381 research outputs found
Spin Driven Jahn-Teller Distortion in a Pyrochlore system
The ground-state properties of the spin-1 antiferromagnetic Heisenberg model
on the corner-sharing tetrahedra, pyrochlore lattice, is investigated. By
breaking up each spin into a pair of 1/2-spins, the problem is reduced to the
equivalent one of the spin-1/2 tetrahedral network in analogy with the valence
bond solid state in one dimension. The twofold degeneracy of the spin-singlets
of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to
tetragonal structural transition. It is proposed that the present mechanism is
responsible for the phase transition observed in the spin-1 spinel compounds
ZnVO and MgVO.Comment: 4 pages, 3 eps figures, REVTeX, to appear in Phys. Rev. Let
Metal-insulator Crossover Behavior at the Surface of NiS_2
We have performed a detailed high-resolution electron spectroscopic
investigation of NiS and related Se-substituted compounds
NiSSe, which are known to be gapped insulators in the bulk at all
temperatures. A large spectral weight at the Fermi energy of the room
temperature spectrum, in conjunction with the extreme surface sensitivity of
the experimental probe, however, suggests that the surface layer is metallic at
300 K. Interestingly, the evolution of the spectral function with decreasing
temperature is characterized by a continuous depletion of the single-particle
spectral weight at the Fermi energy and the development of a gap-like structure
below a characteristic temperature, providing evidence for a metal-insulator
crossover behavior at the surfaces of NiS and of related compounds. These
results provide a consistent description of the unusual transport properties
observed in these systems.Comment: 12 pages, 3 figure
Spin-Peierls phases in pyrochlore antiferromagnets
In the highly frustrated pyrochlore magnet spins form a lattice of corner
sharing tetrahedra. We show that the tetrahedral ``molecule'' at the heart of
this structure undergoes a Jahn-Teller distortion when lattice motion is
coupled to the antiferromagnetism. We extend this analysis to the full
pyrochlore lattice by means of Landau theory and argue that it should exhibit
spin-Peierls phases with bond order but no spin order. We find a range of Neel
phases, with collinear, coplanar and noncoplanar order. While collinear Neel
phases are easiest to generate microscopically, we also exhibit an interaction
that gives rise to a coplanar state instead.Comment: REVTeX 4, 14 pages, 12 figures (best viewed in color
High-energy spectroscopic study of the III-V nitride-based diluted magnetic semiconductor GaMnN
We have studied the electronic structure of the diluted magnetic
semiconductor GaMnN ( = 0.0, 0.02 and 0.042) grown on Sn-doped
-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn
-edge x-ray absorption have indicated that the Mn ions are in the
tetrahedral crystal field and that their valence is divalent. Upon Mn doping
into GaN, new state were found to form within the band gap of GaN, and the
Fermi level was shifted downward. Satellite structures in the Mn 2 core
level and the Mn 3 partial density of states were analyzed using
configuration-interaction calculation on a MnN cluster model. The deduced
electronic structure parameters reveal that the - exchange coupling in
GaMnN is stronger than that in GaMnAs.Comment: 6pages, 10figures. To be published to Phys. Rev.
Itinerant ferromagnetism in half-metallic CoS_2
We have investigated electronic and magnetic properties of the pyrite-type
CoS_2 using the linearized muffin-tin orbital (LMTO) band method. We have
obtained the ferromagnetic ground state with nearly half-metallic nature. The
half-metallic stability is studied by using the fixed spin moment method. The
non-negligible orbital magnetic moment of Co 3d electrons is obtained as in the local spin density approximation (LSDA). The calculated
ratio of the orbital to spin angular momenta / = 0.15 is
consistent with experiment. The effect of the Coulomb correlation between Co 3d
electrons is also explored with the LSDA + U method. The Coulomb correlation at
Co sites is not so large, eV, and so CoS_2 is possibly
categorized as an itinerant ferromagnet. It is found that the observed
electronic and magnetic behaviors of CoS_2 can be described better by the LSDA
than by the LSDA + U.Comment: 4 pages, 3 postscript figure
Hyperthermic effects of dissipative structures of magnetic nanoparticles in large alternating magnetic fields
Targeted hyperthermia treatment using magnetic nanoparticles is a promising cancer therapy. However, the mechanisms of heat dissipation in the large alternating magnetic field used during such treatment have not been clarified. In this study, we numerically compared the magnetic loss in rotatable nanoparticles in aqueous media with that of non-rotatable nanoparticles anchored to localised structures. In the former, the relaxation loss in superparamagnetic nanoparticles has a secondary maximum because of slow rotation of the magnetic easy axis of each nanoparticle in the large field in addition to the known primary maximum caused by rapid Néel relaxation. Irradiation of rotatable ferromagnetic nanoparticles with a high-frequency axial field generates structures oriented in a longitudinal or planar direction irrespective of the free energy. Consequently, these dissipative structures significantly affect the conditions for maximum hysteresis loss. These findings shed new light on the design of targeted magnetic hyperthermia treatments
Numerical Study of Aging in the Generalized Random Energy Model
Magnetizations are introduced to the Generalized Random Energy Model (GREM)
and numerical simulations on ac susceptibility is made for direct comparison
with experiments in glassy materials. Prominent dynamical natures of spin
glasses, {\it i.e.}, {\em memory} effect and {\em reinitialization}, are
reproduced well in the GREM. The existence of many layers causing continuous
transitions is very important for the two natures. Results of experiments in
other glassy materials such as polymers, supercooled glycerol and orientational
glasses, which are contrast to those in spin glasses, are interpreted well by
the Single-layer Random Energy Model.Comment: 8 pages, 9 figures, to be submitted to J. Phys. Soc. Jp
Conceptual design of a wide-field near UV transient survey in a 6U CubeSat
A conceptual design of a wide-field near UV transient survey in a 6U CubeSat is presented. Ultraviolet is one of the frontier in the transient astronomy. To open up the discovery region, we are developing a 6U CubeSat for transient exploration. The possible targets will be supernova shock-breakouts, tidal disruption events, and the blue emission from NS-NS mergers in very early phase. If we only focused on nearby/bright sources, the required detection limit is around 20 mag (AB). To avoid the background and optical light, we chose a waveband of 230-280 nm. As an imaging detector, we employ a delta-doped back-illuminated CMOS. In addition to delta doping, the multi-layer coating directly deposited on the detector enables both a high in-band UV QE and the ultra-low optical rejection ratio. Taking into account these specifications, even an 8 cm telescope can achieve the detection limit of 20 magAB. The expected FoV is larger than 60 deg^2
Magnetic Coupling between 3He and 19F at Low Temperatures
Measurements of relaxation phenomena between liquid 3He and 19F nuclei in small fluorocarbon particles are reported. Magnetic cross relaxation between the 19F in the substrate and the liquid is observed in measurements between 1 K and 0.6 mK. The spin temperatures remain strongly coupled in magnetic fields up to 125 mT. Moreover it is observed that there is a decrease in magnetic relaxation time at the onset of superfluidity in the liquid 3He and that the thermal relaxation times are remarkably short at all temperatures
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