Kyung-Hwan Jin

While solar‐driven photoelectrochemical (PEC) water splitting offers a promising route for sustainable hydrogen production, sluggish interfacial charge transfer and long‐term instability remain critical bottlenecks. To address these limitations, a chemically integrated Ti‐squarate metal–organic framework (IEF‐11)/ n ‐GaN nanowire (NW) hybrid photoanode is designed via a (3‐aminopropyl) triethoxysilane (APTES) interlayer for efficient PEC water splitting. Density functional theory (DFT) calculations support energetically favorable interfacial bonding and the associated charge redistribution across the APTES‐mediated interface, highlighting the dual role of APTES in chemical anchoring and interfacial electronic modulation. This interfacial modification is accompanied by a reduced work function and a cathodic shift in the apparent flat‐band potential. Consequently, the hybrid photoanode demonstrates accelerated charge‐transfer kinetics, featuring reduced charge‐transfer resistance and a 10.4‐fold enhancement in applied‐bias photon‐to‐current efficiency (ABPE) compared to pristine NWs. Furthermore, this chemical anchoring supports the retention of principal structural and chemical features after 100 h of continuous operation, thereby promoting durable solar‐driven hydrogen production.

Kagome‐lattice magnets provide a fertile ground for exploring exotic electronic phases due to topological band structures and complex magnetic orders. Yet, how these ingredients dictate unconventional magnetotransport responses, crucial for possible spintronic applications, has remained elusive. Here, we show that a prototypical kagome antiferromagnet exhibits significant magnetic‐field‐driven redistribution of Berry curvature and the colossal anomalous Hall conductivity (AHC). Under high magnetic fields, spin canting induces a pronounced spin splitting of the flat and Dirac bands near the Fermi level, dramatically enhancing the Berry curvature. The resulting AHC reaches 2.5 10 S/cm, far exceeding the intrinsic Berry curvature predictions and ranking among the highest reported in magnetic systems. This colossal AHC enhancement is attributed to a synergy between field‐tuned Berry curvature and enhanced skew scattering in the clean limit. Our findings highlight the critical role of magnetic‐field‐controlled spin splitting in engineering Berry curvature and anomalous magnetotransport in clean kagome magnets.

Modulation of the electronic energy levels in semiconducting materials is an essential technology for fabrication of advanced optoelectronic devices. Despite active studies on metal halide perovskite materials, a versatile modulation methodology capable of spatially uniform and controllable adjustment of the Fermi level remains elusive. Herein, we demonstrate that electron beam irradiation enables systematic tuning of work function in widely used formamidinium lead triiodide (FAPbI ₃ ) perovskite thin films. The electron‐beam irradiation was found to induce iodine‐rich surface defect states, including interstitial iodine/polyiodide‐like species, thereby reducing the electron density near the Pb‐I coordination sites and leading to acceptor‐like surface electronic modulation. Variations in acceleration voltage and exposure time produced systematic work‐function modulation, reflecting the combined influence of incident electron energy and total electron dose on iodine‐related defect redistribution. Exploiting spatial controllability and local work function modulation, a lateral built‐in potential of 0.62 V in FAPbI ₃ thin films was achieved. The proof‐of‐concept device based on the surface‐potential‐defined lateral junction formed by electron‐beam irradiation exhibited clear rectifying current–voltage behavior.

Quantum condensed states in solids often reveal their fundamental nature via interactions with impurities, as epitomized by Yu–Shiba–Rusinov (YSR) bound states at magnetic impurities in superconductors. Although analogous bound states are predicted within quantum condensates of excitons, their existence has been elusive. Here we visualize in-gap electronic states bound to impurities inside an exciton condensate phase of a van der Waals crystal Ta₂Pd₃Te₅ at 4.4 K, utilizing scanning tunnelling microscopy and spectroscopy. We find that the energies of in-gap states are correlated with the excitonic band gap, which is systematically tuned by local strain and carrier injection. Our theoretical analyses reveal that these in-gap states can be explained by charge dipoles associated with Pd atoms on Ta sites through a charge-exciton version of the YSR mechanism. Our findings introduce the analogue of YSR physics in exciton condensates and a microscopic tool to probe and control quantum properties in exciton condensates persisting up to room temperature.

We demonstrate electrically switchable, non-volatile dipoles in graphene/thin hBN/α-RuCl₃alpha-RuCl3 heterostructures, stabilized purely by interfacial charge transfer across an atomically thin dielectric barrier. This mechanism requires no sliding or twisting to explicitly break inversion symmetry and produces robust ferroelectric-like hysteresis loops that emerge prominently near 30 K. Systematic measurements under strong in-plane and out-of-plane magnetic fields reveal negligible effects on the hysteresis characteristics, confirming that the primary mechanism driving the dipole switching is electrostatic. Our findings establish a distinct and robust route to electrically tunable ferroelectric phenomena in van der Waals heterostructures, opening opportunities to explore the interplay between interfacial charge transfer and temperature-tuned barrier crossing of dipole states at the atomic scale.

Excitons are bosonic quasiparticles with a variety of applications in optoelectronics, photosynthesis, and dissipationless informatics, and their lifetime can become sufficiently long to form a quantum condensate. While exciton condensation has been predicted to occur as a ground state of a solid, so-called an excitonic insulator, whose material realization has been elusive. Here we report the observation of direct photoemission signals from excitons in a ground state of a very recent excitonic insulator candidate Ta 2 Pd 3 Te 5 below its metal-insulator transition temperature using orbital-selective angle-resolved photoemission spectroscopy. It is confirmed that the excitons have a lower energy than the valence band maximum to possibly drive the phase transition. This measurement further discloses the size and the unusual odd parity of the exciton wave function. The present finding opens an avenue toward applications of coherent excitons in solid systems and searching for exotic quantum phases of exciton condensates.

Domain walls (DWs) have been recognized to play crucial roles in various interesting properties and functionalities in quantum materials. A notable example is DWs in charge density wave (CDW), which are believed to provide metallic electron channels for emerging superconductivity. However, electronic states of DWs and the microscopic mechanism toward superconductivity have been elusive. Here, we clarify the atomic/electronic structure of DWs of the chiral CDW emerging in 1T-TiSe2, using scanning tunneling microscopy and density functional calculations. We reveal unambiguously the microscopic origin of chiral CDW as the C2 distortion in Se layers and its interlayer coupling. We further identify unique DWs connecting CDW domains of opposite chirality. The DWs are endowed with no in-gap state due to the characteristic multibands around the band gap, which defies the widely believed notion for CDW DWs. These results provide an important insight into the role of DWs in emerging superconductivity.

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron-hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta2NiSe5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta2NiSe5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal T_d-MoTe₂ by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.