5,568 research outputs found
The reason why doping causes superconductivity in LaFeAsO
The experimental observation of superconductivity in LaFeAsO appearing on
doping is analyzed with the group-theoretical approach that evidently led in a
foregoing paper (J. Supercond 24:2103, 2011) to an understanding of the cause
of both the antiferromagnetic state and the accompanying structural distortion
in this material. Doping, like the structural distortions, means also a
reduction of the symmetry of the pure perfect crystal. In the present paper we
show that this reduction modifies the correlated motion of the electrons in a
special narrow half-filled band of LaFeAsO in such a way that these electrons
produce a stable superconducting state
The structural distortion in antiferromagnetic LaFeAsO investigated by a group-theoretical approach
As experimentally well established, undoped LaFeAsO is antiferromagnetic
below 137K with the magnetic moments lying on the Fe sites. We determine the
orthorhombic body-centered group Imma (74) as the space group of the
experimentally observed magnetic structure in the undistorted lattice, i.e., in
a lattice possessing no structural distortions in addition to the
magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic
band" with Bloch functions that can be unitarily transformed into optimally
localized Wannier functions adapted to the space group Imma. This finding is
interpreted in the framework of a nonadiabatic extension of the Heisenberg
model of magnetism, the nonadiabatic Heisenberg model. Within this model,
however, the magnetic structure with the space group Imma is not stable but can
be stabilized by a (slight) distortion of the crystal turning the space group
Imma into the space group Pnn2 (34). This group-theoretical result is in
accordance with the experimentally observed displacements of the Fe and O atoms
in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)]
A family of Schr\"odinger operators whose spectrum is an interval
By approximation, I show that the spectrum of the Schr\"odinger operator with
potential for f continuous and , is an interval.Comment: Comm. Math. Phys. (to appear
Critical Point of an Interacting Two-Dimensional Atomic Bose Gas
We have measured the critical atom number in an array of harmonically trapped
two-dimensional (2D) Bose gases of rubidium atoms at different temperatures. We
found this number to be about five times higher than predicted by the
semi-classical theory of Bose-Einstein condensation (BEC) in the ideal gas.
This demonstrates that the conventional BEC picture is inapplicable in an
interacting 2D atomic gas, in sharp contrast to the three-dimensional case. A
simple heuristic model based on the Berezinskii-Kosterlitz-Thouless theory of
2D superfluidity and the local density approximation accounts well for our
experimental results
Current-Driven Domain-Wall Dynamics in Curved Ferromagnetic Nanowires
The current-induced motion of a domain wall in a semicircle nanowire with
applied Zeeman field is investigated. Starting from a micromagnetic model we
derive an analytical solution which characterizes the domain-wall motion as a
harmonic oscillation. This solution relates the micromagnetic material
parameters with the dynamical characteristics of a harmonic oscillator, i.e.,
domain-wall mass, resonance frequency, damping constant, and force acting on
the wall. For wires with strong curvature the dipole moment of the wall as well
as its geometry influence the eigenmodes of the oscillator. Based on these
results we suggest experiments for the determination of material parameters
which otherwise are difficult to access. Numerical calculations confirm our
analytical solution and show its limitations
Rippled area formed by surface plasmon polaritons upon femtosecond laser double-pulse irradiation of silicon: the role of carrier generation and relaxation processes
The formation of laser-induced periodic surface structures (LIPSS, ripples)
upon irradiation of silicon with multiple irradiation sequences consisting of
femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800
nm) is studied numerically using a rate equation system along with a
two-temperature model accounting for one- and two-photon absorption and
subsequent carrier diffusion and Auger recombination processes. The temporal
delay between the individual equal-energy fs-laser pulses was varied between
and ps for quantification of the transient carrier densities in
the conduction band of the laser-excited silicon. The results of the numerical
analysis reveal the importance of carrier generation and relaxation processes
in fs-LIPSS formation on silicon and quantitatively explain the two time
constants of the delay dependent decrease of the Low-Spatial-Frequency LIPSS
(LSFL) area observed experimentally. The role of carrier generation, diffusion
and recombination are quantified individually.Comment: 5 pages, 5 figures, Conference On Laser Ablation (COLA) 2013. The
final publication is available at http://link.springer.com. Accepted for
publication in Applied Physics
Phase diagram of the frustrated, spatially anisotropic S=1 antiferromagnet on a square lattice
We study the S=1 square lattice Heisenberg antiferromagnet with spatially
anisotropic nearest neighbor couplings , frustrated by a
next-nearest neighbor coupling numerically using the density-matrix
renormalization group (DMRG) method and analytically employing the
Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of
the anisotropy, within both methods we find quantum fluctuations to stabilize
the N\'{e}el ordered state above the classically stable region. Whereas SBMFT
suggests a fluctuation-induced first order transition between the N\'{e}el
state and a stripe antiferromagnet for and an
intermediate paramagnetic region opening only for very strong anisotropy, the
DMRG results clearly demonstrate that the two magnetically ordered phases are
separated by a quantum disordered region for all values of the anisotropy with
the remarkable implication that the quantum paramagnetic phase of the spatially
isotropic - model is continuously connected to the limit of
decoupled Haldane spin chains. Our findings indicate that for S=1 quantum
fluctuations in strongly frustrated antiferromagnets are crucial and not
correctly treated on the semiclassical level.Comment: 10 pages, 10 figure
A catalytic asymmetric total synthesis of (-)-perophoramidine
We report a catalytic asymmetric total synthesis of the ascidian natural product perophoramidine. The synthesis employs a molybdenum-catalyzed asymmetric allylic alkylation of an oxindole nucleophile and a monosubstituted allylic electrophile as a key asymmetric step. The enantioenriched oxindole product from this transformation contains vicinal quaternary and tertiary stereocenters, and is obtained in high yield along with high levels of regio-, diastereo-, and enantioselectivity. To install the second quaternary stereocenter in the target, the route utilizes a novel regio- and diastereoselective allylation of a cyclic imino ether to deliver an allylated imino ether product in near quantitative yield and with complete regio- and diastereocontrol. Oxidative cleavage and reductive amination are used as final steps to access the natural product.John Stauffer Memorial FellowshipStanford Graduate Fellowshi
Inelastic H and D atom scattering from Au(111) as benchmark for theory.
Efficient transfer of translational energy to electron-hole pair excitation involving multiple collisions dominates H atom collisions with metal surfaces. For this reason, H atom interaction with metal surfaces cannot be modeled within the commonly used Born-Oppenheimer approximation (BOA). This fact makes H atom scattering from metal surfaces an ideal model system for dynamics that go beyond the BOA. We chose the H/Au(111) system as a model system to obtain a detailed dataset that can serve as a benchmark for theoretical models developed for describing electronically nonadiabatic processes at metal surfaces. Therefore, we investigate the influence of various experimental parameters on the energy loss in detail including isotopic variant, incidence translational energy, incidence polar and azimuthal angles, and outgoing scattering angles
Relativistic electronic dressing
We study the effects of the relativistic electronic dressing in
laser-assisted electron-hydrogen atom elastic collisions. We begin by
considering the case when no radiation is present. This is necessary in order
to check the consistency of our calculations and we then carry out the
calculations using the relativistic Dirac-Volkov states. It turns out that a
simple formal analogy links the analytical expressions of the differential
cross section without laser and the differential cross section in presence of a
laser field.Comment: 11 pages, 18 figures, Late
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