78 research outputs found
The Frequency Dependent Conductivity of Electron Glasses
Results of DC and frequency dependent conductivity in the quantum limit, i.e.
hw > kT, for a broad range of dopant concentrations in nominally uncompensated,
crystalline phosphorous doped silicon and amorphous niobium-silicon alloys are
reported. These materials fall under the general category of disordered
insulating systems, which are referred to as electron glasses. Using microwave
resonant cavities and quasi-optical millimeter wave spectroscopy we are able to
study the frequency dependent response on the insulating side of the
metal-insulator transition. We identify a quantum critical regime, a Fermi
glass regime and a Coulomb glass regime. Our phenomenological results lead to a
phase diagram description, or taxonomy, of the electrodynamic response of
electron glass systems
Measurements of the Complex Conductivity of NbxSi1-x Alloys on the Insulating Side of the Metal-Insulator Transition
We have conducted temperature and frequency dependent transport measurements
in amorphous Nb_x Si_{1-x} samples in the insulating regime. We find a
temperature dependent dc conductivity consistent with variable range hopping in
a Coulomb glass. The frequency dependent response in the millimeter-wave
frequency range can be described by the expression with the exponent somewhat smaller than one. Our ac
results are not consistent with extant theories for the hopping transport.Comment: 4 pages with 3 figures; published version has a different title from
original (was: "Electrodynamics in a Coulomb glass"
Electrodynamics of a Coulomb Glass in n-type Silicon
Optical measurements of the real and imaginary frequency dependent
conductivity of uncompensated n-type silicon are reported. The experiments are
done in the quantum limit, , across a broad doping range
on the insulating side of the Metal-Insulator transition (MIT). The observed
low energy linear frequency dependence shows characteristics consistent with
theories of a Coulomb glass, but discrepancies exist in the relative magnitudes
of the real and imaginary components. At higher energies we observe a crossover
to a quadratic frequency dependence that is sharper than expected over the
entire dopant range. The concentration dependence gives evidence that the
Coulomb interaction energy is the relevant energy scale that determines this
crossover.Comment: 5 pages, 4 figures; accepted for publication in Phys. Rev. Let
Sensorimotor Experience Influences Recovery of Forelimb Abilities but Not Tissue Loss after Focal Cortical Compression in Adult Rats
Sensorimotor activity has been shown to play a key role in functional outcome after extensive brain damage. This study was aimed at assessing the influence of sensorimotor experience through subject-environment interactions on the time course of both lesion and gliosis volumes as well as on the recovery of forelimb sensorimotor abilities following focal cortical injury. The lesion consisted of a cortical compression targeting the forepaw representational area within the primary somatosensory cortex of adult rats. After the cortical lesion, rats were randomly subjected to various postlesion conditions: unilateral C5–C6 dorsal root transection depriving the contralateral cortex from forepaw somatosensory inputs, standard housing or an enriched environment promoting sensorimotor experience and social interactions. Behavioral tests were used to assess forelimb placement during locomotion, forelimb-use asymmetry, and forepaw tactile sensitivity. For each group, the time course of tissue loss was described and the gliosis volume over the first postoperative month was evaluated using an unbiased stereological method. Consistent with previous studies, recovery of behavioral abilities was found to depend on post-injury experience. Indeed, increased sensorimotor activity initiated early in an enriched environment induced a rapid and more complete behavioral recovery compared with standard housing. In contrast, severe deprivation of peripheral sensory inputs led to a delayed and only partial sensorimotor recovery. The dorsal rhizotomy was found to increase the perilesional gliosis in comparison to standard or enriched environments. These findings provide further evidence that early sensory experience has a beneficial influence on the onset and time course of functional recovery after focal brain injury
Differential surface density and modulatory effects of presynaptic GABAB receptors in hippocampal cholecystokinin and parvalbumin basket cells
Dielectric properties and dynamical conductivity of LaTiO3: From dc to optical frequencies
We provide a complete and detailed characterization of the
temperature-dependent response to ac electrical fields of LaTiO3, a
Mott-Hubbard insulator close to the metal-insulator transition. We present
combined dc, broadband dielectric, mm-wave, and infrared spectra of ac
conductivity and dielectric constant, covering an overall frequency range of 17
decades. The dc and dielectric measurements reveal information on the
semiconducting charge-transport properties of LaTiO3, indicating the importance
of Anderson localization, and on the dielectric response due to ionic
polarization. In the infrared region, the temperature dependence of the phonon
modes gives strong hints for a structural phase transition at the magnetic
ordering temperature. In addition, a gap-like electronic excitation following
the phonon region is analyzed in detail. We compare the results to the
soft-edge behavior of the optical spectra characteristic for Mott-Hubbard
insulators. Overall a consistent picture of the charge-transport mechanisms in
LaTiO3 emerges.Comment: 11 pages, 8 figures, 1 tabl
Cellular and synaptic phenotypes lead to disrupted information processing in <i>Fmr1-KO</i> mouse layer 4 barrel cortex
Late-Quaternary Environments of the United States, Volume 1, The Late Pleistocene. Stephen C. Porter, editor. University of Minnesota Press, Minneapolis, 1983. xiv + 407 pp., figures, tables, references. 45.00 (cloth).
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