3,440 research outputs found
Renormalization group and other calculations for the oneâdimensional spinâ1/2 dimerized Heisenberg antiferromagnet
A zero-temperature renormalization group (RG) approach is applied to the one-dimensional, spin-1/2 anti ferromagnetic Heisenberg dimerized (alternating) chain. Specifically, the ground state energy and lowest-lying spectral excitations are examined. The calculation indicates the existence of a gap in the spectrum of the dimerized chain which vanishes only in the limit of a uniform spin chain in contrast to a recent Green\u27s function approach. The RG results are in reasonable agreement with numerical extrapolations on the exact eigenvalue spectrum of finite chains of up to 12 spins. Both methods are compared with several other approximate treatments of the Heisenberg system. and tested by comparison with exact results for the spin-1/2 XY dimerized chain
Crystal Structure and Magnetism of the Linear-Chain Copper Oxides Sr5Pb3-xBixCuO12
The title quasi-1D copper oxides (0=< x =<0.4) were investigated by neutron
diffraction and magnetic susceptibility studies. Polyhedral CuO4 units in the
compounds were found to comprise linear-chains at inter-chain distance of
approximately 10 A. The parent chain compound (x = 0), however, shows less
anisotropic magnetic behavior above 2 K, although it is of substantially
antiferromagnetic (mu_{eff}= 1.85 mu_{B} and Theta_{W} = -46.4 K) spin-chain
system. A magnetic cusp gradually appears at about 100 K in T vs chi with the
Bi substitution. The cusp (x = 0.4) is fairly characterized by and therefore
suggests the spin gap nature at Delta/k_{B} ~ 80 K. The chain compounds hold
electrically insulating in the composition range.Comment: To be published in PR
Proportion Regulation in Globally Coupled Nonlinear Systems
As a model of proportion regulation in differentiation process of biological
system, globally coupled activator-inhibitor systems are studied. Formation and
destabilization of one and two cluster state are predicted analytically.
Numerical simulations show that the proportion of units of clusters is chosen
within a finite range and it is selected depend on the initial condition.Comment: 11 pages (revtex format) and 5 figures (PostScript)
Substance use and depressive and anxiety symptoms among out-of-school adolescent girls and young women in Cape Town, South Africa
Background. There is a high prevalence of substance use among youth in South Africa (SA), and adolescent girls and young women (AGYW) experience high rates of depression and anxiety. Substance use behaviours and mental health are associated with other public health problems among AGYW such as HIV and unintended pregnancy. Therefore, understanding the relationship between substance use and mental health is imperative to improve AGYWâs health.Objectives. To examine the association between heavy drinking, marijuana, methamphetamine and methaqualone (Mandrax) use and depressive and anxiety symptoms among AGYW aged 16 - 19 years who have dropped out of school in Cape Town, SA.Methods. Data for this report come from the baseline data of 500 participants of an ongoing cluster-randomised trial assessing the efficacy of a young woman-focused intervention to reduce substance use and HIV risk. After AGYW consented/assented to participate, they completed a urine drug screen and a baseline questionnaire.Results. Logistic and negative binomial regressions, controlling for clustering at the neighbourhood level, revealed that frequency of depressive symptoms was significantly and positively related to a positive drug screen for Mandrax (ÎČ=0.07; p=0.03). All other associations between the frequency of depressive symptoms and substance use were not statistically significant (ps>0.05). The associations between frequency of anxiety symptoms and substance use were not statistically significant (ps>0.05).Conclusions. Our findings highlight the need to address substance use, especially Mandrax use and its associated risk, and depression in an integrated, youth-friendly setting
Mandrax use, sexual risk, and opportunities for pre-exposure prophylaxis among out-of-school adolescent girls and young women in Cape Town, South Africa
Background. In South Africa (SA), adolescent girls and young women (AGYW) aged 15 - 24 years account for nearly 25% of all new HIV infections in the country. The intersection of substance use and sexual risk continues to drive the HIV epidemic among AGYW. For example, methaqualone, also known as Mandrax, has sedative effects that may affect womenâs ability to negotiate condom use during sex, refuse sex without a condom, or consent to sex, thereby increasing their risk for HIV. Consequently, it is critical to understand how Mandrax use affects HIV risk among AGYW and to assess awareness of and willingness to use biomedical HIV prevention methods, such as pre-exposure prophylaxis (PrEP), among AGYW who use Mandrax.Objectives. To examine the role of Mandrax use in sexual behaviours and investigate the extent to which AGYW who use Mandrax are aware of and willing to initiate PrEP.Methods. Data for this report were derived from baseline and 6-month follow-up data provided by 500 AGYW participating in a cluster-randomised trial assessing the efficacy of a young woman-focused intervention to reduce substance use and HIV risk. AGYW who self-identified as black African or coloured, reported using substances, reported condomless sex in the past 3 months, and had discontinued school early were recruited from 24 community clusters across Cape Town, SA. Following consent/assent, participants provided biological specimens to test for recent drug use (including Mandrax) and completed the self-report questionnaire.Results. Logistic regression analysis revealed that the AGYW who had a positive test result for Mandrax use were less likely to use a condom with their main partner (p=0.01), and almost three times more likely to use alcohol and/or other drugs before or during their last act of sexual intercourse (p<0.001), compared with the AGYW who had a negative Mandrax test result. Mandrax use was not significantly related to PrEP awareness (p>0.10) or willingness to use PrEP (p>0.10), but 70% of AGYW who used Mandrax were willing to initiate PrEP.Conclusion. The study findings highlight how Mandrax use may contribute to HIV risk among SA AGYW. Key decision-makers should consider incorporating substance use prevention efforts into existing HIV reduction programmes and equip youth-friendly clinics with the resources to identify AGYW who use Mandrax and offer them PrEP
Susceptibility calculations for alternating antiferromagnetic chains
Earlier work of Duffy and Barr consisting of exact calculations on alternating antiferromagnetic Heisenberg spinâ1/2 chains is extended to longer chains of up to 12 spins, and subsequent extrapolations of thermodynamic properties, particularly the susceptibility, are extended to the weak alternation region close to the uniform limit. This is the region of interest in connection with the recent experimental discovery of spinâPeierls systems. The extrapolated susceptibility curves are compared with corresponding curves calculated from the model of Bulaevskii, which has been used extensively in approximate theoretical treatments of a variety of phenomena. Qualitative agreement is observed in the uniform limit and persists for all degrees of alternation, but quantitative differences of about 10% are present over the whole range, including the isolated dimer limit. Potential application of the new susceptibility calculations to experiment is discussed
The Double-Time Green's Function Approach to the Two-Dimensional Heisenberg Antiferromagnet with Broken Bonds
We improved the decoupling approximation of the double-time Green's function
theory, and applied it to study the spin- two-dimensional
antiferromagnetic Heisenberg model with broken bonds at finite temperature. Our
decoupling approximation is applicable to the spin systems with spatial
inhomogeneity, introduced by the local defects, over the whole temperature
region. At low temperatures, we observed that the quantum fluctuation is
reduced in the neighborhood of broken bond, which is in agreement with previous
theoretical expectations. At high temperatures our results showed that the
quantum fluctuation close to the broken bond is enhanced. For the two parallel
broken bonds cases, we found that there exists a repulsive interaction between
the two parallel broken bonds at low temperatures.Comment: Revtex, 6 pages, 5 Postscript figures (include
Hawking's radiation in non-stationary rotating de Sitter background
Hawking's radiation effect of Klein-Gordon scalar field, Dirac particles and
Maxwell's electromagnetic field in the non-stationary rotating de Sitter
cosmological space-time is investigated by using a method of generalized
tortoise co-ordinates transformation. The locations and the temperatures of the
cosmological horizons of the non-stationary rotating de Sitter model are
derived. It is found that the locations and the temperatures of the rotating
cosmological model depend not only on the time but also on the angle. The
stress-energy regularization techniques are applied to the two dimensional
analog of the de Sitter metrics and the calculated stress-energy tensor
contains the thermal radiation effect.Comment: 13 pages, LaTex format, accepted for publication Astrophysics and
Space Science, Springer; Journal ID: 10509, Article ID: 606, Date 2011-01-1
Pyrochlore Antiferromagnet: A Three-Dimensional Quantum Spin Liquid
The quantum pyrochlore antiferromagnet is studied by perturbative expansions
and exact diagonalization of small clusters. We find that the ground state is a
spin-liquid state: The spin-spin correlation functions decay exponentially with
distance and the correlation length never exceeds the interatomic distance. The
calculated magnetic neutron diffraction cross section is in very good agreement
with experiments performed on Y(Sc)Mn2. The low energy excitations are
singlet-singlet ones, with a finite spin gap.Comment: 4 pages, 4 figure
Quasi-Elastic Scattering in the Inclusive (He, t) Reaction
The triton energy spectra of the charge-exchange C(He,t) reaction
at 2 GeV beam energy are analyzed in the quasi-elastic nucleon knock-out
region. Considering that this region is mainly populated by the charge-exchange
of a proton in He with a neutron in the target nucleus and the final proton
going in the continuum, the cross-sections are written in the distorted-wave
impulse approximation. The t-matrix for the elementary exchange process is
constructed in the DWBA, using one pion- plus rho-exchange potential for the
spin-isospin nucleon- nucleon potential. This t-matrix reproduces the
experimental data on the elementary pn np process. The calculated
cross-sections for the C(He,t) reaction at to triton
emission angle are compared with the corresponding experimental data, and are
found in reasonable overall accord.Comment: 19 pages, latex, 11 postscript figures available at
[email protected], submitted to Phy.Rev.
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