Angle-resolved Photoemission Spectroscopy Research Articles

Magnetic interactions at the surface of topological insulators provide a versatile route to engineer exotic quantum states. Breaking time-reversal symmetry (TRS) at the topological surface state (TSS) enables the opening of a Dirac gap, which is essential for realizing quantum anomalous Hall physics. This work investigates the impact of submonolayer deposition of magnetic rare-earth adatoms on the prototypical topological insulator Bi2Te3. Scanning tunneling microscopy (STM) supported by first-principle calculations, core-level photoemission spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), X-ray magnetic circular dichroism (XMCD) and quasiparticle interference (QPI) mapping are combined to reveal direct evidence of local interactions between erbium (Er) atoms and the substrate, leading to significant modifications of the TSS. XMCD measurements confirm the out-of-plane magnetic anisotropy for Er adatoms on Bi2Te3 , which induces a warping transition of the Fermi surface from a snowflake to a star-of-David-like geometry, along with a Dirac point gap opening and spectral splitting near the Γ point. QPI maps confirm the reconstructed surface band topology through modified scattering patterns consistent with TRS breaking. Our results identify a microscopic mechanism for magnetic interaction at the surface of a topological insulator and establish magnetic rare-earth doping as an effective strategy to tailor topological electronic states with atomic-scalecontrol.

van der Waals (vdW) heterostructures, which combine bidimensional materials of different properties, enable a range of quantum phenomena. Herein, we present a comparative study between the electronic properties of mono- and bilayer of platinum diselenide (PtSe2) grown on hexagonal boron nitride (h-BN) and graphene substrates using molecular beam epitaxy (MBE). Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), the electronic structure of PtSe2/graphene and PtSe2/h-BN vdW heterostructures is investigated in a systematic manner. In contrast to PtSe2/h-BN, the electronic structure of PtSe2/graphene reveals the presence of interlayer hybridization between PtSe2 and graphene, which is evidenced by minigap openings in the π-band of graphene. Furthermore, our measurements show that the valence band maximum (VBM) of monolayer PtSe2 is located at the Γ point with different binding energies of about -0.9 and -0.55 eV relative to the Fermi level on h-BN and graphene and substrates, respectively. Our results represent a significant advance in the understanding of electronic hybridization between TMDs and different substrates, and they reaffirm the crucial role of the substrate in any nanoelectronic applications based on van der Waals heterostructures.

We report on the electronic properties of a van der Waals heterostructure formed by graphene and a type-II Weyl semimetal (Td-WTe2), hosting features of both Dirac and Weyl semimetals. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunnelling microscopy and spectroscopy (STM/STS), and density functional theory (DFT) calculations, we directly visualise how the interlayer interaction modifies the electronic structures of both materials in the heterostructure. Furthermore, STM reveals that the local twist angle between the graphene and the Weyl semimetal affects the appearance of moiré patterns in the heterostructure, which in turn suggests a twist-angle-dependent modulation of the interlayer interaction. Finally, we also show XPS 4d lines splitting for Te related to the local environment of tellurium atoms. Our findings highlight the importance of controlling twist angles and interlayer interactions for electronics and twistronics applications based on novel quantum materials.

The Ultrafast Transient Experimental Facility (UTEF) at Chongqing University is constructing a next-generation angle-resolved photoemission spectroscopy (ARPES) beamline designed to simultaneously achieve sub-meV energy resolution, continuous photon energy tunability (10-40 eV), high photon flux (>1012 photons s-1 at the sample position), full polarization control, and ultra-low-temperature sample environments (<1.5 K). Leveraging the unique advantages of UTEF's low-energy (0.5 GeV), high-beam-current (500-1000 mA) storage ring, the beamline is capable of generating high-flux EUV radiation, enabling detailed exploration of complex quantum materials. The beamline employs two high-groove-density gratings and one low-groove-density grating to achieve, respectively, an energy resolving power exceeding 100000 and photon fluxes greater than 5×1013 photons s-1. A dual-endstation layout enables flexible operation for both ultra-high-resolution measurements at ultra-low temperatures and large-angle, high-flux spin-resolved experiments. Through comprehensive optical optimization-including mirror coatings, customized grating groove profiles and precision focusing geometry-the system can deliver photon flux exceeding 1014 photons s-1 (0.1% bandwidth)-1 with a spatial beam spot size of approximately 30 µm, while maintaining sub-0.4 meV energy resolution. This work presents the optical design and projected performance of the UTEF ARPES beamline.

We report the development of an instrument combining an ultrafast, high-repetition-rate, polarization-tunable monochromatic extreme ultraviolet (XUV, 21.6eV) beamline and a next-generation momentum microscope end station. This setup enables time- and angle-resolved photoemission spectroscopy of quantum materials, offering multimodal photoemission dichroism capabilities. The momentum microscope simultaneously detects the full surface Brillouin zone over an extended binding energy range. It is equipped with advanced electron optics, including a new type of front lens that supports multiple operational modes. Enhanced spatial resolution is achieved by combining the small XUV beam footprint (33 × 45μm2) with the selection of small regions of interest using apertures positioned in the Gaussian plane of the momentum microscope. This instrument achieves an energy resolution of 44meV and a temporal resolution of 144fs. We demonstrate the capability to perform linear, Fourier, and circular dichroism in photoelectron angular distributions from photoexcited 2D materials. This functionality paves the way for time-, energy-, and momentum-resolved investigations of orbital and quantum geometrical properties underlying the electronic structures of quantum materials driven out of equilibrium.

Abstract Palladium diselenide (PdSe 2 ) is a two-dimensional transition metal dichalcogenide van der Waals material that exhibits a unique wrinkled pentagonal structure that results in strong anisotropic layer-dependent properties. Owing to these unique properties the material has a great potential for applications in electronic, optoelectronic, photonic and thermoelectric devices. Here, we study the electronic properties of monolayer PdSe 2 in a Schottky junction by contacting the material with a tungsten Scanning Tunnelling Microscope (STM) tip. The STM-induced lifting of the 2D layer leads to structural warping of the top layer, which causes significant changes in the electronic properties of PdSe 2 as well as the electrostatic potential across the junction. We compare the STM-determined work function of the warped PdSe 2 monolayer with the work function of bulk PdSe 2 as obtained by Kelvin probe force microscopy (KPFM) and electronic structure obtained by synchrotron-based angle-resolved photoemission spectroscopy (ARPES), and X-ray photoelectron spectroscopy (XPS).

Angle-resolved photoemission spectroscopy is the leading tool for studying the symmetry and structure of the order parameter in superconductors. The recent improvement of the technique made it possible to detect the superconducting energy gap at the surface of topological t- PtBi 2 via observation of the record-breaking narrow line shapes. The promising new physics uncovered requires further investigation of the spectral and gap functions of t- PtBi 2 , but the challenging experimental conditions severely limit the application of conventional ARPES setups. In this work, we use synchrotron-based measurements and show that the gap at the surface Fermi arc in t- PtBi 2 can be detected even with more relaxed experimental conditions than in our previous laser-based studies. At the same time, using simple model of ARPES spectra, we identify the minimum requirements to detect the gap and consider cases where the gap cannot be resolved.

We performed angle-resolved photoemission spectroscopy studies on the triple-layer Bi2Sr2Ca2Cu3O10+δ over a wide doping range. Although the doping level of the inner CuO2 plane is extremely low in underdoped samples, the d-wave SC gap is enhanced to the unprecedentedly large value of Δ0 ~ 80–100 meV at the antinode. This gap persists well above Tc without a Fermi arc, indicating a “nodal metal”. We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO2 plane, sandwiched by nearly optimally-doped two outer planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks show electron-hole symmetry, suggesting d-wave pairing fluctuations. Thus the proximity effect on the innermost CuO2 plane is the strongest in the triple-layer cuprates, which explains why the Tc reaches the maximum at the layer number of three in every multi-layer cuprate family.

Exciton lifetimes play a critical role in the performance of organic optoelectronic devices. In this work, we investigate how the presence of multiple rotational domains and therefore grain boundaries impacts exciton dynamics in thin films of C60/Au(111) using time- and angle-resolved photoemission spectroscopy (TR-ARPES). We find that films with multiple rotational domains exhibit shorter exciton lifetimes and higher susceptibility to exciton-exciton annihilation, even when one domain dominates. Scanning tunneling microscopy (STM) measurements reveal electronic structure changes resulting from a locally reduced dielectric constant at grain boundaries, suggesting a mechanism for lifetime reduction through exciton funneling and other additional decay channels. These findings highlight the critical role of film quality in determining intrinsic exciton lifetimes and show that minuscule amounts of disorder that are nearly undetectable by ensemble measurements can significantly impact dynamics. These results imply that precise structural control is essential for optimizing the performance of organic optoelectronic devices.

Altermagnets represent a novel class of quantum magnets that emerge from specific crystal symmetry operations, particularly rotations or mirror reflections, exhibiting unique momentum-dependent spin-split band structures within the antiferromagnetic regime. The spin textures of altermagnets are fundamentally governed by spin group symmetry, offering unique opportunities for magnetic property control. However, experimentally tuning their spin-splitting via symmetry engineering remains a key challenge. Here, interfacial strain in 10nm CrSb thin films by molecular beam epitaxy (MBE) is engineered to systematically modify its crystal symmetry. Remarkably, high-resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) measurements revealed a transition to a spin-degenerate ground state throughout the entire Brillouin zone, quantitatively demonstrating strain control over altermagnetic spin splitting. Although bulk altermagnetic theory anticipates spin-splitting, thin films exhibit spin degeneracy instead, which this first-principles calculations trace to interfacial strain-mediated surface magnetic state stabilization. In CrSb thin films, strain-induced magnetic reconstruction preserves PT symmetry (spatial inversion followed by time reversal), thereby enforces spin degeneracy across the Brillouin zone, as evidenced by the quantitative agreement between calculated spin/electronic structures and spin-ARPES measurements. This strategy establishes a new paradigm for tailoring altermagnetic spin splitting through engineered symmetry, addressing a critical gap in spectroscopic investigations of altermagnetic systems.

Distinct Amplitude Mode Dynamics Upon Resonant and Off-resonant Excitation Across the Charge Density Wave Energy Gap in LaTe₃ Investigated by Time- and Angle-resolved Photoemission Spectroscopy

Phase change materials (PCMs) are well-known for their reversible and rapid switching between crystalline and amorphous phases through thermal excitations mediated by strong electrical or laser pulses. This crystal-to-amorphous transition is accompanied by a remarkable contrast in optical and electronic properties, making PCMs useful in nonvolatile data storage applications. Here, we combine electrical transport and angle resolved photoemission spectroscopy (ARPES) measurements to study the electronic structure of bulk Ge2Sb2Te5−5xSe5x (GSST) for 0≤x≤0.8, where x represents the amount of Se substituting Te in Ge2Sb2Te5—a prototypical PCM. The single-particle density of states (SDOS) derived from the integrated ARPES data display metallic behavior for all x, as evidenced by the presence of a finite density of states in the vicinity of the chemical potential. Transport measurements also display clear signatures of metallic transport, consistent with the SDOS data. The temperature dependence of the resistance indicates the onset of moderate electron–electron Coulomb interaction effects at low temperatures for x≥0.6. At the same time, the magnetoresistance data show signatures of weak antilocalization for x≥0.6. An analysis of the temperature dependence of the phase coherence length suggests that electron dephasing is primarily due to inelastic electron–electron scattering. We find that these effects are enhanced with increasing x, portraying GSST as a PCM with electronic interactions that can be tuned via chemical doping.

We present the experimental realization of Dirac fermions with a π-Berry phase in PbBi6Te10, a 3D topological insulator, as observed through Shubnikov–de Haas (SdH) oscillations in electronic transport measurements. These oscillations highlight the non-trivial nature of the material's surface states. The carriers exhibit a relativistic character, with a Fermi velocity of ∼105ms−1. Their high mobility μSdH (∼ 10 × μe) and low effective mass confirm the surface origin of the topological surface states. Hall resistivity measurements reveal that PbBi6Te10 is electron-dominated. Angle-resolved photoemission spectroscopy (ARPES) displays the hexagonal warping effect near the Fermi energy. Angle-dependent magnetoresistance follows the sin2(θ) fitting, indicating the anisotropy in the Fermi surface, consistent with the ARPES measurements. These results provide a potential platform for device applications of PbBi6Te10 due to the carriers' high mobility and low effective mass.

Dirac Fermions onthe surface of the topological insulator arespin-momentum locked and topologically protected, making them interestingfor spintronics and quantum computing applications. When in proximityto magnetism and superconductivity, these electronic states couldresult in quantum anomalous Hall effect and Majorana Fermions, respectively.An even more dramatic enrichment of the topological insulators’physics is expected for moiré superlattices, where, analogouslyto the twisted graphene layers, electronic correlations could be stronglyenhanced, a task previously notoriously difficult to achieve in topologicalmatter. Until now, the experimental confirmation of such moiréproperties has remained elusive. Here, we grow the two-dimensionalvan der Waals magnetic insulators FeX2 (where X = Cl orBr) on top of the topological insulator Bi2Se3 and establish a moiré superlattice formation at the interface.By means of scanning tunneling microscopy and angle-resolved photoemissionspectroscopy, we investigate the electronic properties of the formedmoiré superlattice and demonstrate its tunability via the filmchoice. We reveal replicated Dirac cones and focus on their intersections,which, in the case of FeBr2/Bi2Se3, occur below the Fermi level. We identify the signatures of smallgaps at the intersections around the M̅i points that we attribute to the moiré interaction.These findings point to the specific type of magnetic moirépotential that breaks the time-reversal symmetry at these points butnot at the Γ̅ point. Our observations provide an intriguingscenario of correlated topological phases induced by moirésuperlattice that may result in topological superconductivity, highChern number phases, and exotic noncollinear magnetic textures.

For several decades, it was widely believed that a noninteracting disordered electronic system could only undergo an Anderson metal-insulator transition due to Anderson localization. However, numerous recent theoretical works have predicted the existence of a disorder-driven non-Anderson phase transition that differs from Anderson localization. The frustration lies in the fact that this non-Anderson disorder-driven transition has not yet been experimentally demonstrated in any system. Here, using angle-resolved photoemission spectroscopy, we present a case study of observing the non-Anderson disorder-driven transition by visualizing the electronic structure of the Weyl semimetal NdAlSi on surfaces with varying amounts of disorder. Our observations reveal that strong disorder can effectively suppress all surface states in the Weyl semimetal NdAlSi, including the topological surface Fermi arcs. This disappearance of surface Fermi arcs is associated with the vanishing of the topological invariant, indicating a quantum phase transition from a Weyl semimetal to a diffusive metal. These observations provide direct experimental evidence of the non-Anderson disorder-driven transition occurring in real quantum systems, a finding long anticipated by theoretical physicists.

Kagome materials serve as a versatile platform where an interplay of flat bands, Dirac Fermions, and Van Hove singularities enables the emergence of exotic strongly correlated phenomena. Recently, it was predicted that an ideal single layer kagome lattice may host high-order Van Hove singularities (HOVHSs) characterized by extremely flat dispersions, leading to drastic changes in electronic behavior. However, experimentally, HOVHSs have been observed up to now only in a narrow range of materials, mostly in graphene layers, but not in metal-semiconductor interfaces. Here, we report the discovery of HOVHSs in the monolayer-thick kagome metal LaTl3 epitaxially synthesized on the Si(111) substrate. The scanning tunneling microscopy observations and ab initio calculations indicate the kagome-like ordering of the LaTl3 layer, while the angle-resolved photoemission spectroscopy measurements and theoretical predictions uncover a rich and complex landscape of various Van Hove singularities emerged in the system, including high-order ones, which can significantly affect the anomalous Hall response and enable the unique high electron-correlation regime in the system. The discovered properties make the LaTl3 kagome monolayer a highly attractive material for ultracompact nanoelectronic devices.

Moiré superlattices, engineered through precise stacking of van der Waals (vdW) layers, hold immense promise for exploring strongly correlated and topological phenomena. However, these applications have been held back by the common preparation method: tear-and-stack of Scotch tape exfoliated monolayers, which suffer from low efficiency and reproducibility, twist angle inhomogeneity, interfacial contamination, and micrometer sizes. Here, we report an effective strategy to construct highly consistent mixed-dimensional and twisted bilayer vdW moiré structures with high production throughput, near-unity yield, pristine interfaces, precisely controlled twist angles, and macroscopic scale (up to centimeters) with enhanced thermal stability. We demonstrate the versatility across various vdW materials, including transition metal dichalcogenides, graphene, and hBN. The expansive size and high quality of moiré structures enable reciprocal-space high-resolution mapping of the superlattices and back-folded moiré mini band structures with low energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES). In particular, we identify the backfolded bands at the K point of twisted transition metal dichalcogenide moiré structures. This technique will have broad applications in both fundamental studies and the mass production of twistronic devices.

Understanding the spatial variation of the conduction band in semiconductor materials is essential for unraveling charge transport phenomena, especially at interfaces, defects, and in nanoscale devices. However, direct experimental access to the conduction band with the required energy and spatial resolution has remained a major challenge. Here we demonstrate that off-axis electron holography enables direct measurements of the conduction band edge in semiconductor systems with nm-scale spatial resolution. We are able to map conduction band profiles across semiconductor heterostructures, revealing band bending, band offsets, and interface dipoles with unprecedented clarity. This method provides access to the local electronic landscape in real space, without relying on indirect modeling or assumptions about doping profiles. Applied to an InP/In0.53Ga0.47As high-speed double heterojunction bipolar transistor for the next generation of telecommunications, our approach uncovers previously inaccessible details of band alignment, opening new avenues for exploring the electronic structure in functional nanostructures.

Abstract Uranium, the heaviest natural element, exhibits rich and complex physical behavior, including three types of charge density wave transitions and superconductivity at low temperatures. It is widely believed that these phenomena are closely linked to the properties of 5f electrons, which are highly susceptible to external perturbations. To elucidate the detailed electronic structure, particularly the 5f-electron signatures, we fabricated high-quality single-crystal uranium films on W (110) substrates using molecular beam epitaxy and investigated their fine electronic properties and temperature-dependent evolution by angle-resolved photoemission spectroscopy (ARPES). Our experiments reveal three electron pockets around the Γ point and direct hybridization between 5f electrons and conduction electrons at regions distant from Γ. The Kondo temperature extracted from ARPES and electrical resistance measurements is approximately 131 K, indicating that the 5f electrons transition from a high-temperature localized state to a low-temperature itinerant state in the thin film system. Additionally, we observe another flat band with an energy scale of 148 meV. The detailed electronic structure and direct evidence of the localized-to-itinerant transition of 5f electrons provided in this study advance the understanding of strong electronic correlations in uranium-based materials.

A Berry phase of odd multiples of π inferred from quantum oscillations (QOs) has often been treated as evidence for nontrivial reciprocal space topology. However, disentangling the Berry phase values from the Zeeman effect and the orbital magnetic moment is often challenging. In centrosymmetric compounds, the case is simpler as the orbital magnetic moment contribution is negligible. Although the Zeeman effect can be significant, it is usually overlooked in most studies of QOs in centrosymmetric compounds. Here, we present a detailed study on the nonmagnetic centrosymmetric SrGa2 and BaGa2, which are predicted to be Dirac nodal line semimetals based on density functional theory (DFT) calculations. Evidence of the nontrivial topology is found in magnetotransport measurements. The Fermi surface topology and band structure are carefully studied through a combination of angle-dependent QOs, angle-resolved photoemission spectroscopy (ARPES), and DFT calculations, where the nodal line is observed in the vicinity of the Fermi level. Strong de Haas–van Alphen fundamental oscillations associated with higher harmonics are observed in both compounds, which are well fitted by the Lifshitz-Kosevich (LK) formula. However, even with the inclusion of higher harmonics in the fitting, we found that the Berry phases cannot be unambiguously determined when the Zeeman effect is included. We revisit the LK formula and analyze the phenomena and outcomes that were associated with the Zeeman effect in previous studies. Our experimental results confirm that SrGa2 and BaGa2 are Dirac nodal line semimetals. Additionally, we highlight the often overlooked role of spin-damping terms in Berry phase analysis.