Recent Research Activities of Single-crystal Growth Groups in Korea

Neutron spectroscopy in the layered quantum magnet SrCu₂(BO₃)₂ and in transition metal phosphorus trisulfides (MPS₃)

Topological semimetals have attracted much attention and become a hot subject in condensed matter physics, and single crystal growth is the basis of the physical investigation on these materials. At present, the research of topological materials has formed a cooperation circle:presenting materials by theoretical calculation; single crystal growth; verification by experiments on single crystals. Single crystal growth has become a bridge between theory and experiment. Here in this paper, we introduce the single crystal growth of the topological semimetals presented in recent years, including topological Dirac semimetals, Weyl semimetals, Node-Line semimetals and other new classes of topological materials. The detailed growth methods are summarized in this paper for each material.

The AV3Sb5 prototype kagome materials have been demonstrated as a versatile platform for exploring exotic properties in condensed matter physics, including charge density waves, superconductivity, non-trivial electron topology, as well as topological superconductivity. Here we identify that ANb3Bi5 (A = K, Rb, Cs) exhibit non-trivial coexisting superconductivity and topological properties via first-principles calculations. The negative formation energy and the absence of imaginary phonon dispersion demonstrate both thermodynamics and dynamics stabilities of ANb3Bi5 (A = K, Rb, Cs) under ambient conditions. By analytically solving the Allen-Dynes-modified McMillan formula, the superconducting transition temperatures are predicted to be 2.11, 2.15 and 2.21 K for KNb3Bi5, RbNb3Bi5, and CsNb3Bi5, respectively. More importantly, the kagome materials proposed here can be classified into Z2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\mathbb{Z}}}_{2}$$\\end{document} topological metals due to the non-trivial topological index and the obvious surface states around the Fermi level. Such coexistence of superconductivity and non-trivial band characters in ANb3Bi5 (A = K, Rb, Cs) offer us more insights to study the relationship between superconductivity and topological properties, and to design innate topological superconductors.

Topological superconductors (TSCs) provide an ideal platform for investigating fundamental quantum phenomena, including non‐Abelian anyons and quantum entanglement, positioning them as a key area of focus in contemporary condensed matter physics research. Besides the widely explored transition metal sulfides (TMSs), transition metal nitrides (TMNs) may exhibit exceptional superconducting properties and non‐trivial topological quantum numbers. Here, we report a first‐principles study of the XMoN 4 (X = transition metal elements) family and predict their superconducting and topological properties at ambient pressure. Through the first‐principles calculations, we identify 7 stable superconductors, and NbMoN 4 exhibits the highest value of 47 K. The nesting of the Fermi surfaces and the flat bands caused by the d‐orbitals of metal atoms result in a high density of states, enhancing the pairing of electrons and phonons, thus leading to high T c . We further study the topological properties and find that the stabilized compound HgMoN 4, whose T c is 35 K at ambient pressure, also exhibits surface states and a Z 2 index of (1, 001). Our work predicts a stable superconductor with topological properties under ambient pressure from a constructed family of TMNs, expanding the study of combining the topological properties and superconductivity in bulk‐phase materials.

Topological electronic materials exhibit many novel physical properties, such as low dissipation transport and high carrier mobility. These extraordinary properties originate from their non-trivial topological electronic structures in momentum space. In recent years, topological phase transitions based on topological electronic materials have gradually become one of the hot topics in condensed matter physics. Using first-principles calculations, we explore the topological phase transitions driven by in-plane strain in ternary pyrochlore oxide Tl<sub>2</sub>Ta<sub>2</sub>O<sub>7</sub>. Firstly, we analyze the atomic-orbital-resolved band structure and find that the O (p<sub><i>x</i></sub>+p<sub><i>y</i></sub>) and p<sub><i>z</i></sub> orbitals of the system near the Fermi level have band inversion, indicating the emergence of topological phase transitions in the system. Then the tight-binding models are constructed to calculate the <i>Z</i><sub>2</sub> topological invariants, which can determine the topologically non-trivial feature of the system. Finally, topological properties such as surface states and a three-dimensional Dirac cone are studied. It is found that Tl<sub>2</sub>Ta<sub>2</sub>O<sub>7</sub> without strain is a semimetal with a quadratic band touching point at Fermi level, while the in-plane strain can drive the topological phase transition via breaking crystalline symmetries. When the system is under the –1% in-plane compression strain and without considering the spin orbit coupling (SOC), the application of strain results in two triply degenerate nodal points formed in the –<i>Z</i> to <i>Γ</i> direction and <i>Γ</i> to <i>Z</i> direction, respectively. When the SOC is included, there are two fourfold degenerate Dirac points on the –<i>Z</i> to <i>Γ</i> path and <i>Γ</i> to <i>Z</i> path<i>,</i> respectively. Thus, the –1% in-plane compression strain makes the system transit from the quadratic contact point semimetal to a Dirac semimetal. When 1% in-plane expansion strain is applied and the SOC is neglected, there exists one band intersection along <i>Y→</i><i>Γ</i>. When the SOC is taken into consideration, the gap is opened. Therefore, the 1% in-plane expansion strain drives Tl<sub>2</sub>Ta<sub>2</sub>O<sub>7</sub> into a strong topological insulator. In addition, the system is also expected to have strong correlation effect and superconductivity due to the possible flat band. This work can guide the study of topological phase transitions in three-dimensional materials and provide a good material platform for the design of low-dissipation electronic devices.

Transition-metal monosilicide RhGe has been reported to exhibit weak itinerant ferromagnetism, superconductivity, and topological properties. In this study, we report the high-pressure growth of high-quality RhGe single crystals up to millimeter size using a flux method. Transport measurements reveal metallic behavior in RhGe from 2 K to 300 K with Fermi liquid behavior at low temperatures. However, no superconductivity was observed with variations in the Ge composition. Magnetic characterizations indicate that RhGe exhibits paramagnetic behavior between 2 K and 300 K. The high-quality and large-size RhGe single crystals pave the way for further investigation of their topological properties using spectroscopic techniques.

Topology has recently become a focus in condensed matter physics, arising in the context of the quantum Hall effect and topological insulators. In both of these cases, the topology of the system is defined through bulk properties ('topological invariants') but detected through surface properties. Here we measure three topological invariants of a quantum Hall material-photonic Landau levels in curved space-through local electromagnetic and gravitational responses of the bulk material. Viewing the material as a many-port circulator, the Chern number (a topological invariant) manifests as spatial winding of the phase of the circulator. The accumulation of particles near points of high spatial curvature and the moment of inertia of the resultant particle density distribution quantify two additional topological invariants-the mean orbital spin and the chiral central charge. We find that these invariants converge to their global values when probed over increasing length scales (several magnetic lengths), consistent with the intuition that the bulk and edges of a system are distinguishable only for sufficiently large samples (larger than roughly one magnetic length). Our experiments are enabled by applying quantum optics tools to synthetic topological matter (here twisted optical resonators).Combined with advances in Rydberg-mediated photon collisions, our work will enable precision characterization of topological matter in photon fluids.

Chirality--that is, left or right handedness--is present in many scientific areas, and particularly in condensed matter physics. Inversion symmetry breaking relates chirality with skyrmions, which are protected field configurations with particle-like and topological properties. Here we show that a kagome magnet, with Heisenberg and Dzyaloshinskii-Moriya interactions, causes non-trivial topological and chiral magnetic properties. We also find that under special circumstances, skyrmions emerge as excitations, having stability even at room temperature. Chiral magnonic edge states of a kagome magnet offer, in addition, a promising way to create, control and manipulate skyrmions. This has potential for applications in spintronics, that is, for information storage or as logic devices. Collisions between these particle-like excitations are found to be elastic at very low temperature in the skyrmion-skyrmion channel, albeit without mass-conservation. Skyrmion-antiskyrmion collisions are found to be more complex, where annihilation and creation of these objects have a distinct non-local nature.

A bstract The presence of a topological phase in a topological many-body system can be distinguished through the analysis of topological invariants. In the present study, the topological invariants for the strongly coupled holographic semimetals have been systematically computed, especially focusing on the holographic Weyl-Nodal line coexisting semimetal. The topological invariants that we calculate include the Weyl charge, the topological charges for a nodal ring ζ 0 , ζ 1 , ζ 2 and an additional mirror symmetry protected topological invariant, $$ {\overset{\sim }{\zeta}}_2 $$ ζ ~ 2 , that we herein introduce. In addition, the effective band structures and topological invariants in the critical phases of holographic semimetals are investigated, including the case of Weyl, nodal line and Weyl-Nodal line coexisting semimetals. The findings indicate the presence of notable and unique features inherent to strongly coupled topological semimetals, including band-crossing ordering interchange and multi Fermi surfaces, which provide a valuable platform for experimental investigations of strongly coupled semimetals in condensed matter physics.

With the discovery and development of topological materials, topological physics has attracted enormous research interest in the fields of contemporary condensed matter physics. Topological property, which describes such a property that physical quantity remains invariant under continuous transformation (such as Chern number), has been revealed in a variety of materials, including topological insulators, topological semimetals (such as Weyl or Dirac semimetals), topological magnetic materials, etc. One-dimensional chiral magnetic soliton, similar to magnetic skyrmion, is a type of magnetic configuration with topological origin and quasi-particle property, which has shown tremendous physical properties and device functionalities. In this review, we mainly focus on a chiral helimagnet, called Cr<sub>1/3</sub>NbS<sub>2</sub>, which possesses chiral magnetic soliton lattice and other more spin configurations under different conditions. We systematically summarize the work on Cr<sub>1/3</sub>NbS<sub>2</sub>, discussing its crystal symmetry, band structure, magnetic interactions, rich magnetic phases, and the physics of associated phase transitions. In particular, the layered crystal structure of Cr<sub>1/3</sub>NbS<sub>2</sub> enables us to control the soliton number through tuning the layer number or crystal thickness. Our review provides a comprehensive summary of Cr<sub>1/3</sub>NbS<sub>2</sub> in order to draw more attention to this interesting material. Moreover, we envision that our work could offer useful guidance to the researchers working on topological and chiral magnetic materials, and thus introducing topological or chiral magnetism into two-dimensional layered materials and promoting the development of modern magnetism and spintronics. Therefore, this review mainly focuses on a magnet, called Cr<sub>1/3</sub>NbS<sub>2</sub>. We systematically summarize the work on Cr<sub>1/3</sub>NbS<sub>2</sub>, discussing its crystal symmetry, band structure, magnetic interaction, rich magnetic phases and the interesting physical phenomena occurring at each phase transition. In addition, the layered crystal structure of Cr<sub>1/3</sub>NbS<sub>2</sub> also enables us to use the layer number or crystal thickness to modulate and control its rich magnetic phases. We believe that our review provides a comprehensive summary of Cr<sub>1/3</sub>NbS<sub>2</sub>, which can make people have a better understanding of a typical topological magnetic material, thereby enriching the material types of magnets and low-dimensional material family and promoting the development of magnetism and spintronics applications, such as in magnetic memory devices, spintronic devices, and quantum information devices.

Physics is said to be universal when it emerges regardless of the underlying microscopic details. A prominent example is the Efimov effect, which predicts the emergence of an infinite tower of three-body bound states obeying discrete scale invariance when the particles interact resonantly. Because of its universality and peculiarity, the Efimov effect has been the subject of extensive research in chemical, atomic, nuclear and particle physics for decades. Here we employ an anisotropic Heisenberg model to show that collective excitations in quantum magnets (magnons) also exhibit the Efimov effect. We locate anisotropy-induced two-magnon resonances, compute binding energies of three magnons and find that they fit into the universal scaling law. We propose several approaches to experimentally realize the Efimov effect in quantum magnets, where the emergent Efimov states of magnons can be observed with commonly used spectroscopic measurements. Our study thus opens up new avenues for universal few-body physics in condensed matter systems.

Single crystal growth and structural characterization of iron telluride doped with chromium and zinc

Laser-cooled lanthanide atoms are ideal candidates with which to study strong and unconventional quantum magnetism with exotic phases. Here, we use state-of-the-art closed-coupling simulations to model quantum magnetism for pairs of ultracold spin-6 erbium lanthanide atoms placed in a deep optical lattice. In contrast to the widely used single-channel Hubbard model description of atoms and molecules in an optical lattice, we focus on the single-site multichannel spin evolution due to spin-dependent contact, anisotropic van der Waals, and dipolar forces. This has allowed us to identify the leading mechanism, orbital anisotropy, that governs molecular spin dynamics among erbium atoms. The large magnetic moment and combined orbital angular momentum of the 4f -shell electrons are responsible for these strong anisotropic interactions and unconventional quantum magnetism. Multichannel simulations of magnetic Cr atoms under similar trapping conditions show that their spin evolution is controlled by spin-dependent contact interactions that are distinct in nature from the orbital anisotropy in Er. The role of an external magnetic field and the aspect ratio of the lattice site on spin dynamics is also investigated.

Topology is central to understanding and engineering materials that display robust physical phenomena immune to imperfections. Different topological phases of matter are characterized by topological invariants. In energy-conserving (Hermitian) systems, these invariants are determined by the winding of eigenstates in momentum space. In non-Hermitian systems, a topological invariant is predicted to emerge from the winding of the complex eigenenergies. Here, we directly measure the non-Hermitian topological invariant arising from exceptional points in the momentum-resolved spectrum of exciton polaritons. These are hybrid light-matter quasiparticles formed by photons strongly coupled to electron-hole pairs (excitons) in a halide perovskite semiconductor at room temperature. We experimentally map out both the real (energy) and imaginary (linewidth) parts of the spectrum near the exceptional points and extract the novel topological invariant—fractional spectral winding. Our work represents an essential step toward realization of non-Hermitian topological phases in a condensed matter system.

In a spontaneously dimerized quantum antiferromagnet, spin-1/2 excitations (spinons) are confined in pairs by strings akin to those confining quarks in non-Abelian gauge theories. The system has multiple degenerate ground states (vacua) and domain walls between regions of different vacua. For two vacua, we demonstrate that spinons on a domain wall are liberated, in a mechanism strikingly similar to domain-wall deconfinement of quarks in variants of quantum chromodynamics. This observation not only establishes a novel phenomenon in quantum magnetism, but also provides a new direct link between particle physics and condensed-matter physics. The analogy opens doors to improving our understanding of particle confinement and deconfinement by computational and experimental studies in quantum magnetism.