1,483 research outputs found
Inside the brain of an elite athlete: The neural processes that support high achievement in sports
Events like the World Championships in athletics and the Olympic Games raise the public profile of competitive sports. They may also leave us wondering what sets the competitors in these events apart from those of us who simply watch. Here we attempt to link neural and cognitive processes that have been found to be important for elite performance with computational and physiological theories inspired by much simpler laboratory tasks. In this way we hope to inspire neuroscientists to consider how their basic research might help to explain sporting skill at the highest levels of performance
Quantum Monte Carlo method using phase-free random walks with Slater determinants
We develop a quantum Monte Carlo method for many fermions that allows the use
of any one-particle basis. It projects out the ground state by random walks in
the space of Slater determinants. An approximate approach is formulated to
control the phase problem with a trial wave function . Using
plane-wave basis and non-local pseudopotentials, we apply the method to Si
atom, dimer, and 2, 16, 54 atom (216 electrons) bulk supercells. Single Slater
determinant wave functions from density functional theory calculations were
used as with no additional optimization. The calculated binding
energy of Si2 and cohesive energy of bulk Si are in excellent agreement with
experiments and are comparable to the best existing theoretical results.Comment: 5 pages, Latex, with 1 fi
Microscopic calculation of the phonon dynamics of SrRuO compared with LaCuO
The phonon dynamics of the low-temperature superconductor SrRuO
is calculated quantitatively in linear response theory and compared with the
structurally isomorphic high-temperature superconductor LaCuO. Our
calculation corrects for a typical deficit of LDA-based calculations which
always predict a too large electronic -dispersion insufficient to
describe the c-axis response in the real materials. With a more realistic
computation of the electronic band structure the frequency and wavevector
dependent irreducible polarization part of the density response function is
determined and used for adiabatic and nonadiabatic phonon calculations. Our
analysis for SrRuO reveals important differences from the lattice
dynamics of - and -doped cuprates. Consistent with experimental evidence
from inelastic neutron scattering the anomalous doping related softening of the
strongly coupling high-frequency oxygen bond-stretching modes (OBSM) which is
generic for the cuprate superconductors is largely suppressed or completely
absent, respectively, depending on the actual value of the on-site Coulomb
repulsion of the Ru4d orbitals. Also the presence of a characteristic
-mode with a very steep dispersion coupling strongly with the
electrons is missing in SrRuO. Moreover, we evaluate the
possibility of a phonon-plasmon scenario for SrRuO which has been
shown recently to be realistic for LaCuO. In contrast to
LaCuO in SrRuO the very low lying plasmons are
overdamped along the c-axis.Comment: 30 pages, 16 figures, 4 tables, 33 reference
High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding
A first principles embedded cluster approach is used to calculate O chemical
shielding tensors, sigma, in prototypical transition metal oxide ABO_3
perovskite crystals. Our principal findings are 1) a large anisotropy of sigma
between deshielded sigma_x ~ sigma_y and shielded sigma_z components (z along
the Ti-O bond); 2) a nearly linear variation, across all the systems studied,
of the isotropic sigma_iso and uniaxial sigma_ax components, as a function of
the B-O-B bond asymmetry. We show that the anisotropy and linear variation
arise from large paramagnetic contributions to sigma_x and sigma_y due to
virtual transitions between O(2p) and unoccupied B(nd) states. The calculated
isotropic delta_iso and uniaxial delta_ax chemical shifts are in good agreement
with recent BaTiO_3 and SrTiO_3 single crystal 17O NMR measurements. In PbTiO_3
and PbZrO_3, calculated delta_iso are also in good agreement with NMR powder
spectrum measurements. In PbZrO_3, delta_iso calculations of the five
chemically distinct sites indicate a correction of the experimental
assignments. The strong dependence of sigma on covalent O(2p)-B(nd)
interactions seen in our calculations indicates that 17O NMR spectroscopy,
coupled with first principles calculations, can be an especially useful tool to
study the local structure in complex perovskite alloys.Comment: 12 pages, 3 figures, and 3 Table
Kinetic Monte Carlo Simulations of Crystal Growth in Ferroelectric Alloys
The growth rates and chemical ordering of ferroelectric alloys are studied
with kinetic Monte Carlo (KMC) simulations using an electrostatic model with
long-range Coulomb interactions, as a function of temperature, chemical
composition, and substrate orientation. Crystal growth is characterized by
thermodynamic processes involving adsorption and evaporation, with
solid-on-solid restrictions and excluding diffusion. A KMC algorithm is
formulated to simulate this model efficiently in the presence of long-range
interactions. Simulations were carried out on Ba(Mg_{1/3}Nb_{2/3})O_3 (BMN)
type materials. Compared to the simple rocksalt ordered structures, ordered BMN
grows only at very low temperatures and only under finely tuned conditions. For
materials with tetravalent compositions, such as (1-x)Ba(Mg_{1/3}Nb_{2/3})O_3 +
xBaZrO_3 (BMN-BZ), the model does not incorporate tetravalent ions at
low-temperature, exhibiting a phase-separated ground state instead. At higher
temperatures, tetravalent ions can be incorporated, but the resulting crystals
show no chemical ordering in the absence of diffusive mechanisms.Comment: 13 pages, 16 postscript figures, submitted to Physics Review B
Journa
Adverse Effects of Systemic Immunosuppression in Keratolimbal Allograft
Purpose. Keratolimbal allograft (KLAL) is a treatment for limbal stem cell deficiency. One disadvantage is systemic immunosuppression to avoid rejection. Our purpose was to examine the adverse effects of systemic immunosuppression in KLAL. Methods. A retrospective case review of 16 patients with KLAL who received systemic immunosuppression consisting of a corticosteroid, an antimetabolite, and/or a calcineurin inhibitor was performed. Patients were monitored for signs, symptoms, or laboratory evidence of toxicity. Results. Eleven of 16 patients (68%) experienced an adverse effect. The average age of those with adverse effects was 43.5 years and without was 31.4 years. Ten of 11 patients (91%) had resolution during mean followup of 16.4 months. No serious adverse effects occurred. The most common included anemia, hyperglycemia, elevated creatinine, and elevated liver function tests. Prednisone and tacrolimus were responsible for the most adverse effects. Patients with comorbidities were more likely to experience an adverse effect (82% versus 20%, P = 0.036). Conclusions. KLAL requires prolonged systemic immunosuppression. Our data demonstrated that systemic immunosuppression did not result in serious adverse effects in our population and is relatively safe with monitoring for toxicity. In addition, we demonstrated that adverse effects are more likely in older patients with comorbidities
On the Substitution of Identicals in Counterfactual Reasoning
It is widely held that counterfactuals, unlike attitude ascriptions, preserve the referential transparency of their constituents, i.e., that counterfactuals validate the substitution of identicals when their constituents do. The only putative counterexamples in the literature come from counterpossibles, i.e., counterfactuals with impossible antecedents. Advocates of counterpossibilism, i.e., the view that counterpossibles are not all vacuous, argue that counterpossibles can generate referential opacity. But in order to explain why most substitution inferences into counterfactuals seem valid, counterpossibilists also often maintain that counterfactuals with possible antecedents are transparency‐preserving. I argue that if counterpossibles can generate opacity, then so can ordinary counterfactuals with possible antecedents. Utilizing an analogy between counterfactuals and attitude ascriptions, I provide a counterpossibilist‐friendly explanation for the apparent validity of substitution inferences into counterfactuals. I conclude by suggesting that the debate over counterpossibles is closely tied to questions concerning the extent to which counterfactuals are more like attitude ascriptions and epistemic operators than previously recognized
Ground states of a one-dimensional lattice-gas model with an infinite range nonconvex interaction. A numerical study
We consider a lattice-gas model with an infinite range pairwise noncovex
interaction. It might be relevant, for example, for adsorption of alkaline
elements on W(112) and Mo(112). We study a competition between the effective
dipole-dipole and indirect interactions. The resulting ground state phase
diagrams are analysed (numerically) in detail. We have found that for some
model parameters the phase diagrams contain a region dominated by several
phases only with periods up to nine lattice constants. The remaining phase
diagrams reveal a complex structure of usually long periodic phases. We also
discuss a possible role of surace states in phase transitions.Comment: 16 pages, 5 Postscript figures; Physical Review B15 (15 August 1996),
in pres
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
The equilibrium structure, energy bands, phonon dispersions, and s- and
d-channel electron-phonon interactions (EPIs) are calculated for the
infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA
and the linear-response full-potential LMTO method were used. In the
equilibrium structure, oxygen is found to buckle slightly out of the plane and,
as a result, the characters of the energy bands near EF are found to be similar
to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in
accord with previous LDA calculations for YBa2Cu3O7. This supports the common
belief that the EPI mechanism alone is insufficient to explain HTSC.
Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This
is surprising and indicates that the EPI could enhance some other d-wave
pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute
significantly to the EPI, although these contributions are proportional to the
static buckling and would vanish for flat planes. These numerical results can
be understood from a generic tight-binding model originally derived from the
LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the
buckling-modes and the influence of the spin-fluctuations should be
investigated.Comment: 19 pages, 9 Postscript figures, Late
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