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Butterfly hysteresis loop at non-zero bias field in antiferromagnetic molecular rings: cooling by adiabatic magnetization

At low temperatures, the magnetization of the molecular ferric wheel NaFe$_6$ exhibits a step at a critical field $B_c$ due to a field-induced level-crossing. By means of high-field torque magnetometry we observed a hysteretic behavior at the level-crossing with a characteristic butterfly shape which is analyzed in terms of a dissipative two-level model. Several unusual features were found. The non-zero bias field of the level-crossing suggests the possibility of cooling by adiabatic magnetization.Comment: 4 pages, 5 figures, REVTEX4, to appear in PR

Proton NMR for Measuring Quantum-Level Crossing in the Magnetic Molecular Ring Fe10

The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a function of temperature and magnetic field (up to 15 T) in the molecular magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic field values corresponding to a crossing between the ground state and the excited states of the molecule. We propose that this is due to a cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir of the Zeeman levels of the molecule. This effect provides a powerful tool to investigate quantum dynamical phenomena at level crossing.Comment: Four pages, to appear in Phys.Rev.Let

Model Exact Low-Lying States and Spin Dynamics in Ferric Wheels; Fe6_6 to Fe12_{12}

Using an efficient numerical scheme that exploits spatial symmetries and spin-parity, we have obtained the exact low-lying eigenstates of exchange Hamiltonians for ferric wheels up to Fe$_{12}$. The largest calculation involves the Fe$_{12}$ ring which spans a Hilbert space dimension of about 145 million for M$_s$=0 subspace. Our calculated gaps from the singlet ground state to the excited triplet state agrees well with the experimentally measured values. Study of the static structure factor shows that the ground state is spontaneously dimerized for ferric wheels. Spin states of ferric wheels can be viewed as quantized states of a rigid rotor with the gap between the ground and the first excited state defining the inverse of moment of inertia. We have studied the quantum dynamics of Fe$_{10}$ as a representative of ferric wheels. We use the low-lying states of Fe$_{10}$ to solve exactly the time-dependent Schr\"odinger equation and find the magnetization of the molecule in the presence of an alternating magnetic field at zero temperature. We observe a nontrivial oscillation of magnetization which is dependent on the amplitude of the {\it ac} field. We have also studied the torque response of Fe$_{12}$ as a function of magnetic field, which clearly shows spin-state crossover.Comment: Revtex, 24 pages, 8 eps figure

Rotational modes in molecular magnets with antiferromagnetic Heisenberg exchange

In an effort to understand the low temperature behavior of recently synthesized molecular magnets we present numerical evidence for the existence of a rotational band in systems of quantum spins interacting with nearest-neighbor antiferromagnetic Heisenberg exchange. While this result has previously been noted for ring arrays with an even number of spin sites, we find that it also applies for rings with an odd number of sites as well as for all of the polytope configurations we have investigated (tetrahedron, cube, octahedron, icosahedron, triangular prism, and axially truncated icosahedron). It is demonstrated how the rotational band levels can in many cases be accurately predicted using the underlying sublattice structure of the spin array. We illustrate how the characteristics of the rotational band can provide valuable estimates for the low temperature magnetic susceptibility.Comment: 14 pages, 7 figures, to be published in Phys. Rev.

Current contraceptive management in Australian general practice: An analysis of BEACH data

Objective: To determine current contraceptive management by general practitioners in Australia. Design, setting and participants: Analysis of data from a random sample of 3910 Australian GPs who participated in the Bettering the Evaluation and Care of Health (BEACH) survey, a continuous cross-sectional survey of GP activity, between April 2007 and March 2011. Consultations with female patients aged 12-54 years that involved all forms of contraception were analysed. Main outcome measures: GP and patient characteristics associated with the management of contraception; types of contraception used; rates of encounters involving emergency contraception. Results: Increased age, ethnicity, Indigenous status and holding a Commonwealth Health Care Card were significantly associated with low rates of encounters involving management of contraception. The combined oral contraceptive pill was the most frequently prescribed method of contraception, with moderate prescription of long-acting reversible contraception (LARC), especially among women aged 34-54 years. Rates of consultations concerned with emergency contraception were low, but involved high rates of counselling, advice or education (48%) compared with encounters for general contraception (> 20%). Conclusion: A shift towards prescribing LARC, as recommended in clinical guidelines, has yet to occur in Australian general practice. Better understanding of patient and GP perspectives on contraceptive choices could lead to more effective contraceptive use

Emergency contraception in Australia: The desired source of information versus the actual source of information

Objective: To determine long-term trends in emergency contraception (EC) management by general practitioners in Australia. Design, setting and participants: Data from April 2000 to March 2012 were drawn from the BEACH (Bettering the Evaluation and Care of Health) program, a continuous cross-sectional survey of GP activity. We analysed consultations involving EC management, unwanted pregnancy management and emergency contraceptive pill (ECP) prescribing per 1000 GP encounters with women aged 14-54 years. Summary statistics were calculated with 95% confidence intervals. Results: In 2000-2001, GPs managed EC problems at a rate of 5.50 per 1000 encounters (95% CI, 4.37-6.63). From 2004, after the ECP became available over the counter (OTC) in pharmacies, EC management, which includes ECP prescription, progressively declined. By 2011-2012, only 1.43 EC problems were managed per 1000 encounters (95% CI, 0.84-2.02) and only 0.48 ECP prescriptions were provided per 1000 encounters (95% CI, 0.14-0.82). Yet the management rate of unwanted pregnancy problems stayed relatively constant (rate in 2000-2001, 0.95 per 1000 encounters; 95% CI, 0.40-1.50; rate in 2011-2012, 0.88 per 1000 encounters; 95% CI, 0.41-1.36). Conclusion: Low rates of EC management by GPs since ECP became available OTC suggest that women may be obtaining information on EC elsewhere. Further investigation is needed to uncover the sources of this information and its acceptability and application by Australian women

An investigation of oxidation and reduction of C 60 , the excited states, energy gaps and stability using semi-empirical and ab initio methods

Abstract We have used semi-empirical and ab initio methods to investigate the various oxidation and reduction states of C 60 with their respective more energetically stable excited states resulting from variations of the charge from 112 to 212 electrons in the system with singlet, doublet, triplet and quadruplet multiplicities. We have analysed the various conformations, energy variations, uHOMO 2 LUMOu, system charges, average interatomic distances as well as regions of minimum energies resulting from the simulation of the possible reaction routes.

Mapping the Evolution of Optically-Generated Rotational Wavepackets in a Room Temperature Ensemble of D2_2

A coherent superposition of rotational states in D$_2$ has been excited by nonresonant ultrafast (12 femtosecond) intense (2 $\times$ 10$^{14}$ Wcm$^{-2}$) 800 nm laser pulses leading to impulsive dynamic alignment. Field-free evolution of this rotational wavepacket has been mapped to high temporal resolution by a time-delayed pulse, initiating rapid double ionization, which is highly sensitive to the angle of orientation of the molecular axis with respect to the polarization direction, $\theta$. The detailed fractional revivals of the neutral D$_2$ wavepacket as a function of $\theta$ and evolution time have been observed and modelled theoretically.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev. A. Full reference to follow.

Influences on condom use: A secondary analysis of women's perceptions from the Australian Contraceptive ChOice pRoject (ACCORd) trial.

Background and objectives Women’s ability to negotiate condom use helps prevent sexually transmissible infections (STIs) and unintended pregnancies. The aim of this study was to assess the relationship between substance use, risk perception and the certainty of using condoms in several hypothetical situations. Methods This is a secondary analysis from the Australian Contraceptive ChOice pRoject (ACCORd) cluster randomised controlled trial. Descriptive statistics and logistic regression were used for the analysis. Results At baseline, contraceptive questions were answered by 698 women attending 57 general practices in Melbourne, Australia. Condom use was reported by 47%. Of those using condoms as the sole form of contraception (n = 137), 20% used them inconsistently. Dual protection was used by 58% of women (188/325). Condoms and the pill were more frequently used than condoms and longer-acting contraceptives. Women were less likely to be confident negotiating condom use when using substances. Discussion Substance use and the concurrent use of other forms of contraception impact use of condoms. Even when condoms are the sole form of contraception with willing partners, use is inconsistent, leaving women at risk of pregnancy and STI