We report the basic thermodynamic properties of the new geometrically frustrated β-pyrochlore bulk superconductor RbOs 2 O 6 with a critical temperature T c = 6.4 K. Specific heat measurements are performed in magnetic fields up to 12 T. The electronic density of states at the Fermi level in the normal state results in γ = (33.7 ± 0.2) mJ/mol f.u. /K 2 . In the superconducting state, the specific heat follows conventional BCS-type behavior down to 1 K, i.e. over three orders of magnitude in specific heat data. The upper critical field slope at T c is 1.2 T/K, corresponding to a Maki-parameter α = 0.64 ± 0.1. From the upper critical field µ 0 H c2 ≈ 6 T at 0 K, we estimate a Ginzburg-Landau coherence length ξ ≈ 74 Å. RbOs 2 O 6 is the second reported metallic AB 2 O 6 type pyrochlore compound after KOs 2 O 6 , and one of only three pyrochlore superconductors in addition to Cd 2 Re 2 O 7 and KOs 2 O 6 .

To assess electron correlation and electron-phonon coupling in the recently discovered ␤-pyrochlores KOs 2 O 6 and RbOs 2 O 6 , we have performed specific heat measurements in magnetic fields up to 14 T. We present data from high-quality single-crystalline KOs 2 O 6 , showing that KOs 2 O 6 is a strong-coupling superconductor with a coupling parameter ep Ϸ 1.0-1.6 ͑RbOs 2 O 6 : ep Ϸ 1͒. The estimated Sommerfeld coefficient of KOs 2 O 6 , ␥ = 76-110 mJ/ mol K 2 , is twice that of RbOs 2 O 6 ͑␥ =44 mJ/mol K 2 ͒. Using strong-coupling corrections, we extract useful thermodynamic parameters of KOs 2 O 6 . Quantifying ep allows us to determine the mass enhancement over the calculated band electronic density of states. A significant contribution in addition to the electron-phonon term of c = 1.7-4.3 is deduced. In an effort to understand the origin of the enhancement mechanism, we also investigate an unusual energetically low-lying phonon. There are three phonon modes per RbOs 2 O 6 , suggestive of the phonon source being the rattling motion of the alkali-metal ion. This dynamic instability of the alkali-metal ions causes large scattering of the charge carriers which shows up in an unusual temperature dependence of the electrical resistivity.

We develop a general procedure for the analysis of bulk thermodynamic data of a superconductor for samples containing a metallic nonsuperconducting second phase. The method is based on the condensation energy and it allows the extraction of the intrinsic properties of a superconductor even for nonideal samples. Applying this procedure to the recently discovered geometrically frustrated ␤-pyrochlore superconductor RbOs 2 O 6 ͑T c = 6.4 K͒ yields a Sommerfeld coefficient as high as 79 J/g/K 2 ͑44 mJ/ mol f.u. /K 2 ͒. RbOs 2 O 6 is inferred to be a strong type-II superconductor ͓͑T c ͒ =23͔ in the intermediate-coupling regime similar to niobium ͑ ep Ϸ 1͒. From the upper critical field 0 H c2 Ϸ 6 T at 0 K, we estimate a Ginzburg-Landau coherence length Ϸ 74 Å. The condensation energy is 860 J/g ͑483 mJ/ mol f.u. ͒ resulting in 1 / ͑8͒ · ͑␥ 1 T c 2 ͒ / ⌬U 1 Ϸ 0.15, a value well in the range of conventional phonon-mediated superconductors. The superconducting electronic specific heat indicates conventional s-wave pairing. The experimental Sommerfeld coefficient of 44 mJ/ mol f.u. /K 2 is about four times larger than the one found in band structure calculations. Together with the electron-phonon coupling constant ep Ϸ 1 this leaves an additional add Ϸ 2.1 for enhancement due to other mechanisms.

RbOs 2 O 6 , the third superconducting pyrochlore oxide (known so far), has been synthesized by encapsulation and by high pressure techniques. Suitable post chemical treatment of the as-prepared sample allowed us to eliminate the impurity phases. Bulk superconductivity with T c = 6.4 K was observed in magnetisation and specific heat measurements. The transition temperature of RbOs 2 O 6 was found to be the same for both preparation methods. Structural investigations showed that Rb atoms occupy the 8b site in the pyrochlore lattice with a lattice parameter of 10.1137(1) Å.Recently materials in which the magnetic interactions are geometrically frustrated have attracted considerable interest [1]. In compounds crystallizing in the pyrochlore structure, with general composition A 2 B 2 O 7 , the corner-sharing BO 6 octahedra form a three-dimensional (3D) network of tetrahedrally arranged B atoms, leading to geometrically frustration of antiferromagnetic coupling among them. Transition metal oxides with a pyrochlore type structure exhibit a wide range of interesting physical properties, varying from highly insulating to metallic.The pyrochlores containing 5d transition metal as B-cation were found to be bad metals in general [2]. The discovery of superconductivity in a rhenium oxide Cd 2 Re 2 O 7 with T c = 1 K was remarkable, as it represents the first example of a superconducting pyrochlore [3-5]. Recently, Yonezawa et al discovered superconductivity below T c = 9.5 K in β-pyrochlore KOs 2 O 6 [6]. Hiroi et al reported detailed studies of this compound (by electrical resistivity, magnetic susceptibility and specific heat) and estimated a value of upper critical magnetic field of 38 T which exceeds the Pauli paramagnetic limit and suggests unconventional superconductivity [7].Here we present the experimental details of the synthesis and post chemical treatment of RbOs 2 O 6 which allowed us to obtain single phase material. Moreover, we propose another method suitable for the synthesis of superconducting pyrochlores: high pressure high temperature technique in a cubic anvil apparatus. Preliminary X-ray diffraction study revealed that Rb cation occupies 8b site in a pyrochlore lattice The influence of different synthetic approaches on the phase formation and superconducting properties of RbOs 2 O 6 is discussed. After this work was completed, two papers by Yonezawa et al appeared, reporting the synthesis in an evacuated quartz tube of Rb and Cs members of AOs 2 O 6 family, namely RbOs 2 O 6 and CsOs 2 O 6 [8-9]. The transition temperature was found to decrease with increasing the ionic radius of A cation (6.3 K and 3.3 K for RbOs 2 O 6 and CsOs 2 O 6 , respectively). Experimental detailsSince osmium can adopt various oxidation states and OsO 4 is a volatile compound, closed crucibles should be used for the preparation of compounds in the Rb-Os-O system. We applied two methods to obtain RbOs 2 O 6 powder samples: encapsulation in an evacuated quartz tube and high pressure high temperature technique. In the first method a...

No abstract