Investigations of the nonohmic behavior of NbSe3 by the method of microwave harmonic mixing

The transition metal trichalcogenide NbSe3, belonging to the group of quasi-onedimensional conductors, exhibits new exciting phenomena in the mechanism of electrical conduction. It shows two independent charge density waves (CDW's) caused by superlattice formation below two phase transition temperatures T1=144 K and T2=59 K. Due to the depinning of CDW's the electrical conductivity is dramatically raised at threshold fields of a few 100 mV/cm e.g. for the low temperature phase transition. This research concentrates on the investigation of the nonohmic behavior of NbSe3 by the extremely sensitive method of phase resolved microwave harmonic mixing, which is done as a function of temperature, microwave power and sample position. The measurements show that below T1 and T2 the amplitude ΔU of MHM is suddenly raised with decreasing temperature due to the development of CDW's in the NbSe3-crystal. The transitions into the CDW-states occur without any change in phase shift psi, while below T2 structures are observed in the temperature dependence of psi. The experimental determination of incident, reflected and transmitted microwave powers allows the calculation of maximum electric field strengths in the samples, which are necessary to obtain the absolute value of the deviations from Ohm's law, namely the nonlinearity coefficient β. Two different theories are examined to explain the experimental facts. It is found that the model of photon assisted tunneling of CDW's as proposed by Bardeen yields agreement with results of microwave harmonic mixing in NbSe3, while the theory of an overdamped oscillator by Gruener et al. does not seem to be applicable. The discussion of theoretical shortcomings leads to the opinion that MHM allows the observation of the dominant CDW-domain in the crystal without influence of coupling processes between CDW's. (Author)