Bulk-sensitive Photoemission Spectroscopy Research Articles

Discovery of Weyl Fermion Semimetals and Topological Fermi Arc States

Weyl semimetals are conductors whose low-energy bulk excitations are Weyl fermions, whereas their surfaces possess metallic Fermi arc surface states. These Fermi arc surface states are protected by a topological invariant associated with the bulk electronic wave functions of the material. Recently, it has been shown that the TaAs and NbAs classes of materials harbor such a state of topological matter. We review the basic phenomena and experimental history of the discovery of the first Weyl semimetals, starting with the observation of topological Fermi arcs and Weyl nodes in TaAs and NbAs by angle and spin-resolved surface and bulk sensitive photoemission spectroscopy and continuing through magnetotransport measurements reporting the Adler–Bell–Jackiw chiral anomaly. We hope that this article provides a useful introduction to the theory of Weyl semimetals, a summary of recent experimental discoveries, and a guideline to future directions.

Bulk nature of layered perovskite iridates beyond the Mott scenario: An approach from a bulk-sensitive photoemission study

We present genuine bulk Ir 5d jeff states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s- and p-polarized lights. HAXPES spectra of Sr2IrO4 and Ba2IrO4 are well reproduced by the quasiparticle density of states calculated by the local density approximation with dynamical mean-field theory (LDA+DMFT). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-low-energy bulk-sensitive photoemission spectroscopy reveals “bad metallic” states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Néel temperature as predicted by the LDA+DMFT.Received 27 October 2013Revised 2 March 2014DOI:https://doi.org/10.1103/PhysRevB.89.121111©2014 American Physical Society

Bulk-Sensitive Photoemission Spectroscopy of TlFe2Se2

We have investigated the electronic structures of the parent compound of an iron chalcogenide superconductor TlFe2Se2 by bulk-sensitive photoemission spectroscopy (PES). Valence-band PES demonstrates the energy gap opening at the Fermi level due to the shift of Fe 3d state to the higher binding energy side as compared to the calculated density of states, being consistent with the insulating behaviour in transport measurements.

Electronic properties of embedded MnAs nano-clusters in a GaAs matrix and (Ga,Mn)As films: Evidence of distinct metallic character

We investigated the electronic properties of MnAs nano-clusters embedded in GaAs by bulk sensitive photoemission spectroscopy and cross-sectional scanning tunneling microscopy/spectroscopy. We report experimental evidences that the clusters are metallic MnAs, in close resemblance to MnAs thin films, and display a sharp interface with the surrounding GaAs. These results are supported by the comparison with GaMnAs and MnAs film in the same experimental condition. Furthermore, we observe a clear Coulomb blockade effect, as due to confinement and their nanometric size.

Electron correlation in the FeSe superconductor studied by bulk-sensitive photoemission spectroscopy

We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high bulk sensitivity. The large Fe 3d spectral weight is located in the vicinity of the Fermi level (EF), which is demonstrated to be a coherent quasi-particle peak. Compared with the results of the band structure calculation with local-density approximation, Fe 3d band narrowing and the energy shift of the band toward EF are found, suggesting an importance of the electron correlation effect in FeSe. The self energy correction provides the larger mass enhancement value (Z^-1=3.6) than in Fe-As superconductors and enables us to separate a incoherent part from the spectrum. These features are quite consistent with the results of recent dynamical mean-field calculations, in which the incoherent part is attributed to the lower Hubbard band.

Novel behaviors in rare-earth-filled skutterudites studied by bulk-sensitive photoemission spectroscopy

Abstract The electronic structures of rare-earth-filled skutterudites have been investigated by high-resolution photoemission spectroscopy. In order to reveal their bulk nature, the bulk-sensitive photoemission spectroscopy with soft X-ray (hv∼800 – 1100 eV) and hard X-ray (hv∼8000 eV) is a very powerful tool since the rare-earths' valence and electronic structures on the surface are often different from those in the bulk. Our experimental results clearly show the coexistence of the strongly mixed-valence state and the heavy-fermion state in SmOs4Sb12 and the quasi-particle peak near the Fermi level in PrFe4P12 induced by the hybridization between the rare-earth 4f and conduction electrons.

High-intensity xenon plasma discharge lamp for bulk-sensitive high-resolution photoemission spectroscopy

We have developed a highly brilliant xenon (Xe) discharge lamp operated by microwave-induced electron cyclotron resonance (ECR) for ultrahigh-resolution bulk-sensitive photoemission spectroscopy (PES). We observed at least eight strong radiation lines from neutral or singly ionized Xe atoms in the energy region of 8.4-10.7 eV. The photon flux of the strongest Xe I resonance line at 8.437 eV is comparable to that of the He Ialpha line (21.218 eV) from the He-ECR discharge lamp. Stable operation for more than 300 h is achieved by efficient air-cooling of a ceramic tube in the resonance cavity. The high bulk sensitivity and high-energy resolution of PES using the Xe lines are demonstrated for some typical materials.

Bulk sensitive photo emission spectroscopy of [formula omitted] compounds

This work reports about bulk-sensitive, high energy photoelectron spectroscopy from the valence band of CoTiSb excited by photons from 1.2 to 5 keV energy. The high energy photoelectron spectra were taken at the KMC-1 high energy beamline of BESSY II employing the recently developed Phoibos 225 HV analyser. The measurements show a good agreement to calculations of the electronic structure using the LDA scheme. It is shown that the high energy spectra reveal the bulk electronic structure better compared to low energy XPS spectra.

Bulk-Sensitive Photoemission Spectroscopy for Heavy-Fermion Pr Compounds Using Hard X-Ray

Bulk-sensitive photoemission spectroscopy using hard x-ray up to h ν=5450 eV has been performed for heavy-fermion Pr compounds. A significant spectral intensity of the |3 d 9 4 f 3 > final state was observed in Pr 3 d core-level spectra of PrFe 4 P 12 at both h ν=2450 and 5450 eV. The spectra obtained by the configuration-interaction cluster model calculation reproduce the experimental spectra for well-hybridized PrFe 4 P 12 and localized Pr metal. Results of the calculations suggest that the Kondo behavior observed in PrFe 4 P 12 arise from the mixing of the |4 f 2 > and |4 f 3 > configurations due to the c – f hybridization. Whereas a contribution possibly from a disordered surface to the spectrum can be negligible for the fractured PrFe 4 P 12 , they cannot be disregarded for the scraped PrSn 3 and Pr metal even in the high-energy x-ray region.

Bulk-sensitive photoemission spectroscopy of Pr-based filled skutterudites

Bulk electronic structures of Pr-based filled skutterudites and their reference materials have been investigated by bulk-sensitive photoemission spectroscopy using soft and hard X-rays. For Pr 3 d – 4 f resonance, a strong intensity in the vicinity of Fermi level ( E F ) is observed only for PrFe 4P 12. Furthermore, PrFe 4P 12 has the significant spectral weight of the | 3 d 9 4 f 3 〉 final state in Pr 3 d core-level spectrum. We interpret that the observed prominent intensity just below E F arises from a Kondo resonance due to the strong hybridization between the Pr 4 f and conduction states.

Surface electronic structure and Verwey-transition of magnetite studied by photoemission spectroscopy

We report on a detailed study of single crystal surfaces with (111) and (100) orientations by means of photoemission spectroscopy (PES). The surfaces were prepared by sputtering, annealing, and post-oxidizing and characterized using LEED, XPS, and STM. The surface electronic structure turns out to be strongly dependent on the surface orientation and preparation conditions. This also holds for more bulk sensitive PES measurements using photons with an energy at the Fe 2p resonance threshold. Thus we conclude that the surface region extends much deeper into the bulk than the topmost atomic layer.