Ultrahigh-resolution Angle-resolved Photoemission Research Articles

Nodeless electron pairing in CsV3Sb5-derived kagome superconductors.

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order and lattice geometry1-9. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive10-17. In particular, consensus on the electron pairing symmetry has not been achieved so far18-20, 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 CsV3Sb5-derived kagome superconductors-Cs(V0.93Nb0.07)3Sb5 and Cs(V0.86Ta0.14)3Sb5-using ultrahigh-resolution and low-temperature angle-resolved photoemission spectroscopy. 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. Our comprehensive characterizations of the superconducting gap provide indispensable information on the electron pairing symmetry of kagome superconductors, and advance our understanding of the superconductivity and intertwined electronic orders in quantum materials.

Electronic structure of La (0001) thin films on W (110) studied by photoemission spectroscopy and first principle calculations

Surface states that have a dz2 symmetry around the center of the surface Brillouin zone (BZ) have been regarded common in closely-packed surfaces of rare-earth metals. In this work, we report the electronic structure of dhcp La (0001) thin films by ultra-high energy resolution angle-resolved photoemission spectroscopy (ARPES) and first principle calculations. Our first principle analysis is based on the many-body approach, therefore, density function theory (DFT) combined with dynamic mean-field theory (DMFT). The experimentally observed Fermi surface topology and band structure close to the Fermi energy qualitatively agree with first principle calculations when using a renormalization factor of between 2 and 3 for the DFT bands. Photon energy dependent ARPES measurements revealed clear k Z dependence for the hole-like band around the BZ center, previously regarded as a surface state. The obtained ARPES results and theoretical calculations suggest that the major bands of dhcp La (0001) near the Fermi level originate from the bulk La 5 d orbits as opposed to originating from the surface states.

Surface Fermi arc connectivity in the type-II Weyl semimetal candidateWTe2

We perform ultrahigh resolution angle-resolved photoemission experiments at a temperature T=0.8 K on the type-II Weyl semimetal candidate WTe$_{2}$. We find a surface Fermi arc connecting the bulk electron and hole pockets on the (001) surface. Our results show that the surface Fermi arc connectivity to the bulk bands is strongly mediated by distinct surface resonances dispersing near the border of the surface-projected bulk band gap. By comparing the experimental results to first-principles calculations we argue that the coupling to these surface resonances, which are topologically trivial, is compatible with the classification of WTe$_{2}$ as a type-II Weyl semimetal hosting topological Fermi arcs. We further support our conclusion by a systematic characterization of the bulk and surface character of the different bands and discuss the similarity of our findings to the case of topological insulators.

Large momentum-dependence of the main dispersion ‘kink’ in the high-Tc superconductor Bi2Sr2CaCu2O8+δ

Ultrahigh resolution angle-resolved photoemission spectroscopy with low-energy photons is used to study the detailed momentum dependence of the well-known nodal ‘kink’ dispersion anomaly of Bi2Sr2CaCu2O8+δ. We find that the kink's location transitions smoothly from a maximum binding energy of about 65 meV at the node of the d-wave superconducting gap to 55 meV roughly one-third of the way to the antinode. Meanwhile, the self-energy spectrum corresponding to the kink dramatically sharpens and intensifies beyond a critical point in momentum space. We discuss the possible bosonic spectrum in energy and momentum space that can couple to the k-space dispersion of the electronic kinks.

Emerging coherence with unified energy, temperature, and lifetime scale in heavy fermion YbRh2Si2

We present ultrahigh resolution angle-resolved photoemission data of the prototypical Kondo lattice system YbRh${}_{2}$Si${}_{2}$, using a 7 eV laser photon source. Detailed temperature dependent measurements reveal the development of coherent states, in the form of sharp, weakly dispersing peaks at the lowest energy of single-electron spectra, below a characteristic temperature. The characteristic temperature, obtained from angle-resolved photoemission, is intriguingly of exactly the same scale as the energy and the lifetime of the coherent state.

Observation of two fine structures related to the hidden order in the spectral functions of URu2Si2

We studied the spectral functions of URu${}_{2}$Si${}_{2}$ in the hidden-order state by means of ultrahigh-resolution angle-resolved photoemission spectroscopy. High signal-to-noise ratio data uncover the existence of two anomalous fine structures, namely a ``splitting'' in the previously reported narrow dispersive band, and a small, dispersionless ``satellite'' structure. These structures can be explained by neither ordinary bands nor the development of a hybridization of two states; thus our observation should evoke the consideration of complicated many-body effects. As a possible origin, we suggest the existence of electron-mode coupling in the ordered state.

Angle-resolved photoemission observation of the superconducting-gap minimum and its relation to the nesting vector in the phonon-mediated superconductorYNi2B2C

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy to directly study the large superconducting (SC) gap anisotropy of YNi 2 B 2 C. We succeed in measuring momentum (k) dependence of SC gap for individual Fermi surface (FS) sheets, which demonstrates complexity of SC gap in a phonon-mediated superconductor. Within measured k regions on FS sheets, we find a pointlike minimum of SC gap, whose k positions can be connected by the known nesting vector. This shows close correlation between the nesting vector and node formation.

Xenon-plasma light ultrahigh-resolution ARPES study of low-energy single-particle excitation gap in (Bi,Pb)2Sr2CuO6

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy of (Bi,Pb)2Sr2CuO6 by using a newly developed xenon-plasma light source to clarify the origin of the pseudogap (PG). We determined the comprehensive momentum and temperature dependences of the superconducting (SC) gap and the PG, and revealed a smooth evolution of the PG from the SC gap. We also found a linear scaling behavior of the characteristic PG temperature with the SC gap size regardless of the momentum location. These experimental results strongly suggest that the observed PG is caused by the precursor pairing.

Magnetic Phase Transition of CeSb Studied by Low-Energy Angle-Resolved Photoemission Spectroscopy

We have performed low-energy ultrahigh-resolution angle-resolved photoemission spectroscopy on CeSb to elucidate the bulk electronic structure responsible for the magnetic phase transition. By using a newly developed xenon-plasma discharge lamp, we clearly observed the splitting of Sb 5 p 3/2 and Ce 5 d bands near the Fermi level as well as their strong hybridization below the Neel temperature ( T N =16 K). We also revealed distinct differences in the band structure between the antiferropara- and antiferro-magnetic phases. These experimental results are discussed in terms of the p – f / p – d mixing effect, the exchange splitting, and the magnetic band-folding. The present result also explains the emergence of fine structures in the optical conductivity in the magnetic phase.

Evolution of a Pairing-Induced Pseudogap from the Superconducting Gap of(Bi,Pb)2Sr2CuO6

We have performed an ultrahigh-resolution angle-resolved photoemission spectroscopy study of slightly overdoped (Bi,Pb)2Sr2CuO6 to elucidate the origin of the pseudogap. By using a newly developed xenon-plasma light source, we determined the comprehensive momentum and temperature dependencies of the superconducting gap and the pseudogap. We found that the antinodal pseudogap persists far above the superconducting transition temperature and is smoothly connected to the nodal gap. The characteristic temperature of the pseudogap scales well with the superconducting gap size irrespective of the momentum location. The present experimental results point to the pairing origin of the pseudogap.

Quasiparticle lifetime in graphite studied by ultrahigh-resolution ARPES

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) to elucidate the nature of quasiparticle dynamics in graphite. We found fairly sharp quasiparticle peak of π band around K( H) point in the vicinity of the Fermi level, together with the strong mass renormalization of the band (kink). The imaginary part of electron self-energy (Im Σ) shows a sudden drop below 0.18 eV, indicative of a strong electron–phonon coupling. The linear energy dependence of Im Σ at higher binding energies provides an evidence for the deviation from the conventional Fermi liquid theory.

Low-energy excitations in graphite studied by ultrahigh-resolution angle-resolved photoemission spectroscopy

We report ultrahigh-resolution angle-resolved photoemission results on kish graphite. We found an evidence for a sharp quasiparticle peak around K(H) point. Self-energy analysis of quasiparticle lifetime indicates presence of a strong electron–phonon coupling and the linear energy-dependence of scattering rate which shows marked deviation from the conventional Fermi-liquid theory.

High-resolution ARPES study of many-body interactions in high-Tc cuprate superconductors

We report our recent ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) results on several hole- and electron-doped high-Tc cuprate superconductors. We have studied the momentum and temperature dependence of the kink in the dispersion near the Fermi level and its “magnetic isotope effect” in hole-doped superconductors. The momentum dependence of pseudogap and superconducting gap has been studied for electron-doped superconductors. All the experimental results unambiguously indicate that the magnetic interaction is essential in characterizing the electronic structure and the superconductivity.

Anomalous Quasiparticle Lifetime and Strong Electron-Phonon Coupling in Graphite

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on high-quality single crystals of graphite to elucidate the character of low-energy excitations. We found evidence for a well-defined quasiparticle (QP) peak in the close vicinity of the Fermi level comparable to the nodal QP in high-T(c) cuprates, together with the mass renormalization of the band at an extremely narrow momentum region around the K(H) point. Analysis of the QP lifetime demonstrates the presence of strong electron-phonon coupling and linear energy dependence of the QP scattering rate indicative of a marked deviation from the conventional Fermi-liquid theory.

Fermi surface and edge-localized states in graphite studied by high-resolution angle-resolved photoemission spectroscopy

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on graphite single crystal (kish graphite). We have successfully determined the electronic band structure in the close vicinity of the Fermi level and found an extremely small hole-like Fermi surface centered at the K(H) point. We also found a weakly dispersive band near the Fermi level around the K(H) point, which is not predicted by the bulk band calculation. The origin of this anomalous feature is discussed in relation to the electronic states characteristic of the stepped surfaces.

Para- to antiferro-magnetic phase transition of CeSb studied by ultrahigh-resolution angle-resolved photoemission spectroscopy

Temperature-dependent angle-resolved photoemission spectroscopy has been performed on CeSb to study the origin of its complicated magnetic phase transition. In paramagnetic phase ( T=30 K), we have found that the electronic structure near the Fermi level (E F) consists of the hole-like Sb 5p band at the Γ point and the electron-like Ce 5d bands at the X point. With decreasing temperature across T N ( T N=10 K), both the energy shift and the energy splitting of the bands appear along the ΓX high-symmetry line. While the energy shift of the bands is consistent with the pf mixing model, the energy splitting has not been expected in the model so far. On the other hand, by comparing with the recent calculation based on the pf+dp mixing model, we found a qualitative agreement between the experiment and the calculation. This result suggests the importance of the dp mixing effect to interpret the mechanism of the magnetic phase transition of CeSb.

Ultrahigh-resolution angle-resolved photoemission spectroscopy of La and Ce monochalcogenides

High quality single crystals of $\mathrm{Ce}{X}_{c}$ and $\mathrm{La}{X}_{c}$ with ${X}_{c}=\mathrm{S},$ Se, or Te were grown, and a systematic study of the energy band structure was carried out by ultrahigh-resolution angle-resolved photoemission spectroscopy. In $\mathrm{La}{X}_{c},$ the bottom of the conduction d bands shifts toward higher binding energy with decreasing atomic weight of the chalcogen, i.e., with decreasing lattice constant in the same way as that caused by the physical pressure, whereas the top of the valence p bands more drastically moves to higher binding energy in the opposite direction of that caused by the physical pressure. Accordingly, the energy gap between the conduction and valence bands increases from LaTe to LaS. On the other hand, the electronic structure of $\mathrm{Ce}{X}_{c}$ is well represented by that of the corresponding $\mathrm{La}{X}_{c}$ plus the localized $4f$ state except for CeTe, in which a new band related to the $4f$ state is observed. Considering the systematic change of the electronic structure from CeS to CeTe, this band is likely to originate from the hybridization of the valence p-band and $4f$ state. The role of the $p\ensuremath{-}f$ hybridization for their magnetic properties is discussed.

Ultrahigh-resolution angle-resolved photoemission study of LaX (X=S, Se, Te)

Systematic study of the energy band structures for LaX (X=S, Se, Te) was carried out by ultrahigh-resolution angle-resolved photoemission spectroscopy . The band gap between the conduction and valence bands was clearly observed in every LaX. The conduction and valence bands show the systematic changes from LaS to LaTe with regard to their energy positions and band widths .

Band structure ofUPd3studied by ultrahigh-resolution angle-resolved photoemission spectroscopy

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on ${\mathrm{UPd}}_{3}$ to study the band structure near the Fermi level ${(E}_{F})$ and the Fermi surface. We found three Fermi surfaces (FS's): two hole pockets with a dominant Pd $4d$ character at the $\mathit{\ensuremath{\Gamma}}$ (A) point and an electron pocket with a U $6d$ nature at the K (H) point. These FS's are qualitatively well reproduced in the band structure calculation based on the localized U-$5f$-electron model, while remarkable quantitative discrepancies are observed near ${E}_{F}.$ The U $5f$ states are located 0.4--1.0 eV below ${E}_{F}$ and do not contribute to the Fermi surface. These results indicate the strongly localized nature of U $5f$ electrons in ${\mathrm{UPd}}_{3}.$

Fermi surface and superconducting gap of 2H-NbSe2 using low-temperature ultrahigh-resolution angle-resolved photoemission spectroscopy

We have studied the Fermi surface (FS) and superconducting gap of 2H-NbSe2, using angle-resolved photoemission spectroscopy with an energy resolution of 2.2meV and an angle resolution of ±0.13°. We succeeded in distinguishing all FS sheets predicted from band calculations and found that the observed hexagonal Fermi surface nesting vector centered at the Γ point is larger than the CDW nesting vector reported from neutron diffraction measurements. Furthermore, a superconducting gap at one FS sheet is successfully observed.