4,168 research outputs found
Amplitude-mode dynamics of polariton condensates
We study the stability of collective amplitude excitations in non-equilibrium
polariton condensates. These excitations correspond to renormalized upper
polaritons and to the collective amplitude modes of atomic gases and
superconductors. They would be present following a quantum quench or could be
created directly by resonant excitation. We show that uniform amplitude
excitations are unstable to the production of excitations at finite
wavevectors, leading to the formation of density-modulated phases. The physical
processes causing the instabilities can be understood by analogy to optical
parametric oscillators and the atomic Bose supernova.Comment: 4 pages, 2 figure
Theoretical Analysis of Electronic and Magnetic Properties of NaVO: Crucial Role of the Orbital Degrees of Freedom
Using realistic low-energy model with parameters derived from the
first-principles electronic structure calculation, we address the origin of the
quasi-one-dimensional behavior in orthorhombic NaVO, consisting of the
double chains of edge-sharing VO octahedra. We argue that the geometrical
aspect alone does not explain the experimentally observed anisotropy of
electronic and magnetic properties of NaVO. Instead, we attribute the
unique behavior of NaVO to one particular type of the orbital ordering,
which respects the orthorhombic symmetry. This orbital ordering acts to
divide all states into two types: the `localized' ones, which are
antisymmetric with respect to the mirror reflection , and
the symmetric `delocalized' ones. Thus, NaVO can be classified as the
double exchange system. The directional orientation of symmetric orbitals,
which form the metallic band, appears to be sufficient to explain both
quasi-one-dimensional character of interatomic magnetic interactions and the
anisotropy of electrical resistivity.Comment: 16 pages, 4 figure
Vibrations and diverging length scales near the unjamming transition
We numerically study the vibrations of jammed packings of particles
interacting with finite-range, repulsive potentials at zero temperature. As the
packing fraction is lowered towards the onset of unjamming at
, the density of vibrational states approaches a non-zero value in
the limit of zero frequency. For , there is a crossover
frequency, below which the density of states drops towards zero.
This crossover frequency obeys power-law scaling with .
Characteristic length scales, determined from the dominant wavevector
contributing to the eigenmode at , diverge as power-laws at the
unjamming transition.Comment: Submitted to PRL, 4 pages + 7 .eps figure
Resistivity scaling and electron relaxation times in metallic nanowires
We study the resistivity scaling in nanometer-sized metallic wires due to
surface roughness and grain-boundaries, currently the main cause of electron
scattering in nanoscaled interconnects. The resistivity has been obtained with
the Boltzmann transport equation, adopting the relaxation time approximation
(RTA) of the distribution function and the effective mass approximation for the
conducting electrons. The relaxation times are calculated exactly, using
Fermi's golden rule, resulting in a correct relaxation time for every sub-band
state contributing to the transport. In general, the relaxation time strongly
depends on the sub-band state, something that remained unclear with the methods
of previous work. The resistivity scaling is obtained for different roughness
and grain-boundary properties, showing large differences in scaling behavior
and relaxation times. Our model clearly indicates that the resistivity is
dominated by grain-boundary scattering, easily surpassing the surface roughness
contribution by a factor of 10.Comment: 19 pages, 5 figure
Can analyses of electronic patient records be independently and externally validated? The effect of statins on the mortality of patients with ischaemic heart disease: a cohort study with nested case-control analysis
Objective To conduct a fully independent and external validation of a research study based on one electronic health record database, using a different electronic database sampling the same population.
Design Using the Clinical Practice Research Datalink (CPRD), we replicated a published investigation into the effects of statins in patients with ischaemic heart disease (IHD) by a different research team using QResearch. We replicated the original methods and analysed all-cause mortality using: (1) a cohort analysis and (2) a case-control analysis nested within the full cohort.
Setting Electronic health record databases containing longitudinal patient consultation data from large numbers of general practices distributed throughout the UK.
Participants CPRD data for 34 925 patients with IHD from 224 general practices, compared to previously published results from QResearch for 13 029 patients from 89 general practices. The study period was from January 1996 to December 2003.
Results We successfully replicated the methods of the original study very closely. In a cohort analysis, risk of death was lower by 55% for patients on statins, compared with 53% for QResearch (adjusted HR 0.45, 95% CI 0.40 to 0.50; vs 0.47, 95% CI 0.41 to 0.53). In case-control analyses, patients on statins had a 31% lower odds of death, compared with 39% for QResearch (adjusted OR 0.69, 95% CI 0.63 to 0.75; vs OR 0.61, 95% CI 0.52 to 0.72). Results were also close for individual statins.
Conclusions Database differences in population characteristics and in data definitions, recording, quality and completeness had a minimal impact on key statistical outputs. The results uphold the validity of research using CPRD and QResearch by providing independent evidence that both datasets produce very similar estimates of treatment effect, leading to the same clinical and policy decisions. Together with other non-independent replication studies, there is a nascent body of evidence for wider validity
Lattice Green's function for crystals containing a planar interface
Flexible boundary condition methods couple an isolated defect to a
harmonically responding medium through the bulk lattice Green's function; in
the case of an interface, interfacial lattice Green's functions. We present a
method to compute the lattice Green's function for a planar interface with
arbitrary atomic interactions suited for the study of line defect/interface
interactions. The interface is coupled to two different semi-infinite bulk
regions, and the Green's function for interface-interface, bulk-interface and
bulk-bulk interactions are computed individually. The elastic bicrystal Green's
function and the bulk lattice Green's function give the interaction between
bulk regions. We make use of partial Fourier transforms to treat in-plane
periodicity. Direct inversion of the force constant matrix in the partial
Fourier space provides the interface terms. The general method makes no
assumptions about the atomic interactions or crystal orientations. We simulate
a screw dislocation interacting with a twin boundary in Ti using
flexible boundary conditions and compare with traditional fixed boundary
conditions results. Flexible boundary conditions give the correct core
structure with significantly less atoms required to relax by energy
minimization. This highlights the applicability of flexible boundary conditions
methods to modeling defect/interface interactions by \textit{ab initio}
methods
Screening model for nanowire surface-charge sensors in liquid
The conductance change of nanowire field-effect transistors is considered a
highly sensitive probe for surface charge. However, Debye screening of relevant
physiological liquid environments challenge device performance due to competing
screening from the ionic liquid and nanowire charge carriers. We discuss this
effect within Thomas-Fermi and Debye-Huckel theory and derive analytical
results for cylindrical wires which can be used to estimate the sensitivity of
nanowire surface-charge sensors. We study the interplay between the nanowire
radius, the Thomas-Fermi and Debye screening lengths, and the length of the
functionalization molecules. The analytical results are compared to
finite-element calculations on a realistic geometry.Comment: 4 pages including 2 figures. Accepted for AP
Quantum Manifestation of Elastic Constants in Nanostructures
Generally, there are two distinct effects in modifying the properties of
low-dimensional nanostructures: surface effect (SS) due to increased
surface-volume ratio and quantum size effect (QSE) due to quantum confinement
in reduced dimension. The SS has been widely shown to affect the elastic
constants and mechanical properties of nanostructures. Here, using Pb nanofilm
and graphene nanoribbon as model systems, we demonstrate the QSE on the elastic
constants of nanostructures by first-principles calculations. We show that
generally QSE is dominant in affecting the elastic constants of metallic
nanostructures while SS is more pronounced in semiconductor and insulator
nanostructures. Our findings have broad implications in quantum aspects of
nanomechanics
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