2,651 research outputs found
Frequency-dependent photothermal measurement of thermal diffusivity for opaque and non-opaque materials; Application to crystals of TIPS-pentacene
We propose the use of a frequency-dependent photothermal measurement as a
complement to light-flash, i.e. time-dependent, measurements to determine the
through-plane thermal diffusivity of small, thin samples, e.g. semiconducting
polymers and small organic molecule crystals. The analysis is extended from its
previous use with some opaque conducting polymers to materials with finite
absorption coefficients, such as crystals of 6,13-bis(triisopropylsilylethynyl
pentacene ("TIPS-pentacene"). Taking into account the finite absorption
coefficients of the latter gives a value of diffusivity, D=0.10 mm2/s, much
smaller than previously estimated and more consistent with its expected value.
We also briefly discuss the effects of coating samples for the measurement to
improve their optical properties.Comment: 11 pages, 6 figures, to be published in Journal of Applied Physic
Characterization of the Torsional Piezoelectric-like Response of Tantalum Trisulfide Associated with Charge-Density-Wave Depinning
We have studied the frequency and voltage dependence of voltage-induced
torsional strains in orthorhombic TaS3 [V. Ya. Pokrovskii, et al, Phys. Rev.
Lett. 98, 206404 (2007)] by measuring the modulation of the resonant frequency
of an RF cavity containing the sample. The strain has an onset voltage below
the charge-density-wave (CDW) threshold voltages associated with changes in
shear compliance and resistance, suggesting that the strain is associated with
polarization of the CDW rather than CDW current. Measurements with square-wave
voltages show that the strain is very sluggish, not even reaching its dc value
at a frequency of 0.1 Hz, but the dynamics appear to be very sample dependent.
By applying oscillating torque while biasing the sample with a dc current, we
have also looked for strain induced voltage in the sample; none is observed at
the low biases where the voltage-induced strains first occur, but an induced
voltage is observed at higher biases, probably associated with strain-dependent
CDW conductance.Comment: 11 pages, including 3 figures, to be published in Phys. Rev. B (Rapid
Comm.
Lattice Universes in 2+1-dimensional gravity
Lattice universes are spatially closed space-times of spherical topology in
the large, containing masses or black holes arranged in the symmetry of a
regular polygon or polytope. Exact solutions for such spacetimes are found in
2+1 dimensions for Einstein gravity with a non-positive cosmological constant.
By means of a mapping that preserves the essential nature of geodesics we
establish analogies between the flat and the negative curvature cases. This map
also allows treatment of point particles and black holes on a similar footing.Comment: 14 pages 7 figures, to appear in Festschrift for Vince Moncrief (CQG
Electro-optic measurement of carrier mobility in an organic thin-film transistor
We have used an electro-optic technique to measure the position-dependent
infrared absorption of holes injected into a thin crystal of the organic
semiconductor, 6,13-bis(triisopropylsilylethynyl)-pentacene incorporated in a
field-effect transistor. By applying square-wave voltages of variable frequency
to the gate or drain, one can measure the time it takes for charges to
accumulate on the surface, and therefore determine their mobility.Comment: 11 pages, 4 figures, to be published in Applied Physics Letter
Dynamics of Charge Flow in the Channel of a Thin-Film Field-Effect Transistor
The local conductivity in the channel of a thin-film field-effect transistor
is proportional to the charge density induced by the local gate voltage. We
show how this determines the frequency- and position-dependence of the charge
induced in the channel for the case of "zero applied current": zero
drain-source voltage with charge induced by a square-wave voltage applied to
the gate, assuming constant mobility and negligible contact impedances. An
approximate expression for the frequency dependence of the induced charge in
the center of the channel can be conveniently used to determine the charge
mobility. Fits of electro-optic measurements of the induced charge in organic
transistors are used as examples.Comment: 9 pages including table + 3 figures; submitted to Jnl. Appl. Phy
Electro-Reflectance Spectra of Blue Bronze
We show that the infrared reflectance of the quasi-one dimensional
charge-density-wave (CDW) conductor K0.3MoO3 (blue bronze) varies with position
when a voltage greater than the CDW depinning threshold is applied. The spatial
dependence and spectra associated with these changes are generally as expected
from the electro-transmission [B.M. Emerling, et al, Eur. Phys. J. B 16, 295
(2000)], but there are some differences which might be associated with changes
in the CDW properties on the surface. We have examined the electro-reflectance
spectrum associated with CDW current investigation for light polarized parallel
to the conducting chains for signs of expected current-induced intragap states,
and conclude that the density of any such states is at least an order of
magnitude lower than expected.Comment: 1)submitted to Eur. Phys. J B 2) revised (July 24) to a) better
emphasize results and b) with new figure insets to make paper more
self-containe
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