High frequency conductivity in the charge density wave semiconductor TaS3

We report the observation of frequency dependent conductivity σ (ω) in the charge density wave (CDW) semiconductor TaS₃. Based on σ (ω) and other evidences, we sugest that three different temperature regions occur in this compound: 1-D fluctuating CDW region above T_MI, a coherent CDW state below T_MI, and a CDW glass state at low temperatures.     

A first-principles study of the electronic structure of the sulvanite compounds

We have investigated by means of first-principles total energy calculations the electronic structure of the sulvanite compounds: Cu₃VS₄, Cu₃NbS₄ and Cu₃TaS₄; the later is a possible candidate as a p-type transparent conductor with potential applications in solar cells and electrochromic devices. The calculated electronic structure shows that these compounds are indirect band gap semiconductors, with the valence band maximum located at the R-point and the conduction band minimum located at the X-point. The character of the valence band maximum is dominated by Cu d-states and the character of the conduction band minimum is due to the d-states of the group five elements. From the calculated charge density and electron localisation function we can conclude that the sulvanite compounds are polar covalent semiconductors.     

Broad band noise associated with the current carrying charge density wave state in TaS3

We report the observation of low frequency broad band noise in the current carrying charge density wave (CDW) state of orthorhombic TaS₃. The noise amplitude is proportional to the number of condensed electrons, and we suggest that the broad band noise reflects the dynamics of the internal deformations of the CDW condensate.     

Study of the optical properties of pyridine intercalated niobium disulphide by electron energy loss spectroscopy

High energy electron energy loss spectroscopy (EELS) has been used to study the influence of the intercalation with pyridine on the optical properties of the host material 2H-NbS₂. The optical joint density of states function OJDOS(ω) of the intercalated material is found to be well represented by a linear superposition of the OJDOS of pure NbS₂ and that of pyridine. A charge transfer from pyridine to NbS₂ of (0.27±0.05) electrons per NbS₂ formula unit is deduced from the energy shift of the d-band plasmon loss relative to the pure host material. Assuming this shift to be caused by a change in the charge density contributing to the d-band plasmon is confirmed by an unchanged effective mass of the electrons in the d_z2 band upon intercalation as has been deduced from the dispersion of the plasmon in question.     

Growth,crystal structure and transport properties of quasi one-dimensional conductors NbS3

We report synthesis of quasi-one-dimensional conductor NbS₃, TEM studies and transport properties of the prepared samples. The grown NbS₃ whiskers are Peierls conductors known as phase II with the transitions at T_P1=365 K and T_P2=150 K. CDW1 is stable and not so sensitive to growth conditions. It can slide and be synchronized by the external microwave irradiation up to 16 GHz. CDW2 strongly depends on growth conditions. Nevertheless, it also shows sliding and synchronization.     

Dynamic properties of an incommensurate charge density wave in monoclinic TaS3 at low temperatures

The permittivity of monoclinic TaS₃, a quasi-one-dimensional conductor with an incommensurate charge density wave (CDW), as a function of frequency and temperature has been studied. At low temperatures and at frequencies below 1 MHz, the temperature dependence of the real part of the permittivity shows a maximum shift to lower temperatures with decreasing frequency. The temperature dependence of the relaxation time consists of two branches corresponding to macroscopic regions of CDWs with long and short relaxation times τ on the microscopic scale. With decreasing temperature, the growth of τ for large CDW regions is faster than thermal activation and shows a tendency to diverge at a finite temperature while the growth of τ due to the relaxation on the microscopic scale is slower than the activation rate. Our results show that with decreasing temperature the m-TaS₃ quasi-one-dimensional conductor goes over to a glasslike state due to the strong pinning of CDWs by randomly distributed impurities and the formation of mutually interacting solitary CDW collective excitations. Zh. éksp. Teor. Fiz. 111, 988–1000 (March 1997)     

Raman spectroscopy of intercalated layered structure compounds

The Raman spectra of intercalated 1TTaS₂ and 2HTaSe₂ have been measured for the first time. Normal symmetry allowed optic phonons and charge density wave induced modes are observed in both systems. The spectra of the ethylenediamine intercalated compounds are qualitatively similar to those of the pure materials with small quantitative shifts in frequency and changes in the relative intensities of the CDW induced modes. The qualitative similarities in the spectra of the pure and the intercalated compounds reflect the two dimensional character of these materials and show that the dynamic properties of the charge density wave states are largely determined by intralayer effects.     

Interaction of both charge density waves in NbSe3 from interlayer tunneling experiments

An interlayer tunneling technique has been used for spectroscopy of charge density wave (CDW) energy gaps (Δ_1,2) in NbSe₃ subsequently opened at the Fermi surface on decreasing temperature at T _p1 = 145 K (CDW1) and at T _p2 = 60 K (CDW2). We found that the CDW2 formation is accompanied by an increase of the CDW1 gap below T _p2. The maximum enhancement of Δ₁, δΔ₁ is about 10%. The effect observed has been predicted theoretically as resulting from the joint phase locking of both CDWs with the underlying crystalline lattice below T _p2. The text was submitted by the authors in English.     

Photo-induced enhancement of charge density wave current in the quasi-one-dimensional conductor TaS3

In the near vicinity of Peierls transition temperature T_P, we have measured the V-I characteristics of the quasi-one-dimensional conductor TaS₃ under dark and photo-irradiation conditions. It is found that a significant enhancement of CDW current occurs only around the threshold voltage V_t under photo-irradiation. This effect can be interpreted as a result of screening of pinning potential for CDW condensate by photo-excited quasi-particles (QP's). Further the distribution of pinning potential intensity is reflected in the behavior of V-I characteristics near V_t. Our finding suggests that the strength of pinning potential can be controlled by the photo-excited QP's in quasi-1D conductors.     

a.c. Conductivity of nearly commensurate charge density waves and application to NbSe3 and TaS3

The a.c. conductivity of an overdamped one dimensional sine-Gordon system with a low density of kinks is evaluated. This corresponds to a nearly commensurate charge or spin density wave with a kink-lattice representing the deviation from commensurability. The results explain the unusually broad crossover regime of NbSe₃ as compared with the similar but more commensurate compound TaS₃. When TaS₃ becomes commensurate (T ? 130 K) we predict that its a.c. response increases and its shape slightly sharpens.     

Breathing charge density waves in intrinsic Josephson junctions

We demonstrate the creation of a charge density wave (CDW) along a stack of coupled Josephson junctions (JJs) in layered superconductors. Electric charge in each superconducting layer oscillates around some average value, forming a breathing CDW. We show the transformation of a longitudinal plasma wave to CDW in the state corresponding to the outermost branch. Transition between different types of CDW’s related to the inner branches of IV characteristic is demonstrated. The effect of the external electromagnetic radiation on the states corresponding to the inner branches differs crucially from the case of the single JJ. The Shapiro steps in the IV characteristics of the junctions in the stack do not correspond directly to the frequency of radiation ω. The system of JJs behaves like a single whole system: the Shapiro steps or their harmonics in the total IV characteristics appear at voltage $\sum {V_l } = N_R \frac{m} {n}\omega$ , where V _l is the voltage in the lth junction, N _R is the number of JJs in the rotating state, and m and n are integers.     

Harmonic and subharmonic shapiro steps in orthorhombic Tas3

Observation of harmonic and subharmonic Shapiro steps in orthorhombic TaS₃ in the presence of combined rf and dc drive is reported, demonstrating that both the harmonic and subharmonic steps are a general characteristic of charge-density-wave response. Compared to NbSe₃, the noise spectrum is typically much broader in TaS₃, leading to poorly defined steps at small rf voltages. The steps sharpen with larger amplitudes of rf. Increased rf amplitude is also accompanied by an increase in the CDW current density at a given step, but this increase approaches a limiting value for large rf levels. The results are interpreted as evidence for increased coherence of the CDW response in the presence of large rf driving fields.     

Dephasing of Andreev pairs entering a charge density wave

A Cooper pair from a s-wave superconductor (S) entering a conventional charge density wave (CDW) below the Peierls gap dephases on the Fermi wavelength while one particle states are localized on the CDW coherence length ξ_CDW. It is thus practically impossible to observe a Josephson current through a CDW. The paths following different sequences of impurities interfere destructively, due to the different electron and hole densities in the CDW. The same conclusion holds for averaging over the conduction channels in the ballistic system. We apply two microscopic approaches to this phenomenon: (i) a Blonder, Tinkham, Klapwijk (BTK) approach for a single highly transparent S-CDW interface; and (ii) the Hamiltonian approach for the Josephson effect in a clean CDW and a CDW with non magnetic disorder. The Josephson effect through a spin density wave (SDW) is limited by the coherence length ξ_SDW, not by the Fermi wave-length. A Josephson current through a SDW might be observed in a structure with contacts on a SDW separated by a distance ξ_SDW.     

Properties of strained TaS<Subscript>3</Subscript> samples in the state of charge density wave and in the normal state

The uniaxial strain of quasi-one-dimensional conductor whiskers of orthorhombic TaS₃ at a strain higher than ε _c ~ 0.8% leads to a sharp increase in the coherence of the properties of a charge density wave (CDW), which manifests itself in its motion in fields higher than threshold field E _t . During uniaxial elongation, TaS₃ is shown to exhibit the following unusual properties even in weak fields: Peierls transition temperature T _P depends nonmonotonically on ε, one-dimensional fluctuations weaken near T _P , and the coherence length of a charge density increases at T < T _P . Investigations in fields higher than E _t show that the ultracoherent properties of CDW exist in a wide temperature range and are retained when temperature increases up to T _P . These properties of CDW make it possible to observe a sharp increase in E _t near T _P and an almost jumplike increase in E _t at T < 90 K. The increase in E _t at T _P is explained by a decrease in the coherence volume of CDW because of a fluctuational suppression of the Peierls gap.     

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, K_0.3MoO₃, and NbS₃(I).     

Current induced deformation of charge density waves in orthorhombic TaS3

The low electric field ohmic resistance R of orthorhombic TaS₃ measured at 90 and 120 K well below the Peierls transition temperature depends on the product of a temperature difference ΔT applied along the sample and the sign of a previously applied current pulse if this pulse is larger than threshold for non-ohmic conductivity. This resistance change is about ΔR/RΔT ∽ 1×10^-3 K^-1 for a pure sample and ΔR/RΔT ∽ 6×10^-3 K^-1 for a slightly electron irradiated one at 90 K. The relative resistance change is insensitive to the sample length. We deduce that the CDW current changes inhomogeneously the Peierls gap E_g. ΔE_g < O at the contact where the CDW current enters and ΔE_g > O at the exit. The effect is attributed to a CDW current induced inhomogeneous deformation of the CDW itself.     

Reflectivity studies of Ti- and Ta-dichalcogenides: Phonons

The i.r.-spectra of the 1T phases of TiS₂, TiSe₂, TaS₂ and TaSe₂ reveal features attributed to large free carrier densities and to i.r.-active longwavelength phonons. We find a single phonon in TiS₂, TiSe₂ and TaS₂, but six phonons in TaSe₂, The oscillator strengths are significantly higher in the Ti-compounds yielding large values of the infrared effective charge, ～6e, and electron-phonon coupling constant, 0.4–1.0. The observation of many modes in TaSe₂ is attributed to a superlattice distortion driven by a charge density wave.     

Raman spectra of several layer compounds with charge density waves

Raman scattering studies of the transition metal dichalcogenide layer compounds 1TTaS₂, 1TTaSe₂, 2H-TaSe₂ and 1TVSe₂ are reported. The materials were studied with commensurate and incommensurate charge density waves (CDWs) as well as in the absence of CDWs. Large qualitative changes observed in the Raman spectra are associated with the formation of CDWs and the change from an incommensurate to a commensurate phase.