Charge Density Wave Condensate Research Articles

Charge-density-wave transport in orthorhombic TaS3. III. Narrow-band "noise"

Narrow-band noise measurements are reported for the linear chain Peierls semiconductor, orthorhombic Ta${\mathrm{S}}_{3}$. In the nonlinear conductivity region, sharp narrow-band noise peaks are observed with several harmonics. The amplitudes of the peaks are temperature dependent, starting from zero just below the charge-density-wave (CDW) transition at ${T}_{P}\ensuremath{\sim}215$ K, and remaining approximately constant below $T=170$ K. Between ${T}_{P}$ and approximately 100 K, where both the field- and the frequency-dependent conductivities suggest highly coherent response of the CDW condensate, the noise spectrum is simple (fundamental and harmonics). At low temperatures the noise spectrum becomes quite complicated, with many fundamental and harmonic peaks present, due to a loss of the transverse coherence in the material and indicative of a disordered CDW state. At intermediate temperatures a linear relationship is found between the excess CDW current and the fundamental noise frequency. Analysis in terms of coherent motion of the CDW indicates that the characteristic distance associated with the narrow-band noise is approximately equal to one CDW wave-length $\ensuremath{\lambda}$ and that pinning is dominated by impurities rather than by commensurability. A comparison of the data is made with the predictions of the classical model of CDW transport and with Bardeen's theory of coherent current oscillations. The noise observations are compared to and contrasted with related measurements performed on the similar CDW material Nb${\mathrm{Se}}_{3}$.

Charge density wave transport in a novel inorganic chain compound, (TaSe 4) 2I

We report the observation of nonlinear conductivity with a well-defined threshold electric field E T, and frequency dependent ac conductivity, in a novel linear chain compound, (TaSe 4) 2I. The material undergoes a phase transition to a semiconducting phase at T ∼ 260 K, and nonlinear and frequency dependent transport is observed below this temperature. We argue that the material is a new example of collective mode transport provided by a Peierls-Fröhlich charge density wave condensate.

AC-DC coupling experiments in the spin density wave state of tetramethyltetraselenofulvalene-hexafluorophosphate [(TMTSF) 2PF 6

Experiments performed on (TMTSF) 2PF 6 in the spin density wave (SDW) state in the presence of joint ac and dc excitations do not show evidence for coupling between dc and ac response. Irradiation by microwaves does not lead to strong dc nonlinearity, and attempts to resurrect the ESR signal by applied dc currents were also unsuccessful. While the strongly frequency dependent response is indicative of collective effects, the response of the SDW ground state is fundamentally different from that of a charge density wave condensate.

Charge-density-wave transport in orthorhombic TaS3. I. Nonlinear conductivity

dc conductivity measurements are reported on the linear chain compound Ta${\mathrm{S}}_{3}$ (orthorhombic form), which undergoes a phase transition to a charge-density-wave (CDW) state at ${T}_{P}=215$ K. Strong resistive fluctuations, associated with the buildup of one-dimensional correlations, are observed above ${T}_{P}$. Below the transition nonlinear conductivity is observed when the electric field $E$ is larger than a threshold field ${E}_{T}$. ${E}_{T}$ is approximately 2 V/cm, and is weakly temperature dependent. The functional dependence of the conductivity $\ensuremath{\sigma}$ on $E$ is analyzed in terms of various models. A good agreement is found with Bardeen's tunneling model, which leads to a field-dependent conductivity $\ensuremath{\sigma}(E)\ensuremath{\sim}(1\ensuremath{-}\frac{{E}_{T}}{E})\mathrm{exp}(\ensuremath{-}\frac{{E}_{0}}{E})$, but Fleming's empirical formula also provides a good fit to the data. The low-threshold field and the strong field dependence indicate small pinning energies and large coherence and correlation lengths. At low temperatures the coherent response of the CDW condensate is absent, and we argue that, because of the loss of coupling between the CDW segments, a disordered CDW state develops. These observations are compared with those made on a similar linear chain compound, Nb${\mathrm{Se}}_{3}$.

Transport studies near phase transitions in NbSe3

The non-Ohmic transport properties of Nb${\mathrm{Se}}_{3}$ are studied near the two phase transitions at ${T}_{1}$ and ${T}_{2}$. From the non-Ohmic data a parameter $\ensuremath{\alpha}$ is obtained which provides information on the fraction of Fermi surface affected by the phase transitions. By relating $\ensuremath{\alpha}$ to a BCS-type gap and modifying the calculations of Fedders and Martin on the spin-density-wave state in Cr we have obtained a theoretical curve for $\ensuremath{\alpha}$ which agrees well with the experimental data. This good agreement provides new indirect evidence for the charge-density-wave (CDW) state in Nb${\mathrm{Se}}_{3}$. The derivative of the low-field resistivity versus temperature has also been measured and is shown to be divergent at ${T}_{1}$ and ${T}_{2}$. The index of this divergence is presented and its significance to the dimensionality of the system is discussed. Finally, we briefly discuss the possibility of explaining the non-Ohmicity below ${T}_{1}$ in terms of nonlinear excitations in a CDW condensate.

Creation of Soliton Pairs by Electric Fields in Charge-Density—Wave Condensates

The creation probability of soliton-antisoliton pairs due to quantum-mechanical tunneling in the presence of an electric field in charge-density---wave condensates is calculated. This process gives rise to an additional electric conductivity strongly nonlinear in the electric field, which may account in part for the anomalous conductivity observed in Nb${\mathrm{Se}}_{3}$.