Effect of charge density waves on the tunnel spectra of the Bi2Sr2CaCu2O8+δ superconductor

The differential tunnel conductance G _S of the junction between a normal metal and a superconductor with a charge density wave (CDW) is calculated as a function of the voltage V across the junction. The results are averaged over the spread of superconducting and CDW energy gaps in the nanoscale-inhomogeneous superconductor. It is shown that, if both order parameters are nonzero, a dip-hump structure is formed beyond the superconducting gap of G _S (V). If the phase of the CDW order parameter is not equal to π/2, a dip-hump structure will appear solely or mainly for one sign of the bias polarity. The results agree with the experimental data for Bi₂Sr₂CaCu₂O_8+δ and other high-temperature oxides     

Novel charge density wave transition in crystals of R5Ir4Si10

We review the observation of novel charge density wave (CDW) transitions in ternary R₅Ir₄Si₁₀ compounds. A high quality single crystal of Lu₅Ir₄Si₁₀ shows the formation of a commensurate CDW along c-axis below 80 K in the (h, 0, l) plane that coexists with BCS type superconductivity below 3.9 K. However, in a single crystal of Er₅Ir₄Si₁₀, one observes the development of a 1D-incommensurate CDW at 155 K, which then locks into a purely commensurate state below 55 K. The well-localized Er³ moments are antiferromagnetically ordered below 2.8 K which results in the coexistence of strongly coupled CDW with local moment antiferromagnetism in Er₅Ir₄Si₁₀. Unlike conventional CDW systems, extremely sharp transition (width ∼ 1.5 K) in all bulk properties along with huge heat capacity anomalies in these compounds makes this CDW transition an interesting one.     

Pion pair production in photon-photon collisions

The process γγ→π⁺π^? was measured using the detector MD-1 at VEPP-4. The two-photon reactionse ⁺ e ^?, μ⁺ μ^? and π⁺ π^? pair production were separated using scintillation counters, Cherenkov counters and shower-range chambers. A radiation widthГ _γγ(f ₂(1270))=3.1±0.35±0.35 keV was obtained.     

Infrared signature of the charge-density-wave gap in ZrTe<Subscript>3</Subscript>

The chain-like ZrTe₃ compound undergoes a charge-density-wave (CDW) transition at T_CDW=63 K, most strongly affecting the conductivity perpendicular to the chains. We measure the temperature (T) dependence of the optical reflectivity from the far infrared up to the ultraviolet with polarized light. The CDW gap Δ(T) along the direction perpendicular to the chains is compatible for T<T_CDW with the behavior of an order parameter within the mean-field Bardeen-Cooper-Schrieffer (BCS) theory. Δ(T) also persists well above T_CDW, which emphasizes the role played by fluctuation effects.     

Electron diffraction study of the charge-density wave superlattice in 2H-NbSe2

In addition to the well-known 3a₀ CDW superlattice, we have observed in 2H-NbSe₂ a 2a₀ superlattice and a well-defined elliptical contour of diffuse intensity between the 3a₀ CDW spots. The 2a₀ superlattice is indicative of a CDW formation through the saddle-point nesting on the Fermi-surface.     

Higher-twist contribution to meson pair production in hadron collisions

The contribution of higher-twist mechanisms to π⁺ π^?,K ⁺ K ^? and ρ° ρ° production in high-energy \(\bar p\) p collisions, with the two mesons emitted back to back at 90° in the c.m. frame and at moderate or largep _T , has been calculated and compared with the minimal-twist contribution, i.e. double jet fragmentation. The signal/background ratio is shown to depend critically on Δp, a parameter defining the upper limit for non-detection of accompanying particles; and, to a lesser extent, on the meson wave function. It is shown that, while there is little hope for checking higher-twist contributions in pion pair production, such contributions may show up clearly in kaon and rho pair production. In the last case, it may be possible to use polarization effects, in addition to cross-section measurements, in order to show the occurrence of higher-twist mechanisms.     

On the origin of quasi-periodic current noise in charge-density wave conductors: Experiments on orthorhombic tantalum trisulphide

The quasi-periodic components, which appear in the voltage V developed across a charge-density wave (CDW) conductor when the steady current I exceeds the threshold current I_T at which motion of the CDW begins, have been observed in orthorhombic TaS₃ at 77 K at frequencies v down to less than 1 Hz. Although its relation to the nonlinear part of the current when I?I_T indicates that v is, or is a low multiple of, the ‘washboard’ frequency v/λ, where v is the velocity and λ the wavelength of the CDW, the amplitude of the periodic variation of V when v<1 kHz is much too great to represent a modulation of the Frohlich current. The alternative is that the variation of V arises from a periodic modulation of the single-electron conductivity, resulting from distortion of the CDW as it moves through the crystal. Such a modulation of conductivity has been demonstrated experimentally, by interrupting a current I>I_T at different stages of the cyclic variation of V, and then using a current I<I_T to observe the resistance of the specimen when the CDW is at rest. Mechanisms whereby a periodic dependence of resistance on the position of the CDW may arise are briefly discussed.     

Phase analysis of non-coherent processes in a moving CDW

Earlier it has been found that the signal produced by a moving CDW contains along with fundamental of narrow band frequency (ω₁) and its harmonics (ω_n) a non-coherent part—a lot of non-coherent oscillations. Due to a rather broad frequency range the power associated with non-coherent oscillations is comparable to or even greater than that for fundamental frequency, and hence, can play an essential role in CDW dynamics.     

Puzzle of low temperature phase of α-(ET)2 salts

α-(ET)₂ salts split into two groups: one superconducting and another with mysterious low temperature phase (LTP). The LTP does not exhibit X-ray signiture of conventional CDW or magnetic one for SDW. But the magnetic phase diagram suggests strongly a kind of CDW. We have analysed the threshold electric field of unconventional CDW (UCDW) both for H=0 and H≠0. The threshold electric field for UCDW with H=0 and for 3D weak-pinning limit describes reasonably well the one determined in α-(ET)₂KHg(SCH)₄. The result for H≠0 is also presented. We propose that the LTP in α-(ET)₂ salts is most likely UCDW.     

Hysteresis in the electrical properties of orthorhombic tantalum trisulphide: Evidence for an incommensurate-commensurate charge-density wave transition?

A study of electrical conduction in orthorhombic TaS₃ has revealed the existence of thermal hysteresis throughout the temperature range 55 K < T < 205 K. This is attributed to variability in the wavevector q of the charge-density wave (CDW) which develops below T_p = 215 K, and confirms the recent finding, from electron diffraction, that at temperatures not too far below T_p the CDW is incommensurate with the underlying lattice. Evidence that q becomes commensurate, at least along the chain direction at 55 K is provided by the vanishing of hysteresis at that temperature, and also by a rise in the threshold field for continuous motion of the CDW.From its dependence on temperature it is concluded that between T_p and 55 K the conduction in the linear regime is better described as that of a Peierls semi-metal, rather than that of a Peierls intrinsic semiconductor. At most temperatures within that range electrical hysteresis also is observed, and a detailed study of this leads to the tentative conclusion that translation of the CDW conveys negative charge, carried presumably by negatively-charged discommensurations. The mechanisms of conduction below 55 K remain uncertain.     

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.     

Mixing experiments in NbSe3

The response of NbSe₃ to combined a.c. and d.c. fields has been characterized at T = 125 K below the first charge-density wave (CDW) transition. The a.c. and d.c. conductances were measured, along with the rectification and harmonic mixing at megahertz frequencies due to the CDW non linearity. These experimental results are shown to be consistent with a revised tunneling theory of CDW depinning.     

Deformation of a charge density wave in the vicinity of a point contact with a normal metal

The current-voltage characteristics of Cu-K_0.3MoO₃ point contacts between a metal and a semiconductor with a charge density wave (CDW) are studied for various diameters of the contacts in a wide range of temperatures T and voltages V. In the interval 80 K ? T ? 150 K, the current-voltage characteristics are correctly described in the framework of a semiconductor model: screening of an external electric field causes CDW deformation, shifts the chemical potential of quasiparticles, and changes the point contact resistance. It is shown that the chemical potential is above the middle of the Peierls gap in equilibrium and approaches the middle upon an increase in temperature. The current-voltage characteristics of point contacts with a diameter d ? 100 Å exhibit a sharp decrease in resistance for |V| > V _t , which is associated with the beginning of local CDW sliding within the contact region. The V _t (d, T) dependence can be explained by the size effect in the CDW phase slip.     

Search for massive photon pair production at the CERN intersecting storage rings

A search for massive photon pair production at \(\sqrt s = 63 GeV\) has been carried out on the data sample previously employed for the electron pair production study. Positive evidence is reported form _γγ>6GeV, with a production cross-section similar to Drell-Yan electron pairs. The ratio γγ/π⁰π⁰ was measured to be ～10^?3 for ap _T of each γ or γ or π⁰ above 3 GeV/c.     

Non-local conduction by charge-density waves in niobium triselenide

The threshold field E_T for the onset of nonlinear conduction in NbSe₃ has been measured using both the normal 4-terminal arrangement, and a transposed version with current flow between the inner pair and voltage measured between the outer. When the distance between the inner terminals is small, and the temperature between 60 and 110 K, E_T is significantly greater for the transposed than for the normal arrangement. The phenomenon is thought to demonstrate that the nonlinear conduction, which arises from motion of the charge-density wave (CDW) forming at 144 K, is non-local over distances of the order of 100μm.Further experiments indicate that E_T is greater with the transposed arrangement because the CDW then has to be broken near the inner terminals, in order that its central section might move independently. Evidence is presented that the breaking process at an inner terminal depends on whether current flows from or towards it, and that above 110 K breaking occurs even when the normal configuration is used, apparently because the CDW is then too weak to overcome the pinning introduced by the contact material.No corresponding effects have been observed in nonlinear conduction by the CDW which forms at 59 K.     

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.     

Joint density of states and charge density wave in 2H-structured transition metal dichalcogenides

The joint density of states of two different 2H-structured transition metal dichalcogenides (TMDs) with and without charge density wave (CDW), Na_0.05TaS₂ and Cu_0.09NbS₂, respectively, are compared. While there is a clear maximum at the 3×3 charge density wavevector for Na_0.05TaS₂, the joint density of states for Cu_0.09NbS₂ does not show such behavior, consistent with the absence of CDW in the system. Our results illustrate that the joint density of states well represents the charge instability in 2D systems.