N. P. Ong

Hall effect of a linear-chain metal: NbSe3

Abstract We present measurements of the low-field Hall constant RH as a function of field and temperature in the linear-chain metal NbSe3, The data range from 2 K to 200 K in temperature and up to 15 kG in field. At the two phase transitions T1 (142 K) and T2 (58 K) RH shows an abrupt increase in magnitude but no change in sign. Below 30 K RH becomes strongly field dependent. The zero-field-limit RH changes from n-type to p-type at 15 K. It is argued that the results are consistent with a two-band model in which the difference in population p - n is equal to 3 × 1018 cm-3 below T2. The hall data are also consistent with a model in which both a loss of carriers and a drastic change in the conductivity anisotropy occur at T1 and T2 and lend support to the charge-density-wave hypothesis which has been used to characterize the two transitions.

Superconductivity in Nb.95Ta.05Se3

Abstract We have measured the resistivity of NbSe 3 doped with 5% Ta from room temperature down to 0.5K and compared our results with similar measurements on pure NbSe 3 . The pure sample remains normal to the lowest temperature (0.5K), whereas the doped sample has a sharp transition to the superconducting state with T c = 1.5 ± 0.2 K . Measurements of the critical magnetic field indicate that the Ta doped samples are homogeneous, anisotropic three dimensional superconductors.

Young's modulus of NbSe3☆

Abstract Using the Barmatz vibrating reed technique we have measured the Youngs modulus of NbSe3. An anomaly is observed at T1 (142 K) but not at T2 (58 K). Taken together with published specific heat data our results provide upper (lower) bounds for the anisotropy of the transition-temperature-stress-dependence at T1 (T2).

Thermopower of doped and damaged NbSe3

Abstract We have measured the thermoelectric power of pure NbSe3 as well as samples which have been substitutionally doped with isoelectronic Ta and the charged impurity Ti and separate samples which have been radiation damaged by 2.5 MeV protons. We find that 5% Ta doping supresses the lower temperature charge density wave transition. In contrast, the radiation damaged samples and 0.1% Ti samples with larger residual resistivities then the Ta doped samples retain the CDW transitions. A discussion is given of the difference between doping and radiation damage.

Effects of radiation damage on the charge density wave transitions in NbSe3

Abstract We have measured the electrical resistivity of NbSe 3 samples which have been radiation damaged with 2.5 MeV protons up to a defect concentration of 0.5%. We find that, unlike substitutional impurities, the defects do not destroy the charge density wave (CDW) transitions and the samples do not go superconducting. The defects become more effective scatterers below the CDW transitions so that the defect resistivity is temperature dependent. The defects pin the CDWs randomly so that carriers in the unnested regions can be scattered by the CDW. This leads to an enhancement of the defect resistivity. The resistivity of the highly damaged samples is still increasing with decreasing temperatures to below 1 K.

Frequency pulling and heterodyning observed in voltage spectrum of the Fröhlich conductor NbSe3

Abstract The voltage power spectrum of NbSe 3 has been studied in the upper charge-density wave (CDW) state where only one CDW exists. In the non-Ohmic regime sharp spikes appear in the spectrum. The frequency of these spikes increases monotonically with current as in the lower CDW state. We study the “fine structure” of the spectrum and find evidence for “frequency pulling” between the modes. A linear coupled-mode treatment is used to model this effect. There is also evidence of heterodyning of an intrinsic (current independent) 2MHz mode. We briefly discuss these phenomena in terms of Frohlich sliding conductivity. Difficulties with the simple two-fluid model as well as models based on solid state turbulence are pointed out.

Thermal conductivity of NbSe3

Abstract The thermal conductivity, κ, of NbSe 3 has been measured by novel self-heating techniques that allowed the electric field dependence of κ to also be measured. Measurements were made from 35 K to room temperature. Above the charge density wave transitions, the phonon thermal conductivity is 4–7 times the electron thermal conductivity, and it rises smoothly below the transitions, indicating that phonon-phonon scattering predominates. Phonon mean free parths have been estimated at 187 A° at 60 K and 60 A° at 150 K. No clear anomalies were observed at the phase transitions, giving upper limits to changes in the phonon mean free path. No field dependence of κ was observed.

Microwave and Hall Studies of TaS3 and NbS3

Abstract The microwave conductivity of TaS3 and NbS3 have been measured. In both compounds the conductivity at 9.8 GHz shows very large enhancement over the dc value. A brief discussion based on contributions from charge-density-wave oscillation is given for TaS3. The Hall effect at room temperature for TaS3 is found to be |RH| = 6.3 × 10−4 C/m3. This corresponds to a carrier density of 1 × 1022 cm−3 and a mobility of 2 cm2/vs. A comparison with the figure obtained from conduction noise measurements will be made.

Far-infrared shifted cyclotron resonance in a charge-density-wave system, NbSe3

Abstract Cyclotron resonance studies on the charge-density-wave metal NbSe 3 at 2 Kelvin reveal two types of free carriers (with effective mass 0 3 .18m 0 and 0.71 m 0 respectively.) The measured lifetimes agree with earlier estimates of the dc mobility. An unexplained shift of the resonance field of the form ω 0 2 + ω c 2 is observed when the laser electric field is polarized perpendicular to the chain direction.

Far-infrared and high-field magnetotransport studies on a new superlattice: HgTe/CdTe

Abstract Far-infrared (FIR) photoconductance studies on HgTe/CdTe superlattice (thickness 110/220 A.) show an absorption line near 16 cm -1 associated with a resonant acceptor state. Quantum oscillations in ϱ xx and ϱ xy are consistent with an electron density n s of 6 × 10 11 cm -2 . Combining the ϱ xx data with the FIR data we deduce a Fermi energy of 45 mev. A temperature dependent high-field anomaly in ϱ xx near 20 T suggestive of a new electronic phase is also observed.