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Nanomaterials based on monoatomic bismuth (Bi) are attracting particular attention because they are candidates of two-dimensional (2D) topological insulators and Rashba metals useful for spintronic applications. We report convenient selective fabrication of two different types of ultrathin Bi films, bismuthene and α-Bi on hydrogen-terminated SiC(0001), by combining the molecular-beam-epitaxy (MBE) method and the low-temperature and low-pressure hydrogen chemical etching of SiC. We have succeeded in selectively fabricating these two different Bi phases by simply tuning the substrate temperature during the MBE process. We observed that while bismuthene and α-Bi showed a similar low-energy electron diffraction pattern of the (√3 × √3)R30° periodicity, angle-resolved photoemission spectroscopy revealed a sizable difference in the band structure; bismuthene shows a massive Dirac cone, a signature of 2D topological insulators, whereas α-Bi exhibits an insulating behavior with a large band gap of more than 1.8 eV. We discuss the underlying mechanism of selective fabrication in terms of hydrogen desorption from the hydrogen-terminated SiC substrate.

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., To be published in Physical Review B

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 2. 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.

Nitrogen (N) doping is one of the most effective approaches to tailor the chemical and physical properties of graphene. By the interplay between N dopants and 3D curvature of graphene lattices, N-doped 3D graphene displays superior performance in electrocatalysis and solar-energy harvesting for energy and environmental applications. However, the electrical transport properties and the electronic states, which are the key factors to understand the origins of the N-doping effect in 3D graphene, are still missing. The electronic properties of N-doped 3D graphene are systematically investigated by an electric-double-layer transistor method. It is demonstrated that Urbach-tail-like localized states are located around the neutral point of N-doped 3D graphene with the background metallic transport channels. The dual nature of electronic states, generated by the synergistic effect of N dopants and 3D curvature of graphene, can be the electronic origin of the high electrocatalysis, enhanced molecular adsorption, and light absorption of N-doped 3D graphene.

One of the key challenges in the research of topological insulators (TIs) is to establish a means to intentionally manipulate Dirac fermions. While the chemical substitution is commonly applied to ...

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 $\mu$m (horizontal) x 12 $\mu$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., Comment: 21 pages, 8 figures

Topological Dirac semimetals (TDSs) offer an excellent opportunity to realize outstanding physical properties distinct from those of topological insulators. Since TDSs verified so far have their own problems such as high reactivity in the atmosphere and difficulty in controlling topological phases via chemical substitution, it is highly desirable to find a new material platform of TDSs. By angle-resolved photoemission spectroscopy combined with first-principles band-structure calculations, we show that ternary compound BaMg2Bi2 is a TDS with a simple Dirac-band crossing around the Brillouin-zone center protected by the C3 symmetry of crystal. We also found that isostructural SrMg2Bi2 is an ordinary insulator characterized by the absence of band inversion due to the reduction of spin–orbit coupling. Thus, XMg2Bi2 (X = Sr, Ba, etc.) serves as a useful platform to study the interplay among crystal symmetry, spin–orbit coupling, and topological phase transition around the TDS phase.

Realization of topological superconductors (TSCs) hosting Majorana fermions is an exciting challenge in materials science. Majorana fermions are predicted to emerge at vortex cores of two-dimensional (2D) TSCs and at both ends of one-dimensional (1D) TSCs; these two types of TSCs have been explored independently in different materials. Here, a system which has a potential to access both 1D and 2D TSCs in a single platform, Pb(111)/Bi(111) heterostructure, is proposed. One to twenty bilayers (BLs) of Bi(111) ultra-thin films are epitaxially fabricated on TlBiSe$_2$, and an intriguing evolution of electronic states upon variation of Bi-layer thickness is revealed by angle-resolved photoemission spectroscopy. The metallic quantum-well states at 1–2BLs are found to turn into the Rashba states at 5–20BLs, via the semiconducting states at 3BL. Fabrication of a Pb(111) film on 20BL Bi(111) enables observation of the proximity-induced superconductivity in Bi(111) as evident from a 1 meV energy gap at 5 K; these energy and temperature scales are considerably larger than those of Rashba superconductors. The Pb/Bi heterostructure serves as a versatile platform to study the interplay among proximity-induced superconductivity, band structure, and topology.

We have performed angle-resolved photoemission spectroscopy on epitaxial VTe2 films to elucidate the relationship between the fermiology and charge-density waves (CDW). We found that a two-dimensional triangular pocket in 1 monolayer (ML) VTe2 is converted to a strongly warped quasi-one-dimensional (1D) Fermi surface in the 6ML counterpart, likely associated with the 1T-to-1T' structural phase transition. We also revealed a metallic Fermi edge on the entire Fermi surface in 6ML at low temperature distinct from anisotropic pseudogap in 1ML, signifying a contrast behavior of CDW that is also supported by first-principles band-structure caluculations. The present result points to the importance of simultaneously controlling the structural phase and fermiology to manipulate the CDW properties in ultrathin transition-metal dichalcogenides., Comment: To be published in Physical Review B

3D integration of graphene has attracted attention for realizing carbon-based electronic devices. While the 3D integration can amplify various excellent properties of graphene, the influence of 3D curved surfaces on the fundamental physical properties of graphene has not been clarified. The electronic properties of 3D nanoporous graphene with a curvature radius down to 25-50 nm are systematically investigated and the ambipolar electronic states of Dirac fermions are essentially preserved in the 3D graphene nanoarchitectures, while the 3D curvature can effectively suppress the slope of the linear density of states of Dirac fermion near the Fermi level are demonstrated. Importantly, the 3D curvature can be utilized to tune the back-scattering-suppressed electrical transport of Dirac fermions and enhance both electron localization and electron-electron interaction. As a result, nanoscale curvature provides a new degree of freedom to manipulate 3D graphene electrical properties, which may pave a new way to design new 3D graphene devices with preserved 2D electronic properties and novel functionalities.

We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ was epitaxially grown on a van der Waals ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in six quintuple-layer (6QL) ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in its 2QL counterpart. Intriguingly, this gap is much larger than those for ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ 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.

The authors probe a ferromagnetic state emerging on an atomic surface of a nonmagnetic layered metal PdCoO2. Measurements of PdCoO2 ultrathin films reveal that spin-polarized Pd-derived electrons are flowing at the surface under the influence of triangular lattices of magnetic Co ions.

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., Comment: 7 pages, 4 figures

Realization of topological superconductors (TSCs) hosting Majorana fermions is a central challenge in condensed-matter physics. One approach is to use the superconducting proximity effect (SPE) in heterostructures, where a topological insulator contacted with a superconductor hosts an effective p-wave pairing by the penetration of Cooper pairs across the interface. However, this approach suffers a difficulty in accessing the topological interface buried deep beneath the surface. Here, we propose an alternative approach to realize topological superconductivity without SPE. In a Pb(111) thin film grown on TlBiSe2, we discover that the Dirac-cone state of substrate TlBiSe2 migrates to the top surface of Pb film and obtains an energy gap below the superconducting transition temperature of Pb. This suggests that a Bardeen-Cooper-Schrieffer superconductor is converted into a TSC by the topological proximity effect. Our discovery opens a route to manipulate topological superconducting properties of materials., Realizing topological superconductivity is essential for applicable fault-tolerant quantum computation. Here, Trang et al. report migration of Dirac-cone from TlBiSe2 substrate to top surface of superconducting Pb film due to topological proximity effect, suggesting realization of topological superconductivity.

One of the key challenges in condensed-matter physics is to establish a topological superconductor that hosts exotic Majorana fermions. Although various heterostructures consisting of conventional ...

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., 6 pages, 3 figures

Realization of unconventional physical properties in two-dimensional (2D) transition-metal dichalcogenides (TMDs) is currently one of the key challenges in condensed-matter systems. However, the electronic properties of 2D TMDs remain largely unexplored compared to those of their bulk counterparts. Here, we report the fabrication of a high-quality monolayer NbSe2 film with a trigonal prismatic structure by molecular beam epitaxy, and the study of its electronic properties by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and electrical transport measurements, together with first-principles band-structure calculations. In addition to a charge density wave (CDW) with 3 × 3 periodicity and superconductivity below 1.5 K, we observed sizable (~0.1 eV) band splitting along the Γ-K cut in the Brillouin zone due to inversion symmetry breaking in the monolayer crystal. This splitting is highly anisotropic in k space, leading to a spin-split van-Hove singularity in the band structure. The present results suggest the importance of spin–orbit coupling and symmetry breaking for unconventional superconductivity and CDW properties in monolayer TMDs. The interplay between symmetry breaking, superconductivity and charge density wave in monolayer NbSe2 is unveiled by spectroscopic techniques. A team led by Takafumi Sato at Tohoku University used molecular beam epitaxy to fabricate atomically thin NbSe2 films on bilayer graphene, and investigated their electronic properties by a combination of electrical transport measurements, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy. A charge density wave transition with a strong periodic charge modulation was detected below 1.5 K, accompanied by the emergence of superconductivity. Owing to the inversion symmetry breaking occurring in monolayer NbSe2, a band splitting developed along the Γ-Κ direction of the Brillouin zone. Such band splitting was found to be related to the robustness of the observed superconductivity, as well as the formation of the charge density wave.

We have fabricated bismuth (Bi) ultrathin films on a charge-density-wave (CDW) compound 1T-TaS2 and elucidated electronic states by angle-resolved photoemission spectroscopy and first-principles band-structure calculations. We found that the Bi film on 1T-TaS2 undergoes a structural transition from (111) to (110) upon reducing the film thickness, accompanied by a drastic change in the energy band structure. We also revealed that while two-bilayer-thick Bi(110) film on Si(111) is characterized by a dispersive band touching the Fermi level (EF), the energy band of the same film on 1T-TaS2 exhibits holelike dispersion with a finite energy gap at EF. We discuss the origin of such intriguing differences in terms of the CDW proximity effect.

We report the selective fabrication of monolayer TaSe2 with trigonal-prismatic (1H) or octahedral (1T) crystal structure on bilayer graphene by the molecular-beam-epitaxy method. By angle-resolved photoemission spectroscopy, we found that monolayer 1H-TaSe2 shows a metallic electronic structure with a dispersive band across the Fermi level (EF) similar to the bulk crystal, while monolayer 1T-TaSe2 has a sizable energy gap at EF indicative of the insulating nature in contrast to the metallic nature of bulk 1T-TaSe2. We discuss the origin and implications of differences in the electronic structure among monolayer and bulk TaSe2 by comparing the experimental results with first-principles band-structure calculations as well as previous studies on bulk TaSe2.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on hydrogen-exposed monolayer graphene grown on SiC(0001). In the valence-band region, we observed the π and σ bands of graphene together with the buffer-layer bands, which hybridize with each other at specific momentum points. Upon hydrogen exposure, the ARPES-spectral intensity of buffer layer is reduced and the band hybridization is weakened, suggesting the intercalation of hydrogen atoms between graphene and buffer layer. On further hydrogen exposure, we observed the enhancement of the band gap accompanied with the evolution of in-gap states. These results are interpreted in terms of the partial transformation of C 2pz orbitals into the sp3 hybridized orbitals with covalent bonding between carbon and hydrogen atoms. The present results pave an important path toward feasible application of graphene-based hydrogen-storage devices.

We fabricated monolayer silicene on a thin silver (Ag) crystal film grown on hydrogen-terminated Si(111), and studied the crystal and the electronic structure by low-energy electron diffraction (LEED) and high-resolution angle-resolved photoemission spectroscopy (ARPES). The fabricated sample exhibits the 3 × 3 LEED pattern characteristic of silicene and the band dispersions with the periodicity of the silicene Brillouin zone, indicative of successful fabrication of silicene on Ag/H-Si(111). However, we observed no signature for the Dirac-cone state at the K point, while the observed band structure exhibits an unexpected van-Hove singularity with a saddle point at the binding energy of 1 eV. Such a singularity has not been reproduced in so-far existing band structure calculations, requesting sophisticated calculations incorporating the complicated strong hybridization between silicene and Ag orbitals.

We have performed angle-resolved photoemission spectroscopy on a heterostructure consisting of topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and iron chalcogenide FeTe fabricated on ${\mathrm{SrTiO}}_{3}$ substrate by molecular-beam-epitaxy technique. This system was recently found to show the superconductivity albeit nonsuperconducting nature of each constituent material. Upon interfacing FeTe with two quintuple layers of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, we found that the Dirac-cone surface state of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ 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 ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ through the interface. The present result points to the importance of hole-doped FeTe interface for the occurrence of unconventional superconductivity.

We have performed angle-resolved photoemission spectroscopy on transition-metal dichalcogenide $1T\text{\ensuremath{-}}{\mathrm{HfTe}}_{2}$ to elucidate the evolution of electronic states upon potassium (K) deposition. In pristine ${\mathrm{HfTe}}_{2}$, an in-plane hole pocket and electron pockets are observed at the Brillouin-zone center and corner, respectively, indicating the semimetallic nature of bulk ${\mathrm{HfTe}}_{2}$, with dispersion perpendicular to the plane. In contrast, the band structure of heavily K-dosed ${\mathrm{HfTe}}_{2}$ is obviously different from that of bulk and resembles the band structure calculated for monolayer ${\mathrm{HfTe}}_{2}$. It was also observed that lightly K-dosed ${\mathrm{HfTe}}_{2}$ is characterized by quantized bands originating from bilayer and trilayer ${\mathrm{HfTe}}_{2}$, indicative of staging. The results suggest that the dimensionality crossover from 3D (dimensional) to 2D electronic states due to systematic K intercalation takes place via staging in a single sample. The study provides a strategy for controlling the dimensionality and functionality of novel quantum materials.

We investigated the electronic structure of monolayer ${\mathrm{VTe}}_{2}$ grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer ${\mathrm{VTe}}_{2}$ 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 $1T\text{\ensuremath{-}}{\mathrm{VTe}}_{2}$. We have revealed that monolayer $1T\text{\ensuremath{-}}{\mathrm{VTe}}_{2}$ at low temperatures is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer ${\mathrm{VSe}}_{2}$ which undergoes a charge density wave (CDW) transition to an insulator at low temperatures. The present result suggests an importance of the Fermi surface topology for characterizing the CDW properties of monolayer transition-metal dichalcogenides.

The authors investigate quantum phase transitions in single unit-layer NbSe${}_{2}$. The superconductor--normal state transition caused by the surface-normal magnetic field is found to be intermediated by a quasimetallic state. This behavior is consistent with the ``Bose metal'' picture, in which finite dissipation is caused by the breaking of phase coherence due to the strong gauge-field fluctuation. The results of scaling analyses reveal a complex but essential phase diagram of single unit-layer NbSe${}_{2}$ as a two-dimensional superconductor.

The discovery of high-temperature (high-Tc) superconductivity in one-monolayer FeSe on SrTiO3 has attracted tremendous attention. Subsequent studies suggested the importance of cooperation between intra-FeSe-layer and interfacial interactions to enhance Tc. However, the nature of intra-FeSe-layer interactions, which would play a primary role in determining the pairing symmetry, remains unclear. Here we have performed high-resolution angle-resolved photoemission spectroscopy of one-monolayer and alkaline-metal-deposited multilayer FeSe films on SrTiO3, and determined the evolution of quasiparticle band dispersion across Tc. We found that the band dispersion in the superconducting state deviates from the Bogoliubov-quasiparticle dispersion expected from the normal-state band dispersion with a constant gap size. This suggests highly anisotropic pairing originating from small momentum transfer and/or mass renormalization due to electron&ndash, boson coupling. This band anomaly is interpreted in terms of the electronic interactions within the FeSe layers that may be related to the high-Tc superconductivity in electron-doped FeSe.

We report the direct evidence for superconductivity in Ca-intercalated bilayer graphene C6CaC6, which is regarded as the thinnest limit of Ca-intercalated graphite. We performed the electrical transport measurements with the in situ 4-point-probe method in ultrahigh vacuum under zero- or nonzero-magnetic field for pristine bilayer graphene, Li-intercalated bilayer graphene (C6LiC6) and C6CaC6 fabricated on SiC substrate. We observed that the zero-resistance state occurs in C6CaC6 with the onset temperature (T(c)(onset)) of 4 K, while the T(c)(onset) is gradually decreased upon applying the magnetic field. This directly proves the superconductivity origin of the zero resistance in C6CaC6. On the other hand, both pristine bilayer graphene and C6LiC6 exhibit nonsuperconducting behavior, suggesting the importance of intercalated atoms and its species to drive the superconductivity.

We have fabricated bismuth (Bi) ultrathin films on a charge-density-wave (CDW) compound 1T-TaS

In the one-monolayer FeSe film on SrTiO${}_{3}$, which is a new member of high-temperature superconductor family, there exists two competing proposals on the nature of the ground state in the parent undoped film: strongly correlated insulator (as in copper-oxide superconductors) versus moderately correlated metal (as in iron-based superconductors). Here, the authors provide experimental evidence for the metallic nature by performing $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ angle-resolved photoemission spectroscopy on an undoped one-monolayer FeSe film prepared by careful post-growth annealing. The authors demonstrate the materialization of an ideal two-dimensional Dirac electron system, where only the Dirac-cone band crosses the Fermi level, unlike in the multilayer counterpart.

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., 27 page, 12 figures

The recent discovery of possible high-temperature (T(c)) superconductivity over 65 K in a monolayer FeSe film on SrTiO3 (refs 1-6) triggered a fierce debate on how superconductivity evolves from bulk to film, because bulk FeSe crystal exhibits a T(c) of no higher than 10 K (ref. 7). However, the difficulty in controlling the carrier density and the number of FeSe layers has hindered elucidation of this problem. Here, we demonstrate that deposition of potassium onto FeSe films markedly expands the accessible doping range towards the heavily electron-doped region. Intriguingly, we have succeeded in converting non-superconducting films with various thicknesses into superconductors with T(c) as high as 48 K. We also found a marked increase in the magnitude of the superconducting gap on decreasing the FeSe film thickness, indicating that the interface plays a crucial role in realizing the high-temperature superconductivity. The results presented provide a new strategy to enhance and optimize T(c) in ultrathin films of iron-based superconductors.

We have performed low-energy ultrahigh-resolution photoemission spectroscopy (PES) of A15 compound V3Si with a xenon-plasma discharge lamp to elucidate the bulk superconducting gap. Below the superconducting transition temperature (Tc = 15.9 K), we found a sharp quasiparticle peak at the Fermi level in the PES spectrum. The gap spectrum is well fitted by a single s-wave superconducting-gap function together with a dip structure at ∼30 meV suggestive of a strong electron-phonon coupling. The anomalous in-gap state previously observed in the PES measurement with high-energy photons is absent or negligibly small in the present bulk-sensitive measurement. The present PES result shows that V3Si is a single-gap s-wave superconductor.

One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (T_c). It was found in FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap (DeltaE_{h-e}) between the hole and electron pockets in the normal state. The change in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change in T_c. Furthermore, the strength of nematicity is also found to be characterized by DeltaE_{h-e}. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap., 6 pages, 3 figures

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on trigonal tellurium consisting of helical chains in the crystal. Through the band-structure mapping in the three-dimensional Brillouin zone, we found a definitive evidence for the band splitting originating from the chiral nature of crystal. A direct comparison of the band dispersion between the ARPES results and the first-principles band-structure calculations suggests the presence of Weyl nodes and tiny spin-polarized hole pockets around the H point. The present result opens a pathway toward studying the interplay among crystal symmetry, band structure, and exotic physical properties in chiral crystals., 6 pages, 3 figures

We have performed high-resolution angle-resolved photoemission spectroscopy of oxygen-adsorbed monolayer graphene grown on 6H–SiC(0 0 0 1). We found that the energy gap between the π and π ∗ bands gradually increases with oxygen adsorption to as high as 0.45 eV at the 2000 L oxygen exposure. A systematic shrinkage of the π ∗ electron Fermi surface was also observed. The present result strongly suggests that the oxidization is a useful technique to create and control the band gap in monolayer graphene.

We report three-dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore-size dependence, together with unique angular independence. The free-standing, large-scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.

We performed in-situ electrical transport measurements for bilayer graphene grown on SiC(0 0 0 1) substrate, Li-intercalated bilayer graphene, and after that desorbing Li atoms by heating. Bilayer graphene after desorbing intercalated Li atoms showed a higher resistivity and different behavior in magnetoconductance compared to pristine bilayer graphene. We observed the weak localization of carriers at low temperatures in all the three samples and analyzed the experimental results with the extended Hikami-Larkin-Nagaoka equation to investigate the transport properties. The result shows that the magnetoconductance of pristine bilayer graphene is described by the AB stacking structure model and the phase breaking scattering is dominated by the electron-electron scattering. The intra-valley scattering occurs most frequently probably due to dopants in SiC substrate. However, in Li-desorbed graphene, the magnetoconductance can be described by neither AB nor AA-stacking model, suggesting the coexistence of domains with several different stacking structures.

We have performed high-resolution angle-resolved photoemission spectroscopy study of electron-doped multilayer FeSe films on SrTiO3. We determined the evolution of the electronic structure in a wide temperature range and found temperature-insensitive nature of the Fermi surface in contrast to the strong temperature dependence observed in non-doped FeSe. Such a striking doping dependence would provide a key to understanding the origin of unconventional fermiology and physical properties in FeSe.

Graphene is a single-layer carbon sheet with a honeycomb structure, and bilayer graphene consists of two graphene sheets with AB stacking. In trilayer graphene, the third graphene sheet has two possible stacking sequences, A or C, when it is overlaid on bilayer graphene. It has been theoretically predicted that trilayer graphene exhibits a variety of novel electronic properties with/without a Dirac-cone band, depending on the stacking sequence. In this regard, trilayer graphene has a high potential for widening the capability of graphene-based electronic devices. However, the difficulty of selective fabrication has hindered the progress of research. Here, we report the first success in the selective fabrication of quasi-free-standing trilayer graphene with ABA or ABC stacking grown epitaxially on hydrogen-terminated silicon carbide. Angle-resolved photoemission spectroscopy (ARPES) clearly demonstrated that our trilayer graphene with ABA stacking has a massless Dirac-like band near the Fermi level, while that with ABC stacking shows a parabolic non-Dirac-like band dispersion. Strict control of temperature and pressure during annealing is shown to be crucial for controllably stacking graphene by a team in Japan. Graphene is a single layer of carbon atoms in a hexagonal arrangement. When two graphene layers are stacked, some carbon atoms in the top layer sit above the center of each hexagon of the lower layer. A third layer either sits directly aligned with the bottom layer (ABA stacking) or is again displaced from the previous two (ABC stacking). These two arrangements exhibit very different properties. Takashi Takahashi from Tohoku University and colleagues deterministically created either ABA or ABC trilayer graphene by first annealing bilayer graphene on a hydrogen-terminated silicon carbide substrate. They found that the alignment of the third layer is determined by the temperature and pressure of this anneal. Quasi-free-standing trilayer graphene with ABA or ABC stacking was selectively synthesized on hydrogen-terminated silicon carbide. The electronic structure was investigated by angle-resolved photoemission spectroscopy. While ABA graphene exhibits a massless Dirac-cone-like band at the K point in the Brillouin zone, ABC graphene was found to show a parabolic non-Dirac-like band. The present success in selective fabrication of ABA and ABC graphene would open a pathway toward graphene-based nano electronic devices with variable layer number and stacking sequence such as high-speed transistor and photodetector.

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) to elucidate the nature of quasiparticle dynamics in graphite. We found fairly sharp quasiparticle peak of π band around K ( H ) point in the vicinity of the Fermi level, together with the strong mass renormalization of the band (kink). The imaginary part of electron self-energy (Im Σ ) shows a sudden drop below 0.18 eV, indicative of a strong electron–phonon coupling. The linear energy dependence of Im Σ at higher binding energies provides an evidence for the deviation from the conventional Fermi liquid theory.