Han Woong Yeom

Chiral charge‐density waves (CDWs) are a rare form of electronic order with handedness, enabling novel couplings among light, lattice, and topology in quantum materials. Yet the microscopic origin of chirality has remained elusive, even in the prototypical chiral CDW system 1 T ‐TiSe ₂ . Using ab initio calculations, we investigate the structural stability and electronic structure of competing CDW models in 1 T ‐TiSe ₂ . We show that a chiral monoclinic () structure is stabilized by the cooperative condensation of multiple phonon modes and reproduces the chiral topography and energy gap observed experimentally. We further find an exceptionally soft potential landscape around this energy‐minimized structure, along the direction that removes the remaining rotational symmetry, allowing the lattice to fluctuate through fully asymmetric configurations while its average structure remains chiral. This dynamical chiral CDW naturally reconciles the chiral domains seen by time‐averaged topographic probes with the complete loss of symmetry detected by bulk spectroscopy, and clarifies how chiral CDWs emerge from the interplay of lattice instability and fluctuation.

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.

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.

Topological solitons, which connect degenerate ground states with phase shifts, have the potential to carry topologically protected information for dissipationless informatics. While topological solitons have been discussed actively in one-dimensional materials, a soliton with a geometric Z topology has been recently found in a van der Waals material of 1T-TaS₂ as a form of a mobile kink along domain walls in its charge-density-wave phase. However, details of kink kinetics and the mechanism of their motion have been obscured. Here, we reveal the kinetics and dynamics of kink solitons by tracking the motions of several kink solitons over a relatively long time using real-time scanning tunneling microscopy measurements. We disclose two different types of kink motions, that is, small (one unit cell) or large (a few unit cells) jumps. The energetics and atomistic details of kink jumps were revealed by density functional calculations. Furthermore, the average frequency of jumping modes is significantly enhanced by the injection of the tunneling current. Our work makes a step toward realizing control over the motion of kink solitons, which must be secured for soliton applications.

Magnetic impurities in strongly correlated electronic systems serve as sensitive probes to a wide range of many-body quantum phenomena. Broken symmetries in such a system can lead to inequivalent lattice sites, and magnetic impurities may interact selectively with particular orbitals or sublattices. However, the microscopic mechanisms behind such site-specific interactions have been poorly understood. Here, we explore the behavior of individual Fe adatoms on a cluster-Mott charge density-wave (CDW) system of 1T-TaS₂ by utilizing scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Our measurements uncover pronounced site-dependent electronic states of CDW clusters with Fe adatoms, indicating distinct local coupling to cluster-Mott states. DFT calculations identify three distinct types of interactions: hybridization with localized correlated electrons, distorting the CDW cluster, and charge transfer. In particular, the hybridization of Fe 3d and half-filled Ta 5d_z² orbitals suppresses the Mott insulating state for an adatom at the center of a CDW cluster. While the results underscore a crucial role of the direct orbital hybridization and the limitation of the prevailing single-site Kondo impurity model, they suggest the possibility of controlling entangled interactions separately in a cluster-Mott insulator.

Magnetic impurities in strongly correlated electronic systems serve as sensitive probes to a wide range of many-body quantum phenomena. Broken symmetries in such a system can lead to inequivalent lattice sites, and magnetic impurities may interact selectively with particular orbitals or sublattices. However, the microscopic mechanisms behind such site-specific interactions have been poorly understood. Here, we explore the behavior of individual Fe adatoms on a cluster-Mott charge-density-wave (CDW) system of 1T-TaS2 utilizing scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Our measurements uncover pronounced site-dependent electronic states of CDW clusters with Fe adatoms, indicating distinct local coupling to cluster-Mott states. DFT calculations identify three distinct types of interactions; hybridization with localized correlated electrons, distorting the CDW cluster, and charge transfer. In particular, the hybridization of Fe 3d and half-filled Ta 5dz2 orbitals suppresses the Mott insulating state for an adatom at the center of a CDW cluster. While the results underscore a crucial role of the direct orbital hybridization and the limitation of the prevailing single-site Kondo impurity model, they suggest the possibility of controlling entangled interactions separately in a cluster Mott insulator.

While localized topological modes in quantum materials, such as topological solitons and Majorana fermions, promise loss‐less delivery of classical and quantum information, manipulating individual topological modes has been highly challenging. The manipulation of Z ₄ topological solitons is reported in a 1D charge density wave (CDW) insulator of indium atomic wires on a silicon surface. Using the current injection from a scanning tunneling microscopy tip, individual defects‐pinned solitons can be created, translated, and annihilated. Moreover, this method is applied to induce bi‐soliton processes such as transforming a soliton into a different soliton, fissioning one into two solitons, and fusing and dissociating two solitons, where soliton topological charges are manipulated. The theoretical calculations attribute the mechanism of manipulation to local destabilization of the CDW structure by hole doping. This work demonstrates addressable control over most of the possible operations between individual and paired Z ₄ solitons and secures a way to investigating dynamics of quantum solitons and to processing topologically‐protected multi‐valued information in electronic systems.

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.

Excitons are bosonic quasiparticles with a variety of applications in optoelectronics, photosyn thesis, 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 Ta2Pd3Te5 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.