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In this study, we analyze the evolution of current-voltage characteristics of a quasi-one-dimensional TaS3 conductor under uniaxial tension in the temperature range of 150–40 K. At all temperatures, elongation is found to lead to formation of an “ultracoherent” charge density wave (CDW) the main feature of which is a sharp increase in conductivity under an electric field that is higher than the threshold: above 100 K, the transition in the dependence on e occurs almost abruptly while, below 90 K, elongation leads to an intermediate region in which the threshold field increases sharply and is poorly defined due to weak nonlinear conductivity. The result proves the occurrence of a spatially non-uniform state of the CDW below 90 K.

Field-effect transistor structures based on whiskers of layered quasi-one-dimensional semiconductor TiS3 have been fabricated. The dependences of the conductivity σ on the gate voltage Vg, as well as the current-voltage characteristics of whiskers (“source-drain”) at different Vg values, have been measured in the temperature range of 4.2-300 K. As the temperature decreases, the sensitivity of the conductivity to the gate voltage, α ≡ 1/σdσ/dVg, increases in the range from 300 to 80 K and decreases sharply below 80 K, where the nonlinear conductivity begins to depend on Vg. The results can be explained by the formation of an electronic crystal at low temperatures.

Mechanical vibrations of whiskers of the quasi-one-dimensional conductor TaS3 that appear at the application of an ac voltage above the threshold value for the sliding of a charge density wave have been studied by the heterodyning method. It has been shown that the highest frequency modes correspond to the excitation of longitudinal ultrasonic standing waves. A sharp minimum of the frequencies of ultrasonic resonances at a critical tension of ec ≈ 0.7% has been detected in the region of transition of the charge density wave to a new state.

The sliding of the room-temperature charge density wave (CDW) in the monoclinic phase of $\mathrm{Nb}{\mathrm{S}}_{3}$ under rf power is studied. The threshold field, ${E}_{\mathrm{t}}$, and Shapiro steps' width, \ensuremath{\delta}E, show aperiodic Bessel-type oscillations as a function of rf voltage. Here we demonstrate experimentally that, if presented as a function of CDW path length in each half period of the rf voltage, ${E}_{\mathrm{t}}$ and \ensuremath{\delta}E show periodic oscillations, the period being equal to the CDW wavelength. The result is found to be universal for different compounds and gives clear understanding of the synchronization effects in terms of forced oscillations of a particle in a periodic potential.

Quasi-one-dimensional conductors with charge-density wave (CDW), which are used to excite and detect mechanical oscillations, are considered. The effect of elongation e of whiskers of a quasi-one-dimensional conductor (orthorhombic TaS3) on the excited resonance oscillation modes is studied by means of heterodyning. Dependences of the oscillation frequency on e allow one to distinguish bending and torsional modes. The magnitude of the detected signal increases sharply at ec ≈ 0.6% in the region of CDW transition into the ultracoherent (UC) state. This effect is attributed to an anomalously strong piezoresistive response; in the UC state (e > ec), the signal virtually vanishes.

Heterodyne mixing with frequency modulation is employed for detection of torsional resonances of whiskers in rhombic TaS3 (typical quasi-1D conductor) with charge-density wave (CDW). The dependences of both torque and the feedback signal of torsion on the internal properties of CDW are distinctive features of the proposed method. Sample placement in the vicinity of the electrode that serves as gate is unnecessary. Electromechanical and elastic properties of the CDW systems can be studied with the aid of the proposed method, at least, in the megahertz frequency range.

The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS3 on the direction and magnitude of the magnetic field B have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at T < 100 K. Below T0 ≈ 50 K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance (B ∥ c) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at T0. The negative magnetoresistance (at B ∥ c) below T0 is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at B ∥ ab) is explained by the effect of the magnetic field on the spectrum of electronic states.

We report I–V curves of quasi one-dimensional charge-density wave (CDW) conductors under simultaneous impacts of rf voltage at two frequencies, F and f, with F ≫ f. We find that each Shapiro step (ShS) induced by F-irradiation behaves as a “host” surrounded with “satellites” induced by f-irradiation, e.g., by ShSs at the CDW currents corresponding to F ± f. The subharmonic “hosts” are surrounded with subharmonic “satellites,” e.g., by ShSs at the CDW currents corresponding to F/2 ± f/2. The hosts and satellites show oscillations of their widths as a function of rf amplitude. We demonstrate that each satellite can be treated as a conventional ShS, if its host ShS is taken as a reference point at which the CDW is resting. Thus, with the help of rf irradiation one can emulate CDW samples with tunable threshold voltages and extremely coherent sliding modes.

Charge-density wave (CDW) transport is studied in the whiskers of orthorhombic TaS3 suspended between piezoelectric actuators. It is found that when the frequency of rf voltage applied to the actuators coincides with a vibration resonance of the whisker, the I–V curves show Shapiro-step-like features, similar with those under rf voltage applied directly to the sample. We provide evidence that the features observed are coupled with the periodic lattice deformation. The effect of vibrations appears surprisingly strong: a time-dependent strain not exceeding 10−4 results in a periodic stop of the CDW. The result suggests an area of electromechanical effects inherent to sliding CDWs.

The Shapiro steps (ShSs) developing in sliding charge-density wave (CDW) under rf irradiation are considered in terms of the CDW travel in the periodic (washboard) potential. We demonstrate that treating the CDW as an inertialess object whose velocity is defined by the instantaneous voltage one can predict the positions of the ShSs in dc voltages for the given amplitude of rf voltage. Moreover, the approximation allows indicating the rf voltages at which the ShSs' magnitudes show maxima and minima. The only information necessary in advance is the I–V characteristic without rf irradiation and the cross-sectional area of the sample. Thus, one can recover the course of an I–V curve in average and the positions of ShSs on it, though not their forms.

The uniaxial strain of quasi-one-dimensional conductor whiskers of orthorhombic TaS3 at a strain higher than e 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 e, 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.

The temperature and voltage dependences of conductivity of the monoclinic phase of NbS 3 are reported. The extremely high coherence of the room-temperature charge-density wave (CDW) allows its synchronization (observation of Shapiro steps) under RF electric field up to T P1 =365 K, the temperature of the CDW transition, and even above it. The following regularity is established: the higher is the RF frequency, the higher is the utmost temperature of Shapiro steps observation. Thus, RF field can synchronize CDW fluctuations. The possible contributions of CDW fluctuations to linear and non-linear conductivity are discussed. The result present the NbS 3 whiskers as high-temperature nano-sized RF detectors.

We report studies of the Seebeck coefficient $S$ of the quasi-one-dimensional compound $\mathrm{Nb}{\mathrm{S}}_{3}$, together with the temperature dependence of its specific conductivity ${\ensuremath{\sigma}}_{\mathrm{s}}$ and heat capacity ${c}_{p}$. The monoclinic phase $(\mathrm{Nb}{\mathrm{S}}_{3}$-II) is studied over the temperature range $T=80--400$ K, which covers two charge density wave (CDW) transitions at ${T}_{\mathrm{P}1}=360$ K (CDW-1) and ${T}_{\mathrm{P}2}=150$ K (CDW-2). The $S(T)$ curves show features in the vicinities of both CDW transitions and appear to be correlated with the value of ${\ensuremath{\sigma}}_{\mathrm{s}}(300\phantom{\rule{0.28em}{0ex}}\mathrm{K})$: The increase of $S$ below ${T}_{\mathrm{P}1}$ in the high-Ohmic samples reveals a complete dielectrization of the electronic spectrum, while in the low-Ohmic samples $S$ decreases below ${T}_{\mathrm{P}1}$ and even becomes negative below ${T}_{\mathrm{P}2}$. The magnitude of $S$ in low-Ohmic samples at $Tl{T}_{\mathrm{P}2}$ is well below ${k}_{\mathrm{B}}/e\ensuremath{\approx}86\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}/\mathrm{K},{k}_{B}$ being the Boltzmann constant and $e$ the elementary charge, which is surprisingly low for a usual CDW semiconducting state. Our results suggest that at ${T}_{\mathrm{P}1}$ the main electronic band with $p$-type carriers becomes gapped, while some $n$-type carriers can remain in a separate band with low density of states. These carriers, whose concentration is defined by the compositional doping, are gapped at ${T}_{\mathrm{P}2}$. We also report $S$ for the triclinic dielectric phase of $\mathrm{Nb}{\mathrm{S}}_{3}\phantom{\rule{4pt}{0ex}}(\mathrm{Nb}{\mathrm{S}}_{3}\text{\ensuremath{-}}\mathrm{I})$. Its low absolute value, $\ensuremath{\sim}{k}_{\mathrm{B}}/e$, and the anomalous temperature dependence demonstrate that $\mathrm{Nb}{\mathrm{S}}_{3}\text{\ensuremath{-}}\mathrm{I}$ is neither a semiconductor nor a CDW conductor in the usual sense.

We report the evolution of charge density wave states under pressure for two NbS3 phases: triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and x-ray diffraction (XRD) techni ...

The basic and the charge density wave (CDW) structures of the monoclinic ${\mathrm{NbS}}_{3}$-II polymorph were studied by synchrotron x-ray diffraction, ab initio calculations, simulation of electron diffraction patterns, and by atomic-resolution transmission electron and low-temperature scanning tunneling microscopies. It is confirmed that the basic structure belongs to the space group $P{2}_{1}/m$ and is described with a unit cell, formed of four pairs of symmetry-related trigonal prismatic (TP) columns [${a}_{0}=0.96509(8)\phantom{\rule{4pt}{0ex}}\mathrm{nm}$, ${b}_{0}=0.33459(2)\phantom{\rule{4pt}{0ex}}\mathrm{nm}$, ${c}_{0}=1.9850(1)\phantom{\rule{4pt}{0ex}}\mathrm{nm}$, and ${\ensuremath{\beta}}_{0}=110.695{(4)}^{\ensuremath{\circ}}$]. The incommensurate components of the two CDWs, $\stackrel{P\vec}{{q}_{1}}=(0,\phantom{\rule{4pt}{0ex}}0.298,0)$ and $\stackrel{P\vec}{{q}_{2}}=(0,\phantom{\rule{4pt}{0ex}}0.352,\phantom{\rule{4pt}{0ex}}0)$, are related as ${q}_{{1}_{b}}+2{q}_{{2}_{b}}\ensuremath{\approx}1$. Both CDWs form their own modulation patterns with unit cells (${a}_{\mathrm{m}}=2{a}_{0}$, ${b}_{\mathrm{m}}={b}_{0}/{q}_{\mathrm{jb}}$, ${c}_{\mathrm{m}}={c}_{0}$, ${\ensuremath{\beta}}_{m}={\ensuremath{\beta}}_{0}$) and are ordered along adjacent isosceles TP columns either pairwise or with both columns modulated by either the $\stackrel{P\vec}{{q}_{1}}$ or $\stackrel{P\vec}{{q}_{2}}$ CDW only. The CDWs are ordered according to one of the two possible modulation pattern space groups, $Cm$ or $C2/m$. If considered as long-period commensurate, the entire modulated structure with both CDWs included is described within experimental error with an enlarged unit cell ($a=2{a}_{0}$, $b=37{b}_{0}$, $c={c}_{0}$, and $\ensuremath{\beta}={\ensuremath{\beta}}_{0}$) and with all atoms displaced from their average positions in accord with the specified modulation pattern.

We review the features of the charge density wave (CDW) conductor $\mathrm{Nb}{\mathrm{S}}_{3}$ (phase II) and include several additional results from transport, compositional, and structural studies. Particularly, we highlight three central results: (1) In addition to the previously reported CDW transitions at ${T}_{\mathrm{P}1}=360\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{P}2}=150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, a third CDW transition occurs at a much higher temperature ${T}_{\mathrm{P}0}\ensuremath{\approx}620\ensuremath{-}650\phantom{\rule{0.16em}{0ex}}\mathrm{K}$; evidence for the nonlinear conductivity of this CDW is presented. (2) We show that the CDW associated with the ${T}_{\mathrm{P}2}$ transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. (3) We demonstrate the exceptional coherence of the ${T}_{\mathrm{P}1}$ CDW at room temperature. The effects of uniaxial strain on the CDW transition temperature and transport are reported.

We report studies of low-field conductivity, \ensuremath{\sigma}, of nanosized samples of orthorhombic $\mathrm{Ta}{\mathrm{S}}_{3}$ as a function of strain, \ensuremath{\epsilon}. The \ensuremath{\sigma}(\ensuremath{\epsilon}) curves show steplike changes associated with the ``quantization'' of the wave vector, $q$, of the charge-density wave. Based on the effect we have revealed the change of the $q$-vector with strain: its in-chain component (normalized by the reciprocal lattice constant) is found to increase with sample stretch. A similar $q$(\ensuremath{\epsilon}) dependence results from the analysis of the \ensuremath{\sigma}(\ensuremath{\epsilon}) hysteresis for macroscopic samples. This means that the strain-induced anomalies cannot be explained by the transition of the CDW to fourfold commensurability with the pristine lattice (lock-in transition), as it was supposed earlier. We also discuss the metastable length states and the elastic anomalies in $\mathrm{Ta}{\mathrm{S}}_{3}$ in light of the observed $q$(\ensuremath{\epsilon}) dependence.

It is found that below 80 K o-TaS 3 demonstrates spontaneous resistance fluctuations growing when the temperature is decreasing. The average relaxation time, τ 0 , obtained from the noise spectra demonstrates activated behavior, τ 0 ∞exp(W/T), W≃1300 K for 50