Microscopic Description of Band Structure at Very Extended Shapes in the A ~ 110 Mass Region

Recent experiments have confirmed the existence of rotational bands in the A \~ 110 mass region with very extended shapes lying between super- and hyper-deformation. Using the projected shell model, we make a first attempt to describe quantitatively such a band structure in 108Cd. Excellent agreement is achieved in the dynamic moment of inertia J(2) calculation. This allows us to suggest the spin values for the energy levels, which are experimentally unknown. It is found that at this large deformation, the sharply down-sloping orbitals in the proton i_{13/2} subshell are responsible for the irregularity in the experimental J(2), and the wave functions of the observed states have a dominant component of two-quasiparticles from these orbitals. Measurement of transition quadrupole moments and g-factors will test these findings, and thus can provide a deeper understanding of the band structure at very extended shapes.Comment: 4 pages, 3 eps figures, final version accepted by Phys. Rev. C as a Rapid Communicatio

Gate-Voltage Control of Chemical Potential and Weak Anti-localization in Bismuth Selenide

We report that Bi$_2$Se$_3$ thin films can be epitaxially grown on SrTiO$_{3}$ substrates, which allow for very large tunablity in carrier density with a back-gate. The observed low field magnetoconductivity due to weak anti-localization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest much suppressed bulk conductivity at large negative gate-voltages and a possible role of surface states in the WAL phenomena. This work may pave a way for realizing three-dimensional topological insulators at ambient conditions.Comment: 5 pages, 4 figures

Distribution of Spectral Lags in Gamma Ray Bursts

Using the data acquired in the Time To Spill (TTS) mode for long gamma-ray bursts (GRBs) collected by the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory (BATSE/CGRO), we have carefully measured spectral lags in time between the low (25-55 keV) and high (110-320 keV) energy bands of individual pulses contained in 64 multi-peak GRBs. We find that the temporal lead by higher-energy gamma-ray photons (i.e., positive lags) is the norm in this selected sample set of long GRBs. While relatively few in number, some pulses of several long GRBs do show negative lags. This distribution of spectral lags in long GRBs is in contrast to that in short GRBs. This apparent difference poses challenges and constraints on the physical mechanism(s) of producing long and short GRBs. The relation between the pulse peak count rates and the spectral lags is also examined. Observationally, there seems to be no clear evidence for systematic spectral lag-luminosity connection for pulses within a given long GRB.Comment: 20 pages, 4 figure

An SU(4) Model of High-Temperature Superconductivity and Antiferromagnetism

We present an SU(4) model of high-temperature superconductivity having many similarities to dynamical symmetries known to play an important role in microscopic nuclear structure physics and in elementary particle physics. Analytical solutions in three dynamical symmetry limits of this model are found: an SO(4) limit associated with antiferromagnetic order; an SU(2) X SO(3) limit that may be interpreted as a d-wave pairing condensate; and an SO(5) limit that may be interpreted as a doorway state between the antiferromagnetic order and the superconducting order. The model suggests a phase diagram in qualitative agreement with that observed in the cuprate superconductors. The relationship between the present model and the SO(5) unification of superconductivity and antiferromagnetic order proposed by Zhang is discussed.Comment: A long paper extended from the early version cond-mat/9903150; accepted by Phys. Rev.

Effect of the Thickness of Undoped GaN Interlayers Between Multiple Quantum Wells and the P-Doped Layer on the Performance of GaN Light-Emitting Diodes

InGaN based light-emitting diodes (LEDs) with undoped GaN interlayer of variant thicknesses grown by metal-organic chemical vapor deposition technique have been investigated. It was found that the thickness of undoped GaN interlayers affected LEDs\u27 performance greatly. The LED with 50 nm undoped GaN interlayer showed higher light output power and lower reverse-leakage current compared with the others at 20 mA. Based on electrical and optical characteristics analysis and numerical simulation, these improvements are mainly attributed to the improvement of the quality of depletion region by inserting an undoped GaN layer, as well as reduction of the Shockley-Read-Hall recombination in InGaN/GaN MQWs