CDW Conductor Research Articles

Photoconduction in CDW conductors

Photoconduction study of quasi-1D conductors allows to distinguish between the single-particle and collective linear conduction, investigate the effect of screening on collective transport and obtain interesting new details of the electronic energy structure of pure and doped CDW conductors. Here we present results of photoconduction study in quasi-1D conductors o-TaS3, K0.3MoO3, and NbS3(I).

Solitons: From charge density waves to FFLO in superconductors

This short review aims to summarize on "What the Charge Density Waves can tell to other inhomogeneous states in strongly correlated systems, particularly to spin-polarized superconductors". We shall update on expanding observations of solitons in quasi 1D CDW conductors and link them to the growing information and demands related to inhomogeneous spin-polarized states in superconductors. The related theory, existent or awaited for, stretches from solitons in 1D models to vortex-like elementary excitations in 2D,3D ordered incommensurate CDWs and superconductors.

Photoconduction and photocontrolled collective phenomena in Peierls conductor TaS3

Light illumination of thin crystals of CDW conductor TaS$_3$ is found to result in dramatic changes of both linear ($G$) and nonlinear conduction. The increase of $G$ is accompanied by suppression of the collective conduction, growth of the threshold field $E_T$, and appearance of the switching and hysteretic behavior in the nonlinear conduction. The effects in the nonlinear conduction are associated with increase of CDW elasticity due to illumination that leads in particular to appearance of a relation $E_T\propto G^{1/3}$ expected for the one-dimensional pinning.

Transverse correlations and plasticity in the CDW conductor NbSe3studied by X-ray microbeam diffraction

In whisker-like samples of the quasi-1D conductor NbSe 3 , the presence of longitudinal steps causes shearing of the CDW, and leads to a loss of transverse correlations. We use a microdiffraction setup with a spatial resolution of 300 nm and an angular sensitivity of 5 mdeg to image the resulting CDW contrast between thick and thin portions of the sample. Microdiffraction in the b* - c* plane shows that depinning on the thick, weakly pinned side is accompanied by the loss of diffraction intensity, demonstrating a loss of correlations in qualitative agreement with previous X-ray diffraction topography measurements 1 , but with an order-of-magnitude improvement in spatial resolution. Microdiffraction images in the a* - b* plane reveal a sharp increase in the full width at half maximum in an approximately 1 micron thick region near the step edge and a rotation of the CDW wavevector that varies with applied field. We use the extremal value of the CDW wavevector rotation to estimate the shear modulus of this electronic crystal.

Photoconduction and photocontrolled collective effects in the Peierls conductor TaS3

Light illumination of thin crystals of CDW conductor TaS3 is found to result in dramatic changes of both linear (G) and nonlinear conduction. The increase of G is accompanied by suppression of the collective conduction, growth of the threshold field E T , and appearance of the switching and hysteretic behavior in the nonlinear conduction. The effects in the nonlinear conduction are associated with increase of CDW elasticity due to illumination that leads, in particular, to the appearance of a relation E T ∝ G 1/3 expected for the one-dimensional pinning.

Low-temperature specific heat in the charge-density-wave state of ZrTe 3 and NbTe 4

Low-temperature specific heat of CDW compounds ZrTe 3 and NbTe 4, measured between 1.5 and 10 K , exhibited an anomaly which deviates from the regular phonon term; an excess specific-heat contribution appeared around T=6 K . The excess contribution was well presented in terms of the phason contribution of CDW. The pinning frequency estimated from the analysis was quite high in comparison to the value of common CDW conductors, which is not inconsistent with the linear relation between current and voltage.

Modification of charge density wave fluctuations by charge perturbations

Spectral density of fluctuations of the CDW phase are calculated taking into account electric field induced by phase fluctuations. The approach based upon the fluctuation-dissipation theorem (FDT) combined with equations of linear response of the CDW conductor is used. Fluctuating electric field is found to suppress fluctuations of the phase, while fluctuations of the electric potential are sizeable. This suggests that transition from the CDW to the normal state (which is usually observed well below the mean-field transition temperature) may he provoked by fluctuations of the chemical potential, rather than by destruction of the CDW coherence between conducting chains due to phase fluctuations.

Dynamics of charge density wave phase-slips in mesoscopic samples

Theoretical model of current conversion from normal to collective current in mesoscopic samples of CDW conductors is presented. The current conversion is described in terms of phase slips (PS) induced by strain of the CDW and stimulated by thermal fluctuations. Kernels of PS situated near contacts are surrounded by long-range perturbations of the CDW phase. If spacing between contacts is shorter than decaying length of these perturbations then temporal evolution of PS at different contacts becomes correlated, and PS voltage decreases with spacing between the contacts decreasing. The results are in qualitative agreement with experimental study of current conversion in submicron NbSe3 wires.

Fabricated structures for studying mesoscopic physics of charge density waves

We present new methods for patterning and controlling the cross-section of the CDW conductor niobium triselenide crystals to sub-micron dimensions using photolithography and electron beam lithography. Experiments demonstrating these techniques are presented: accurate four-probe measurement of the c-axis resistivity as a function of temperature, and measurements of width and thickness-dependent size effects on the CDW condensate. In all samples, a region with near-perfect rectangular cross-section is left to provide a control, eliminating the need to compare different samples and reduces the role of sample-to-sample variations in the analyses.

Imaging field-dependent structure in charge-density waves by X-ray diffraction topography

We have imaged field-dependent CDW structure in NbSe 3 single crystals by X-ray diffraction topography. Just above E T , the CDW shears along longitudinal steps in crystal thickness associated with small-angle grain boundaries, and at high fields transverse correlations recover. These results demonstrate X-ray topography as an effective probe of CDW structure, and together with earlier transport measurements establish that extrinsic sources dominate the most obvious manifestations of plasticity in this CDW conductor.

Temporally ordered collective creep in NbSe3 : A new window on low-temperature charge-density-wave dynamics

We have observed a new form of collective creep in the CDW conductor NbSe 3 . This creep provides fundamental insight into the relation between local and collective pinning and the mechanism of CDW dissipation, and has broad implications for understanding the CDW response at low temperatures.

Dragging a Charge-Density-Wave Soliton by an Electromagnetic or Acoustic Wave

It is demonstrated that a traveling electromagnetic or acoustic wave drags a phase soliton in a nearly commensurated CDW conductor at a velocity proportional to the square of the wave's amplitude. The traveling-wave-induced drift of the charged soliton gives rise to specific photoelectric (actually, radioelectric) and acoustoelectric effects in the conductor containing “frozen” unipolar solitons. These effects can be used to indirectly detect the CDW solitons, and to estimate their size (the cutoff wavelength of the driving wave coincides with a characteristic size of the soliton).

Search for photoinduced absorption in CDW conductors

Search for photoinduced absorption in CDW conductors

Charge density wave- and contact-effects in NbSe 3 observed with microwave harmonic mixing

It is shown that the method of P hase RE solved H armonic M ixing (PREHM) at microwave frequencies provides a powerful tool in the investigation of the non-ohmic behavior of CDW conductors. The observed PREHM signal is investigated in view of the contacts necessary for the detection of the PREHM signal is possibly being a source of nonlinear behavior, too, and hence contributing to the signal. For comparison, harmonic mixing by a microwave diode has been observed for the first time in order to investigate the behavior of a rectifying contact. The measurements show that in NbSe 3 there is a clear dominance of the CDW contribution over a negligible contact contribution at temperatures below T 1 = 144K.

Glassy behaviour of the ground state of quasi one-dimensional charge density wave conductors

The low-temperature specific heat measurements of CDW conductors : NbSe 3, TaS 3, (TaSe 4) 2l, K 0.3MoO 3 and KCP show characteristic properties at T ⩽ 1 K. They are : (a) an excess contribution in addition to the phonon one, following a power T v law with the exponent v comprised between 0.2 and 0.8 depending on the compound, and (b) a non-exponential energy relaxation following an energy input in the sample. The relaxation rates are largely increased when the thermal perturbation is applied longer.

Electromechanical Properties of Charge-Density-Wave Conductors: AC-DC Coupling Effects

We calculate perturbatively the Shapiro-Type anomalies in Young's modulus Y and internal friction δ of the CDW conductors when a combined ac and dc electric field presents within a classical model of two incommensurate, interacting, many-body system proposed by Sneddon. It is found that this model produces the experimental results very well at the weak pinning. Some experiments are suggested.

Breaking of the CDW coherence in various inhomogeneous situations

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.