Superconductivity in rubidium-substituted Ba${}_{1\ensuremath{-}x}$Rb${}_{x}$Ti${}_{2}$Sb${}_{2}$O

Superconductivity in rubidium-substituted Ba $_{1 - x}$ Rb $_{x}$ Ti $_{2}$ Sb $_{2}$ O

Fabian von Rohr, Reinhard Nesper, and Andreas Schilling

Phys. Rev. B 89, 094505 – Published 7 March 2014

Abstract

We report on the synthesis and the physical properties of Ba $_{1 - x}$ Rb $_{x}$ Ti $_{2}$ Sb $_{2}$ O ( $x \leq 0.4$ ) by x-ray diffraction, superconducting quantum interference device magnetometry, resistivity, and specific-heat measurements. Upon hole doping by substituting Ba with Rb, we find superconductivity with a maximum $T_{c} = 5.4$ K. Simultaneously, the charge-density-wave transition temperature is strongly reduced from $T_{CDW} \approx 55$ K in the parent compound ${BaTi}_{2} {Sb}_{2} O$ and seems to be suppressed for $x \geq 0.2$ . The bulk character of the superconducting state for the optimally doped sample ( $x = 0.2$ ) is confirmed by the occurrence of a well developed discontinuity in the specific heat at $T_{c}$ , with $Δ C / T_{c} \approx 22$ mJ/mol K $^{2}$ , as well as a large Meissner-shielding fraction of $\approx$ 40%. The isotropically averaged lower and upper critical fields of the optimally doped sample ( $x = 0.2$ ) are estimated to $μ_{0} H_{c, 1} (0) \approx 3.8$ mT and $μ_{0} H_{c, 2} (0) \approx 2.3$ T, respectively, indicating that these compounds are strongly type-II superconductors.

Received 6 November 2013
Revised 16 January 2014

DOI:https://doi.org/10.1103/PhysRevB.89.094505

Authors & Affiliations

Fabian von Rohr^1,2,*, Reinhard Nesper², and Andreas Schilling¹

¹Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
²Laboratory of Inorganic Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland

^*vonrohr@physik.uzh.ch

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Vol. 89, Iss. 9 — 1 March 2014

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Figure 1
(a) The powder x-ray diffraction (XRD) pattern at ambient temperature for the sample of nominal composition ${Ba}_{0.7} {Rb}_{0.3} {Ti}_{2} {Sb}_{2} O$ ( $x = 0.3$ ). The crystal structure of the compound is shown in the inset. The vertical dark green lines show the theoretical Bragg peak positions for this phase. The blue pattern on the bottom is the difference plot between the theoretical pattern and the observed intensities. (b) Enlarged XRD data around the (111) reflections as functions of increasing $x$ , demonstrating the change in the lattice parameters with Rb content. The dotted line is a guide to the eye. (c) Cell parameters for the different compositions used in this study. The dashed line represents an idealized linear change of the cell parameters.
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Figure 2
Resistivities of Ba $_{1 - x}$ Rb $_{x}$ Ti $_{2}$ Sb $_{2}$ O for $x = 0$ , 0.02, 0.05, 0.1, and 0.2 in a temperature range between 1.8 and 100 K. The CDW transition manifests itself in a sudden increase of the resistivity for the samples $x = 0$ , 0.02, 0.05, and 0.1 (marked with a black arrow). This anomaly is absent for the sample with $x = 0.2$ . For increasing rubidium contents $x$ (up to $x = 0.2$ ), the normal-state resistivities increase, the CDW ordering transition temperatures $T_{CDW}$ decrease, while the critical temperatures $T_{c}$ increase.
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Figure 3
Phase diagram of the electronic properties of Ba $_{1 - x}$ Rb $_{x}$ Ti $_{2}$ Sb $_{2}$ O derived from resistivity and magnetization measurements ( $T_{c}$ for the parent compound $x = 0$ was taken from Ref. [8]). The inset shows the magnetic susceptibility of Ba $_{1 - x}$ Rb $_{x}$ Ti $_{2}$ Sb $_{2}$ O for $x = 0.02$ , 0.05, 0.1, 0.15, 0.2, 0.3, and 0.4 measured between 1.75 and 10 K in an external magnetic field of $μ_{0} H = 1$ mT in the zero-field-cooled (ZFC) mode. The onsets of the transition to superconductivity $T_{c, onset}$ are marked with arrows. The samples $x = 0.3$ and 0.4 do not display a bulk diamagnetic shielding, and their magnetic susceptibilities are displayed with dashed lines.
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Figure 4
(a) and (b) Normalized resistivity $ρ (T) / ρ (10 K)$ for the optimally doped sample $x = 0.2$ and $x = 0.05$ , respectively. Field-dependent resistivity measurements are shown for magnetic fields between 0 and 3 T, varied by 0.05 T steps, in a temperature range between 1.8 and 8 K. The dashed lines denote the 50% criterion used to determine $H_{c 2} (T)$ , shown in (c). The solid lines indicate the extrapolated slopes $\frac{d H_{c 2}}{d T}$ used for the WHH approximation [Eq. (1)].
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Figure 5
Reduced specific heat $C / T$ vs temperature $T$ of the optimally doped sample ( $x = 0.2$ ), together with an inset showing the same data after subtraction of the normal-state contribution (dashed line in the main panel; see text). The solid lines in the inset represent an entropy-conserving construction to obtain the discontinuity and $Δ C / T_{c}$ and $T_{c}^{cal}$ .
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Figure 6
ZFC field dependence of the magnetization $m (H)$ of the optimally doped sample with $x = 0.2$ , for temperatures between 1.75 and 5 K (in 0.25 K steps), in magnetic fields $μ_{0} H$ between 0 and 15 mT. The dashed line shows the ideal diamagnetic shielding. Upper inset: ZFC and FC magnetic susceptibility of the sample with $x = 0.2$ . Lower inset: The temperature dependence of the lower critical field $H_{c 1}$ of the sample with $x = 0.2$ (see text). The dashed line is a fit to Eq. (4) with $T_{c}$ fixed to 5.4 K.
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Physical Review B

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Superconductivity in rubidium-substituted Ba $_{1 - x}$ Rb $_{x}$ Ti $_{2}$ Sb $_{2}$ O

Fabian von Rohr, Reinhard Nesper, and Andreas Schilling

Phys. Rev. B 89, 094505 – Published 7 March 2014

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Physical Review B

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Superconductivity in rubidium-substituted Ba1−xRbxTi2Sb2O

Fabian von Rohr, Reinhard Nesper, and Andreas Schilling

Phys. Rev. B 89, 094505 – Published 7 March 2014

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Superconductivity in rubidium-substituted Ba $_{1 - x}$ Rb $_{x}$ Ti $_{2}$ Sb $_{2}$ O