Electronic-Structure-Driven Magnetic Ordering in a Kondo Semiconductor ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$

We report the anisotropic changes in the electronic structure of a Kondo semiconductor ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$ across an anomalous antiferromagnetic ordering temperature ($T_0$) of 29 K, using optical conductivity spectra. The spectra along the $a$ and $c$ axes indicate that an energy gap due to the hybridization between conduction bands and nearly local $4f$ states, namely the $c\mathrm{\text{−}}f$ hybridization gap, emerges from a higher temperature continuously across $T_0$. Along the $b$ axis, on the other hand, another energy gap with a peak at 20 meV becomes visible at 39 K ($>T_0$) and fully opens at $T_0$ because of a charge instability. This result implies that the appearance of the energy gap, as well as the change in the electronic structure along the $b$ axis, induces the antiferromagnetic ordering below $T_0$.

Figure 1

Low-energy portion of the temperature-dependent polarized reflectivity [$R(\omega )$] spectra of ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$ along three principal axes. (Inset) The entire reflectivity spectra at 300 K along the $a$ axis (solid lines), $b$ axis (dashed lines), and $c$ axis (dotted lines).

Figure 2

Temperature-dependent polarized optical conductivity [$\sigma (\omega )$] spectra of ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$ in the photon-energy region below 70 meV. Each of the lines is shifted by $1.25\times {10}^3{\Omega }^{-1}{\mathrm{cm}}^{-1}$ for clarity.

Figure 3

Temperature-dependent optical conductivity [$\sigma (\omega )$] spectra in $E\parallel a$ (a), $E\parallel b$ (b), and $E\parallel c$ (c) at temperatures from 10 to 40 K. (d) Temperature dependence of representatives of spectral change. ${\sigma }_x(5\mathrm{meV})$ [$x=a$, $b$, $c$ ($x$ is axis name)] and ${\sigma }_b(20\mathrm{meV})$ are the intensities of the $\sigma (\omega )$ spectra at 5 and 20 meV, respectively.

Figure 4

(a) Ratio spectra of optical conductivity [$\sigma (\omega )$] at 10, 20, 28, and 36 K, derived from that at 40 K along the $b$ axis. The residual conductivity is indicated by the horizontal dashed line. (Inset) Expected density of states (DOS) near $E_F$ and corresponding energy levels of “Peak”, “Midpoint”, and “Onset” in $\sigma (\omega ,T)/\sigma (\omega ,T=40\mathrm{K})$ spectra. (b) Temperature dependence of the peak energy (solid squares), the energy at the midpoint (solid circles), and the onset (solid triangles) indicated in (a). The inelastic neutron peaks of ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$ (open circles, Ref. [12]) and of ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ (open diamonds, Ref. [13]) in which the temperature is normalized against $T_0$ are also plotted. The solid, dotted, and dashed lines are the temperature-dependent energy gaps predicted by BCS theory with $T_c=28.7\mathrm{K}$ ($T_0$), 31 K, and 39 K ($T^{*}$), respectively.