6,986 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)]
Triplon mean-field analysis of an antiferromagnet with degenerate Shastry-Sutherland ground states
We look into the quantum phase diagram of a spin-
antiferromagnet on the square lattice with degenerate Shastry-Sutherland ground
states, for which only a schematic phase diagram is known so far. Many exotic
phases were proposed in the schematic phase diagram by the use of exact
diagonalization on very small system sizes. In our present work, an important
extension of this antiferromagnet is introduced and investigated in the
thermodynamic limit using triplon mean-field theory. Remarkably, this
antiferromagnet shows a stable plaquette spin-gapped phase like the original
Shastry-Sutherland antiferromagnet, although both of these antiferromagnets
differ in the Hamiltonian construction and ground state degeneracy. We propose
a sublattice columnar dimer phase which is stabilized by the second and third
neighbor antiferromagnetic Heisenberg exchange interactions. There are also
some commensurate and incommensurate magnetically ordered phases, and other
spin-gapped phases which find their places in the quantum phase diagram.
Mean-field results suggest that there is always a level-crossing phase
transition between two spin gapped phases, whereas in other situations, either
a level-crossing or a continuous phase transition happens
Effective interactions between star polymers
We study numerically the effective pair potential between star polymers with
equal arm lengths and equal number of arms. The simulations were done for
the soft core Domb-Joyce model on the simple cubic lattice, to minimize
corrections to scaling and to allow for an unlimited number of arms. For the
sampling, we used the pruned-enriched Rosenbluth method (PERM). We find that
the potential is much less soft than claimed in previous papers, in particular
for . While we verify the logarithmic divergence of , with
being the distance between the two cores, predicted by Witten and Pincus, we
find for that the Mayer function is hardly distinguishable from that for
a Gaussian potential.Comment: 5 pages, 5 figure
Spin Stiffness in the Hubbard model
The spin stiffness of the repulsive Hubbard model that occurs
in the hydrodynamic theory of antiferromagnetic spin waves is shown to be the
same as the thermodynamically defined stiffness involved in twisting the order
parameter. New expressions for are derived, which enable easier
interpretation, and connections with superconducting weight and gauge
invariance are discussed.Comment: 21 Pages LaTeX2e, to be published in Journal of Physics
Hardware-In-the-loop Validation of Autonomous Interplanetary Navigation Algorithms for Interplanetary Cruises with the Optical Star Stimulator
An atom fiber for guiding cold neutral atoms
We present an omnidirectional matter wave guide on an atom chip. The
rotational symmetry of the guide is maintained by a combination of two current
carrying wires and a bias field pointing perpendicular to the chip surface. We
demonstrate guiding of thermal atoms around more than two complete turns along
a spiral shaped 25mm long curved path (curve radii down to 200m) at
various atom--surface distances (35-450m). An extension of the scheme for
the guiding of Bose-Einstein condensates is outlined
Long-Time Asymptotics for the Korteweg-de Vries Equation via Nonlinear Steepest Descent
We apply the method of nonlinear steepest descent to compute the long-time
asymptotics of the Korteweg-de Vries equation for decaying initial data in the
soliton and similarity region. This paper can be viewed as an expository
introduction to this method.Comment: 31 page
Superfluid flow past an obstacle in annular Bose–Einstein condensates
We investigate the flow of a one-dimensional nonlinear Schrödinger model with periodic boundary conditions past an obstacle, motivated by recent experiments with Bose–Einstein condensates in ring traps. Above certain rotation velocities, localized solutions with a nontrivial phase profile appear. In striking difference from the infinite domain, in this case there are many critical velocities. At each critical velocity, the steady flow solutions disappear in a saddle-center bifurcation. These interconnected branches of the bifurcation diagram lead to additions of circulation quanta to the phase of the associated solution. This, in turn, relates to the manifestation of persistent current in numerous recent experimental and theoretical works, the connections to which we touch upon. The complex dynamics of the identified waveforms and the instability of unstable solution branches are demonstrated
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