Laser-based Angle-resolved Photoemission Spectroscopy Research Articles

A versatile laser-based apparatus for time-resolved ARPES with micro-scale spatial resolution.

We present the development of a versatile apparatus for 6.2eV laser-based time and angle-resolved photoemission spectroscopy with micrometer spatial resolution (time-resolved μ-ARPES). With a combination of tunable spatial resolution down to ∼11 μm, high energy resolution (∼11meV), near-transform-limited temporal resolution (∼280fs), and tunable 1.55eV pump fluence up to 3 mJ/cm2, this time-resolved μ-ARPES system enables the measurement of ultrafast electron dynamics in exfoliated and inhomogeneous materials. We demonstrate the performance of our system by correlating the spectral broadening of the topological surface state of Bi2Se3 with the spatial dimension of the probe pulse, as well as resolving the spatial inhomogeneity contribution to the observed spectral broadening. Finally, after in situ exfoliation, we performed time-resolved μ-ARPES on a ∼30 μm flake of transition metal dichalcogenide WTe2, thus demonstrating the ability to access ultrafast electron dynamics with momentum resolution on micro-exfoliated materials.

Laser-based angle-resolved photoemission spectroscopy with micrometer spatial resolution and detection of three-dimensional spin vector

We have developed a state-of-the-art apparatus for laser-based spin- and angle-resolved photoemission spectroscopy with micrometer spatial resolution (µ-SARPES). This equipment is realized by the combination of a high-resolution photoelectron spectrometer, a 6 eV laser with high photon flux that is focused down to a few micrometers, a high-precision sample stage control system, and a double very-low-energy-electron-diffraction spin detector. The setup achieves an energy resolution of 1.5 (5.5) meV without (with) the spin detection mode, compatible with a spatial resolution better than 10 µm. This enables us to probe both spatially-resolved electronic structures and vector information of spin polarization in three dimensions. The performance of µ-SARPES apparatus is demonstrated by presenting ARPES and SARPES results from topological insulators and Au photolithography patterns on a Si (001) substrate.

Intrinsic electronic structure and nodeless superconducting gap of YBa2Cu3O7–δ observed by spatially-resolved laser-based angle resolved photoemission spectroscopy

The spatially-resolved laser-based high-resolution angle resolved photoemission spectroscopy (ARPES) measurements have been performed on the optimally-doped YBa2Cu3O7–δ (Y123) superconductor. For the first time, we found the region from the cleaved surface that reveals clear bulk electronic properties. The intrinsic Fermi surface and band structures of Y123 were observed. The Fermi surface-dependent and momentum-dependent superconducting gap was determined which is nodeless and consistent with the d+is gap form.

Observation of flat band, Dirac nodal lines and topological surface states in Kagome superconductor CsTi3Bi5

Kagome lattices of various transition metals are versatile platforms for achieving anomalous Hall effects, unconventional charge-density wave orders and quantum spin liquid phenomena due to the strong correlations, spin-orbit coupling and/or magnetic interactions involved in such a lattice. Here, we use laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations to investigate the electronic structure of the newly discovered kagome superconductor CsTi3Bi5, which is isostructural to the AV3Sb5 (A = K, Rb or Cs) kagome superconductor family and possesses a two-dimensional kagome network of titanium. We directly observe a striking flat band derived from the local destructive interference of Bloch wave functions within the kagome lattice. In agreement with calculations, we identify type-II and type-III Dirac nodal lines and their momentum distribution in CsTi3Bi5 from the measured electronic structures. In addition, around the Brillouin zone centre, {{mathbb{Z}}}_{2} nontrivial topological surface states are also observed due to band inversion mediated by strong spin-orbit coupling.

Testing electron–phonon coupling for the superconductivity in kagome metal CsV3Sb5

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 CsV3Sb5, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV3Sb5. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV3Sb5. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=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 CsV3Sb5. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V0.93Nb0.07)3Sb5. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV3Sb5.

Observation of electronic structure and electron-boson coupling in the low-dimensional superconductor Ta4Pd3Te16

Ternary telluride ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$ is a layered superconductor with quasi-one-dimensional characteristics. It has attracted great research interests due to the possible nodal superconductivity and its interplay with adjacent charge-density wave as well as proposed nontrivial band topology. Here, we report low-energy electronic band structure of ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$ by means of high-resolution laser-based angle-resolved photoemission spectroscopy. We acquired the Fermi surface and detailed band dispersions, directly reflecting its multiband nature. Furthermore, dispersion kinks at \ensuremath{\sim}8 meV below the Fermi level are unambiguously observed, indicative of strong electron-boson coupling. We suggest these kinks are probably due to electron-phonon coupling with effective coupling constants quantified to be \ensuremath{\sim}0.72 and \ensuremath{\sim}0.48 for the ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{2}$ bands, respectively. Our results provide valuable information for future studies on the mechanism of superconductivity as well as its intimacy to charge-density wave in ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$.

Band-selective Holstein polaron in Luttinger liquid material A0.3MoO3 (A\u2009=\u2009K, Rb)

(Quasi-)one-dimensional systems exhibit various fascinating properties such as Luttinger liquid behavior, Peierls transition, novel topological phases, and the accommodation of unique quasiparticles (e.g., spinon, holon, and soliton, etc.). Here we study molybdenum blue bronze A0.3MoO3 (A = K, Rb), a canonical quasi-one-dimensional charge-density-wave material, using laser-based angle-resolved photoemission spectroscopy. Our experiment suggests that the normal phase of A0.3MoO3 is a prototypical Luttinger liquid, from which the charge-density-wave emerges with decreasing temperature. Prominently, we observe strong renormalizations of band dispersions, which are recognized as the spectral function of Holstein polaron derived from band-selective electron-phonon coupling in the system. We argue that the strong electron-phonon coupling plays an important role in electronic properties and the charge-density-wave transition in blue bronzes. Our results not only reconcile the long-standing heavy debates on the electronic properties of blue bronzes but also provide a rare platform to study interesting excitations in Luttinger liquid materials.

Origins of electronic bands in the antiferromagnetic topological insulator MnBi2Te4

Despite the rapid progress in understanding the first intrinsic magnetic topological insulator MnBi$_2$Te$_4$, its electronic structure remains a topic under debates. Here we perform a thorough spectroscopic investigation into the electronic structure of MnBi$_2$Te$_4$ via laser-based angle-resolved photoemission spectroscopy. Through quantitative analysis, we estimate an upper bound of 3 meV for the gap size of the topological surface state. Furthermore, our circular dichroism measurements reveal band chiralities for both the topological surface state and quasi-2D bands, which can be well reproduced in a band hybridization model. A numerical simulation of energy-momentum dispersions based on a four-band model with an additional step potential near the surface provides a promising explanation for the origin of the quasi-2D bands. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of MnBi$_2$Te$_4$, and provides an important framework to understand the electronic structures of other relevant topological materials MnBi$_{2n}$Te$_{3n+1}$.

Observation of topological superconductivity in a stoichiometric transition metal dichalcogenide 2M-WS2

Topological superconductors (TSCs) are unconventional superconductors with bulk superconducting gap and in-gap Majorana states on the boundary that may be used as topological qubits for quantum computation. Despite their importance in both fundamental research and applications, natural TSCs are very rare. Here, combining state of the art synchrotron and laser-based angle-resolved photoemission spectroscopy, we investigated a stoichiometric transition metal dichalcogenide (TMD), 2M-WS2 with a superconducting transition temperature of 8.8 K (the highest among all TMDs in the natural form up to date) and observed distinctive topological surface states (TSSs). Furthermore, in the superconducting state, we found that the TSSs acquired a nodeless superconducting gap with similar magnitude as that of the bulk states. These discoveries not only evidence 2M-WS2 as an intrinsic TSC without the need of sensitive composition tuning or sophisticated heterostructures fabrication, but also provide an ideal platform for device applications thanks to its van der Waals layered structure.

Direct observation of electronic structures in WTe2 flakes with a small number of layers by micro-focused laser-based angle-resolved photoemission spectroscopy

Direct observation of electronic structures in WTe₂ flakes with a small number of layers by micro-focused laser-based angle-resolved photoemission spectroscopy

Spectroscopic evidence of bilayer splitting and strong interlayer pairing in the superconductor KCa2Fe4As4F2

In high temperature cuprate superconductors, the interlayer coupling between the CuO$_2$ planes plays an important role in dictating superconductivity, as indicated by the sensitive dependence of the critical temperature (T$_C$) on the number of CuO$_2$ planes in one structural unit. In Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ superconductor with two CuO$_2$ planes in one structural unit, the interaction between the two CuO$_2$ planes gives rise to band splitting into two Fermi surface sheets (bilayer splitting) that have distinct superconducting gap. The iron based superconductors are composed of stacking of the FeAs/FeSe layers; whether the interlayer coupling can cause similar band splitting and its effect on superconductivity remain unclear. Here we report high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) measurements on a newly discovered iron based superconductor, KCa$_2$Fe$_4$As$_4$F$_2$ (T$_C$=33.5\,K) which consists of stacking FeAs blocks with two FeAs layers separated by insulating Ca$_2$F$_2$ blocks. Bilayer splitting effect is observed for the first time that gives rise to totally five hole-like Fermi surface sheets around the Brilliouin zone center. Band structure calculations reproduce the observed bilayer splitting by identifying interlayer interorbital interaction between the two FeAs layers within one FeAs block. All the hole-like pockets around the zone center exhibit Fermi surface-dependent and nodeless superconducting gap. The gap functions with short-range antiferromagetic fluctuations are proposed and the gap symmetry can be well understood when the interlayer pairing is considered. The particularly strong interlayer pairing is observed for one of the bands. Our observations provide key information on the interlayer coupling and interlayer pairing in understanding superconductivity in iron based superconductors.

A combined laser-based angle-resolved photoemission spectroscopy and two-photon photoemission spectroscopy study of Td–WTe2

Laser-based angle-resolved photoemission spectroscopy and two-photon photoemission spectroscopy are employed to study the valence electronic structure of the Weyl semimetal candidate Td–WTe2 along two high symmetry directions and for binding energies between ≈ −1 eV and 5 eV. The experimental data show a good agreement with band structure calculations. Polarization dependent measurements provide further information on initial and intermediate state symmetry properties with respect to the mirror plane of the Td structure of WTe2.

Band-dependent superconducting gap in SrFe2(As0.65P0.35)2 studied by angle-resolved photoemission spectroscopy

The isovalent-substituted iron pnictide compound SrFe2(As1−xPx)2 exhibits multiple evidence for nodal superconductivity via various experimental probes, such as the penetration depth, nuclear magnetic resonance and specific heat measurements. The direct identification of the nodal superconducting (SC) gap structure is challenging, partly because the presence of nodes is not protected by symmetry but instead caused by an accidental sign change of the order parameter, and also because of the three-dimensionality of the electronic structure. We have studied the SC gaps of SrFe2(As0.65P0.35)2 in three-dimensional momentum space by synchrotron and laser-based angle-resolved photoemission spectroscopy. The three hole Fermi surfaces (FSs) at the zone center have SC gaps with different magnitudes, whereas the SC gaps of the electron FSs at the zone corner are almost isotropic and kz-independent. As a possible nodal SC gap structure, we propose that the SC gap of the outer hole FS changes sign around the Z-X [(0, 0, 2π) − (π, π, 2π)] direction.

Structure and magneto-electric properties of Co-based ferromagnetic films grown on the Pb0.71Sn0.29Te crystalline topological insulator

Co, Ni, Co55Fe45 and Co40Fe40B20 layers were grown on Pb0.71Sn0.29Te (111) crystalline topological insulator films by conventional and laser molecular beam epitaxy (LMBE) methods. It was demonstrated that the Co40Fe40B20 ferromagnetic films were grown epitaxially on the crystalline topological insulator surface, with clear epitaxial relations. Obtained Co40Fe40B20 layers have a bcc crystal structure with a crystalline (111) plane parallel to the (111) plane of Pb0.71Sn0.29Te. The use of reciprocal space three-dimensional mapping of reflection high electron diffraction (RHEED) patterns made it possible to determine the epitaxial relations of the film and substrate. Using laser based angle-resolved photoemission spectroscopy (LARPES) the clean Pb0.71Sn0.29Te surface with Dirac-like dispersion of the topological surface states was demonstrated. The stable surface topological state of Pb0.71Sn0.29Te was observed at least at the monolayer thick Co coverage. It was shown that Co (or Ni) and Co55Fe45 may be used as a pair of contacts (injector and detector) with different coercivities for spin transport measurements in the “metal – topological insulator” systems. The measurements of magnetic and transport properties show distinct hysteresis – type dependence of the magnetoresistance in the range of quadratic I(U) dependence.

Evolution of incommensurate superstructure and electronic structure with Pb substitution in (Bi2−xPbx)Sr2CaCu2O8+δ superconductors**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0300300 and 2017YFA0302900), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant Nos. XDB07020300 and XDB25000000), the National Natural Science Foundation of China (Grant Nos. 11334010 and 11534007), and the

High-quality Bi2−xPbxSr2CaCu2O8+δ (Bi2212) single crystals have been successfully grown by the traveling solvent floating zone technique with a wide range of Pb substitution (x = 0–0.8). The samples are characterized by transmission electron microscope (TEM) and measured by high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) with different photon energies. A systematic evolution of the electronic structure and superstructure with Pb substitution has been revealed for the first time. The superstructure shows a significant change with Pb substitution and the incommensurate modulation vector () decreases with increasing Pb substitution. In the meantime, the superstructure intensity from ARPES measurements also decreases dramatically with increasing Pb concentration. The superstructure in Bi2212 can be effectively suppressed by Pb substitution and it nearly disappears with a Pb substitution of x=0.8. We also find that the superstructure bands in ARPES measurements depend sensitively on the photon energy of lasers used; they can become even stronger than the main band when using a laser photon energy of 10.897 eV. These results provide important information on the origin of the incommensurate superstructure and its control and suppression in bismuth-based high temperature superconductors.

High-Resolution Photoemission on Sr2RuO4 Reveals Correlation-Enhanced Effective Spin-Orbit Coupling and Dominantly Local Self-Energies

We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The self-energies for the $\beta$ and $\gamma$ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects, but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies, and provides strong evidence for an electronic origin of the observed non-linear frequency dependence of the self-energies, leading to `kinks' in the quasiparticle dispersion of Sr2RuO4.

Low-energy electron-mode couplings in the surface bands of Sr2RuO4 revealed by laser-based angle-resolved photoemission spectroscopy

The low-energy electron-mode couplings are of interest in the study of superconductors as they could be the essential excitations to form electron pairs. The authors use laser-based angle-resolved photoemission spectroscopy and find a kink structure around 8 meV in the bands of Sr${}_{2}$RuO${}_{4}$. It is most pronounced for the $d$${}_{x\phantom{\rule{0}{0ex}}y}$ orbital, which implies that its mechanism is the electron coupling to ferromagnetic spin fluctuations. The results suggest that the ferromagnetic fluctuations are a promising candidate for the pairing mechanism in the unconventional superconductivity of Sr${}_{2}$RuO${}_{4}$.

Multiple topological states in iron-based superconductors

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.

Detailed electronic structure of three-dimensional Fermi surface and its sensitivity to charge density wave transition in ZrTe3 revealed by high resolution laser-based angle-resolved photoemission spectroscopy**Project supported by the National Basic Research Program of China (Grant No. 2015CB921301), the National Natural Science Foundation of China (Grant Nos. 11574360, 11534007, and 11334010), and the Strategic Priority Research Program (B) of the Chinese

The detailed information of the electronic structure is the key to understanding the nature of charge density wave (CDW) order and its relationship with superconducting order in the microscopic level. In this paper, we present a high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) study on the three-dimensional (3D) hole-like Fermi surface around the Brillouin zone center in a prototypical quasi-one-dimensional CDW and superconducting system ZrTe3. Double Fermi surface sheets are clearly resolved for the 3D hole-like Fermi surface around the zone center. The 3D Fermi surface shows a pronounced shrinking with increasing temperature. In particular, the quasiparticle scattering rate along the 3D Fermi surface experiences an anomaly near the charge density wave transition temperature of ZrTe3 (∼63 K). The signature of electron–phonon coupling is observed with a dispersion kink at ∼20 meV; the strength of the electron–phonon coupling around the 3D Fermi surface is rather weak. These results indicate that the 3D Fermi surface is also closely connected to the charge-density-wave transition and suggest a more global impact on the entire electronic structure induced by the CDW phase transition in ZrTe3.

Temperature Evolution of Energy Gap and Band Structure in the Superconducting and Pseudogap States of Bi2Sr2CaCu2O8+δ Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy**Supported by the National Key Research and Development Program of China under Grant No 2016YFA0300300, the National Natural Science Foundation of China under Grant No 11334010, the National Basic Research Program of China under Grant No 2015CB921300, and

We carry out detailed momentum-dependent and temperature-dependent measurements on Bi2Sr2CaCu2O8+δ (Bi2212) superconductor in the superconducting and pseudogap states by super-high resolution laser-based angle-resolved photoemission spectroscopy. The precise determination of the superconducting gap for the nearly optimally doped Bi2212 ( K) at low temperature indicates that the momentum-dependence of the superconducting gap deviates from the standard d-wave form ()). It can be alternatively fitted by including a high-order term ()) in which the next nearest-neighbor interaction is considered. We find that the band structure near the antinodal region smoothly evolves across the pseudogap temperature without a signature of band reorganization which is distinct from that found in Bi2Sr2CuO6+δ superconductors. This indicates that the band reorganization across the pseudogap temperature is not a universal behavior in cuprate superconductors. These results provide new insights in understanding the nature of the superconducting gap and pseudogap in high-temperature cuprate superconductors.