513 research outputs found
Square vortex lattice at anomalously low magnetic fields in electron-doped NdCeCuO
We report here on the first direct observations of the vortex lattice in the
bulk of electron-doped NdCeCuO single crystals. Using
small angle neutron scattering, we have observed a square vortex lattice with
the nearest-neighbors oriented at 45 from the Cu-O bond direction,
which is consistent with theories based on the d-wave superconducting gap.
However, the square symmetry persists down to unusually low magnetic fields.
Moreover, the diffracted intensity from the vortex lattice is found to decrease
rapidly with increasing magnetic field.Comment: 4 pages, 4 Figures, accepted for publication in Phys. Rev. Let
The Absence of Vortex Lattice Melting in a Conventional Superconductor
The state of the vortex lattice extremely close to the superconducting to
normal transition in an applied magnetic field is investigated in high purity
niobium. We observe that thermal fluctuations of the order parameter broaden
the superconducting to normal transition into a crossover but no sign of a
first order vortex lattice melting transition is detected in measurements of
the heat capacity or the small angle neutron scattering (SANS) intensity.
Direct observation of the vortices via SANS always finds a well ordered vortex
lattice. The fluctuation broadening is considered in terms of the Lowest Landau
Level theory of critical fluctuations and scaling is found to occur over a
large H_{c2}(T) range
Unconventional magnetic phase separation in -CoVO
We have explored the magnetism in the non-geometrically frustrated spin-chain
system -CoVO which possesses a complex magnetic exchange
network. Our neutron diffraction patterns at low temperatures (
= 6.6 K) are best described by a model in which two magnetic
phases coexist in a volume ratio 65(1) : 35(1), with each phase consisting of a
single spin modulation. This model fits previous studies and our observations
better than the model proposed by Lenertz in J. Phys. Chem. C 118,
13981 (2014), which consisted of one phase with two spin modulations. By
decreasing the temperature from , the minority phase of our
model undergoes an incommensurate-commensurate lock-in transition at =
5.6 K. Based on these results, we propose that phase separation is an
alternative approach for degeneracy-lifting in frustrated magnets
Exploring the fragile antiferromagnetic superconducting phase in CeCoIn5
CeCoIn5 is a heavy fermion Type-II superconductor which exhibits clear
indications of Pauli-limited superconductivity. A variety of measurements give
evidence for a transition at high magnetic fields inside the superconducting
state, when the field is applied either parallel to or perpendicular to the c
axis. When the field is perpendicular to the c axis, antiferromagnetic order is
observed on the high-field side of the transition, with a magnetic wavevector
of (q q 0.5), where q = 0.44 reciprocal lattice units. We show that this order
remains as the magnetic field is rotated out of the basal plane, but the
associated moment eventually disappears above 17 degrees, indicating that the
anomalies seen with the field parallel to the c axis are not related to this
magnetic order. We discuss the implications of this finding.Comment: Accepted Physical Review Letters, September 2010. 4 pages, 4 figure
Convergence of SPH simulations of self-gravitating accretion discs: Sensitivity to the implementation of radiative cooling
Recent simulations of self-gravitating accretion discs, carried out using a
three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate,
have been interpreted as implying that three-dimensional global discs fragment
much more easily than would be expected from a two-dimensional local model.
Subsequently, global and local two-dimensional models have been shown to
display similar fragmentation properties, leaving it unclear whether the
three-dimensional results reflect a physical effect or a numerical problem
associated with the treatment of cooling or artificial viscosity in SPH. Here,
we study how fragmentation of self-gravitating disc flows in SPH depends upon
the implementation of cooling. We run disc simulations that compare a simple
cooling scheme, in which each particle loses energy based upon its internal
energy per unit mass, with a method in which the cooling is derived from a
smoothed internal energy density field. For the simple per particle cooling
scheme, we find a significant increase in the minimum cooling time scale for
fragmentation with increasing resolution, matching previous results. Switching
to smoothed cooling, however, results in lower critical cooling time scales,
and tentative evidence for convergence at the highest spatial resolution
tested. We conclude that precision studies of fragmentation using SPH require
careful consideration of how cooling (and, probably, artificial viscosity) is
implemented, and that the apparent non-convergence of the fragmentation
boundary seen in prior simulations is likely a numerical effect. In real discs,
where cooling is physically smoothed by radiative transfer effects, the
fragmentation boundary is probably displaced from the two-dimensional value by
a factor that is only of the order of unity.Comment: 9 pages, 11 figures, MNRAS in pres
Glass phases of flux lattices in layered superconductors
We study a flux lattice which is parallel to superconducting layers, allowing
for dislocations and for disorder of both short wavelength and long wavelength.
We find that the long wavelength disorder has a significant effect on the phase
diagram -- it produces a first order transition within the Bragg glass phase
and leads to melting at strong disorder. This then allows a Friedel scenario of
2D superconductivity.Comment: 5 pages, 1 eps figure, Revte
The Hunt for Exomoons with Kepler (HEK): III. The First Search for an Exomoon around a Habitable-Zone Planet
Kepler-22b is the first transiting planet to have been detected in the
habitable-zone of its host star. At 2.4 Earth radii, Kepler-22b is too large to
be considered an Earth-analog, but should the planet host a moon large enough
to maintain an atmosphere, then the Kepler-22 system may yet possess a telluric
world. Aside from being within the habitable-zone, the target is attractive due
to the availability of previously measured precise radial velocities and low
intrinsic photometric noise, which has also enabled asteroseismology studies of
the star. For these reasons, Kepler-22b was selected as a target-of-opportunity
by the 'Hunt for Exomoons with Kepler' (HEK) project. In this work, we conduct
a photodynamical search for an exomoon around Kepler-22b leveraging the
transits, radial velocities and asteroseismology plus several new tools
developed by the HEK project to improve exomoon searches. We find no evidence
for an exomoon around the planet and exclude moons of mass >0.5 Earth masses to
95% confidence. By signal injection and blind retrieval, we demonstrate that an
Earth-like moon is easily detected for this planet even when the
time-correlated noise of the data set is taken into account. We provide updated
parameters for the planet Kepler-22b including a revised mass of <53 Earth
masses to 95% confidence and an eccentricity of 0.13(-0.13)(+0.36) by
exploiting Single-body Asterodensity Profiling (SAP). Finally, we show that
Kepler-22b has a >95% probability of being within the empirical habitable-zone
but a <5% probability of being within the conservative habitable-zone.Comment: 19 pages, 11 figures, 7 tables. Accepted in ApJ. Planet-moon transit
animations available at https://www.cfa.harvard.edu/~dkipping/kepler22.htm
Polarized Neutron Laue Diffraction on a Crystal Containing Dynamically Polarized Proton Spins
We report on a polarized-neutron Laue diffraction experiment on a single
crystal of neodynium doped lanthanum magnesium nitrate hydrate containing
polarized proton spins. By using dynamic nuclear polarization to polarize the
proton spins, we demonstrate that the intensities of the Bragg peaks can be
enhanced or diminished significantly, whilst the incoherent background, due to
proton spin disorder, is reduced. It follows that the method offers unique
possibilities to tune continuously the contrast of the Bragg reflections and
thereby represents a new tool for increasing substantially the signal-to-noise
ratio in neutron diffraction patterns of hydrogenous matter.Comment: 5 pages, 3 figure
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