Superlattice spots observed by X-ray and electron diffraction in the tetragonal, metallic, quasi-one-dimensional compounds NbTe4 and TaTe4 are interpreted in terms of charge-density waves. It is shown that three slightly different types of columns occur in the crystals. In NbTe4 at room temperature two types of chains have incommensurate distortions with wavevectors q1=(0, 0, 0.311c*) and q2=(0.5a*, 0.5b*, 0.344c*): the third type has a very weak distortion with q3=(0.5a*, 0, 1/3c*). The wavevectors for TaTe4 are as above except that the component along the metal chain is exactly 1/3c* in each case. The vectors q1 and q2 are independent of temperature but can be varied by substitution of Ta for Nb. These are structurally the simplest inorganic crystal structures in which CDW have been observed and are the only reported crystals in which three CDW coexist.

The effect of low-temperature annealing on the superconducting transition temperature and electrical resistance of single crystals of YBa2Cu3O7−δ with an oxygen deficiency δ=0.5–0.6 quenched from temperatures of 620–650 °C is considered. The transition temperature increases during annealing, while the resistance decreases. The isothermal relaxation of resistance is measured and used for estimating the activation energy of the relaxation process, which coincides with the activation energy of oxygen diffusion. The obtained results are attributed to oxygen ordering in Cu-O planes without changing its concentration. The observed step form of resistive transitions to the superconducting state is interpreted as the formation of clusters characterized by different oxygen concentrations and ordering. The cluster size is estimated.

Erratum: NbS 3 : A unique quasi-one-dimensional conductor with three charge density wave transitions [Phys. Rev. B 95, 035110 (2017)]

A series of hydrogenated cubic C15 and hexagonal C14 ZrCr 2 Laves phases were studied by means of semiempirical extended Hu ¨ckel tight-binding, ab initio density functional theory methods and maximally localized Wannier functions, with a goal to find the most energetically favorable positions of interstitial H atoms in the host unit cells. We consider situations with one or two H atoms per primitive cell. Crystalorbital overlap population studies, performed for the C15 structure, show repulsion between two hydrogen atoms in close proximity. This is in accord with the ab initio calculations, performed for the hydrogenated C14 and C15 structures, which clearly favor two separated hydrogen atoms instead of the formation of moleculelike pairs in five-coordinated trigonal-bipyramidal environments.

The effect of hydrostatic pressure on the resistance and resistive transition to the superconducting state in YBa2Cu3O7−δ single crystals with different oxygen contents is studied experimentally. It is found that, following the application (or removal) of pressure, the resistance relaxes to an equilibrium value that depends on pressure, the relaxation time at room temperature being about two days. It is shown that the value of the superconducting transition temperature depends on the pressure, but the width and shape of the transition depend mainly on the degree of resistance relaxation to the equilibrium value. It is concluded that the decrease in resistance under hydrostatic pressure is associated with the ordering of labile oxygen in the Cu–O plane. The possible mechanisms of variation of the superconducting transition temperature are discussed.

Through transport, compositional and structural studies, we review the features of the chargedensity wave (CDW) conductor of NbS3 (phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at TP 1 = 360 K and TP 2 = 150 K, another CDW transition occurs at a much higher temperature TP 0 = 620-650 K; evidence for the non-linear conductivity of this CDW is presented. 2) We show that CDW associated with the TP 2 -transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. 3) We show exceptional coherence of the TP 1-CDW at room-temperature. Additionally, we report on the effects of uniaxial strain on the CDW transition temperatures and transport.

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