arXiv subjects

Yu-Mi Wu

Publications and source records attributed to Yu-Mi Wu.

7 recordsLinked to original sources

arXiv2025

Scalable and deterministic construction of moir\'e superlattice in 2D materials using stressor films

Moir\'e superlattice in two-dimensional (2D) materials provides a powerful platform to engineer emergent electronic states, yet the construction of moir\'e superlattices remains lab-scale, involving much trial and error and with little control. Here, we demonstrate the construction of a heterostrain-induced moir\'e superlattice in transition metal dichalcogenides using a scalable process that deterministically induces strain to 2D materials. By applying patterned thin-film stressors and probing the resulting structures with scanning transmission electron microscopy, we directly resolve the induced heterostrain, lattice deformations, and stacking variations that produce the moir\'e superlattice. We find that uniaxial and biaxial heterostrain give rise to distinct moir\'e patterns, including stripes and distorted hexagonal patterns. With this approach, we create in-plane polar distortions and thus in-plane polarization at the domain boundaries of the moir\'e superlattice in MoS$_2$. The deterministic and scalable construction of moir\'e patterns using a well-established scalable process opens opportunities to design new moir\'e geometries in 2D materials.

cond-mat.mtrl-sci↗

arXiv2024

Unraveling nano-scale effects of topotactic reduction in LaNiO$_2$ crystals

Infinite-layer nickelates stand as a promising frontier in the exploration of unconventional superconductivity. Their synthesis through topotactic oxygen reduction from the parent perovskite phase remains a complex and elusive process. This study delves into the nano-scale effects of the topotactic lattice transformation within LaNiO$_2$ crystals. Leveraging high-resolution scanning transmission electron microscopy and spectroscopy, our investigations uncover a panorama of structural alterations, including grain boundaries and coherent twin boundaries, triggered by reduction-induced transformations. In addition, our analyses unveil the formation of an oxygen-rich disordered transition phase encircling impurities and pervading crystalline domains, and the internal strain is accommodated by grain boundary formation. By unraveling these nano-scale effects, our findings provide insights into the microscopic intricacies of the topotactic reduction process elucidating the transition from the perovskite to the infinite-layer phase within nickelate bulk crystals.

cond-mat.mtrl-sci↗

arXiv2023

Unconventional crystal structure of the high-pressure superconductor La$_3$Ni$_2$O$_7$

The discovery of high-temperature superconductivity in La$_3$Ni$_2$O$_7$ at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO$_6$ octahedral bilayers stacked along the $c$-axis direction was consistently posited in initial studies on La$_3$Ni$_2$O$_7$. Here we reassess this structure in optical floating zone-grown La$_3$Ni$_2$O$_7$ single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO$_6$ octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La$_3$Ni$_2$O$_7$, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.

cond-mat.supr-con↗

arXiv2023

Topotactically induced oxygen vacancy order in nickelate single crystals

The strong structure-property coupling in rare-earth nickelates has spurred the realization of new quantum phases in rapid succession. Recently, topotactic transformations have provided a new platform for the controlled creation of oxygen vacancies and, therewith, for the exploitation of such coupling in nickelates. Here, we report the emergence of oxygen vacancy ordering in Pr$_{0.92}$Ca$_{0.08}$NiO$_{2.75}$ single crystals obtained via a topotactic reduction of the perovskite phase Pr$_{0.92}$Ca$_{0.08}$NiO$_{3}$, using CaH$_2$ as the reducing agent. We unveil a brownmillerite-like ordering pattern of the vacancies by high-resolution scanning transmission electron microscopy, with Ni ions in alternating square-pyramidal and octahedral coordination along the pseudocubic [100] direction. Furthermore, we find that the crystal structure acquires a high level of internal strain, where wavelike modulations of polyhedral tilts and rotations accommodate the large distortions around the vacancy sites. Our results suggest that atomic-resolution electron microscopy is a powerful method to locally resolve unconventional crystal structures that result from the topotactic transformation of complex oxide materials.

cond-mat.mtrl-sci↗

arXiv2021

Topotactic transformation of single-crystals: from perovskite to infinite-layer nickelates

Topotactic transformations between related crystal structures are a powerful emerging route for the synthesis of novel quantum materials. Whereas most such "soft chemistry" experiments have been carried out on polycrystalline powders or thin films, the topotactic modification of single crystals -- the gold standard for physical property measurements on quantum materials -- has proven remarkably difficult. Here, we report the topotactic reduction of La$_{1-x}$Ca$_{x}$NiO$_3$ single-crystals to La$_{1-x}$Ca$_{x}$NiO$_{2+δ}$ using CaH$_2$ as the reducing agent. The transformation from the three-dimensional perovskite to the quasi-two-dimensional infinite-layer phase was thoroughly characterized by X-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Our work demonstrates that the infinite-layer structure can be realized as a bulk phase in crystals with micrometer-sized single-domains. The electronic properties of these specimens resemble those of epitaxial thin films rather than powders with similar compositions.

cond-mat.supr-con↗

arXiv2017

Quasiparticle Interference in ZrSiS - Strongly Band-Selective Scattering Depending on Impurity Lattice Site

Scanning tunneling microscopy visualizations of quasiparticle interference (QPI) enable powerful insights into the k-space properties of superconducting, topological, Rashba and other exotic electronic phases, but their reliance on impurities acting as scattering centers is rarely scrutinized. Here we investigate QPI at the vacuum-cleaved (001) surface of the Dirac semimetal ZrSiS. We find that interference patterns around impurities located on the Zr and S lattice sites appear very different, and can be ascribed to selective scattering of different sub-sets of the predominantly Zr 4d-derived band structure, namely the m = 0 and m = +/-1 components. We show that the selectivity of scattering channels requires an explanation beyond the different bands' orbital characteristics and their respective charge density distributions over Zr and S lattices sites. Importantly, this result shows that the usual assumption of generic scattering centers allowing observations of quasiparticle interference to shed light indiscriminately and isotropically upon the \textit{q}-space of scattering events does not hold, and that the scope and interpretation of QPI observations can therefore be be strongly contingent on the material defect chemistry. This finding promises to spur new investigations into the quasiparticle scattering process itself, to inform future interpretations of quasiparticle interference observations, and ultimately to aid the understanding and engineering of quantum electronic transport properties.

cond-mat.mtrl-sci↗

arXiv2017

Observation of surface superstructure induced by systematic vacancies in the topological Dirac semimetal Cd$_{3}$As$_{2}$

The Dirac semimetal phase found in Cd$_{3}$As$_{2}$ is protected by a $C_{4}$ rotational symmetry derived from a corkscrew arrangement of systematic Cd vacancies in its complicated crystal structure. It is therefore surprising that no microscopic observation, direct or indirect, of these systematic vacancies has so far been described. To this end, we revisit the cleaved (112) surface of Cd$_{3}$As$_{2}$ using a combined approach of scanning tunneling microscopy and \textit{ab initio} calculations. We determine the exact position of the (112) plane at which Cd$_{3}$As$_{2}$ naturally cleaves, and describe in detail a structural periodicity found at the reconstructed surface, consistent with that expected to arise from the systematic Cd vacancies. This reconciles the current state of microscopic surface observations with those of crystallographic and theoretical models, and demonstrates that this vacancy superstructure, central to the preservation of the Dirac semimetal phase, survives the cleavage process and retains order at the surface.

cond-mat.mtrl-sci↗