Quantum Crystal Workshop 2024

During the crystalline growth process, one consistently observes the emergence of grain boundaries or twin boundaries. Nevertheless, the genesis of these two-dimensional imperfections varies based on whether they manifest within a bulk crystal or a thin film. Grain boundaries delineate the divisions between grains in a polycrystal, while twin boundaries may manifest in bulk single crystals. To illustrate, we can consider a crystal with a cubic high-temperature phase undergoing a phase transition to a lower temperature, resulting in a change in crystal system. It is during this transition that lost symmetry becomes evident at the twin boundary. Specifically, if a crystal with a tetragonal system of 4/m2/m2/m at high temperatures transitions to an orthorhombic system of mm2 at lower temperatures, the twin boundary includes either 2-fold rotational symmetry or mirror symmetry.

Thin films similarly exhibit grain boundaries and twin boundaries. The grain boundaries in thin films also mirror their polycrystalline nature, although their origin of the twin boundaries differs significantly from them found in bulk materials. Achieving a single-crystal thin film devoid of grain boundaries and featuring a perfectly flat surface demands a nuanced set of techniques. In this presentation, I will delve into the intricacies of cultivating thin film single crystals without such grain boundaries and provide a detailed exploration of the mechanisms governing twin boundary formation.

During my graduate course, I gained interest in the influence of defects and composition on the physical properties of materials from my research projects on intriguing material systems. The advances in thin film techniques open a new era for strain and interface engineering, providing opportunities to manipulate the physical properties of materials. To suggest solid evidence supporting the ideas for our studies, I needed to prepare many individual samples with a systematic variation. I needed to pay special care and attention to prevent possible problems in the sample preparation process while considering time and cost limits.  

In this presentation, I will talk about combinatorial methods and machine learning, which resolve many possible issues and thus accelerate the discovery of exotic physics and functional materials. Our very recent work will be shown: 1) the fabrication of antiperovskite Sr3SnO thin-film library and 2) the design of the color of copper/copper oxide to highlight the versatility of combinatorial methodology and machine learning for studying condensed matter physics. I will discuss their necessity for our next project on borides. 

The significance of producing high-quality, defect-free single crystals in the field of solid-state physics cannot be overemphasized. Single crystals, characterized by their uniform atomic structure, serve as fundamental building blocks for investigating the intrinsic properties of materials. In solid-state physics, producing defect-free single crystals is of utmost importance as these crystals provide an ideal platform for probing the intricate electronic, magnetic, and optical behaviors inherent in condensed matter systems.

The absence of defects in single crystals ensures the purity of material properties, allowing for a detailed exploration of phenomena such as electronic band structures, magnetic ordering, and optical responses. Researchers in solid-state physics heavily rely on defect-free single crystals to uncover and understand quantum mechanical effects, phase transitions, and the underlying principles governing material behavior.

Furthermore, the careful production of high-quality single crystals is essential for the technological advancements based on solid-state physics principles, including semiconductor devices and advanced electronic components. As we delve deeper into the complexities of condensed matter physics, the creation of defect-free single crystals remains a cornerstone, enabling breakthroughs in both theoretical understanding and practical applications within the field.

The hydrothermal method has proven to be a highly effective strategy for tailoring the properties of kagome antiferromagnetic materials, known for hosting intriguing quantum spin liquids, spin nematic, and supersolid phases. Various synthesis conditions, including temperature, pressure, and reaction time, play pivotal roles in controlling the size, morphology, and magnetic characteristics of the resulting compounds. In this talk, we provide an overview of our recent activities in the hydrothermal synthesis of kagome antiferromagnetic compounds featuring distinct spin numbers: (i) s=1/2 YCu3(OH)6.5Br2.5, (ii) s=1 (CH3NH2)2NaV3F12, and (iii) s=2 CsMn3F(SeO3). Comprehensive discussions on their crystal structures and magnetic properties will be presented, unveiling exciting prospects for uncovering exotic states of matter in condensed matter physics.

The spin-triplet pairing in superconductivity is of great interest to find exotic topological and quantum states of matter such as Majorana fermions, spin fluctuation-mediated superconductivity, etc. There has been much effort to find unconventional ferromagnetic superconductors, but the materials candidates are rare and limited such as Uranium-based superconductors. Here we found evidence of diamagnetically aligned spin-triplet superconductivity in Fe-based high entropy alloy NbTaTiZrFe compound. The significant diamagnetic signal below 42 K with a superconducting transition temperature Tc = 7 K in zero-field-cooled magnetic susceptibility changes to ferromagnetic transition in field-cooled conditions. The diamagnetic spin-triplet pairing is supported by various experimental and theoretical evidences such as significant diamagnetic signal at low fields with strong ferromagnetic coercive force Hcoer = 1800 Oe, ferromagnetic spin flip signal only for the diamagnetic state, and metallic ferromagnetism confirmed by the scanning magnetic force microscopy and spin-resolved density of states. The spin-triplet diamagnetic/ferromagnetic superconductivity is controllable by offset field conditions (direct- and oscillation-off) to remove a residual magnetic field at room temperature. The diamagnetically aligned spin-triplet superconductivity is an unprecedented finding and can be a good platform to investigate the Majorana fermion and its potential applicability to quantum computation.

Traditional perovskite oxide substrates, such as cubic SrTiO3 and orthorhombic R(Al/Sc/Lu)O3 (R = rare earth), have been widely used but are limited by their inherent symmetry and lattice constants, restricting the variety of attainable materials and functionalities. Recently, we have successfully developed a novel perovskite oxide substrate, BaZrO3, which is characterized by its unusually large lattice constant. This presentation will showcase this innovative substrate and examine the physical properties of both the substrate and the epitaxial film. Utilizing BaZrO3, we aim to broaden the spectrum of materials and explore uncharted functionalities in complex oxide heterostructures.

Kagome metals, composed of corner-sharing triangles, exhibits distinctive electronic band structures such as van Hove singularities or flat bands resulting from their geometrical complexities. Theoretically, it is anticipated that these peculiar band structures in kagome metals can host intriguing physical phenomena, such as unconventional charge order, superconductivity or nematicity. However, despite the abundance of candidates with kagome lattice structure, exemplary instances realizing these theories are scarce. In this talk, I would like to share our recent progress in growing high-quality single crystals of kagome metals, which display various types of specific electronic bands near the Fermi level, facilitating the emergence of peculiar states. Furthermore, we uncovered intriguing physical properties originating from their characteristic band structures through transport and optical measurements.

Quantum materials (QMs) have garnered significant attention due to their captivating quantum properties, notably quantum entanglement. This interest is motivated by the potential of quantum materials to offer innovative prospects for quantum applications. However, despite the heightened interest, the development of quantum materials suitable for practical applications remains a formidable challenge. This challenge arises from the difficulty of identifying quantum entanglement within materials using existing experimental equipment that lacks the necessary sensitivity. The simplest and most effective approach to addressing the challenge is to grow large-sized, high-quality single crystals that maximize the signals in measurements beyond the sensitivity of experimental tools. In this talk, I would like to introduce recent research that utilizes the Czochralski method to grow large-sized, high-quality single crystals of quantum materials. 

Recent trend in materials research is require quick turn-around from sample synthesis to its characterization. Thus, it is desirable to establish a full research capability in-house. In this talk, we present several years of effort to build such a laboratory in Korea Atomic Energy Research Institute (KAERI) for single crystal growth and characterization. We employ various growth techniques including flux growth, chemical vapor transport, and optical floating zone method. Samples are characterized by X-rays as well as powder and single crystal neutron diffraction. Further characterization is performed by low-temperature/high magnetic field physical properties measurement system, mostly targeted, but not limited, to quantum materials study.

In this talk, I would like to introduce single crystal growth of the exotic material, electrides and their hydrides. In electrides, interstitial anionic electrons (IAEs) in the quantized energy levels at cavities of positively charged lattice framework possess their own magnetic moment and interact with each or surrounding cations, behaving as quasi-atoms and inducing diverse magnetism. Recently, the reversible and irreversible structural and magnetic transitions by the substitution of the quasi-atomic IAEs in the two-dimensional Gd2C and YTiGe electrides with hydrogens and subsequent dehydrogenation of their hydrides. A quasi-lattice of anionic electrons and hydrogen anions in the single crystalline electrides and their hydrides can deliver a new perspective on condensed matter physics and materials science, which has never been accessed by the known hydrides. 2D hydrogen lattice and possible super-ionic hydrogen conduction will be discussed in the meeting.

This study investigates the anisotropic magnetic response of the kagome antiferromagnetic compound Fe1-xCoxSn, where x ranges from 0 to 0.14. Magnetic phase diagrams for single crystals with x < 0.14 reveal the influence of magnetic fields on planar, tilted, and axial orderings. Both magnetic susceptibility (M/H) and differential magnetic susceptibility (dM/dH) are utilized to discern subtle differences in the magnetic response of tilted and axial spin states within the antiferromagnetic order. Magnetic susceptibility scaling is exclusively applicable in planar and axial spin states, revealing distinct behaviors. Axial spins undergo spin-flops, while planar and tilted spins exhibit continuous changes. The emergence of the axial state is attributed to dopants acting as obstacles within the kagome lattice.

Topological nodal-line materials, characterized by one-dimensional nodes of bulk band crossings, provide a unique quantum system serving as the parent state of various topological and many-body phases. The line node carries a $\pi$ Berry flux in the momentum space and dictates unusual scattering and topological transport of the quasiparticles nearby. This aspect of nodal-line fermions, however, is often hidden in many candidate materials, whose low-energy electronic structures possess complex multiple nodal lines with a finite energy dispersion, together with topologically trivial bands. In this talk, we will present our recent effort to grow high-quality single crystals with different kinds of nodal-line states at the Fermi level, dominating their transport properties. Various magnetotransport phenomena, originating from their characteristic Fermi surface, pseudospin texture, and the spin-orbit coupling will be discussed.