Superconductivity in the geometrically frustrated pyrochlore ${\mathrm{RbOs}}_{2}{\mathrm{O}}_{6}$

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Superconductivity in the geometrically frustrated pyrochlore ${RbOs}_{2} O_{6}$

M. Brühwiler, S. M. Kazakov, N. D. Zhigadlo, J. Karpinski, and B. Batlogg

Phys. Rev. B 70, 020503(R) – Published 15 July 2004

Abstract

We report the basic thermodynamic properties of the 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 $γ = (33.7 \pm 0.2) {mJ ∕ mol}_{f.u.} ∕ K^{2}$ . 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 $α = 0.64 \pm 0.1$ . From the upper critical field $μ_{0} H_{c 2} \approx 6 T$ at $0 K$ , we estimate a Ginzburg-Landau coherence length $ξ \approx 74 Å$ . ${RbOs}_{2} O_{6}$ is the second reported metallic ${A B}_{2} O_{6}$ 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 ${KOs}_{2} O_{6}$ .

Received 19 March 2004

DOI:https://doi.org/10.1103/PhysRevB.70.020503

Authors & Affiliations

M. Brühwiler^*, S. M. Kazakov, N. D. Zhigadlo, J. Karpinski, and B. Batlogg

Laboratory for Solid State Physics, ETH Zürich, 8093 Zürich, Switzerland

^*Electronic address: bruehwiler@solid.phys.ethz.ch

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Vol. 70, Iss. 2 — 1 July 2004

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Figure 1
(Color online) The powder x-ray diffraction pattern of the purified sample, where all reflections can be indexed on the basis of a pyrochlore face-centered unit cell with lattice parameter $a = 10.1137 (1) Å$ . The inset shows an $Os$ atom and its relationship to its surrounding. Every $Os$ atom forms the shared corner of two $Os$ tetrahedra in a network. The exchange pathways to the nearest neighbors, leading by one of the coordinating oxygen atoms, are shown by dashed lines. The $Rb$ cations are omited in this illustration for clarity.Reuse & Permissions
Figure 2
Specific heat $C_{p} ∕ T$ vs $T^{2}$ for magnetic fields from 0 to $4 T$ in $0.5 T$ steps and $12 T$ . From the normal-state curve at $12 T$ , we extract the electronic specific heat coefficient $γ = \lim_{T \to 0 K} C_{p} ∕ T = (33.7 \pm 0.2) {mJ ∕ mol}_{f.u.} ∕ K^{2}$ . The Debye temperature $Θ_{D} (T)$ below $4 K$ is slightly temperature dependent, with a value of $Θ_{D} (1 K) \approx 230 K$ and $Θ_{D} (4 K) \approx 240 K$ . The inset shows the $0 T$ and normal state specific heat on a linear $T$ scale. Also shown is 23.4% of the normal state $C_{p}$ , expected to be present in the superconducting $C_{p}$ , and used for our analysis. The normalized specific heat jump at $T_{c}$ , $Δ C_{p} ∕ (76.6 % γ T_{c}) = 1.38$ , is close to the weak-coupling BCS value of $1.43$ .Reuse & Permissions
Figure 3
Superconducting specific heat calculated by subtracting 23.4% of the normal state specific heat at $12 T$ , plotted on a semilogarithmic scale vs $T_{c} ∕ T$ . The corresponding superconducting volume fraction is thus about 77%, similar to the estimated fraction in ${KOs}_{2} O_{6}$ (Ref. 6). The line indicates the expected behavior from BCS assuming an isotropic gap: For $2.5 < T_{c} ∕ T < 6$ , the specific heat approximately follows an exponential behavior $8.5 γ T_{c} exp (- 1.44 T_{c} ∕ T)$ . Down to $1 K$ , the superconducting specific heat decreases in close quantitative agreement with this conventional superconducting behavior.Reuse & Permissions
Figure 4
Upper critical field $H_{c 2}$ extracted from specific heat measurements. A power law fit $H_{c 2} (T) = H_{c 2} (0) (1 - {(T ∕ T_{c})}^{n})$ (dashed line) gives an exponent of $n = 1.0 \pm 0.1$ and a critical temperature $T_{c} = (6.37 \pm 0.03) K$ . The initial slope of the critical boundary at $T_{c}$ is $- d (μ_{0} H_{c 2}) ∕ d T ∣_{T = T_{c}} = 1.2 T ∕ K$ . The dotted line is a fit to the WHH formula in the orbital limit $(λ_{SO} = \infty)$ using the calculated Maki parameter $α = 0.64$ . Also shown are the Pauli limiting field $H_{p 0}$ $(⋆)$ and the orbital limiting field $H_{c 2}^{*}$ $(◇)$ . The magnetic susceptibility at low temperatures, depicted in the inset, shows the diamagnetic transition into a bulk superconducting state. The magnetization measurements have been performed at $H = 3.3 Oe$ in zero-field-cooled (ZFC) and field-cooled (FC) states.Reuse & Permissions

Physical Review B

covering condensed matter and materials physics

Superconductivity in the geometrically frustrated pyrochlore ${RbOs}_{2} O_{6}$

M. Brühwiler, S. M. Kazakov, N. D. Zhigadlo, J. Karpinski, and B. Batlogg

Phys. Rev. B 70, 020503(R) – Published 15 July 2004

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Physical Review B

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Superconductivity in the geometrically frustrated pyrochlore RbOs2O6

M. Brühwiler, S. M. Kazakov, N. D. Zhigadlo, J. Karpinski, and B. Batlogg

Phys. Rev. B 70, 020503(R) – Published 15 July 2004

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Superconductivity in the geometrically frustrated pyrochlore ${RbOs}_{2} O_{6}$