A New Type of Charge-Density-Wave Pinning in Orthorhombic TaS3 Crystals with Quenching Defects

Diminishing in the concentration of quenching defects during thermocycling of orthorhombic TaS₃ samples in the temperature range below the Peierls transition temperature T < T_P is observed. It makes it possible to study the character of pinning of the charge density wave by these defects. A number of fundamental differences from pinning by ordinary local pinning centers—impurities and point defects—have been found. We conclude that quenching defects are extended (non-local) objects (presumably, dislocations) that can diffuse from the crystal during low-temperature thermocycling due to their strong interaction with the charge density wave, which is intrinsic for Peierls conductors. The presence of these defects leads to a previously unknown non-local type of the charge-density-wave pinning that acts on T_P and the threshold field for the onset of charge-density-wave sliding, E_T, differently in comparison with the local pinning centers.

     

On the tremendous effect of an electric field on the crystal lattice of quasi-one-dimensional conductors with a charge density wave

The deformation of the lattice of quasi-one-dimensional conductors with a charge density wave deformed by an electric field has been considered. In the case of the “strong” interaction between the charge density wave and lattice, the effect of the field can be compared to the usual piezoelectric effect with a tremendous piezoelectric modulus that is larger than the value in ionic crystals by a factor of L _c/λ (λ is the period of the charge density wave and L _c is the coherence length reaching several millimeters upon the sliding of the charge density wave). The interaction between the charge density wave and lattice is likely attributed to the possibility of the interband redistribution of charges (rearrangement of covalent bonds) in the process of the deformation of certain compounds with the charge density wave. The observed and expected effects provide a way of the creation of fundamentally new actuators including those of nanometer sizes.     

Infrared conductivity from spin and charge density waves: (TMTSF) 2X

Infrared conductivity from an incommensurate spin density wave occurs due to even-order charge density wave harmonics which interact with the host lattice. Phonon states within the density-wave-induced energy gap for single-particle excitations lead to conductivity much different from that of an incommensurate charge density wave including counter-ion ordering. The conductivity expected for relaxed and quenched states of (TMTSF)₂ClO₄ is discussed.     

Nonlinear conductivity and narrow band noise in NbSe3

A charge density wave system near commensurability and with strong damping is considered, as a model for NbSe₃. The observed threshold field is associated with depinning of a commensurate part of the charge density, while the excess charge, in form of phase kinks, contributes just to the ohmic conductivity. The characteristic length associated with the frequency generation is one lattice constant.     

Rigidity of charge density wave current under inhomogeneous conditions in the blue bronze Rb0.3MoO3

The narrow band voltage oscillation due to sliding charge density wave domains in the blue bronze Rb_0.3MoO₃ is shown to be unaffected by a variation of temperature along the sample. This proves the non-local nature of the relation between charge density wave velocity, driving electric field and temperature. The largest temperature gradient applied would give rise to a broadening equal to about 30 times the bandwidth of the oscillation investigated, were the charge density wave velocity determined by a local relation.     

Optical investigation of the quasi two-dimensional monophosphate tungsten bronzes K x P 4 W 8 O 32 (x = 0 - 1.57)

The potassium doped monophosphate tungsten bronzes K_xP₄W₈O₃₂ are two-dimensional metals which show a metal-to-metal transition at a critical temperature which depends on the doping level. The metal-to-metal transition is accompanied by the formation of a commensurate charge density wave with wave vector (π/b,0) which is independent of the doping level. Undoped P₄W₈O₃₂, on the other hand, has two metal-to-metal transitions which are connected to the formation of incommensurate charge density waves. We measured the infrared reflectivity of the series K_xP₄W₈O₃₂ (x = 0 - 1.57) in the spectral range from 100 to 10 000 cm^-1 for room temperature and well below the critical temperature. Polarization-dependent infrared spectra find a two-dimensional behavior in the normal and the charge density wave state and show signatures of hybridization between one- and two-dimensional conduction bands. In undoped P₄W₈O₃₂ the essentials of the charge density wave state can be understood from the nesting vectors of the calculated Fermi surface and two gaps are observed in the infrared spectra. The gap sizes are a factor of about 2.5 bigger than the predictions from mean-field theory in the weak-coupling limit which suggests medium- or strong electron-phonon coupling. For potassium doped K_xP₄W₈O₃₂ one gap is observed in the charge density wave state. The energetics of the charge density formation may be dominated by the energy required for the lattice modulation. Received 27 April 2001 and Received in final form 21 September 2001     

Anatomy of Gossamer superconductivity

There are many systems in which two order parameters compete with each other. Of particular interest are systems in which these order parameters are both unconventional. In this contribution, we examine the representative example of a d-wave superconductor in the presence of a d-wave density wave, which has been suggested as a model for the pseudogap phase in the high-T_c superconductors. The physical properties of unconventional superconductivity in the presence of an anisotropic charge density wave are investigated within mean field theory. This model describes many features that were anticipated by an earlier phenomenological treatment of Tallon and Loram. In addition, the quasiparticle density of states in the presence of these two order parameters is calculated, which should be accessible by scanning tunneling microscopy.     

Bond stretching phonon anomalies due to incommensurate charge density wave instabilities in high-Tc cuprates

Phonon anomalies observed in various high T_c cuprates are analyzed theoretically within the Hubbard-Holstein model in the limit of strong local electron correlations and in presence of long-range Coulomb interaction. The phonon self-energy is evaluated by taking into account the charge collective modes that become critical upon doping approaching an instability towards an incommensurate charge density wave (ICDW) driven by electron correlations. The doping dependence of phonon softening features and the highly distinctive phonon self-energy dependence on the wave vector agree with experiments. We discuss relevance of dynamical corrections to the density correlation function to achieve a sizeable bond-stretching phonon softening with a kink-like profile away from the zone boundary.     

Sm atomic dynamics in a charge density wave compound SmNiC2

¹⁴⁹Sm nuclear resonant inelastic scattering was carried out in a charge density wave compound SmNiC₂. We have investigated temperature dependences of the Sm partial phonon density of states and recoil-free fraction at the Sm site and the average sound velocity estimated from the Sm partial density of states. The Sm partial density of states exhibits temperature dependence, suggesting that the phonon modes between 20 and 25 meV may correlate with the charge density wave. Temperature dependence of the recoil-free fraction is difficult to prove the correlation with either the charge density wave or ferromagnetic ordering. The average sound velocity obtained by the Sm partial phonon density of states exhibits temperature dependence, agreeing qualitatively with very recent elastic constant measurements.     

欠掺杂高温超导体中的涡旋电荷结构相变

利用平均场t-t′-U-V-V_c模型,通过自洽求解Bogoliubov-de Gennes方程,研究了高温超导体中涡旋结构的相变.发现增大原位排斥势U,自旋密度波、电荷密度波以及d波序参量由棋盘结构转变为条纹结构.模型哈密顿量中引入合适强度的长程库仑势后,欠掺杂高温超导体样品中也可以出现二维或者棋盘结构,结果与文献报道的扫描隧道显微镜实验结果一致. 关键词： 高温超导 涡旋结构 长程库仑势     

Charge density waves in nanocrystalline thin films of blue bronze K0.3MoO3

Thin granular films of charge density wave (CDW) system K_0.3MoO₃ were prepared by pulsed laser deposition and investigated by various standard characterization methods such as GI-XRD, electric transport, TOF-ERDA, AFM and UV–visible spectroscopy. While all these methods indicate that the thin films consist of nanometer grains of K_0.3MoO₃, it is only the non-destructive femtosecond time-resolved spectroscopy (fsTRS) that demonstrates the charge density wave nature of the ground state and therefore proves directly the presence of K_0.3MoO₃. Furthermore, the comparison of the fsTRS data obtained in thin films and in single crystals shows the reduction of the charge density wave transition temperature and of the photoinduced signal strength in granular thin films with respect to single crystals, which is attributed to the granularity and crystal growth morphology. Our results establish fsTRS technique as the essential tool for the detection and characterization of complex ground states in nano-sized systems.     

Quantum coherent effects in layered nanostructures

Magnetotransport in structures based on NbSe₃, Bi₂Sr₂CaCu₂O_{8 + x} , and graphite has been examined. The stimulation of the charge density wave gap by a high magnetic field in NbSe₃ has been detected and studied using interlayer tunneling spectroscopy. It has been shown that this stimulation is of orbital character and the effect of the Zeeman splitting of the ground state of the charge density wave on this stimulation has been analyzed. The second harmonic of the gap singularity of the charge density wave in tunneling spectra, which is associated with the cascade relaxation of quasiparticles with the excess energy 2Δ. The effects caused by the interaction between Josephson vortices and Abrikosov vortex filaments have been studied on Bi₂Sr₂CaCu₂O_{8 + x} mesas. Studies of the magnetoresistance of thin graphite with nanoholes reveal a significant contribution to the magnetotransport from Dirac fermions and provide experimental evidence of the existence of edge states around nanoholes.     

Two wave collapses and a strange soliton in current carrying plasmas

A current driven plasma is analyzed in the two-fluid approach. A Lagrangian treatment of the charge neutral system results in a nonlinear scalar wave equation, which exhibits two different singular wave solutions. One wave collapse is associated with a density excavation, the other with a density compression. The latter represents the scenario of wave breaking and survives under charge separation. A new soliton solution is found just above threshold of linear instability of the charge nonneutral system in case of hot electrons. It is reminiscent of an ion acoustic KdV-soliton but differs from it in several respects such as in the propagation speed, in the strength and polarity of the electrostatic potential and in the spatial width. It is a finite ion temperature effect and disappears in the cold ion limit T_i → 0.     

Mössbauer studies of the charge-density-wave state of 57FeK0.30MoO3

Mössbauer studies have been performed between 6 K and 300 K on single crystals of ⁵⁷Fe-doped K_0.30MoO₃, which show a Peierls transition at 180 K towards on incommensurate charge density wave state. The spectra show three doublets. A strong line broadening below 120 K indicates that the Fe impurities are strong pinning centers for the charge density wave above ∼ 120 K and become progressively weak pinning ones below, in agreement with theoretical models.     

Effect of a Gaussian white noise on the charge density wave dynamics in a one dimensional compound

The effect of a white Gaussian noise (WGN) on the charge density wave (CDW) dynamics properties of a one-dimensional conductor is studied numerically in the weak pinning limit. In agreement with a recent experimental work on the charge density wave conductor K_0.3MoO₃, the addition of a Gaussian noise affects the CDW dynamics of the system. The results are discussed in the context of CDW dislocations.     

Temperature and pressure dependence of the non-linear properties of NbSe3

We report on the non-linear properties associated with charge density wave formation in NbSe₃ as a function of pressure up to 5 kbar. Although the charge density gap is reduced by more a factor of two, the minimum critical electric field is independent of pressure. A discussion of the non-linear behaviour of NbSe₃ is given.     

A simple interpretation of quantum mirages

In an interesting new experiment, the electronic structure of a magnetic atom adsorbed on the surface of Cu(1 1 1), observed by scanning tunneling microscopy, was projected into a remote location on the same surface. The purpose of the present paper is to interpret this experiment with a model Hamiltonian, using ellipses of the size of the experimental ones, containing about 2300 atoms. The charge distribution for the different wave functions is analyzed, in particular, for those with energy close to the Fermi energy of copper E_F. Some of them show two symmetric maxima located on the principal axis of the ellipse but not necessarily at the foci. If a Co atom is adsorbed at the site where the wave function with energy E_F has maximum and the interaction is small, the main effect of the adsorbed atom will be to split this particular wave function into two. The total charge density will remain the same but the local density of states will present a dip at E_F at any site where the charge density is large enough. We relate the presence of this dip to the observation of quantum mirages. Our interpretation suggests that other sites, apart from the foci of the ellipses, can be used for projecting atomic images and also indicates the conditions for other non-magnetic adsorbates to produce mirages.     

Quantum mechanisms of density wave transport

We report on new developments in the quantum picture of correlated electron transport in charge and spin density waves. The model treats the condensate as a quantum fluid in which charge soliton domain wall pairs nucleate above a Coulomb blockade threshold field. We employ a time-correlated soliton tunneling model, analogous to the theory of time-correlated single electron tunneling, to interpret the voltage oscillations and nonlinear current-voltage characteristics above threshold. An inverse scaling relationship between threshold field and dielectric response, originally proposed by Grüner, emerges naturally from the model. Flat dielectric and other ac responses below threshold in NbSe₃ and TaS₃, as well as small density wave phase displacements, indicate that the measured threshold is often much smaller than the classical depinning field. In some materials, the existence of two distinct threshold fields suggests that both soliton nucleation and classical depinning may occur. In our model, the ratio of electrostatic charging to pinning energy helps determine whether soliton nucleation or classical depinning dominates.     

Thermoreflectance spectroscopy of systems showing charge density wave transitions

The thermoreflectance spectra of 1T-TaS₂, -TaSe₂ and alloys of these compounds have been measured and show substantial changes at the different charge density wave transitions. The changes include the appearance of new structure and the modification of the shape and sign of the modulated spectra. These changes are due to the new gaps introduced into the band structure by the presence of a charge density wave.     

Prerequisites for chiral charge order

The chiral charge density wave state which was recently discovered in TiSe₂ can be understood as a combination of orbital and charge order. Here, we discuss the prerequisite material properties for this type of chiral charge order to emerge. We find that although both the lattice and orbital structure constrain the set of candidate materials, there remains a class of materials in which chiral charge order is expected to emerge.