Non-linear conductivity of monoclinic TaS3

Strongly non-ohmic conduction is observed in the semiconducting charge-density-wave (CDW) state of monoclinic TaS₃ above a very sharp threshold field. These features could be interpreted in terms of depinning or tunneling of pinned CDW under applied electric field. We also found that large noise appears with the onset of the non-linear conductivity and fades away as the current through the samples is increased, which behaves as the decreasing function of frequency.     

Transverse conductivity behavior at threshold electric field for CDW sliding in NbSe3

We have measured the dynamical properties which occur in the transverse direction to the conducting chains in NbSe₃ single-crystals, when the CDW slides along the chains. A sharp decrease in transverse conductivity takes place above an electric field less than the longitudinal threshold one for CDW sliding; that may result from induced phase shifts between CDW chains. Under the joint application of dc and rf driving fields voltage Shapiro steps for longitudinal transport are observed as usual but also pronounced current Shapiro steps in transverse direction. The possible mechanisms of this effect as well as a tentative new view on the origin of the narrow band noise in CDW compounds are discussed.     

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.     

Size dependence of current oscillations in the charge density wave compound (TaSe4)2I

The volume dependence of the current oscillations has been examined in the current carrying charge density wave (CDW) state of (TaSe₄)₂I. The spectral width of the oscillations broadens as the specimen cross section is increased, indicating a nonuniform distribution of CDW velocity in the spicemens. The observed behavior is contrasted with that found in the more isotropic compound NbSe₃.     

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.     

Electric field dependent dielectric function in NbSe3

Dielectric function of quasi-one-dimensional conductor NbSe₃ has been measued in the vicinity of Peierls transition near T_p=145K by use of novel technique. The low frequency noise in V/I/ characteristics is attributed to the Gaussian type of position fluctuations of pinned charge density wave (CDW) and fundamental frequency of the noise is found to be identical with spring frequency of restoring forces.     

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.     

横向交、直流电流对蓝青铜K0.3MoO3的I-V特性的调制效应

通过I-V特性测量，研究了横向交、直流电流对蓝青铜K_0.3MoO₃中电荷密度波(CDW)动力学行为的影响.实验结果表明，无论是直流还是交流，随着横向调制电流的增大，CDW滑移的阈值电场均会相应地减小；但横向交流电流的调制效应较小，可能更接近本征的效应.考察了横向交流电流的调制效应与其频率的依赖关系. 关键词： 0.3MoO₃单晶')" href="#">K_0.3MoO₃单晶 电荷密度波 横向电流调制效应     

Transient conductivity and conduction noise in NbSe3

In the non-Ohmic regime of NbSe₃, the transient conductivity under pulsed field and the conduction noise under dc bias were investigated. The results under pulsed-field with two steps show the number of carriers (CDWs) causing the extra conductivity is field independent. The noise spectrum of the crystal on which the pulsed experiment was carried out has only one fundamental narrow band noise and exhibits no enhancement of the noise level. It is concluded for this crystal that the CDW slides uniformly over the whole crystal. It is also confirmed from measurements under two successive pulsed field that the configuration of the CDW changes as it begins to slide.     

Charge-density-wave partial gap opening in quasi-2D KMo6O17 purple bronze studied by angle resolved photoemission spectroscopy

Low dimensional (LD) metallic oxides have been a subject of continuous interest in the last two decades, mainly due to the electronic instabilities that they present at low temperatures. In particular, charge density waves (CDW) instabilities associated with a strong electron-phonon interaction have been found in Molybdenum metallic oxides such as KMo₆O₁₇ purple bronze. We report an angle resolved photoemission (ARPES) study from room temperature (RT) to T ∼40 K well below the Peierls transition temperature for this material, with CDW transition temperature T_CDW ∼120 K. We have focused on photoemission spectra along ΓM high symmetry direction as well as photoemission measurements were taken as a function of temperature at one representative k_F point in the Brillouin zone in order to look for the characteristic gap opening after the phase transition. We found out a pseudogap opening and a decrease in the density of states near the Fermi energy, E_F, consistent with the partial removal of the nested portions of the Fermi surface (FS) at temperature below the CDW transition. In order to elucidate possible Fermi liquid (FL) or non-Fermi liquid (NFL) behaviour we have compared the ARPES data with that one reported on quasi-1D K_0.3MoO₃ blue bronze.     

Spin- and charge-density-wave orders in La1.87Sr0.13Cu0.99Fe0.01O4

We have performed elastic neutron-scattering measurements on La_1.87Sr_0.13Cu_0.99Fe_0.01O₄ to study the magnetic impurity effects on the stripe correlations in high-T_c cuprates. Both charge-density-wave (CDW) and spin-density-wave (SDW) orders are observed in the low-temperature-orthorhombic (LTO) phase for the first time. With decreasing the temperature, CDW order first appears at followed by the SDW order at lower temperature of . The incommensurability for the CDW order (ε) was found to be 0.224±0.002 r.l.u., which is approximately twice of that for SDW order (δ) of 0.115±0.003 r.l.u. Both ordering sequence and relation of ε≈2δ are the same for SDW and CDW orders observed in the low-temperature-tetragonal (LTT) phase of La_1.60-xNd_0.4Sr_xCuO₄ (LNSCO), suggesting that the similar stripe correlations exist in the cuprate oxides, irrespective of crystal structure.     

Spectral purity and phase coherence of narrow-band noise in NbSe3

An unusually simple narrow band noise spectrum with a single fundamental and seven harmonics due to depinned charge density wave motion in NbSe₃ is reported. The simplicity of the spectrum is attributed to a very uniform current distribution associated with a short, thin sample of high perfection. The distribution of harmonics is near to that of the sawtooth waveform generated by a relaxation oscillator. As indicated by the amplitude ratio of the fundamental to the first harmonic, this distribution has only a moderate variation over three orders of magnitude in frequency. An electrical relaxation oscillator analog which reproduces most of the properties of CDW motion in NbSe₃ is discussed. A search for phase memory of the narrow band noise in NbSe₃ showed it to be less than one period.     

Frequency pulling and heterodyning observed in voltage spectrum of the Fröhlich conductor NbSe3

The voltage power spectrum of NbSe₃ has been studied in the upper charge-density wave (CDW) state where only one CDW exists. In the non-Ohmic regime sharp spikes appear in the spectrum. The frequency of these spikes increases monotonically with current as in the lower CDW state. We study the “fine structure” of the spectrum and find evidence for “frequency pulling” between the modes. A linear coupled-mode treatment is used to model this effect. There is also evidence of heterodyning of an intrinsic (current independent) 2MHz mode. We briefly discuss these phenomena in terms of Fröhlich sliding conductivity. Difficulties with the simple two-fluid model as well as models based on solid state turbulence are pointed out.     

Thermal expansion measurements of the quasi-one-dimensional conductor K0.30MoO3

The charge density wave (CDW) dynamics of the quasi-one-dimensional conductor K_0.30MoO₃ shows two different regimes depending on the temperature: a strongly damped CDW motion above ∼50 K and CDW motion with almost no damping below ∼50 K. In a search for a characterization of this CDW behaviour, we performed thermal expansion measurements on K_0.30MoO₃ single crystals in the temperature range 4-250 K. In addition to the anomaly observed at the Peierls transition at 180 K along the [102] direction, an anomaly is observed at ∼50 K along the [−201] and [102] directions. The results are discussed in relation with the change in the CDW rigidity at ∼50 K.     

Charge density wave noise in NbSE3

An extension of the noise measurements of Fleming and Grimes is reported for frequencies up to 100 MHz and at various temperatures. Somewhat below the lower transition temperature T₂, the frequency of the noise varies quadratically with the applied dc voltage. At lower temperatures a very different behavior is observed. We attribute the narrowband noise to coupled relaxation oscillators and suggest that the voltage-frequency relation reflects the distribution of CDW segments.     

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.     

Electronic structure and anomalous photoemission line-shape of quasi-2D oxide η-Mo4O11

The η-Mo₄O₁₁ compound is a layered two-dimensional (2D) metallic system whose reduced dimensionality originates non-linear properties as charge density wave (CDW) instabilities. We report on synchrotron radiation angle resolved photoemission spectroscopy (ARPES) measurements in order to obtain a detailed picture of the electronic structure of this material. The symmetry of the states near the Fermi level (E_F) has been discussed in relation to the photoemission symmetry selections rules. Our results are in excellent agreement with previous tight-binding calculations and support the hidden nesting concept proposed to explain the CDW instabilities exhibited by this family of compounds. In addition, a very peculiar photoemission line-shape has been found with the presence of localized non-dispersive states. Some possible explanations are discussed.     

Coupling of the CDW and the water mode in K2Pt(CN)4Br0.3⋯3.2H2O

In previous work we have observed the amplitude mode of the charge density wave (CDW) in K₂Pt(CN)₄Br_0.3?3.2H₂O (KCP) by means of Raman scattering. New measurements made on deuterated material, K₂Pt(CN)₄Br_0.3?3.2D₂O (KCP1), show the same mode but shifted from 44 to 38 cm_?1, maintaining the symmetry properties and temperature dependence of frequency and linewidth. This considerable isotope effect is interpreted in terms of a coupling of the CDW with the water stretching mode, which by the deuteration is shifted from 3494 cm^?1 in KCP to 2560 cm^?1 in KCP1 according to the change in atomic mass. Both of these modes exhibit A₁(z) symmetry. At 5 K the resulting decoupled frequency of the CDW amplitude mode is 57 cm^?1, and the coupling energy about 140 cm^?1. A discussion of the temperature dependence of various important quantities is given. The present results show that the water molecules, which are located in between the Pt chains are strongly involved in the eigenvector of the CDW amplitude mode.     

X-ray study of field induced deformation of sliding CDW in K0.30MoO3

A deformation of the CDW is investigated under the electric field whose magnitude is intensionally made inhomogeneous in the one-dimensional conductor K_0.30MoO₃. It is verified that the CDW's deformation, which occurs in the transverse (2a¹-c¹)-direction, is caused by the field gradient in the sample. In the sample where the CDW deformation is found without the intensional inhomogeneity of the electroc field, the deformation is not uniform from point to point. Possible relations are discussed between the structural change and the electrical polarization observed in pulse measurements of the conductivity.