2,770 research outputs found
Dynamical conductivity at the dirty superconductor-metal quantum phase transition
We study the transport properties of ultrathin disordered nanowires in the
neighborhood of the superconductor-metal quantum phase transition. To this end
we combine numerical calculations with analytical strong-disorder
renormalization group results. The quantum critical conductivity at zero
temperature diverges logarithmically as a function of frequency. In the
metallic phase, it obeys activated scaling associated with an
infinite-randomness quantum critical point. We extend the scaling theory to
higher dimensions and discuss implications for experiments.Comment: 4 pages, 2 figures; (v2) minor typos corrected, published versio
From stripe to checkerboard order on the square lattice in the presence of quenched disorder
We discuss the effects of quenched disorder on a model of charge density wave
(CDW) ordering on the square lattice. Our model may be applicable to the
cuprate superconductors, where a random electrostatic potential exists in the
CuO2 planes as a result of the presence of charged dopants. We argue that the
presence of a random potential can affect the unidirectionality of the CDW
order, characterized by an Ising order parameter. Coupling to a unidirectional
CDW, the random potential can lead to the formation of domains with 90 degree
relative orientation, thus tending to restore the rotational symmetry of the
underlying lattice. We find that the correlation length of the Ising order can
be significantly larger than the CDW correlation length. For a checkerboard CDW
on the other hand, disorder generates spatial anisotropies on short length
scales and thus some degree of unidirectionality. We quantify these disorder
effects and suggest new techniques for analyzing the local density of states
(LDOS) data measured in scanning tunneling microscopy experiments.Comment: 10 pages, 11 figures; added referenc
Simulating a dual beam combiner at SUSI for narrow-angle astrometry
The Sydney University Stellar Interferometer (SUSI) has two beam combiners,
i.e. the Precision Astronomical Visible Observations (PAVO) and the
Microarcsecond University of Sydney Companion Astrometry (MUSCA). The primary
beam combiner, PAVO, can be operated independently and is typically used to
measure properties of binary stars of less than 50 milliarc- sec (mas)
separation and the angular diameters of single stars. On the other hand, MUSCA
was recently installed and must be used in tandem with the for- mer. It is
dedicated for microarcsecond precision narrow-angle astrometry of close binary
stars. The performance evaluation and development of the data reduction
pipeline for the new setup was assisted by an in-house computer simulation tool
developed for this and related purposes. This paper describes the framework of
the simulation tool, simulations carried out to evaluate the performance of
each beam combiner and the expected astrometric precision of the dual beam
combiner setup, both at SUSI and possible future sites.Comment: 28 pages, 23 figures, accepted for publication in Experimental
Astronomy. The final publication is available at http://link.springer.co
Cenozoic stress field in the southwestern Antarctic Peninsula from brittle mesostructures in Wright Peninsula, Adelaide Island
Palaeostresses inferred from brittle mesostructures in the southern Wright Peninsula show a stress field characterized by compressional, strike-slip and extensional regime stress states. The compressional stress (σ 1 ) shows a main NW-SE direction and the extensional stress (σ 3 ) shows a relative scattering with two main modes: NE-SW to E-W and NW-SE. The maximum horizontal stress (σ y ) has a bimodal distribution with NW-SE and NE-SW direction. The compressional orientation is related to subduction of the former Phoenix Plate under the Antarctic Plate from the Early Jurassic to the Early Miocene. Extensional structures within a broad-scale compressional stress field can be related to both the decrease in relative stress magnitudes from active margins to intraplate regions and stretching processes occurring in eastern Adelaide Island, which develop a fore-arc or intra-arc basin from the Early Miocene. Stress states with NW-SE-trending σ 1 are compatible with the dominant pattern established for the western Antarctic Peninsula. NW-SE orientations of σ 3 suggest the occurrence of tectonic forces coming from fore-arc extension along the western Antarctic Peninsula.This work was supported by the projects CGL2005-03256/ANT and CGL2007-28812E/ANT of the Spanish Ministry of Education and Scienc
Evidence for gapped spin-wave excitations in the frustrated Gd2Sn2O7 pyrochlore antiferromagnet from low-temperature specific heat measurements
We have measured the low-temperature specific heat of the geometrically
frustrated pyrochlore Heisenberg antiferromagnet Gd2Sn2O7 in zero magnetic
field. The specific heat is found to drop exponentially below approximately 350
mK. This provides evidence for a gapped spin-wave spectrum due to an anisotropy
resulting from single ion effects and long-range dipolar interactions. The data
are well fitted by linear spin-wave theory, ruling out unconventional low
energy magnetic excitations in this system, and allowing a determination of the
pertinent exchange interactions in this material
Low temperature specific heat and possible gap to magnetic excitations in the Heisenberg pyrochlore antiferromagnet Gd2Sn207
The Gd2Sn2O7 pyrochlore Heisenberg antiferromagnet displays a phase
transition to a four sublattice Neel ordered state at a temperature near 1 K.
Despite the seemingly conventional nature of the ordered state, the specific
heat has been found to be described in the temperature range 350-800 mK by an
anomalous T-squared power law. A similar temperature dependence has also been
reported for Gd2Ti2O7, another pyrochlore Heisenberg material. Such anomalous
T-squared behavior in Cv has been argued to be correlated to an unusual
energy-dependence of the density of states which also seemingly manifests
itself in low-temperature spin fluctuations found in muon spin relaxation
experiments. In this paper, we report calculations of Cv that consider spin
wave like excitations out of the Neel order observed in Gd2Sn2O7 and argue that
the parametric T-squared behavior does not reflect the true low-energy
excitations of Gd2Sn2O7. Rather, we find that the low-energy excitations of
this material are antiferromagnetic magnons gapped by single-ion and dipolar
anisotropy effects, and that the lowest temperature of 350 mK considered in
previous specific heat measurements accidentally happens to coincide with a
crossover temperature below which magnons become thermally activated and Cv
takes an exponential form. We argue that further specific heat measurements
that extend down to at least 100 mK are required in order to ascribe an
unconventional description of magnetic excitations out of the ground state of
Gd2Sn2O7 or to invalidate the standard picture of gapped excitations proposed
herein.Comment: 12 pages, 13 figures; shortened introduction and added 1 figur
Second phalanx shortening osteotomy. An innovative technique for long second toe syndrome
AbstractLong second-toe syndrome, although frequent and disabling, has been little described. Current surgical techniques often lead to loss of function. Based on anatomical and biomechanical observations, the present study reports a second phalanx shortening osteotomy technique. The procedure is relatively non-invasive, involving self-stabilizing segment resection osteotomy of the second phalanx. Results for the first 23 feet undergoing the procedure were analyzed retrospectively. Assessment comprised clinical examination, radiography and AOFAS and FAAM scores. Mean follow-up was 19±9.9months. Second phalanx shortening osteotomy proved reliable, respecting the biomechanics of the toe
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