Enhanced superconducting transition temperatures in the rocksalt-type superconductors ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}\mathrm{Te}$ $(x\ensuremath{\le}0.5)$

Enhanced superconducting transition temperatures in the rocksalt-type superconductors ${In}_{1 - x} {Sn}_{x} Te$ $(x \leq 0.5)$

Kaya Kobayashi, Yukio Ai, Harald O. Jeschke, and Jun Akimitsu

Phys. Rev. B 97, 104511 – Published 26 March 2018

Abstract

We investigate superconductivity in ${In}_{1 - x} {Sn}_{x} Te$ ( $x \leq 0.5$ ) synthesized at high pressures of up to 2 GPa and observe an enhancement of the superconducting transition temperature $T_{c}$ for increasing tin concentration $x$ . These compounds have not been accessible in rocksalt structure via conventional ambient pressure synthesis. While the lattice constant smoothly increases with $x$ , $T_{c}$ saturates around $x = 0.4$ . Electronic structure calculations indicate that the $T_{c}$ modulation is brought on by the change of the density of states in the vicinity of the Fermi energy $[N (E_{F})]$ . However, differences between the calculated $N (E_{F})$ and the observed electronic specific-heat coefficient indicate that the phonon dispersion plays an important role in the system and that the mechanism of superconductivity may not be the same in the entire doping range.

Received 3 January 2018
Revised 12 March 2018

DOI:https://doi.org/10.1103/PhysRevB.97.104511

Physics Subject Headings (PhySH)

Electronic structure Pressure effects Superconductivity

Topological superconductors

AC susceptibility measurements Density functional calculations Resistivity measurements X-ray powder diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Kaya Kobayashi^1,2, Yukio Ai², Harald O. Jeschke¹, and Jun Akimitsu¹

¹Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan
²Department of Physics, Okayama University, Okayama 700-8530, Japan

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Vol. 97, Iss. 10 — 1 March 2018

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Figure 1
(a) Powder x-ray diffraction patterns for ${In}_{1 - x} {Sn}_{x} Te (x = 0 \sim 0.5)$ . Peaks are indexed as shown in the lower part of the panel. Except for a BN peak for $x = 0.3$ ( $▽$ ) and small InTe peaks for 0.4 ( $◯$ ), no impurity peaks were observed. (b) Normalized peak of (200) for ${In}_{1 - x} {Sn}_{x} Te (x = 0 \sim 0.5)$ . Increasing $x$ brings the peak to lower $θ$ , indicating the increasing lattice constant, $a$ . (c) Lattice constant $a$ increases when increasing Sn doping in InTe. The uncertainty of the lattice constant stays negligibly small for all $x$ including those with impurity peaks ( $x = 0.3$ and 0.4).
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Figure 2
(a) Superconducting transition temperatures ( $T_{c} s$ ) obtained from the susceptibility measurement are plotted against Sn doping $x$ in ${In}_{1 - x} {Sn}_{x} Te (x = 0 \sim 0.5)$ . $T_{c}$ is enhanced by Sn doping and saturates above $x = 0.4$ . Small doping levels show a variation of $T_{c}$ between samples resulting in larger error bars. (b) Temperature dependence of susceptibility $χ$ for ${In}_{0.5} {Sn}_{0.5} Te$ . A clear $T_{c}$ is obtained when the signal deviates from 0.
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Figure 3
(a) Temperature dependence of the resistivity in various magnetic fields ( $B = 0 \sim 1.5$ T) for ${In}_{0.5} {Sn}_{0.5} Te$ . Gradual shift of superconducting transition $T_{c}$ is observed when increasing $B$ . (b) Obtained upper critical fields ( $H_{c 2}$ ) from $ρ$ data presented in the left panel. The lower critical-field ( $H_{c 1}$ ) data is enlarged in the inset panel. $H_{c 2}$ is well described by the Werthamer-Helfand-Hohenberg equation (dashed line); for details see the text.
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Figure 4
(a) Heat-capacity analysis of ${In}_{0.7} {Sn}_{0.3} Te$ and InTe samples. The curves are measured without a magnetic field (B). Samples show $T_{c} = 4.99$ K ( ${In}_{0.7} {Sn}_{0.3} Te$ ) and 3.2 K (InTe). The inset panels depict the $Δ C$ to show the superconducting transitions in the heat capacity for both compounds. The magnitude of the jump at the transition depends on the doping level of Sn. (b) (Color online.) The heat capacity measured in $B = 3$ T for ${In}_{0.7} {Sn}_{0.3} Te$ and InTe. The well-fitting $T^{3}$ dependence is seen for two compounds with largely different TC. The fitted curves are depicted in red and black for ${In}_{0.7} {Sn}_{0.3} Te$ and InTe, respectively.
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Figure 5
$x$ dependence of superconducting transition temperature and specific-heat coefficient. In the high $x$ region, data is taken from a previous report [12] and depicted as filled symbols. TC is enhanced in the mixed state of Sn and In from both ends (SnTe and InTe). The change on the InTe side is gradual compared to the SnTe side. The obtained $γ (0)$ parameter is roughly proportional to the In ratio in the compounds, contrasting with the nonmonotonic change of TC.
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Figure 6
Electronic structure of ${In}_{1 - x} {Sn}_{x} Te$ . (a) Comparison between scalar (GGA) and fully relativistic (GGA+SO) densities of states of pure InTe. (b) Evolution of the density of states at the Fermi level for ${In}_{1 - x} {Sn}_{x} Te$ as function of $x$ determined by virtual crystal approximation (VCA, circles) and coherent potential approximation (CPA, squares). (c) $k$ -integrated spectral function from CPA for different Sn content $x$ .
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Enhanced superconducting transition temperatures in the rocksalt-type superconductors ${In}_{1 - x} {Sn}_{x} Te$ $(x \leq 0.5)$

Kaya Kobayashi, Yukio Ai, Harald O. Jeschke, and Jun Akimitsu

Phys. Rev. B 97, 104511 – Published 26 March 2018

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Enhanced superconducting transition temperatures in the rocksalt-type superconductors In1−xSnxTe (x≤0.5)

Kaya Kobayashi, Yukio Ai, Harald O. Jeschke, and Jun Akimitsu

Phys. Rev. B 97, 104511 – Published 26 March 2018

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Enhanced superconducting transition temperatures in the rocksalt-type superconductors ${In}_{1 - x} {Sn}_{x} Te$ $(x \leq 0.5)$