Time-correlated soliton tunneling in charge and spin density waves

We consider a model in which an electric field induces quantum nucleation of kink-antikink pairs in a pinned charge or spin density wave. Pair nucleation events, prevented by Coulomb blockade below a pair creation threshold, become correlated in time above threshold. The model provides a natural explanation for the observed (i) small density wave polarization below threshold in NbSe (3), (ii) narrow band noise, (iii) coherent oscillations, and (iv) mode-locking at high drift frequencies.     

Effect of viscosity on collective zener tunneling in charge density wave systems

Zeitschrift für Physik B Condensed Matter - The theory of Caldeira and Leggett on tunneling in the presence of viscous dissipation is extended and applied to the problem of collective Zener...     

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.     

Interaction of both charge density waves in NbSe3 from interlayer tunneling experiments

An interlayer tunneling technique has been used for spectroscopy of charge density wave (CDW) energy gaps (Δ_1,2) in NbSe₃ subsequently opened at the Fermi surface on decreasing temperature at T _p1 = 145 K (CDW1) and at T _p2 = 60 K (CDW2). We found that the CDW2 formation is accompanied by an increase of the CDW1 gap below T _p2. The maximum enhancement of Δ₁, δΔ₁ is about 10%. The effect observed has been predicted theoretically as resulting from the joint phase locking of both CDWs with the underlying crystalline lattice below T _p2. The text was submitted by the authors in English.     

Spin-dependent tunneling in junctions containing metals with charge density waves in a magnetic field

The dependences of the differential tunneling conductance G on the voltage V across a junction in an external magnetic field H are calculated for two types of junctions involving normal or superconducting metals with charge density waves (CDWs). Junctions of the first type are asymmetric CDW metal (CDWM)-insulator-ferromagnet junctions. The results of calculations for these junctions demonstrate that there occurs splitting between the components of the conductance G(V) corresponding to the tunneling of electrons with spins aligned with the magnetic field H and opposite to it, as is the case with junctions containing a superconducting electrode instead of the CDWM electrode. Junctions of the second type are junctions between two normal or superconducting CDWM electrodes. For junctions with at least one normal CDWM electrode and H ≠ 0, the conductance G(V) also exhibits spin splitting. The form of the conductance G(V) for tunnel junctions of both types depends on the phase of the order parameter of the charge density waves.     

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) “interrupted” ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically T _g≃ 100-200 mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal, C. Monthus, Phys. Rev. Lett. 80, 3539 (1998)]. The typical relaxation time scales like ∼τexp(T _g/T). The glass cross-over temperature T_g related to correlations among solitons is equal to the average energy barrier and scales like T _g∼ 2xξΔ. x is the concentration of defects, ξ the correlation length of the CDW or SDW and Δ the charge or spin gap. Received 12 December 2001     

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.     

The influence of impurities on the quasiparticle tunneling current between weakly coupled quasi-one-dimensional charge density waves

The dc and ac conductivity of a tunneling junction between two impure quasi-one-dimensional charge density wave (CDW) systems is calculated. The non-magnetic impurities are considered in the self-consistent Born approximation (SCBA). Impurities modify the density of states (DOS) of the pure CDW system for quasiparticles inside the energy region of the gap 2(T). As in the pure case, the theory predicts in addition to a tunneling current which is proportional to the product of the DOS a term proportional to the cosine of the order parameter phase difference. In the case of a normal state/CDW junction, analytical expressions are obtained forT=0 showing deviations from the pure case. The linear ac conductivity is obtained by the scaling relation between the dc and the ac response.     

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.     

Energy relaxation in disordered charge and spin density waves

We investigate collective effects in the strong pinning model of disordered charge and spin density waves (CDWs and SDWs) in connection with heat relaxation experiments. We discuss the classical and quantum limits that contribute to two distinct contribution to the specific heat (a Cv T-2 contribution and a Cv T contribution respectively), with two different types of disorder (strong pinning versus substitutional impurities). From the calculation of the two level system energy splitting distribution in the classical limit we find no slow relaxation in the commensurate case and a broad spectrum of relaxation times in the incommensurate case. In the commensurate case quantum effects restore a non vanishing energy relaxation, and generate stronger disorder effects in incommensurate systems. For substitutional disorder we obtain Friedel oscillations of bound states close to the Fermi energy. With negligible interchain couplings this explains the power-law specific heat Cv T observed in experiments on CDWs and SDWs combined to the power-law susceptibility (T)T-1+ observed in the CDW o-TaS3.     

Pseudogaps in incommensurate charge density waves and one-dimensional semiconductors

We consider pseudogap effects for electrons interacting with gapless modes. We study generic 1D semiconductors with acoustic phonons and incommensurate charge density waves. We calculate the subgap absorption as it can be observed by means of photoelectron or tunneling spectroscopy. Within the formalism of functional integration and adiabatic approximation, the probabilities are described by nonlinear configurations of an instanton type. Particularities of both cases are determined by the topological nature of stationary excited states (acoustic polarons or amplitude solitons) and by the presence of gapless phonons that change the usual dynamics to the quantum dissipation regime. Below the free-particle edge, the pseudogap starts with an exponential (stretched exponential for gapful phonons) decrease of the transition rates. Deeply within the pseudogap, they are dominated by a power law, in contrast to a nearly exponential law for gapful modes.     

Fermi surface and charge density waves in niobium diselenide

The Fermi energy of niobium diselenide is calculated and the result is used to compute the wave-vector dependent susceptibility χ(q). This is found to have a fairly narrow peak at a wave-vector in very good agreement with that of the periodic lattice distortion developed by the solid at low temperature and such a peak can be seen to arise from good nesting of the Fermi surface.     

Breathing charge density waves in intrinsic Josephson junctions

We demonstrate the creation of a charge density wave (CDW) along a stack of coupled Josephson junctions (JJs) in layered superconductors. Electric charge in each superconducting layer oscillates around some average value, forming a breathing CDW. We show the transformation of a longitudinal plasma wave to CDW in the state corresponding to the outermost branch. Transition between different types of CDW’s related to the inner branches of IV characteristic is demonstrated. The effect of the external electromagnetic radiation on the states corresponding to the inner branches differs crucially from the case of the single JJ. The Shapiro steps in the IV characteristics of the junctions in the stack do not correspond directly to the frequency of radiation ω. The system of JJs behaves like a single whole system: the Shapiro steps or their harmonics in the total IV characteristics appear at voltage $\sum {V_l } = N_R \frac{m} {n}\omega$ , where V _l is the voltage in the lth junction, N _R is the number of JJs in the rotating state, and m and n are integers.     

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.     

Lattice modes and charge density waves in 1T-TaS2

The far-infrared reflectivity of 1T-TaS₂ was measured at 100, 295 and 390 K. At the two highest temperatures, the spectra were featureless. Overall reflectivity dropped abruptly in cooling through 350 K and through 190 K. These transitions are correlated to CDW formation. At 100 K five strong bands and at least two weak bands were observed. These are attributed to lattice vibration which can be excited optically as a result of the breakdown of selection rules due to the formation of a large superlattice by the onset of CDW.     

Magnetic field dependence of charge density waves in PbTe

Previous work by the authors indicated the possible existence of charge density waves in PbTe in the presence of strong magnetic fields so that the extreme quantum limit conditions prevail. The present work follows the transition as the field is lowered so that successively higher Landau levels become occupied. The transition persists at increasingly lower temperatures, having a maximum value, T_cmax, identifiable with each value, N, of the last (partially) occupied Landau level.     

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.