1,072 research outputs found
Equilibrium Low Temperature Heat Capacity of the Spin Density Wave compound (TMTTF)2 Br: effect of a Magnetic Field
We have investigated the effect of the magnetic field (B) on the very
low-temperature equilibrium heat capacity ceq of the quasi-1 D organic compound
(TMTTF)2Br, characterized by a commensurate Spin Density Wave (SDW) ground
state. Below 1K, ceq is dominated by a Schottky-like contribution, very
sensitive to the experimental time scale, a property that we have previously
measured in numerous DW compounds. Under applied field (in the range 0.2- 7 T),
the equilibrium dynamics, and hence ceq extracted from the time constant,
increases enormously. For B = 2-3 T, ceq varies like B2, in agreement with a
magnetic Zeeman coupling. Another specific property, common to other
Charge/Spin density wave (DW) compounds, is the occurrence of metastable
branches in ceq, induced at very low temperature by the field exceeding a
critical value. These effects are discussed within a generalization to SDWs in
a magnetic field of the available Larkin-Ovchinnikov local model of strong
pinning. A limitation of the model when compared to experiments is pointed out.Comment: 10 pages, 11 figure
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
Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering
Diffuse X-ray data for mixed stack organic charge-transfer crystals
approaching the neutral-ionic phase transition can be quantitatively explained
as due to the softening of the optical phonon branch. The interpretation is
fully consistent with vibrational spectra, and underlines the importance of
electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure
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
Linear viscoelastic properties of high reclaimed asphalt content mixes with biobinders
The use of high Reclaimed Asphalt (RA) content mixtures together with binders produced from renewable resources (biobinders) is one of the current challenges in pavement engineering research. On one hand, RA has been used for decades but there are still some concerns about its performance, especially when high contents are used (>30%). On the other hand, biobinders are relatively new materials which have to be deeply characterised and studied in order to develop good-practices for their use. In this paper, linear viscoelastic properties of biobinders and bio-mixtures manufactured with high-RA content and biobinders are analysed and discussed. High-modulus mixtures with 50% RA were selected for the mix design. Binders and mixtures were tested over a wide range of asphalt service temperatures and frequencies by means of DSR and two-point bending tests respectively. Results show that biobinders have an important effect on mixtures behaviour. However, no direct links between their linear viscoelastic properties were found. Bio-asphalt mixtures still need further development for commercial exploitation; however the take-away fact of this investigation is that it is possible to manufacture asphalt-like mixtures with acceptable viscoelastic properties while being composed only of RA and non-petroleum based binders
Modulation of charge-density waves by superlattice structures
We discuss the interplay between electronic correlations and an underlying
superlattice structure in determining the period of charge density waves
(CDW's), by considering a one-dimensional Hubbard model with a repeated
(non-random) pattern of repulsive (U>0) and free (U=0) sites. Density matrix
renormalization group diagonalization of finite systems (up to 120 sites) is
used to calculate the charge-density correlation function and structure factor
in the ground state. The modulation period can still be predicted through
effective Fermi wavevectors, k_F*, and densities, and we have found that it is
much more sensitive to electron (or hole) doping, both because of the narrow
range of densities needed to go from q*=0 to \pi, but also due to sharp
2k_F*-4k_F* transitions; these features render CDW's more versatile for actual
applications in heterostructures than in homogeneous systems.Comment: 4 pages, 5 figures, to appear in Phys Rev
Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCrS from first principles
We have studied the dielectric properties of the ferromagnetic spinel
CdCrS from first principles. Zone-center phonons and Born effective
charges were calculated by frozen-phonon and Berry phase techniques within
LSDA+U. We find that all infrared-active phonons are quite stable within the
cubic space group. The calculated static dielectric constant agrees well with
previous measurements. These results suggest that the recently observed
anomalous dielectric behavior in CdCrS is not due to the softening of a
polar mode. We suggest further experiments to clarify this point
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.
Spin-triplet superconductivity in quasi-one dimension
We consider a system with electron-phonon interaction, antiferromagnetic
fluctuations and disconnected open Fermi surfaces. The existence of odd-parity
superconductivity in this circumstance is shown for the first time. If it is
applied to the quasi-one-dimensional systems like the organic conductors
(TMTSF)_2X we obtain spin-triplet superconductivity with nodeless gap. Our
result is also valid in higher dimensions(2d and 3d).Comment: 2 page
Photoemission study of the metal-insulator transition in VO_2/TiO_2(001) : Evidence for strong electron-electron and electron-phonon interaction
We have made a detailed temperature-dependent photoemission study of
VO_2/TiO_2(001) thin films, which show a metal-insulator transition at \sim 300
K. Clean surfaces were obtained by annealing the films in an oxygen atmosphere.
Spectral weight transfer between the coherent and incoherent parts accompanying
the metal-insulator transition was clearly observed. We also observed a
hysteretic behavior of the spectra for heating-cooling cycles. We have derived
the ``bulk'' spectrum of the metallic phase and found that it has a strong
incoherent part. The width of the coherent part is comparable to that given by
band-structure calculation in spite of its reduced spectral weight, indicating
that the momentum dependence of the self-energy is significant. This is
attributed to by ferromagnetic fluctuation arising from Hund's rule coupling
between different d orbitals as originally proposed by Zylbersztejn and Mott.
In the insulating phase, the width of the V 3d band shows strong temperature
dependence. We attribute this to electron-phonon interaction and have
reproduced it using the independent boson model with a very large coupling
constant.Comment: 7 pages, 7 figures, submitted to Phys. Rev.
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