Hybridization gap formation in the Kondo insulator ${\mathrm{YbB}}_{12}$ observed using time-resolved photoemission spectroscopy

A detailed low-energy electronic structure of a Kondo insulator ${\mathrm{YbB}}_{12}$ was revealed by a synergetic combination of ultra-high-resolution laser photoemission spectroscopy (PES) and time-resolved PES. The former confirmed a 25-meV pseudogap corresponding to the Kondo temperature of this material, and more importantly, it revealed that a 15-meV gap and a Kondo-peak feature developed below a crossover temperature of $T^{*}\sim 110\mathrm{K}$. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below $\sim T^{*}$. This is a clear manifestation of photoexcited carriers due to the bottleneck in the recovery dynamics, which is interpreted as a developing hybridization gap of a hard gap.

Figure 1

Temperature dependence of the valence-band spectra of ${\mathrm{YbB}}_{12}$ in the near-$E_F$ region recorded by a 7-eV laser. (a) Near-$E_F$ valence-band spectra at various temperatures. The inset shows a magnified view at $E_F$. (b) Experimental density of states (DOS). (c) Intensity of the experimental DOS relative to that at 225 K. (d) Spectral intensity at $E_F$, 15, 30, and 45 meV as functions of temperature. The solid lines are guides to the eye. Here, although the lowest temperature was in the range of 7 to 8 K, we describe it as 7 K for simplicity.

Figure 2

(a) TrPES intensity map at 6 K as functions of binding energy and delay time with a logarithmic color scale. (b) Delay time profiles at the binding energies of $-5$ and $-35\mathrm{meV}$, corresponding to the triangles shown in panel (a). (c) TrPES spectra recorded at 6 K displayed on a logarithmic intensity scale. $\mathrm{\Delta }I\left(E_B\right)$ (see text) is also shown on a linear intensity scale.

Figure 3

(a) Time dependence of the integrated intensities in the unoccupied region ($-45\mathrm{meV}<E_B<E_F$) at several temperatures. Dashed lines correspond to the zero intensity. (b) The long-range decay behaviors ($\gg 100\mathrm{ps}$) at 8 and 175 K. (c) Temperature dependence of the integrated intensity between 10 and 90 ps obtained from data shown in (a). The intensities were normalized to the lowest-temperature data. The characteristic temperature ($T^{*}$) shown in Fig. 1 is indicated by a dashed line at 110 K.

Figure 4

Schematic electron- ($e$-) hole ($h$) recombination processes in (a) the metallic state ($T\gg T^{*}$), (b) the crossover state ($T\lesssim T^{*}$), and (c) the insulating ground state ($T=0$). $|c\rangle ,|f\rangle$, and $|cf\rangle$ correspond to the states of the conduction band, the localized $f$ state, and the $c-f$ hybridized band, respectively.