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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.

Hydrogen fuel cells offer a clean and sustainable energy conversion solution. The bipolar separator plate, a critical component in fuel cells, plays a vital role in preventing reactant gas cross-contamination and facilitating efficient ion transport in a fuel cell. High chromium ferritic stainless steel with an artificially formed thin chromium oxide passive film has recently gained attention due to its superior electrical conductivity and corrosion resistance, making it a suitable material for separators. In this study, we investigate the microscopic electrical conductivity of the intrinsic passive oxide film on such ferritic stainless steel. Through advanced surface characterization techniques such as current sensing atomic force microscopy and scanning tunneling microscopy/spectroscopy, we discover highly conductive regions within the film that vary depending on location. These findings provide valuable insights into the behavior of the passive oxide film in fuel cells. By understanding the microscopic electrical properties, we can enhance the design and performance of separator materials in hydrogen fuel cells. Ultimately, this research contributes to a broader understanding of separator materials and supports the wider application of hydrogen fuel cells., Comment: 6 pages, 4 figures. The following article has beenaccepted by Journal of the Korean Physical Society. After it is published, it will be found at the following DOI

Graphene, with its unique band structure, mechanical stability, and high charge mobility, holds great promise for next-generation electronics. Nevertheless, its zero band gap challenges the control of current flow through electrical gating, consequently limiting its practical applications. Recent research indicates that atomic oxygen can oxidize epitaxial graphene in a vacuum without causing unwanted damage. In this study, we have investigated the effects of chemisorbed atomic oxygen on the electronic properties of epitaxial graphene, using scanning tunneling microscopy (STM). Our findings reveal that oxygen atoms effectively modify the electronic states of graphene, resulting in a band gap at its Dirac point. Furthermore, we demonstrate that it is possible to selectively induce desorption or hopping of oxygen atoms with atomic precision by applying appropriate bias sweeps with an STM tip. These results suggest the potential for atomic-scale tailoring of graphene oxide, enabling the development of graphene-based atomic-scale electronic devices., Comment: 5 pages, 3 figures. The following article has been accepted by Applied Physics Letters. After it is published, it will be found at the following DOI

Piezoelectric motors are widely used in various applications where both precision positioning and miniaturization are required. Either inertial or quasi-static motors are commonly employed because of their high accuracy, which demands consistent sliding friction between moving sliders and their static counterparts for reliable operation. In general, slider wear is unavoidable after long-term use. Especially, the wear often leads to more serious cold welding in vacuum, which also refers to friction welding induced by direct contact between similar metal surfaces. Non-metallic coatings can prevent such unwanted cold welding in ultrahigh vacuum (UHV) applications. However, the practical reliability of available coatings under UHV conditions still remains to be elucidated. Here, we systematically investigate the practical reliability of commonly used UHV-compatible lubricant coatings for piezoelectric motors in vacuum. We demonstrate that polytetrafluoroethylene (PTFE) shows the most reliable long-term operation in vacuum, while other coatings eventually lead to wear-induced cold welding and motor failure. Our finding provides a simple and effective way to improve the long-term performance of UHV piezoelectric motors by coating the slider surface with PTFE., Comment: 4 pages, 3 figures. The following article has been accepted by Review of Scientific Instruments. After it is published, it will be found at https://doi.org/10.1063/5.0147344

Lower thermal stability due to thinning often leads to unprecedented hidden phases in low-dimensional materials. Such hidden phases can coexist or compete with preexisting electronic phases. We investigate hidden phases observed in atomically thin (6-8 layers) 1T-TaS$_2$ with scanning tunneling microscopy. First, we can electrically induce a hidden stripe phase at room temperature. Such a uniaxial stripe phase has three equivalent orientations by breaking three-fold symmetry of 1T-TaS$_2$. We also reveal that the hidden stripe phase coexists with nearly commensurate charge-density-wave phase. Next, we observe that the emergent stripe phase spontaneously appears without any electric excitation on a tiny flake ($160\times80$ nm$^2$). Our findings may provide a plausible explanation for the previously observed phase transition and two-fold optical response in thin 1T-TaS$_2$ devices at room temperature. Furthermore, the hidden stripe phase would be crucial to understand exotic CDW-related phenomena in 1T-TaS$_2$ for potential applications., Comment: 6 pages, 5 figures

Quasi-two-dimensional (Quasi-2D) van der Waals (vdW) materials can be mechanically or chemically exfoliated down to monolayer because of their strong intralayer bonding and the weak interlayer vdW interaction. Thanks to this unique property, one can often find exotic thickness-dependent electronic properties from these quasi-2D vdW materials, which can lead to band gap opening, emerging superconductivity, or enhanced charge density waves with decreasing thickness. Surface-sensitive scanning tunneling microscopy (STM) can provide direct observation of structural and electronic characteristics of such layered materials with atomic precision in real space. However, it is very challenging to preserve the intrinsic surfaces of air-sensitive quasi-2D materials between preparation and measurement. In addition, vdW 2D crystals after exfoliation are extremely hard to explore with a typical STM setup due to their small size (< 10 {\mu}m). Here, we present a straightforward method compatible with any STM setup having optical access: (1) exfoliating and/or stacking layered materials in a glove box, (2) transferring them to an ultra-high vacuum STM chamber using a suitcase without exposure to air, and (3) navigating surface to locate exfoliated vdW 2D flakes with different thicknesses. We successfully demonstrated that the clean surfaces of the air-sensitive Fe$_3$GeTe$_2$ can be effectively protected from unwanted oxidation during transfer. Furthermore, our method provides a simple but useful way to access a specific tiny stack of layered materials without any ex-situ fabrication processes for STM navigation. Our experimental improvement will open up a new way to investigate air-sensitive layered vdW materials with various thicknesses via surface-sensitive techniques including STM., Comment: 5 pages, 5 figures

We investigate mechanically exfoliated thin 1T-TaS$_2$ with scanning tunneling microscopy at room temperature. Sample preparation without air exposure enables access to intrinsic charge-density-wave (CDW) phases of thin 1T-TaS$_2$. At room temperature, we can observe the expected nearly commensurate CDW (NCCDW) phase on thin flakes similar to bulk 1T-TaS$_2$. Further analysis reveals that the CDW domains in the NCCDW phase become smaller and have more anisotropic shape with decreasing thickness in the range of 8-28 layers. Our findings demonstrate that the anisotropic CDW nature of thin 1T-TaS$_2$ would be crucial to understand its exotic CDW-related phenomena and demand a systematic study on its correlation between the thickness-driven CDW domain anisotropy and the intermediate CDW states in thin 1T-TaS$_2$., Comment: 5 pages, 4 figures

Tuning dimensionality in van der Waals materials with finite interlayer coupling has introduced various electronic phase transitions by conventional mechanical exfoliation. Particularly when the electronic order is tied to the modulation of the interlayer coupling, such dimensional tunability has a strong impact on its stability and properties, which has rarely been investigated experimentally. Here, we demonstrate a dimensional crossover of charge order in IrTe$_2$ from genuine two- to quasi-three-dimension using low-temperature scanning tunneling microscopy and spectroscopy. Employing atomically thin IrTe$_2$ flakes ranging from monolayer to multilayer, we observe a gradual phase transition of charge order and exponential decay of Coulomb gap with increasing thickness. Moreover, we find a suppression of the density of states emerging at an abrupt lateral interface between two- and three-dimension. These findings are attributed to the interplay between the strongly coupled layers and substrate-driven perturbation, which can provide a new insight into the dimensional crossover of strongly coupled layered materials with hidden electronic phases., Comment: 8 pages, 6 figures

Chirality-driven optical properties in charge density waves are of fundamental and practical importance. Here, we investigate the interaction between circularly polarized light and emergent chiral stacking orders in quasi-one-dimensional (quasi-1D) charge-density waves (CDW) with density-functional theory calculations. In our specific system, self-assembled In nanowires on Si(111) surface, spontaneous mirror symmetry breaking leads to symmetrically distinct four degenerate quasi-1D CDW structures, which exhibit geometrical chirality. Such geometrical chirality may naturally induce optically active phenomena even when the quasi-1D CDW structures are stacked perpendicular to the CDW chain direction. Indeed, we find that left- and right-chiral stacking orders show distinct circular dichroism responses while a nonchiral stacking order does no circular dichroism. Such optical responses are attributed to the existence of glide mirror symmetry of the CDW stacking orders. Our findings suggest that the CDW chiral stacking orders can lead to diverse active optical phenomena such as chirality-dependent circular dichroism, which can be observed in scanning tunneling luminescence measurements with circularly polarized light., Comment: 5 pages, 4 figures + Supplemental Material, accepted in Phys. Rev. Lett

In a recent article, Huda et al. demonstrated tuneable topological domain wall states in the c(2$\times$2) chlorinated Cu(100). Their system allows to experimentally tune the domain wall states using atom manipulation by the tip of a scanning tunneling microscope (STM). They have realized topological domain wall states of two prototypical 1D models such as trimer and coupled dimer chains. However, they did not distinguish trivial domain wall states from topological ones in their models. As a result, all states of a specific domain wall are not topological but trivial. Here, we show why the specific domain wall states are trivial and how to make them topological. This topological consideration would provide more clear insight on future studies on topological domain wall states in artificial atomic chains.

Topological phases and their topological features are enriched by the fundamental time-reversal, particle-hole, and chiral as well as crystalline symmetries. While one-dimensional (1D) generalized Su-Schrieffer-Heeger (SSH) systems show various topological phenomena such as topological solitons and topological charge pumping, it remains unclear how such symmetry protects and relates such topological phenomena. Here we show that the generalized time-reversal, particle-hole, and chiral symmetry operators consistently explain not only the symmetry transformation properties between the ground states but also the topological features of the topological solitons in prototypical quasi-1D systems such as the SSH, Rice-Mele, and double-chain models. As a consequence, we classify generalized essential operators into three groups: Class I and class II operators connect ground states in between after spontaneous symmetry breaking while class III operators give the generalized particle-hole and chiral symmetries to ground states. Furthermore, class I operators endow the equivalence relation between topological solitons while class II and III operators do the particle-hole relations. Finally, we demonstrate three distinct types of topological charge pumping and soliton chirality from the viewpoint of class I, II, and III operators. We build a general framework to explore the topological features of the generalized 1D electronic system, which can be easily applied in various condensed matter systems as well as photonic crystal and cold atomic systems., Comment: 18 pages, 7 figures, 8 tables

We demonstrate the existence of atomically-sized topological solitons in a quasi one-dimensional charge density wave system: indium atomic wires on Si(111). Performing joint scanning tunneling microscopy and density-functional calculations, we show that the Si(111)-(8x2)In surface features two conceptually different types of abrupt phase flip structures. One is caused by In adatoms and is, hence, non-solitonic in nature. The other one is an abrupt left-chiral soliton.

Chirality manifests in various forms in nature. However, there is no evidence of the chirality in one-dimensional charge density wave (CDW) systems. Here, we have explored the chirality among quasi-one-dimensional CDW ground states with the aid of scanning tunneling microscopy, symmetry analysis, and density functional theory calculations. We discovered three distinct chiralities emerging in the form of two-dimensional chiral stacking orders composed of degenerate CDW ground states: right-, left-, and nonchiral stacking orders. Such chiral stacking orders correspond to newly introduced chiral winding numbers. Furthermore, we observed that these chiral stacking orders are intertwined with chiral vortices and chiral domain walls, which play a crucial role in engineering the chiral stacking orders. Our findings suggest that the unexpected chiral stacking orders can open a way to investigate the chirality in CDW systems, which can lead to diverse phenomena such as circular dichroism depending on chirality., Comment: 5 pages, 4 figures + Supplemental Material, accepted in Phys. Rev. B, https://journals.aps.org/prb/accepted/7d073Ye7Fc510c7178d74b69816582e083b0a5dd0

Superconductivity in the vicinity of a competing electronic order often manifests itself with a superconducting dome, centred at a presumed quantum critical point in the phase diagram. This common feature, found in many unconventional superconductors, has supported a prevalent scenario that fluctuations or partial melting of a parent order are essential for inducing or enhancing superconductivity. Here we present a contrary example, found in IrTe2 nanoflakes of which the superconducting dome is identified well inside the parent stripe charge ordering phase in the thickness-dependent phase diagram. The coexisting stripe charge order in IrTe2 nanoflakes significantly increases the out-of-plane coherence length and the coupling strength of superconductivity, in contrast to the doped bulk IrTe2. These findings clarify that the inherent instabilities of the parent stripe phaseare sufficient to induce superconductivity in IrTe2 without its complete or partial melting. Our study highlights the thickness control as an effective means to unveil intrinsic phase diagrams of correlated vdW materials., Comment: 20 pages, 4 Figures