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Nitrogen-vacancy (NV) scanning magnetometry (Image: qnami.ch).

Single-spin-based quantum magnetometry

A nitrogen-vacancy (NV) can be used as an ultrasensitive magnetometer (~nT). The two-level states in a single electron are sensitive to the magnetic field around it so combined with a tip-based scanning technique at cryogenic temperature, the magnetization distribution of a system can be explored. 

Light-matter interactions

   To study various condensed matter physics, I utilize light-matter interaction phenomena. Light can reveal hidden physics and I have been dealing with spin/orbital angular momentum transport, topological effects, nanoscale heat transport, charge transport, and so on using optical technique. I am usually interested in magneto-optical Kerr effect (MOKE) scanning microscopy, time-domain thermoreflectance (TDTR), electro-optic effects, terahertz time-domain spectroscopy (THz-TDS) with various external perturbation sources (optical pump, electrical bais, heating and so on). Recently, I have been studying nitrogen-vacancy (NV) center scanning magnetometry to clarify the physical origin of the various quantum materials, such as orbital Hall physics, in Dr. Uri Vool's group in Max-Planck-Institute for Chemical Physics of Solids. If you are interested in my research topics, please feel free to contact me (choiyg1024@gmail.com)

Spin/orbital transport

  Spin/orbital Hall effects can pave the way for spintronic applications. Especially, the importance of the orbital Hall effect has been considered as a key point to the giant torque effects. The orbital Hall effect refers to the generation of electron orbital angular momentum flow transverse to an external electric field. Contrary to the common belief that the orbital angular momentum is quenched in solids, theoretical studies predict that the orbital Hall effect can be strong and is a fundamental origin of the spin Hall effect in many transition metals. [1] [2] [3]

Topological effects

  The optical Hall effect is an ideal platform to investigate high-order nonlinear processes owing to the intense E-field of a short-pulsed laser. A strong nonlinear optical effect has been often discovered with topologically non-trivial materials, associated with topological quantities, such as the Berry curvature, responsible for various nonlinear optical effects, including shift current, second-harmonic generation, and nonlinear Kerr rotation. [4] [5]


Pseudo-hydrodynamics

  The hydrodynamic flow of electrons can lead to a nonlocal macroscopic signature of viscosity. The whirlpools drive the electric current against the applied electric field, resulting in a negative nonlocal voltage near narrow electrodes. The sign-flipping voltage pattern, which depends on the geometry of the sample, is an experimentally detectable signature of the electronic viscous behavior. [10] [11]

Nanoscale heat transport

  With an advent of exfoliation technique for van der Waals (vdW) materials, there have been tremendous progresses in 2D nanotechnology using graphene, transition metal dichalcogenides (TMD), and so on. The cross-plane thermal resistance in ultrathin 2D materials should be contributed to mainly by the boundary (Kapitza) resistance due to their infinitesimal thickness. [6] [7] [8] [9]

[1] “Observation of the orbital Hall effect in a light metal Ti”, Choi et al. arXiv
[2] “Investigation of stiffness and damping constant of spin waves in a soft magnet Co100-xBx alloy”, Choi et al. Appl. Phys. Lett. 121, 012404 (2022)
[3] “Time-resolved measurement of magnetization vectors driven by pulsed spin–orbit torque”, Choi et al. Appl. Phys. Lett. 119, 22404 (2021)
[4] “Nonlinear optical Hall effect in Weyl semimetal WTe2” Choi et al. arXiv
[5] “Ultrafast transient photocarrier dynamics of the bulk-insulating topological insulator Bi1.5Sb0.5Te1.7Se1.3” Choi et al. Phys. Rev. B 97, 075307 (2018)
[6] “Covalent-bonding-induced strong phonon scattering in the atomically thin WSe2 layer” Choi et al. Sci. Rep. 9, 7612 (2019)
[7] “Enhanced magnetic and thermoelectric properties in epitaxial polycrystalline SrRuO3 thin films” Woo et al. Nanoscale 10, 4377 (2018)
[8] “Nano-scale heat transport through the hetero-interface of SrRuO3 thin films” Jeong et al. Nanotechnology 30, 374001 (2019)|
[9] “Influence of stacking disorder on cross-plane thermal transport properties in TMPS3 (TM = Mn, Ni, Fe)”. Ju et al. Appl. Phys. Lett. 117, 063103 (2020).
[10] “Pseudo-hydrodynamic flow of quasiparticles in semimetal WTe2 at room temperature”, Choi et al. arXiv
[11] “Direct visualization of charge accumulations in an electrical anisotropic semimetal”, Choi et al. J. Korean Phys. Soc. 81, 267 (2022)