Superconductivity in Cu${}_x$IrTe${}_2$ driven by interlayer hybridization

The change in the electronic structure of layered Cu${}_x$IrTe${}_2$ has been characterized by transport and spectroscopic measurements, combined with first-principles calculations. The Cu intercalation suppresses the monoclinic distortion, giving rise to the stabilization of the trigonal phase with superconductivity. Thermopower and Hall resistivity measurements suggest the multiband nature with hole and electron carriers for this system, which is masked by the predominance of the hole carriers enhanced by the interlayer hybridization in the trigonal phase. Rather than the instability of the Ir $d$ band, a subtle balance between the interlayer and intralayer Te-Te hybridizations is proposed as a main factor dominating the structural transition and the superconductivity.

Figure 1

(a) In-plane and out-of-plane lattice constants and (b) electronic phase diagram of Cu${}_x$IrTe${}_2$ as a function of $x$. Solid lines in (a) are a guide to the eyes. Filled circles and open circles in (b) indicate the critical temperatures for structural phase transition on heating and cooling, respectively. Filled squares correspond to the superconducting transition temperature. The inset shows Ir $L$${}_3$-edge x-ray absorption spectra for IrO${}_2$, Ir, and IrTe${}_2$ taken at room temperature. The spectra for Ir and IrO${}_2$ are shifted upward for clarity. The absorption peak positions are indicated by the filled triangles.

Figure 2

(a) Temperature dependence of normalized resistivity $\rho$ (T)$/\rho$ (300 K) of Cu${}_x$IrTe${}_2$ with $x$ ($0\le x\le 0.1$). $\rho$ (T)$/\rho$ (300 K) for Cu-doped samples are shifted upwards by 0.2 for clarity. Temperature dependence of (b) $\rho$ (T)$/\rho$ (3.5 K) without external magnetic fields and (c) magnetic susceptibility under a magnetic field of 10 Oe.

Figure 3

Temperature dependence of (a) Seebeck coefficient measured on cooling for Cu${}_x$IrTe${}_2$, (b) calculated Seebeck coefficient, and (c) Hall coefficient. The broken lines in panel (a) are the fit to the data at low temperatures based on the equation $S=\alpha T$. The inset shows the variation of $\alpha$ as a function of the Cu content $x$.

Figure 4

Calculated density of states of IrTe${}_2$ in (a) the trigonal phase and (b) the monoclinic phase. The respective band structures near the Fermi level are shown in (c) and (d). The solid (red) and dashed (red) lines correspond to the band dispersions obtained with and without inclusion of SOI, respectively. (e) Relative positioning of bonding and antibonding states in each phase. The monoclinic phase has additionally nonbonding states [corresponding to the states spanning energies from $-3$ eV to $-4.6$ eV in (b)] which are mainly localized on Ir atoms. The electron density distribution of these states is shown in the inset. For the trigonal phase, the interplay between the CFS and SOI at the $\Gamma$ and A points is schematically shown in (f).