Spin tunneling and topological selection rules for integer spins

We present topological interference effects for the tunneling of a single large spin, which are caused by the symmetry of a general class of magnetic anisotropies. The interference originates from spin Berry phases associated with different tunneling paths exposed to the same dynamics. Introducing a generalized path integral for coherent spin states, we evaluate transition amplitudes between ground as well as low-lying excited states. We show that these interference effects lead to topological selection rules and spin-parity effects for integer spins that agree with quantum selection rules and which thus provide a generalization of the Kramers degeneracy to integer spins. Our results apply to the molecular magnets Mn12 and Fe8.Comment: 4 pages, 3 EPS figures, REVTe

Reply to the comment of Chudnovsky&Garanin on "Spin relaxation in Mn12-acetate"

Reply to the comment of E.M. Chudnovsky and D.A. Garanin on Europhys. Lett. 46, 692 (1999).Comment: 2 pages, Latex (europhys.sty

Discrete Fourier Transform in Nanostructures using Scattering

In this paper we show that the discrete Fourier transform can be performed by scattering a coherent particle or laser beam off a two-dimensional potential that has the shape of rings or peaks. After encoding the initial vector into the two-dimensional potential, the Fourier-transformed vector can be read out by detectors surrounding the potential. The wavelength of the laser beam determines the necessary accuracy of the 2D potential, which makes our method very fault-tolerant.Comment: 6 pages, 5 EPS figures, REVTe

The Grover algorithm with large nuclear spins in semiconductors

We show a possible way to implement the Grover algorithm in large nuclear spins 1/2<I<9/2 in semiconductors. The Grover sequence is performed by means of multiphoton transitions that distribute the spin amplitude between the nuclear spin states. They are distinguishable due to the quadrupolar splitting, which makes the nuclear spin levels non-equidistant. We introduce a generalized rotating frame for an effective Hamiltonian that governs the non-perturbative time evolution of the nuclear spin states for arbitrary spin lengths I. The larger the quadrupolar splitting, the better the agreement between our approximative method using the generalized rotating frame and exact numerical calculations.Comment: 11 pages, 18 EPS figures, REVTe

Ultrastructural alterations in capillaries of the diabetic hypertensive rat retina: protective effects of ACE inhibition

Aims/hypothesis: The ACE inhibitor cilazapril was administered to diabetic hypertensive rats to evaluate its ability to influence the development of retinal capillary alterations. Methods: Normotensive (strain: Wistar Kyoto) and genetically hypertensive (strain: spontaneously hypertensive) rats were rendered diabetic by intravenous injections of streptozotocin. Half of the diabetic animals received cilazapril with their daily food. At 20 weeks of diabetes, endothelial cells, pericytes and extracellular matrix were assessed by ultrastructural morphometry. Each experimental group consisted of seven animals. Results: Cilazapril normalised systolic arterial pressure in diabetic hypertensive rats (137±2mmHg compared with 188±16mmHg in non-medicated diabetic hypertensive rats, p<0.001). The number of endothelial intercellular junctions was reduced in untreated diabetic hypertensive rats (0.15±0.05, p<0.02, vs 0.47±0.20 in non-diabetic normotensive rats). In diabetic hypertensive animals treated with cilazapril, this loss was attenuated (0.32±0.16, p<0.05). The significant thickening of the basement membrane observed in the diabetic normotensive (132.8±19.4nm) and diabetic hypertensive (150.3±20.2nm) groups was decreased by cilazapril in the diabetic hypertensive group (116.7±11.0nm, p<0.01), but was unaffected in the normotensive (131.9±17.3nm) group. No protective effect of the drug was observed in either group on pericytes. Conclusions/interpretation: Long-term administration of an effective antihypertensive therapy normalises endothelial alterations and basement membrane thickness in diabetic hypertensive conditions, and thus may account for the well-known improvement of the blood-retinal barrier observed during antihypertensive treatmen

Crystal Field -AS_z^2 Does Not Produce One-Phonon Transitions With Delta S_z=+-2 [Comment on EPL 46, 692 (1999) by Leuenberger and Loss]

Recently Leuenbeger and Loss suggested a theory of phonon-assisted relaxation in a molecular nanomagnet Mn-12 that "contrary to previous results is in reasonably good agreement ... with all experimental parameter values known so far". The purpose of this Comment is to show that the model of Leuenberger and Loss and its comparison with experiment are premised upon their incorrect use of the linear formula for the strain tensor. The spin-phonon coupling introduced by Leuenberger and Loss disappears if the nonlinear term in the strain tensor is taken into account.Comment: 2 pages, no figures, submitted to EP

Weak antiferromagnetic coupling in molecular ring is predicted correctly by density functional theory plus Hubbard U

We apply density functional theory with empirical Hubbard U parameter (DFT+U) to study Mn-based molecular magnets. Unlike most previous DFT+U studies, we calibrate U parameters for both metal and ligand atoms using five binuclear manganese complexes as the benchmarks. We note delocalization of the spin density onto acetate ligands due to pi-back bonding, inverting spin polarization of the acetate oxygen atoms relative to that predicted from superexchange mechanism. This inversion may affect the performance of the models that assume strict localization of the spins on magnetic centers for the complexes with bridging acetate ligands. Next, we apply DFT+U methodology to Mn-12 molecular wheel and find antiparallel spin alignment for the weakly interacting fragments Mn-6, in agreement with experimental observations. Using the optimized geometry of the ground spin state instead of less accurate experimental geometry was found to be crucial for this good agreement. The protocol tested in this study can be applied for the rational design of single molecule magnets for molecular spintronics and quantum computing applications

Metabolic response of early-lactating cows exposed to transport and high altitude grazing conditions

The metabolic response of dairy cows to high as opposed to low altitude conditions (2000 m v. 400 m above sea level) was determined. In the first experiment, four cows were subjected to a series of measurements before, during and after transport from lowland to high altitude pasture. During transport, cortisol, l-lactate and non-esterified fatty acids were significantly elevated but decreased within 1 to 3 days to initial levels. After transport, β-hydroxybutyrate and the thyroid hormones immediately increased and returned within 3 weeks to initial levels. Plasma urea increased during transport and subsequently was at an intermediate level due to the different diet. There were no direct carry-over effects of transport on metabolic traits during pasturing. In the second experiment, three groups of six different dairy cows were either grazed in one of two consecutive years or kept inside (2nd year only). Lowland sojourn lasted for 4 weeks, and high altitude period for 8 weeks. At the end of high altitude sojourn, both outside and inside groups were found still to have significantly higher plasma cortisol values than at lowland. Thyroid hormones and ketosis related metabolites sharply increased at the start of the alpine period and were elevated for 1 to 3 weeks thereafter. According to the hormonal and metabolic profiles, the permanently housed cows did not benefit from the less adverse climatic conditions and the lower physical strain. Plasma urea closely reflected dietary changes in the ratio of nitrogen to fermentable organic matter. Plasma protein, albumin, creatinine, and liver enzyme activities were not affected by transport or high altitude sojourn in both experiments. The results indicate that the metabolic response to transport and high altitude conditions can be mostly explained by the efforts to cover the additional energy requirements. Overall the data suggest a wide but nevertheless limited ability of early-lactating cows to adapt to high altitude condition