Magnetotransport properties of ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$: Similarities to ${\mathrm{URu}}_2{\mathrm{Si}}_2$

We report on magnetotransport properties of the Kondo semiconducting compound ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$, focusing on its exotic phase below $T_0=27$ K. An excess thermal conductivity $\kappa$ emerges below $T_0$ and is gradually suppressed by magnetic field, strikingly resembling those observed in the hidden-order phase of ${\mathrm{URu}}_2{\mathrm{Si}}_2$. Our analysis indicates that low-energy magnetic excitation is the most likely origin, as was also proposed for ${\mathrm{URu}}_2{\mathrm{Si}}_2$ recently based on inelastic x-ray scattering measurements of phonon dynamics, despite the largely reduced magnetic moments. Likewise, other transport properties such as resistivity, thermopower, and Nernst effect exhibit distinct features characterizing the very different charge dynamics above and below $T_0$, sharing similarities to ${\mathrm{URu}}_2{\mathrm{Si}}_2$, too. Given the exotic nature of the ordered phase in both compounds, whether a unified interpretation to all these observations exists appears to be extremely interesting.

Figure 1

(a) Electrical resistivity $\rho \left(T\right)$ measured in various magnetic fields applied perpendicular to electrical current, the magnetoresistance ${\mathrm{MR}}_{8\mathrm{T}}=({\rho }_{8\mathrm{T}}-{\rho }_{0\mathrm{T}})/{\rho }_{0\mathrm{T}}$, and the derivative of $\rho$ with respect to $T$ for $B=0$ T. Dotted line represents a thermal activation behavior of $\rho \left(T\right)$ observed between 30 and 80 K. (b) Hall mobility ${\mu }_{\mathrm{H}}$ as a function of temperature [10].

Figure 2

Thermal conductivity $\kappa \left(T\right)$ measured in different magnetic fields. The electronic contribution ${\kappa }_{\mathrm{e}}\left(T\right)$ (crosses) was estimated from the WF law on the basis of the measured values of resistivity in zero field. The solid line represents the phononic thermal conductivity ${\kappa }_{\mathrm{ph}}$ of the nonmagnetic analog ${\mathrm{LaRu}}_2{\mathrm{Al}}_{10}$ and the dotted curve $\kappa \left(T\right)$ of single-crystalline ${\mathrm{CeFe}}_2{\mathrm{Al}}_{10}$ along the most heat-conductive direction ($a$ axis). The dashed curve at temperatures below $T_0$ is an extrapolation of the measured $\kappa$ for ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$, ignoring the excess contribution below $T_0$ and following essentially the same $T$ dependence of ${\kappa }_{\mathrm{ph}}$ of ${\mathrm{LaRu}}_2{\mathrm{Al}}_{10}$. By applying magnetic field, the excess heat conductance below $T_0$ is significantly reduced.

Figure 3

The excess thermal conductivity ${\kappa }_{\mathrm{m}}$ in the ordered phase and its suppression in magnetic field, denoted as the difference of thermal conductivity measured in zero and applied magnetic fields, i.e., $\mathrm{\Delta }\kappa ={\kappa }_{0\mathrm{T}}-{\kappa }_{\mathrm{B}}$. Note the appearance of nonzero $\mathrm{\Delta }\kappa$ in only the ordered phase and its strong field dependence.

Figure 4

(a) Thermopower $S\left(T\right)$ measured in different magnetic fields for ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$. The curve of $\rho \left(T\right)$ is also shown in order to highlight the similarity of the sharp peak at 22 K in both $S\left(T\right)$ and $\rho \left(T\right)$. Inset: The magnetothermopower defined $\mathrm{\Delta }S=S_{8\mathrm{T}}-S_{0\mathrm{T}}$. (b) Nernst coefficient $\nu$ measured in different fields shown as $-\nu$ vs $T$.