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Ultrafast photoexcitation is an emerging route to selective control of phase transitions. However, it is difficult to determine which modes govern the transformation and how effectively they are targeted by photoexcitation. This is…
Nonequilibrium hidden states, both transient and long-lived, provide a unique window into thermally inaccessible regimes of strong coupling between microscopic degrees of freedom in quantum materials. Understanding the physical origin of…
Photoinduced phase transitions from insulating to metallic states, accompanied by structural re-arrangements, have been recently reported in complex transition-metal oxides. However, the optical control of a purely electronic phase…
We study the nonequilibrium dynamics of photoexcited electrons in the narrow-gap Mott insulator VO$_2$. The initial stages of relaxation are treated using a quantum Boltzmann equation methodology, which reveals a rapid ($\sim$ femtosecond…
Achieving fundamental understanding of insulator-to-metal transitions (IMTs) in strongly correlated systems and their persistent and reversible control via nonequilibrium drive are prime targets of current condensed matter research.…
Ultrafast photoexcitation offers a novel approach to manipulating quantum materials. One of the long-standing goals in this field is to achieve optical control over topological properties. However, the impact on their electronic structures,…
Photo-excitation is a very powerful way to instantaneously drive a material into a novel quantum state without any fabrication, and variable ultrafast techniques have been developed to observe how electron-, lattice-, and spin-degrees of…
Transition metal dichalcogenides with superperiodic lattice distortions have been widely investigated as the platform of ultrafast structural phase manipulations. Here we performed ultrafast electron diffraction on room-temperature…
Novel phases of matter with unique properties that emerge from quantum and topological protection present an important thrust of modern research. Of particular interest is to engineer these phases on demand using ultrafast external stimuli,…
Ta$_2$NiSe$_5$ is an excitonic insulator candidate showing the semiconductor/semimetal-to-insulator (SI) transition below $T_{\text{c}}$ = 326 K. However, since a structural transition accompanies the SI transition, deciphering the role of…
We reveal an ultrafast purely electronic phase transition in \TNS, which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks…
Laser excitation has emerged as a means to expose hidden states of matter and promote phase transitions on demand. Such laser induced transformations are often rendered possible owing to the delivery of spatially and/or temporally…
Strain engineering is a powerful technology which exploits stationary external or internal stress of specific spatial distribution for controlling the fundamental properties of condensed materials and nanostructures. This advanced technique…
Photonic topological insulators (PTIs) have been proposed as an analogy to topological insulators in electronic systems. In particular, two-dimensional PTIs have gained attention for the integrated circuit applications. However, controlling…
Time-resolved multi-terahertz (THz) spectroscopy is used to observe an ultrafast, non-thermal electronic phase change in SnSe driven by interband photoexcitation with 1.55 eV pump photons. The transient THz photoconductivity spectrum is…
Utilizing the time-dependent density-matrix renormalization group technique, we numerically prove photoinduced pairing states in the extended Falicov-Kimball model (EFKM) at half filling, both with and without internal SU(2) symmetry. In…
This article reviews experimental work on the ultrafast electron dynamics in the topological surface state (TSS) of three-dimensional (3D) topological insulators (TIs) observed with time- and angle-resolved two-photon photoemission (2PPE).…
The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behavior, including non-thermal phases and photoinduced phase…
We report on an ultrafast terahertz free-carrier dynamic study of a photo-excited WTe2 thin film. In the photo-excited state, we observe a metastable electronic state featuring negative differential terahertz photoconductivity and reduced…
The nature of the insulator-to-metal phase transition in vanadium dioxide (VO2) is one of the longest-standing problems in condensed-matter physics. Ultrafast spectroscopy has long promised to determine whether the transition is primarily…