3rd Quantum Crystal Workshop

For pioneering research in condensed matter physics, acquiring new materials with novel electronic correlations or emergent properties is essential. Among recent research trends, two material classes have received particular attention: Kagome-lattice materials and altermagnet. Each of these classes has attracted attention as a next-generation material platform due to their unique electronic structures and electromagnetic symmetries.

Materials that combine distinct properties within a single phase are of fundamental and technological interest. In this talk, I will present a universal strategy to achieve strong bulk polarity that not only coexists with its originally incompatible properties, such as metallicity, but also synergistically enables exceptional functionalities. A combination of thin-film synthesis, atomic-scale imaging, and theoretical calculations reveals that A-site selective atomic gradients induce strong polar states in otherwise centrosymmetric perovskite oxides ABO3. These polar states unconventionally coexist with various preexisting properties, leading to bulk polar metallicity with tunable nonreciprocal transport, high-κ/low-loss dielectricity with an equivalent oxide thickness below 0.1 nm, and giant pyroelectricity. This work will facilitate the development of new multifunctional materials with unusual coexisting and synergistic properties.

Among various transition metal oxides, topotactic oxides represent a special class of materials that undergo composition-driven structural phase transitions. These transitions are often accompanied by significant electronic and magnetic changes, leading to intriguing multifunctional properties. Importantly, the process is reversible, making these materials promising candidates for programmable matter. Recently, interest in topotactic phase transitions has surged due to their connection to nickelate superconductivity and thermal switching behavior. 

In this presentation, I will discuss our recent studies on the real-time observation of oxygen vacancy formation in cobalt-containing complex oxides. We will also explore the potential realization of tunable tri-states and their applications in thermal switching devices. In particular, we focus on low-temperature structural and selective chemical modulation in SrFe₀.₅Co₀.₅Oₓ [1], where the brownmillerite structure provides a versatile platform for designing responsive and energy-efficient materials.

In this talk, I'll try to convey several difficulties and physics related on the magnetic ordering and superconductivity in several important materials grown in my lab, mostly by the flux methods.  This might help audience gain insights on their own growth efforts on the related materials system.

Kagome lattices provide a fertile ground for exploring correlated and topological quantum phenomena arising from the interplay between geometry, magnetism, and electronic structure. While stoichiometric Mn₃Sn has served as a prototypical kagome antiferromagnet exhibiting anomalous Hall and chiral transport phenomena, we recently synthesized a new compound, Zr₃Mn₃Sn₄Ga, that realizes a hetero-kagome bilayer composed of magnetic [Mn₃Ga] and non-magnetic [Zr₃Sn₄] sheets. Single-crystal studies reveal a commensurate antiferromagnetic order with k = (1/3, 1/3, 0) at Tₙ ≈ 87 K, accompanied by anisotropic magnetoresistance and nonlinear Hall responses, reflecting strong magneto-electronic coupling. Resonant photoemission spectroscopy identifies Mn 3d and Zr 4d states as distinct contributors to the valence bands, evidencing inter-kagome hybridization. These results establish Zr₃Mn₃Sn₄Ga as the first bulk hetero-kagome antiferromagnet, providing a promising model system for studying quantum magnetism, interlayer interactions, and topological electronic states in single crystals.

Materials properties have been traditionally determined mainly by 3 factors: 1. Element, 2. Bonding nature, and 3. Crystal Structure. Recent studies try to new strategy for material design beyond these factors to accomplish the expectable functionality, thus can be called as “transcendent functional materials”. Here I introduce 3 kinds of material design strategies to attain transcendent functionality. The first one is two-dimensional (2D) dichalcogenides consisting with post-transition metal whose electrical conductivity can be very widely controlled via conventional substitutional doping engineering. The second one is 2D metal-rich chalcogenides that can mimic highly pressurized material state even under an ambient pressure condition. The last one is new p-type transparent conductive oxide materials whose band gap is determined not by conventional orbital energy but by strong electron correlations. The detailed materials design concepts and their experimental realization including theoretical analysis will be discussed.

Hydrothermal synthesis is a powerful and versatile method for preparing frustrated quantum magnets, enabling controlled single-crystal growth in sealed aqueous environments under high temperature and pressure, typically between 180–700 °C. This approach offers distinct advantages over conventional flux or melt techniques, such as milder synthesis conditions and reduced levels of crystalline defects, making it especially suited for stabilizing complex lattices prone to frustration.

As a case study, the trillium-lattice compound (NH₄)(H₃O)Ti₂(PO₄)₃ was successfully grown as pure black tetragonal single crystals under hydrothermal conditions. This material hosts a three-dimensional chiral network of corner-sharing triangles, giving rise to a random singlet ground state distinguished by thermodynamic and ESR signatures of weakly coupled and isolated orphan spins coexisting with dynamic singlet correlations.

A second example involves the growth of Gd-substituted kagome antiferromagnets YCu₃(OD)₆.₅Br₂.₅, where hydrothermal methods enable the preparation of high-quality crystals suitable for advanced magnetic studies. Thermodynamic and ESR measurements on these compounds reveal Dirac spinon excitations and suggest the emergence of Kondo screening, highlighting the interplay between magnetic impurities and a spin-liquid host in such frustrated lattices.

The recent discovery of superconductors with exceptionally high transition temperatures (Tc) has sparked great interest in both the scientific community and society, opening new avenues for superconductivity research. Despite this progress, a major challenge remains in discovering superconductors that are stable at ambient pressure while maintaining a high Tc. In this presentation, I will introduce my ongoing research on the synthesis of superconductors using high-temperature and high-pressure techniques, and discuss strategies for identifying next-generation superconductors and understanding the physical principles governing novel superconductivity.

Altermagnetism has recently been emerged as a distinct class of magnetic states, differing from conventional ferromagnetism and antiferromagnetism. It features zero net magnetization with alternating spin polarization, driven by an anisotropic local crystal environment across sublattices, resulting in unconventional spin-polarized band structure in momentum space. Recent reports have highlighted anisotropic spin splitting in MnTe₂, dependent on crystal orientation. In this study, we synthesized high-quality MnTe₂ single crystals using the chemical vapor transport (CVT) method and characterized them using X-ray diffraction. Temperature-dependent magnetization and resistivity measurements confirm that MnTe₂ is an antiferromagnetic semiconductor with a Néel temperature of TN ≈ 87 K. Notably, an anisotropic Hall effect was observed, which depends strongly on the current direction: for I ∥ [100], the Hall resistivity shows a linear dependence with a positive slope, whereas for I ∥ [110], it exhibits a large nonlinear dependence with a negative slope. These anisotropic Hall responses are attributed to direction-dependent spin splitting in the band structure.

CeRh2As2 has been attracting attention due to its unusual multiphase superconductivity. While the superconducting transition temperature (Tc) is relatively low, Tc = 0.35 K, the upper critical field (Hc2) is as high as 15 T. This field-resilient superconductivity evidences spin-triplet pairing. Indeed, clear thermodynamic anomalies within the superconducting (SC) state indicate a field-induced transition between a spin-singlet even- to a spin-triplet odd-parity SC phase. CeRh2As2 crystallizes in the centrosymmetric CaBe2Ge2-type structure. The lack of local inversion symmetry at the Ce site gives rise to staggering antisymmetric spin-orbit couplings within the unit cell. This allows sublayer degrees of freedom for the SC order parameter to realize spin-triplet superconductivity. The lesson from the study of CeRh2As2 encourages to explore further locally-noncentrosymmetric heavy-fermion materials. In this talk, I will briefly summarize recent progress in understanding CeRh2As2 and discuss a strategy to search for further new materials with an exotic spin-orbit related phase.

In conventional copper systems, grain boundaries and twin boundaries are often intermixed, making it difficult to distinguish their individual roles. Grain boundaries typically induce diffusive scattering, which shortens the electron mean free path and disrupts coherence. However, when grain boundaries are entirely eliminated and only twin boundaries remain, the intrinsic role of twin boundaries can be clearly understood. This highlights why twin boundaries should not be treated as mere defects.

In this presentation, we focus on the role of twin boundaries in governing electron transport in a clean Cu(111) system.

In this presentation, I will discuss fabrication of transition metal oxide based thin films & heterostructures. Using pulsed laser epitaxy (PLE), our group specializes in the realization of artificial lattices with atomic-scale precision. Oxide superlattice has become one of the most celebrated synthetic crystals, in which one can now find textbook examples in high quality samples. Zone folding of acoustic phonons, low dimensional behavior, and octahedral tilt propagation can be observed in oxide superlattices. On the other hand, hybrid heterostructures enable synergetic behavior between the transition metal oxides and 2D layered materials. I will discuss challenges and opportunities of direct fabrication of hybrid heterostructures.