The NJL model and strange quark matter

The stability of strange quark matter is studied within the Nambu Jona-Lasinio model with three different parameter sets. The model Lagrangian contains 4-fermion (with and without vector interaction) and 6-fermion terms; the minimum energy per baryon number as a function of the strangeness fraction of the system is compared to the masses of hyperons having the same strangeness fraction, and coherently calculated in the same version of the model, and for the same parameter set. The results show that in none of the different parameter sets strangelets are stable, and in some cases a minimum in the energy per baryon does not even exist.Comment: 8 pages, 2 figures, reference added, typos corrected, version to appear in Europhys. Let

Helium star evolutionary channel to super-Chandrasekhar mass type Ia supernovae

Recent discovery of several overluminous type Ia supernovae (SNe Ia) indicates that the explosive masses of white dwarfs may significantly exceed the canonical Chandrasekhar mass limit. Rapid differential rotation may support these massive white dwarfs. Based on the single-degenerate scenario, and assuming that the white dwarfs would differentially rotate when the accretion rate $\dot{M}>3\times 10^{-7}M_{\odot}\rm yr^{-1}$, employing Eggleton's stellar evolution code we have performed the numerical calculations for $\sim$ 1000 binary systems consisting of a He star and a CO white dwarf (WD). We present the initial parameters in the orbital period - helium star mass plane (for WD masses of $1.0 M_{\odot}$ and $1.2 M_{\odot}$, respectively), which lead to super-Chandrasekhar mass SNe Ia. Our results indicate that, for an initial massive WD of $1.2 M_{\odot}$, a large number of SNe Ia may result from super-Chandrasekhar mass WDs, and the highest mass of the WD at the moment of SNe Ia explosion is 1.81 $M_\odot$, but very massive ($>1.85M_{\odot}$) WDs cannot be formed. However, when the initial mass of WDs is $1.0 M_{\odot}$, the explosive masses of SNe Ia are nearly uniform, which is consistent with the rareness of super-Chandrasekhar mass SNe Ia in observations.Comment: 6 pages, 7 figures, accepted for publication in Astronomy and Astrophysic

Cosmology in nonrelativistic general covariant theory of gravity

Horava and Melby-Thompson recently proposed a new version of the Horava-Lifshitz theory of gravity, in which the spin-0 graviton is eliminated by introducing a Newtonian pre-potential $\phi$ and a local U(1) gauge field $A$. In this paper, we first derive the corresponding Hamiltonian, super-momentum constraints, the dynamical equations, and the equations for $\phi$ and $A$, in the presence of matter fields. Then, we apply the theory to cosmology, and obtain the modified Friedmann equation and the conservation law of energy, in addition to the equations for $\phi$ and $A$. When the spatial curvature is different from zero, terms behaving like dark radiation and stiff-fluid exist, from which, among other possibilities, bouncing universe can be constructed. We also study linear perturbations of the FRW universe with any given spatial curvature $k$, and derive the most general formulas for scalar perturbations. The vector and tensor perturbations are the same as those recently given by one of the present authors [A. Wang, Phys. Rev. D{\bf 82}, 124063 (2010)] in the setup of Sotiriou, Visser and Weinfurtner. Applying these formulas to the Minkowski background, we have shown explicitly that the scalar and vector perturbations of the metric indeed vanish, and the only remaining modes are the massless spin-2 gravitons.Comment: Revtex4, no figures. Gauge freedom was clarified and typos were corrected. Version to appear in Physical Reviews

Control of spin relaxation in semiconductor double quantum dots

We propose a scheme to manipulate the spin relaxation in vertically coupled semiconductor double quantum dots. Up to {\em twelve} orders of magnitude variation of the spin relaxation time can be achieved by a small gate voltage applied vertically on the double dot. Different effects such as the dot size, barrier height, inter-dot distance, and magnetic field on the spin relaxation are investigated in detail. The condition to achieve a large variation is discussed.Comment: 5 pages, 4 figures, to be published in PR

X-ray photoelectron spectroscopy investigation of the mixed anion GaSb/InAs heterointerface

X-ray photoelectron spectroscopy has been used to measure levels of anion cross-incorporation and to study interface formation for the mixed anion GaSb/lnAs heterojunction. Anion cross-incorporation was measured in 20 Å thick GaSb layers grown on lnAs, and 20 Å thick InAs layers grown on GaSb for cracked and uncracked sources. It was found that significantly less anion cross-incorporation occurs in structures grown with cracked sources. Interface formation was investigated by studying Sb soaks of InAs surfaces and As soaks of GaSb surfaces as a function of cracker power and soak time. Exchange of the group V surface atoms was found to be an increasing function of both cracker power and soak time. We find that further optimization of current growth parameters may be possible by modifying the soak time used at interfaces

Study of interface asymmetry in InAs–GaSb heterojunctions

We present reflection high energy electron diffraction, secondary ion mass spectroscopy, scanning tunneling microscopy and x‐ray photoelectron spectroscopy studies of the abruptness of InAs–GaSb interfaces. We find that the interface abruptness depends on growth order: InAs grown on GaSb is extended, while GaSb grown on InAs is more abrupt. We first present observations of the interfacial asymmetry, including measurements of band alignments as a function of growth order. We then examine more detailed studies of the InAs–GaSb interface to determine the mechanisms causing the extended interface. Our results show that Sb incorporation into the InAs overlayer and As exchange for Sb in the GaSb underlayer are the most likely causes of the interfacial asymmetry

A method for determining an optimum shape of a class of thin shells of revolution

Optimum shape of convex thin shell of revolution with respect to volume, weight and length - mathematical functio