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Three charge density waves (CDWs), two of which are formed above room temperature, are observed in the NbS3 monoclinic phase (NbS3-II). The charge density and mobility in the high-field limit have been determined for each of three CDWs in this work using the synchronization effect of CDWs in high-frequency fields. It has been found that the mobility of each CDW in this limit is approximately equal to the normal-state mobility of quasiparticles condensed in it. Furthermore, correlation has been observed between the temperature dependences of mobilities of CDWs and quasiparticles. The results of this work refresh problems of a mechanism of the limit conductivity of CDWs and of the distribution of CDWs between atomic chains in the unit cell.

A new polytype of the quasi-one-dimensional conductor NbS3 with high-temperature charge density waves has been synthesized. The transmission electron microscopy study at room temperature has revealed only one of the two superstructures existing in the previously studied monoclinic phase with the period along chains close to b/0.352. In addition, a new incommensurate distortion of the lattice with the period close to 2b has been observed. The transport properties of the samples under study are in agreement with the study of the superstructure. The main lattice constants are somewhat different from the respective parameters in the previously studied monoclinic phase. The new phase can assumingly be intermediate between the two main known phases of NbS3, monoclinic and triclinic. Its properties can be explained within a model where a unit cell of the monoclinic phase can be considered as the superposition of unit cells of the triclinic phase.

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.

Problem formulating. The quasi one-dimensional conductor TaS3 of the orthorhombic modification shows strong electromechanical coupling of the sliding charge density wave (CDW) with the lattice of crystals. However, until now the effect of the dynamic deformation of the sample on the CDW sliding has not been studied. Goal. Search and study of a new effect: the influence of sample vibration on CDW dynamics. Result. The conductivity of TaS3 samples is studied as a function of the electric field and the frequency of mechanical deformation of the sample. It is found that when the vibration frequency of the piezoelectric actuators deforming the sample coincides with one of the mechanical resonances of the sample, suppression of the threshold field and features in the form of Shapiro steps are observed on the I–V characteristics. The effect is similar (though not equivalent) to that of RF voltage applied directly to the sample at the same frequency. Evidence is provided that these features are not coupled with the effect of electrical interference. Practical meaning. The result opens up a new area of electromechanical effects in quasi one-dimensional conductors with CDW and can be used for detecting and processing of mechanical oscillations in the microwave range.

The voltage dependences of conductivity, $\ensuremath{\sigma}(V)$, of the monoclinic phase of ${\mathrm{NbS}}_{3}$ are reported. The extremely high coherence of the room-temperature charge density wave (CDW) allows its synchronization by a rf field up to ${T}_{\mathrm{P}1}\ensuremath{\approx}365\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the temperature of the Peierls transition, and above it by about 10 K. The ratio of nonlinear current, ${I}_{\mathrm{nl}}=I\ensuremath{-}V\ensuremath{\sigma}(0)$, to the fundamental frequency of the CDW sliding, ${f}_{\mathrm{f}}$, falls as T increases across ${T}_{\mathrm{P}1}$. This result is considered in terms of spontaneous phase slippage near ${T}_{\mathrm{P}1}$, which could give both single-particle and collective contributions to linear conductivity. The two contributions are considered, and the former is found to dominate. The temperature-frequency dependence of ${I}_{\mathrm{nl}}/{f}_{f}$ is consistent with the model of thermally activated phase slippage.

The layered quasi one-dimensional compound TiS3 is remarkable for its optical properties, especially, photoconductivity. Up to now, photoconductivity in TiS3 was studied only at room temperature. Here, we report photoconductivity, δσ, of the TiS3 whiskers in the temperature range of 5–300 K under irradiation in the near-infrared region. With a temperature decrease from room temperature down to T ∼ 100 K, δσ grows moderately, dominated by the variation of the mobility of the electrons. The estimates based on the values of δσ give the recombination time ∼3–25 μs at 300 K. The results show that above ∼100 K, TiS3 behaves as a semiconductor, where electron–hole pairs are excited across the gap, while the recombination goes through transitions of the excited electrons to a donor level with a long lifetime. Below 60 K, a drop of δσ is likely to reveal a phase transition of electrons into a collective state. Another feature in δσ( T) is seen around 17 K, resembling the temperature variation of the conductivity. This feature indicates a drop in the mobility of the electrons. While the behavior of δσ( T) is consistent with the transition of electrons into a collective state, it is strikingly different from δσ( T) of the known quasi one-dimensional conductors with charge-density waves.

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 report on high temperature (T) studies of conductivity, σ, of the monoclinic phase of NbS3 (NbS3-II). The compound is shown to be stable up to T∼550 K. Threshold non-linear conductivity and Shapiro steps on the I – V curves can be seen up to TP0 ≈ 450 – 475 K clearly demonstrating sliding of a charge-density wave (CDW). The feature in σ(T) shows that TP0 is the temperature of the formation of this CDW. The charges condensed in this CDW show relatively high density and, at the same time, extremely low mobility. Their mobility appears low in the single-particle states as well, giving a plausible clue to the surprising dielectric-like temperature variation of σ above TP0.

It has recently been proved that, in alloys, e.g., based on the Ti–Ni system, a shape memory effect (SME) is preserved down to the nanoscale sizes of an active alloy layer and demonstrated ultrasmall-sized and fully functional microand nanomechanical devices: actuators and nanotweezers that are fabricated via standard microelectronic technologies relying on composite materials with the SME. In the nearest future, such achievements will enable the creation of the next-generation microand nanomechanical devices whose sizes are quite comparable with those inherent to, e.g., carbon nanotubes, graphene sheets, viruses, etc. Mathematical simulation methods are used to study how the shape-memory micromechanical devices can be activated by means of resistive pulse heating. A decrease in the overall sizes of heating elements (from 1 mm to 10 μm) is demonstrated to be accompanied by the fact that the speed of operation increases sharply from 102 to 105 s–1 and, simultaneously, the energy consumption diminishes from 10–3 to 10–8 J per operation. The preliminary results of experiments whereby the control of the composite nanotweezer exhibiting the SME is perfected with the help of the automatic pulsed heating technology, as well as the prospects for creating high-speed and high-performance microrobotic systems incorporating newly developed components, are discussed.

The current–voltage characteristics, Hall effect and magnetoresistance of TiS 3 single crystal whiskers have been studied experimentally. Below 60 K the I – V curves are non-linear along all crystallographic directions and exhibit a power law behavior. The Hall voltage shows a pronounced maximum at around 60 K. The results indicate a change of the conductivity mechanism at low temperatures. Possible explanations of the observed effects are discussed.

We have developed techniques for stretching whiskers up to 1.5–3%, including those of nanometer cross-section. An exceptionally high coherence of the CDW transport for TaS 3 and NbS 3 has been observed under the uniaxial strain, e . For TaS 3 samples with transverse dimensions ∼0.1–1 μm for e approaching e с the CDW coherence falls down and a new, ultra-coherent, CDW phase begins to form. At e ~ e с the two phases coexist within a very narrow range δe ~2×10 −4 . Further stretching results in a complete vanishing of the transport of the incoherent CDW. The threshold fields, E t , of the ultra-coherent CDW, as well as the dissipation, appear by an order of magnitude lower; giant negative differential resistance is observed. The narrow-band noise shows up to 10 harmonics of the fundamental frequency. RF irradiation results in complete synchronization of the CDW sliding. The CDW transformation shows features of the 1st order transition. We also report hysteresis of resistance vs. e , from which we conclude that the transition cannot be associated with the lock-in of the CDW: the strain draws the CDW away from 4-fold commensurability with the lattice.

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.

Nonlinear conductance along the crystallographic axis c across the layered whiskers of the TiS3 quasi-one-dimensional semiconductor has been discovered. It has been shown that the current-voltage characteristics in all three directions along the a, b, and c axes obey a power law with the exponent increasing with a decrease in temperature. Possible mechanisms of the nonlinear conductance including the motion of condensed electrons, excitation and dissociation of electron-hole pairs in two-dimensional layers, and interlayer tunneling under the conditions of the Coulomb blockade with a charge spreading over the layers are considered.

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.

We report synthesis of quasi-one-dimensional conductor NbS 3 , TEM studies and transport properties ofthe prepared samples. The grown NbS 3 whiskers are Peierls conductors known as phase II with thetransitions at T P1 ¼365 K and T P2 ¼150 K. CDW1 is stable and not so sensitive to growth conditions. Itcan slide and be synchronized by the external microwave irradiation up to 16 GHz. CDW2 stronglydepends on growth conditions. Nevertheless, it also shows sliding and synchronization.& 2012 Elsevier B.V. All rights reserved. 1. IntroductionNbS 3 , phase II, is standing out for its extremely high Peierlstransition temperature, T P ¼ 330–355 K [1]. Studies of this phasewere complicated because of the difficulties with its synthesis.The number of NbS 3 polytypes is not quite clear yet. The mostreproducible in synthesis is phase I (triclinic), high-ohmic at all T;it does not show clear evidence for CDW transport. Phase II ismonoclinic with 8Nb chains per unit cell and shows approximatetrimerisation below T

The crystal structure and transport properties of TiS 3 whiskers in the plane of layers (ab) have been studied. Maxima of the logarithmic derivative of resistance, dln R/d(1/T), are observed at 17, 60 and 120 K both along and across the chains. Strong nonlinearity of the current-voltage characteristics has been revealed in both directions. Nonlinear conductivity along the chains is observed up to T=60 K, while in the transverse direction it is observed up to T=130 K. The results indicate possible phase transitions of electrons to collective states, probably, charge density waves. © 2012 Elsevier B.V. All rights reserved.

Features have been observed in the current dependences of the torsion angle, φ(I), for samples of a quasi-one-dimensional conductor TaS3 irradiated by a high-frequency (HF) field. The features appear at the same current values as the Shapiro steps on the current-voltage characteristics, i.e., correspond to the synchronization of the motion of a charge-density wave (CDW) by the HF field. The shape of the features in the φ(I) dependences indicates a decrease in the inhomogeneous deformation of the CDW under the synchronization conditions. The investigation of torsion appears to be a highly sensitive method for determining the spatial coherence of the CDW.

The structure and transport properties of single crystal whiskers of the TiS3 quasi-one-dimensional semiconductor have been investigated. The anisotropy of the conductivity in the plane of layers (ab) has been measured as a function of the temperature. The anisotropy at 300 K is 5 and increases with a decrease in the temperature. Features on the temperature dependences of the conductivity along and across the chains are observed at 59 and 17 K. Near the same temperatures the form of the current-voltage characteristics measured along the chains is qualitatively changed. The current-voltage characteristics below 60 K exhibit nonlinearity and have a threshold form below 10 K. The results indicate possible phase transitions and the collective conduction mechanism at low temperatures.

The resistance of TiS3 single crystal whiskers has been measured as a function of temperature and electric field in the range of 4.2–340 K. At temperatures below 60 K, a strong nonlinearity of the current-voltage characteristics has been revealed. Below 10 K, the current-voltage characteristics are of the threshold type. The results are attributed to the transition of electrons to a collective state, probably, with the formation of a charge density wave.

We report the effects of electric field induced deformations of quasi one-dimensional conductors with charge-density wave (CDW). The most pronounced sort of deformation is torsional strain (TS). The TS is found to comprise two contributions. The features of the 1st—the larger one—are threshold hysteretic dependence on electric field and high relaxation time τ : For o-TaS 3 τ ∼10 −2 s at T =80 K and falls as exp(900 K/ T ) with increasing T . The 2nd contribution is linear in electric field and does not drop with frequency increase. The amplitude of this contribution falls abruptly with T approaching the Peierls transition temperature T P from below. Similar features of TS are demonstrated for other CDW compounds: (TaSe 4 ) 2 I, K 0.3 MoO 3 and NbS 3 type II, for which T P ≈360 K. We attribute the 1st and the 2nd contributions to large (hysteretic) and small (near-equilibrium) CDW deformations, respectively, likely—shear at the surface. The TS is observed also above T P : For TaS 3 and (TaSe 4 ) 2 I typical torsional amplitude is 10 −1 °/V in the resonance regimes, corresponding to the piezomodulus ∼10 −9 m/V. A separate study of TS was performed at room temperature with AFM technique. Apart from this (‘intrinsic’) effect, we observe electrostatic contribution to the TS. In contrast to the intrinsic response, the electrostatic one is proportional to the potential either over the sample, or over an additional electrode (‘gate’) placed nearby, but not to the difference of potentials between the sample ends. It is typically 2 orders of magnitude less. The intrinsic TS reveals a new electromechanical effect at room temperature, presumably associated with the excitations of the pinned mode of the CDW fluctuations. Its observation opens prospects for application of quasi one-dimensional conductors as micro- and nano-actuators. Basing on the electrostatic-induced TS we propose prototypes of electrometers and a technique for measurements of work function changes. Using a cryogenic transmission electronic microscope we observe also flexural deformation of TaS 3 in the double-clamped mode, visible both in resonant and static modes.

We discuss deformation of quasi 1-dimensional conductors with charge-density wave (CDW) under the electric field deforming the CDW. In case of ``strong'' CDW-lattice interaction the effect can be treated in terms of the converse piezoelectric effect with enormous piezomodulus, $\sim L_c/\lambda$ times larger than that in ionic crystals ($\lambda$ is the CDW wavelength, $L_c$ is the CDW coherence length, mm scale in the sliding state). The CDW-lattice interaction is likely to be defined by the interband charge transfer (rearrangement of the covalent bonds) with lattice deformation, possible in a number of CDW compounds. The resulting effects, observed or expected, are paving a way towards new-principle actuators, including nanosized ones.

The influence of magnetic field and microwave irradiation on dynamical phase separation in submicron Bi2Sr2CaCu2O8+x bridges has been studied. Strong effect on the shape and metastable character of the step-like I-V characteristics are found. Under a weak field H < 2 Oe and low level microwave irradiation the step-like structure of the I-V characteristics smears out and disappears completely. The average frequency of switching between metastable states grows by 5 orders under increase of magnetic field by only 1 Oe. This behavior is explained in terms of the model of dynamical vortex lines.

The current-voltage characteristics of Cu-K0.3MoO3 point contacts between a metal and a semiconductor with a charge density wave (CDW) are studied for various diameters of the contacts in a wide range of temperatures T and voltages V. In the interval 80 K ≲ T ≲ 150 K, the current-voltage characteristics are correctly described in the framework of a semiconductor model: screening of an external electric field causes CDW deformation, shifts the chemical potential of quasiparticles, and changes the point contact resistance. It is shown that the chemical potential is above the middle of the Peierls gap in equilibrium and approaches the middle upon an increase in temperature. The current-voltage characteristics of point contacts with a diameter d ≳ 100 A exhibit a sharp decrease in resistance for |V| > V t , which is associated with the beginning of local CDW sliding within the contact region. The V t (d, T) dependence can be explained by the size effect in the CDW phase slip.

A simple consideration demonstrates that if the temperature of a second-order phase transition is suppressed by fluctuations, the dominating effect at the transition is a maximum, but not a jump of specific heat, c p . One can attribute a certain value of latent heat to the transition; an estimate of the latent heat is proposed and compared with the behavior of c p and of thermal expansion at the Peierls and superconducting transitions.

In thin BSCCO (2212) whisker-based superconducting bridges with width and length ≤ 1 μm we observe random telegraph noise at discrete values of current. We show that the noise is associated with spontaneous processes of addition and removal of discrete vortex trains and reflects the regular structure of steps on the I–V curves. The average lifetime of the trains τ falls down with increasing temperature T in a complex way: intervals of steep drop (∼ an order of magnitude per Kelvin) are separated with a plateau of τ(T) with τ ∼ 10−2 s. The 70 GHz irradiation with 100 μW power results in the growth of the average switching frequency by 5 orders of magnitude; thus the HTSC bridges could be efficient detectors of microwave radiation. We discuss the peculiar features of τ(T).

The Hall effect in single crystal BSCCO whiskers has been measured, with the current flowing along the a and b crystallographic directions. The ab-plane anisotropy of the Hall anomaly is found. The magnitude of the anomaly, as well as its anisotropy, are strongly affected by the structural defects.

The resistive state above the Larkin-Ovchinnikov (LO) instability current has been experimentally studied. Analysis of the stepwise I–V curves of thin superconducting bridges ≤1 μm wide, etched from the Bi2Sr2Ca1Cu2Oy (Bi2212) whiskers, reveals the following scenario of vortex dynamics: first, a pair of current-induced Abrikosov vortices enter the bridge, then they are accelerated by the current above the critical velocity of the LO instability and approach the limiting velocity of the normal phase propagation, of the order of 107 cm/s. Each vortex leaves a tail of normal phase behind itself, which is equivalent to a phase slip line. At low temperatures the tails cover the whole width of the bridge—the I–V curves between the steps become Ohmic.

We describe a simple technique for study of (thermal) expansion of whisker-like samples. At the heart of the method is the measurement of the shift of the sample middle, which is lifted over the substrate, while the ends of the sample are fixed at the substrate. The optical interference technique allows one to detect the relative length change about 5×10−7. Simultaneous electric measurements of the sample can be performed. We illustrate the technique with the study of TaS3, a quasi-one-dimensional conductor with charge-density waves: thermal expansivity has been measured and hysteresis in thermal expansion is found.

We have applied the method of point-contact (PC) spectroscopy to conductors with a charge-density-wave (CDW) ground state. Recent experimental results of the investigation of characteristics of PC formed between a normal metal (N) and different CDW conductors are reported. For a uniform N–CDW boundary, we show that the interaction of carriers injected from the normal metal with the CDW results in their reflection without changes in the sign of the charge but with a momentum transfer 2pF to the condensate of electron–hole pairs carried away from the N–CDW interface. In the case of a non-uniform N–CDW boundary (direct point contact), a strong local CDW deformation, due to a band bending effect, was observed in materials with a semiconducting ground state. The chemical potential can be strongly modified, changing the conductivity type in the vicinity of the PC. It was shown that, in the case of a CDW conductor with a metal or semimetal ground state, the band bending effect is negligible (because of the screening of the electric field by remaining normal carriers) and the spectroscopy of the Peierls energy gap is possible.

We report steps of conductivity between discrete conducting states in microsamples of quasi-one-dimensional conductors K0.3MoO3 and NbSe3. The steps reveal single phase-slip events, and the discrete states reveal “quantization” of the wave vector q of the charge-density wave. The “quantization” is observed due to the coherence of the CDW within the sample volume and tight boundary conditions at the contacts. The distribution of steps in temperature gives the temperature dependences of the q-vectors, while the steps' height provides the carriers' mobilities. For the case of NbSe3 we give the 1st direct confirmation of the extremely high mobility of the “pocket” holes at low temperatures.

We have performed investigations with a low-temperature scanning electron microscope (SEM) on TaS 3 , in order to visualize the structures of the charge density waves, both at rest and sliding. In contrast to the usual SEM methods, we recorded the electric response of the sample in dependence of the beam position. A two-dimensional picture of deformations is obtained from the analysis of the images. The main features of the picture agree with the results obtained by other methods and are consistent with the model discussed.

Measurements of the current-voltage characteristics of micron-size bridges made of Bi2Sr2CaCu2Ox single-crystal whiskers are carried out. It is found that, at temperatures below the superconducting transition temperature, the current-voltage characteristics exhibit quasi-periodic voltage jumps with segments of constant differential resistance whose value is proportional to the jump number. For the narrowest bridges (0.5–1 ¼m), up to ten voltage jumps are observed. The result of the experiment is explained by the formation of vortex lines under the current effect.

When an electric field is applied to a Peierls quasi-one-dimensional conductor K0.3MoO3, a change in conductivity is observed in the vicinity of the point contact. This change is due to a strong local distortion of the charge density wave in the pinned state. In this case, the chemical potential can be varied over a wide range by changing the conductivity from the n type to the p type. The local position of the chemical potential can be determined by the method proposed; in particular, the low-temperature anomaly of conductivity is explained.

In the Peierls state very thin samples of TaS3 ~cross-section area ;10 23 mm 2 ) are found to demonstrate smearing of the I-V curves near the threshold field. With approaching the Peierls transition temperature, T P , the smearing evolves into smooth growth of conductance from zero voltage interpreted by us as the contribution of fluctuations to the nonlinear conductance. We identify independently the fluctuation contribution to the linear conductance near T P . Both linear and nonlinear contributions depend on temperature with close activation energies ;(224)310 3 K and apparently reveal the same process. We reject creep of the continuous charge-density waves ~CDW’s! as the origin of this effect and show that it is spontaneous phase slippage that results in creep of the CDW. A model is proposed, accounting for both the linear and nonlinear parts of the fluctuation conduction up to T P .

Very thin samples of TaS 3 (cross-section area ∼ 10 -3 μm 2 ) are found to demonstrate smearing of the I-V curves near the threshold field in the Peierls state. With approaching the Peierls transition temperature, Tp, the smearing evolves into smooth growth of conductance from zero voltage, interpreted by us as the contribution of fluctuations to the non-linear conductance. The fluctuation contribution to the linear conductance near Tp is identified independently. Both linear and non-linear contributions depend on temperature with close activation energies ∼ (2 - 4) x 10 3 K and apparently reveal the same process. We reject creep of the continuous charge-density waves (CDW) as an origin of the effect and show that it is spontaneous phase slippage that results to creep of the CDW. A model is proposed accounting for both the linear and non-linear parts of the fluctuation conduction up to Tp.

Besides the superlattice with the period equal to twice the Fermi wave vector q =2 k F , charge density waves (CDW) also show a spatial structure formation with a length scale much larger than the period of the superlattice. Following the theory of Fukuyama, Lee, and Rice (FLR), values of some microns are expected for the extension of the FLR domains. We have performed investigations with a low-temperature scanning electron microscope (SEM) on K 0.30 MoO 3 and TaS 3 , in order to visualize these domain-like structures. In contrast to the usual SEM methods, we recorded the electric response (e.g., the thermovoltage) of the sample in dependence of the beam position. The analysis of the images obtained reveals values for different parameters that well agree with the ones obtained by other methods.