Emergent photovoltage on SmB6 surface upon bulk-gap evolution revealed by pump-and-probe photoemission spectroscopy

Prolonged duration of nonequilibrated Dirac fermions in neutral topological insulators

Topological insulators (TIs) possess spin-polarized Dirac fermions on their surface but their unique properties are often masked by residual carriers in the bulk. Recently, (Sb_1−xBi_x)₂Te₃ was introduced as a non-metallic TI whose carrier type can be tuned from n to p across the charge neutrality point. By using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast carrier dynamics in the series of (Sb_1−xBi_x)₂Te₃. The Dirac electronic recovery of ∼10 ps at most in the bulk-metallic regime elongated to >400 ps when the charge neutrality point was approached. The prolonged nonequilibration is attributed to the closeness of the Fermi level to the Dirac point and to the high insulation of the bulk. We also discuss the feasibility of observing excitonic instability of (Sb_1−xBi_x)₂Te₃.

High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system

The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively, the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm² is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm².

CeRu4Sn6: a strongly correlated material with nontrivial topology

Topological insulators form a novel state of matter that provides new opportunities to create unique quantum phenomena. While the materials used so far are based on semiconductors, recent theoretical studies predict that also strongly correlated systems can show non-trivial topological properties, thereby allowing even the emergence of surface phenomena that are not possible with topological band insulators. From a practical point of view, it is also expected that strong correlations will reduce the disturbing impact of defects or impurities, and at the same increase the Fermi velocities of the topological surface states. The challenge is now to discover such correlated materials. Here, using advanced x-ray spectroscopies in combination with band structure calculations, we infer that CeRu₄Sn₆ is a strongly correlated material with non-trivial topology.

Gigantic surface lifetime of an intrinsic topological insulator

The interaction between light and novel two-dimensional electronic states holds promise to realize new fundamental physics and optical devices. Here, we use pump-probe photoemission spectroscopy to study the optically excited Dirac surface states in the bulk-insulating topological insulator Bi_{2}Te_{2}Se and reveal optical properties that are in sharp contrast to those of bulk-metallic topological insulators. We observe a gigantic optical lifetime exceeding 4  μs (1  μs=10^{-6}  s) for the surface states in Bi_{2}Te_{2}Se, whereas the lifetime in most topological insulators, such as Bi_{2}Se_{3}, has been limited to a few picoseconds (1  ps=10^{-12}  s). Moreover, we discover a surface photovoltage, a shift of the chemical potential of the Dirac surface states, as large as 100 mV. Our results demonstrate a rare platform to study charge excitation and relaxation in energy and momentum space in a two-dimensional system.