257 research outputs found
Electronic structure of the muonium center as a shallow donor in ZnO
The electronic structure and the location of muonium centers (Mu) in
single-crystalline ZnO were determined for the first time. Two species of Mu
centers with extremely small hyperfine parameters have been observed below 40
K. Both Mu centers have an axial-symmetric hyperfine structure along with a
[0001] axis, indicating that they are located at the AB_{O,//} and BC_{//}
sites. It is inferred from their small ionization energy (~6 meV and 50 meV)
and hyperfine parameters (~10^{-4} times the vacuum value) that these centers
behave as shallow donors, strongly suggesting that hydrogen is one of the
primary origins of n type conductivity in as-grown ZnO.Comment: 4 pages, 4 figures, submitted to PR
Scaling of impact fragmentation near the critical point
We investigated two-dimensional brittle fragmentation with a flat impact
experimentally, focusing on the low impact energy region near the
fragmentation-critical point. We found that the universality class of
fragmentation transition disagreed with that of percolation. However, the
weighted mean mass of the fragments could be scaled using the pseudo-control
parameter multiplicity. The data for highly fragmented samples included a
cumulative fragment mass distribution that clearly obeyed a power-law. The
exponent of this power-law was 0.5 and it was independent of sample size. The
fragment mass distributions in this regime seemed to collapse into a unified
scaling function using weighted mean fragment mass scaling. We also examined
the behavior of higher order moments of the fragment mass distributions, and
obtained multi-scaling exponents that agreed with those of the simple biased
cascade model.Comment: 6 pages, 6 figure
Field-Induced Uniform Antiferromagnetic Order Associated with Superconductivity in PrLaCeCuO
Strong correlation between field-induced antiferromagnetic (AF) order and
superconductivity is demonstrated for an electron-doped cuprate superconductor,
PrLaCeCuO (PLCCO). In addition to the specimen with
(which is close to the AF phase boundary, ), we show that
the one with ( K at zero field) also exhibits the
field-induced AF order with a reduced magnitude of the induced moment. The
uniform muon Knight shift at a low magnetic field ( Oe) indicates
that the AF order is not localized within the cores of flux lines, which is in
a marked contrast with theoretical prediction for hole-doped cuprates. The
presence of anomalous non-diagonal hyperfine coupling between muons and Pr ions
is also demonstrated in detail.Comment: 8 pages, 5 figures, to be published in J. Phys. Soc. Jp
Nonlocal Effects and Shrinkage of the Vortex Core Radius in YNi2B2C Probed by muSR
The magnetic field distribution in the vortex state of YNi2B2C has been
probed by muon spin rotation (muSR). The analysis based on the London model
with nonlocal corrections shows that the vortex lattice has changed from
hexagonal to square with increasing magnetic field H. At low fields the vortex
core radius, rho_v(H), decreases with increasing H much steeper than what is
expected from the sqrt(H) behavior of the Sommerfeld constant gamma(H),
strongly suggesting that the anomaly in gamma(H) primarily arises from the
quasiparticle excitations outside the vortex cores.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
A Giant Crater on 90 Antiope?
Mutual event observations between the two components of 90 Antiope were
carried out in 2007-2008. The pole position was refined to lambda0 =
199.5+/-0.5 eg and beta0 = 39.8+/-5 deg in J2000 ecliptic coordinates, leaving
intact the physical solution for the components, assimilated to two perfect
Roche ellipsoids, and derived after the 2005 mutual event season (Descamps et
al., 2007). Furthermore, a large-scale geological depression, located on one of
the components, was introduced to better match the observed lightcurves. This
vast geological feature of about 68 km in diameter, which could be postulated
as a bowl-shaped impact crater, is indeed responsible of the photometric
asymmetries seen on the "shoulders" of the lightcurves. The bulk density was
then recomputed to 1.28+/-0.04 gcm-3 to take into account this large-scale
non-convexity. This giant crater could be the aftermath of a tremendous
collision of a 100-km sized proto-Antiope with another Themis family member.
This statement is supported by the fact that Antiope is sufficiently porous
(~50%) to survive such an impact without being wholly destroyed. This violent
shock would have then imparted enough angular momentum for fissioning of
proto-Antiope into two equisized bodies. We calculated that the impactor must
have a diameter greater than ~17 km, for an impact velocity ranging between 1
and 4 km/s. With such a projectile, this event has a substantial 50%
probability to have occurred over the age of the Themis family.Comment: 30 pages, 3 Tables, 8 Figures. Accepted for publication in Icaru
Field-Induced Quasiparticle Excitation in Ca(AlSi): Evidence for unconventional Superconductivity
The temperature () and magnetic field () dependence of the magnetic
penetration depth, , in Ca(AlSi) exhibits
significant deviation from that expected for conventional BCS superconductors.
In particular, it is inferred from a field dependence of () at 2.0 K that the quasiparticle excitation is strongly enhanced by the
Doppler shift. This suggests that the superconducting order parameter in
Ca(AlSi) is characterized by a small energy scale
K originating either from anisotropy or multi-gap
structure.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Muonium as a shallow center in GaN
A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter
was observed for the first time in single-crystalline GaN below 25 K. It has a
highly anisotropic hyperfine structure with axial symmetry along the [0001]
direction, suggesting that it is located either at a nitrogen-antibonding or a
bond-centered site oriented parallel to the c-axis. Its small ionization energy
(=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value)
indicate that muonium in one of its possible sites produces a shallow state,
raising the possibility that the analogous hydrogen center could be a source of
n-type conductivity in as-grown GaN.Comment: 4 figures, to be published in Phys. Rev. Letter
Transition from damage to fragmentation in collision of solids
We investigate fracture and fragmentation of solids due to impact at low
energies using a two-dimensional dynamical model of granular solids. Simulating
collisions of two solid discs we show that, depending on the initial energy,
the outcome of a collision process can be classified into two states: a damaged
and a fragmented state with a sharp transition in between. We give numerical
evidence that the transition point between the two states behaves as a critical
point, and we discuss the possible mechanism of the transition.Comment: Revtex, 12 figures included. accepted by Phys. Rev.
New model for surface fracture induced by dynamical stress
We introduce a model where an isotropic, dynamically-imposed stress induces
fracture in a thin film. Using molecular dynamics simulations, we study how the
integrated fragment distribution function depends on the rate of change and
magnitude of the imposed stress, as well as on temperature. A mean-field
argument shows that the system becomes unstable for a critical value of the
stress. We find a striking invariance of the distribution of fragments for
fixed ratio of temperature and rate of change of the stress; the interval over
which this invariance holds is determined by the force fluctuations at the
critical value of the stress.Comment: Revtex, 4 pages, 4 figures available upon reques
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