Superconductivity in the tungsten bronzeRbxWO3(0.20⩽x⩽<

Abstract

We have measured the magnetic susceptibility of the Rb x WO 3 compound ͑0.20ഛ x ഛ 0.33͒ and examined its structural properties and lattice dynamics, using elastic and inelastic neutron scattering ͑INS͒ experiments, in order to gain further insight into the unusual features of its superconducting state, namely, ͑i͒ the stabilizing effect resulting from the reduction of rubidium content, i.e., of the conduction electron density ͓what we shall name the "T c ͑x͒ paradox"͔, and ͑ii͒ the destabilizing effect of the ordering of the Rb ions. We also performed density-functional calculations of the phonon dispersion in the "stoichiometric" Rb 0.33 WO 3 and Cs 0.33 WO 3 to identify the main features of the phonon spectra. These calculations give a very satisfactory description of the INS data and confirm the assignment to these bronzes of a lower ͑orthorhombic͒ symmetry than previously proposed. Our results contradict the previous interpretations of the T c ͑x͒ paradox and of the ordering effect: ͑i͒ no general softening of the lattice accompanies the increase of the Rb-vacancy population and ͑ii͒ no general decrease of the electron density of states D EF distinguishes the ordered nonsuperconducting Rb 0.25 WO 3 from its neighboring disordered parents. It appears, therefore, that the electron-electron coupling in this system probably proceeds through well-defined electronic states and phonons. This is a feature these "hexagonal" tungsten bronzes ͑HTB͒ apparently share with several high-T c materials. We discuss what could be the mechanisms responsible for the very selective electron-phonon coupling in the HTB.

Spontaneous Formation of Zigzag Chains at the Metal-Insulator Transition in theβ-PyrochloreCsW2O6

“…In the low temperature structure, Δ 2 = 0.0184 and 0.0687 for W1 and W2, respectively. Even at this level, however, there is no clear indication of charge ordering in the sitesboth W1 and W2 are in much more regular octahedra than are found for 5d 0 W 6+ in WO3, Δ 2 = 0.714 and 0.659 [20], and much closer to what is found for the uniform charge state in the hexagonal tungsten bronze Rb0.33WO3, where Δ 2 = 0.119 and 0.088 [21]. Further there is no clear indication of orbital ordering of partially occupied 5d orbitals within the chains below the phase transition.…”

Theoretical investigation of twin boundaries in WO3 : Structure, properties, and implications for superconductivity

“…While BCS theory has been shown to capture some aspects of the behavior of doped WO 3 [9,21,75,76], the absence of superconductivity in the α 2 phase, and the decrease in T c with increasing doping are not well understood (the latter has even been described as the "T c paradox"! [9,14,16,18,73,77,78]), and we explore these aspects here.…”

Rattling-Induced Superconductiviy in the β-Pyrochlore Oxides AOs2O6

“…A recent INS study on the Cs and Rb compounds revealed the existence of local modes through the detection of a sharp peak at low energy in the phonon DOS. 108) However, no clear relationship between T c and the energy was observed, and, thus, it was suggested that the A vibrations are not crucial for the superconductivity.…”