arXiv subjects

Katsuaki Sugawara

Publications and source records attributed to Katsuaki Sugawara.

18 recordsLinked to original sources

arXiv2025

Direct observation of band structure modifications from monolayer WSe2 to Janus WSSe

Janus monolayer transition metal dichalcogenides (TMDs), created by post-growth substitution of the top chalcogen layer, represent a new direction for engineering 2D crystal properties. However, their rapid ambient degradation and the difficulty of obtaining large-area monolayer samples have limited the available experimental probes, leaving their detailed electronic structure near the Fermi level largely unexplored. In this work, by performing micro-focused angle-resolved photoemission spectroscopy (μ-ARPES) on an identical sample transformed from monolayer WSe2 to Janus WSSe via a H2 plasma-assisted chalcogen-exchange method, we reveal the evolution of its electronic band structure. We observe ARPES signature consistent with the Rashba-type spin splitting due to broken horizontal mirror symmetry, and a significant upward shift of the highest valence band at the Γ-point by approximately 160 meV. These direct observations clarify the key electronic modifications that govern the material's properties and provide a pathway for band engineering in Janus TMDs.

cond-mat.mtrl-sci↗

arXiv2025

Giant Rashba splitting in PtTe/PtTe$_2$ heterostructure

Achieving a large spin splitting is highly desirable for spintronic devices, which often requires breaking of the inversion symmetry. However, many atomically thin films are centrosymmetric, making them unsuitable for spintronic applications. Here, we report a strategy to achieve inversion symmetry breaking from a centrosymmetric transition metal dichalcogenide (TMDC) bilayer PtTe$_2$, leading to a giant Rashba spin splitting. Specifically, the thermal annealing turns one layer of PtTe$_2$ sample into a transition metal monochalcogenide (TMMC) PtTe through Te extraction, thus forming PtTe/PtTe$_2$ heterostructure with inversion symmetry breaking. In the naturally-formed PtTe/PtTe$_2$ heterostructure, we observe a giant Rashba spin splitting with Rashba coefficient of $α_{R}$ = 1.8 eV$\cdot$$Å$, as revealed by spin- and angle-resolved photoemission spectroscopy measurements. Our work demonstrates a convenient and effective pathway for achieving pronounced Rashba splitting in centrosymmetric TMDC thin films by creating TMMC/TMDC heterostructure, thereby extending their potential applications to spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Unusual temperature dependence of the band structure associated with local atomic distortion in monolayer 1T'-WTe2

The ground state of monolayer 1T'-WTe2 has been a target of intensive debate on whether or not it is a two-dimensional topological insulator (2D TI) associated with exciton formation. We investigated the band structure of an epitaxial monolayer 1T'-WTe2 film grown on graphene/SiC(0001) in a wide temperature range of T = 40 - 400 K by angle-resolved photoemission spectroscopy (ARPES). We observed an electron band above the Fermi level (EF) slightly away from the Γ point, together with four hole bands below EF just at the Γ point. This signifies an indirect band gap exceeding 0.1 eV in support of the 2D-TI phase with the inverted band structure. We uncovered an unexpectedly large downward shift of valence bands upon cooling, accompanied with an upward shift of the conduction band. Comparison of the ARPES-derived band structure with first-principles band calculations suggests that the observed band shift is ascribed to the systematic local atomic distortion of tungsten atoms, which should be incorporated into the interpretation of unusual transport properties of 1T'-WTe2.

cond-mat.mtrl-sci↗

arXiv2025

Selective Fabrication of Monolayer 1H- and 1T'-WTe2

We selectively fabricated monolayers of octahedral (1H) and distorted trigonal (1T') WTe2 on graphene/SiC(0001) by controlling the substrate temperature during epitaxy. Angle-resolved photoemission spectroscopy, combined with first-principles band-structure calculations, has revealed several drastic differences between these two polymorphs. The 1T' phase exhibits a semiconducting character with a nearly-zero energy gap, while the 1H phase shows a large band gap and the band splitting at the K/K' point. The present results pave a pathway toward developing nanoelectronic devices based with WTe2.

cond-mat.mes-hall↗

arXiv2024

In-situ topotactic chemical reaction for spectroscopies

Topotactic chemical reaction (TCR) is a chemical process that transforms one crystalline phase to another while maintaining one or more of the original structural frameworks, typically induced by the local insertion, removal, or replacement of atoms in a crystal. The utilization of TCR in atomic-layer materials and surfaces of bulk crystals leads to exotic quantum phases, as highlighted by the control of topological phases, the emergence of two-dimensional (2D) superconductivity, and the realization of 2D ferromagnetism. Advanced surface-sensitive spectroscopies such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) are leading techniques to visualize the electronic structure of such exotic states and provide us a guide to further functionalize material properties. In this review article, we summarize the recent progress in this field, with particular emphasis on intriguing results obtained by combining spectroscopies and TCR in thin films.

cond-mat.mtrl-sci↗

arXiv2022

Charge order with unusual star-of-David lattice in monolayer NbTe2

Interplay between fermiology and electron correlation is crucial for realizing exotic quantum phases. Transition-metal dichalcogenide (TMD) 1T-TaS2 has sparked a tremendous attention owing to its unique Mott-insulating phase coexisting with the charge-density wave (CDW). However, how the fermiology and electron correlation are associated with such properties has yet to be claried. Here we demonstrate that monolayer 1T-NbTe2 is a new class of two-dimensional TMD which has the star-of-David lattice similarly to bulk TaS2 and isostructural monolayer NbSe2, but exhibits a metallic ground state with an unusual lattice periodicity root19xroot19 characterized by the sparsely occupied star-of-David lattice. By using angle-resolved photoemission and scanning-tunneling spectroscopies in combination with first-principles band-structure calculations, we found that the hidden Fermi-surface nesting and associated CDW formation are a primary cause to realize this unique correlated metallic state with no signature of Mott gap. The present result points to a vital role of underlying fermiology to characterize the Mott phase of TMDs.

cond-mat.mtrl-sci↗

arXiv2022

Development of a versatile micro-focused angle-resolved photoemission spectroscopy system with Kirkpatrick-Baez mirror optics

Angle-resolved photoemission spectroscopy using a micro-focused beam spot (micro-ARPES) is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axes motion, and are vibrationally separated from the ARPES measurement system. Spatial mapping of the Auphotolithography pattern on Si signifies the beam spot size of 10 $μ$m (horizontal) x 12 $μ$m (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly associated with tiny sample bending near the edge of a cleaved topological insulator single crystal.

cond-mat.mes-hall↗

arXiv2021

Robust charge-density wave strengthened by electron correlations in monolayer 1T-TaSe2 and 1T-NbSe2

Combination of low-dimensionality and electron correlation is vital for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity. Transition-metal dichalcogenide (TMD) 1T-TaS2 has evoked great interest owing to its unique nonmagnetic Mott-insulator nature coupled with a charge-density-wave (CDW). To functionalize such a complex phase, it is essential to enhance the CDW-Mott transition temperature TCDW-Mott, whereas this was difficult for bulk TMDs with TCDW-Mott < 200 K. Here we report a strong-coupling 2D CDW-Mott phase with a transition temperature onset of ~530 K in monolayer 1T-TaSe2. Furthermore, the electron correlation derived lower Hubbard band survives under external perturbations such as carrier doping and photoexcitation, in contrast to the bulk counterpart. The enhanced Mott-Hubbard and CDW gaps for monolayer TaSe2 compared to NbSe2, originating in the lattice distortion assisted by strengthened correlations and disappearance of interlayer hopping, suggest stabilization of a likely nonmagnetic CDW-Mott insulator phase well above the room temperature. The present result lays the foundation for realizing monolayer CDW-Mott insulator based devices operating at room temperature.

cond-mat.str-el↗

arXiv2020

Dirac Fermion kinetics in three-dimensionally curved graphene

Three dimensionally curved graphene with a wide range of curvature radii from 25 nm to 1000 nm demonstrates that nano-scale curvature is a new degree of freedom to tune the transport properties of graphene by manipulating 2D electron kinetics on 3D curved surfaces.

cond-mat.mes-hall↗

arXiv2020

Modulation of Dirac electrons in epitaxial Bi2Se3 ultrathin films on van-der-Waals ferromagnet Cr2Si2Te6

We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator Bi2Se3 was epitaxially grown on a van-der-Waals ferromagnet Cr2Si2Te6 (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in 6 quintuple-layer (QL) Bi2Se3 on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in 2QL counterpart. Intriguingly, this gap is much larger than those for Bi2Se3 films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small characteristic length and energy scale of the magnetic proximity effect. The present results suggest a crucial role of interfacial coupling for modulating Dirac electrons in topological-insulator hybrids.

cond-mat.mtrl-sci↗

arXiv2020

Unusual temperature evolution of band structure of Bi(111) studied by angle-resolved photoemission spectroscopy and density functional theory

We have performed angle-resolved photoemission spectroscopy of Bi(111) thin films grown on Si(111), and investigated the evolution of band structure with temperature. We revealed an unexpectedly large temperature variation of the energy dispersion for the Rashba-split surface state and the quantum-well states, as seen in the highly momentum-dependent energy shift as large as 0.1 eV. A comparison of the band dispersion between experiment and first-principles band-structure calculations suggests that the interlayer spacing at the topmost Bi bilayer expands upon temperature increase. The present study provides a new pathway for investigating the interplay between lattice and electronic states through the temperature dependence of band structure.

cond-mat.mes-hall↗

arXiv2019

Dichotomy of superconductivity between monolayer FeS and FeSe

The discovery of high-temperature (Tc) superconductivity in monolayer FeSe on SrTiO3 raised a fundamental question whether high Tc is commonly realized in monolayer iron-based superconductors. Tetragonal FeS is a key material to resolve this issue because bulk FeS is a superconductor with Tc comparable to that of isostructural FeSe. However, difficulty in synthesizing tetragonal monolayer FeS due to its metastable nature has hindered further investigations. Here we report elucidation of band structure of monolayer FeS on SrTiO3, enabled by a unique combination of in-situ topotactic reaction and molecular-beam epitaxy. Our angle-resolved photoemission spectroscopy on FeS and FeSe revealed marked similarities in the electronic structure, such as heavy electron doping and interfacial electron-phonon coupling, both of which have been regarded as possible sources of high Tc in FeSe. However, surprisingly, high-Tc superconductivity is absent in monolayer FeS. This is linked to the weak superconducting pairing in electron-doped multilayer FeS in which the interfacial effects are absent. Our results strongly suggest that the cross-interface electron-phonon coupling enhances Tc only when it cooperates with the pairing interaction inherent to the superconducting layer. This finding provides a key insight to explore new heterointerface high-Tc superconductors.

cond-mat.supr-con↗

arXiv2019

Monolayer VTe2: Incommensurate Fermi surface nesting and suppression of charge density waves

We investigated the electronic structure of monolayer VTe2 grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer VTe2 takes the octahedral 1T structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi-surface topology between ARPES results and first-principles band calculations for octahedral monolayer 1T-VTe2. We have revealed that monolayer 1T-VTe2 at low temperature is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer VSe2 which undergoes a CDW transition to insulator at low temperature. The present result suggests an importance of Fermi-surface topology for characterizing the CDW properties of monolayer TMDs.

cond-mat.mtrl-sci↗

arXiv2019

Electronic structure of Bi2Te3/FeTe heterostructure: implications for unconventional superconductivity

We have performed angle-resolved photoemission spectroscopy on a heterostructure consisting of topological insulator Bi2Te3 and iron chalcogenide FeTe fabricated on SrTiO3 substrate by molecular-beam-epitaxy technique. This system was recently found to show superconductivity albeit non-superconducting nature of each constituent material. Upon interfacing FeTe with two quintuple layers of Bi2Te3, we found that the Dirac-cone surface state of Bi2Te3 is shifted toward higher binding energy, while the holelike band at the Fermi level originating from FeTe moves toward lower binding energy. This suggests that electron charge transfer takes place from FeTe to Bi2Te3 through the interface. The present result points to importance of hole-doped FeTe interface for the occurrence of unconventional superconductivity.

cond-mat.supr-con↗

arXiv2018

Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D-2D crossover in monolayer VSe2

One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials, as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene. Transition-metal dichalcogenides (TMDs) provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change, since they exhibit a variety of electronic/magnetic states owing to quantum confinement. Here we report an anomalous metal-insulator transition induced by 3D-2D crossover in monolayer 1T-VSe2 grown on bilayer graphene. We observed a complete insulating state with a finite energy gap on the entire Fermi surface in monolayer 1T-VSe2 at low temperatures, in sharp contrast to metallic nature of bulk. More surprisingly, monolayer 1T-VSe2 exhibits a pseudogap with Fermi arc at temperatures above the charge-density-wave temperature, showing a close resemblance to high-temperature cuprates. This similarity suggests a common underlying physics between two apparently different systems, pointing to the importance of charge/spin fluctuations to create the novel electronic states, such as pseudogap and Fermi arc, in these materials.

cond-mat.mtrl-sci↗

arXiv2017

Three-dimensional porous graphene networks expand graphene-based electronic device applications

Graphene with excellent 2D characters has been required to expand their potentials for meeting applicational demands in recent years. As one avenue, several approaches for fablicating 3D porous graphene network structures have been developed to realize multi-functional graphene materials with 2D graphene. Here we overview characteristics of 3D porous graphene for future electronic device applications along with physical insights into "2D to 3D graphene", where the characters of 2D graphene such as massless Dirac fermions are well preserved. The present review thus summarizes recent 3D porous graphene studies with a perspective for providing new and board graphene electronic device applications.

cond-mat.mtrl-sci↗

arXiv2016

Electric Properties of Dirac Fermions Captured into 3D Nanoporous Graphene Networks

Graphene, as a promising material of post-silicon electronics, opens a new paradigm for the novel electronic properties and device applications. On the other hand, the 2D feature of graphene makes it technically challenging to be integrated into 3D transistors with a sufficient processor capacity. Although there are many attempts to assemble 2D graphene into 3D structures, the characteristics of massless Dirac fermions cannot be well preserved in these materials for transistor applications. Here we report a high-performance graphene transistor by utilizing 3D nanoporous graphene which is comprised of an interconnected single graphene sheet and a commodious open porosity to infuse an ionic liquid for a tunable electronic state by applying electric fields. The 3D nanoporous graphene transistor, with high carrier mobility of 5000-7500 cm$^2$V$^{-1}$s$^{-1}$, exhibits two to three orders of magnitude higher electric conductance and capacitance than those of 2D graphene devices, along with preserved ambipolor electronic nature of Dirac cones. Moreover, the 3D graphene networks with Dirac fermions turn out to exhibit a unique nonlinear Hall resistance in a wide range of the gate voltages. The high quality 3D nanoporous graphene EDLT may open a new field for utilizing Dirac fermions in 3D network structures for various fundamental and practical applications.

cond-mat.mtrl-sci↗

arXiv2010

Direct Evidence for the Dirac-Cone Topological Surface States in Ternary Chalcogenide TlBiSe2

We have performed high-resolution angle-resolved photoemission spectroscopy on TlBiSe2, which is a member of the ternary chalcogenides theoretically proposed as candidates for a new class of three-dimensional topological insulators. By measuring the energy band dispersions over the entire surface Brillouin zone, we found a direct evidence for a non-trivial surface metallic state showing a X-shaped energy dispersion within the bulk band gap. The present result unambiguously establishes that TlBiSe2 is a strong topological insulator with a single Dirac cone at the Brillouin-zone center. The observed bulk band gap of 0.4 eV is the largest among known topological insulators, making TlBiSe2 the most promising material for studying room-temperature topological phenomena.

cond-mat.mes-hall↗