Electronic structure for sliding charge density waves

We present a model for the electron system in NbSe₃ based on its quasi one-dimensional metallic properties. In a one-dimensional metal phonon drag of 2K_F-phonons takes place at temperatures higher than θ_D, since the phonon-electron scattering rate τ^?1_ph?el is greater than the phonon-phonon rate τ^?1_ph?ph. this situation is in contrast to the situation in three dimensional metals, where phonon drag takes place only at very low temperatures. Our model explains the transport properties of the material including the electrical conductivity anistropy, the conductivity in a strong electric field, and the Hall effect data.     

Multidimensional elastic waves excited by oscillating domain walls

Magnetoelastic excitations with a fine structure in the 50kHz range have been observed in the study of the domain wall resonance (DWR) in magnetic garnet thin films. DWR excites standing transverse elastic waves which have a resonance frequency given by $\text{f=}\text{nv}\text{2d}$, where f is the frequency, n is an integer, v=3.5×10⁵ cm/sec is the transversal velocity of the elastic wave, and d=0.05 cm is thickness of the film/substrate system. A fine structure associated with each of these modes has been identified as due to two dimensional bulk elastic waves by using a set of parallel microstrip lines. The dispersion relation of these elastic waves is ω²=v²(k²₁+k²₂), where ω is the radial frequency, k₁ and k₂ are the wave vectors in the orientation perpendicular and parallel to the sample surface respectively. In the case of k₁?k₂, $\text{f=}\text{f}{}_0\text{+v}{}^2\text{k}{}^2{}_2\text{f}{}_0$, where f₀ is the resonance when k₂=0. The experimental results are in excellent agreement with this model. A linear dispersion, observed when using a shorted slot-line structure, is understood as the excitation of three dimensional modes due to the complex structure of the slot-line and the sample geometry.     

Phenomenology of current oscillations by sliding charge density waves

Bardeen's suggestion that the dc current associated with the sliding of a charge density wave instead of the dc electric field determines the dynamics of the phase, is formulated in a simple but self-consistent way. The resulting equation of motion for the phase reduces to that of the classical model of a rigid charge density wave far above threshold with a new, nondissipative scaling frequency. It is suggested that the harmonic content of the narrow band noise monitored as function of dc bias may decide between both interpretations. The model may also have relevance to the observation of subharmonics and chaos in charge density wave systems.     

Bistability of non-linear conductivity in insulators with sliding charge density waves

At low temperatures in semiconducting compounds with a sliding charge density wave (CDW), it is shown that the nonlinear d.c current voltage characteristic should be bistable. The high velocity branch describes free sliding of the CDW, motion which becomes undamped at zero temperature (“Fröhlich superconductivity”). In the low-velocity branch the CDW is highly deformed, with a velocity low enough that backflow currents of thermally excited quasiparticles can screen the local electric fields produced by the moving CDW. The results are compared with recent experiments on the blue bronze K_0.3MoO₃.     

Stability of uniformly driven domain walls

An infinite ferromagnetic medium with orthorombic crystal field anisotropy and with a constantdc field applied along the easy axis is considered in the framework of micromagnetics. It is shown that in the presence of Gilbert damping the uniformly driven plane domain wall (Walker wall) with positive Döring mass is linearly stable for any strengthH of the applied field below a critical valueH ₀, where it connects to an unstable wall configuration with negative Döring mass. The critical fieldH ₀ is lower than the existence limitH _W (Walker limit) of the solitary travelling-wave type domain wall solutions, and represents the threshold of the field regionH _{0 W} where negative wall mobilities occur. The instability atH=H ₀ is associated with a nonuniform localized relaxation mode (corrugating mode) of the plane domain wall. If coupling to the external circuit is taken into account, negative-mobility walls also become unstable against uniform perturbations for sufficiently small circuit resistance.Work supported by the Swiss National Science Foundation     

Coupled charge density waves in nearly one dimensional systems

A mean field theory of coupling between charge density waves CDW in linear chain systems is described. It is shown that interchain coupling can stabilize CDW's and lead to a semiconducting behavior. The model is applied to semiconducting TCNQ salts.     

Conductivity from charge or spin density waves

Below a Peierls transition the coupled electron phonon collective mode plays an important role in the conductivity of one-dimensional metal models such as have been recently postulated for various organic compounds. Within the jellium model, or in an incommensurate situation, the mode frequency goes to zero for q → 0 and is responsible for the infinite conductivity first proposed by Fröhlich. Impurities, lattice commensurability and three dimensional ordering introduce a gap into the mode spectrum. The low frequency conductivity and a large dielectric constant are predicted. Similar effects are predicted for a spin density wave.     

Pinning of charge density waves by irradiation induced defects in orthorhombic TaS3

Effects of electron irradiation induced damage on dc conductivity, threshold field and complex dielectric constant at 9 GHz are investigated in a wide temperature range. Whereas the single particle gap in the low temperature phase is not influenced by irradiation, transport properties associated with collective excitation of charge density waves (CDW) are sensitive to the presence of new pinning centres. The results are discussed in the framework of the semiclassical model of Grüner, Zawadowski and Chaikin. The pinning frequency determined from the threshold field agrees well with that of calculated from the complex dielectric constant.     

Linear and non-linear ac response of the stochastic classical model for sliding charge density waves

A comprehensive study of the ac response properties of the classical stochastic model for sliding charge density waves (CDW) in quasi one-dimensional metals is made by numerically solving the associated Fokker-Planck equation. Above the conductivity threshold a noise mechanism is indispensable to give finite line widths to the resonances of the applied ac signal of frequency with the narrow band noise frequency _OSC inherent in the model. In the present investigation a current noise of strengthT _N proportional to the CDW current is used in the Fokker-Planck equation in order to model the broad band current noise frequently observed above threshold. The present model thus incorporates three characteristic frequency scales: _OSC,T _N ,and a crossover frequency _OSC. Results are evaluated for the ac conductivity (;E ₀,E ) as function of frequency , dc bias electric fieldE ₀ and ac signal field strengthE . ForE 0 the linear ac response is obtained by a separate treatment of the Fokker-Planck equation. The resonances near =_OSC are studied in detail. Strong ac signals reduce the response at the fundamental resonance and lead to a harmonic interference structure nearn=_OSC. The overall agreement of the present results with recent measurements of the linear ac response is not good. In reality our results seem to be superimposed on a background not reproduced by the classical model with one cross over frequency. However, the peak in Im (;E ₀,E =0) vs.E ₀, when the narrow band noise frequency is near , is well reproduced. The spectral width of this peak which corresponds to the inductive dip in the susceptibility is studied as function of current noise strengthT _N .The results stress the need for a complete Fokker-Planck treatment sinceT _N is not simply related to the line width.     

Transportation of pinned charge density waves

We investigated the transport of pinned charge density waves (CDWs) that is observed in low dimensional materials. We treated pinned CDWs as moving CDWs that were confined within a typical quantum well amongst the many different types where pinning occurs at the barrier. We calculated the current flowing out of the quantum well by confined CDWs. The calculated conductivity is in good correspondence with experimental data in TTF–TCNQ, where the measured Fröhlich–Peierls temperature is 60 K much higher than the theoretical value of 20 K. The voltage dependence of the conductivity was calculated, where this is easily transformed into the dependence of electric field. The magnetic susceptibility was also calculated with a similar trend of experimental data. The susceptibility is a diamagnetic contribution by CDWs in addition to the constant background Pauli paramagnetic part.     

Narrow band current oscillation amplitudes in the stochastic classical model for sliding charge density waves

The first theoretical investigation of the amplitudes of the narrow band current oscillations generated by sliding charge density waves is given using the stochastic classical model with a current noise source. In contrast to the classical model without fluctuations, the power spectrum S(ω) of the current-current correlation function has finite peaks S²_n and non- vanishing line widths Γ_n near ω = nω_osc where ω_osc is the fundamental frequency of the current oscillations. For weak current noise, analytical expressions for s_n and Γ_n are given which agree with the exact numerical treatment of S(ω) using the Fokker-Planck approach. The results are in accord with the observed asymptotic decrease of the fundamental amplitude s₁ with increasing d.c. bias. They also confirm the validity of the weak noise limit for sliding charge density waves.     

Pinning of domain walls in two-layer ferromagnetic nanowire with scattering fields of nanoparticles

The results of a micromagnetic simulation of the pinning-depinning processes of a domain wall (DW) in a rectangular ferromagnetic nanowire (NW) consisting of two magnetic layers with scattering fields of two rectangular two-layer nanoparticles (NPs) located on NW opposite sides and oriented perpendicular to its axis are presented. The features of magnetization reversal of this system in the external magnetic field are studied depending on direction of the magnetic moments of the nanoparticle layers. The value of the depinning field in such a system depends essentially on mutual orientation of NP magnetic moments and NW magnetization. The possibility to realize a magnetic logic cell performing the “conjunction” operation of ternary logic is discussed.     

Nonlinear magnetoinductive waves and domain walls in composite metamaterials

We describe novel physics of nonlinear magnetoinductive waves in left-handed composite metamaterials. We derive the coupled equations for describing the propagation of magnetoinductive waves, and show that in the nonlinear regime the magnetic response of a metamaterial may become bistable. We analyze modulational instability of different nonlinear states, and also demonstrate that nonlinear metamaterials may support the propagation of domain walls (kinks) connecting the regions with the positive and negative magnetization.     

Pseudogap and charge density waves in two dimensions

Using angle-resolved photoemission spectroscopy we demonstrate that a normal-state pseudogap exists above T(N-IC) in one of the most studied two-dimensional charge-density wave (CDW) dichalcogenides 2H-TaSe(2). The initial formation of the incommensurate CDW is confirmed as being driven by a conventional nesting instability, which is marked by a pseudogap. The magnitude, character, and anisotropy of the 2D-CDW pseudogap bear considerable resemblance to those seen in superconducting cuprates.     

Domain walls and ising-BLOCH transitions in parametrically driven systems

Parametrically driven systems sustaining sech solitons are shown to support a new kind of localized state. These structures are walls connecting two regions oscillating in antiphase that form in the parameter domain where the sech soliton is unstable. Depending on the parameter set the oppositely phased domains can be either spatially uniform or patterned. Both chiral (Bloch) and nonchiral (Ising) walls are found, which bifurcate one into the other via an Ising-Bloch transition. While Ising walls are at rest Bloch walls move and may display secondary bifurcations leading to chaotic wall motion.     

The control of gap width in the low-dimensional systems with charge density waves instability

The strong photoeffect was discovered experimentally in InSe crystals at temperature 4.2 K as a result of laser radiation action at fixed wavelengths of 337 and 195 micron. Effect was explained with appearance of the gap in the continuum of conduction band. The photoeffect value was found to depend strongly on electric current through the crystal.     

Shear modulus and plasticity of a driven charge density wave

We have probed the effects of transverse variations in pinning strength on charge-density-wave (CDW) structure in NbSe3 by x-ray micro-beam diffraction. In ribbonlike crystals having a large longitudinal step in thickness, the CDW first depins on the thick side of the step, causing rotations of the CDW wave vector. By measuring these rotations as a function of position and electric field, the corresponding shear strains are determined, allowing the CDW's shear modulus to be estimated. These results demonstrate the usefulness of x-ray microdiffraction as a tool in studying collective dynamics in electronic crystals.     

On the coexistence of superconductivity and charge density waves

We study a simple tight-binding (Hubbard) model for electrons interacting via a short range attractive potential. We show that on a cubic lattice, for a half-filled band the ground state may feature both superconductivity and a charge density wave. We examine the response of such a ground state to an external magnetic field and describe the effect of the charge density wave on the Meissner effect.