Tae-Hwan Kim

One-dimensional electronic transport reveals many-body quantum phenomena and serves as a foundation for quantum devices, sensors, and reconfigurable electronics. Here, we demonstrate that surface-exposed ferroelastic twin walls in epitaxial WO ₃ films function as intrinsic one-dimensional channels, confining electrons to screen surface-bound charges induced by flexoelectric polarization. Scanning tunneling microscopy uncovers a self-organized nematic-like phase forming periodic stripes perpendicular to the walls. Conductive atomic force microscopy and angle-resolved photoemission spectroscopy resolve quantized one-dimensional states and reveal conductance enhancements exceeding two orders of magnitude along the twin walls relative to the surrounding semi-insulating surface regions at room temperature. Temperature-dependent transport follows an Efros–Shklovskii variable-range hopping law, with a crossover near 140 K and a localization length of 39 Å—identical to the stripe period—signifying Coulomb-coupled electrons confined within stripe-induced potential wells along the walls. These findings establish WO ₃ ferroelastic walls as a tunable platform for engineering one-dimensional correlated electron states.

Excitonic insulating phase, a long‐sought exotic quantum state, is formed by spontaneous condensation of electron‐hole pairs or excitons. Stabilizing such a fragile many‐body state in reduced dimensions requires precise control of multiple electronic parameters and external perturbations, which makes its material realization challenging, particularly at high temperatures. Here, using van der Waals layers of as a material platform, we show that the excitonic insulating phase is stable up to 190 K in the two‐dimensional limit. The distinctive signatures of the excitonic insulating phase transition, such as the hybridization gap opening and critical fluctuations of the excitonic order observed in Raman spectroscopy, persist even in a monolayer, although systematically suppressed with thickness. The large gap ratio, nearly independent of thickness, suggests the importance of strong exciton‐phonon coupling in maintaining a high‐temperature excitonic instability, while substrate‐induced strain disorder, as observed by scanning tunneling microscopy, lowers in monolayer than in the bulk. Our findings establish the monolayer as a promising model system for studying and manipulating correlated exciton‐lattice coupling at the two dimensional limit.

MXenes have a characteristic quasi-two-dimensional structure composed of covalently bonded transition metal (M) and carbon or nitrogen (X) sublayers, [Mn+1Xn], stacked with an interlayer spacing. Conventional MAX-etched MXenes inevitably have inhomogeneous surface functional groups (Tx), while chemical vapor deposition (CVD) has so far yielded only ultrathin α-Mo2C bulk crystals, failing to form a layered structure. Here, we report a CVD approach that enables the formation of layered [Mon+1Cn] structures through controlled carburization of predeposited Mo films without Tx sources. The resulting films are composed of multiple MXene phases, including Mo2C, Mo3C2, and Mo4C3 layered motifs. Regulated carbon diffusion results in discrete [Mon+1Cn] slabs with a well-defined interlayer spacing. In addition, a signature Tx-free, Mo-exposed surface exhibits enhanced nitrogen reduction reaction (NRR) activity compared with a CVD-grown α-Mo2C film, achieving an NH3 yield of 6.52 μg h–1 cm–2. Given the broad applicability to other MXenes, we anticipate that this work will stimulate further fundamental research and application on MXenes.

The exchange of angular momentum among electron spin, orbital, and lattice degrees of freedom governs the topology, transport, and magnetization dynamics of magnetic materials. Probing the interactions among magnons, phonons, and electron orbital angular momentum has become a global focus in condensed-matter physics, yet the underlying mechanisms—operating on nanometer and femtosecond–nanosecond scales and under non-equilibrium conditions, such as applied current/voltage, optical excitation, and thermal gradients—are difficult to probe with conventional static measurements. To address this challenge, our Center for Quantum Dynamics of Angular-Momentum advances operando imaging that synchronizes stimuli and responses to achieve real-time spatiotemporal resolution across multiple platforms. This overview introduces five representative operando instruments developed and deployed at our Center: (i) scanning tunneling microscopy (STM), (ii) time resolved magneto-optic Kerr effect (MOKE) microscopy/spectroscopy, (iii) magnetic force microscopy (MFM), and (iv) diamond nitrogen-vacancy (NV) center quantum microscopy, (v) scanning electron microscopy with polarization analysis (SEMPA), providing complementary access to angular momentum dynamics in solid state systems.

We present the design and implementation of an advanced manipulator system for use in low-temperature scanning tunneling microscope (STM) setups, specifically in an ultrahigh vacuum (UHV) chamber equipped with a bottom-loading cryostat and a superconducting magnet. This system integrates UHV-compatible endoscope assemblies, providing visual assistance for the precise alignment and transfer of the STM scanner shuttle to its receptacle. Additionally, the manipulator features an 85 K precooling stage, enabling molecule deposition on metallic substrates and STM imaging to verify surface conditions before transferring the shuttle to its receptacle at 4.2 K. This new approach enhances the operational efficiency of STM systems, particularly for low-temperature measurements in high magnetic fields, and addresses the challenges of sample contamination and thermal shock during the transfer process.

This study presents a newly constructed dilution-refrigerator ultrahigh vacuum (UHV) scanning tunneling microscope (STM) with a 9/2/2 T superconducting vector magnet capable of achieving electron temperatures as low as 76 mK. Our design emphasizes robust thermal contacts, particularly with the sample holder through a thin insulating layer. Additionally, we focus on effective shielding and grounding against radio-frequency electromagnetic interference by integrating the critical electronics as a physically and electrically integral component of the STM setup. Scanning tunneling spectroscopy results obtained from a superconducting aluminum substrate and a gold tip indicate superior energy resolution, with a higher aspect ratio of the superconducting coherence peak in the dI/dV spectra compared to other dilution-refrigerator UHV STMs. Given that only a handful of UHV STMs with dilution refrigerators have reached electron temperatures below 100 mK, these results demonstrate the effectiveness of our design and methodology in achieving low electron temperatures.

In this study, we investigate the discrepancy between the estimate of Q. He [], who observed a remarkable collapse of the exciton gap in Ta 2 NiSe 5 due to the electrostatic field between the scanning tunneling microscope (STM) tip and the sample, and that of a recent angle-resolved photoemission spectroscopy investigation [C. Chen , ]. It is proposed that a critical factor contributing to this discrepancy is due to He 's assumption of a constant work function of the STM tip. This assumption led to an underestimation of the tip-induced electric field. Using a literature value for the sample work function, a more substantial electric field strength is obtained, which resolves the apparent conflict between the doping estimates of these two techniques. Furthermore, our findings highlight the importance of the STM tip condition, which can significantly impact the tip work function and, consequently, influence the doping estimation in experiments involving tip-induced electric fields. Published by the American Physical Society 2024

Chirality, a fundamental concept from biological molecules to advanced materials, is prevalent in nature. Yet, its intricate behavior in specific topological systems remains poorly understood. Here, we investigate the emergence of hidden chiral domain wall states using a double-chain Su-Schrieffer-Heeger model with interchain coupling specifically designed to break chiral symmetry. Our phase diagram reveals single-gap and double-gap phases based on electronic structure, where transitions occur without topological phase changes. In the single-gap phase, we reproduce chiral domain wall states, akin to chiral solitons in the double-chain model, where chirality is encoded in the spectrum and topological charge pumping. In the double-gap phase, we identify hidden chiral domain wall states exhibiting opposite chirality to the domain wall states in the single-gap phase, where the opposite chirality is confirmed through spectrum inversion and charge pumping as the corresponding domain wall slowly moves. By engineering gap structures, we demonstrate control over hidden chiral domain states. Our findings open avenues to investigate novel topological systems with broken chiral symmetry and potential applications in diverse systems.