Search

By method of dielectric spectroscopy we have obtained experimental data concerning the 4k F charge density wave (CDW) associated with the charge ordering (CO) state of Wigner type in (TMTTF) 2 X conductors. A divergence of the relaxation time was found at the critical transition temperature T CO into the CO state. Below T CO a low frequency shoulder appears in the frequency dependence of the imaginary part of the dielectric permittivity. Under a moderate pressure, the peak in the real part of the dielectric permittivity, corresponding to the transition into the CO state, shifts to low temperature and broadens. The substitution of hydrogen by deuterium in TMTTF molecules results in an increase of T CO (isotopic effect). The results obtained confirm the ferroelectric character of the CO state; they also favour long range Coulomb interactions as the main driven force for the formation of this state.

We present experimental evidence and a corresponding theory for the ferroelectric transition in the family of quasi-one-dimensional conductors $(\mathrm{TMTTF}{)}_{2}X$. We interpret this new transition in the frame of the combined Mott-Hubbard state taking into account the double action of the spontaneous charge disproportionation on the TMTTF molecular stacks and of the $X$ anionic potentials.

We have determined the spatial distribution of the local charge-density-wave (CDW) strain in the sliding state of NbSe 3 . The strain is measured by monitoring the spatially-varying shift q(x) of the CDW satellite wave vector between current contacts. Experiments were carried out at T = 90 K in the upper CDW state using high spatial resolution (30-50 μm) X-ray diffraction. Applying direct currents about twice the threshold value, we observe a steep exponential decrease of the shift within a few hundred microns from the contact followed by a linear variation of q in the central section of the sample. This latter regime is attributed to transverse pinning of the CDW dislocation loops (DL), while the exponential regime is controlled by the finite DL nucleation rate. Additional to these data in the stationary state of the sliding CDW, we investigated the relaxation of the CDW strain q(t) upon switching off the current (T = 75 K). Using time-resolved high-spatial resolution X-ray diffraction, we observe at 800 μm from the electrode a decay law of the stretched exponential type: q(t) = q o exp(-(t/τ) μ )), with r = 283 ms and μ = 0.37.

We present results on the normal-collective current conversion processes and associated deformation profile for a charge density wave in the sliding state. High resolution x-ray measurements of the satellite positional shift have been performed on ${\mathrm{NbSe}}_{3}$ at 90 K. The shift $q$ has been determined in the vicinity of the contacts by applying direct currents. We observe a steep variation of $q$ near the contact, modeled in terms of dislocation loops (DL) nucleated at host defects. A small constant gradient in the sample's central part indicates incomplete conversion, consistent with DL pinning.

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.

A well-formed energy gap Δ is observed in the energy spectrum of the quasi-one-dimensional orthorhombic conductor TaS3 at temperatures much lower than the Peierls transition temperature TP. As the temperature increases, in the region T>TP/2 there is a growth of the density of states in the gap and a relative decrease of the density at energies greater than Δ. In addition, absorption lines which probably correspond to soliton states in a charge-density wave are observed in the gap.

We show that the newly discovered phase transitions in (TMTTF)2X (X=PF6, AsF6, SbF6, ReO4) are characterised by a huge divergent peak in the low frequency dielectric constant. These phase transitions can be explained by combined effect of charge disproportionation on TMTTF molecules and the uniform shift of anions, yielding thus a macroscopic ferroelectric polarisation. Anion ordering transition, which occurs for non-centrosymmetrical anions, has a specific effect depending on the distortion vector: it destabilises the ferroelectric state in the (TMTTF)2ReO4 and brings an antiferroelectric character to the transition in (TMTTF)2SCN.

The spatial distribution of the electric field E between two current terminals along the quasi-one-dimensional TaS3 conductor has been studied using a multicontact configuration. In the low-temperature range (30 VT the spatial distribution of E is governed by an additional voltage developed near both current terminals to maintain the conversion of the current of electrons in the sliding charge-density-wave condensate.

We have found that the real part of the dielectric constant of the quasi-one-dimensional conductors with a charge density wave (CDW) shows a peak in the low temperature range ( T

The spatial distribution of electric field E between two current terminals to quasi-one-dimensional conductor TaS 3 has been studied using the multicontact configuration. In the low temperature range (30–50 K) at low voltage V between current terminals we observed strong increase of E near negative terminal, while near the positive terminal we observed the lowering of E. The observed asymmetry of the spatial distribution of E is supposed to be related to the formation of two Schottky barriers on the metal-Peierls semiconductor contacts which are biased one in reverse and another in forward directions.

We have studied the nonlinear conductivity \ensuremath{\sigma}(E,T) and the narrow-band noise in quasi-one-dimensional ${\mathrm{TaS}}_{3}$ at low temperatures down to liquid helium. We have found that the ratio between the charge-density-wave (CDW) current ${\mathit{I}}_{\mathit{c}}$ and the narrow-band-noise frequency f, ${\mathit{I}}_{\mathit{c}}$/f, begins to sharply decrease below \ensuremath{\sim}80 K with a slope correlated with the number of defects in the samples. We have carried out the analysis of our experimental data on the basis of the equation for phase slippage in narrow superconducting channels. We show that this equation well describes the \ensuremath{\sigma}(E,T) dependences at low temperatures in the high electric-field range above the threshold value. The value of the energy barrier for phase slippage has been estimated. We suggest that the decrease of the ${\mathit{I}}_{\mathit{c}}$/f ratio is mainly caused by the growth of the relaxation time of the CDW response at low temperatures. Our results confirm that the fundamental mechanism which determines the motion of the CDW and its excitations is the phase slippage both in the high- and in the low-temperature ranges.

We have measured the conductivity of orthorhombic and monoclinic TaS 3 as well as blue bronze K 0.3 MoO 3 as a function of electric field (10 −2 V/cm to 500 V/cm) at the low temperature down to 4.2 K. Two threshold fields E T ′ and E T are revealed and their temperature dependences are analyzed. The results obtained are explained on the base of supposition that the onset of non-linear conductivity is associated with the beginning of the motion of defects and dislocations in CDW superlattice. At the electric field range E T ′ T this motion has random character, but at E>E T the slippage of CDW phase with periodical nucleation and coherent motion of these dislocations becomes main process.

Polarized infrared reflectance and transmittance spectra have been measured on NbS3 crystals with two-fold commensurate superstructure (type I) which is in a sense similar to polyacetylene. From these data spectra of absorption coefficient at the temperatures 300 and 8.5 K have been calculated. It was shown that the absorption in the spectral range v >6700 cm−1 is provided by electron-hole excitations across the Peierls gap. In the energy range v

We prepared conducting Langmuir-Blodgett (LB) films of charge-transfer complexes using (C 16 H 33 -TCNQ) 1−x (C 17 H 35 -DMTTF) x mixture and measured conductivity in a wide temperature range and Hall constant at the room temperature. Electron microscopy and electron diffraction technique were used to make more clear the causes of some peculiarities of temperature dependencies of conductivity.

The spatial inhomogeneity of the electrical characteristics of high-T/sub c/ thin films was studied by optical laser probing. It was shown that the characteristic length L of inhomogeneous current distribution in high-T/sub c/ films may be much larger than the grain size a; for example, in polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-x/ film with critical current densities j/sub c/(4.2 K) approximately 10/sup 3/-10/sup 4/ A/cm/sup 2/ and grain size a approximately 1 mu m, the characteristic length L was equal to approximately 15 mu m at T>T/sub c/ and approximately 100 mu m at T

High spatial resolution (30–100 μm) X-ray measurements of the satellite positional shift q have been carried out in the upper charge-density-wave state of NbSe 3 . We determined the spatial distribution of the CDW plastic deformations between electrodes under applied currents. With direct current we observe a steep exponential decrease of q with position x along the sample near the contacts followed by a linear dependence in the central part of the sample. This second regime is attributed to transverse pinning of the CDW dislocations while the first is dominated by phase slip processes. The observed differences between data from direct current and pulsed current measurements indicate a significant, strongly spatially dependent relaxational behaviour of the CDW deformations.

The paper presents the results of experimental study of optical properties, do and ac conductivity, and dielectric constant of orthorhombic TaS3 in a wide range of temperatures and electric fields. The results are interpreted in terms of an essential role played by solitons in determining the properties of TaS3.

The nonlinear conductivity in orthorhombic and monoclinic TaS3 has been measured as a function of temperature from the Peierls transition temperature down to 4.2 K in the electric field range 10-2-500 V cm-1. At liquid-helium temperature a sharp increase in conductivity is measured in a very limited variation range of voltages. A general description based on phase-slip processes is given to explain these nonlinear properties in the whole temperature range.

The non uniform spatial distribution of the electric field E along the quasi-one-dimensional conductor (o-TaS3) has been studied in the low-temperature range (T < 50 K) for a d.c. voltage V applied between current terminals less than the threshold value VT. We have found a strong increase of E near negative terminal with respect to its value in the middle of the sample, on the contrary E is reduced near the positive terminal. Such distribution of E is supposed to be related to the formation of two Schottky barriers at the metal-Peierls conductor contacts. With increasing V, both the size of non-uniform E region and appropriate band bending are enlarged. We suggest that these effects are related to space rearrangement of CDW deformation (solitons and dislocations) which also exist in CDW at V < VT. In the range V > VT the electric field increases near both contacts and spatial distribution of E becomes more symmetrical.

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

The temperature dependent conductivity, I–V curves and optical transmittance have been measured on quasi-one-dimensional conductor NbS 3 with two-fold commensurate superstructure. The existence of a fundamental absorption edge has been found at the energy in agreement with the thermal activation energy of conductivity. The dependence of transmittance on radiation polarization has been studied. The I–V curves show a strong non-linearity which grows with decreasing temperature. Qualitative explanations of these data are proposed, in particular taking into account the existence of various types of soliton-like excitations in such materials.

The conductivity and dielectric constant e′ of quasi-one-dimensional orthorhombic TaS3 crystals are measured in the frequency range 3 to 105 Hz and temperature range 4.2 to 40 K. Below 20 K the frequency-dependent component of conductivity is found to follow a σac ∼ ω8 law (s ≈ 0.8) and increase linearly with temperature; above 20 K no temperature dependence is found, while the frequency dependence is retained. The conductivity of samples with some disorder is shown to be due to variable range hopping near the chemical potential level. The value of E' is found to increase at lower frequencies and higher temperatures. High values of the dielectric constant (≈ 106) may be due to polarization of the pinned charge density wave. [Russian Text Ignored].

Bolometric spectra of orthorhombic TaS 3 have been measured in the range of the fundamental absorption edge. The transition from the pseudogap to the fairly pronounced Peierls gap in the electronic spectrum was observed with decreasing temperature. However, the absorption edge remains broadened by the order of value of the gap. The results are in the qualitative agreement with the appropriate theories.

In small orthorhombic TaS3 samples we have observed the following: the step-like dependence of resistivity on temperature and electric field, the negative differential resistance at the current slightly above the threshold; a shift, splitting and locking of the narrow band noise frequencies.

The V-I characteristic of Josephson junction with an external parallel admittance, the high frequency impedance and high frequency response of a Josephson junction to small external signal are calculated by analytical method. The experimentally obtained spectral response curves for the superconducting point contacts in the frequency range 5-30 cm-1 are presented. The condition need for achieving high sensitivity of superconducting point contacts and superconducting bridges are determined.

We present high resolution X-ray diffraction measurements on NbSe3 of the charge-density-wave (CDW) relaxation from the deformation created by the sliding of the CDW. The data are taken in the temperature range 75 K < T < 105 K in the upper-CDW phase of NbSe, and cover spatial positions up to $8OO~\mu$m from the current contact. Convenient fits to the data are obtained by a stretched exponential decay profile yielding exponents μ in the range of 0.4–0.7 and relaxation time-scales τ of the order of 1–150 ms. becoming faster with increasing sample temperature and slowing down with increasing distance from the current contact.