Pressure Induced 18 K Superconductivity and Two Superconducting Phases in CuIr2S4

Abstract

We report pressure-induced superconductivity in the spinel CuIr$_{2}$S$_{4}$ with a transition temperature ($T_{\text{c}}$) reaching \textbf{18.2 K}, establishing a new record for this class of materials and surpassing the decades-old limit of 13.7 K. Our electrical transport and synchrotron X-ray diffraction studies up to 224 GPa reveal the emergence of \textbf{two distinct superconducting phases} from a charge-ordered insulating state. The first phase (SC-I) appears around 18 GPa, and forms a dome-shaped superconducting region in which the resistivity exhibits a pronounced, field- and current-sensitive drop without reaching strict zero above our base temperature. Above 111.8 GPa, a second, lower-$T_{\text{c}}$ phase (SC-II) emerges and coexists with SC-I over a broad pressure range, and SC-II ultimately develops a true zero-resistance state above 122.2 GPa. These superconducting phases are intimately linked to a cascade of structural transitions that systematically distort the frustrated pyrochlore lattice of Ir atoms. Our results expand the potential for superconductivity in spinels and demonstrate a pathway to high-$T_{\text{c}}$ pairing directly from a correlated insulating state driven by lattice tuning.

Figures (4)

• Figure 1: Pressure‑induced superconductivity in CuIr$_2$S$_4$. (a) The crystal structure of CuIr$_2$S$_4$, space group Fd-3m. The Ir sublattice forms a network of corner‑sharing tetrahedra and a perfect 2D Kagome‑like net Ortiz2019New. (b, c) Temperature dependence of resistance of CuIr$_2$S$_4$ under various pressures up to 224 GPa. (d, e) An expanded view of the low‑temperature resistive region under pressures ranging from 18.1–111.8 GPa (d) and 122.2–224 GPa (e). At 111.8 GPa, a new superconducting phase emerges (marked with green arrow) and is enhanced with increasing pressure. The criterion for determining the superconducting transition temperature ($T_{\rm c}$) is shown with a black cross‑wire. $T_c$ of SC-I and SC-II here is defined as the onset of the resistance drop.
• Figure 2: Evolution of the superconducting transition in CuIr$_2$S$_4$ under magnetic fields. (a) Temperature dependence of the electrical resistance at 49.5 GPa under various applied currents. Inset: enlarged view of $R(T)$ at 49.5 GPa under different currents. (b, c) Temperature-dependent resistivity under varying magnetic fields at 18.3 GPa and 56.2 GPa, respectively. (d, e) Temperature-dependent resistivity under different magnetic fields at 154.5 GPa and 208 GPa, respectively. Inset of (d) shows the best fit of $T_{\rm c}$ vs. $\mu_0 H_{c2}(0)$ using the G–L formula [Eq. (1)]. (f, g) Fits of $T_{\rm c}$ vs. $\mu_0 H_{c2}(0)$ using the G–L formula for the SC-I and SC-II phases, respectively. (h, i) Pressure dependence of the zero-temperature upper critical field $\mu_0H_{c2}(0)$ obtained from the G–L fit [Eq. (1)] and the WHH model [Eq. (2)], along with the normalized slope $-\left( \frac{1}{T_c} \left. \left[ \frac{d\mu_0 H_{c2}(T)}{dT} \right] \right|_{T_c} \right)$. Error bars for $\mu_0 H_{c2}(0)$ are derived from the covariance matrix of the Ginzburg--Landau fit and are typically $\pm 1$--3 T. (j) Magnetic field dependence of the resistance measured at $T = 2$ K under various pressures.
• Figure 3: Pressure‑induced structural phase transition in CuIr$_2$S$_4$. (a) XRD patterns measured in CuIr$_2$S$_4$ under high pressure up to 40.1 GPa with an incident wavelength $\lambda = 0.68883\,\text{\AA}$. Asterisks, triangles and dotted lines indicate the presence of extra peaks. (b) Pressure‑induced transformation of crystalline from metallic cubic phase to triclinic insulator phase and finally orthorhombic superconductive phase. (c, d) Synchrotron XRD patterns with subtracted background of CuIr$_2$S$_4$ at selected pressures up to 219.4 GPa and incident wavelength $\lambda = 0.434\,\text{\AA}$. Arrows indicate the presence of extra peaks. Black asterisks indicate the Au peaks for pressure calibration.
• Figure 4: Electronic structures and pressure--temperature phase diagram of CuIr$_2$S$_4$. (a–d) Band structure of CuIr$_2$S$_4$ obtained by DFT calculations under different pressures at (a) 0 GPa, (b) 5.4 GPa, (c) 14.1 GPa and (d) 21.4 GPa. The orbital characters of bands are represented by different colors, and the projected weights are indicated by marker sizes. (e) Pressure--temperature phase diagram of CuIr$_2$S$_4$. Black dashed lines denote the phase boundaries of structures under pressure at room temperature. Different symbols represent the $T_{\rm MIT}$ and $T_{\rm c}$ of SC‑I and SC‑II measured in different runs. Slight variations in $T_c$ observed between different experimental runs within the SC-I regime can be attributed to differences in diamond culet size, sample dimensions, and pressure calibrations used in each run. Transport anisotropy is determined from the van der Pauw measurements of $R_{12}$ and $R_{14}$ detailed in the SI (Figs. S6–S7).