Gravitational fields with space-times of Bianchi type IX

Spatially homogeneous space-times of Bianchi type IX are considered. A general scheme for the derivation of exact solutions of Einstein’s equations corresponding to perfect fluid plus pure radiation fields is outlined. Some simple rotating Bianchi type IX cosmological models are presented. The details of these solutions are also discussed. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

Isotropisation of flat homogeneous Bianchi type <Emphasis Type="Italic">I</Emphasis> model with a non minimally coupled and massive scalar field

In previous works, we studied the isotropisation of some Bianchi class A models with a minimally coupled scalar field. In this paper we extend these results, in the special case of a Bianchi type I model, to a non minimally coupled scalar field. The Universe isotropisation for the Brans-Dicke and low energy string theories are studied.     

Bianchi Type <Emphasis Type="Italic">VI</Emphasis><Subscript>0</Subscript> Magnetized Barotropic Bulk Viscous Fluid Massive String Universe in General Relativity

A model of a cloud formed by massive strings is studied in the context of the usual general relativity. This model is used as a source of Bianchi type VI ₀ massive with magnetic field and bulk viscosity. To get a determinate model, we assume that the expansion (θ) in the model is proportional to the shear (σ) and also the fluid obeys the barotropic equation of state. The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field and bulk viscosity is discussed.     

Thermal entanglement in two-qutrit spin-1 anisotropic Heisenberg model with inhomogeneous magnetic field

The thermal entanglement of a two-qutrit spin-1 anisotropic Heisenberg XXZ chain in an inhomogeneous magnetic field is studied in detail. The effects of the external magnetic field (B), a parameter b which controls the inhomogeneity of B, and the bilinear interaction parameters J_{x=Jy≠Jz on the thermal variation of the negativity are studied in detail. It is found that negativity N decreases when the values of magnetic field, inhomogeneity b and temperature are increasing. In addition, N remains at higher temperatures for higher values of Jz and lower values of B and b.}     

The 1/D expansion for classical magnets: Low-dimensional models with magnetic field

The field-dependent magnetizationm(H, T) of one- and two-dimensional classical magnets described by theD-component vector model is calculated analytically in the whole range of temperature and magnetic fields with the help of the 1/D expansion. In the first order in 1/D the theory reproduces with a good accuracy the temperature dependence of the zero-field susceptibility of antiferromagnets with maximum atT|J ₀|/D (J ₀ is the Fourier component of the exchange interaction) and describes for the first time the singular behavior of (H, T) at small temperatures and magnetic fields: lim _T0 lim _H0 (H, T)=1/(2|J ₀|)(1–1/D) and lim _H0 lim _T0 (H, T)=1/(2|J ₀|).     

Determining the sign of g factor via time-resolved Kerr rotation spectroscopy with a rotatable magnetic field

Time-resolved Kerr rotation spectroscopy is used to determine the sign of the g factor of carriers in a semiconductor material,with the help of a rotatable magnetic field in the plane of the sample.The spin precession signal of carriers at a fixed time delay is measured as a function of the orientation of the magnetic field with a fixed strength B.The signal has a sine-like form and its phase determines the sign of the g factor of carriers.As a natural extension of previous methods to measure the (time-resolved) photoluminescence or time-resolved Kerr rotation signal as a function of the magnetic field strength with a fixed orientation,such a method gives the correct sign of the g factor of electrons in GaAs.Furthermore,the sign of carriers in a (Ga,Mn)As magnetic semiconductor is also found to be negative.     

The Discrete N-Vector Ferromagnet: Connection to a Percolation with Frustration Features

We extend the Kasteleyn–Fortuin formalism to the discrete N-vector ferromagnet. We show that the free energy and the correlation functions of this model are related, when the number of states tends to 1, to the mean number of clusters and to the pair connectedness of a polychromatic bond percolation type problem which combines frustration and connectivity features.     

Biomagnetic measurements utilising a superparamagnetic marker: a feasibility study

Summary A preliminary study is here reported on a new potential marker for biomagnetic measurements. The marker consists of superparamagnetic polymer microspheres which were detected in the presence of external steady magnetic fields by means of an r.f.-SQUID magnetometer. The particles were prepared in samples differing in the concentration value and immersed in a homogeneous magnetic field of variable intensity. A simple model was taken into account for the distribution of the microspheres in the samples, so that the theoretical values were compared to the marker field values measured by the biomagnetic sensor. The overall sensitivity of the experimental apparatus and the minimum concentration value of the marker were then estimated.     

Collective field in Hamiltonian and Lagrangian (path-integral) approach: An application to O(N) Heisenberg spin operator

Collective field approach to O(N) Heisenberg spin system is discussed. Hamiltonian formulation is reviewed and connection with largeN limit is shown. Collective field is then introduced in the Lagrangian path-integral formulation and 1/N corrections to various quantities like mass-gap, beta-function are computed. This research was supported in part by the National Science Foundation Grant No. PHY-78-24888.