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Register a new userRobust hybridization gap in a Kondo Insulator YbB${}_{12}$ probed by femtosecond optical spectroscopy
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In heavy fermions the relaxation dynamics of photoexcited carriers has been found to be governed by the low energy indirect gap, E$_{g}$, resulting from hybridization between localized moments and conduction band electrons. Here, carrier relaxation dynamics in a prototype Kondo insulator YbB${}_{12}$ is studied over large range of temperatures and over three orders of magnitude. We utilize the intrinsic non-linearity of dynamics to quantitatively determine microscopic parameters, such as electron-hole recombination rate. The extracted value reveals that hybridization is accompanied by a strong charge transfer from localized 4f-levels. The results imply the presence of a hybridization gap up to temperatures of the order of E$_{g}$/k$_{B}approx200$ K, which is extremely robust against electronic excitation. Finally, below 20 K the data reveal changes in the low energy electronic structure, attributed to short-range antiferromagnetic correlations between the localized levels.
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A detailed low-energy electronic structure of a Kondo insulator YbB$_{12}$ was revealed by a synergetic combination of ultrahigh-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 $T^ast sim 110$ K. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below $sim$$T^ast$. This is a clear manifestation of photoexcited carriers due to the bottleneck in the recovery dynamics, which is interpreted as a developing hybridization gap of a hard gap.
Kondo insulators have recently aroused great interest because they are promising materials that host a topological insulator state caused by the strong electron interactions. Moreover, recent observations of the quantum oscillations in the insulating state of Kondo insulators have come as a great surprise. Here, to investigate the surface electronic state of a prototype Kondo insulator YbB$_{12}$, we measured transport properties of single crystals and microstructures. In all samples, the temperature dependence of the electrical resistivity is insulating at high temperatures and the resistivity exhibits a plateau at low temperatures. The magnitude of the plateau value decreases with reducing sample thickness, which is quantitatively consistent with the surface electronic conduction in the bulk insulating YbB$_{12}$. Moreover, the magnetoresistance of the microstructures exhibits a weak-antilocalization effect at low field. These results are consistent with the presence of topologically protected surface state, suggesting that YbB$_{12}$ is a candidate material of the topological Kondo insulator. The high field resistivity measurements up to $mu_0H$ = 50 T of the microstructures provide supporting evidence that the quantum oscillations of the resistivity in YbB$_{12}$ occurs in the insulating bulk.
We investigate the quasiparticle dynamics in the prototype heavy fermion CeCoIn$_5$ using ultrafast optical pump-probe spectroscopy. Our results indicate that this material system undergoes hybridization fluctuations before full establishment of the heavy electron coherence, as the temperature decreases from $sim$120 K ($T^dagger$) to $sim$55 K ($T^*$ ). We reveal that the observed anomalous phonon softening and damping reduction below $T^*$ are directly associated with opening of an indirect hybridization gap. We also discover a distinct collective mode with an energy of $sim$8 meV, which may be the experimental evidence of the predicted unconventional density wave. Our observations provide critical informations for understanding the hybridization dynamics in heavy fermion materials.
We report the ultrafast optical pump-probe spectroscopy measurements on the recently discovered quantum critical ferromagnet CeRh$_6$Ge$_4$. Our experimental results reveal the two-stage development of the hybridization between localized $f$ moments and conduction electrons with lowering temperature, as evidenced by (1) the presence of hybridization fluctuation for temperatures from $sim$85 K ($T^*$) to $sim$140 K ($T^dagger$), and (2) the emergence of collective hybridization below the coherence temperature, $T^*$, marked by the opening of an indirect gap of 2$Delta$ $approx$12 meV. We also observe three coherent phonon modes being softened anomalously below $T^*$, reflecting directly their coupling with the emergent coherent heavy electrons. Our findings establish the universal nature of the hybridization process in different heavy fermion systems.
We report a detailed and comparative study of the single crystal CeCoInGa$_3$ in both experiment and theory. Resistivity measurements reveal the typical behavior of Kondo lattice with the onset temperature of coherence, $T^*approx 50,$K. The magnetic specific heat can be well fitted using a spin-fluctuation model at low temperatures, yielding a large Sommerfeld coefficient, $gammaapprox172,$mJ/mol K$^2$ at 6 K, suggesting that this is a heavy-fermion compound with a pronounced coherence effect. The magnetic susceptibility exhibits a broad field-independent peak at $T_{chi}$ and shows an obvious anisotropy within the $bc$ plane, reflecting the anisotropy of the coherence effect at high temperatures. These are compared with strongly correlated calculations combining first-principles band structure calculations and dynamical mean-field theory. Our results confirm the onset of coherence at about 50 K and reveal a similar anisotropy in the hybridization gap, pointing to a close connection between the hybridization strength of the low-temperature Fermi-liquid state and the high-temperature coherence effect.
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