5,702 research outputs found
Exciton doublet in the Mott-Hubbard LiCuVO insulator identified by spectral ellipsometry
Spectroscopic ellipsometry was used to study the dielectric function of
LiCuVO, a compound comprised of chains of edge-sharing CuO
plaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K.
For photon polarization along the chains, the data reveal a weak but
well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral
weight is strongly enhanced upon cooling near the magnetic ordering
temperature. We identify these features as an exciton doublet in the
Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction
between electrons on nearest and next-nearest neighbor sites along the chains.
Our results and methodology can be used to address the role of the long-range
Coulomb repulsion for compounds with doped copper-oxide chains and planes.Comment: 4 pages with 4 figures and EPAPS supplementary online material (3
pages with 4 figures), accepted in Phys. Rev. Let
Evidence of secondary relaxations in the dielectric spectra of ionic liquids
We investigated the dynamics of a series of room temperature ionic liquids
based on the same 1-butyl-3-methyl imidazolium cation and different anions by
means of broadband dielectric spectroscopy covering 15 decades in frequency
(10^(-6)-10^9 Hz), and in the temperature range from 400 K down to 35 K. An
ionic conductivity is observed above the glass transition temperature T_{g}
with a relaxation in the electric modulus representation. Below T_{g}, two
relaxation processes appear, with the same features as the secondary
relaxations typically observed in molecular glasses. The activation energy of
the secondary processes and their dependence on the anion are different. The
slower process shows the characteristics of an intrinsic Johari-Goldstein
relaxation, in particular an activation energy E_{beta}=24k_{B}T_{g} is found,
as observed in molecular glasses.Comment: Major revision, submitted to Phys. Rev. Let
Cooperative motion and growing length scales in supercooled confined liquids
Using molecular dynamics simulations we investigate the relaxation dynamics
of a supercooled liquid close to a rough as well as close to a smooth wall. For
the former situation the relaxation times increase strongly with decreasing
distance from the wall whereas in the second case they strongly decrease. We
use this dependence to extract various dynamical length scales and show that
they grow with decreasing temperature. By calculating the frequency dependent
average susceptibility of such confined systems we show that the experimental
interpretation of such data is very difficult.Comment: 7 pages of Latex, 3 figure
Helicoidal magnetic order in a clean copper oxide spin chain compound
We report susceptibility, specific heat, and neutron diffraction measurements
on NaCuO, a spin-1/2 chain compound isostructural to LiCuO,
which has been extensively investigated. Below 13 K, we find a long-range
ordered, incommensurate magnetic helix state with a propagation vector similar
to that of LiCuO. In contrast to the Li analogue, substitutional
disorder is negligible in NaCuO. We can thus rule out that the helix is
induced by impurities, as was claimed on the basis of prior work on
LiCuO. A spin Hamiltonian with frustrated longer-range exchange
interactions provides a good description of both the ordered state and the
paramagnetic susceptibility.Comment: 4 pages, 4 figures Improved Fig.1 and 4. Minor rephrasing. Reference
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What is the Entanglement Length in a Polymer Melt ?
We present results of molecular dynamics simulations of very long model
polymer chains analyzed by various experimentally relevant techniques. The
segment motion of the chains is found to be in very good agreement with the
repatation model. We also calculated the plateau-modulus G_N. The predicitions
of the entanglement length N_e from G_N and from the mean square displacements
of the chains segments disagree by a factor of about 2.2(2), indicating an
error in the prefactor in the standard formula for G_N. We show that recent
neutron spin echo measurements were carried out for chain lengths which are too
small for a correct determination of N_e.Comment: 5 pages, 4 figures, RevTe
Electronic structure and thermoelectric properties of CuRh(1-x)MgxO2
Electronic structure calculations using the augmented spherical wave method
have been performed for CuRhO2. For this semiconductor crystallizing in the
delafossite structure, it is found that the valence band maximum is mainly due
to the 4d t2g orbitals of Rh^{3+}. The structural characterizations of
CuRh(1-x)MgxO2 show a broad range of Mg^{2+} substitution for Rh^{3+} in this
series, up to about 12%. Measurements of the resistivity and thermopower of the
doped systems show a Fermi liquid-like behavior for temperatures up to about
1000K, resulting in a large weakly temperature dependent power factor. The
thermopower is discussed both within the Boltzmann equation approach as based
on the electronic structure calculations and the temperature independent
correlation functions ratio approximation as based on the Kubo formalism.Comment: 9 pages, 12 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
The relaxation dynamics of a simple glass former confined in a pore
We use molecular dynamics computer simulations to investigate the relaxation
dynamics of a binary Lennard-Jones liquid confined in a narrow pore. We find
that the average dynamics is strongly influenced by the confinement in that
time correlation functions are much more stretched than in the bulk. By
investigating the dynamics of the particles as a function of their distance
from the wall, we can show that this stretching is due to a strong dependence
of the relaxation time on this distance, i.e. that the dynamics is spatially
very heterogeneous. In particular we find that the typical relaxation time of
the particles close to the wall is orders of magnitude larger than the one of
particles in the center of the pore.Comment: 9 pages of Latex, 4 figure
Ferromagnetism and Lattice Distortions in the Perovskite YTiO
The thermodynamic properties of the ferromagnetic perovskite YTiO are
investigated by thermal expansion, magnetostriction, specific heat, and
magnetization measurements. The low-temperature spin-wave contribution to the
specific heat, as well as an Arrott plot of the magnetization in the vicinity
of the Curie temperature K, are consistent with a
three-dimensional Heisenberg model of ferromagnetism. However, a magnetic
contribution to the thermal expansion persists well above , which
contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a
comparison with the corresponding model system EuS. The pressure dependences of
and of the spontaneous moment are extracted using thermodynamic
relationships. They indicate that ferromagnetism is strengthened by uniaxial
pressures and is weakened by uniaxial
pressures and hydrostatic pressure.
Our results show that the distortion along the - and -axes is further
increased by the magnetic transition, confirming that ferromagnetism is favored
by a large GdFeO-type distortion. The c-axis results however do not fit
into this simple picture, which may be explained by an additional
magnetoelastic effect, possibly related to a Jahn-Teller distortion.Comment: 12 pages, 13 figure
Field theory of self-avoiding walks in random media
Based on the analogy with the quantum mechanics of a particle propagating in
a {\em complex} potential, we develop a field-theoretical description of the
statistical properties of a self-avoiding polymer chain in a random
environment. We show that the account of the non-Hermiticity of the quantum
Hamiltonian results in a qualitatively different structure of the effective
action, compared to previous studies. Applying the renormalisation group
analysis, we find a transition between the weak-disorder regime, where the
quenched randomness is irrelevant, and the strong-disorder regime, where the
polymer chain collapses. However, the fact that the renormalised interaction
constants and the chiral symmetry breaking regularisation parameter flow
towards strong coupling raises questions about the applicability of the
perturbative analysis.Comment: RevTeX, 9 pages; accepted for publication in J. Phys.
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