoluminescence (PL) spectrum. Monochromatic light from a xenon lamp is used as excitation source. PL spectrum shows two broad peaks, first near the band gap in UV (4.4-4.8eV) and another in the sub band gap region (< 4.0 eV). Band gap PL along with RRS peaks are reported for the first time. Photoluminescence excitation spectrum (PLE) shows a peak at 4.8 eV. Peak positions and full width at half maximum (FWHM) of RRS peaks depend upon the excitation energy. Dispersions of the fundamental and the third harmonic of the stretching mode of O-H with excitation energy are about 800 cm-1/eV and 2000 cm-1/eV respectively. This dispersion is much higher than reported in any other material.Comment: 20 page

Theory of traveling filaments in bistable semiconductor structures

We present a generic nonlinear model for current filamentation in semiconductor structures with S-shaped current-voltage characteristics. The model accounts for Joule self-heating of a current density filament. It is shown that the self-heating leads to a bifurcation from static to traveling filament. Filaments start to travel when increase of the lattice temperature has negative impact on the cathode-anode transport. Since the impact ionization rate decreases with temperature, this occurs for a wide class of semiconductor systems whose bistability is due to the avalanche impact ionization. We develop an analytical theory of traveling filaments which reveals the mechanism of filament motion, find the condition for bifurcation to traveling filament, and determine the filament velocity.Comment: 13 pages, 5 figure

Investigating the Role of Islet Cytoarchitecture in Its Oscillation Using a New β-Cell Cluster Model

The oscillatory insulin release is fundamental to normal glycemic control. The basis of the oscillation is the intercellular coupling and bursting synchronization of β cells in each islet. The functional role of islet β cell mass organization with respect to its oscillatory bursting is not well understood. This is of special interest in view of the recent finding of islet cytoarchitectural differences between human and animal models. In this study we developed a new hexagonal closest packing (HCP) cell cluster model. The model captures more accurately the real islet cell organization than the simple cubic packing (SCP) cluster that is conventionally used. Using our new model we investigated the functional characteristics of β-cell clusters, including the fraction of cells able to burst fb, the synchronization index λ of the bursting β cells, the bursting period Tb, the plateau fraction pf, and the amplitude of intracellular calcium oscillation [Ca]. We determined their dependence on cluster architectural parameters including number of cells nβ, number of inter-β cell couplings of each β cell nc, and the coupling strength gc. We found that at low values of nβ, nc and gc, the oscillation regularity improves with their increasing values. This functional gain plateaus around their physiological values in real islets, at nβ∼100, nc∼6 and gc∼200 pS. In addition, normal β-cell clusters are robust against significant perturbation to their architecture, including the presence of non-β cells or dead β cells. In clusters with nβ>∼100, coordinated β-cell bursting can be maintained at up to 70% of β-cell loss, which is consistent with laboratory and clinical findings of islets. Our results suggest that the bursting characteristics of a β-cell cluster depend quantitatively on its architecture in a non-linear fashion. These findings are important to understand the islet bursting phenomenon and the regulation of insulin secretion, under both physiological and pathological conditions

Simulation of melting of two-dimensional Lennard-Jones solids

We study the nature of melting of a two-dimensional (2D) Lennard-Jones solid using large-scale Monte Carlo simulation. We use systems of up to 102 400 particles to capture the decay of the correlation functions associated with translational order (TO) as well as the bond-orientational (BO) order. We study the role of dislocations and disclinations and their distribution functions. We computed the temperature dependence of the second moment of the TO parameter (ΨG) as well as of the order parameter Ψ6 associated with BO order. By applying finite-size scaling of these second moments, we determined the anomalous dimension critical exponents η(T) and η6(T) associated with power-law decay of the ΨG and Ψ6 correlation functions. We also computed the temperature-dependent distribution of the order parameters ΨG and Ψ6 on the complex plane that supports a two-stage melting with a hexatic phase as an intermediate phase. From the correlation functions of ΨG and Ψ6, we extracted the corresponding temperature-dependent correlation lengths ξ(T) and ξ6(T). The analysis of our results leads to a consistent picture strongly supporting a two-stage melting scenario as predicted by the Kosterlitz, Thouless, Halperin, Nelson, and Young (KTHNY) theory where melting occurs via two continuous phase transitions, first from solid to a hexatic fluid at temperature Tm, and then from the hexatic fluid to an isotropic fluid at a critical temperature Ti. We find that ξ(T) and ξ6(T) have a distinctly different temperature dependence, each diverging at different temperature, and that their finite-size scaling properties are consistent with the KTHNY theory. We also used the temperature dependence of η and η6 and their theoretical bounds to provide estimates for the critical temperatures T m and Ti, which can also be estimated using the Binder ratio. Our results are within error bars, the same as those extracted from the divergence of the correlation lengths. © 2011 American Physical Society

Quantum simulation of 3He impurities and of 4He interstitials in solid 4He

We have studied the role of an atomic 3He impurity and an interstitial 4He atom in two-dimensional (2D) and three-dimensional (3D) solid 4He using path-integral Monte Carlo simulation. We find that when a substitutional 3He impurity is introduced, the impurity becomes localized and occupies an ideal lattice site. When an interstitial 3He impurity is introduced in the 4He solid, we find that the impurity becomes localized at a substitutional position and, thus, promotes the extra 4He atom to the interstitial space. As a consequence we find that the one-body density matrix (OBDM) and the superfluid fraction, for the case of a 4He solid with an interstitial impurity, are very similar to those calculated for a 4He solid with a 4He interstitial atom. Namely, while the off-diagonal OBDM approaches zero exponentially with increasing particle displacement for the &quot;pure&quot; solid, an interstitial 4He atom or a 3He impurity appear to enhance it at long distances. Finally, the effective mass of the 3He impurity quasiparticle in 2D and 3D crystalline 4He is estimated. © 2010 The American Physical Society