1,214 research outputs found
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
Carbon Isotope Constraints on the Deglacial CO2 Rise from Ice Cores
The stable carbon isotope ratio of atmospheric CO2 (d13Catm) is a key parameter in deciphering past carbon cycle changes. Here we present d13Catm data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in d13Catm during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the d13Catm evolution. During the Last Glacial Maximum, d13Catm and atmospheric CO2 concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then
Quantum Computing in Molecular Magnets
Shor and Grover demonstrated that a quantum computer can outperform any
classical computer in factoring numbers and in searching a database by
exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm
requires both superposition and entanglement of a many-particle system, the
superposition of single-particle quantum states is sufficient for Grover's
algorithm. Recently, the latter has been successfully implemented using Rydberg
atoms. Here we propose an implementation of Grover's algorithm that uses
molecular magnets, which are solid-state systems with a large spin; their spin
eigenstates make them natural candidates for single-particle systems. We show
theoretically that molecular magnets can be used to build dense and efficient
memory devices based on the Grover algorithm. In particular, one single crystal
can serve as a storage unit of a dynamic random access memory device. Fast
electron spin resonance pulses can be used to decode and read out stored
numbers of up to 10^5, with access times as short as 10^{-10} seconds. We show
that our proposal should be feasible using the molecular magnets Fe8 and Mn12.Comment: 13 pages, 2 figures, PDF, version published in Nature, typos
correcte
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
Spin relaxation in Mn12-acetate
We present a comprehensive derivation of the magnetization relaxation in a
Mn12-acetate crystal based on thermally assisted spin tunneling induced by
quartic anisotropy and weak transverse magnetic fields. The overall relaxation
rate as function of the magnetic field is calculated and shown to agree well
with data including all resonance peaks. The Lorentzian shape of the resonances
is also in good agreement with recent data. A generalized master equation
including resonances is derived and solved exactly. It is shown that many
transition paths with comparable weight exist that contribute to the relaxation
process. Previously unknown spin-phonon coupling constants are calculated
explicitly.Comment: 4 pages,4 EPS figures,LaTeX(europhys.sty);final version accepted for
EP
Are invasive marsh frogs (Pelophylax ridibundus) replacing the native P. lessonae /P. esculentus hybridogenetic complex in Western Europe? Genetic evidence from a field study.
The water-frog L-E system, widespread in Western Europe, comprises the pool frog Pelophylax lessonae and the hybridogenetic edible frog P. esculentus, which originated from hybridization between pool frogs and marsh frogs (P. ridibundus). In P. esculentus, the lessonae (L) genome is eliminated during meiosis and has to be gained anew each generation from a P. lessonae partner, while the ridibundus (R') genome is transmitted clonally. It therefore accumulates deleterious mutations, so that R'R' offspring from P. esculentus x P. esculentus crosses are normally unviable. This system is now threatened by invasive P. ridibundus (RR) imported from Eastern Europe and the Balkans. We investigated the genetic interactions between invasive marsh frogs and native water frogs in a Swiss wetland area, and used genetic data collected in the field to validate several components of a recently postulated mechanism of species replacement. We identified neo-ridibundus individuals derived from crosses between invasive ridibundus and native esculentus, as well as newly arisen hybridogenetic esculentus lineages stemming from crosses between invasive ridibundus (RR) and native lessonae (LL). As their ridibundus genomes are likely to carry less deleterious mutations, such lineages are expected to produce viable ridibundus offspring, contributing to species replacement. However, such crosses with invasive ridibundus only occurred at a limited scale; moreover, RR x LL crosses did not induce any introgression from the ridibundus to the lessonae genome. We did not find any ridibundus stemming from crosses between ancient esculentus lineages. Despite several decades of presence on the site, introduced ridibundus individuals only represent 15% of sampled frogs, and their spatial distribution seems shaped by specific ecological requirements rather than history of colonization. We therefore expect the three taxa to coexist stably in this area
Temperature-dependent spin gap and singlet ground state in BaCuSi2O6
Bulk magnetic measurements and inelastic neutron scattering were used to
investigate the spin-singlet ground state and magnetic gap excitations in
BaCuSi2O6, a quasi-2-dimensional antiferromagnet with a bilayer structure. The
results are well described by a model based on weakly interacting
antiferromagnetic dimers. A strongly temperature-dependent dispersion in the
gap modes was found. We suggest that the observed excitations are analogous to
magneto-excitons in light rare-earth compounds, but are an intrinsic property
of a simple Heisenberg Hamiltonian for the S=1/2 magnetic bilayer.Comment: 10 pages, 4 figures, REVTeX and PS for text, PS for figures direct
download: http://papillon.phy.bnl.gov/preprints/bacusio.htm
Mode-selective coupling of coherent phonons to the Bi2212 electronic band structure
Cuprate superconductors host a multitude of low-energy optical phonons. Using
time- and angle-resolved photoemission spectroscopy, we study coherent phonons
in BiSrCaYCuO. Sub-meV
modulations of the electronic band structure are observed at frequencies of
and THz. For the dominant mode at 3.94 THz, the
amplitude of the band energy oscillation weakly increases as a function of
momentum away from the node. Theoretical calculations allow identifying the
observed modes as CuO-derived phonons. The Bi- and Sr-derived
modes which dominate Raman spectra in the relevant frequency range are
absent in our measurements. This highlights the mode-selectivity for phonons
coupled to the near-Fermi-level electrons, which originate from CuO
planes and dictate thermodynamic properties.Comment: 7 pages, 3 figure
Toxic effects of estradiol E2 on development in the European tree frog (Hyla arborea)
Estrogenic hormones are a major environmental threat to aquatic wildlife. Here we report chronic toxic effects of exposure to the naturally excreted estrogen, 17β-estradiol (E2), on the larval and subadult development of the European tree frog (Hyla arborea), by an experimental setting and long-term monitoring. In addition to the documented impact on sexual development and mating behavior, the general toxicity of human-released estrogens may contribute to global amphibian declines
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