Superconductivity in the Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ spinel

We report the observation of superconductivity in the CuIr${}_2$Se${}_4$ spinel induced by partial substitution of Pt for Ir. The optimal doping level for superconductivity in Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ is $x$ $=$ 0.2, where $T_c$ is 1.76 K. A superconducting $T_c$ vs composition dome is established between the metallic, normal conductor CuIr${}_2$Se${}_4$ and semiconducting CuIrPtSe${}_4$. Electronic structure calculations show that the optimal $T_c$ occurs near the electron count of a large peak in the calculated electronic density of states and that CuIrPtSe${}_4$ is a band-filled insulator. Characterization of the superconducting state in this heavy metal spinel through determination of $\Delta C/\gamma T_c$ indicates that it is BCS-like. The relatively high upper critical field at the optimal superconducting composition [$H_c$${}_2$(0) $=$ 3.2 T] is much larger than that reported for analogous rhodium spinels and is comparable to or exceeds the Pauli field (${\mu }_0{H}_P$), suggesting that strong spin-orbit coupling may influence the superconducting state. Further, comparison to doped CuIr${}_2$S${}_4$ suggests that superconductivity in iridium spinels is not necessarily associated with the destabilization of a charge-ordered spin-paired state through doping.

Figure 1

(a) Composition dependence of the room-temperature lattice parameter for Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ (0.0 ≤ $x$ ≤ 0.5). Inset: The spinel crystal structure: CuSe4 tetrahedra, blue; (Ir,Pt) octahedra, gray; Se ions, green. (b) XRD patterns of Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ (0.1 ≤ $x$ ≤ 0.5) compounds heated at 1123 K for 96 h and Cu(Ir${}_{0.5}$Pt${}_{0.5}$)${}_2$Se${}_4$ heated for 96+48+72 h.

Figure 2

The temperature dependence of the electrical resistivity of polycrystalline samples of the Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ (0.05 ≤ $x$ ≤ 0.4) compounds without magnetic field. Inset: enlarged view of low-temperature region (0.4–3 K) of the electrical resistivity of Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ (0.1 ≤ $x$ ≤ 0.35).

Figure 3

Low-temperature resistivity at various applied fields for (a) Cu(Ir${}_{0.8}$Pt${}_{0.2}$)${}_2$Se${}_4$ and (b) Cu(Ir${}_{0.7}$Pt${}_{0.3}$)${}_2$Se${}_4$. Inset shows the temperature dependence of the upper field ($H_c$${}_2$).

Figure 4

(a) Temperature dependence of the electrical resistivity of polycrystalline sample of CuIrPtSe${}_4$. Inset: temperature dependence of the Seebeck coefficient for CuIrPtSe${}_4$. (b) Temperature dependence of the resistivity as log${}_{10}\rho$ vs (1/$T$). Inset: low-temperature data plotted as log${}_{10}\rho$ vs (1/$T$)${}^{1/4}$.

Figure 5

Temperature dependence of specific heat $C_p$ of Cu(Ir${}_{0.8}$Pt${}_{0.2}$)${}_2$Se${}_4$ measured under magnetic fields 0 and 5 T, presented in the form of $C_{\mathrm{el}}$/$T$ vs $T$ (main panel) and $C_p$/$T$ vs $T^2$ (inset). The fitting of the low temperature with the range 2–7 K heat capacity data obtained in the magnetic field 5 T.

Figure 6

(a) The Pt content dependence of electronic specific-heat coefficients (γ) and (b) the Debye temperature (${\Theta }_D$) obtained from low-temperature fits of specific heats. (c) The electronic phase diagram for Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ (0 ≤ $x$ ≤ 0.5) as a function of Pt content $x$.

Figure 7

(a) The calculated density of states and the Fermi surface for Cu(Ir${}_{0.7}$Pt${}_{0.3}$)${}_2$Se${}_4$. The inset shows the dependence of the calculated DOS at $E_F$ on the doping level. Two different projections of the Fermi surface are shown. The colors are a guide for the eye to emphasize the topography. (b) The band structure close to $E_F$ of Cu(Ir${}_{0.7}$Pt${}_{0.3}$)${}_2$Se${}_4$. A VHS with a very flat band is visible at the Γ point.