No abstract

The thermal conductivity of polyacetylene has been measured in the temperature range of 0.8 to 300 K. For T<70 K we present also data of AsF5-doped samples and of doped samples which have been compensated with (CH3)2NH into the insulating state. The undoped sample shows a T2-temperature dependence up to 12 K followed by a decreasing power law at higher temperatures and a thermal conductivity value of 4 mW/K cm at room temperature. Doping and even compensating increases the thermal conductivity below 12 K. This additional conductivity is therefore explained by a change in the phonon boundary scattering at the interfaces between crystalline and amorphous regions of polyacetylene and not by a contribution of charge carriers to the thermal conductivity.

We report some unusual observations on the NMR spectrum and the clustering rate of o-H 2 at low ortho concentration in solid H 2 at temperatures between 25 mK and 2 K. Evidence points towards motional narrowing of the HD impurities, the proton signal of which is observed for T < 0.15 K, and a hopping of o-B. 2 from one pair configuration into another. Furthermore, there is an unexpected increase of the clustering rate of the other o-H 2 molecules as the temperature decreases below ~ 0.1 K.

The NMR spectrum of isolated ortho-Hz pairs has been measured on two single crystals of H2 with ortho concentrations below about 2%. The experiments were carried out over a temperature range between 0.025 and 1 K, and spectra were recorded with the magnetic field applied in a number of directions. Extending the previous analysis of the spectrum for isolated ortho-H2 to the present data led to the determination of the crystal orientation. The complicated pair spectrum includes the signals from the nine nonequivalent pair orientations in the crystal and is a function of the two lowest rotational states. These states are split by an energy A, which is different for the pairs with axis respectively in and out of the basal plane of the crystal (denoted IP and OP), namely A(OP)/kB ~-64 mK and A(IP)/kB < 5 inK.A detailed calculation of the NMR spectrum for each pair as a function of the applied field direction and temperature is carried out. The limiting cases are the high-temperature region ksT >> A and the low-temperature regime k~T << A. Therefore at the lowest temperatures attained, the NMIR spectrum of the OP and IP pairs should behave differently. There is good agreement between experiment and theory, which is demonstrated in a detailed discussion. While at high temperatures, the transition rate 1"2 between the rotational states appears to be large in comparision with the NMR splitting constant d = 57.68 kHz, there is evidence that g2 becomes smaller than d at temperatures lower than about 60 mK for the OP pairs, but not for the IP pairs. The NMR spectrum intensity for both types of pairs decreases as a function of time at constant temperature, as best seen from experiments below O. 1 K, and the corresponding time constant is appreciably shorter for the IP than for the OP pairs. This phenomenon might possibly be caused by the gradual disappearance of pairs in favor of larger ortho-H2 cluster formation.

The time-dependent effects in the NMR spectrum of ortho-Hz impurities in two single crystals of Hz are investigated over the temperature range between 0.02 and 3 K and for an ortho-Hz mole fraction X between 0.1% and 2%. The disappearance of the signal from isolated o-H2 impurities, after cooling the crystals to a given temperature, and the simultaneous increase of the signal from isolated o-H2 pairs indicates a clustering process. The characteristic times from both spectra are comparable and pass over a flat maximum near T = 0.3 K. The clustering process then accelerates drastically as T is decreased below 0.1 K. The characteristic times are only weakly dependent on the concentration X. Furthermore, the subsequent decay of the o-H2 pair signals with time suggests a slow diffusion of pairs and formation of larger clusters. It is found that the period associated with the decay of in-plane pairs (having their axis parallel to the basal plane) is considerably shorter than that for the out-of-plane pairs. The resui'ts are discussed in the light of the theory of hopping impurities (resonant ortho-para conversion) and possible coherent tunneling effects at low temperamres. Finally we briefly report some studies of the longitudinal relaxation time for the o-H2 pair NMR lines.