Optical-Pump Terahertz-Probe Spectroscopy of the Topological Crystalline Insulator Pb_1–xSn_xSe through the Topological Phase Transition

Abstract

Topological crystalline insulators, topological insulators whose properties are guaranteed by crystalline symmetry, can potentially provide a promising platform for terahertz optoelectronic devices, as their properties can be tuned on demand when layered in heterostructures. We perform the first optical-pump terahertz-probe spectroscopy of topological crystalline insulators, using them to study the dynamics of Pb1–x Sn x Se as a function of temperature. At low temperatures, excitation of Dirac fermions leads to an increase in terahertz transmission; from this negative photoconductivity, the intrasubband relaxation rate of 6 ps is extracted. At high temperatures, where only massive Fermions exist, the free-carrier losses induced by the pump reduce the terahertz transmission for the duration of the 27 ps interband lifetime. Both effects are present at temperatures near the topological-to-trivial transition. Our experimental observations provide critical details for potential applications of Pb1–x Sn x Se and provide a direct measurement of the topological character of Pb1–x Sn x Se heterostructures.

Observation of interplay between phonon chirality and electronic band topology

“…1C at 250 K and zero magnetic field. The minima observed at ∼0.9 THz (TO 1 ) and ∼1.6 THz (TO 2 ) are consistent with two anharmonicity-split optical phonon modes observed in neutron scattering measurements on PbTe (56), similar to features observed in Pb 1−x Sn x Se (57) and common in all the studied samples; see the Supplementary Materials (58). The large widths of the phonon resonances are related to alloy-inherent disorder.…”

Negative Photoconductivity: Bizarre Physics in Semiconductors

“…To date, almost all commercialized photodetector devices are based on positive photoconductivity. The opposite NPC phenomenon has been widely demonstrated in inorganic semiconductors. − Various types of mechanisms/hypotheses have been proposed behind the NPC generation in these semiconductors. In the following few subsections, we will discuss the NPC phenomenon in inorganic semiconductors and the possible mechanism behind this feature.…”

Ferroic Berry Curvature Dipole in a Topological Crystalline Insulator at Room Temperature

“…At 5 K, from the experimental result shown in Figure c (the ferroelectric distortion is oriented along the x direction by the pulsed electric field), χ = V a − b b 2 ω / false( I b ω false) 2 was found to be 8.29 × 10 5 V/A 2 in the PST. Using τ = 6 ps and the conductivity measured by the van der Pauw method (Figure f), the BCD of the PST was estimated to be 150 nm. It is notable that this estimation provides a lower limit because bulk conduction is not completely suppressed in the actual experiments.…”