Ultrahigh-resolution Photoemission Spectroscopy Research Articles

Hybridization gap in Kondo insulators CeRhSb and CeRhAs studied by ultrahigh-resolution photoemission spectroscopy

Ultrahigh-resolution photoemission spectroscopy (ΔE∼8meV) has been performed on Kondo insulators CeRhAs and CeRhSb. We found that the 4f-derived density of states shows a depletion (pseudogap) at EF in contrast to metallic Kondo materials. It was also found that the size of both 4f-derived and conduction-electron derived pseudogaps scales well with the Kondo temperature.

Electronic structure and superconducting gap of silicon clathrateBa8Si46studied with ultrahigh-resolution photoemission spectroscopy

We study the electronic structure and superconducting transition of silicon clathrate ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}{(T}_{c}=8\mathrm{K})$ using photoemission spectroscopy. We observe a narrow band at the Fermi level ${(E}_{F}),$ whose width (\ensuremath{\sim}0.3 eV) is substantially smaller than that of band structure calculations (\ensuremath{\sim}1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K $[2\ensuremath{\Delta}{(0)/k}_{B}{T}_{c}=3.51].$ Fine structures associated with phonons are observed within 70 meV of ${E}_{F}.$ These results characterize ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ as a weak-coupling superconductor most probably driven by phonon.

Crystal-field splitting in CeB6 observed by ultrahigh-resolution photoemission spectroscopy

Abstract We report ultrahigh-resolution (Δ E =8 meV) photoemission spectroscopy on CeB 6 to study the electronic structure and the crystal-field splitting. The photoemission spectrum of CeB 6 near E F shows two prominent structures; a broad one near E F and a relatively sharp peak at 280 meV. The former is ascribed to the “Kondo resonance peak” at E F while the latter being from the spin–orbit satellite (Ce 4f 7/2 ). In the broad structure near E F , we clearly observed an additional small structure around 60 meV from E F . Numerical simulations based on the single-impurity Anderson model with non-crossing approximation show that this satellite is due to the crystal-field splitting ( Γ 8 – Γ 7 ) of the Ce 4f 5/2 state.

Ultrahigh-resolution photoemission and resonant-inverse photoemission spectroscopy on CePd3

Abstract We carried out ultrahigh-resolution photoemission (URPE) and resonant-inverse photoemission spectroscopy on CePd3 and CePd2.9Cu0.1 compounds. The URPE spectra of CePd3 and CePd2.9Cu0.1 clearly show small humps in the vicinity of Fermi level well below the Kondo temperature. The hump is gradually formed below the characteristic coherent temperature T*. We conclude that it gives direct evidence of the appearance of the small pseudogap below T* in CePd3 and CePd2.9Cu0.1.

Ultrahigh-resolution photoemission study of CePd3: absence of Kondo insulator gap

Abstract We have performed ultrahigh-resolution (Δ E =8 meV) photoemission spectroscopy to examine the possibility of CePd 3 being a ‘Kondo insulator’ as suggested by a recent optical measurement [Phys. Rev. B 53 (1996) R2948]. We found that the photoemission spectrum of CePd 3 does not show any indication of (pseudo) gap at E F , differing from the well-established temperature-dependent (pseudo) gap in Kondo insulators. The Ce 4 f spectrum clearly shows the ‘Kondo resonance peak’ at E F , indicating the intrinsic metallic nature of CePd 3 . These results strongly suggest that CePd 3 is not a Kondo insulator but a conventional Kondo metal as suggested by thermodynamic measurements.

Ultra-high resolution photoemission spectroscopy of simple metals: direct observation of superconducting gap and phonon-induced fine structures

Abstract We have performed ultra-high resolution photoemission spectroscopy of simple metals, Nb ( T c =9.26 K) and Pb ( T c =7.19 K) in order to study the changes in the density of states across the transition temperatures. The spectra of both compounds measured at 5.3 K show an opening of the superconducting gap accompanied by a leading-edge shift and an appearance of a sharp peak just below the Fermi level. Furthermore, the Pb spectrum clearly shows peak-dip-hump structures due to strong electron–phonon coupling, as seen in the high- T c cuprates. This makes it a characteristic of strong-coupling superconductivity.

Ultrahigh-resolution photoemission spectroscopy of Ni borocarbides: direct observation of the superconducting gap and a change in gap anisotropy by impurity

We have performed ultrahigh-resolution photoemission spectroscopy of Y(Ni1-xPtx)2B2C ( x = 0.0 and 0.2) in order to study the changes in the density of states across the superconducting transition. Because of a drastic increase in energy resolution, we clearly observe the opening of superconducting gaps across T(c) in both compounds. Furthermore, we find a small but significant difference in the superconducting-state spectral shape. This can be explained in terms of reduction in gap anisotropy by introducing impurities and provides spectroscopic evidence for an anisotropic s-wave gap in YNi2B2C.

Small and large pseudogaps in high- T c superconductors observed by ultrahigh-resolution photoemission spectroscopy

Two different (small and large) pseudogaps have been observed by ultrahigh-resolution photoemission spectroscopy in optimally doped Bi 2Sr 2CaCu 2O 8 and La 1.85Sr 0.15CuO 4. The small pseudogap is smoothly connected from the superconducting gap and closes slightly above T c , while the large one appears as a remarkable depletion of the density of states in a wider energy range near E F and persists far above T c . The small pseudogap is likely associated with pairing and the large one is ascribed to the development of antiferromagnetic correlation.

Pseudogap formation in Kondo insulators CeRhSb and CeRhAs studied by ultrahigh-resolution photoemission spectroscopy

We report an ultrahigh-resolution (Δ E∼8 meV) temperature-dependent photoemission spectroscopy on the Kondo insulators CeRhAs and CeRhSb. We have observed a pseudogap at the Fermi level at low temperature for both compounds, which is gradually filled-in with increasing temperature. We found that the size of pseudogap is well scaled with the Kondo temperature ( T K) while the temperature evolution is dominated by another characteristic temperature much lower than T K.