Infrared study of the valence transition compound $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$ using cleaved surfaces

Optical reflectivity $\left[R\left(\omega \right)\right]$ of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$ single crystals has been measured across its first order valence transition at $T_v\simeq 42\mathrm{K}$, using both polished and cleaved surfaces. $R\left(\omega \right)$ measured on cleaved surfaces $\left[R_c\left(\omega \right)\right]$ was found much lower than that on polished surface $\left[R_p\left(\omega \right)\right]$ over the entire infrared region. Upon cooling through $T_v$, $R_c\left(\omega \right)$ showed rapid changes over a temperature range of less than $2\mathrm{K}$ and showed only minor changes with further cooling. In contrast, $R_p\left(\omega \right)$ showed much more gradual and continuous changes across $T_v$, similar to previously reported data on polished surfaces. The present result on cleaved surfaces demonstrates that the microscopic electronic structures of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$ indeed undergo sudden changes upon the valence transition, which is consistent with its first order nature. The gradual temperature evolution of $R_p\left(\omega \right)$ is most likely due to the compositional and/or Yb-In site disorders caused by polishing.

Figure 1

(a) Resistivity $\left(\rho \right)$ of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$ single crystal as a function of temperature $\left(T\right)$. Two vertical lines indicate 41 and $44\mathrm{K}$. (b) Photograph of cleaved (fractured) surfaces of a $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$ single crystal. The dark areas indicate flat, specular surfaces.

Figure 2

(Color online) (a) Reflectivity spectra $\left(R\right)$ of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$ measured on cleaved and polished surfaces above and below $T_v$. (b) Optical conductivity spectra $\left(\sigma \right)$ obtained from the reflectivity using the Kramers-Kronig relations. The spectra in the extrapolated region are indicated by broken curves. (c) Effective carrier density $N_{\mathrm{eff}}$ per formula unit (f.u.) calculated from $\sigma \left(\omega \right)$ using the optical sum rule.

Figure 3

(Color online) Reflectivity spectra of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$ single crystals measured on (a) polished and (b) cleaved surfaces. Note that the vertical scale is different between (a) and (b).

Figure 4

(Color online) $R_c\left(\omega \right)$ and $R_p\left(\omega \right)$ integrated from $0.2\text{to}0.55\mathrm{eV}$ as a function of temperature. The vertical line is drawn at $46\mathrm{K}$ and is a guide to the eye.