arXiv subjects

Han K. D. Le

Publications and source records attributed to Han K. D. Le.

3 recordsLinked to original sources

arXiv2026

From interface-limited to Auger-dominated carrier dynamics in $π$-SnS

Metastable cubic tin(II) sulfide ($π$-SnS) is an earth-abundant semiconductor whose three-dimensionally bonded chiral lattice may overcome the short minority-carrier lifetime of orthorhombic SnS while maintaining a near-ideal bandgap for tandem photovoltaics. Despite its promise, ultrafast carrier cooling and recombination mechanisms over illumination density remain poorly constrained. We use core-level extreme-ultraviolet attosecond transient absorption spectroscopy at the Sn $4d$ edge to track carrier injection, cooling, and recombination in $π$-SnS with element- and orbital-specific sensitivity. Following femtosecond near-infrared excitation, the Sn $4d\rightarrow$CB onset exhibits conduction-band state filling and a carrier-induced edge shift, enabling extraction of density-dependent kinetics. The transient response follows a biexponential decay with a fast hot-carrier cooling component and a slower recombination component. At low carrier densities, recombination is consistent with interface-limited processes, whereas above $\sim1\times10^{20}$ cm$^{-3}$ both cooling and recombination accelerate, indicating a crossover to carrier-carrier interaction-dominated dynamics. Coherent phonon oscillations with a period of $\sim188$ fs reveal coupling between electronic excitation and lattice motion. These results provide a comprehensive picture of nonequilibrium carrier and phonon dynamics in cubic SnS, reveal a change of mechanisms over a range of carrier densities, and establish the value of using attosecond transient absorption spectroscopy to study ultrafast processes in complex semiconductors that have optoelectronic and energy-conversion applications.

cond-mat.mtrl-sci↗

arXiv2025

Coherent phonon motions and ordered vacancy compound mediated quantum path interference in Cu-poor CuIn$_{x}$Ga$_{(1-x)}$Se$_2$ (CIGS) with attosecond transient absorption

In this study, coherent phonon motion is observed in bandgap excited CuIn$_{x}$Ga$_{(1-x)}$Se$_2$ (CIGS) utilizing extreme ultraviolet (XUV) attosecond transient absorption spectroscopy across the Se M$_{4,5}$ absorption edge. Two frequencies of coherent phonon motion are resolved, a low frequency mode attributed through Raman measurements to the $A_{1g}$ phonon motion of a Cu-deficient ordered vacancy compound (OVC), while the high frequency mode originates from the $A_{1g}$ phonon motion in the chalcopyrite phase. The two oscillations lead to modulations in the XUV differential absorption $ΔA(ε,τ)$ due to energy shifts of the Se M$_{4,5}$ edge, with a minima occuring approximately 1 ps after the band gap excitation. The hot carrier cooling time of holes and electrons are disentangled and the observed slower cooling of holes is attributed to the higher density of hole states in the valence band. We also observe fast oscillations (18.6(3) fs period) across the Se absorption edge, which are interpreted to originate from quantum path interference between the electronic conduction bands of the chalcopyrite CIGS and OVC phases, opening the possibility towards quantum coherent metrology in photovoltaics on the femtosecond timescale. The complex interplay between the chalcopyrite and OVC phases are revealed in this investigation through both coherent vibrational and electronic motions.

cond-mat.mtrl-sci↗

arXiv2025

Coherently Coupled Carrier and Phonon Dynamics in Elemental Tellurium Probed by XUV Transient Absorption

The narrow bandgap semiconductor elemental tellurium (Te) has a unique electronic structure due to strong spin-orbit splitting and a lack of inversion symmetry of it's helical lattice. Using broadband extreme ultraviolet core-level transient absorption, we measure simultaneously the coherently coupled photo-induced carrier and lattice dynamics at the Te N$_{4,5}$ edge initiated by a few-cycle NIR pulse. Ultrafast excitation of carriers leads to a coherently excited A$_{\rm{1}}$ phonon oscillation and the generation of a hot carrier population distribution that oscillates in temperature, and the phonon excursion and hot carrier temperature are $π$ out of phase with respect to each other. The depths of modulation suggest a significant coupling between the electronic and lattice degrees of freedom in Te. A long-lived shift of the absorption edge suggests an excited state of Te in a new equilibrium potential energy surface that lives on the order of the carrier recombination timescale. The observed phonon-induced oscillations of the hot carriers are supportive of a change in the metallicity, whereby Te becomes more metallic with increasing phonon-induced displacement. Additionally, near the Fermi level we observe an energy-dependent phase of the displacive excitation of the A$_{\rm{1}}$ phonon mode. The discovery of coherent coupling between the lattice and hot carriers in Te provides the basis to investigate coherent interactions between spin and orbital degrees of freedom. The results spectrally and temporally resolve the correlation between photo-excited hot carriers and coherent lattice excitations, providing insight on the optical manipulation of the Te electronic structure at high carrier densities exceeding $10^{21}\,\mathrm{cm}^{-3}$.

cond-mat.mtrl-sci↗