Hydrogenation of thin Y-Ba-Cu-O films: Electrical transport and structure properties ofYBa2Cu3

Abstract

c-axis-oriented YBa,Cu307 (1:2:3)and YBa2Cu408 (1:2:4) thin films were charged with hydrogen at 463 K and a H2 pressure of 100 mbar. The hydrogen concentrations and depth profiles were measured with the ' N nuclear reaction method. In contrast to earlier studies on bulk samples where a constant T, ,"", is reported, we find in our 1:2:3films that the superconducting transition temperature decreases continuously with increasing hydrogen concentration and that the transitions remain relatively sharp, e.g. , T, ,"", =38 K and b, T, =6.2 K for [H]/cell -=0.6. The electrical resistivity and Hall-effect measurements in the normal state indicate that the main effect of hydrogen doping is a reduction of the charge carrier concentration, whereas the carrier mobility is not changed significantly. At an average hydrogen concentration of [H]/cell =-2 in both systems, a new hydride phase is observed, which for 1:2:3is characterized by a c-axis expansion of 16% and for 1:2:4by a c-axis expansion of 1.5%.

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

“…The superconducting onset temperature remains constant at 92K throughout the doping regime, until superconductivity is completely destroyed [728]. However, in epitaxial films, a sharp transition is found at all H dopings, and T c falls smoothly to zero as the H concentration increases [729]. Moreover, it is believed that the phase separation has more to do with the H than with the holes [726].…”

Phase diagrams for the YBa2Cu3O7 family — ANNO 1996

“…Typical for this newly recognized perovskite structure is that it has the c-axis extended by almost 200 pm as compared with the original material, according to Ref. [587], and this can be corroborated by the data in Ref. [588] (although probably erroneously referred to the 124 or 147 phases) and in Ref.…”

“…A somewhat clearer picture can be extracted from a combination of several recent studies adopting special methods like the 15N nuclear reaction method [587], HREM [588] and PND at variable temperatures [589], as well as special substrates like the thin film arrangement in Ref. [587]. The data show consistently that the incorporation of hydrogen proceeds in two steps.…”

“…During the formation of this phase, the release of water seen [589] by TG is comparable with that during the first step, whereas the continuing reduction of copper, particularly at Cu(1), is manifested by the superstructure features seen by HREM [588], by iodometry [591] and apparently also by the appearance [592,593] of the antiferromagnetic ordering typical for the w -~ 0 versions of the original phase. The additional hydrogen-containing defects are tentatively located [587] in the Cu(1) coordination sphere and this provides a good possible explanation for the c-axis extension: Whereas the Cu(1) atom is reduced to Cu(I) and should presumably adopt the linear (dumbbell) coordination in the c direction, there are now hydroxide groups OH-located around Cu(1) in high concentrations. For bond-valence reasons and also due to the size of the hydroxide groups, the c-axis bond distance at Cu(1) must then extend substantially.…”

“…Their characterization, however, has proved extremely difficult, particularly due to the analytical problems with hydrogen being the lightest atom (TG, PXD) and an incoherently scattering element (PND), as well as due to the inhomogeneities and the enhanced reactivity of the products. A somewhat clearer picture can be extracted from a combination of several recent studies adopting special methods like the 15N nuclear reaction method [587], HREM [588] and PND at variable temperatures [589], as well as special substrates like the thin film arrangement in Ref. [587].…”