Abstract

Iridates have garnered significant interest in the past decade for possible new quantum states of matter including unconventional superconductors, quantum spin liquids, and correlated analogs of topological insulators/semi-metals. However, it is very difficult to grow single crystals of them in high purity and stoichiometry. In this talk, I will discuss on our efforts to grow them using an entirely new method called aerosol-assisted chemical vapor deposition. Recently, we have succeeded to grow high quality single crystals of Sr2IrO4. I will compare the quality of our new crystals with those grown by conventional flux method. I will also discuss some of the new experiments that becomes possible with our new samples.

Interfaces and surfaces of quantum materials have gained significance due to their utility as scalable and controllable platforms for manipulating physical properties such as superconductivity, multiferroicity, and topological states of matter. For decades, researchers have explored manipulating interfaces/surfaces in these materials through thin-film growth techniques. However, studying the surfaces/interfaces of quantum materials often poses experimental challenges. In this talk, I explore controlled electronic structures and novel phenomena using angle-resolved photoemission spectroscopy (ARPES) and oxide thin-film growth methods.

Compositional gradient in oxide heterostructure could be a new control parameter as it not only breaks inversion symmetry but also changes electronic/magnetic/optical properties of materials in mesoscopic levels. In this talk, I will discuss the effect of compositional gradient in ferroelectric BaTiO3 thin film and resultant longitudinal switching of ferroelectric polarization in the mesoscopic length scale.

Cracks in thin films have traditionally been considered as undesirable failures that disrupt the normal epitaxial structures in oxide films. In these crack systems, unusual electrical transport properties have emerged [1], which are related crucially to the local symmetry breaking via strain gradients at and near the cracks. Therefore, given a well-controlled crack structure, cracks can act as functional components in electronic systems. Here, we explore the quantum electronic conduction properties of oxide heterostructures incorporating strain-induced cracks. Polar textures with topological magnetic edge states emerge due to the strain gradient, and the textures can be controlled by an external electric field. These states can induce discrete conduction levels similar to the Coulomb staircase phenomenon. This study may unleash an exotic dimension of nanoelectronics research. 

[1] Y. Yeo et al., Configurable Crack Wall Conduction in a Complex Oxide, Nano Letters 23, 398 (2023).

Remote epitaxy, a recent discovery allowing epitaxial growth of thin films on single-crystalline substrates through graphene, has clearly shown that the utility of graphene goes far beyond the established applications. This novel process allows creation of freestanding and flexible single-crystalline membranes of exotic materials not possible previously. In particular, complex-oxide materials exhibit many desirable physical properties that can advance conventional electronics, photonics, and quantum devices, such as ferromagnetism, ferroelectricity, piezoelectricty, pyroelectricty, and other unique optoelectric properties absent in convention bulk or 2d materials. Moreover, the combination of two or more of these materials could lead to a discovery of new physical coupling phenomena and functionalities. However, monolithic growth of dissimilar single-crystalline complex-oxide films have been impossible due to the widely different crystalline lattice structure as well as the thermal budget. To solve these restrictions, I will present the experimental process of optimizing remote epitaxy of complex-oxide materials to produce freestanding single-crystalline membranes with crystalline structures such as perovskite, spinel, and garnet. Additionally, I will discuss results of artificially integrating two very dissimilar complex-oxide membranes together via a simple transfer process, creating new heterostructures consisting of 2D and 3D materials. Such demonstration of artificial 3D-3D and 2D-3D heterostructures will open up a new playground for material scientists, physicist, and device engineers to explore these materials with unrestricted manipulation and integration possibilities. The details of the process of remote epitaxy of complex-oxides, as well as potential future applications will also be discussed.

We present the successful growth of impurity-free Yb garnet single crystals using the laser floating zone (LDFZ) method. Yb garnets are considered as candidate material to realize three-dimensional quantum spin liquid (QSL) state, with Yb3+ ions possessing an effective spin Jeff =1/2, arranged in hyper-kagome lattice, realizing a strong magnetic frustrated structure. A recent study on polycrystalline Yb garnets containing Sc has revealed that the long-range magnetic order is suppressed down to 130 mK [1]. However, their polycrystalline sample contains an unknown impurity phase that may give rise to structural disorder and destruct the possible QSL state. In addition, large single crystals are necessary to investigate magnetic anisotropy and search for non-trivial magnetic excitations by neutron scattering that are predicted for QSLs. Single-crystal synthesis was conducted by varying multiple growth-condition parameters such as oxygen pressure, growth rate, laser power, and ratio between constituent elements. As a result, we found that non-stoichiometric ratio of starting materials of garnet-B sites is essential to achieve impurity-free phase of YbSGG single crystals. However, the impurity-free phase was limited to only at the initial stage of LDFZ growth due to high volatility of the molten zone. We also present several crystallographic and bulk measurements to check the quality of the single-crystals.

Topological semimetals have garnered significant attention due to their unique electronic properties. The coexistence of correlation behavior such as charge density wave and magnetic excitation in topological properties has great interest in condensed matter physics for exotic quantum phenomena. The LaBiTe3 has been reported to undergo a topological phase transition, while EuBiTe3 exhibits insulating behavior characterized by variable range hopping transport. Our investigations reveal the presence of magnon excitations at low temperatures and a topological semimetallic band structure in La-doped Eu1-xLaxBiTe3 single crystals. Furthermore, we observe CDW-like behavior in Fe-doped Ni1-xFexTe2 single crystals. Through Angle Resolved Photoemission Spectroscopy (ARPES), we identify a band structure featuring CDW nesting alongside a topological band structure. These findings underscore the intricate interplay between magnetic excitations and electronic structures in topological semimetals, offering insights into their potential applications in advanced electronic and spintronic devices.

The heterostructured quantum materials can generate nontrivial quantum phenomena such as superconductivity, spin hall effect, magnetism, and qubits. These heterostructures were generally created by epitaxy techniques necessitated by stringent lattice matching between layers. Due to this requirement, realizing novel heterostructures between lattice-mismatched materials has been a decades-long obstacle. Remote epitaxy can be a solution by heterostructuring free-standing 3D or 2D materials with various lattices. This emerging synthesis technique can produce single-crystalline and free-standing layers and structures by utilizing 2D van der Waals materials as sacrificial layers. Crystalline 3D or 2D layers detached from substrate can be transferred onto other materials to create quantum heterostructures with exotic interfaces. In this presentation, I will introduce the integration of oxide layers via remote epitaxy and discuss its challenges and potentials for advancing quantum heterostructures.

In recent years, there exist a growing body of experimental results that both conventional and unconventional superconductivity can be emergent or enhanced in the vicinity of quantum critical points of competing or neighboring orders. Here, I will present recent progresses on the several kagome metals studied in my research group to shed light into the mechanism of creating superconductivity with either conventional or unconventional characteristics. We show that a recently found, another CDW system CsV3Sb5, having the 2D kagome lattice structure and exhibiting competing nematic and superconducting transitions as a function of temperature, can also induce an unexpected quantum critical point of nematic order associated with the enhancement of the first SC dome. According to our on going investigations, the fluctuation of the nematic order that can be derived from different phases of each 3Q CDW order may boost up superconductivity of conventional/unconventional origins at the nematic quantum criticality. We further show our results on the evolution of quasiparticle excitations via thermal conductivity measurements under finite magnetic field in a series of samples Cs(V,Ti)3Sb5. We find evidence that thermal conductivity is fully gapped at zero field in most of samples, and the thermal conductivity is quickly approaching the normal state value even well inside the SC state, suggesting gapless state can be achieved in this system under very low magnetic field. We also present electronic Raman scattering date on Ni3In showing evidence of anisotropic Kondo state might be formed in this compound due to highly anisotropic hybridization between localized and itinerant electrons. 

The rare-earth nickel bismuth (RNiBi2), which belongs to ZrCuSi2-type structure composed of alternating stacking of Bi square net and RNi0.8Bi layer, have paid significant attention because of the coexistence of superconductivity and antiferromagnetism. Previous research on CeNi0.8Bi2 reported that the light electron from Bi (6p) square net is responsible to the superconductivity (TC ~ 4.2 K) and the heavy electron from Ce (4f) from RNi0.8Bi layer is responsible for the antiferromagnetism (TN ~ 5.0 K) [1]. On contrary, studies on RNi1-xBi2±y suggested that second phase associated with Ni-Bi and Bi contamination is responsible for the superconductivity at low temperatures [2]. In this study, we investigate the transport and magnetic properties of GdNi0.8Bi2.4 single crystals. X-ray diffraction result reveals no evidence of secondary phase related with Ni-Bi and Bi contamination, and the stoichiometric composition of Gd:Ni:Bi = 1:0.8:2.2 is determined by electron probe micro analyzer. We observe signals of superconducting transition at TC ~ 4.4 K and antiferromagnetic order at TN ~ 10.8 K. The upper critical fields are found to be very anisotropic with HC2 ~ 13 T and 5 T for in-plane (IP) and out-of-plane (OP) orientations, respectively. Moreover, the zero-field-cooled and field-cooled superconducting volume fractions at 50 Oe are 0.5% for IP and 25% for OP. This low volume fraction could come from the coexistence of antiferromagnetic phase. More experiments such as specific heat are needed in order to manifest the origin of superconductivity observed in GdNi0.8Bi2.2. 

[1] Phys. Rev. Lett. 106, 057002 (2011). 

[2] J. Alloys Comp. 554, 304 (2013).

Oxide heterostructures exhibit the emergence of quantum phenomena such as colossal magnetoresistance, magnetism, and superconductivity. At the same time, they are widely utilized in various applications such as clean energy, neuromorphic devices, and innovative technologies leveraging interface states. Their remarkable versatility has spurred extensive research efforts. Recent progress in oxide thin film deposition techniques, particularly in soft chemical synthesis and the fabrication of freestanding oxide membranes, has driven significant advancements in this field. In this talk, we will discuss the latest advancements in pulsed laser deposition techniques based on these perspectives.

In this work, we report a brief overview of the techniques most commonly used to grow 2D and 3D bulk single crystals and thin films, from insulators and semiconductors to metals. The goal of this study is to motivate more researchers for crystal growth by sharing examples of the growth and properties of 2D and 3D materials while providing general explanations and considerations for several growth techniques. Single crystal growth techniques are expected to provide the information needed to plan and conduct experiments to further investigate the physical properties and potential applications of materials. 

A double-helical (DH) order, characterized by a pair of alternatingly nested single helices, represents a unique noncollinear spin texture of antiferromagnets, emerging from unevenly frustrated exchange interactions. Leveraging a frustrated planar-anisotropic spin model, this study presents a comprehensive characterization of the intricate noncollinear magnetic phases of the DH antiferromagnet YMn6Sn6, which exhibits a high Néel temperature (TN = 345 K). Notably, a mere 0.3% magnetocrystalline anisotropy energy compared to the interlayer exchange interaction significantly influences the formation of field-driven magnetic phases and the evolution of anisotropic properties. Visualizations of spin textures verify that the DH ground state in an in-plane magnetic field transforms into a distorted conical spiral (DCS) state, contrary to previous simplistic illustrations of conical spirals. This rapid suppression of the c-axis component for Mn moments in DCS state naturally leads to a fan-like state. Furthermore, each spin state during field rotation is constructed, which is directly correlated with the reversal behavior of angle-dependent magnetic torques. These findings highlight the crucial impact of magnetic anisotropy on DH antiferromagnetism and support the development of noncollinear antiferromagnet-based spintronics.