Topological surface states of semimetal TaSb.

Topological surface states, protected by the global symmetry of the materials, are the keys to understanding various novel electrical, magnetic, and optical properties. TaSb is a newly discovered topological material with unique transport phenomena, including negative magnetoresistance and resistivity plateau, whose microscopic understanding is yet to be reached. In this study, we investigate the electronic band structure of TaSb using angle-resolved photoemission spectroscopy and density functional theory.

Spin-orbit-splitting-driven nonlinear Hall effect in NbIrTe.

The Berry curvature dipole (BCD) serves as a one of the fundamental contributors to emergence of the nonlinear Hall effect (NLHE). Despite intense interest due to its potential for new technologies reaching beyond the quantum efficiency limit, the interplay between BCD and NLHE has been barely understood yet in the absence of a systematic study on the electronic band structure. Here, we report NLHE realized in NbIrTe that persists above room temperature coupled with a sign change in the Hall conductivity at 150 K.

Controlling structure and interfacial interaction of monolayer TaSe on bilayer graphene.

Tunability of interfacial effects between two-dimensional (2D) crystals is crucial not only for understanding the intrinsic properties of each system, but also for designing electronic devices based on ultra-thin heterostructures. A prerequisite of such heterostructure engineering is the availability of 2D crystals with different degrees of interfacial interactions. In this work, we report a controlled epitaxial growth of monolayer TaSe with different structural phases, 1H and 1 T, on a bilayer graphene (BLG) substrate using molecular beam epitaxy, and its impact on the electronic properties of the heterostructures using angle-resolved photoemission spectroscopy.

Electronic Structure of Above-Room-Temperature van der Waals Ferromagnet FeGaTe.

FeGaTe, a recently discovered van der Waals ferromagnet, demonstrates intrinsic ferromagnetism above room temperature, necessitating a comprehensive investigation of the microscopic origins of its high Curie temperature (). In this study, we reveal the electronic structure of FeGaTe in its ferromagnetic ground state using angle-resolved photoemission spectroscopy and density functional theory calculations. Our results establish a consistent correspondence between the measured band structure and theoretical calculations, underscoring the significant contributions of the Heisenberg exchange interaction () and magnetic anisotropy energy to the development of the high- ferromagnetic ordering in FeGaTe.

Enhanced thermoelectric performance of SnSe by controlled vacancy population.

The thermoelectric performance of SnSe strongly depends on its low-energy electron band structure that provides high density of states in a narrow energy window due to the multi-valley valence band maximum (VBM). Angle-resolved photoemission spectroscopy measurements, in conjunction with first-principles calculations, reveal that the binding energy of the VBM of SnSe is tuned by the population of Sn vacancy, which is determined by the cooling rate during the sample growth. The VBM shift follows precisely the behavior of the thermoelectric power factor, while the effective mass is barely modified upon changing the population of Sn vacancies.

Signatures of the exciton gas phase and its condensation in monolayer 1T-ZrTe.

The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The material realization of EI has been challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the preformed exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking.

A Novel × Superstructure in Epitaxially Grown 1T-TaTe.

The spontaneous formation of electronic orders is a crucial element for understanding complex quantum states and engineering heterostructures in 2D materials. A novel × charge order in few-layer-thick 1T-TaTe transition metal dichalcogenide films grown by molecular beam epitaxy, which has not been realized, is report. The photoemission and scanning probe measurements demonstrate that monolayer 1T-TaTe exhibits a variety of metastable charge density wave orders, including the × superstructure, which can be selectively stabilized by controlling the post-growth annealing temperature.

Large-gap insulating dimer ground state in monolayer IrTe.

Monolayers of two-dimensional van der Waals materials exhibit novel electronic phases distinct from their bulk due to the symmetry breaking and reduced screening in the absence of the interlayer coupling. In this work, we combine angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 × 1 dimer ground state in monolayer 1T-IrTe that has a large band gap in contrast to the metallic bilayer-to-bulk forms of this material. First-principles calculations reveal that phonon and charge instabilities as well as local bond formation collectively enhance and stabilize a charge-ordered ground state.

Tunable Kondo Resonance at a Pristine Two-Dimensional Dirac Semimetal on a Kondo Insulator.

The proximity of two different materials leads to an intricate coupling of quasiparticles so that an unprecedented electronic state is often realized at the interface. Here, we demonstrate a resonance-type many-body ground state in graphene, a nonmagnetic two-dimensional Dirac semimetal, when grown on SmB, a Kondo insulator, via thermal decomposition of fullerene molecules. This ground state is typically observed in three-dimensional magnetic materials with correlated electrons.

Controlling the Magnetic Anisotropy of the van der Waals Ferromagnet FeGeTe through Hole Doping.

Identifying material parameters affecting properties of ferromagnets is key to optimized materials that are better suited for spintronics. Magnetic anisotropy is of particular importance in van der Waals magnets, since it not only influences magnetic and spin transport properties, but also is essential to stabilizing magnetic order in the two-dimensional limit. Here, we report that hole doping effectively modulates the magnetic anisotropy of a van der Waals ferromagnet and explore the physical origin of this effect.

Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides.

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe and WS monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively.

Anisotropic Dirac Fermions in BaMnBi and BaZnBi.

We investigate the electronic structure of BaMnBi and BaZnBi using angle-resolved photoemission spectroscopy and first-principles calculations. Although they share similar structural properties, we show that their electronic structure exhibit dramatic differences. A strong anisotropic Dirac dispersion is revealed in BaMnBi with a decreased asymmetry factor compared with other members of AMnBi (A = alkali earth or rare earth elements) family.

Emergence of Kondo Resonance in Graphene Intercalated with Cerium.

The interaction between a magnetic impurity, such as cerium (Ce) atom, and surrounding electrons has been one of the core problems in understanding many-body interaction in solid and its relation to magnetism. Kondo effect, the formation of a new resonant ground state with quenched magnetic moment, provides a general framework to describe many-body interaction in the presence of magnetic impurity. In this Letter, a combined study of angle-resolved photoemission (ARPES) and dynamic mean-field theory (DMFT) on Ce-intercalated graphene shows that Ce-induced localized states near Fermi energy, E, hybridized with the graphene π-band, exhibit gradual increase in spectral weight upon decreasing temperature.

Persistent Charge-Density-Wave Order in Single-Layer TaSe.

We present the electronic characterization of single-layer 1H-TaSe grown by molecular beam epitaxy using a combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and density functional theory calculations. We demonstrate that 3 × 3 charge-density-wave (CDW) order persists despite distinct changes in the low energy electronic structure highlighted by the reduction in the number of bands crossing the Fermi energy and the corresponding modification of Fermi surface topology. Enhanced spin-orbit coupling and lattice distortion in the single-layer play a crucial role in the formation of CDW order.

Temperature-Dependent Electron-Electron Interaction in Graphene on SrTiO.

The electron band structure of graphene on SrTiO substrate has been investigated as a function of temperature. The high-resolution angle-resolved photoemission study reveals that the spectral width at Fermi energy and the Fermi velocity of graphene on SrTiO are comparable to those of graphene on a BN substrate. Near the charge neutrality, the energy-momentum dispersion of graphene exhibits a strong deviation from the well-known linearity, which is magnified as temperature decreases.

Hole doping, hybridization gaps, and electronic correlation in graphene on a platinum substrate.

The interaction between graphene and substrates provides a viable route to enhance the functionality of both materials. Depending on the nature of electronic interaction at the interface, the electron band structure of graphene is strongly influenced, allowing us to make use of the intrinsic properties of graphene or to design additional functionalities in graphene. Here, we present an angle-resolved photoemission study on the interaction between graphene and a platinum substrate.

Spin-resolved photoemssion study of epitaxially grown MoSe2 and WSe2 thin films.

Few-layer thick MoSe2 and WSe2 possess non-trivial spin textures with sizable spin splitting due to the inversion symmetry breaking embedded in the crystal structure and strong spin-orbit coupling. We report a spin-resolved photoemission study of MoSe2 and WSe2 thin film samples epitaxially grown on a bilayer graphene substrate. We only found spin polarization in the single- and trilayer samples-not in the bilayer sample-mostly along the out-of-plane direction of the sample surface.

Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films.

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2.

Magnetic effects in sulfur-decorated graphene.

The interaction between two different materials can present novel phenomena that are quite different from the physical properties observed when each material stands alone. Strong electronic correlations, such as magnetism and superconductivity, can be produced as the result of enhanced Coulomb interactions between electrons. Two-dimensional materials are powerful candidates to search for the novel phenomena because of the easiness of arranging them and modifying their properties accordingly.

Observation of the intrinsic bandgap behaviour in as-grown epitaxial twisted graphene.

Twisted graphene is of particular interest due to several intriguing characteristics, such as its the Fermi velocity, van Hove singularities and electronic localization. Theoretical studies recently suggested the possible bandgap opening and tuning. Here, we report a novel approach to producing epitaxial twisted graphene on SiC (0001) and the observation of its intrinsic bandgap behaviour.

Ultrafast quenching of electron-boson interaction and superconducting gap in a cuprate superconductor.

Ultrafast spectroscopy is an emerging technique with great promise in the study of quantum materials, as it makes it possible to track similarities and correlations that are not evident near equilibrium. Thus far, however, the way in which these processes modify the electron self-energy--a fundamental quantity describing many-body interactions in a material--has been little discussed. Here we use time- and angle-resolved photoemission to directly measure the ultrafast response of self-energy to near-infrared photoexcitation in high-temperature cuprate superconductor.

Angle-resolved photoemission spectroscopy study on graphene using circularly polarized light.

We have investigated graphene using circularly polarized light via angle-resolved photoemission spectroscopy. We observe that photoelectron intensity rotates around a constant energy contour towards the opposite direction upon changing the chirality of light. Interestingly, the circular dichroism is found to be asymmetric with respect to the Dirac energy, which is not explained by the Berry phase effect (Liu et al 2011 Phys.

Many-body interactions in quasi-freestanding graphene.

The Landau-Fermi liquid picture for quasiparticles assumes that charge carriers are dressed by many-body interactions, forming one of the fundamental theories of solids. Whether this picture still holds for a semimetal such as graphene at the neutrality point, i.e.