arXiv subjects

Hide Takagi

Publications and source records attributed to Hide Takagi.

3 recordsLinked to original sources

arXiv2020

Detecting electron-phonon couplings during photo-induced phase transition

Photo-induced phase transitions have been intensively studied owing to the ability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it has currently been a central issue how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photo-induced phase transitions. Here, we develop a new analysis method, frequency-domain angle-resolved photoemission spectroscopy, to gain precise insight into electron-phonon couplings during photo-induced insulator-to-metal transitions for Ta$_2$NiSe$_5$. We demonstrate that multiple coherent phonons generated by displacive excitations show band-selective coupling to the electrons. Furthermore, we find that the lattice modulation corresponding to the 2 THz phonon mode, where Ta lattice is sheared along the a-axis, is the most relevant for the photo-induced semimetallic state.

cond-mat.str-el↗

arXiv2018

Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$

Femtosecond time-resolved mid-infrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta$_2$NiSe$_5$ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C = 0.30~\pm~0.08$~mJ/cm$^2$ due to optical absorption saturation. This limits the optical excitation density in Ta$_2$NiSe$_5$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta$_2$NiSe$_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.

cond-mat.str-el↗

arXiv2017

Ultrafast Electronic Band Gap Control in an Excitonic Insulator

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta$_2$NiSe$_5$ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of $F_{C} = 0.2$ mJ cm$^{-2}$, the band gap $narrows$ transiently, while it is $enhanced$ above $F_{C}$. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta$_2$NiSe$_5$, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta$_2$NiSe$_5$ with light on the femtosecond time scale.

cond-mat.str-el↗