Accessing non-equilibrium states of correlated materials through targeted ultrafast optical excitation
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Abstract
Strongly correlated materials provide rich platforms through which to explore interesting non-equilibrium states using the pathways provided by the complex interactions of spin, orbit, charge, and lattice degrees of freedom. Ultrafast laser pulses allow us to disentangle the transient dynamics of these non-equilibrium states and potentially open the door to tunable control of material properties or the discovery of novel phases. Here, we make use of ultrafast terahertz time-domain spectroscopy to map the response of correlated materials following targeted photoexcitation. Iron-based superconductors display interesting interactions of nematicity, magnetic order, and unconventional superconductivity. The electronic band structure and magnetic properties of these correlated materials are extremely sensitive to the anion height over the Fe square lattice layer. The coherent excitation of the A_{1g} phonon using ultrafast laser pulses presents a means to access optical control of the anion height; this mechanism can be used to tune the properties and phases of iron-based superconductors. The existing literature has shown that an upward displacement of the anion height in BaFe_2As_2 favors an enhanced Fe magnetic moment and conditions for magnetic order. Despite this, no direct observations of enhanced spin-density-wave order have been made. Here, we investigate the transient dynamics of BaFe_2As_2 in the A_{1g} phonon-driven state. In the magnetic phase, the formation of a transient gap is observed at early time delays. This feature is seen in both the spin-density-wave state and paramagnetic normal state, persisting up to room temperature. Van der Waals magnets are a particularly interesting material family for investigating light-matter interactions and spin-correlated excitations, with promise in spintronic and optoelectronic applications. In particular, the van der Waals antiferromagnet NiPS_3 has attracted extensive interest due to its ultranarrow exciton features, which are closely linked to magnetic ordering. Here, we explore the nature of the spin-orbit-entangled exciton series in NiPS_3 through tailored ultrafast laser pulses. We report a photo-induced state with a lifetime of 17 ps that only appears with resonant 1.476-eV pumping of the spin-orbit-entangled exciton in the antiferromagnetic phase. The long-lived response exhibits a negative change in the terahertz optical conductivity, and we interpret the state to be a population inversion between closely spaced excitonic levels. These results present NiPS_3 as a potential candidate to achieve long-lived lasing at terahertz frequencies in reduced dimensions. Finally, we apply the selective photoexcitation scheme to investigate the photoexcited carrier response in the antiferromagnetic and paramagnetic phases of NiPS_3. The pump fluence and photon energy dependence of the transient change in spectral weight are used to discern the dominant carrier generation mechanisms, identify the onset of interband transitions, and estimate the spin-orbit-entangled exciton binding energy. These results provide key insights to validate the electronic structure and quantify the exciton characteristics of NiPS_3.
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2025