Response Of Charge-density Wave Research Articles

Experimental and Theoretical Studies of the Surface Oxidation Process of Rare‐Earth Tritellurides

AbstractRecent studies have established Van der Waals (vdW) layered and 2D rare‐earth tritellurides (RTe3) as superconductors and near room‐temperature charge density wave (CDW) materials. Their environmental stability raises natural concern owing to aging/stability effects observed in other tellurium‐based layered crystals. Here, the results establish the stability and environmental aging characteristics of these RTe3 systems involving a variety of metals such as La, Nd, Sm, Gd, Dy, and Ho. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results show that all the RTe3 sheets oxidize to form thin TeOx layers that are primarily confined to the surface, edges, and grain boundaries. Time‐resolved in situ Raman spectroscopy measurements are used to understand the kinetics of the oxidization process for different lanthanide metal cations and establish their relative stability/resilience to oxidization. Overall results indicate that the vdW layers show higher air stability as the 4f electron number decreases going from Ho to La, resulting in the most stable LaTe3 compared to the least stable HoTe3. Comprehensive quantum mechanical simulations reveal that environmental degradation originates from a strong oxidizing reaction with O2 molecules, while humidity (H2O) plays a negligible role unless Te vacancies are present. Moreover, the simulations explain the effects of 4f electrons on the work function and Te vacancies formation, which directly impact the aging characteristics of RTe3 layers. Interestingly, optical and electrical measurements show that the CDW response is still observed in aged RTe3 layers owing to the presence of underlying pristine/nonoxidized RTe3 layers, except CDW transition temperatures increase due to the thickness effect. Overall results offer the first in‐depth environmental aging studies on these materials, which can be applied to engineer and design their chemical stability, surface properties, and overall CDW characteristics.

Impurity Effect on the Inter-Chain Dielectric Response of the CDW Condensate in K0.3MoO3

We study the effect of impurity on the charge-density-wave (CDW) state via low-frequency dielectric measurements, especially for the inter-chain direction. The extremely high dielectric constant seen in non-doped K0.3MoO3 is strongly reduced by W- or V-doping, demonstrating that the three-dimensional (3D) phase rigidity is an innate characteristic of CDW condensate. In the impurity-doped K0.3MoO3, the 3D rigidity has been lost and the CDW state exhibits a different nature in which the dielectric response of CDW cannot be explained in terms of the screened phason model.

Phase diagrams of (La,Y,Sr,Ca)14Cu24O41: Switching between the ladders and the chains

The most comprehensive charge response study of the intrinsically hole doped, spin-chain and spin-ladder composites is overviewed. Results of dc and electric-field-dependent resistivity, low frequency dielectric, and optical spectroscopy in all crystallographic directions are used to build phase diagrams of the underdoped materials (hole count= 6 - y per formula unit, f.u.) and of the fully doped, 6 holes per f.u. The underdoped materials are insulators with hopping transport along the chains, which behave as a one-dimensional disordered system. For the fully doped materials the charge transport switches to the ladders due to transfer of holes from the chains. Two-dimensional (2D) charge-density wave (CDW) ground state is formed, i.e. besides CDW phason response along the ladders for 0 ≤ x ≤ 9, we also found it along the ladder rungs for x ≤ 6. However, CDW sliding conductivity, as observed in the standard CDW materials, is not observed in either of directions. Normal to the ladder planes no CDW response was found, and transport is presumably hopping-like. For the first time in any system, CDW response away from the principal direction was identified. For x > 9 both CDW in ladders and CO in chains are supressed.

Surface phason-polaritons in charge density wave films

The coupled non-radiative excitations of the electromagnetic field and phasons in films with a quasi one-dimensional charge density wave (CDW) are evaluated for P-polarization and CDW conducting axis inside the film. The prominent features are two surface phason-polariton branches extending from the CDW pinning frequency to the frequency of the longitudinal optical phason. These surface phason-polariton states are confined to a finite band of longitudinal wave numbers. Besides surface polaritons, infinite series of guided wave modes are found which extend to large wave numbers. These differences to usual phonon-polaritons are caused by the extreme anisotropy of the electric CDW response. At finite temperatures, quasi-particles are thermally excited. Their dissipation leads to polariton damping. Significant level shifts of surface phason-polaritons and guided wave modes are also found. They are due to thermal dressing of the longitudinal optical phason via quasi-particles. The zero and finite temperature results including the case of neglected retardation are displayed in detail.

Phase slippage and narrow-band noise in TaS3 at low temperatures.

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.

Charge-density-wave dynamics in (Ta1-xNbxSe4)2I alloys.

Frequency-dependent conductivity and phase-resolved harmonic-mixing studies are reported on alloys of the linear-chain compound (${\mathrm{TaSe}}_{4}$${)}_{2}$I with Nb in the microwave and millimeter-wave spectral range. The frequency-dependent conductivity is evaluated in the charge-density-wave (CDW) state, and the collective-mode response is analyzed in terms of a harmonic-oscillator response with an effective mass ${\mathit{m}}^{\mathrm{*}}$, a pinning frequency ${\mathrm{\ensuremath{\omega}}}_{0}$, and relaxation time ${\mathrm{\ensuremath{\tau}}}^{\mathrm{*}}$ as parameters. We find that the magnitude, temperature, and concentration dependence of the effective mass ${\mathit{m}}^{\mathrm{*}}$ is well described in terms of the mean-field description of the dynamical CDW response. The pinning frequency ${\mathrm{\ensuremath{\omega}}}_{0}$ increases with increasing impurity concentration x, and the concentration dependence is suggestive of strong-impurity pinning. We also evaluate the temperature and concentration dependence of the relaxation time ${\mathrm{\ensuremath{\tau}}}^{\mathrm{*}}$, and compare this parameter with various models.

Dielectric relaxation in o-TaS3: Charge-density-wave screening and near-commensurability effects.

The temperature dependence of the rf dielectric response has been measured from 223 down to 100 K for the charge-density-wave (CDW) conductor o-${\mathrm{TaS}}_{3}$. A temperature-independent response close to the transition gives way to an Arrhenius behavior below 150 K. All features of the experimental data are quantitatively reproduced by a simple phenomenological model which includes normal-carrier screening of the CDW. Data taken below 150 K are shown to be consistent with the appearance of a second pinned mode, now identified as a universal feature associated with a temperature-activated CDW response. Near-commensurability effects are proposed to account for the observed increase in the static dielectric constant and decrease in the dc threshold field between 150 and 100 K.

Charge-density-wave transport in quasi-one-dimensional conductors. II. ac-dc interference phenomena.

We present a detailed experimental study of ac-dc interference phenomena in charge-density-wave (CDW) transport. Harmonic and subharmonic ``steps'' of constant CDW current are observed when ac and dc voltages are applied together, and the widths of these steps exhibit Bessel-like oscillations as a function of both ac amplitude and frequency. We now observe similar oscillations in the dc threshold field, and thereby complete the analogy between these phenomena and similar effects in Josephson junctions. By minimizing crystal defects and contact effects, we have obtained complete harmonic and subharmonic phase locking in very long ${\mathrm{NbSe}}_{3}$ crystals under a broad range of conditions, including cases where the total time-dependent voltage remains above the dc threshold ${V}_{T}$. We analyze our experimental results with a single-coordinate model used previously to account for subharmonic steps. It is based on the tunneling theory and assumes CDW motion in a highly nonsinusoidal pinning potential. This model provides an excellent and semiquantitative account of all our results; it also provides a detailed interpretation of interference features observed in the ac conductance, electric-field-induced anomalies observed in the CDW elastic response, and the inductive loops observed in phase-space plots of the CDW's response to ac fields. The quality of the agreement provides strong evidence both for an effective pinning potential of the proposed form and for a single-degree-of-freedom description of CDW transport.

Charge-density-wave transport in quasi-one-dimensional conductors. I. Current oscillations.

We present a detailed experimental study of current oscillation phenomena in charge-density-wave (CDW) transport. The amplitude and harmonic content of the quasiperiodic component of the current oscillations remain large and approximately constant even at very high electric fields, suggesting that the form and magnitude of the impurity pinning potential are approximately independent of applied field. A simple single-coordinate model of CDW motion in a nonsinusoidal pinning potential, motivated by the quantum tunneling theory of CDW depinning, accounts for all the qualitative features of our results. Nearly perfect velocity coherence seems to be characteristic of weak-impurity-pinned CDW's in high-quality ${\mathrm{NbSe}}_{3}$. However, other types of crystal defects together with contact effects act to broaden the observed distribution of CDW velocities. This velocity distribution plays a crucial role in the ``ringing'' observed in the CDW response to a current pulse, and also in generating the broadband noise.

Moving charge-density waves in K 0.30MoO 3

A number of anisotropic materials exhibit non-linear charge transport resulting from a charge-density wave (CDW) that moves in an applied electric field. In these materials the CDW is pinned to the lattice by impurities at low electric fields, but may be depinned by d.c. fields in excess of a threshold value. A moving CDW is a new form of charge transport in a solid and it is associated with a number of interesting non-linear electrical phenomena. These include enhanced d.c. and a.c. conductivity, a periodic response to a d.c. bias, noise, and metastable conductivity states. The ‘blue bronze’ K 0.30MoO 3 has proved to be an interesting material in which to study CDW transport because of its large sample size and large metastable effects. This paper reviews a.c. measurements of the pinned and unpinned CDW in K 0.30MoO 3. In addition, d.c. measurements and X-ray scattering measurements of field-cooled samples are reported. The unifying feature of these measurements is that the non-linear CDW response is dominated by a large number of degrees of freedom. One must therefore view CDW transport as motion of a deformable medium rather than the sliding of a rigid object.