725 research outputs found
Current-induced transverse spin wave instability in a thin nanomagnet
We show that an unpolarized electric current incident perpendicular to the
plane of a thin ferromagnet can excite a spin-wave instability transverse to
the current direction if source and drain contacts are not symmetric. The
instability, which is driven by the current-induced ``spin-transfer torque'',
exists for one current direction only.Comment: 4 pages, 2 figure
Current induced transverse spin-wave instability in thin ferromagnets: beyond linear stability analysis
A sufficiently large unpolarized current can cause a spin-wave instability in
thin nanomagnets with asymmetric contacts. The dynamics beyond the instability
is understood in the perturbative regime of small spin-wave amplitudes, as well
as by numerically solving a discretized model. In the absence of an applied
magnetic field, our numerical simulations reveal a hierarchy of instabilities,
leading to chaotic magnetization dynamics for the largest current densities we
consider.Comment: 14 pages, 10 figures; revtex
Magnetic Field Dependent Tunneling in Glasses
We report on experiments giving evidence for quantum effects of
electromagnetic flux in barium alumosilicate glass. In contrast to expectation,
below 100 mK the dielectric response becomes sensitive to magnetic fields. The
experimental findings include both, the complete lifting of the dielectric
saturation by weak magnetic fields and oscillations of the dielectric response
in the low temperature resonant regime. As origin of these effects we suggest
that the magnetic induction field violates the time reversal invariance leading
to a flux periodicity in the energy levels of tunneling systems. At low
temperatures, this effect is strongly enhanced by the interaction between
tunneling systems and thus becomes measurable.Comment: 4 pages, 4 figure
Evidence for Magnetic Field Induced Changes of the Phase of Tunneling States: Spontaneous Echoes in (KBr)(KCN) in Magnetic Fields
Recently, it has been discovered that in contrast to expectations the
low-temperature dielectric properties of some multi-component glasses depend
strongly on magnetic fields. In particular, the low-frequency dielectric
susceptibility and the amplitude of coherent polarization echoes show striking
non-monotonic magnetic field dependencies. The low-temperature dielectric
response of these materials is governed by atomic tunneling systems. We now
have investigated the coherent properties of tunneling states in a crystalline
host in magnetic fields up to 230mT. Two-pulse echo experiments have been
performed on a KBr crystal containing about 7.5% CN. Like in glasses, but
perhaps even more surprising in the case of a crystalline system, we observe a
very strong magnetic field dependence of the echo amplitude. Moreover, for the
first time we have direct evidence that magnetic fields change the phase of
coherent tunneling systems in a well-defined way. We present the data and
discuss the possible origin of this intriguing effect.Comment: 4 pages, 3 figures, submitted to PR
Dielectric Susceptibility and Heat Capacity of Ultra-Cold Glasses in Magnetic Field
Recent experiments demonstrated unexpected, even intriguing properties of
certain glassy materials in magnetic field at low temperatures. We have studied
the magnetic field dependence of the static dielectric susceptibility and the
heat capacity of glasses at low temperatures. We present a theory in which we
consider the coupling of the tunnelling motion to nuclear quadrupoles in order
to evaluate the static dielectric susceptibility. In the limit of weak magnetic
field we find the resonant part of the susceptibility increasing like
while for the large magnetic field it behaves as 1/B. In the same manner we
consider the coupling of the tunnelling motion to nuclear quadrupoles and
angular momentum of tunnelling particles in order to find the heat capacity.
Our results show the Schotky peak for the angular momentum part, and
dependence for nuclear quadrupoles part of heat capacity, respectively. We
discuss whether or not this approach can provide a suitable explanation for
such magnetic properties.Comment: 10 pages, 1 figur
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Evaluation of coupled ocean-atmosphere simulations of northern hemisphere extratropical climates in the mid-Holocene
We have used the BIOME4 biogeography–biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean–atmosphere general circulation models to mid-Holocene changes in orbital forcing in the mid- to high-latitudes of the northern hemisphere. All the models produce: (a) a northward shift of the northern limit of boreal forest, in response to simulated summer warming in high-latitudes. The northward shift is markedly asymmetric, with larger shifts in Eurasia than in North America; (b) an expansion of xerophytic vegetation in mid-continental North America and Eurasia, in response to increased temperatures during the growing season; (c) a northward expansion of temperate forests in eastern North America, in response to simulated winter warming. The northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in North America are supported by palaeovegetation data. The expansion of xerophytic vegetation in mid-continental North America is consistent with palaeodata, although the extent may be over-estimated. The simulated expansion of xerophytic vegetation in Eurasia is not supported by the data. Analysis of an asynchronous coupling of one model to an equilibrium-vegetation model suggests vegetation feedback exacerbates this mid-continental drying and produces conditions more unlike the observations. Not all features of the simulations are robust: some models produce winter warming over Europe while others produce winter cooling. As a result, some models show a northward shift of temperate forests (consistent with, though less marked than, the expansion shown by data) and others produce a reduction in temperate forests. Elucidation of the cause of such differences is a focus of the current phase of the Palaeoclimate Modelling Intercomparison Project
Modular Invariance and Uniqueness of Conformal Characters
We show that the conformal characters of various rational models of
W-algebras can be already uniquely determined if one merely knows the central
charge and the conformal dimensions. As a side result we develop several tools
for studying representations of SL(2,Z) on spaces of modular functions. These
methods, applied here only to certain rational conformal field theories, may be
useful for the analysis of many others.Comment: 21 pages (AMS TeX), BONN-TH-94-16, MPI-94-6
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