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Hyobin Yoo

Publications and source records attributed to Hyobin Yoo.

16 recordsLinked to original sources
arXiv2026

Magneto-Optical Detection of Anisotropic Spin Currents in Altermagnetic RuO2

Altermagnets are a recently identified class of collinear antiferromagnets that host large spin-split electronic bands, offering a promising platform for efficient spin-current generation. Among proposed candidates, the metallic oxide RuO2 is predicted to exhibit strong altermagnetic spin splitting; however, whether it sustains robust magnetic order beyond the ultrathin thickness limit remains unresolved. Here, we employ optical probes to investigate charge-to-spin conversion in a 12-nm-thick (101)-oriented RuO2 film grown on sapphire. Polarization-resolved second-harmonic generation reveals nonlinear optical responses consistent with the surface symmetry and N\'eel order of RuO2. Under an applied current, both second-harmonic generation and polar magneto-optical Kerr effect measurements detect a pronounced, directionally anisotropic spin polarization, exhibiting enhanced signals for current along [010] and strongly suppressed responses for current along [-101], in agreement with the symmetry of the altermagnetic spin-splitter effect. Non-magnetic or Rashba-type mechanisms cannot explain this symmetry-selective response. Scanning transmission electron microscopy further reveals that substantial strain persists even in relatively thick films, providing a possible explanation for the observed behavior. Therefore, these results establish RuO2 as an efficient spin source and demonstrate the potential of altermagnets for field-free spintronic devices.

cond-mat.mtrl-sci↗
arXiv2026

Vertical Nb Josephson junctions fabricated by direct metal deposition on both surfaces of freestanding graphene layers

Vertical integration of superconducting electronics requires fabrication strategies that preserve pristine interfaces while accommodating oxidation-sensitive elemental superconductors. However, existing van der Waals-based vertical Josephson junctions largely rely on transfer-based assembly schemes that are incompatible with elemental materials such as niobium (Nb). Here, we introduce a freestanding van der Waals membrane architecture that enables deposition-based fabrication of vertical Josephson junctions through double-sided processing of a single suspended two-dimensional layer. Using multilayer graphene suspended across lithographically defined through-holes in a SiNx membrane, we realize vertical Nb/multilayer graphene/Nb Josephson junctions without ambient exposure of buried interfaces. The resulting devices exhibit clear Josephson coupling, including reproducible supercurrents and a temperature dependence of the critical current consistent with short-junction behaviour. Well-defined magnetic interference patterns governed by the membrane-defined aperture geometry, together with sub-gap features that track a Bardeen-Cooper-Schrieffer (BCS)-like superconducting gap, further confirm the junction quality. This platform establishes a scalable route to vertical superconducting devices based on oxidation-sensitive elemental superconductors and van der Waals materials.

cond-mat.supr-con↗
arXiv2024

Double-sided van der Waals epitaxy of topological insulators across an atomically thin membrane

Atomically thin van der Waals (vdW) films provide a novel material platform for epitaxial growth of quantum heterostructures. However, unlike the remote epitaxial growth of three-dimensional bulk crystals, the growth of two-dimensional (2D) material heterostructures across atomic layers has been limited due to the weak vdW interaction. Here, we report the double-sided epitaxy of vdW layered materials through atomic membranes. We grow vdW topological insulators (TIs) Sb$_2$Te$_3$ and Bi$_2$Se$_3$ by molecular beam epitaxy on both surfaces of atomically thin graphene or hBN, which serve as suspended 2D vdW "$\textit{substrate}$" layers. Both homo- and hetero- double-sided vdW TI tunnel junctions are fabricated, with the atomically thin hBN acting as a crystal-momentum-conserving tunnelling barrier with abrupt and epitaxial interface. By performing field-angle dependent magneto-tunnelling spectroscopy on these devices, we reveal the energy-momentum-spin resonant tunnelling of massless Dirac electrons between helical Landau levels developed in the topological surface states at the interface.

cond-mat.mtrl-sci↗
arXiv2024

Unconventional domain tessellations in moir\'e-of-moir\'e lattices

Imposing incommensurable periodicity on the periodic atomic lattice can lead to complex structural phases consisting of locally periodic structure bounded by topological defects. Twisted trilayer graphene (TTG) is an ideal material platform to study the interplay between different atomic periodicities, which can be tuned by twist angles between the layers, leading to moir\'e-of-moir\'e lattices. Interlayer and intralayer interactions between two interfaces in TTG transform this moir\'e-of-moir\'e lattice into an intricate network of domain structures at small twist angles, which can harbor exotic electronic behaviors. Here we report a complete structural phase diagram of TTG with atomic scale lattice reconstruction. Using transmission electron microscopy combined with a new interatomic potential simulation, we show several large-scale moir\'e lattices, including triangular, kagome, and a corner-shared hexagram-shaped domain pattern. Each domain is bounded by a two-dimensional network of domain wall lattices. In the limit of small twist angles, two competing structural orders-rhombohedral and Bernal stackings-with a slight energy difference, cause unconventional lattice reconstruction with spontaneous symmetry breaking and nematic instability, highlighting the importance of long-range interlayer interactions across entire van der Waals layers. The diverse tessellation of distinct domains, whose topological network can be tuned by the adjustment of the twist angles, establishes TTG as a platform for exploring the interplay between emerging quantum properties and controllable nontrivial lattices.

cond-mat.mtrl-sci↗
arXiv2022

Non-Abelian topological defects and strain mapping in 2D moir\'e materials

We present a general method to analyze the topological nature of the domain boundary connectivity that appeared in relaxed moir\'e superlattice patterns at the interface of 2-dimensional (2D) van der Waals (vdW) materials. At large enough moir\'e lengths, all moir\'e systems relax into commensurated 2D domains separated by networks of dislocation lines. The nodes of the 2D dislocation line network can be considered as vortex-like topological defects. We find that a simple analogy to common topological systems with an $S^1$ order parameter, such as a superconductor or planar ferromagnet, cannot correctly capture the topological nature of these defects. For example, in twisted bilayer graphene, the order parameter space for the relaxed moir\'e system is homotopy equivalent to a punctured torus. Here, the nodes of the 2D dislocation network can be characterized as elements of the fundamental group of the punctured torus, the free group on two generators, endowing these network nodes with non-Abelian properties. Extending this analysis to consider moir\'e patterns generated from any relative strain, we find that antivortices occur in the presence of anisotropic heterostrain, such as shear or anisotropic expansion, while arrays of vortices appear under twist or isotropic expansion between vdW materials. Experimentally, utilizing the dark field imaging capability of transmission electron microscopy (TEM), we demonstrate the existence of vortex and antivortex pair formation in a moir\'e system, caused by competition between different types of heterostrains in the vdW interfaces. We also present a methodology for mapping the underlying heterostrain of a moir\'e structure from experimental TEM data, which provides a quantitative relation between the various components of heterostrain and vortex-antivortex density in moir\'e systems.

cond-mat.mes-hall↗
arXiv2022

Torsional Periodic Lattice Distortions and Diffraction of Twisted 2D Materials

Twisted 2D materials form complex moir\'e structures that spontaneously reduce symmetry through picoscale deformation within a mesoscale lattice. We show twisted 2D materials contain a torsional displacement field comprised of three transverse periodic lattice distortions (PLD). The torsional PLD amplitude provides a single order parameter that concisely describes the structural complexity of twisted bilayer moir\'es. Moreover, the structure and amplitude of a torsional periodic lattice distortion is quantifiable using rudimentary electron diffraction methods sensitive to reciprocal space. In twisted bilayer graphene, the torsional PLD begins to form at angles below 3.89{\deg} and the amplitude reaches 8 pm around the magic angle of 1.1{\deg}. At extremely low twist angles (e.g. below 0.25{\deg}) the amplitude increases and additional PLD harmonics arise to expand Bernal stacked domains separated by well defined solitonic boundaries. The torsional distortion field in twisted bilayer graphene is analytically described and has an upper bound of 22.6 pm. Similar torsional distortions are observed in twisted WS$_2$, CrI$_3$, and WSe$_2$ / MoSe$_2$.

cond-mat.mtrl-sci↗
arXiv2021

Emergent Interfacial Superconductivity between Twisted Cuprate Superconductors

Twisted interfaces between stacked van der Waals cuprate crystals enable tunable Josephson coupling between in-plane anisotropic superconducting order parameters. Employing a novel cryogenic assembly technique, we fabricate Josephson junctions with an atomically sharp twisted interface between Bi2Sr2CaCu2O8+x crystals. The Josephson critical current density sensitively depends on the twist angle, reaching the maximum value comparable to that of the intrinsic junctions at small twisting angles, and is suppressed by almost 2 orders of magnitude yet remains finite close to 45 degree twist angle. Through the observation of fractional Shapiro steps and the analysis of Fraunhofer patterns we show that the remaining superconducting coherence near 45 degree is due to the co-tunneling of Cooper pairs, a necessary ingredient for high-temperature topological superconductivity.

cond-mat.supr-con↗
arXiv2020

Dual-gated graphene devices for near-field nano-imaging

Graphene-based heterostructures display a variety of phenomena that are strongly tunable by electrostatic local gates. Monolayer graphene (MLG) exhibits tunable surface plasmon polaritons, as revealed by scanning nano-infrared experiments. In bilayer graphene (BLG), an electronic gap is induced by a perpendicular displacement field. Gapped BLG is predicted to display unusual effects such as plasmon amplification and domain wall plasmons with significantly larger lifetime than MLG. Furthermore, a variety of correlated electronic phases highly sensitive to displacement fields have been observed in twisted graphene structures. However, applying perpendicular displacement fields in nano-infrared experiments has only recently become possible (Ref. 1). In this work, we fully characterize two approaches to realizing nano-optics compatible top-gates: bilayer $\text{MoS}_2$ and MLG. We perform nano-infrared imaging on both types of structures and evaluate their strengths and weaknesses. Our work paves the way for comprehensive near-field experiments of correlated phenomena and plasmonic effects in graphene-based heterostructures.

cond-mat.mes-hall↗
arXiv2020

Spatially correlated incommensurate lattice modulations in an atomically thin high-temperature Bi_{2.1}Sr_{1.9}CaCu_{2.0}O_{8+{\delta}} superconductors

Strong variations in superconducting critical temperatures in different families of the cuprate perovskites, even with similar hole doping in their copper-oxygen planes, suggest the importance of lattice modulation effects. The one-dimensional incommensurate lattice modulation (ILM) of Bi_2Sr_2CaCu_2O_{8+y}, with the average atomic positions perturbed beyond the unit cell, offers an ideal test ground for studying the interplay between superconductivity and the long-range incommensurate lattice fluctuations. Here we report Scanning nano X-ray Diffraction (SnXRD) imaging of incommensurate lattice modulations in Bi_{2.1}Sr_{1.9}CaCu_{2.0}O_{8+{\delta}} Van der Waals heterostructures of thicknesses down to two-unit cells. Using SnXRD, we probe that the long-range and short-range incommensurate lattice modulations in bulk sample surface with spatial resolution below 100 nm. We find that puddle-like domains of ILM of size uniformly evolving with dimensionality. In the 2-unit cell thin sample, it is observed that the wavevectors of the long- and short-range orders become anti-correlated with emerging spatial patterns having a directional gradient. The emerging patterns, originated by tiny tuning of lattice strain, induce static mesoscopic charge density waves. Our findings thus demonstrate that the strain can be used to tune and control the electromagnetic properties of two-dimensional high-temperature superconductors.

cond-mat.mtrl-sci↗
arXiv2020

Nano-photocurrent mapping of local electronic structure in twisted bilayer graphene

We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene (TBG) enabling access to the local electronic phenomena at length scales as short as 20 nm. We show that the photocurrent changes sign at carrier densities tracking the local superlattice density of states of TBG. We use this property to identify domains of varying local twist angle by local photo-thermoelectric effect. Consistent with the photocurrent study, infrared nano-imaging experiments reveal optical conductivity features dominated by twist-angle dependent interband transitions. Our results provide a fast and robust method for mapping the electronic structure of TBG and suggest that similar methods can be broadly applied to probe electronic inhomogeneities of moir\'e superlattices in other van der Waals heterostructures.

cond-mat.mes-hall↗
arXiv2020

Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe$_2$/MoSe$_2$ bilayers

Structural engineering of van der Waals heterostructures via stacking and twisting has recently been used to create moir\'e superlattices, enabling the realization of new optical and electronic properties in solid-state systems. In particular, moir\'e lattices in twisted bilayers of transition metal dichalcogenides (TMDs) have been shown to lead to exciton trapping, host Mott insulating and superconducting states, and act as unique Hubbard systems whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures also feature atomic reconstruction and domain formation. Unfortunately, due to the nanoscale sizes (~10 nm) of typical moir\'e domains, the effects of atomic reconstruction on the electronic and excitonic properties of these heterostructures could not be investigated systematically and have often been ignored. Here, we use near-0$^o$ twist angle MoSe$_2$/MoSe$_2$ bilayers with large rhombohedral AB/BA domains to directly probe excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane (z) electric dipole moments in opposite directions. The dipole orientation of ground-state $\Gamma$-K interlayer excitons (X$_{I,1}$) can be flipped with electric fields, while higher-energy K-K interlayer excitons (X$_{I,2}$) undergo field-asymmetric hybridization with intralayer K-K excitons (X$_0$). Our study reveals the profound impacts of crystal symmetry on TMD excitons and points to new avenues for realizing topologically nontrivial systems, exotic metasurfaces, collective excitonic phases, and quantum emitter arrays via domain-pattern engineering.

cond-mat.mes-hall↗
arXiv2018

Sign reversing Hall effect in atomically thin high temperature superconductors

We fabricate van der Waals heterostructure devices using few unit cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ for magnetotransport measurements. The superconducting transition temperature and carrier density in atomically thin samples can be maintained to close to that of the bulk samples. As in the bulk sample, the sign of the Hall conductivity is found to be opposite to the normal state near the transition temperature but with a drastic enlargement of the region of Hall sign reversal in the temperature-magnetic field phase diagram as the thickness of samples decreases. Quantitative analysis of the Hall sign reversal based on the excess charge density in the vortex core and superconducting fluctuations suggests a renormalized superconducting gap in atomically thin samples at the 2-dimensional limit.

cond-mat.supr-con↗
arXiv2018

Measuring the Local Twist Angle and Layer Arrangement in Van der Waals Heterostructures

The properties of Van der Waals heterostructures are determined by the twist angle and the interface between adjacent layers as well as their polytype and stacking. Here we describe the use of spectroscopic Low Energy Electron Microscopy (LEEM) and micro Low Energy Electron Diffraction ({\mu}LEED) methods to measure these properties locally. We present results on a MoS$_{2}$/hBN heterostructure, but the methods are applicable to other materials. Diffraction spot analysis is used to assess the benefits of using hBN as a substrate. In addition, by making use of the broken rotational symmetry of the lattice, we determine the cleaving history of the MoS$_{2}$ flake, i.e., which layer stems from where in the bulk.

cond-mat.mtrl-sci↗
arXiv2018

Atomic and electronic reconstruction at van der Waals interface in twisted bilayer graphene

Control of the interlayer twist angle in two-dimensional (2D) van der Waals (vdW) heterostructures enables one to engineer a quasiperiodic moir\'e superlattice of tunable length scale. In twisted bilayer graphene (TBG), the simple moir\'e superlattice band description suggests that the electronic band width can be tuned to be comparable to the vdW interlayer interaction at a 'magic angle', exhibiting strongly correlated behavior. However, the vdW interlayer interaction can also cause significant structural reconstruction at the interface by favoring interlayer commensurability, which competes with the intralayer lattice distortion. Here we report the atomic scale reconstruction in TBG and its effect on the electronic structure. We find a gradual transition from incommensurate moir\'e structure to an array of commensurate domain structures as we decrease the twist angle across the characteristic crossover angle, $\theta_c$ ~1\deg. In the twist regime smaller than $\theta_c$ where the atomic and electronic reconstruction become significant, a simple moir\'e band description breaks down. Upon applying a transverse electric field, we observe electronic transport along the network of one-dimensional (1D) topological channels that surround the alternating triangular gapped domains, providing a new pathway to engineer the system with continuous tunability.

cond-mat.mtrl-sci↗
arXiv2017

Heterointerface effects in the electro-intercalation of van der Waals heterostructures

Molecular-scale manipulation of electronic/ionic charge accumulation in materials is a preeminent challenge, particularly in electrochemical energy storage. Layered van der Waals (vdW) crystals exemplify a diverse family of materials that permit ions to reversibly associate with a host atomic lattice by intercalation into interlamellar gaps. Motivated principally by the search for high-capacity battery anodes, ion intercalation in composite materials is a subject of intense study. Yet the precise role and ability of heterolayers to modify intercalation reactions remains elusive. Previous studies of vdW hybrids represented ensemble measurements at macroscopic films/powders, which do not permit the isolation and investigation of the chemistry at individual 2-dimensional (2D) interfaces. Here, we demonstrate the intercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures comprised of deterministically stacked hexagonal boron nitride, graphene (G) and molybdenum dichalcogenide (MoX2; X = S, Se) layers are fabricated, enabling the direct resolution of intermediate stages in the intercalation of discrete heterointerfaces and the extent of charge transfer to individual layers. Operando magnetoresistance and optical spectroscopy coupled with low-temperature quantum magneto-oscillation measurements show that the creation of intimate vdW heterointerfaces between G and MoX2 engenders over 10-fold accumulation of charge in MoX2 compared to MoX2/MoX2 homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX2 by at least 0.5 V. Beyond energy storage, our new combined experimental and computational methodology to manipulate and characterize the electrochemical behavior of layered systems opens up new pathways to control the charge density in 2D (opto)electronic devices.

cond-mat.mtrl-sci↗