Van Hove Singularity and Enhanced Superconductivity in Ca-Intercalated Bilayer Graphene Induced by Confinement Epitaxy.

We demonstrate the impact of high-density calcium introduction into Ca-intercalated bilayer graphene on a SiC substrate, wherein a metallic layer of Ca has been identified at the interface. We have discerned that the additional Ca layer engenders a free-electron-like band, which subsequently hybridizes with a Dirac band, leading to the emergence of a van Hove singularity. Coinciding with this, there is an increase in the critical temperature for superconductivity. These findings allude to the manifestation of Ca-driven confinement epitaxy, augmenting superconductivity through the enhancement of attractive interactions in a pair of electron and hole bands with flat dispersion around the Fermi level.

Observation of plaid-like spin splitting in a noncoplanar antiferromagnet.

Spatial, momentum and energy separation of electronic spins in condensed-matter systems guides the development of new devices in which spin-polarized current is generated and manipulated1-3. Recent attention on a set of previously overlooked symmetry operations in magnetic materials4 leads to the emergence of a new type of spin splitting, enabling giant and momentum-dependent spin polarization of energy bands on selected antiferromagnets5-10. Despite the ever-growing theoretical predictions, the direct spectroscopic proof of such spin splitting is still lacking. Here we provide solid spectroscopic and computational evidence for the existence of such materials. In the noncoplanar antiferromagnet manganese ditelluride (MnTe2), the in-plane components of spin are found to be antisymmetric about the high-symmetry planes of the Brillouin zone, comprising a plaid-like spin texture in the antiferromagnetic (AFM) ground state. Such an unconventional spin pattern, further found to diminish at the high-temperature paramagnetic state, originates from the intrinsic AFM order instead of spin-orbit coupling (SOC). Our finding demonstrates a new type of quadratic spin texture induced by time-reversal breaking, placing AFM spintronics on a firm basis and paving the way for studying exotic quantum phenomena in related materials.

Two-dimensional heavy fermion in a monoatomic-layer Kondo lattice YbCu2.

The Kondo effect between localized f-electrons and conductive carriers leads to exotic physical phenomena. Among them, heavy-fermion (HF) systems, in which massive effective carriers appear due to the Kondo effect, have fascinated many researchers. Dimensionality is also an important characteristic of the HF system, especially because it is strongly related to quantum criticality. However, the realization of the perfect two-dimensional (2D) HF materials is still a challenging topic. Here, we report the surface electronic structure of the monoatomic-layer Kondo lattice YbCu2 on a Cu(111) surface observed by synchrotron-based angle-resolved photoemission spectroscopy. The 2D conducting band and the Yb 4f state, located very close to the Fermi level, are observed. These bands are hybridized at low-temperature, forming the 2D HF state, with an evaluated coherence temperature of about 30 K. The effective mass of the 2D state is enhanced by a factor of 100 by the development of the HF state. Furthermore, clear evidence of the hybridization gap formation in the temperature dependence of the Kondo-resonance peak has been observed below the coherence temperature. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions.

Semiconducting Electronic Structure of the Ferromagnetic Spinel HgCr_{2}Se_{4} Revealed by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy.

We study the electronic structure of the ferromagnetic spinel HgCr_{2}Se_{4} by soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) and first-principles calculations. While a theoretical study has predicted that this material is a magnetic Weyl semimetal, SX-ARPES measurements give direct evidence for a semiconducting state in the ferromagnetic phase. Band calculations based on the density functional theory with hybrid functionals reproduce the experimentally determined band gap value, and the calculated band dispersion matches well with ARPES experiments. We conclude that the theoretical prediction of a Weyl semimetal state in HgCr_{2}Se_{4} underestimates the band gap, and this material is a ferromagnetic semiconductor.

Stacking of a Cofacially Stacked Iron Phthalocyanine Dimer on Graphite Achieved High Catalytic CH4 Oxidation Activity Comparable to That of pMMO.

Numerous biomimetic molecular catalysts inspired by methane monooxygenases (MMOs) that utilize iron or copper-oxo species as key intermediates have been developed. However, the catalytic methane oxidation activities of biomimetic molecule-based catalysts are still much lower than those of MMOs. Herein, we report that the close stacking of a µ-nitrido-bridged iron phthalocyanine dimer onto a graphite surface is effective in achieving high catalytic methane oxidation activity. The activity is almost 50 times higher than that of other potent molecule-based methane oxidation catalysts and comparable to those of certain MMOs, in an aqueous solution containing H2O2. It was demonstrated that the graphite-supported µ-nitrido-bridged iron phthalocyanine dimer oxidized methane, even at room temperature. Electrochemical investigation and density functional theory calculations suggested that the stacking of the catalyst onto graphite induced partial charge transfer from the reactive oxo species of the µ-nitrido-bridged iron phthalocyanine dimer and significantly lowered the singly occupied molecular orbital level, thereby facilitating electron transfer from methane to the catalyst in the proton-coupled electron-transfer process. The cofacially stacked structure is advantageous for stable adhesion of the catalyst molecule on the graphite surface in the oxidative reaction condition and for preventing decreases in the oxo-basicity and generation rate of the terminal iron-oxo species. We also demonstrated that the graphite-supported catalyst exhibited appreciably enhanced activity under photoirradiation owing to the photothermal effect.

Direct evaluation of hole effective mass of SnS-SnSe solid solutions with ARPES measurement.

Valence band dispersions of single-crystalline SnS1-xSex solid solutions were observed by angle-resolved photoemission spectroscopy (ARPES). The hole effective masses, crucial factors in determining thermoelectric properties, were directly evaluated. They decrease slightly with increasing Se content in the low Se composition range but sharply in the high Se composition range.

Dirac semimetal phase and switching of band inversion in XMg2Bi2 (X = Ba and Sr).

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.

Acceptance-cone-tunable electron spectrometer for highly-efficient constant energy mapping.

We have developed an acceptance-cone-tunable (ACT) electron spectrometer for the highly efficient constant-energy photoelectron mapping of functional materials. The ACT spectrometer consists of the hemispherical deflection analyzer with the mesh-type electrostatic lens near the sample. The photoelectron trajectory can be converged by applying a negative bias to the sample and grounding the mesh lens and the analyzer entrance. The performance of the present ACT spectrometer with neither rotating nor tilting of the sample is demonstrated by the wide-angle observation of the well-known π-band dispersion of a single crystalline graphite over the Brillouin zone. The acceptance cone of the spectrometer is expanded by a factor of 3.30 when the negative bias voltage is 10 times as high as the kinetic energy of photoelectrons.

Non-trivial surface states of samarium hexaboride at the (111) surface.

The peculiar metallic electronic states observed in the Kondo insulator, samarium hexaboride (SmB6), has stimulated considerable attention among those studying non-trivial electronic phenomena. However, experimental studies of these states have led to controversial conclusions mainly due to the difficulty and inhomogeneity of the SmB6 crystal surface. Here, we show the detailed electronic structure of SmB6 with angle-resolved photoelectron spectroscopy measurements of the three-fold (111) surface where only two inequivalent time-reversal-invariant momenta (TRIM) exist. We observe the metallic two-dimensional state was dispersed across the bulk Kondo gap. Its helical in-plane spin polarisation around the surface TRIM indicates that SmB6 is topologically non-trivial, according to the topological classification theory for weakly correlated systems. Based on these results, we propose a simple picture of the controversial topological classification of SmB6.

Electronic-dimensionality reduction of bulk MoS2 by hydrogen treatment.

A reduction in the electronic-dimensionality of materials is one method for achieving improvements in material properties. Here, a reduction in electronic-dimensionality is demonstrated using a simple hydrogen treatment technique. Quantum well states from hydrogen-treated bulk 2H-MoS2 are observed using angle resolved photoemission spectroscopy (ARPES). The electronic states are confined within a few MoS2 layers after the hydrogen treatment. A significant reduction in the band-gap can also be achieved after the hydrogen treatment, and both phenomena can be explained by the formation of sulfur vacancies generated by the chemical reaction between sulfur and hydrogen.

Large-Gap Magnetic Topological Heterostructure Formed by Subsurface Incorporation of a Ferromagnetic Layer.

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future "topotronics" devices.

Single-Crystal Pentacene Valence-Band Dispersion and Its Temperature Dependence.

The electronic structures of the highest occupied molecular orbital (HOMO) or the HOMO-derived valence bands dominate the transport nature of positive charge carriers (holes) in organic semiconductors. In the present study, the valence-band structures of single-crystal pentacene and the temperature dependence of their energy-momentum dispersion relations are successfully demonstrated using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). For the shallowest valence band, the intermolecular transfer integral and effective mass of the holes are evaluated as 43.1 meV and 3.43 times the electron rest mass, respectively, at room temperature along the crystallographic direction for which the widest energy dispersion is expected. The temperature dependence of the ARUPS results reveals that the transfer integral values (hole effective mass) are enhanced (reduced) by ∼20% on cooling the sample to 110 K.

Surface Kondo effect and non-trivial metallic state of the Kondo insulator YbB12.

A synergistic effect between strong electron correlation and spin-orbit interaction has been theoretically predicted to realize new topological states of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001) using angle-resolved photoelectron spectroscopy. The SS shows temperature-dependent reconstruction corresponding to the Kondo effect observed for bulk states. Despite the low-temperature insulating bulk, the reconstructed SS with c-f hybridization is metallic, forming a closed Fermi contour surrounding on the surface Brillouin zone and agreeing with the theoretically expected behaviour for SS on TKIs. These results demonstrate the temperature-dependent holistic reconstruction of two-dimensional states localized on KIs surface driven by the Kondo effect.

Direct spectroscopic evidence for phase competition between the pseudogap and superconductivity in Bi2Sr2CaCu2O(8+δ).

In the high-temperature (T(c)) cuprate superconductors, a growing body of evidence suggests that the pseudogap phase, existing below the pseudogap temperature T*, is characterized by some broken electronic symmetries distinct from those associated with superconductivity. In particular, recent scattering experiments have suggested that charge ordering competes with superconductivity. However, no direct link of an interplay between the two phases has been identified from the important low-energy excitations. Here, we report an antagonistic singularity at T(c) in the spectral weight of Bi2Sr2CaCu2O(8+δ) as compelling evidence for phase competition, which persists up to a high hole concentration p ~ 0.22. Comparison with theoretical calculations confirms that the singularity is a signature of competition between the order parameters for the pseudogap and superconductivity. The observation of the spectroscopic singularity at finite temperatures over a wide doping range provides new insights into the nature of the competitive interplay between the two orders and the complex phase diagram near the pseudogap critical point.

Surface Tomonaga-Luttinger-Liquid State on Bi/InSb(001).

A 1D metallic surface state was created on an anisotropic InSb(001) surface covered with Bi. Angle-resolved photoelectron spectroscopy (ARPES) showed a 1D Fermi contour with almost no 2D distortion. Close to the Fermi level (E_{F}), the angle-integrated photoelectron spectra showed power-law scaling with the binding energy and temperature. The ARPES plot above E_{F}, obtained thanks to a thermally broadened Fermi edge at room temperature, showed a 1D state with continuous metallic dispersion across E_{F} and power-law intensity suppression around E_{F}. These results strongly suggest a Tomonaga-Luttinger liquid on the Bi/InSb(001) surface.

Persistence of the superconducting condensate far above the critical temperature of YBa2(Cu,Zn)3O(y) revealed by c-axis optical conductivity measurements for several Zn concentrations and carrier doping levels.

The superconductivity precursor phenomena in high temperature cuprate superconductors is studied by direct measurements of the superconducting condensate with the use of the c-axis optical conductivity of YBa2(Cu1-xZnx)3Oy for several doping levels (p) as well as for several Zn concentrations. Both the real and imaginary parts of the optical conductivity clearly show that the superconducting carriers persist up to the high temperatures Tp that is higher than the critical temperature Tc but lower than the pseudogap temperature T*. Tp increases with reducing doping level like T*, but decreases with Zn substitution unlike T*.

Distinct Fermi-momentum-dependent energy gaps in deeply underdoped Bi2212.

We used angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2Ca(1-x)YxCu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increased with underdoping, a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near-nodal regime where a coherent peak in the spectrum can be observed, did not increase with less doping, a behavior not previously observed in the single particle spectra. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high-transition temperature superconductivity.