Relativistic virial relation for cosmological structures

Starting with the relativistic Boltzmann equation for a system of particles defined by a distribution function, we have derived the virial relation for a spherical structure within an expanding background in the context of general relativity. This generalized form of the virial relation is then applied to the static case of a spherically symmetric structure to see the difference in the simplest case to the Newtonian relation. A relativistic Mass-Temperature relation for this simple case is also derived which can be applied to compact objects in astrophysics. Our general virial relation is then applied to the non-static case of a structure within an expanding universe where an extra term, usually missed in studies of structures in the presence of the dark energy, appears.Comment: 8 page

Relativistic rotation curve for cosmological structures

Using a general relativistic exact model for spherical structures in a cosmological background, we have put forward an algorithm to calculate the test particle geodesics within such cosmological structures in order to obtain the velocity profile of stars or galaxies. The rotation curve thus obtained is based on a density profile and is independent of any mass definition which is not unique in general relativity. It is then shown that this general relativistic rotation curves for a toy model and a NFW density profile are almost identical to the corresponding Newtonian one, although the general relativistic masses may be quite different

Photocurrent Generation in a Metallic Transition Metal Dichalcogenide

Light induced current in two-dimensional (2D) layered materials emerges from mechanisms such as photothermoelectric effect, photovoltaic effect or nonlocal hot carrier transport. Semiconducting layered transition metal dichalcogenides have been studied extensively in recent years as the generation of current by light is a crucial process in optoelectronic and photovoltaic devices. However, photocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Typically, high thermal conductivity and low absorption of the visible spectrum prevent photothermal current generation in metals. Here, we report photoresponse from two-terminal devices of mechanically exfoliated metallic 3R-NbS$_2$ thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/NbS$_2$ junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/NbS$_2$ junctions, now a negative photoresponse from all over the NbS$_2$ crystal is evident. Among our results, we realized that the observed photocurrent can be explained by the local heating caused by the laser excitation. These findings show that NbS$_2$ is among a few metallic materials in which photocurrent generation is possible

Brane worlds and dark matter

Two problems related to dark matter is considered in the context of a brane world model in which the confinement of gauge fields on the brane is achieved by invoking a confining potential. First, we show that the virial mass discrepancy can be addressed if the conserved geometrical term appearing in this model is considered as an energy momentum tensor of an unknown type of matter, the so-called X-matter whose equation of state is also obtained. Second, the galaxy rotation curves are explained by assuming an anisotropic energy momentum tensor for the X-matter.Comment: 13 pages, 1 figure, to appear in IJMP

Pomeranchuk effect and spin-gradient cooling of Bose-Bose mixtures in an optical lattice

We theoretically investigate finite-temperature thermodynamics and demagnetization cooling of two-component Bose-Bose mixtures in a cubic optical lattice, by using bosonic dynamical mean field theory (BDMFT). We calculate the finite-temperature phase diagram, and remarkably find that the system can be heated from the superfluid into the Mott insulator at low temperature, analogous to the Pomeranchuk effect in 3He. This provides a promising many-body cooling technique. We examine the entropy distribution in the trapped system and discuss its dependence on temperature and an applied magnetic field gradient. Our numerical simulations quantitatively validate the spin-gradient demagnetization cooling scheme proposed in recent experiments.Comment: 9 pages, 8 figure