Magnetic-Field-Induced Kondo Metal Realized in ${\mathrm{YbB}}_{12}$

The specific heat of the Kondo insulator ${\mathrm{YbB}}_{12}$ has been measured up to 60 T. The Sommerfeld coefficient $\gamma$ significantly increases at around 50 T, where the insulator metal transition occurs with a steep increase of the magnetization. $\gamma$ reaches $67\mathrm{mJ}/(\mathrm{mol}{\mathrm{K}}^2)$ at high fields, which directly indicates that the quasiparticles gain a heavy thermodynamic effective mass and transform into a Kondo metal under magnetic fields. The field-induced Kondo metal has a rather high Kondo temperature around 200 K. The strong Kondo coupling proves that the energy gap collapse does not correspond to the breakdown of the Kondo bound state. The steep increase of the magnetization at the transition manifests the sharp density of states at the Fermi energy formed via the Kondo resonance.

Figure 1

(a) Time profile of the sample temperature and the sequence of applied heat pulses at zero magnetic field. The temperature increase $\mathrm{\Delta }T$ by the heat pulse is evaluated by the difference of the temperatures indicated by two dotted lines. (b) Time profile of the sample temperature and the sequence of applied heat pulses at magnetic fields around 60 T.

Figure 2

(a) The plot of the $C_p/T$ in the $B\text{−}T$ plain with a color gradient scale. (b) The temperature dependence of $C_p/T$ at different magnetic fields. The dashed lines are the least square fits to the experimental data.

Figure 3

(a) The magnetic field dependence of $\gamma$. The magnetic field dependence of $\beta$ is shown in the inset. (b) Temperature measured as a function of applied magnetic field with different initial temperatures. The vertical dashed lines denote the IM transition magnetic field [15].