M. Renard

Nonlocal Properties and Long-Range Phase Coherence in Charge Density Wave Transport

We show nonlocal effects in charge density wave depinning by the reduction of the threshold field of a short segment of a NbSe3 sample when the CDW slides in a neighbouring segment separated by a distance d. When d = 0 the CDW with the faster velocity pulls the slower one whatever the respective segments length which reveals the infinite coherence length of the moving wave.

Breaking of the CDW coherence in various inhomogeneous situations

Abstract Recent works tend to prove that the time-dependent voltage detected in the non-linear state of a CDW conductor is generated by phase-slippage centers or vortices created at some interface where the CDW velocity undergoes a discontinuity ; this evidently is the case near the contacts but can also be achieved in some places in the bulk of the samples when inhomogeneous conditions are imposed. We report some results concerning the breaking of the CDW coherence 1) by electrode injection effects, 2) by thermal gradient.

f-α noise in NbSe3

The authors have measured the voltage noise of NbSe3 samples in the nonlinear state as a function of frequency for temperatures below the lower charge density wave formation temperature. They find that over broad conditions of electric fields and temperatures the noise power spectrum follows a f- alpha dependence with alpha approximately 0.8. They interpret the broad-band noise as due either to the mixing of radiofrequency fields radiated by the domains which form the charge density wave lattice, or alternatively to the dynamics of kinks-antikinks along the chains.

Threshold field, electrical conductivity and time-dependent voltage in transition metal tri- and tetrachalcogenides

Non-linear transport properties have now been measured in numerous one-dimensional transition metal tri- and tetrachalcogenides. For all these compounds the threshold electric field above which the non-linear state appears goes through a minimum in the vicinity of the Peierls temperature transition. The value of this minimum is all the higher as the Peierls transition occurs at a higher temperature. The variation of the fundamental frequency of the time-dependent voltage is shown to follow a (E-Ec)y law (with y ∼ 1.5) near the threshold indicating a collective pinning. Finally non-linear properties of a NbSe3 sample have been studied when both contacts are above the Peierls temperature : in these conditions an a.c. voltage is still detected. This result is discussed in relation with theories on the origin of this voltage generation.

A.C.—D.C. coupling of the motion of charge density waves in NbSe3

Abstract We report measurements of the non-linear d.c. characteristic of NbSe3 in the CDW state when we apply an a.c. field. When this a.c. field is superposed to a d.c. field we have studied the variation of the critical d.c. field for several frequencies and amplitudes of the a.c. field. When there is no d.c. bias, we have measured the d.c. critical field as a function of the frequency and put into evidence new relaxation times in the motion of the CDW.

Study of the Differential Resistance dV/dI as a Function of the Electric Field in the CDW State of NbSe3

Abstract We have measured the variation of the differential resistance as a function of the electric field for temperatures below those at which the two CDWs form in NbSe3. dV/dI decreases above a critical electric field Ec. We have studied the variation of Ec with the amount of impurities as tantalum, zirconium, titanium. The shape of dV/dI is very sensitive to the temperature especially below that where the resistive anomaly due to the lower CDW shows its maximum. We have observed for same samples a very sharp drop with a deep minimum in dV/dI at Ec. Over a large range of electric field below this dip we have measured low frequency noise without decrease in dV/dl. We also have studied the depinning of the CDW by a ac field with large amplitude.

Non-Linear Response of Transition Metal Tri-and Tetra-Chalcogenides

Abstract Electric field dependence of the d.c. electrical conductivity and of the fundamental frequency (i.e. the charge density wave velocity) for the transition metal tri-and tetrachalcogenides exhibiting CDW transport is compared to the tunneling model and to the classical collective pinning theory.

Dynamical Properties of the CDWs in NbSe3

Abstract Above the electric critical field Ec where the CDWs in NbSe3 are depinned, noise is generated in the sample. This noise consists of broad band noise and quasi periodic noise. For many samples we have measured the fundamental frequency ν in the noise as a function of E for the two CDWs. These frequencies can be synchronised by an external rf field as seen by peaks in the dV/dI characteristic. These peaks correspond to an increase in the differential resistance. We show that the current JCDW carried by the CDW is the superposition of a continuous one and a modulation due to the pinning with the fundamental frequency ν that we calculate as a function of the electric field E. When ν is calculated as a function of JCDW we obtain a linear variation where the slope is proportional to the number of electrons affected in the band by the CDW gap. We verify the linear variation between JCDW and ν and we find the same slope for the two CDWs. We deduce that the number of electrons affected by the gap is aroun...