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.
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.
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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.
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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.
