Richa Sapkota

Luttinger liquids emerge in one-dimensional metals with strong electron interactions, exhibiting intriguing near-equilibrium properties such as spin-charge separation and power-law correlations. Although these interactions suggest fast, distinctive out-of-equilibrium dynamics, such phenomena remain largely unexplored on ultrashort timescales. Here, we use femtosecond laser excitation to weakly deplete the electron density in the Luttinger band of Li _0.9 Mo ₆ O ₁₇ and track the response via time- and angle-resolved photoemission spectroscopy. By fitting the measured electron distributions to a finite-temperature Luttinger liquid model, we observe a fast drop in the Luttinger exponent, quantifying the strength of electron interactions. Subsequently, unlike hot electrons in conventional Fermi liquids that slowly relax within picoseconds via electron-phonon coupling, hot electrons in Li _0.9 Mo ₆ O ₁₇ relax within a short time of ~100 femtoseconds, through the excitation of a nonequilibrium collective plasmon. The extremely fast evolution of the Luttinger exponent and electron temperature—including a tens of femtosecond time lag between excitation, recovery, and plasmon-driven modulation—reveals previously unidentified pathways for modulating quantum many-body interactions in low-dimensional materials.

Topological materials are of great interest because they can support metallic edge or surface states that are robust against perturbations, with the potential for technological applications. Here, we experimentally explore the light-induced non-equilibrium properties of two distinct topological phases in NaCd4As3: a topological crystalline insulator (TCI) phase and a topological insulator (TI) phase. This material has surface states that are protected by mirror symmetry in the TCI phase at room temperature, while it undergoes a structural phase transition to a TI phase below 200 K. After exciting the TI phase by an ultrafast laser pulse, we observe a leading band edge shift of >150 meV that slowly builds up and reaches a maximum after ∼0.6 ps and that persists for ∼8 ps. The slow rise time of the excited electron population and electron temperature suggests that the electronic and structural orders are strongly coupled in this TI phase. It also suggests that the directly excited electronic states and the probed electronic states are weakly coupled. Both couplings are likely due to a partial relaxation of the lattice distortion, which is known to be associated with the TI phase. In contrast, no distinct excited state is observed in the TCI phase immediately or after photoexcitation, which we attribute to the low density of states and phase space available near the Fermi level. Our results show how ultrafast laser excitation can reveal the distinct excited states and interactions in phase-rich topological materials.