Novel Quantum States of Exciton-Floquet Composites: Electron-Hole Entanglement and Information.

Coulomb exchange between distinct electron-hole modes, i.e., exciton and Floquet states, in two-dimensional semiconductors is explored. Coherent ultrafast mixing of the exciton and Floquet states under weak optical pumping is investigated through a theoretical description of time-resolved and angle-resolved photoemission spectroscopy (tr-ARPES) in an extended Haldane model that includes the electron-hole Coulomb interaction. Two branches of novel quantum states are found in the form of bosonic exciton-Floquet composites, which result from exchange coupling due to the Coulomb interaction. Furthermore, tr-ARPES could be directly employed for the density matrix element of the biparticle subsystem of photoelectron and hole, and electron-hole entanglement and information could be further explored. This finding suggests a unique platform to study the buildup and dephasing of novel exciton-Floquet composites and to resolve the information carried by them, which would enable the pursuit of new reconfigurable devices based on two-dimensional semiconductors.

Observation of Spin Splitting in Room-Temperature Metallic Antiferromagnet CrSb.

Recently, unconventional antiferromagnets that enable the spin splitting (SS) of electronic states have been theoretically proposed and experimentally realized, where the magnetic sublattices containing moments pointing at different directions are connected by a novel set of symmetries. Such SS is substantial, k-dependent, and independent of the spin-orbit coupling (SOC) strength, making these magnets promising materials for antiferromagnetic spintronics. Here, combined with angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, a systematic study on CrSb, a metallic spin-split antiferromagnet candidate with Néel temperature TN = 703 K, is conducted. The data reveal the electronic structure of CrSb along both out-of-plane and in-plane momentum directions, rendering an anisotropic k-dependent SS that agrees well with the calculational results. The magnitude of such SS reaches up to at least 0.8 eV at non-high-symmetry momentum points, which is significantly higher than the largest known SOC-induced SS. This compound expands the choice of materials in the field of antiferromagnetic spintronics and is likely to stimulate subsequent investigations of high-efficiency spintronic devices that are functional at room temperature.

Magnetotransport and angle-resolved photoemission spectroscopy of MnSb12Te19: a new member of MnSb2nTe3n+1family.

The quest for intrinsically ferromagnetic topological materials is a focal point in the study of topological phases of matter, as intrinsic ferromagnetism plays a vital role in realizing exotic properties such as the anomalous Hall effect (AHE) in quasi-two-dimensional materials, and this stands out as one of the most pressing concerns within the field. Here, we investigate a novel higher order member of the MnSb2nTe3n+1family, MnSb12Te19, for the first time combining magnetotransport and angle-resolved photoemission spectroscopy (ARPES) measurements. Our magnetic susceptibility experiments identify ferromagnetic transitions at temperatureTc= 18.7 K, consistent with our heat capacity measurements (T= 18.8 K). The AHE is observed for the field along thec-axis belowTc. Our study of Shubinikov-de-Haas oscillations provides evidence for Dirac fermions withπBerry phase. Our comprehensive investigation reveals that MnSb12Te19exhibits a FM ground state along with AHE, and hole-dominated transport properties consistent with ARPES measurements.

Coupling Between Electrons and Charge Density Wave Fluctuation and its Possible Role in Superconductivity.

In most charge density wave (CDW) systems of different material classes, ranging from traditional correlated systems in low-dimension to recent topological systems with Kagome lattice, superconductivity emerges when the system is driven toward the quantum critical point (QCP) of CDW via external parameters of doping and pressure. Despite this rather universal trend, the essential hinge between CDW and superconductivity has not been established yet. Here, the evidence of coupling between electron and CDW fluctuation is reported, based on a temperature- and intercalation-dependent kink in the angle-resolved photoemission spectra of 2H-PdxTaSe2. Kinks are observed only when the system is in the CDW phase, regardless of whether a long- or short-range order is established. Notably, the coupling strength is enhanced upon long-range CDW suppression, albeit the coupling energy scale is reduced. Interestingly, the estimation of the superconducting critical temperature by incorporating the observed coupling characteristics into McMillan's equation yields results closely resembling the known values of the superconducting dome. The results thus highlight a compelling possibility that this new coupling mediates Cooper pairs, which provides new insights into the competing relationship not only for CDW but also for other competing orders.

Evolution of the Fermi Surface of 1T-VSe2 across a Structural Phase Transition.

Periodic lattice distortion, known as the charge density wave, is generally attributed to electron-phonon coupling. This correlation is expected to induce a pseudogap at the Fermi level in order to gain the required energy for stable lattice distortion. The transition metal dichalcogenide 1T-VSe2 also undergoes such a transition at 110 K. Here, we present detailed angle-resolved photoemission spectroscopy experiments to investigate the electronic structure in 1T-VSe2 across the structural transition. Previously reported warping of the electronic structure and the energy shift of a secondary peak near the Fermi level as the origin of the charge density wave phase are shown to be temperature independent and hence cannot be attributed to the structural transition. Our work reveals new states that were not resolved in previous studies. Earlier results can be explained by the different dispersion natures of these states and temperature-induced broadening. Only the overall size of the Fermi surface is found to change across the structural transition. These observations, quite different from the charge density wave scenario commonly considered for 1T-VSe2 and other transition metal dichalcogenides, bring fresh perspectives toward correctly describing structural transitions. Therefore, these new results can be applied to material families in which the origin of the structural transition has not been resolved.

Realization of Two-Dimensional Intrinsic Polar Metal in a Buckled Honeycomb Binary Lattice.

Structural topology and symmetry of a two-dimensional (2D) network play pivotal roles in defining its electrical properties and functionalities. Here, a binary buckled honeycomb lattice with C3v symmetry, which naturally hosts topological Dirac fermions and out-of-plane polarity, is proposed. It is successfully achieved in a group IV-V compound, namely monolayer SiP epitaxially grown on Ag(111) surface. Combining first-principles calculations with angle-resolved photoemission spectroscopy, the degeneration of the Dirac nodal lines to points due to the broken horizonal mirror symmetry is elucidated. More interesting, the SiP monolayer manifests metallic nature, which is mutually exclusive with polarity in conventional materials. It is further found that the out-of-plane polarity is strongly suppressed by the metallic substrate. This study not only represents a breakthrough of realizing intrinsic polarity in 2D metallic material via ingenious design but also provides a comprehensive understanding of the intricate interplay of many exotic low-dimensional quantum phenomena.

Light-induced insulator-metal transition in Sr2IrO4 reveals the nature of the insulating ground state.

Sr2IrO4 has attracted considerable attention due to its structural and electronic similarities to La2CuO4, the parent compound of high-Tc superconducting cuprates. It was proposed as a strong spin-orbit-coupled Jeff = 1/2 Mott insulator, but the Mott nature of its insulating ground state has not been conclusively established. Here, we use ultrafast laser pulses to realize an insulator-metal transition in Sr2IrO4 and probe the resulting dynamics using time- and angle-resolved photoemission spectroscopy. We observe a gap closure and the formation of weakly renormalized electronic bands in the gap region. Comparing these observations to the expected temperature and doping evolution of Mott gaps and Hubbard bands provides clear evidence that the insulating state does not originate from Mott correlations. We instead propose a correlated band insulator picture, where antiferromagnetic correlations play a key role in the gap opening. More broadly, our results demonstrate that energy-momentum-resolved nonequilibrium dynamics can be used to clarify the nature of equilibrium states in correlated materials.

Inferior Interfacial Superconductivity in 1 UC FeSe/SrVO3/SrTiO3 with Screened Interfacial Electron-Phonon Coupling.

Single-unit cell (1 UC) FeSe interfaced with TiOx or FeOx exhibits significantly enhanced superconductivity compared to that of bulk FeSe, with interfacial electron-phonon coupling (EPC) playing a crucial role. However, the reduced dimensionality in 1 UC FeSe, which may drive superconducting fluctuations, complicates our understanding of the enhancement mechanisms. We construct a new superconducting interface, 1 UC FeSe/SrVO3/SrTiO3. Here, the itinerant electrons of highly metallic SrVO3 films can screen all high-energy Fuchs-Kliewer phonons, including those of SrTiO3, making it the first FeSe/oxide system with screened interfacial EPC while maintaining the 1 UC FeSe thickness. Despite comparable doping levels, the heavily electron-doped 1 UC FeSe/SrVO3 exhibits a pairing temperature (Tg ∼ 48 K) lower than those of FeSe/SrTiO3 and FeSe/LaFeO3. Our findings disentangle the contributions of interfacial EPC from dimensionality in terms of enhancing Tg in FeSe/oxide interfaces, underscoring the critical importance of interfacial EPC. This FeSe/VOx interface also provides a platform for studying interfacial superconductivity.

Realization of Honeycomb Tellurene with Topological Edge States.

The two-dimensional (2D) honeycomb lattice has attracted intensive research interest due to the appearance of Dirac-type band structures as the consequence of two sublattices in the honeycomb structure. Introducing strong spin-orbit coupling (SOC) leads to a gap opening at the Dirac point, transforming the honeycomb lattice into a 2D topological insulator as a platform for the quantum spin Hall effect (QSHE). In this work, we realize a 2D honeycomb-structured film with tellurium, the heaviest nonradioactive element in Group VI, namely, tellurene, via molecular beam epitaxy. We revealed the gap opening of 160 meV at the Dirac point due to the strong SOC in the honeycomb-structured tellurene by angle-resolved photoemission spectroscopy. The topological edge states of tellurene are detected via scanning tunneling microscopy/spectroscopy. These results demonstrate that tellurene is a novel 2D honeycomb lattice with strong SOC, and they unambiguously prove that tellurene is a promising candidate for a room-temperature QSHE system.

Quasi-Homoepitaxial Growth of Highly Strained Alkali-Metal Ultrathin Films on Kagome Superconductors.

Applying lattice strain to thin films, a critical factor to tailor their properties such as stabilizing a structural phase unstable at ambient pressure, generally necessitates heteroepitaxial growth to control the lattice mismatch with substrate. Therefore, while homoepitaxy, the growth of thin film on a substrate made of the same material, is a useful method to fabricate high-quality thin films, its application to studying strain-induced structural phases is limited. Contrary to this general belief, here the quasi-homoepitaxial growth of Cs and Rb thin films is reported with substantial in-plane compressive strain. This is achieved by utilizing the alkali-metal layer existing in bulk crystal of kagome metals AV3Sb5 (A = Cs and Rb) as a structural template. The angle-resolved photoemission spectroscopy measurements reveal the formation of metallic quantum well states and notable thickness-dependent quasiparticle lifetime. Comparison with density functional theory calculations suggests that the obtained thin films crystalize in the face-centered cubic structure, which is typically stable only under high pressure in bulk crystals. These findings provide a useful approach for synthesizing highly strained thin films by quasi-homoepitaxy, and pave the way for investigating many-body interactions in Fermi liquids with tunable dimensionality.

On-Surface Synthesis of Polyene-Linked Porphyrin Cooligomer.

π-Conjugated molecules are viewed as fundamental components in forthcoming molecular nanoelectronics in which semiconducting functional units are linked to each other via metallic molecular wires. However, it is still challenging to construct such block cooligomers on the surface. Here, we present a synthesis of [18]-polyene-linked Zn-porphyrin cooligomers via a two-step reaction of the alkyl groups on Cu(111) and Cu(110). Nonyl groups (-C9H19) substituted at the 5,15-meso positions of Zn-porphyrin were first transformed to alkenyl groups (-C9H10) by dehydrogenation. Subsequently, homocoupling of the terminal -CH2 groups resulted in the formation of extended [18]-polyene-linked porphyrin cooligomers. The structures of the products at each reaction step were investigated by bond-resolved scanning tunneling microscopy at low temperatures. A combination of angle-resolved photoemission spectroscopy and density functional theory calculations revealed the metallic property of the all trans [18]-polyene linker on Cu(110). This finding may provide an approach to fabricate complex nanocarbon structures on the surface.

Singular Hall Response from a Correlated Ferromagnetic Flat Nodal-Line Semimetal.

Topological quantum phases are largely understood in weakly correlated systems, which have identified various quantum phenomena, such as the spin Hall effect, protected transport of helical fermions, and topological superconductivity. Robust ferromagnetic order in correlated topological materials particularly attracts attention, as it can provide a versatile platform for novel quantum devices. Here, a singular Hall response arising from a unique band structure of flat topological nodal lines in combination with electron correlation in a van der Waals ferromagnetic semimetal, Fe3GaTe2, with a high Curie temperature of Tc = 347 K is reported. High anomalous Hall conductivity violating the conventional scaling, resistivity upturn at low temperature, and a large Sommerfeld coefficient are observed in Fe3GaTe2, which implies heavy fermion features in this ferromagnetic topological material. The scanning tunneling microscopy, circular dichroism in angle-resolved photoemission spectroscopy, and theoretical calculations support the original electronic features of the material. Thus, low-dimensional Fe3GaTe2 with electronic correlation, topology, and room-temperature ferromagnetic order appears to be a promising candidate for robust quantum devices.

Emergence of two distinct phase transitions in monolayer CoSe2 on graphene.

Dimensional modifications play a crucial role in various applications, especially in the context of device miniaturization, giving rise to novel quantum phenomena. The many-body dynamics induced by dimensional modifications, including electron-electron, electron-phonon, electron-magnon and electron-plasmon coupling, are known to significantly affect the atomic and electronic properties of the materials. By reducing the dimensionality of orthorhombic CoSe2 and forming heterostructure with bilayer graphene using molecular beam epitaxy, we unveil the emergence of two types of phase transitions through angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements. We disclose that the 2 × 1 superstructure is associated with charge density wave induced by Fermi surface nesting, characterized by a transition temperature of 340 K. Additionally, another phase transition at temperature of 160 K based on temperature dependent gap evolution are observed with renormalized electronic structure induced by electron-boson coupling. These discoveries of the electronic and atomic modifications, influenced by electron-electron and electron-boson interactions, underscore that many-body physics play significant roles in understanding low-dimensional properties of non-van der Waals Co-chalcogenides and related heterostructures.

Controlling structure and interfacial interaction of monolayer TaSe2 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 TaSe2 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. 1H-TaSe2 exhibits significant charge transfer and band hybridization at the interface, whereas 1 T-TaSe2 shows weak interactions with the substrate. The distinct interfacial interactions are attributed to the dual effects from the differences of the work functions as well as the relative interlayer distance between TaSe2 films and BLG substrate. The method demonstrated here provides a viable route towards interface engineering in a variety of transition-metal dichalcogenides that can be applied to future nano-devices with designed electronic properties.

Temperature Dependence of Relativistic Valence Band Splitting Induced by an Altermagnetic Phase Transition.

Altermagnetic (AM) materials exhibit non-relativistic, momentum-dependent spin-split states, ushering in new opportunities for spin electronic devices. While the characteristics of spin-splitting are documented within the framework of the non-relativistic spin group symmetry, there is limited exploration of the inclusion of relativistic symmetry and its impact on the emergence of a novel spin-splitting in the band structure. This study delves into the intricate relativistic electronic structure of an AM material, α-MnTe. Employing temperature-dependent angle-resolved photoelectron spectroscopy across the AM phase transition, the emergence of a relativistic valence band splitting concurrent with the establishment of magnetic order is elucidated. This discovery is validated through disordered local moment calculations, modeling the influence of magnetic order on the electronic structure and confirming the magnetic origin of the observed splitting. The temperature-dependent splitting is ascribed to the advent of relativistic spin-splitting resulting from the strengthening of AM order in α-MnTe as the temperature decreases. This sheds light on a previously unexplored facet of this intriguing material.

Spin polarization of photoelectrons emitted from spin-orbit coupled surface states of Pb/Ge(111).

We report that the spin vector of photoelectrons emitted from an atomic layer Pb grown on a germanium substrate [Pb/Ge(111)] can be controlled using an electric field of light. The spin polarization of photoelectrons excited by a linearly polarized light is precisely investigated by spin- and angle-resolved photoemission spectroscopy. The spin polarization of the photoelectrons observed in the mirror plane reverses between p- and s-polarized lights. Considering the dipole transition selection rule, the surface state of Pb/Ge(111) is represented by a linear combination of symmetric and asymmetric orbital components coupled with spins in mutually opposite directions. The spin direction of the photoelectrons is different from that of the initial state when the electric field vector of linearly polarized light deviates from p- or s-polarization conditions. The quantum interference in the photoexcitation process can determine the direction of the spin vector of photoelectrons.

Van der Waals Epitaxy of Weyl-Semimetal Td-WTe2.

Epitaxial growth of WTe2 offers significant advantages, including the production of high-quality films, possible long-range in-plane ordering, and precise control over layer thicknesses. However, the mean island size of WTe2 grown by molecular beam epitaxy (MBE) in the literature is only a few tens of nanometers, which is not suitable for the implementation of devices at large lateral scales. Here we report the growth of Td -WTe2 ultrathin films by MBE on monolayer (ML) graphene, reaching a mean flake size of ≃110 nm, which is, on overage, more than three times larger than previous results. WTe2 films thicker than 5 nm have been successfully synthesized and exhibit the expected Td phase atomic structure. We rationalize the epitaxial growth of Td-WTe2 and propose a simple model to estimate the mean flake size as a function of growth parameters that can be applied to other transition metal dichalcogenides (TMDCs). Based on nucleation theory and the Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showing a critical coverage of 0.13 ML above which WTe2 nucleation becomes negligible. The quality of monolayer WTe2 films is demonstrated by electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES), which is in agreement with first-principles calculations performed on free-standing WTe2 and previous reports. We found electron pockets at the Fermi level, indicating a n-type doping of WTe2 with an electron density of n = 2.0 ± 0.5 × 1012 cm-2 for each electron pocket.

Conduction Band Replicas in a 2D Moiré Semiconductor Heterobilayer.

Stacking monolayer semiconductors creates moiré patterns, leading to correlated and topological electronic phenomena, but measurements of the electronic structure underpinning these phenomena are scarce. Here, we investigate the properties of the conduction band in moiré heterobilayers of WS2/WSe2 using submicrometer angle-resolved photoemission spectroscopy with electrostatic gating. We find that at all twist angles the conduction band edge is the K-point valley of the WS2, with a band gap of 1.58 ± 0.03 eV. From the resolved conduction band dispersion, we deduce an effective mass of 0.15 ± 0.02 me. Additionally, we observe replicas of the conduction band displaced by reciprocal lattice vectors of the moiré superlattice. We argue that the replicas result from the moiré potential modifying the conduction band states rather than final-state diffraction. Interestingly, the replicas display an intensity pattern with reduced 3-fold symmetry, which we show implicates the pseudo vector potential associated with in-plane strain in moiré band formation.

ARPES investigation of the electronic structure and its evolution in magnetic topological insulator MnBi2+2nTe4+3n family.

In the past 5 years, there has been significant research interest in the intrinsic magnetic topological insulator family compounds MnBi2+2nTe4+3n (where n = 0, 1, 2 …). In particular, exfoliated thin films of MnBi2Te4 have led to numerous experimental breakthroughs, such as the quantum anomalous Hall effect, axion insulator phase and high-Chern number quantum Hall effect without Landau levels. However, despite extensive efforts, the energy gap of the topological surface states due to exchange magnetic coupling, which is a key feature of the characteristic band structure of the system, remains experimentally elusive. The electronic structure measured by using angle-resolved photoemission (ARPES) shows significant deviation from ab initio prediction and scanning tunneling spectroscopy measurements, making it challenging to understand the transport results based on the electronic structure. This paper reviews the measurements of the band structure of MnBi2+2nTe4+3n magnetic topological insulators using ARPES, focusing on the evolution of their electronic structures with temperature, surface and bulk doping and film thickness. The aim of the review is to construct a unified picture of the electronic structure of MnBi2+2nTe4+3n compounds and explore possible control of their topological properties.

Selective Control of Phases and Electronic Structures of Monolayer TaTe2.

Transition metal dichalcogenide (TMDC) films exhibit rich phases and superstructures, which can be controlled by the growth conditions as well as post-growth annealing treatment. Here, the selective growth of monolayer TaTe2 films with different phases as well as superstructures using molecular beam epitaxy (MBE) is reported. Monolayer 1H-TaTe2 and 1T-TaTe2 films can be selectively controlled by varying the growth temperature, and their different electronic structures are revealed through the combination of angle-resolved photoemission spectroscopy measurements (ARPES) and first-principles calculations. Moreover, post-growth annealing of the 1H-TaTe2 film further leads to a transition from a 19 × 19 $\sqrt {19}{\times }\sqrt {19}$ superstructure to a new 2 × 2 superstructure, where two gaps are observed in the electronic structure and persist up to room temperature. First-principles calculations reveal the role of the phonon instability in the formation of superstructures and the effect of local atomic distortions on the modified electronic structures. This work demonstrates the manipulation of the rich phases and superstructures of monolayer TaTe2 films by controlling the growth kinetics and post-growth annealing.