The Chirality operators for Heisenberg Spin Systems

The ground state of closed Heisenberg spin chains with an odd number of sites has a chiral degeneracy, in addition to a two-fold Kramers degeneracy. A non-zero chirality implies that the spins are not coplanar, and is a measure of handedness. The chirality operator, which can be treated as a spin-1/2 operator, is explicitly constructed in terms of the spin operators, and is given as commutator of Permutation operators.Comment: 7 pages, report IC/94/23. E-mail: [email protected]

Spin Decoherence from Hamiltonian dynamics in Quantum Dots

The dynamics of a spin-1/2 particle coupled to a nuclear spin bath through an isotropic Heisenberg interaction is studied, as a model for the spin decoherence in quantum dots. The time-dependent polarization of the central spin is calculated as a function of the bath-spin distribution and the polarizations of the initial bath state. For short times, the polarization of the central spin shows a gaussian decay, and at later times it revives displaying nonmonotonic time dependence. The decoherence time scale dep ends on moments of the bath-spin distribuition, and also on the polarization strengths in various bath-spin channels. The bath polarizations have a tendency to increase the decoherence time scale. The effective dynamics of the central spin polarization is shown to be describ ed by a master equation with non-markovian features.Comment: 11 pages, 6 figures Accepted for publication in Phys.Rev

Shear flocculation of quartz.

The present study deals with the shear flocculation of quartz in aqueous solutions of dodecylamine. Zetapotential data was correlated with flocculation behavior.It was observed that the particle hydrophobicity and pH were most important since dodecylamine chloride reactions are Ph and concentration dependent

Prevalence of subclinical hypothyroidism in children and adolescents of northern Andhra Pradesh population and its association with hyperlipidemia

Background: The thyroid dysfunction particularly, subclinical hypothyroidism (SCH) is quite a common clinical condition in paediatric population but there is limited data available regarding its prevalence in children and adolescents in our population. The present study was aimed to know the prevalence of thyroid dysfunction particularly SCH in children and adolescents of northern Andhra Pradesh population and its association with hyperlipidemia.Methods: A retrospective study of 600 subjects (Children=272, Adolescents=328) between 6-19 years of age were included and the following parameters were examined: age, sex, total triiodothyronine (tT3), total tetraiodothyronine (tT4), thyroid stimulating hormone (TSH), total cholesterol, triglycerides (TGL), LDL and HDL cholesterol. The subjects were divided into group I and group II on the basis of age (in years), subjects between 6- ≤12 years age were grouped as group I and 12-≤19 years were as group II.Results: Out of 272 children and 328 adolescents studied, the prevalence of thyroid dysfunction was found to be 9.9% and 10.4 % respectively. The prevalence of subclinical hypothyroidism in children was 7.7% where as 4.9% in adolescents. In both the groups, females were predominantly affected with thyroid dysfunction as compared to males. Significantly elevated serum levels of total cholesterol, TSH and TGL were observed in SCH subjects when compared to euthyroid subjects (p<0.05). Statistically significant lower levels of HDL cholesterol were found in SCH as compared to euthyroids (p<0.05). However, no difference was noticed in the levels of total T3, total T4 and LDL cholesterol between SCH and euthyroids.Conclusions: The prevalence of thyroid dysfunction was found to be 10.2% in study population. SCH was observed in 7.7 % and 4.9% respectively in children and adolescent groups. Subclinical hypothyroidism (SCH) was the most predominant thyroid dysfunction found in our studied population with a prevalence of 6.2% (both children and adolescents). Correction of thyroid dysfunction particularly SCH in early childhood is highly essential to prevent the impairment of psychomotor and cognitive development

Effective chiral-spin Hamiltonian for odd-numbered coupled Heisenberg chains

An $L \times \infty$ system of odd number of coupled Heisenberg spin chains is studied using a degenerate perturbation theory, where $L$ is the number of coupled chains. An effective chain Hamiltonian is derived explicitly in terms of two spin half degrees of freedom of a closed chain of $L$ sites, valid in the regime the inter-chain coupling is stronger than the intra-chain coupling. The spin gap has been calculated numerically using the effective Hamiltonian for $L=3,5,7,9$ for a finite chain up to ten sites. It is suggested that the ground state of the effective Hamiltonian is correlated, by examining variational states for the effective chiral-spin chain Hamiltonian.Comment: 9 Pages, Latex, report ICTP-94-28

Molecular structure of Boc-Aib-Aib-Phe-Met-NH<SUB>2</SUB>·DMSO. A fragment of a biologically active enkephalin analogue

The tetrapeptide t-butyloxycarbonyl-&#945;-aminoisobutyryl-&#945;-aminoisobutyryl-L- phenylalanyl-L-methionyl amide crystallizes in the orthorhombic space group P212121 with a= 9.096, b= 18.067, c= 21.701 &#197; and Z= 4. The crystals contain one molecule of dimethyl sulphoxide (DMSO) associated with each peptide. The structure has been solved by direct methods and refined to an R value of 0.103 for 2 672 observed reflections. The peptide adopts a distorted 310 helical structure stabilized by two intramolecular 4 &#8594; 1 hydrogen bonds between the Boc CO and Aib(1) CO groups and the NH groups of Phe(3) and Met(4), respectively. A long hydrogen bond (N...O = 3.35 &#197;) is also observed between Aib(2) CO and one of the terminal amide hydrogens. The DMSO molecule is strongly hydrogen bonded to the Aib(1) NH group. The solid-state conformation agrees well with proposals made on the basis of n.m.r. studies in solution

Low-energy sector of the S=1/2 Kagome antiferromagnet

Starting from a modified version of the the S=1/2 Kagome antiferromagnet to emphasize the role of elementary triangles, an effective Hamiltonian involving spin and chirality variables is derived. A mean-field decoupling that retains the quantum nature of these variables is shown to yield a Hamiltonian that can be solved exactly, leading to the following predictions: i) The number of low lying singlet states increase with the number of sites N like 1.15 to the power N; ii) A singlet-triplet gap remains in the thermodynamic limit; iii) Spinons form boundstates with a small binding energy. By comparing these properties with those of the regular Kagome lattice as revealed by numerical experiments, we argue that this description captures the essential low energy physics of that model.Comment: 4 pages including 3 figure

Fermionic concurrence in the extended Hubbard dimer

In this paper, we introduce and study the fermionic concurrence in a two-site extended Hubbard model. Its behaviors both at the ground state and finite temperatures as function of Coulomb interaction $U$ (on-site) and $V$ (nearest-neighbor) are obtained analytically and numerically. We also investigate the change of the concurrence under a nonuniform field, including local potential and magnetic field, and find that the concurrence can be modulated by these fields.Comment: 5 pages, 7 figure