6 research outputs found
Insulating and Conducting Phases of RbC60
Optical measurements were performed on thin films of RbC,
identified by X-ray diffraction as mostly material. The samples were
subjected to various heat treatments, including quenching and slow cooling from
400K. The dramatic increase in the transmission of the quenched samples, and
the relaxation towards the transmission observed in slow cooled samples
provides direct evidence for the existence of a metastable insulating phase.
Slow cooling results in a phase transition between two electrically conducting
phases.Comment: Minor revisions. Submitted to PRB, RevTeX 3.0 file, 2 postscript
figures included, ir_dop
On-chain electrodynamics of metallic (TMTSF)_2 X salts: Observation of Tomonaga-Luttinger liquid response
We have measured the electrodynamic response in the metallic state of three
highly anisotropic conductors, (TMTSF)_2 X, where X=PF_6, AsF_6, or ClO_4, and
TMTSF is the organic molecule tetramethyltetraselenofulvalene. In all three
cases we find dramatic deviations from a simple Drude response. The optical
conductivity has two features: a narrow mode at zero frequency, with a small
spectral weight, and a mode centered around 200 cm^{-1}, with nearly all of the
spectral weight expected for the relevant number of carriers and single
particle bandmass. We argue that these features are characteristic of a nearly
one-dimensional half- or quarter-filled band with Coulomb correlations, and
evaluate the finite energy mode in terms of a one-dimensional Mott insulator.
At high frequencies (\hbar\omega > t_\perp, the transfer integral perpendicular
to the chains), the frequency dependence of the optical conductivity
\sigma_1(\omega) is in agreement with calculations based on an interacting
Tomonaga-Luttinger liquid, and is different from what is expected for an
uncorrelated one-dimensional semiconductor. The zero frequency mode shows
deviations from a simple Drude response, and can be adequately described with a
frequency dependent mass and relaxation rate.Comment: 12 pages, 7 figures, RevTeX; minor corrections to text and
references; To be published in Phys. Rev. B, 15 July 199
Variable-range hopping in quasi-one-dimensional electron crystals
We study the effect of impurities on the ground state and the low-temperature
dc transport in a 1D chain and quasi-1D systems of many parallel chains. We
assume that strong interactions impose a short-range periodicicity of the
electron positions. The long-range order of such an electron crystal (or
equivalently, a charge-density wave) is destroyed by impurities. The 3D
array of chains behaves differently at large and at small impurity
concentrations . At large , impurities divide the chains into metallic
rods. The low-temperature conductivity is due to the variable-range hopping of
electrons between the rods. It obeys the Efros-Shklovskii (ES) law and
increases exponentially as decreases. When is small, the metallic-rod
picture of the ground state survives only in the form of rare clusters of
atypically short rods. They are the source of low-energy charge excitations. In
the bulk the charge excitations are gapped and the electron crystal is pinned
collectively. A strongly anisotropic screening of the Coulomb potential
produces an unconventional linear in energy Coulomb gap and a new law of the
variable-range hopping . remains
constant over a finite range of impurity concentrations. At smaller the
2/5-law is replaced by the Mott law, where the conductivity gets suppressed as
goes down. Thus, the overall dependence of on is nonmonotonic.
In 1D, the granular-rod picture and the ES apply at all . The conductivity
decreases exponentially with . Our theory provides a qualitative explanation
for the transport in organic charge-density wave compounds.Comment: 20 pages, 7 figures. (v1) The abstract is abridged to 24 lines. For
the full abstract, see the manuscript (v2) several changes in presentation
per referee's comments. No change in result