Possible nucleation of a 2D superconducting phase on WO single crystals surface doped with Na

Abstract

WO3 crystals with a surface composition of Na0.05WO3 were grown. These crystals exhibit a sharp diamagnetic step in magnetization at 91 K, and a magnetic hysteresis below this temperature. As the temperature is lowered below 100 K in transport measurements, a sharp metal to insulator transition is observed, this is followed by a sharp decrease in the resistivity when the temperature is lowered to about 90 K. When the surface of the crystals was covered by gold the depth of the diamagnetic step had decreased considerably. These results indicate a possible nucleation of a superconducting phase on the surface of these crystals. This is a non cuprate system exhibiting a critical temperature in the HTS range. PACS. 74.70.-b Superconducting materials (excluding high-Tc compounds) -74.20.-z Theories and models of superconducting state -74.60.Ec Mixed state, critical fields, and surface sheathTwo dimensional and quasi 2D superconductivity are common phenomena. The first example goes back to 1963 when Saint-James and de Gennes [1] showed that in an ideal sample with anhomogenous order parameter the nucleation of superconducting regions in decreasing field should always occur near the surface of the sample. For a superconductor of the second kind this implies that in fields H : H 2 < H < H 3 there remains a superconducting sheath on some parts of the sample confined to a thickness of the order of the correlation length ξ. In HTS perovskite cuprates the order parameter is spatially modulated along the c axis as the superconductivity is quasi 2D in character and is confined to the CuO planes coupled by Josephson junctions. A question arises about a possible system in which the structure is modulated in such a way so that superconductivity will develop on the surface only, not a type III superconductivity, and will not propagate into the bulk. In this short paper we suggest that WO 3 single crystals doped on the surface with sodium may constitute such a superconductor. This is a non cuprate system with a critical temperature of 91 K.The 5d-transition-metal oxides WO 3 and Na x WO 3 are similar in electronic structure to 3d oxides. They have nearly the ABO 3 perovskite crystal structure, with the W ions occupying the octahedral B cation sites. In WO 3 the A cation site is vacant, while in Na x WO 3 the Na + ions occupy that site. Stoichiometric WO 3 is an insulator, since the W 5d band is empty; when Na ions are added to WO 3 , they donate their 3s electron to the W 5d band, resulting in bulk metallic behavior for x ≥ 0.3 [2,3]. These materials in tetragonal or hexagonal form exhibit also bulk superconductivity at sub-liquid helium temperature [4,5]. Sheet superconductivity at 3 K in twin walls of a non superconducting tetragonal WO 3−x was reported by Aird et al. in 1998 [6,7]. For x < 0.3 the Na x WO 3 sodium tungsten bronzes, formed by doping the insulating host WO 3 with Na + ions, are n-type semiconductors.We prepared single crystals in which the surface is doped with Na + ions following the method described in reference...

Inhomogeneous Stripe Phase Revisited for Surface Superconductivity

“…Our results provide a tool for studying surface superconductivity as a function of doping. Islands of a surface superconducting phase were also observed for the surface-doped tungsten bronzes, WO 3 :Na, at T c = 91.5K [4] . The ARPES results mentioned above suggest that SSC may actually be a rather widespread phenomenon.…”

Superconducting 2D System with Lifted Spin Degeneracy: Mixed Singlet-Triplet State

“…Superconductivity (SC) with critical temperature T c 91 K existing in small islands at the surface of insulating WO 3 (when the surface was doped by Na 1 ions) was reported in Ref. [1]. We speculate that strong near-surface electric fields should be present in the surface layer of these crystals quite similar to the 2D superconducting fullerene and polyacene crystals in the field-effect-transistor geometry [2].…”

Hard-Wired Dopant Networks and the Prediction of High Transition Temperatures in Ceramic Superconductors

“…The embedded clusters are not connected, so the result is "localized nonpercolative superconductivity" still with R = 2 [48]. In the free surface case [45,46], thermal fluctuations disrupt superconductivity above 100 K. Both of these R = 2 points are shown in Figure 2. Considering the rapid progress in nanoscience, it may be possible to obtain similar pairs of percolative and cluster points for other HTSC.…”

“…Now we turn to a much more difficult problem, for which the data base is small, but still robust: a surface monolayer of A x WO 3 , where A is an alkali metal (Na [45] or Cs [46]). While bulk Na x WO 3 exhibits superconductivity only near 1 K, here for Na superconductivity appears around 100 K; for Cs there are two phase transitions, a bulk one with lower T c at higher doping, and a re-entrant percolative one with higher T c at lower doping.…”