4,254 research outputs found
Orbital-Peierls State in NaTiSi2O6
Does the quasi one-dimensional titanium pyroxene NaTiSi2O6 exhibit the novel
{\it orbital-Peierls} state? We calculate its groundstate properties by three
methods: Monte Carlo simulations, a spin-orbital decoupling scheme and a
mapping onto a classical model. The results show univocally that for the spin
and orbital ordering to occur at the same temperature --an experimental
observation-- the crystal field needs to be small and the orbitals are active.
We also find that quantum fluctuations in the spin-orbital sector drive the
transition, explaining why canonical bandstructure methods fail to find it. The
conclusion that NaTiSi2O6 shows an orbital-Peierls transition is therefore
inevitable.Comment: 4 pages, 3 figure
Optical conductivity in A3C60 (A=K, Rb)
We study the optical conductivity in A3C60 (A =K, Rb). The effects of the
electron-phonon interaction is included to lowest order in the coupling
strength lambda. It is shown that this leads to a narrowing of the Drude peak
by a factor 1+lambda and a transfer of weight to a mid-infrared peak at
somewhat larger energies than the phonon energy. Although this goes in the
right direction, it is not sufficient to describe experiment.Comment: 5 pages, revtex, 2 figures more information at
http://librix.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Formal Solution of the Fourth Order Killing equations for Stationary Axisymmetric Vacuum Spacetimes
An analytic understanding of the geodesic structure around non-Kerr
spacetimes will result in a powerful tool that could make the mapping of
spacetime around massive quiescent compact objects possible. To this end, I
present an analytic closed form expression for the components of a the fourth
order Killing tensor for Stationary Axisymmetric Vacuum (SAV) Spacetimes. It is
as yet unclear what subset of SAV spacetimes admit this solution. The solution
is written in terms of an integral expression involving the metric functions
and two specific Greens functions. A second integral expression has to vanish
in order for the solution to be exact. In the event that the second integral
does not vanish it is likely that the best fourth order approximation to the
invariant has been found. This solution can be viewed as a generalized Carter
constant providing an explicit expression for the fourth invariant, in addition
to the energy, azimuthal angular momentum and rest mass, associated with
geodesic motion in SAV spacetimes, be it exact or approximate. I further
comment on the application of this result for the founding of a general
algorithm for mapping the spacetime around compact objects using gravitational
wave observatories.Comment: 5 Page
Non conventional screening of the Coulomb interaction in low dimensional and finite size system
We study the screening of the Coulomb interaction in non polar systems by
polarizable atoms. We show that in low dimensions and small finite size systems
this screening deviates strongly from that conventionally assumed. In fact in
one dimension the short range interaction is strongly screened and the long
range interaction is anti-screened thereby strongly reducing the gradient of
the Coulomb interaction and therefore the correlation effects. We argue that
this effect explains the success of mean field single particle theories for
large molecules.Comment: 4 pages, 5 figure
Majorana spin-flip transitions in a magnetic trap
Atoms confined in a magnetic trap can escape by making spin-flip Majorana
transitions due to a breakdown of the adiabatic approximation. Several papers
have studied this process for atoms with spin or . The present
paper calculates the escape rate for atoms with spin . This problem has
new features because the perturbation which allows atoms to escape
satisfies a selection rule and multi-step
processes contribute in leading order. When the adiabatic approximation is
satisfied the leading order terms can be summed to yield a simple expression
for the escape rate.Comment: 16page
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