Our research program aims to enhance our understanding of physical properties oflow-dimensional correlated electrons using spectroscopic techniques, with particular emphasis on novel properties at heterointerfaces. Inspired by the novel Mott state identified in the nominally Jeff = ½ state of Sr2IrO4, we explore how relativistic spin-orbit coupling reshape the delicate balance among spin, charge, orbital, and lattice degrees of freedom to expose new quantum phases in the presence of electron correlation in 5d oxides. Within this context, we explore metal-insulator transitions and search for links to high-temperature superconductivity. We study how spin-orbit coupling can lead to frustration of long-range magnetic orders and promote quantum entanglement of distant spins. We test the generality of magnetic and electronic phase segregation of doped charge carriers. We address these questions by:
1. Creating model materials: Our research starts from the recognition that highest quality samples reveal intrinsic physics. The objective of our synthesis activity is to identify and to produce materials that will enable new scientific inquiry and/or significantly advance our understanding of important physical phenomena. A vital need is to create materials of a quality that provides idealized platforms for fundamental studies. For line compounds, the perfection of crystalline order is the Holy Grail, whereas for substituted or doped materials the influence of the degree of atomic order/disorder can be critical. Thus in the latter case, we strive to measure and control atomic order. To this end, we have developed a new crystal growth instrument that will (i) allow high throughput of precursor chemicals by assisting their transport with ultrasonic nebulization, (ii) greatly extend the range of chemical species applicable to the growth process, and (iii) allow deterministic control over the purity, chemical composition, and size of the product crystals.
2. Resonant inelastic x-ray scattering (RIXS): RIXS is a photon-in/photon-out spectroscopy that is fundamentally analogous to inelastic neutron scattering for measuring elementary excitations in solids, with a photon replacing the neutron as the probe. RIXS is not only sensitive to magnetic excitations but also to many other types of excitations that are composed of electron-hole pairs, such as excitons, orbitons, triplons, charge transfer excitations, and particle-hole continua to name a few. This makes RIXS extremely useful for the research of quantum spin liquids and high temperature superconductors. For the latter, the fact that RIXS is sensitive to both charge and magnetic excitations makes it an ideal tool to study the elusive connection between the two degrees of freedom. Since the first measurement of magnetic excitations using hard x-ray RIXS in 2010 led by the PI, the technique has seen a dramatic improvement especially in its energy resolution. Now, 25 meV is routinely achieved and the most recent developments have demonstrated sub-10 meV resolution, approaching the theoretical limit.
3. Angle-resolved photoemission spectroscopy (ARPES): ARPES directly probes the electronic band structure of matter by measuring energy and momentum of electrons photoexcited out of the sample. Thus, ARPES provides momentum- and energy-resolved information on the charge dynamics and thus complements RIXS which is more sensitive to spin and orbital degrees of freedom. See Dr. Jae-Young Kim’s research for more details.
4. Raman spectroscopy: In the Raman process, light is inelastically scattered by elementary excitations in solids such as optical phonons, magnons, crystal field excitations, or inter-band electronic excitations. Thus, Raman spectroscopy is useful not only for initial assessment of sample quality but also for in-depth study of zero-momentum modes. Our home-built Raman setup provides high energy resolution and polarization analysis, and can operate at cryogenic temperatures in high magnetic fields. Our system uses long-focal-length (75 cm) spectrometer in combination with Bragg-grating notch filters to resolve (~ 0.1 meV resolution) low-energy (down to ~ 1 meV) excitations in a spectrum. Our system is currently equipped with three different lasers (488 nm, 532 nm, and 633 nm), and upgrade to a supercontinuum laser is underway to enable continuous tuning of the incident photon energy for differentiation of resonance and non-resonant processes.
5. Second harmonic generation (SHG): Nonlinear optical generation from a crystalline material is extremely sensitive to subtle changes in the symmetries of its electronic phases and the underlying lattice structure. We use SHG for detection of subtle lattice distortions below the detection level of standard diffraction techniques, which are driven by electronic orders through spin-orbital-lattice coupling.
6. X-ray diffraction: The high-brilliance x-ray diffractometer installed at Postech plays a complementary role to the RIXS spectrometer installed in the Pohang Light Source to reveal the lattice structure and dynamics. It combines a high-brightness metal-jet x-ray source and a high-resolution x-ray area detector. Our instrument provides a small x-ray beam size of 80 μm and high luminance of 8.7E11 ph/s/mm2. Crystal structure analysis is possible for both single crystals and polycrystalline solids, and lattice dynamics studies through thermal diffusion signals will be implemented by taking advantage of the high-resolution area detector. We are currently developing a program that applies a machine-learning algorithm that can process massive amounts of data generated by the area detector.