1,561 research outputs found
Collective Spin-Density-Wave Response Perpendicular to the Chains of the Quasi One-Dimensional Conductor (TMTSF)2PF6
Microwave experiments along all three directions of the spin-density-wave
model compound (TMTSF)PF reveal that the pinned mode resonance is
present along the and axes. The collective transport is
considered to be the fingerprint of the condensate. In contrast to common quasi
one-dimensional models, the density wave also slides in the perpendicular
direction. The collective response is absent along the least
conducting direction.Comment: 3 pages, 4 figure
A New Scenario on the Metal-Insulator Transition in VO2
The metal-insulator transition in VO2 was investigated using the three-band
Hubbard model, in which the degeneracy of the 3d orbitals, the on-site Coulomb
and exchange interactions, and the effects of lattice distortion were
considered. A new scenario on the phase transition is proposed, where the
increase in energy level separation among the t_2g orbitals caused by the
lattice distortion triggers an abrupt change in the electronic configuration in
doubly occupied sites from an S=1 Hund's coupling state to a spin S=0 state
with much larger energy, and this strongly suppresses the charge fluctuation.
Although the material is expected to be a Mott-Hubbard insulator in the
insulating phase, the metal-to-insulator transition is not caused by an
increase in relative strength of the Coulomb interaction against the electron
hopping as in the usual Mott transition, but by the level splitting among the
t_2g orbitals against the on-site exchange interaction. The metal-insulator
transition in Ti2O3 can also be explained by the same scenario. Such a large
change in the 3d orbital occupation at the phase transition can be detected by
linear dichroic V 2p x-ray absorption measurements.Comment: 5 pages, 5 figures, to be published in J. Phys. Soc. Jpn. Vol. 72 No.
1
Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)X
High-resolution thermal expansion measurements have been performed for
exploring the mysterious "structureless transition" in (TMTTF)X (X =
PF and AsF), where charge ordering at coincides with the
onset of ferroelectric order. Particularly distinct lattice effects are found
at in the uniaxial expansivity along the interstack
-direction. We propose a scheme involving a charge
modulation along the TMTTF stacks and its coupling to displacements of the
counteranions X. These anion shifts, which lift the inversion symmetry
enabling ferroelectric order to develop, determine the 3D charge pattern
without ambiguity. Evidence is found for another anomaly for both materials at
0.6 indicative of a phase transition
related to the charge ordering
Effective band-structure in the insulating phase versus strong dynamical correlations in metallic VO2
Using a general analytical continuation scheme for cluster dynamical mean
field calculations, we analyze real-frequency self-energies, momentum-resolved
spectral functions, and one-particle excitations of the metallic and insulating
phases of VO2. While for the former dynamical correlations and lifetime effects
prevent a description in terms of quasi-particles, the excitations of the
latter allow for an effective band-structure. We construct an
orbital-dependent, but static one-particle potential that reproduces the full
many-body spectrum. Yet, the ground state is well beyond a static one-particle
description. The emerging picture gives a non-trivial answer to the decade-old
question of the nature of the insulator, which we characterize as a ``many-body
Peierls'' state.Comment: 5 pages, 4 color figure
Competing phases in the high field phase diagram of (TMTSF)ClO
A model is presented for the high field phase diagram of (TMTSF)ClO,
taking into account the anion ordering, which splits the Fermi surface in two
bands. For strong enough field, the largest metal-SDW critical temperature
corresponds to the N=0 phase, which originates from two intraband nesting
processes. At lower temperature, the competition between these processes puts
at disadvantage the N=0 phase vs. the N=1 phase, which is due to interband
nesting. A first order transition takes then place from the N=0 to N=1 phase.
We ascribe to this effect the experimentally observed phase diagrams.Comment: 5 pages, 3 figures (to appear in Phys. Rev. Lett.
Low temperature structural effects in the (TMTSF)PF and AsF Bechgaard salts
We present a detailed low-temperature investigation of the statics and
dynamics of the anions and methyl groups in the organic conductors
(TMTSF)PF and (TMTSF)AsF (TMTSF :
tetramethyl-tetraselenafulvalene). The 4 K neutron scattering structure
refinement of the fully deuterated (TMTSF)PF-D12 salt allows locating
precisely the methyl groups at 4 K. This structure is compared to the one of
the fully hydrogenated (TMTSF)PF-H12 salt previously determined at the
same temperature. Surprisingly it is found that deuteration corresponds to the
application of a negative pressure of 5 x 10 MPa to the H12 salt. Accurate
measurements of the Bragg intensity show anomalous thermal variations at low
temperature both in the deuterated PF and AsF salts. Two different
thermal behaviors have been distinguished. Low-Bragg-angle measurements reflect
the presence of low-frequency modes at characteristic energies {\theta} =
8.3 K and {\theta} = 6.7 K for the PF-D12 and AsF-D12 salts,
respectively. These modes correspond to the low-temperature methyl group
motion. Large-Bragg-angle measurements evidence an unexpected structural change
around 55 K which probably corresponds to the linkage of the anions to the
methyl groups via the formation of F...D-CD2 bonds observed in the 4 K
structural refinement. Finally we show that the thermal expansion coefficient
of (TMTSF)PF is dominated by the librational motion of the PF
units. We quantitatively analyze the low-temperature variation of the lattice
expansion via the contribution of Einstein oscillators, which allows us to
determine for the first time the characteristic frequency of the PF6
librations: {\theta} = 50 K and {\theta} = 76 K for the PF-D12 and
PF-H12 salts, respectively
- …