5,632 research outputs found
Muon spin relaxation and rotation study on the solid solution of the two spin-gap systems (CH3)2CHNH3-CuCl3 and (CH3)2CHNH3-CuBr3
Muon-spin-rotation and relaxation studies have been performed on
(CH)CHNHCu(ClBr) with =0.85 and 0.95, which are
solid solutions of the two isomorphic spin-gap systems
(CH)CHNHCuCl and (CH)CHNHCuBr with different
spin gaps. The sample with =0.85 showed a clear muon spin rotation under
zero-field below =11.65K, indicating the existence of a long-range
antiferromagnetic order. A critical exponent of the hyperfine field was
obtained to be =0.33, which agrees with 3D-Ising model. In the other
sample with =0.95, an anomalous enhancement of the muon spin relaxation was
observed at very low temperatures indicating a critical slowing down due to a
magnetic instability of the ground state
Electronic structure of CaSrVO: a tale of two energy-scales
We investigate the electronic structure of CaSrVO using
photoemission spectroscopy. Core level spectra establish an electronic phase
separation at the surface, leading to distinctly different surface electronic
structure compared to the bulk. Analysis of the photoemission spectra of this
system allowed us to separate the surface and bulk contributions. These results
help us to understand properties related to two vastly differing energy-scales,
namely the low energy-scale of thermal excitations (~) and the
high-energy scale related to Coulomb and other electronic interactions.Comment: 4 pages and 3 figures. Europhysics Letters (appearing
Dynamics and thermalization of the nuclear spin bath in the single-molecule magnet Mn12-ac: test for the theory of spin tunneling
The description of the tunneling of a macroscopic variable in the presence of
a bath of localized spins is a subject of great fundamental and practical
interest, and is relevant for many solid-state qubit designs. Instead of
focusing on the the "central spin" (as is most often done), here we present a
detailed study of the dynamics of the nuclear spin bath in the Mn12-ac
single-molecule magnet, probed by NMR experiments down to very low temperatures
(T = 20 mK). We find that the longitudinal relaxation rate of the 55Mn nuclei
in Mn12-ac becomes roughly T-independent below T = 0.8 K, and can be strongly
suppressed with a longitudinal magnetic field. This is consistent with the
nuclear relaxation being caused by quantum tunneling of the molecular spin, and
we attribute the tunneling fluctuations to the minority of fast-relaxing
molecules present in the sample. The transverse nuclear relaxation is also
T-independent for T < 0.8 K, and can be explained qualitatively and
quantitatively by the dipolar coupling between like nuclei in neighboring
molecules. We also show that the isotopic substitution of 1H by 2H leads to a
slower nuclear longitudinal relaxation, consistent with the decreased tunneling
probability of the molecular spin. Finally, we demonstrate that, even at the
lowest temperatures, the nuclear spins remain in thermal equilibrium with the
lattice phonons, and we investigate the timescale for their thermal
equilibration. After a review of the theory of macroscopic spin tunneling in
the presence of a spin bath, we argue that most of our experimental results are
consistent with that theory, but the thermalization of the nuclear spins is
not.Comment: 24 pages, 18 figures. Experimental study of the spin bath dynamics in
quantum nanomagnets, plus an extensive review and application of the theor
Mechanism of Lattice-Distortion-Induced Electric-Polarization Flop in the Multiferroic Perovskite Manganites
Magnetoelectric phase diagrams of the perovskite manganites, Eu1-xYxMnO3 and
Gd1-xTbxMnO3, are theoretically studied. We first construct a microscopic
model, and then analyze the model using the Monte-Carlo method. We reproduce
the diagrams, which contain two different multiferroic states, i.e., the
ab-plane spin cycloid with electric polarization P//a and the bc-plane spin
cycloid with P//c. We reveal that their competition originates from a conflict
between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction,
which is controlled by the second-neighbor spin exchanges enhanced by the
GdFeO3-type distortion. This leads to a P flop from a to c with increasing x in
agreement with the experiments.Comment: 5 pages, 5 figures. Recalculated results after correcting errors in
the assignment of DM vectors. The conclusion is not affecte
A Doubly Nudged Elastic Band Method for Finding Transition States
A modification of the nudged elastic band (NEB) method is presented that
enables stable optimisations to be run using both the limited-memory
quasi-Newton (L-BFGS) and slow-response quenched velocity Verlet (SQVV)
minimisers. The performance of this new `doubly nudged' DNEB method is analysed
in conjunction with both minimisers and compared with previous NEB
formulations. We find that the fastest DNEB approach (DNEB/L-BFGS) can be
quicker by up to two orders of magnitude. Applications to permutational
rearrangements of the seven-atom Lennard-Jones cluster (LJ7) and highly
cooperative rearrangements of LJ38 and LJ75 are presented. We also outline an
updated algorithm for constructing complicated multi-step pathways using
successive DNEB runs.Comment: 13 pages, 8 figures, 2 table
Studies of transverse and longitudinal relaxations of Mn in molecular cluster magnet MnAc
The transverse and longitudinal relaxation rates 1/ and 1/ of
Mn in molecular cluster magnet MnAc have been measured al low
temperatures down to 200mK and in the fields upto 9T. Both of 1/ and
1/ exhibit remarkable decreases with decreasing temperature and with
increasing field, with the relative relation . In the
analysis, we adopt a simple model that the thermal fluctuation of the cluster
spin =10 associated with the spin-phonon interactionis, is only due to the
excitation to the first excited state from the ground state with the average
life-times and (). We show that
1/ is interpreted in terms of the strong collision regime as given by
1/, and that 1/ is understood by the high-frequency limit based
on standard perturbation treatment for the step-wise fluctuating field, thus
being proportional to 1/.Comment: 12 pages, 11 fugures, revtex
Search for Magnetic Monopoles Trapped in Matter
There have been many searches for magnetic monopoles in flight, but few for
monopoles in matter. We have searched for magnetic monopoles in meteorites,
schists, ferromanganese nodules, iron ores and other materials. The detector
was a superconducting induction coil connected to a SQUID (Superconducting
Quantum Interference Device) with a room temperature bore 15 cm in diameter. We
tested a total of more than 331 kg of material including 112 kg of meteorites.
We found no monopole and conclude the overall monopole/nucleon ratio in the
samples is with a 90\% confidence level.Comment: 6 pages, rev tex, no figure
RNA and epigenetic silencing: Insight from fission yeast
Post-translational modifications of histones are critical not only for local regulation of gene expression, but also for higher-order structure of the chromosome and genome organization in general. These modifications enable a preset state to be maintained over subsequent generations and thus provide an epigenetic level of regulation. Heterochromatic regions of the genome are epigenetically regulated to maintain a "silent state" and protein coding genes inserted into these regions are subject to the same epigenetic silencing. The fission yeast Schizosaccharomyces pombe has well characterized regions of heterochromatin and has proven to be a powerful model for elucidation of epigenetic silencing mechanisms. Research in S. pombe led to the breakthrough discovery that epigenetic silencing is not solely a chromatin-driven transcriptional repression and that RNA interference of nascent transcripts can guide epigenetic silencing and associated histone modifications. Over the last 10years, an eloquent integration of genetic and biochemical studies have greatly propelled our understanding of major players and effector complexes for regulation of RNAi-mediated epigenetic silencing in S. pombe. Here, we review recent research related to regulation of the epigenetic state in S. pombe heterochromatin, focusing specifically on the mechanisms by which transcription and RNA processing interact with the chromatin modification machinery to maintain the epigenetically silent state
The Effect of - Magnetic Coupling in Multiferroic MnO Crystals
We have established detailed magnetoelectric phase diagrams of
(EuY)TbMnO () and
(Eu,Y)GdMnO (), whose average ionic radii of
-site (: rare earth) cations are equal to that of Tb, in order to
reveal the effect of rare earth 4 magnetic moments on the magnetoelectric
properties. In spite of the same -site ionic radii, the magnetoelectric
properties of the two systems are remarkably different from each other. A small
amount of Tb substitution on sites () totally destroys
ferroelectric polarization along the a axis (), and an increase in Tb
concentration stabilizes the phase. On the other hand, Gd substitution
() extinguishes the phase, and slightly suppresses the
phase. These results demonstrate that the magnetoelectric properties of
MnO strongly depend on the characteristics of the rare earth 4
moments.Comment: 10 pages, 5 figures Submitted to Journal of the Physical Society of
Japa
Local Heavy Quasiparticle in Four-Level Kondo Model
An impurity four-level Kondo model, in which an ion is tunneling among
4-stable points and interacting with surrounding conduction electrons, is
investigated using both perturbative and numerical renormalization group
methods. The results of numerical renormalization group studies show that it is
possible to construct the ground state wavefunction including the excited ion
states if we take into account the interaction between the conduction electrons
and the ion. The resultant effective mass of quasiparticles is moderately
enhanced. This result offers a good explanation for the enhanced and
magnetically robust Sommerfeld coefficient observed in SmOsSb, some
other filled-skutterudites, and clathrate compounds.Comment: 9 pages, 7 figures. Added references and "Note added
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