Nonlinear conductivity and narrow band noise in NbSe3

A charge density wave system near commensurability and with strong damping is considered, as a model for NbSe₃. The observed threshold field is associated with depinning of a commensurate part of the charge density, while the excess charge, in form of phase kinks, contributes just to the ohmic conductivity. The characteristic length associated with the frequency generation is one lattice constant.     

Charge-density-wave conductivity in NbSe3

The nonlinear conductivity in the charge-density-wave states in NbSe₃ was measured up to the high-field limit. Ohmicity recurs in this limit. The temperature dependence of σ_eo, the extra conductivity in the high-field limit, is in agreement with that of carrier concentration condensed into the CDW, providing an evidence that the nonlinear conductivity is due to CDW motion. The magnitude of σ_eo is very sensitive to the impurity concentration. The mobility of the drifting CDW's is temperature independent and the damping of them is mainly due to the impurity scattering in even pure crystals.     

Thermal conductivity of NbSe3

The thermal conductivity, κ, of NbSe₃ 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.     

Spectrum of the quasiperiodic noise in NbSe3

An additional fundamental frequency in the quasiperiodic noise generated by NbSe₃ is obtained in a model which assumes the presence of objects which move more slowly than the sliding charge density wave. The objects are driven by their interaction with the charge density wave and the lattice potential. The model is in close agreement with the observations of Richard, Monceau and Renard.     

Phase transitions in NbSe3

We describe the physical properties of the transition metal trichalcogenide NbSe₃ including electrical resistivity, magnetic susceptibility, and heat capacity. NbSe₃ undergoes phase transitions at 145 and 59 K. The effect of pressure on these transitions is also reported. We try to explain the physical properties of NbSe₃ in terms of the formation of charge density waves.     

Spectral purity and phase coherence of narrow-band noise in NbSe3

An unusually simple narrow band noise spectrum with a single fundamental and seven harmonics due to depinned charge density wave motion in NbSe₃ is reported. The simplicity of the spectrum is attributed to a very uniform current distribution associated with a short, thin sample of high perfection. The distribution of harmonics is near to that of the sawtooth waveform generated by a relaxation oscillator. As indicated by the amplitude ratio of the fundamental to the first harmonic, this distribution has only a moderate variation over three orders of magnitude in frequency. An electrical relaxation oscillator analog which reproduces most of the properties of CDW motion in NbSe₃ is discussed. A search for phase memory of the narrow band noise in NbSe₃ showed it to be less than one period.     

Magnetic susceptibility of NbSe3 and TaSe3

The magnetic susceptibility of NbSe₃ shows a decrease beginning slightly above its upper charge density wave transition (CDW) of 144 K, but no change within our resolution near the 59 K transition. The change in the density of states at the Fermi level due to the upper transition is 0.14 states-eV/Nb. TaSe₃ on the other hand has a temperature independent susceptibility. In some cases the trichalcogenides are contaminated with their corresponding dichalcogenide. Such contamination can be observed by susceptibility measurements in the case of 2HTaSe₂ but not of 2HNbSe₂. We also report an anomaly in the susceptibility of 4HaNbSe₂, which suggests a CDW transition at 45 K.     

Mixing experiments in NbSe3

The response of NbSe₃ to combined a.c. and d.c. fields has been characterized at T = 125 K below the first charge-density wave (CDW) transition. The a.c. and d.c. conductances were measured, along with the rectification and harmonic mixing at megahertz frequencies due to the CDW non linearity. These experimental results are shown to be consistent with a revised tunneling theory of CDW depinning.     

Shubnikov-de Haas oscillations in NbSe3

We have measured the resistivity of the linear transition metal trichalco-genide NbSe₃ in magnetic fields up to 180 kG. We observe large Shubnikov-de Haas oscillations. The measurements reveal a strong anisotropy in the a.c. plane perpendicular to the chain direction. A ratio of 3 in the cyclotron effective masses for the two orthogonal directions in this plane is determined. Spin splitting occurs for high magnetic field, and we obtain a Landé factor g of 2.45.     

Transverse conductivity behavior at threshold electric field for CDW sliding in NbSe3

We have measured the dynamical properties which occur in the transverse direction to the conducting chains in NbSe₃ single-crystals, when the CDW slides along the chains. A sharp decrease in transverse conductivity takes place above an electric field less than the longitudinal threshold one for CDW sliding; that may result from induced phase shifts between CDW chains. Under the joint application of dc and rf driving fields voltage Shapiro steps for longitudinal transport are observed as usual but also pronounced current Shapiro steps in transverse direction. The possible mechanisms of this effect as well as a tentative new view on the origin of the narrow band noise in CDW compounds are discussed.     

Thermopower of doped and damaged NbSe3

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.     

Evidence for extrinsic superconductivity in NbSe3

Detailed resistivity measurements and X-ray and microscopic examinations revealed that pure NbSe₃ does not show superconductivity at least above 0.38K at ambient pressure. The superconductivity with the sluggish, current-sensitive transition curve, which has been observed frequently, is extrinsic and arises only from the crystal boundaries in the polycrystalline sample.     

Low temperature phase transition in the chain-like compounds NbSe3 and TaSe3

New transitions have been observed at low temperature by electrical measurements on the linear trichalcogenides NbSe₃ and TaSe₃. The resistivity of TaSe₃ drops near zero below 1.5 K which may indicate a superconducting transition. The resistivity of NbSe₃ drops below 2.2 K by 30 to 75% of its residual value at 4.2 K. However for TaSe₃ and NbSe₃ initial susceptibility measurements do not show any increase in diamagnetism indicating a flux expulsion. In addition the transitions below the critical temperatures are strongly non-linear with current density higher than 10^-3 A mm^-2.     

Far infrared optical properties of NbSe3

We have measured the far infrared reflectance of NbSe₃ and used models of the frequency dependent conductivity to fit the data. General arguments show that at 2 K a charge density wave (CDW) energy gap exists between 120 and 190 cm^?1, the relaxation time(s) of the free carriers and CDW pinned mode is >30×10^?12 s, and the ratio of the free carrier concentration to band mass is <2×10²⁰ cm^?3/m₀.     

Chemical bonding and the dimensionality of NbSe3

The electron energy band structure of NbSe₃ is presented for a single one-dimensional prismatic chain and for a two-dimensional layer. In the former case the Fermi level falls in the middle of an isolated band. Interchain bonds in the 2-D crystal lead to semimetallic behaviour and a strong pressure dependence of n(E_F).     

Charge density wave depinning and switching in NbSe3

We have investigated the relation between charge density wave (CDW) depinning and switching in NbSe₃. In the lower CDW state we observe that the critical electric field at which switching occurs is independent of temperature. We also observe that the differential resistance of the sample is independent of applied dc bias beyond the switching threshold, and corresponds to the high-field limit of the pure resistance. We interpret our results in terms of a temperature dependent CDW domain structure in the crystal.     

Electric field dependent dielectric function in NbSe3

Dielectric function of quasi-one-dimensional conductor NbSe₃ has been measued in the vicinity of Peierls transition near T_p=145K by use of novel technique. The low frequency noise in V/I/ characteristics is attributed to the Gaussian type of position fluctuations of pinned charge density wave (CDW) and fundamental frequency of the noise is found to be identical with spring frequency of restoring forces.     

The thermoelectric power of monoclinic TaS3. Comparison with orthorhombic TaS3 and NbSe3.

The thermoelectric power of monoclinic TaS₃ was measured over the temperature range of 90K to 430K. It is positive at high temperatures, negative between the two Peierls transitions and positive at low temperatures. The changes in sign occur at the transitions. The results obtained at high temperatures are compared with those for NbSe₃ and orthorhombic TaS₃.