Research

Overview

Our group specializes in studying the solid state structure via spectroscopical means. Under the supervision of Prof. Bumjoon Kim, researchers and students perform active studies on strongly correlated materials.

Our current interest lies in discovering the nature of ordering phenomena in transition metal oxides, one of the most important strongly correlated electronic systems. Amongst the oxides with transition phenomena our main concern are iridates, of which their characteristics and physical properties can be learned from the following articles

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.

[62] Sr2IrO4/Sr3Ir2O7 superlattice for a model 2D quantum Heisenberg antiferromagnet, Physical Reivew Research, 4, 013229 (2022).

[65] Single crystal growth of iridates without platinum impurities, Physical Review Materials, 6, 103401 (2022).

[?] In preparation.

Spin-orbit coupled Mott-insulator

Over the past few years, Sr₂IrO₄, a single-layer member of the Ruddlesden-Popper series iridates, has received much attention as a close analog of cuprate high-temperature superconductors. Although there is not yet firm evidence for superconductivity, a remarkable range of cuprate phenomenology has been reproduced in electron- and hole-doped iridates including pseudogaps, Fermi arcs, and d-wave gaps. Furthermore, many symmetry-breaking orders reminiscent of those decorating the cuprate phase diagram have been reported using various experimental probes. For details, please refer to our review paper: Square Lattice Iridates.

Inspired by this 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. We study how spin-orbit coupling can lead to frustration of long-range magnetic orders and promote quantum entanglement of distant spins.

[70] Quantum spin nematic phase in a square-lattice iridate, Nature, 625, 264-269 (2024)

Chiral phenomena in solid state materials

Chirality means that something cannot be superimposed onto its mirror image—like how your left and right hands are mirror images but not identical. In nature, chirality shows up in many places, from tiny particles to large living organisms. In solid-state materials, especially in systems like the transition-metal dichalcogenide, scientists have been curious about how chirality can appear.

Our research focuses on identifying the symmetry-breaking mechanisms that give rise to chiral charge phenomena. Using a combination of group theoretical analysis, Raman spectroscopy, and inelastic X-ray scattering, we aim to reveal how distinct symmetry representations of the charge and lattice sectors induce frustration and ultimately lead to a chiral ground state. 

[71] Origin of the chiral charge density wave in transition-metal dichalcogenide, Nature Physics, 20, 1919-1926 (2024)

[?] In preparation

Excitonic insulator

Coulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator (EI). This phase is widely believed to arise from Bose-Einstein condensation of electron-hole pairs (i.e., excitons) when their spontaneous formation is preferred to the energy cost of the band-gap excitation. Recently, we reported divergence of (static) excitonic fluctuation in EI-candidate Ta₂NiSe₅ upon approaching the semiconductor (or semimetal)-insulator transition using Raman spectroscopy. For the details, please refer to our published work: Direct observation of excitonic instability in Ta₂NiSe₅.

[55] Direct observation of excitonic instability in Ta₂NiSe₅, Nature Communications, 12, 1969 (2021)

Quantum magnetism

Quantum materials realize various ordering phenomena where quantized moments (e.g., spins, quadrupoles) are aligned through competition/cooperation between microscopic interactions. For example, Heisenberg interactions are ubiquitous in magnetic materials and play a central role in modeling and designing quantum magnets. Bond-directional interactions offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model, which has a quantum spin liquid as its exact ground state. These magnetic interactions can be carefully tuned by microscopic coupling parameters, such as spin-orbit coupling, which has attracted attention as a novel pathway to realize pseudo-spins (i.e., magnetic moments replacing the spins and obeying unique magnetic interactions). The emergent magnetic moments and interactions are of widespread interest due to the potential to realize exotic states of matter.