First results from KamLAND: evidence for reactor antineutrino disappearance

KamLAND has measured the flux of nu;(e)'s from distant nuclear reactors. We find fewer nu;(e) events than expected from standard assumptions about nu;(e) propagation at the 99.95% C.L. In a 162 ton.yr exposure the ratio of the observed inverse beta-decay events to the expected number without nu;(e) disappearance is 0.611+/-0.085(stat)+/-0.041(syst) for nu;(e) energies >3.4 MeV. In the context of two-flavor neutrino oscillations with CPT invariance, all solutions to the solar neutrino problem except for the "large mixing angle" region are excluded.     

Chromism of soluble polythienylenes

Thermochromism and solvatochromism of solutions of poly(3-alkylthienylenes) are reported. The experimental results indicate the presence of two coexisting phases: polymer in solution and polymer in microcrystalline aggregates. From the concentration independence of the thermochromism, it is concluded that the transition is fundamentally driven by a single chain mechanism, and that the aggregation (microcrystallization) of the poly(3-alkylthienylene) macromolecules can occur only after the single chain conformational change has occurred.     

Dielectric constant and microwave conductivity of the vanadium bronze Na0.33V2O5

Measurements of the dielectric constant and microwave conductivity are reported for the vanadium bronze Na_0.33V₂O₅. The dielectric constant is highly anisotropic and very large parallel to the highly conducting direction, while the conductivity is strongly dispersive. The results can be consistently interpreted by considering Na_0.33V₂O₅ as a quasi-one-dimensional conductor in which, at low temperatures, a pseudo-gap opens in the open-electron density of states due to inter-site correlations.     

Optical studies of pyrolyzed polyacrylonitrile

The results of measurements of the optical absorption spectra, IR absorption spectra, thermogravimetric analysis, and elementary analysis of pyrolyzed polyacrilonitrile are presented as a function of the pyrolysis temperature T_p and of the duration of pyrolysis. At T_p ～ 200°C, an intermediate phase was discovered, containing conjugated CN sequences and a completely unreacted carbon backbone. The optical absorption data imply that the resulting polymer is a semiconductor with a delocalized π-electron system and an energy gap E_g ? 2.5 eV. For T_p > 260°C, the weight loss rapidly increases, and the absorption edge gradually broadens and shifts to lower energies. The resulting polymer (after higher-temperature pyrolysis) contains CN and C?C? C conjugation sequences, but appears to be a complex structure consisting of a mixture of different chemical species.     

Proton NMR studies of the structure of trans-(CH)x

The proton NMR line width in undoped and AsF₅ doped polyacetylene shows an abrupt change as a function of temperature, indicative of motional narrowing. The characteristic temperature for the onset of narrowing decreases with increasing AsF₅ concentration. The results are discussed in terms of large amplitude librational or translational motions of the trans-(CH)_x chains. The magnitudes obtained for the second moment at temperatures below and above the narrowing transition are used to obtain information on the chain packing of trans-(CH)_x within the three-dimensional crystal structure.     

Theory of the rod-to-coil transition in polydiacetylene

A theory is proposed for the rod to coil transition in polydiacetylene 4BCMU and related polymers which is based on the hypothesis that the high-temperature (yellow) phase consists predominantly of the cis structure, while the low-temperature (red) phase is trans. Because the occurrence of a cis-trans interface is energetically costly, the correlation length for either isomer remains long and the transition is sharp, much like the helix-coil transition in the theory of Zimm and Bragg. The transition is driven by the higher entropy of the cis isomer, which is free to coil, unlike the trans or butatriene forms. The theory gives excellent agreement with optical absorption measurements and is consistent with all other experimental data for this system.     

Anisotropic optical properties of pure and doped polyacetylene

Optical absorption and reflection measurements have been carried out on pure and doped polyacetylene films. The absorption data are consistent with a model of (CH)_X as a direct gap quasi-l d semiconductor; doping with AsF₅ or iodine introduces absorption within the gap, but appears to leave the interband transition intact. The anisotropic reflectance from partially aligned films and the increased optical anisotropy upon doping provide evidence of quasi-l d behavior of the semiconducting pure polymer and the metallic doped polymer.     

Elastic properties of one dimensional compounds

We have investigated the elastic properties of several one-dimensional compounds using a vibrating reed technique. A model calculation shows that a vanishing Young's modulus will occur for elastic one-dimensionality. TTF-TCNQ exhibits a small modulus which decreases with increasing temperature extrapolating to zero at T ≈ 500–700K. The uniaxial stress dependence of the metal-insulator transition temperature is calculated to be $\text{¦}\text{d}\text{T}{}_{\mathrm{c}}\text{/}\text{d}\text{σ¦ ～ (10 ± 5)}\text{K/bar}$. KCP shows a much higher modulus and a phase transformation at ～ 35 K.     

Neutron scattering study of 49 K phase transition in TTF-TCNQ

Elastic neutron scattering measurements show the existence of three low temperature phase transitions in TTF-TCNQ and are interpreted within the framework of a recent theoretical investigation by Bak and Emery. Direct evidence for the new 49 K transition is presented in addition to the 38 and 54 K transitions reported previously.     

Voltage-induced metal-insulator transition in polythiophene field-effect transistors

We have extensively studied the carrier transport in regio-regular polythiophene field-effect transistors (FETs) from room temperature to 4.2 K. At low temperatures, Zabrodskii plots (dlnsigma/dlnT) demonstrate that the gate voltage and source-drain voltage combine to induce the insulator-to-metal transition at a carrier density of 5x10(12) cm-2. The carrier transport in the insulating regime is well described by phonon assisted hopping in a disordered Fermi glass with Coulomb interaction between the hopping charge carrier and the opposite charge left behind, as described by Efros and Shklovskii.