RESEARCH

Ultrafast Phenomena in Quantum Materials

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Schematic diagram for ultrafast techniques

Over the limitation of continuous-light-source spectroscopy, ultrafast spectroscopy enables us to study material properties in the time domain. Specifically, pump-probe techniques, using a stronger pump beam to excite a material and a weaker probe beam to monitor pump-induced changes, have been intensively utilized to study ultrafast dynamics in electronic and crystal structures of condensed matters (see the figure above). 

One of my researches has been focused on the studies of ultrafast phenomena in quantum materials. To explore the non-equilibrium dynamics of exotic quantum phases in strongly correlated electron systems and topological materials, I've utilized various ultrashort lights of terahertz, infrared, visible, and X-ray sources for excitation or observation of electronic and lattice dynamics.

Coherent phonon spectroscopy

Coherent phonon oscillations - periodic modulations in the lattice triggered by femtosecond optical pulses - provide a unique opportunity to investigate how the crystal structures are coupled to the other degrees of freedom, such as spin, orbital, and charge, in quantum materials. Probing coherent phonon responses, I've found exotic ultrafast features in the lattice that have not been observed previously in equilibrium states.

  • Spin-phonon coupling in Mott insulator Ca2RuO4 [1]

  • Structural evolution with orbital ordering in Ca2RuO4 [2]

  • Nematic responses in coherent phonon of pnictide BaFe2As2

  • Acoustic phonon generation in Weyl semimetal TaAs [3]

Coherent oscillations of the lowest A1g phonon mode in Ca2RuO4 [1].

The real-time observation on a femtosecond time scale after external stimuli allows us to single out important interactions during the photoinduced excitation and the recovery back to the equilibrium state. In this manner, I've multiple phase transitions arising from competing interactions in quantum matters by disentangling the interplay among charge, lattice, spin, and orbital dynamics, showing their different timescales in the excitation and recovery processes.

Non-equilibrium electrodynamics

  • Structural evolution in correlated metal Sr2RhO4 [4]

  • Spin and orbital ordering in antiferromagnetic Mott insulator Sr2CrO4

  • Lifshitz transition in pyrochlore antiferromagnet Cd2Os2O7 [5]

  • Strain-induced superconductivity enhancement in YBa2Cu3O6+d

  • Review paper of equilibrium/non-equilibrium spectroscopic studies of metal-insulator transitions [6]

Sr2CrO4.tif

Ultrafast signature of orbital ordering at 140 K in Sr2CrO4

Low-to-High-energy Spectroscopic studies on Quantum Materials

Shining a light on condensed matters lets us know more about the material. A variety of sources from low- to high-photon-energy lights, as well as neutron particles, drive resonant excitations of magnetic/lattice collective modes (magnon/phonon), or electronic transitions from the valence and core bands. In addition to the field of ultrafast spectroscopy, I've also studied the spectroscopic properties of quantum matters in their equilibrium states using optical, infrared, and X-ray light sources. 

X-ray absorption spectroscopy of double perovskites

Double perovskite in a formula of A2BB'O6, where A is a rare earth element, B/B' = transition metal (TM) ions, and O is oxygen, is a novel platform for spintronics applications, given by their large magnetoresistance and high Curie temperature. Its magnetic properties are governed by the spin interaction between the neighboring TM ions, B and B'. 

I've studied 3d-5d double perovskite compounds, La2CoIrO6 and La2CoPtO6, by using X-ray absorption and neutron scattering spectroscopy techniques. As a result, I revealed that the spin-orbit entangled character of 5d TM ions plays a significant role to determine the subdued magnetism in these double perovskites.

  • X-ray absorption spectroscopy (XAS) and
    X-ray magnetic circular dichroism (XMCD) [7,8]

  • Neutron scattering spectroscopy [9,10]

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XAS and XMCD spectrum of La2CoIrO6 and La2CoPtO6 [7]

Optical spectroscopy of transition metal compounds

Optical spectroscopy has been widely used to investigate the low-energy physics in quantum materials. Using Fourier transform infrared spectroscopy (FTIR) and ellipsometry technique, I've studied collective lattice excitations (phonon) and electronic structures in transition metal compounds, and how these properties are correlated to the electronic/magnetic grounds states.

  • Spin-orbit entangled 5d Mott insulator Sr2IrO4 [11]

  • Semimetallic Dirac material SrMnBi2 [12]

  • Correlated Van der Waals antiferromagnet NiPS3 [13]

Optical conductivity of SrMnBi2 [12]

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EXPERIMENTAL TECHNIQUES

  • Time-domain terahertz spectroscopy

  • Pump-probe ultrafast optical/terahertz spectroscopy

  • Time-resolved X-ray diffraction/scattering

  • Coherent phonon spectroscopy

  • Cryogenic systems

  • X-ray absorption spectroscopy

  • X-ray magnetic circular/linear dichroism

  • Time-resolved X-ray diffraction/scattering

  • High-harmonics generation

  • Ellipsometry

  • Fourier transform infrared spectroscopy (FTIR)

MATERIALS

  • Strongly correlated electron systems:

    • (Magnetic) Mott insulators

    • Ferroelectrics / Multiferroics

    • Unconventional / High-temperature superconductors

  • Topological Materials: 
    Weyl and Dirac semimetals​

  • (Correlated) Van der Waals magnets