Unconventional charge-density wave in ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ cubic superconductor revealed by optical spectroscopy study

${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ is an interesting compound showing the coexistence of structural phase transition and superconductivity. The structural phase transition at 147 K leads to the formation of a superlattice. We perform optical spectroscopy measurements across the structural phase transition on single-crystal samples of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$. The optical spectroscopy study reveals an unusual temperature-induced spectral weight transfer over a broad energy scale, yielding evidence for the presence of electron correlation effects. Below the structural phase transition temperature an energy gap-like suppression in optical conductivity was observed, leading to the removal of partial itinerant carriers near the Fermi level. Unexpectedly, the suppression appears at a much higher energy scale than expected for the usual charge-density-wave phase transition.

Figure 1

The x-ray diffraction patterns of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ single crystals at room temperature.

Figure 2

Physical properties of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ single crystals. (a) Resistivity $\rho$ versus temperature in the range of 2 to 300 K. (b) Temperature dependence of specific heat C with zero applied field. (c–e) Temperature-dependent magnetic susceptibility (a), resistivity (b), and specific heat (c) from 90 to 190 K, respectively. The dashed line indicates the phase transition beginning at $T^{*}=147$ K.

Figure 3

(a) Low-temperature magnetic susceptibility $\chi$ versus temperature for ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ single crystals in $H=50$ Oe parallel to the (110) plane with zero-field-cooled (ZFC) and field-cooled (FC) processes. (b) Specific heat of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ in the plot of $C/T$ versus $T^2$ with 0 T and 9 T applied field perpendicular to the (110) plane; the solid line is the fitting result using the relation $C=\gamma T+\beta T^3+\eta T^5$ after the superconductivity is completely suppressed by a magnetic field of 9 T.

Figure 4

(a) The (110)-plane reflectance spectra of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ single crystals at various temperatures, with the black arrow marking the onset of the suppression feature. (b) Optical conductivity ${\sigma }_1\left(\omega \right)$ up to 15 000 ${\mathrm{cm}}^{-1}$; the black arrow points at the newly developed peaks.

Figure 5

(a) Experimental ${\sigma }_1\left(\omega \right)$ data at 10 and 300 K up to 15 000 ${\mathrm{cm}}^{-1}$ . The Drude (D1, D2) and the Lorentz (L1–L5) terms acquired from a Drude-Lorentz fit for $T=10$ K below 15 000 ${\mathrm{cm}}^{-1}$ are illustrated at the bottom. (b) Normalized total ${\omega }_p^2$ (black line with squares) and $1/{\tau }_1$ (blue line with circles). Both parameters are normalized to the values of 300 K .

Figure 6

(a) Frequency dependence of the effective mass and (b) scattering rate of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$.