6,641 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)]
Contrast Interferometry Using Bose-Einstein Condensates to Measure h/m and the Fine Structure Constant
The kinetic energy of an atom recoiling due to absorption of a photon was
measured as a frequency using an interferometric technique called ``contrast
interferometry''. Optical standing wave pulses were used as atom-optical
elements to create a symmetric three-path interferometer with a Bose-Einstein
condensate. The recoil phase accumulated in different paths was measured using
a single-shot detection technique. The scheme allows for additional photon
recoils within the interferometer and its symmetry suppresses several random
and systematic errors including those from vibrations and ac Stark shifts. We
have measured the photon recoil frequency of sodium to ppm precision, using
a simple realization of this scheme. Plausible extensions should yield a
sufficient precision to bring within reach a ppb-level determination of
and the fine structure constant
Quantum Phase Transitions in Spin Systems
We discuss the influence of strong quantum fluctuations on zero-temperature
phase transitions in a two-dimensional spin-half Heisenberg system. Using a
high-order coupled cluster treatment, we study competition of magnetic bonds
with and without frustration. We find that the coupled cluster treatment is
able to describe the zero-temperature transitions in a qualitatively correct
way, even if frustration is present and other methods such as quantum Monte
Carlo fail.Comment: 8 pages, 12 Postscipt figures; Accepted for publication in World
Scientifi
Absence of magnetic order for the spin-half Heisenberg antiferromagnet on the star lattice
We study the ground-state properties of the spin-half Heisenberg
antiferromagnet on the two-dimensional star lattice by spin-wave theory, exact
diagonalization and a variational mean-field approach. We find evidence that
the star lattice is (besides the \kagome lattice) a second candidate among the
11 uniform Archimedean lattices where quantum fluctuations in combination with
frustration lead to a quantum paramagnetic ground state. Although the classical
ground state of the Heisenberg antiferromagnet on the star exhibits a huge
non-trivial degeneracy like on the \kagome lattice, its quantum ground state is
most likely dimerized with a gap to all excitations. Finally, we find several
candidates for plateaux in the magnetization curve as well as a macroscopic
magnetization jump to saturation due to independent localized magnon states.Comment: new extended version (6 pages, 6 figures) as published in Physical
Review
Spin Waves in Quantum Antiferromagnets
Using a self-consistent mean-field theory for the Heisenberg
antiferromagnet Kr\"uger and Schuck recently derived an analytic expression for
the dispersion. It is exact in one dimension () and agrees well with
numerical results in . With an expansion in powers of the inverse
coordination number () we investigate if this expression can be
{\em exact} for all . The projection method of Mori-Zwanzig is used for the
{\em dynamical} spin susceptibility. We find that the expression of Kr\"uger
and Schuck deviates in order from our rigorous result. Our method is
generalised to arbitrary spin and to models with easy-axis anisotropy \D.
It can be systematically improved to higher orders in . We clarify its
relation to the expansion.Comment: 8 pages, uuencoded compressed PS-file, accepted as Euro. Phys. Lette
Weakly interacting Bose gas in the one-dimensional limit
We prepare a chemically and thermally one-dimensional (1d) quantum degenerate
Bose gas in a single microtrap. We introduce a new interferometric method to
distinguish the quasicondensate fraction of the gas from the thermal cloud at
finite temperature. We reach temperatures down to (transverse oscillator eigenfrequency )
when collisional thermalization slows down as expected in 1d. At the lowest
temperatures the transverse momentum distribution exhibits a residual
dependence on the line density , characteristic for 1d systems. For
very low densities the approach to the transverse single particle ground state
is linear in .Comment: to appear in Phys. Rev. Let
Non-equilibrium electromagnetic fluctuations: Heat transfer and interactions
The Casimir force between arbitrary objects in equilibrium is related to
scattering from individual bodies. We extend this approach to heat transfer and
Casimir forces in non-equilibrium cases where each body, and the environment,
is at a different temperature. The formalism tracks the radiation from each
body and its scatterings by the other objects. We discuss the radiation from a
cylinder, emphasizing its polarized nature, and obtain the heat transfer
between a sphere and a plate, demonstrating the validity of proximity transfer
approximation at close separations and arbitrary temperatures.Comment: 4 pages, 2 figures, published version, minor changes (e.g. typos
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