4,330 research outputs found
Spin dynamics of Mn12-acetate in the thermally-activated tunneling regime: ac-susceptibility and magnetization relaxation
In this work, we study the spin dynamics of Mn12-acetate molecules in the
regime of thermally assisted tunneling. In particular, we describe the system
in the presence of a strong transverse magnetic field. Similar to recent
experiments, the relaxation time/rate is found to display a series of
resonances; their Lorentzian shape is found to stem from the tunneling. The
dynamic susceptibility is calculated starting from the microscopic
Hamiltonian and the resonant structure manifests itself also in .
Similar to recent results reported on another molecular magnet, Fe8, we find
oscillations of the relaxation rate as a function of the transverse magnetic
field when the field is directed along a hard axis of the molecules. This
phenomenon is attributed to the interference of the geometrical or Berry phase.
We propose susceptibility experiments to be carried out for strong transverse
magnetic fields to study of these oscillations and for a better resolution of
the sharp satellite peaks in the relaxation rates.Comment: 22 pages, 23 figures; submitted to Phys. Rev. B; citations/references
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Evidence of kaon nuclear and Coulomb potential effects on soft K+ production from nuclei
The ratio of forward K+ production on copper, silver and gold targets to that
on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as
a function of the kaon momentum p_K using the ANKE spectrometer at
COSY-Juelich. The strong suppression in the ratios observed for p_K<200-250
MeV/c can be ascribed to a combination of Coulomb and nuclear repulsion in the
K+A system. This opens a new way to investigate the interaction of K+-mesons in
the nuclear medium. Our data are consistent with a K+A nuclear potential of
V_K~20 MeV at low kaon momenta and normal nuclear density. Given the
sensitivity of the data to the kaon potential, the current experimental
precision might allow one to determine V_K to better than 3 MeV.Comment: 9 pages, 3 figures; changed conten
Nonlinear Dynamics of the Perceived Pitch of Complex Sounds
We apply results from nonlinear dynamics to an old problem in acoustical
physics: the mechanism of the perception of the pitch of sounds, especially the
sounds known as complex tones that are important for music and speech
intelligibility
Atomic correlations in itinerant ferromagnets: quasi-particle bands of nickel
We measure the band structure of nickel along various high-symmetry lines of
the bulk Brillouin zone with angle-resolved photoelectron spectroscopy. The
Gutzwiller theory for a nine-band Hubbard model whose tight-binding parameters
are obtained from non-magnetic density-functional theory resolves most of the
long-standing discrepancies between experiment and theory on nickel. Thereby we
support the view of itinerant ferromagnetism as induced by atomic correlations.Comment: 4 page REVTeX 4.0, one figure, one tabl
Genome sequence of the button mushroom Agaricus bisporus reveals mechanisms governing adaptation to a humic-rich ecological niche
Agaricus bisporus is the model fungus for the adaptation, persistence, and growth in the humic-rich leaf-litter environment. Aside from its ecological role, A. bisporus has been an important component of the human diet for over 200 y and worldwide cultivation of the "button mushroom" forms a multibillion dollar industry. We present two A. bisporus genomes, their gene repertoires and transcript profiles on compost andduringmushroomformation.The genomes encode a full repertoire of polysaccharide-degrading enzymes similar to that of wood-decayers. Comparative transcriptomics of mycelium grown on defined medium, casing-soil, and compost revealed genes encoding enzymes involved in xylan, cellulose, pectin, and protein degradation aremore highly expressed in compost. The striking expansion of heme-thiolate peroxidases and β-etherases is distinctive from Agaricomycotina wood-decayers and suggests a broad attack on decaying lignin and related metabolites found in humic acid-rich environment. Similarly, up-regulation of these genes together with a lignolytic manganese peroxidase, multiple copper radical oxidases, and cytochrome P450s is consistent with challenges posed by complex humic-rich substrates. The gene repertoire and expression of hydrolytic enzymes in A. bisporus is substantially different from the taxonomically related ectomycorrhizal symbiont Laccaria bicolor. A common promoter motif was also identified in genes very highly expressed in humic-rich substrates. These observations reveal genetic and enzymatic mechanisms governing adaptation to the humic-rich ecological niche formed during plant degradation, further defining the critical role such fungi contribute to soil structure and carbon sequestration in terrestrial ecosystems. Genome sequence will expedite mushroom breeding for improved agronomic characteristics
Switchable Coupling of Vibrations to Two-Electron Carbon-Nanotube Quantum Dot States
We report transport measurements on a quantum dot in a partly suspended
carbon nanotube. Electrostatic tuning allows us to modify and even switch 'on'
and 'off' the coupling to the quantized stretching vibration across several
charge states. The magnetic-field dependence indicates that only the
two-electron spin-triplet excited state couples to the mechanical motion,
indicating mechanical coupling to both the valley degree of freedom and the
exchange interaction, in contrast to standard models
New determination of the mass of the eta meson at COSY-ANKE
A value for the mass of the eta meson has been determined at the COSY-ANKE
facility through the measurement of a set of deuteron laboratory beam momenta
and associated 3He center-of-mass momenta in the d+p -> 3He+X reaction. The eta
was then identified by the missing-mass peak and the production threshold
determined. The individual beam momenta were fixed with a relative precision of
3 x 10^-5 for values around 3 GeV/c by using a polarized deuteron beam and
inducing an artificial depolarizing spin resonance, which occurs at a
well-defined frequency. The final-state momenta in the two-body d+p -> 3He+eta
reaction were investigated in detail by studying the size of the 3He momentum
ellipse with the forward detection system of the ANKE spectrometer. Final
alignment of the spectrometer for this high precision experiment was achieved
through a comprehensive study of the 3He final-state momenta as a function of
the center-of-mass angles, taking advantage of the full geometrical acceptance.
The value obtained for the mass, m(eta)=(547.873 +- 0.005(stat) +- 0.027(syst))
MeV/c^2, is consistent and competitive with other recent measurements, in which
the meson was detected through its decay products.Comment: 11 pages, 11 figures, 3 tables, published versio
Ordering of dipolar Ising crystals
We study Ising systems of spins with dipolar interactions. We find a simple
approximate relation for the interaction energy between pairs of parallel
lattice columns of spins running along the Ising spin direction. This relation
provides insight into the relation between lattice geometry and the nature of
the ordered state. It can be used to calculate ground state energies. We have
also obtained ground state energies and ordering temperatures T_0 from Monte
Carlo simulations. Simple empirical relations, that give T_0 for simple and
body centered tetragonal lattices in terms of lattice parameters are also
established. Finally, the nature of the ordered state and T_0 are determined
for Fe_8 clusters, which crystallize on a triclinic lattice.Comment: 13 pages, 4 eps figures, to be published in PRB. For related work,
see http://pipe.unizar.es/~jf
Measurement of the analysing power in proton-proton elastic scattering at small angles
The proton analysing power in elastic scattering has been measured
at small angles at COSY-ANKE at 796 MeV and five other beam energies between
1.6 and 2.4 GeV using a polarised proton beam. The asymmetries obtained by
detecting the fast proton in the ANKE forward detector or the slow recoil
proton in a silicon tracking telescope are completely consistent. Although the
analysing power results agree well with the many published data at 796 MeV, and
also with the most recent partial wave solution at this energy, the ANKE data
at the higher energies lie well above the predictions of this solution at small
angles. An updated phase shift analysis that uses the ANKE results together
with the World data leads to a much better description of these new
measurements.Comment: 5 pages, 3 figure
Determination of Deuteron Beam Polarizations at COSY
The vector and tensor polarizations of a deuteron beam have been measured
using elastic deuteron-carbon scattering at 75.6 MeV and deuteron-proton
scattering at 270 MeV. After acceleration to 1170 MeV inside the COSY ring, the
polarizations of the deuterons were checked by studying a variety of nuclear
reactions using a cluster target at the ANKE magnet spectrometer placed at an
internal target position of the storage ring. All these measurements were
consistent with the absence of depolarization during acceleration and provide a
number of secondary standards that can be used in subsequent experiments at the
facility.Comment: 12 pages, 13 figure
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