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Kagome superconductor CsV3Sb5, which exhibits intertwined unconventional charge density wave (CDW) and superconductivity, has garnered significant attention recently. Despite extensive static studies, the nature of these exotic electronic orders remains elusive. In this study, we investigate the non-equilibrium electronic structure of CsV3Sb5 via time- and angle-resolved photoemission spectroscopy. Our results reveal that upon laser excitation, the van Hove singularities immediately shift towards the Fermi level and subsequently oscillate in sync with a 1.3 THz coherent phonon mode. By analyzing the coherent intensity oscillations in the energy-momentum (E-k) map, we find that this coherent phonon is strongly coupled with electronic bands from both Sb and V orbitals. While typically observable only in the CDW state, remarkably, we find that the 1.3-THz coherent phonon mode can be persistently excited at temperatures above T_CDW, suggesting the potential existence of fluctuated CDW in CsV3Sb5. These findings enhance our understanding of the unconventional CDW control of kagome superconductivity., Comment: 11 pages, 5 figures

The electronic structure of the topmost layer in Sr2RuO4 in the close vicinity of the Fermi level is investigated by angle-resolved photoemission spectroscopy (ARPES) with a 7-eV laser. We find that the spin-orbit coupling (SOC) predicted as 100 meV by the density functional theory (DFT) calculations is enormously enhanced in a real material up to 250 meV, even more than that of bulk state (200 meV), by the electron-correlation effect increased by the octahedral rotation in the crystal structure. This causes the formation of highly orbital-mixing small Fermi pockets and reasonably explains why the orbital-selective Mott transition (OSMT) is not realized in perovskite oxides with crystal distortion. Interestingly, Hund's metal feature allows the quasiparticle generation only near EF, restricting the spectral gap opening derived by band hybridization within an extremely small binding energy (< 10 meV). Furthermore, it causes coherent-incoherent crossover, making the Fermi pockets disappear at elevated temperatures. The anomalous Fermi pockets are characterized by the dichotomy of the orbital-isolating Hund's coupling and the orbital-mixing SOC, which is key to understanding the nature of Sr2RuO4.

Recent developments in laser-excited photoemission electron microscopy (laser-PEEM) advance the visualization of electronic nematicity and nematic domain structures in iron-based superconductors. In FeSe and BaFe$_2$(As$_{0.87}$P$_{0.13}$)$_2$ superconductors, it has been reported that the thickness of the electronic nematic domain walls is unexpectedly long, leading to the formation of mesoscopic nematicity wave [T. Shimojima $\textit{et al.}$, Science $\textbf{373}$ (2021) 1122]. This finding demonstrates that the nematic coherence length $\xi_{\rm nem}$ can be decoupled from the lattice domain wall. Here, we report that the electronic domain wall thickness shows a distinct variation in related materials: it is similarly long in FeSe$_{0.9}$S$_{0.1}$ whereas it is much shorter in undoped BaFe$_2$As$_2$. We find a correlation between the thick domain walls and the non-Fermi liquid properties of normal-state resistivity above the nematic transition temperature. This suggests that the nematic coherence length can be enhanced by underlying spin-orbital fluctuations responsible for the anomalous transport properties., Comment: 5 pages, 3 figures

HfO$_2$-based ferroelectrics are one of the most actively developed functional materials for memory devices. However, in HfO$_2$-based ferroelectric devices, dielectric breakdown is a main failure mechanism during repeated polarization switching. Elucidation of the breakdown process may broaden the scope of applications for the ferroelectric HfO$_2$. Here, we report direct observations of a breakdown process in HfO$_2$-based ferroelectric capacitors, by \textit{in-situ} laser-based photoemission electron microscopy (laser-PEEM). We have not only clearly visualized the hard dielectric breakdown (HDB) spot, but also observed the regions responsible for the soft dielectric breakdown (SDB) which is a precursor phenomenon to HDB. It was found that the low-resistance region formed after SDB is wider than the conduction path formed after HDB. Furthermore, our spectromicroscopic analysis revealed that the photoelectron spectrum after SDB shows an enhancement in intensity without spectral-shape modulation, interpreted that the initially existed defects are increased. In the HDB spot, however, an additional shoulder structure was observed. These results provide spectroscopic evidence that the electronic states responsible for the conduction path after SDB are different from those after HDB. Through this work, we propose this microscopic approach as a versatile tool for studying buried materials as they are, accelerating the development of material engineering for advanced electronic devices., Comment: 6 pages

The spin-dependent band structure of CoS$_2$ which is a candidate for a half-metallic ferromagnet was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations, in order to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS$_2$ is not but very close to a half-metal. Band calculations using density functional theory with generalized gradient approximation showed a good agreement with the observed majority spin $e_g$ bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin $e_g$ bands. All those results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems., Comment: 53 pages, 25 figures, 1 table

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO$_2$ planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba$_2$Ca$_5$Cu$_6$O$_{12}$(F,O)$_2$ with inner CuO$_2$ layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.

Identifying ordered phases and their underlying symmetries is the first and most important step toward understanding the mechanism of high-temperature superconductivity; critical behaviors of ordered phases are expected to be correlated with superconductivity. Efforts to find such ordered phases have been focused on symmetry breaking in the pseudogap region while the Fermi liquid-like metal region beyond the so-called critical doping $p_{c}$ has been regarded as a trivial disordered state. Here, we used rotational anisotropy second harmonic generation and uncovered a broken mirror symmetry in the Fermi liquid-like phase in (Bi,Pb)$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ with $p = 0.205 > p_{c}$. By tracking the temperature evolution of the symmetry-breaking response, we verify an order parameter-like behavior with the onset temperature $T_{up}$ at which the strange metal to Fermi liquid-like-metal crossover takes place. Complementary angle-resolved photoemission study showed that the quasiparticle coherence between $\mathrm{CuO_{2}}$ bilayers is enhanced in proportion to the symmetry-breaking response as a function of temperature, indicating that the change in metallicity and symmetry breaking are linked. These observations contradict the conventional quantum disordered scenario for over-critical-doped cuprates and provide new insight into the nature of the quantum critical point in cuprates., Comment: 8 pages, 4 figures

We use a pump-probe, spin-, and angle-resolved photoemission spectroscopy (ARPES) with a 10.7 eV laser accessible up to the Brillouin zone edge, and reveal for the first time the entire band structure, including the unoccupied side, for the elemental bismuth (Bi) with the spin-polarized surface states. Our data identify Bi as in a strong topological insulator phase ($Z_2$=1) against the prediction of most band calculations. We unveil that the unoccupied topological surface states possess spin-polarized saddle points yielding the van Hove singularity, providing an excellent platform for the future development of opto-spintronics., Comment: 6 pages, 4 figures

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below Tc~4.5K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc.These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling., Comment: accepted by PRL

The quantum spin Hall (QSH) effect has attracted extensive research interest because of the potential applications in spintronics and quantum computing, which is attributable to two conducting edge channels with opposite spin polarization and the quantized electronic conductance of 2e2/h. Recently, 2M-WS2, a new stable phase of transition metal dichalcogenides with a 2M structure showing an identical layer configuration to that of the monolayer 1T' TMDs, was suggested to be a QSH insulator as well as a superconductor with critical transition temperature around 8 K. Here, high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES are applied to investigate the electronic and spin structure of the topological surface states (TSS) in the superconducting 2M-WS2. The TSS exhibits characteristic spin-momentum-locking behavior, suggesting the existence of long-sought nontrivial Z2 topological states therein. We expect that 2M-WS2 with co-existing superconductivity and TSS might host the promising Majorana bound states.

The photo-induced band structure variation of a rare-earth-based semiconductor, samarium monosulfide (SmS), was investigated using high-harmonic-generation laser-based time-resolved photoelectron spectroscopy. A nonlinear photo-induced band shift of the Sm 4f multiplets was observed. The first one is a shift to the high-binding-energy side due to a large surface photovoltage (SPV) effect of approximately 93 meV, comparable to the size of the bulk band gap, with a much longer relaxation time than 0.1 ms. The second one is an ultrafast band shift to the low binding energy side, which is in the opposite direction to the SPV shift, suggesting an ultrafast valence transition from divalent to trivalent Sm ions due to photo-excitation. The latter energy shift was approximately 58 meV, which is consistent with the energy gap shift from ambient pressure to the boundary between the black insulator and golden metallic phase with the application of pressure. This suggests that the photo-induced valence transition can reach the phase boundary, but other effects are necessary to realize the golden metallic phase., Comment: 6 pages, 4figures

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal $\rm{CsV_3Sb_5}$, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength,$\lambda$, supporting an unconventional pairing mechanism in $\rm{CsV_3Sb_5}$. However, experimental determination of $\lambda$ is still missing, hindering a microscopic understanding of the intertwined ground state of $\rm{CsV_3Sb_5}$. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate $\lambda$=0.45~0.6 at T=6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in $\rm{CsV_3Sb_5}$. Remarkably, the EPC on the V 3d-band enhances to $\lambda$~0.75 as the superconducting transition temperature elevated to 4.4 K in $\rm{Cs(V_{0.93}Nb_{0.07})_3Sb_5}$. Our results provide an important clue to understand the pairing mechanism in the Kagome superconductor $\rm{CsV_3Sb_5}$., Comment: To appear in Nature Communications

Spin-orbit interaction (SOI) in low-dimensional systems, namely Rashba systems and the edge states of topological materials, is extensively studied in this decade as a promising source to realize various fascinating spintronic phenomena, such as the source of the spin current and spin-mediated energy conversion. Here, we show the odd fluctuation in the spin-orbital texture in a surface Rashba system on Bi/InAs(110)-(2$\times$1) by spin- and angle-resolved photoelectron spectroscopy and a numerical simulation based on a density-functional theory (DFT) calculation. The surface state shows a paired parabolic dispersion with the spin degeneracy lifted by the Rashba effect. Although its spin polarization should be fixed in a particular direction based on the Rashba model, the observed spin polarization varies greatly and even reverses its sign depending on the wavenumber. DFT calculations also reveal that the spin directions of two inequivalent Bi chains on the surface change from nearly parallel (canted-parallel) to anti-parallel in real space in the corresponding wavevector region. These results point out an oversimplification of the nature of spin in Rashba and Dirac systems and provide more freedom than expected for spin manipulation of photoelectrons., Comment: 23 pages, 7 figures

We report time- and angle-resolved photoemission spectroscopy on SnSe which currently attracts great interest due to its extremely high thermoelectric performance. Laser-assisted photoemission signals are observed within $\pm$20 fs of the pump pulse arrival. Around 30-50 fs after the photoexcitation, the conduction band minima are not populated by the photoexcited electrons while the valence bands are considerably broadened. In going from 90 fs to 550 fs after the photoexcitation, the photoexcited carriers are decayed into the multiple conduction band minima. The observed conduction bands are consistent with the band structure calculations. The multiple conduction minima suggest possibility of high and anisotropic thermoelectric performance of n-type SnSe single crystal if it is realized., Comment: 5 pages, 4 figures

The electronic state of WTe2, a candidate of type-II Weyl semimetal, is investigated by using laser-based spin- and angle-resolved photoemission spectroscopy (SARPES). We prepare the pair of WTe2 samples, one with (001) surface and the other with (00-1) surface, by "sandwich method", and measure the band structures of each surface separately. The Fermi arcs are observed on both surfaces. We identify that the Fermi arcs on the two surfaces are both originating from surface states. We further find a surface resonance band, which connects with the Fermi-arc band, forming a Dirac-cone-like band dispersion. Our results indicate that the bulk electron and hole bands are much closer in momentum space than band calculations.

Time- and angle-resolved photoemission spectroscopy has played an important role in revealing the non-equilibrium electronic structures of solid-state materials. The implementation of high harmonic generation to obtain a higher photon energy also allows us to investigate the wide Brillouin zone on a time scale below 100 fs. In this article, we review our recent studies using high-harmonic-generation-laser-based time- and angle-resolved photoemission spectroscopy to study a variety of quantum materials. We reveal many unprecedented phenomena in each system and highlight some representative results.

Similar to light polarization that is selected by a superposition of optical basis, electron spin direction can be controlled through a superposition of spin basis. We investigate such a spin interference occurring in photoemission of the spin-orbit coupled surface state in Bi2Se3 by using spin- and angle-resolved photoemission spectroscopy combined with laser light source (laser-SARPES). Our laser-SARPES with three-dimensional spin detection and tunable laser polarization including elliptical and circular polarization enables us to directly visualize how the direction of the fully-polarized photoelectron spin changes according to the optical phase and orientation of the incident laser polarization. By this advantage of our laser-SARPES, we demonstrate that such optical information can be projected to the three-dimensional spin vector of the photoelectrons. Our results, therefore, present a novel spin-polarized electron source permitting us to optically control the pure spin state pointing to the arbitrary direction., Comment: 3 figures

Spin-momentum locking is essential to the spin-split Fermi surfaces of inversion-symmetry broken materials, which are caused by either Rashba-type or Zeeman-type spin-orbit coupling (SOC). While the effect of Zeeman-type SOC on superconductivity has experimentally been shown recently, that of Rashba-type SOC remains elusive. Here we report on convincing evidence for the critical role of the spin-momentum locking on crystalline atomic-layer superconductors on surfaces, for which the presence of the Rashba-type SOC is demonstrated. In-situ electron transport measurements reveal that in-plane upper critical magnetic field is anomalously enhanced, reaching approximately three times the Pauli limit at $T = 0$. Our quantitative analysis clarifies that dynamic spin-momentum locking, a mechanism where spin is forced to flip at every elastic electron scattering, suppresses the Cooper pair-breaking parameter by orders of magnitude and thereby protects superconductivity. The present result provides a new insight into how superconductivity can survive the detrimental effects of strong magnetic fields and exchange interactions., Comment: This is a post-peer-review, pre-copyedit version of an article published in Nature Communications. The final authenticated version is available online at: http://dx.doi.org/10.1038/s41467-021-21642-1

We use laser-based spin- and angle-resolved photoemission spectroscopy (laser-SARPES) with high-resolution, and experimentally determine, for the first time, the Rashba-parameters of quantum well states (QWSs) systematically changing with the film thickness and the quantum numbers, through the observation of the Ag films grown on an Au(111) substrate. The data are very well reproduced by the theoretical calculations based on the density functional theory. Most importantly, we find a scaling law for the Rashba parameter ($\alpha_{\rm R}$) that the magnitude of $\alpha_{\rm R}$ is scaled by the charge density at the interface and the spin-orbit coupling ratio between the film and the substrate, and it is expressed by a single straight line regardless of the film thickness and the quantum numbers. The new finding not only is crucial to understand the Rashba effect in QWSs but also gives a foundation of film growth engineering to fine-tune the spin splitting in 2D heterostructure systems., Comment: 6 pages, 4 figures

A quantum spin Hall insulator hosts topological states at the one-dimensional edge, along which backscattering by nonmagnetic impurities is strictly prohibited and dissipationless current flows. Its 3D analogue, a weak topological insulator (WTI), possesses similar quasi-1D topological states confined at side surfaces of crystals. The enhanced confinement could provide a route for dissipationless current and better advantages for applications relative to the widely studied strong topological insulators. However, the topological side surface is usually not cleavable and is thus hard to observe by angle-resolved photoemission spectroscopy (ARPES), which has hindered the revealing of the electronic properties of WTIs. Here, we visualize the topological surface states of the WTI candidate ZrTe5 for the first time by spin and angle-resolved photoemission spectroscopy: a quasi-1D band with spin-momentum locking was revealed on the side surface. We further demonstrate that the bulk band gap in ZrTe5 is controlled by strain to the crystal, realizing a more stabilized WTI state or an ideal Dirac semimetal state depending on the direction of the external strain. The highly directional spin-current and the tunable band gap we found in ZrTe5 will provide an excellent platform for applications., Comment: 8 pages, 4 figures. Accepted by Nature Communications

We investigated the nonequilibrium electronic structure of 2H-NbSe$_2$ by time- and angle-resolved photoemission spectroscopy. We find that the band structure is distinctively modulated by strong photo-excitation, as indicated by the unusual increase in the photoelectron intensities around E$_F$. In order to gain insight into the observed photo-induced electronic state, we performed DFT calculations with modulated lattice structures, and found that the variation of the Se height from the Nb layer results in a significant change in the effective mass and band gap energy. We further study the momentum-dependent carrier dynamics. The results suggest that the relaxation is faster at the K-centered Fermi surface than at the $\Gamma$-centered Fermi surface, which can be attributed to the stronger electron-lattice coupling at the K-centered Fermi surface. Our demonstration of band structure engineering suggests a new role for light as a tool for controlling the functionalities of solid-state materials., Comment: 7 pages, 5 figures

Control of the phase transition from topological to normal insulators can allow for an on/off switching of spin current. While topological phase transitions have been realized by elemental substitution in semiconducting alloys, such an approach requires the preparation of materials with various compositions, thus it is quite far from a feasible device application, which demands a reversible operation. Here we use angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES to visualize the strain-driven band structure evolution of the quasi-1D superconductor TaSe3. We demonstrate that it undergoes reversible strain-induced topological phase transitions from a strong topological insulator phase with spin-polarized, quasi-1D topological surface states, to topologically trivial semimetal and band insulating phases. The quasi-1D superconductor TaSe3 provides a suitable platform for engineering the topological spintronics, for example as an on/off switch for spin current robust against impurity scattering.

The superconductivity of high transition temperature (Tc) occurs in copper oxides with carrier-doping to an antiferromagnetic (AF) Mott insulator. This discovery more than thirty years ago immediately led to a prediction about the formation of a small Fermi pocket. This structure, however, has not yet been detected, while it could be a key element in relating high-Tc superconductivity to Mott physics. To address this long-standing issue, we investigate the electronic structure of a five-layered Ba2Ca4Cu5O10(F,O)2 with inner CuO2 planes demonstrated to be cleanest ever in cuprates. Most surprisingly, we find small Fermi surface (FS) pockets closed around (pi/2,pi/2) consistently by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between the superconductivity and AF order in the same CuO2 sheet. Our data further indicate that the superconductivity can occur without contribution from the states near the antinodal region, which are shared by other competing excitations such as the charge density wave (CDW) and pseudogap states. This will have significant implications for understanding the superconductivity and puzzling Fermi arc phenomena in cuprates.

Interfacing bulk conducting topological Bi$_2$Se$_3$ films with s-wave superconductors initiates strong superconducting order in the nontrivial surface states. However, bulk insulating topological (Bi$_{1-x}$Sb$_{x})_2$Te$_3$ films on bulk Nb instead exhibit a giant attenuation of surface superconductivity, even for films only two-layers thick. This massive suppression of proximity pairing is evidenced by ultrahigh-resolution band mappings and by contrasting quantified superconducting gaps with those of heavily n-doped topological Bi$_2$Se$_3$/Nb. The results underscore the limitations of using superconducting proximity effects to realize topological superconductivity in nearly intrinsic systems., Comment: Accepted at Physical Review Letters: https://journals.aps.org/prl/accepted/b0073Y70Ddc12c71a13a3ac871102f6c7e1a1bd7b

Photo-induced phase transitions have been intensively studied owing to the ability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it has currently been a central issue how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photo-induced phase transitions. Here, we develop a new analysis method, frequency-domain angle-resolved photoemission spectroscopy, to gain precise insight into electron-phonon couplings during photo-induced insulator-to-metal transitions for Ta$_2$NiSe$_5$. We demonstrate that multiple coherent phonons generated by displacive excitations show band-selective coupling to the electrons. Furthermore, we find that the lattice modulation corresponding to the 2 THz phonon mode, where Ta lattice is sheared along the a-axis, is the most relevant for the photo-induced semimetallic state.

The van der Waals (vdW) materials with low dimensions have been extensively studied as a platform to generate exotic quantum properties. Advancing this view, a great deal of attention is currently paid to topological quantum materials with vdW structures. Here, we provide a new concept of designing topological materials by the vdW stacking of quantum spin Hall insulators (QSHIs). Most interestingly, a slight shift of inversion center in the unit cell caused by a modification of stacking is found to induce the topological variation from a trivial insulator to a higher-order topological insulator (HOTI). Based on that, we present the first experimental realization of a HOTI by investigating a bismuth bromide Bi4Br4 with angle-resolved photoemission spectroscopy (ARPES). The unique feature in bismuth halides capable of selecting various topology only by differently stacking chains, combined with the great advantage of the vdW structure, offers a fascinating playground for engineering topologically non-trivial edge-states toward future spintronics applications., Comment: Nature Materials, in press. The final version of this article is available online at https://doi.org/10.1038/s41563-020-00871-7

Magnetic topological insulators (TIs) with nontrivial topological electronic structure and broken time-reversal symmetry exhibit various exotic topological quantum phenomena. The realization of such exotic phenomena at high temperature is one of central topics in this area. We reveal that MnBi6Te10 is a magnetic TI with an antiferromagnetic ground state below 10.8 K whose nontrivial topology is manifested by Dirac-like surface states. The ferromagnetic axion insulator state with Z4 = 2 emerges once spins polarized at field as low as 0.1 T, accompanied with saturated anomalous Hall resistivity up to 10 K. Such a ferromagnetic state is preserved even external field down to zero at 2 K. Theoretical calculations indicate that the few-layer ferromagnetic MnBi6Te10 is also topologically nontrivial with a non-zero Chern number. Angle-resolved photoemission spectroscopy experiments further reveal three types of Dirac surface states arising from different terminations on the cleavage surfaces, one of which has insulating behavior with an energy gap of ~ 28 meV at the Dirac point. These outstanding features suggest that MnBi6Te10 is a promising system to realize various topological quantum effects at zero field and high temperature., Comment: 18 pages, 4 figures and 1 table

We fabricated spin-polarized surface electronic states with tunable Fermi level from semiconductor to low-dimensional metal in the Bi/GaSb(110)-(2$\times$1) surface using angle-resolved photoelectron spectroscopy (ARPES) and spin-resolved ARPES. The spin-polarized surface band of Bi/GaSb(110) exhibits quasi-one-dimensional character with the Rashba parameter $\alpha _{\rm R}$ of 4.1 and 2.6 eV\AA \ at the $\bar{\Gamma}$ and $\bar{\rm Y}$ points of the surface Brillouin zone, respectively. The Fermi level of the surface electronic state is tuned in situ by element-selective Ar-ion sputtering on the GaSb substrate. The giant Rashba-type spin splitting with switchable metallic/semiconducting character on semiconductor substrate makes this system a promising candidate for future researches in low-dimensional spintronic phenomena., Comment: 19 pages, 5 figures, 4 tables

In magnetic topological insulators (TIs), the interplay between magnetic order and nontrivial topology can induce fascinating topological quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topological quantum phenomena. In despite of intensive efforts, experimental evidence of the topological surface states (SSs) remains elusive. Here, by combining first-principles calculations and angle-resolved photoemission spectroscopy (ARPES) experiments, we have revealed that EuSn2As2 is an antiferromagnetic TI with observation of Dirac SSs consistent with our prediction. We also observe nearly gapless Dirac SSs in antiferromagnetic TIs MnBi2nTe3n+1 (n = 1 and 2), which were absent in previous ARPES results. These results provide clear evidence for nontrivial topology of these intrinsic magnetic TIs. Furthermore, we find that the topological SSs show no observable changes across the magnetic transition within the experimental resolution, indicating that the magnetic order has quite small effect on the topological SSs, which can be attributed to weak hybridization between the localized magnetic moments, from either 4f or 3d orbitals, and the topological electronic states. This provides insights for further research that the correlations between magnetism and topological states need to be strengthened to induce larger gaps in the topological SSs, which will facilitate the realization of topological quantum phenomena at higher temperatures., Comment: 22 pages, 5 figures, to be published in PRX

Recently, iron-chalcogenide superconductors have emerged as a new and promising platform for studying and manipulating Majorana zero mode (MZM). By combining topological band structure and superconductivity in a multiband material, they provide significant advantages such as higher superconducting transition temperature (Tc) and isolated Majorana mode. However, iron-chalcogenide superconductors, especially Fe(Te,Se), suffer from strong inhomogeneity which may hamper their practical application. On the other hand, some iron-pnictide (Fe-As) superconductors, such as LiFeAs, have been demonstrated to have a similar topological band structure, yet no MZM has been observed in its vortex cores, raising a question of universality of MZM presence in iron-based superconductors. In this work, by using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, we identify the first Fe-As superconductor CaKFe4As4 (Tc = 35 K) which has both the superconducting Dirac surface states and the MZMs inside its vortex cores. The topological band inversion is largely due to the down-shift of the pz band caused by the bilayer band folding in this material. More strikingly, the energies and spatial line profiles of MZM and multiple quantized Caroli-de Gennes-Matricon bound states observed inside the topological vortex can be accurately reproduced by a simple theoretical model derived from a surface Dirac cone, firmly establishing Majorana nature of the zero mode.

Photo-excitation is a very powerful way to instantaneously drive a material into a novel quantum state without any fabrication, and variable ultrafast techniques have been developed to observe how electron-, lattice-, and spin-degrees of freedom change. One of the most spectacular phenomena is photo-induced superconductivity, and it has been suggested in cuprates that the transition temperature Tc can be enhanced from original Tc with significant lattice modulations. Here we show another photo-induced high-Tc superconducting state in the iron-based superconductor FeSe with semi-metallic hole and electron bands. The transient electronic state in the entire Brillouin zone is directly observed by the time- and angle-resolved photoemission spectroscopy using extreme ultraviolet pulses obtained from high harmonic generation. Our results of dynamical behaviors on timescales from 50 fs to 800 ps consistently support the favorable superconducting state after photo-excitation well above Tc. This finding demonstrates that multiband iron-based superconductors emerge as an alternative candidate for photo-induced superconductors.

High surface-mobility, which is attributable to topological protection, is a trademark of three-dimensional topological insulators (3DTIs). Exploiting surface-mobility indicates successful application of topological properties for practical purposes. However, the detection of the surface-mobility has been hindered by the inevitable bulk conduction. Even in the case of high-quality crystals, the bulk state forms the dominant channel of the electrical current. Therefore, with electrical transport measurement, the surface-mobility can be resolved only below-micrometer-thick crystals. The evaluation of the surface-mobility becomes more challenging at higher temperatures, where phonons can play a role. Here, using spectroscopic techniques, we successfully evaluated the surface-mobility of Bi2Te3 (BT) at room temperature (RT). We acquired the effective masses and mean scattering times for both the surface and bulk states using angle-resolved photoemission and terahertz time-domain spectroscopy. We revealed a record-high surface-mobility for BT, exceeding 33,000 cm^2/(Vs) per surface sheet, despite intrinsic limitations by the coexisting bulk state as well as phonons at RT. Our findings partially support the interesting conclusion that the topological protection persists at RT. Our approach could be applicable to other topological materials possessing multiband structures near the Fermi level.

Topological insulators and semimetals as well as unconventional iron-based superconductors have attracted major recent attention in condensed matter physics. Previously, however, little overlap has been identified between these two vibrant fields, even though the principal combination of topological bands and superconductivity promises exotic unprecedented avenues of superconducting states and Majorana bound states (MBSs), the central building block for topological quantum computation. Along with progressing laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) towards high energy and momentum resolution, we have resolved topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level ($E_{\text{F}}$) in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively. The TI and TDS bands can be individually tuned to locate close to $E_{\text{F}}$ by carrier doping, allowing to potentially access a plethora of different superconducting topological states in the same material. Our results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, rendering these materials a promising platform for high-$T_{\text{c}}$ topological superconductivity., Comment: 9 pages, 6 figures. arXiv admin note: substantial text overlap with arXiv:1803.00846, arXiv:1803.00845

Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally-friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realization of photo-induced insulator-to-metal transitions in Ta2Ni(Se1-xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic-insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron-hole coupled systems.

We investigate the band structure, nematic state and superconducting gap structure of two selected FeSe single crystals containing different amount of disorder. Transport and angle-resolved photoemission spectroscopy measurements show that the small amount of disorder has little effect to the band structure and the nematic state of FeSe. However, temperature and magnetic field dependencies of specific heat for the two samples are quite different. Wave-vector-dependent gap structure are obtained from the three dimensional field-angle-resolved specific heat measurements. A small gap with two vertical-line nodes or gap minima along the $k_z$ direction is found only in the sample with higher quality. Such symmetry-unprotected nodes or gap minima are found to be smeared out by small amount of disorder, and the gap becomes isotropic in the sample of lower quality. Our study reveals that the reported controversy on the gap structure of FeSe is due to the disorder-sensitive node-like small gap., Comment: 7 pages, 4 figures

Spin-resolved band structures of L-gap surface states on Ag(111) and Cu(111) are investigated by spin- and angle-resolved photoelectron spectroscopy (SARPES) with a vacuum-ultra-violet laser. The observed spin textures of the Ag(111) and Cu(111) surface states agree with that expected by the conventional Rashba effect. The Rashba parameter of the Ag(111) surface state is estimated quantitatively and is 80% of that of Cu(111). The surface-state wave function is found to be predominantly of even mirror-symmetry with negligible odd contribution by SARPES using a linearly polarized light. The results are consistent with our theoretical calculations for the orbital-resolved surface state.

Transient electron-hole pairs generated in semiconductors can exhibit unconventional excitonic condensation. Anisotropy in the carrier mass is considered as the key to elongate the life time of the pairs, and hence to stabilize the condensation. Here we employ time- and angle-resolved photoemission spectroscopy to explore the dynamics of photo-generated carriers in black phosphorus. The electronic structure above the Fermi level has been successfully observed, and a massive-and-anisotropic Dirac-type dispersions are confirmed; more importantly, we directly observe that the photo-carriers generated across the direct band gap have the life time exceeding 400 ps. Our finding confirms that black phosphorus is a suitable platform for excitonic condensations, and also open an avenue for future applications in broadband mid-infrared BP-based optoelectronic devices., Comment: 13 pages, 5 figures

Given the parallelism between the physical properties of Ce and Yb based magnets and heavy fermions due to the electron-hole symmetry, it has been rather odd that the transition temperature of the Yb based compounds is normally very small, as low as $\sim$ 1 K or even lower, whereas Ce counterparts may often have the transition temperature well exceeding 10 K. Here, we report our experimental discovery of the transition temperature reaching 20 K for the first time in a Yb based compound at ambient pressure. The Mn substitution at the Al site in an intermediate valence state of $\alpha$-YbAlB$_{4}$ not only induces antiferromagnetic transition at a record high temperature of 20 K but also transforms the heavy fermion liquid state in $\alpha$-YbAlB$_{4}$ into a highly resistive metallic state proximate to a Kondo insulator., Comment: 7 pages, 3 figures

We investigate the transient electronic structure of BaFe2As2, a parent compound of iron-based superconductors, by time- and angle-resolved photoemission spectroscopy. In order to probe the entire Brillouin zone, we utilize extreme ultraviolet photons and observe photoemission intensity oscillation with the frequency of the A1g phonon which is antiphase between the zone-centered hole Fermi surfaces (FSs) and zone-cornered electron FSs. We attribute the antiphase behavior to the warping in one of the zone-centered hole FSs accompanying the displacement of the pnictogen height, and find that this displacement is the same direction as that induced by substitution of P for As, where superconductivity is induced by a structural modification without carrier doping in this system., Comment: accepted for publication as a Rapid Commun. in PRB

Topological Dirac semimetals (TDSs) exhibit bulk Dirac cones protected by time reversal and crystal symmetry, as well as surface states originating from non-trivial topology. While there is a manifold possible onset of superconducting order in such systems, few observations of intrinsic superconductivity have so far been reported for TDSs. We observe evidence for a TDS phase in FeTe$_{1-x}$Se$_x$ ($x$ = 0.45), one of the high transition temperature ($T_c$) iron-based superconductors. In angle-resolved photoelectron spectroscopy (ARPES) and transport experiments, we find spin-polarized states overlapping with the bulk states on the (001) surface, and linear magnetoresistance (MR) starting from 6 T. Combined, this strongly suggests the existence of a TDS phase, which is confirmed by theoretical calculations. In total, the topological electronic states in Fe(Te,Se) provide a promising high $T_c$ platform to realize multiple topological superconducting phases., Comment: 6 pages, 4 figures

Topological insulators/semimetals and unconventional iron-based superconductors have attracted major attentions in condensed matter physics in the past 10 years. However, there is little overlap between these two fields, although the combination of topological states and superconducting states will produce more exotic topologically superconducting states and Majorana bound states (MBS), a promising candidate for realizing topological quantum computations. With the progress in laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) with very high energy- and momentum-resolution, we directly resolved the topological insulator (TI) phase and topological Dirac semimetal (TDS) phase near Fermi level ($E_F$) in the iron-based superconductor Li(Fe,Co)As. The TI and TDS phases can be separately tuned to $E_F$ by Co doping, allowing a detailed study of different superconducting topological states in the same material. Together with the topological states in Fe(Te,Se), our study shows the ubiquitous coexistence of superconductivity and multiple topological phases in iron-based superconductors, and opens a new age for the study of high-Tc iron-based superconductors and topological superconductivity., Comment: 6 pages, 3 figures

Topological superconductors, whose edge hosts Majorana bound states or Majorana fermions that obey non-Abelian statistics, can be used for low-decoherence quantum computations. Most of the proposed topological superconductors are realized with spin-helical states through proximity effect to BCS superconductors. However, such approaches are difficult for further studies and applications because of the low transition temperatures and complicated hetero-structures. Here by using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we discover that the iron-based superconductor FeTe1-xSex (x = 0.45, Tc = 14.5 K) hosts Dirac-cone type spin-helical surface states at Fermi level, which open an s-wave SC gap below Tc. Our study proves that the surface states of FeTe0.55Se0.45 are 2D topologically superconducting, and thus provides a simple and possibly high-Tc platform for realizing Majorana fermions., Comment: 10 pages, 5 figures. 1706.05163, 1803.00845 and 1803.00846 are a series of studies on topological superconductivity and topological states in iron-based superconductors

We investigate the superconducting-gap anisotropy in one of the recently discovered BiS$_2$-based superconductors, NdO$_{0.71}$F$_{0.29}$BiS$_2$ ($T_c$ $\sim$ 5 K), using laser-based angle-resolved photoemission spectroscopy. Whereas the previously discovered high-$T_c$ superconductors such as copper oxides and iron-based superconductors, which are believed to have unconventional superconducting mechanisms, have $3d$ electrons in their conduction bands, the conduction band of BiS$_2$-based superconductors mainly consists of Bi 6$p$ electrons, and hence the conventional superconducting mechanism might be expected. Contrary to this expectation, we observe a strongly anisotropic superconducting gap. This result strongly suggests that the pairing mechanism for NdO$_{0.71}$F$_{0.29}$BiS$_2$ is unconventional one and we attribute the observed anisotropy to competitive or cooperative multiple paring interactions.

Experiments of time-resolved x-ray magnetic circular dichroism (Tr-XMCD) and resonant x-ray scattering at a beamline BL07LSU in SPring-8 with a time-resolution of under 50 ps are presented. A micro-channel plate is utilized for the Tr-XMCD measurements at nearly normal incidence both in the partial electron and total fluorescence yield (PEY and TFY) modes at the L2,3 absorption edges of the 3d transition-metals in the soft x-ray region. The ultrafast photo-induced demagnetization within 50 ps is observed on the dynamics of a magnetic material of FePt thin film, having a distinct threshold of the photon density. The spectrum in the PEY mode is less-distorted both at the L2,3 edges compared with that in the TFY mode and has the potential to apply the sum rule analysis for XMCD spectra in pump-probed experiments., Comment: 7 pages, 5 figures