Low-temperature Photoemission Spectroscopy Research Articles

Coexistence of Bulk-Nodal and Surface-Nodeless Cooper Pairings in a Superconducting Dirac Semimetal.

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 T_{c}∼4.5 K. 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.

High-resolution, low-temperature photoemission spectroscopy of Kondo semiconductor CeRhAs and related compounds

Ce 4f states of the Kondo semiconductor CeRhAs, semimetal CeRhSb, and metal CePtSn single crystals were observed directly by high-resolution resonant photoemission spectroscopy. A large gap and a pseudogap were found in CeRhAs and CeRhSb, respectively.

Electron spectroscopies of d,f electron systems at Hiroshima Synchrotron Radiation Center

The Hiroshima Synchrotron Radiation Center has started to deliver the light from 1998. A linear and a helical undulator beamline, dedicated mainly for high energy resolution and low-temperature photoemission spectroscopy, have been installed and applied to studies on research subjects related to d,f electron systems; direct observation of the Ce 4f states in CeRhSb single crystal and metal–insulator transition in Y 1− x Ca x TiO 3 single crystals. Photoemission and inverse photoemission spectroscopies using tunable photon and tunable electron beam have also been developed and employed for research on electronic structure of zincblende and hexagonal MnTe.

Superconducting transitions studied by ultrahigh-resolution photoemission

Superconducting transitions of elemental metals and Ni borocarbides are studied using ultrahigh-resolution and low-temperature photoemission spectroscopy. Photoemission spectra of elemental metals (Nb and Pb) show the appearance of a sharp peak just below the Fermi level and a shift of the leading edge, indicating clear opening of the superconducting gap. Furthermore, Pb spectrum shows a dip and hump structure beyond the peak, characteristic of the strong-coupling superconductivity. On the other hand, comparative study of Y(Ni 1− x Pt x ) 2B 2C ( x=0.0 and 0.2) shows a difference in the slope of the leading edge, which can be explained in terms of a highly anisotropic s-wave gap in YNi 2B 2C, consistent to magnetic field dependent specific heat measurements. These observations demonstrate that photoemission spectroscopy is a useful and reliable technique to study electronic structures and phase transitions at very small energy scales.

HIGH-RESOLUTION, LOW-TEMPERATURE PHOTOEMISSION SPECTROSCOPY AT THE HISOR LINEAR UNDULATOR BEAMLINE

In order to study the electronic states of solids, using high-resolution, low-temperature photoemission spectroscopy (PES) with tunable photon energies, we have constructed a high-resolution undulator beamline operating in the photon-energy range 26–300 eV at the compact 700 MeV electron-storage ring HiSOR located at Hiroshima University. By way of gas-phase experiments, with photon flux larger than 2 × 1010 photons/s, photon-energy resolving powers of E/ΔE > 16000 and 3800 were obtained at hν ~ 48 eV and ~ 244 eV, respectively. High-resolution PES experiments on evaporated Au at 8 K were performed at the beamline with a total energy resolution of ΔE = 12 meV at hν = 48 eV. Among the first experiments from the beamline, a resonant photoemission study of UNiSn is presented. The U 5f derived peak is significantly enhanced at hν = 98 eV.

HIGH-RESOLUTION AND LOW-TEMPERATURE PHOTOEMISSION SPECTROSCOPY AT THE HISOR HELICAL-UNDULATOR BEAMLINE

We report an experimental system dedicated for high-resolution and low-temperature photoemission spectroscopy installed on the helical-undulator beamline of a compact 700 MeV electron-storage ring (HiSOR) at Hiroshima University. A 3 m off-plane Eagle normal-incidence monochromator at the beamline covers the photon-energy range of hν = 4–40 eV. From the photoemission spectra near the Fermi edge of evaporated Au film, the total energy resolution was estimated to be 4.5 and 7.5 meV at hν = 7.1 and 22.6 eV, respectively. We have also performed photoemission measurements of Yb metal, which demonstrated highly bulk-sensitive spectra for the excitation by low energy photon. The high-resolution photoemission spectra of Y 1 - x Ca x TiO 3 with x = 0.41 in the vicinity of the boundary between the metallic and insulating phases exhibited anomalous temperature dependence in comparison with those of normal metals.

High-resolution and low-temperature photoemission study of a Kondo insulator

High-resolution and low-temperature photoemission spectroscopy has been carried out on the Kondo insulator Ce 3Bi 4Pt 3. The Ce 3d spectrum indicates that the Ce ion is close to trivalent at low temperature, corresponding to the Kondo regime for Ce 3Bi 4Pt 3. The Ce 4f spectral weight was obtained by difference, and it showed a sharp peak at the Fermi level and broad structures at 120 and 300 meV. We compared the Ce 4f spectrum with the Ce 4f partial density of states given by the LDA calculation and found reasonable agreement.

High-Resolution Photoemission Study of UNi2Al3and URu2Si2

High-resolution low-temperature photoemission spectroscopy has been performed for UNi 2 Al 3 and URu 2 Si 2 to study the nature of 5 f electrons in U-based heavy fermion materials. Photo- emission spectra of both compounds have a sharp peak at E F and a large broad peak around 2 eV, while only UNi 2 Al 3 exhibits an additional small broad feature at 0.6 eV. The two features in the vicinity of E F (the E F -peak and the small structure at 0.6 eV) are found to have a dominant U 5 f character while the prominent peak around 2 eV is due to the Ni 3 d or Ru 4 d states. Comparison with the band structure calculations and experimental results on UPd 2 Al 3 suggests that the E F -peak is ascribed to itinerant U 5 f electrons while the following broad feature at 0.6 eV represents localized f electrons.

High-resolution photoemission study of CeRu2: The dual character of 4f electrons.

High-resolution low-temperature photoemission spectroscopy was performed for Ce${\mathrm{Ru}}_{2}$ to study the nature of low-lying excitations around the Fermi level (${E}_{F}$). It was found that the photoemission spectrum near ${E}_{F}$ shows two well-resolved peaks at and about 270 meV below ${E}_{F}$, which are ascribed to the tail of the Ce 4 ${f}^{1}$ final state located just above ${E}_{F}$ and its spin-orbit satellite (${\ensuremath{\Delta}}_{\mathrm{SO}}\ensuremath{\sim}270$ meV), respectively. This suggests that the many-body-correlation effect is necessary for understanding the electronic structure near ${E}_{F}$ of Ce${\mathrm{Ru}}_{2}$, although the Ce $4f$ electron exhibits a substantially itinerant character through the strong hybridization with the conduction electrons.

High-resolution photoemission study of UNi 2Al 3 and URu 2Si 2

High-resolution low-temperature photoemission spectroscopy was performed for UNi 2Al 3 and URu 2Si 2 to study the nature of 5 f electrons in U-based heavy fermion materials. Photoemission spectra of both compounds have a sharp peak at E f and a large broad peak around 2 eV, while only UNi 2Al 3 exhibits an additional small broad feature at 0.6 eV. The two features in the vicinity of E f (the E f-peak and the small structure at 0.6 eV) were found to have a dominant U 5 f character. Comparison with the band structure calculations and experimental results on UPd 2Al 3 suggests that the E f-peak is ascribed to itinerant U 5 f electrons while the following broad feature at 0.6 eV represents localized f electrons.

High-energy-resolution photoemission study of CeNiSi2 and CePtSi2.

We studied the electronic structure of isostructural ${\mathrm{CeNiSi}}_{2}$ and ${\mathrm{CePtSi}}_{2}$ using high-resolution low-temperature photoemission spectroscopy. ${\mathrm{CeNiSi}}_{2}$ is a typical valence fluctuation material, while ${\mathrm{CePtSi}}_{2}$ is a low ${\mathit{T}}_{\mathit{K}}$(\ensuremath{\sim}10 K) heavy-fermion system. The valence-band spectra show that Ni 3d derived states appear at energies closer to the Fermi level (${\mathit{E}}_{\mathit{F}}$) compared to the Pt 5d derived states. The near-${\mathit{E}}_{\mathit{F}}$ spectra exhibit the characteristic spin-orbit splitting of 4f states (4${\mathit{f}}_{5/2}$ and 4${\mathit{f}}_{7/2}$), with higher 4${\mathit{f}}_{5/2}$ intensity in ${\mathrm{CeNiSi}}_{2}$ than in ${\mathrm{CePtSi}}_{2}$. Numerical simulations based on the single-impurity Anderson model show that the hybridization between the conduction band and f electrons is stronger in ${\mathrm{CeNiSi}}_{2}$ than in ${\mathrm{CePtSi}}_{2}$. This is qualitatively understood in terms of proximity of Ni 3d states to the f level. The obtained spectroscopic results were discussed in comparison with transport and magnetic measurements. \textcopyright{} 1996 The American Physical Society.