Frequency-Dependent Conductivity in NbSe3

Abstract

Niobium triselenide, NbSe 3 , shows two phase transitions, 1 at 145 K (7\) and 59 K (T 2 ) associated with the development of incommensurate charge-density waves (CDW). These transitions lead to the increase of resistivity which is ascribed to the reduction of the Fermi surface; 2 ' 3 the CDW condensate is assumed to be pinned by impurities to give negligible contribution to the dc conductivity at low electric fields. The conductivity increases when the electric field is larger than a threshold field E T and then saturates at high electric fields. 4 " 6 This and the observation of noise in the nonlinear region 6 * 7 together with the x ray observation of the superstructure at high electric fields 8 demonstrate that the depinning of the CDW is responsible for the increase of the conductivity with increasing electric field. The resistive anomalies are also suppressed at microwave frequencies 2 suggesting that the pinning frequency is less than 10 10 Hz.In this paper we report the first observation of frequency-dependent conductivity associated with the incommensurate CDW states in NbSe 3 . We show that the CDW anomalies are suppressed at extremely low frequencies and also lead to a gigantic dielectric constant. The transition between the low-frequency (/<1 MHz) and high-frequency (/>100 MHz) limit is smooth with no edited by M. L. Klein and J.The observation of a frequency-dependent conductivity (o) and dielectric constant (e) in NbSe 3 is reported. In both charge-density-wave phases a strong frequency dependence and huge dielectric constant are observed below 100 MHz, with greatest effects observed at 42 K. The conductivity cr increases smoothly from the dc value to the high-frequency (f = 100 MHz) limit; this increase is accompanied by the reduction of e. A resistancecapacitance network model is suggested to account for the observed frequency dependence.

Sliding Density Wave in Sr 14 Cu 24 O 41 Ladder Compounds

“…Such was the case in CDW systems (15,16,17,18,19,20). The transfer of spectral weight to the longitudinal channel explains why the oscillator strength in the microwave region (8) is anomalously small.…”

“…This pinning shifts the oscillator strength associated with the collective modes to finite frequencies, with no contribution to the dc conductivity that shows an activated behavior and is entirely determined by quasi-particle excitations out of the condensate. The Arrhenius law for the decay constant Γ(T ) suggests a hydrodynamic origin for the Raman and lowfrequency conductivity modes, for which there is considerable precedent in studies of the dynamics of pinned CDW and SDW systems (15,16,17,18,19,20), and we compare the Sr 14 Cu 24 O 41 ladder system to well-established models of CDW dynamics (21). The pinned collective mode can be described by an oscillator making a collective contribution to the ac conductivity…”

Huge values of room-temperature dielectric constants in pellets of poly(3-methylthiophene)

“…It is well established that in charge-density wave (CDW) systems, there is a dipole moment associated with the phase mode, and huge values of dielectric constant have been measured at low frequencies in conventional charge-density wave materials like NbSe 3 and TaS 3 . 14 The CDW system was modeled 15 as a resistance due to normal electrons in parallel with the CDW condensate contribution that can be represented by a resistance in series with a capacitance. The normal electrons contribute to a frequency-independent conductivity.…”

Dynamical Properties of Quasi-One-Dimensional Conductors: A Phase Hamiltonian Approach

“…Ihe remarkable w-dependence of the ac conductivity even at low frequencies w is another feature associated with the CDW condensate [171,192] . In the case of a strong damping in the classical particle model, the dynamics is an overdamped oscillation yielding the ac conductivity of Equation (5.14).…”