R. S. List

Photoelectron spectroscopic evidence for superconducting gap isotropy in the basal plane of the high-temperature superconductor Bi2Sr2CaCu2O8

Abstract High-resolution angle-resolved photoelectron spectra were obtained at 20K (below T c ) and 90K (above T c ) at several points on the Fermi surface of Bi 2 Sr 2 CaCu 2 O 8 in the basal plane. These points were of different wave function character (CuO, BiO) according to one-electron band calculations. The same value of the superconducting gap Δ was obtained for all these points. Additional points in k-space were examined less extensively, and all were consistent with the same gap. The lack of basal plane gap anisotropy puts a constraint on theories of the superconducting pairing mechanism.

Electronic structure of the gold/Bi2Sr2CaCu2O8 and gold/EuBa2Cu3O7−δ interfaces as studied by photoemission spectroscopy

High‐resolution photoemission has been used to probe the electronic structure of the gold/Bi2Sr2CaCu2O8 and gold/EuBa2Cu3O7−δ interface formed by a low‐temperature (20 K) gold evaporation on cleaved high quality single crystals. We find that the metallicity of the EuBa2Cu3O7−δ substrate in the near surface region (∼5 A) is essentially destroyed by the gold deposition, while the near surface region of Bi2Sr2CaCu2O8 remains metallic. This has potentially wide ranging consequences for the applicability of the different types of superconductors in real devices.

Chemical and electronic properties of the Pt/GaAs(110) interface

The interaction between evaporated Pt and the GaAs (110) surface has been studied at room temperature in ultrahigh vacuum utilizing surface‐sensitive soft x‐ray and ultraviolet photoemission spectroscopies. Detailed curve‐fitting analysis of the substrate and metal core levels indicates dissociation of the GaAs with a reaction between the Pt and the Ga and a strong reaction with the As as well. Evidence of Pt–As compound formation is observed. Above 6.67 monolayers (ML) (10 A), a second reacted component dominates and the As‐core energy shifts smoothly to a final position ∼0.3‐eV‐higher binding energy than As in GaAs. The reacted Ga has a well‐defined energy ∼0.4 eV less bound than the Ga in GaAs. This correlates well with the calculated core level binding energy shift reported by Nogami et al. for dilute Ga in Pt. The Schottky barrier height is established by 0.67 ML and is observed to be 0.83 (0.48)±0.1 eV for n‐type (p‐type) substrates, in agreement with electrical measurements on much thicker diodes. ...

The Si/GaAs (110) heterojunction: Strain, disorder, and valence‐band discontinuity

We have studied the effects of strain on band offsets through a combined photoemission and polarization dependent surface extended x‐ray absorption fine structure (SEXAFS) study of thin Si overlayers on the 4% lattice‐mismatched GaAs (110) substrate. By selectively orienting the heterojunction interface either parallel to or perpendicular to the x‐ray polarization vector, we have independently measured from SEXAFS the Si–Si nearest‐neighbor bond lengths of bonds lying either in the plane or out of the plane of the interface, respectively. The in‐plane bond lengths were larger by a maximum of 2% for our thinnest coverage of 4 monolayers. An ideal pseudomorphic overlayer would have a 4% difference between these bond lengths. We therefore infer a large density of dislocations even in such thin overlayers. From our photoemission measurements we found no resolvable (>0.05 eV) differences in the valence‐band discontinuity between Si overlayers which were either crystalline and 2% anisotropically strained, cryst...

Angle resolved photoemission of NiO(001)

Abstract We report off-normal angle-resolved photoemission data from NiO(001), which is complementary to the earlier normal-emission result. On the one hand, the energy position of the bands along this direction and the dispersion of the lowest oxygen band agree with the band calculation very well. On the other hand, the existence of the valence band satellite and the absence of the energy band in the gap region do not agree with the band calculation. For the first time, interesting movement of the Ni d7 satellite has been observed as a function of the emission angle and the photon energy, which is most likely a result of the satellite dispersion.

Search for a proximity effect induced gap in gold/high Tc junctions

We have used high‐resolution photoemission spectroscopy to search for a proximity effect induced superconducting gap in gold overlayers on c‐axis single crystals of Bi2Sr2CaCu2O8 and a‐axis thin films of YBa2Cu3O7. These two junction types give us a representative sampling of very well characterized near‐ideal interfaces (gold/c‐axis Bi2Sr2CaCu2O8) and junctions in which the geometry more strongly favors the existence of the proximity effect but the interfacial quality may not be as ideal (gold/a‐axis YBa2Cu3O7). In neither of these junction types did we observe any evidence for a proximity effect induced gap, and we place an upper limit of approximately 5 meV on its existence in the junctions that we have studied.

Photoemission from single‐crystal EuBa2Cu3O6+x cleaved below 20 K; Metallic‐to‐insulating surface transformation

Valence band ultraviolet photoemission spectra (UPS) of single‐crystal EuBa2Cu3O6+x (x≳0.6) samples cleaved in vacuum at 20 K demonstrate that the metallic superconducting phase undergoes an irreversible transformation via near‐surface oxygen loss to an insulating state upon annealing above 50 K. Freshly cleaved surfaces at 20 K exhibit a density of states at the Fermi level comparable to that of copper, and have both O(2p) and Cu(3d) character at EF based on the photon energy dependence of the intensity. Reasonably good agreement between band structure calculations and the present data would suggest theoretical models using the band state as a starting point.

Normal State Electronic Structure and the Superconducting Energy Gap in HTSC’s as Determined from Photoemission Spectroscopy

Photoemission spectroscopy is utilized to determine the electronic structure of high-Tc materials. The observation of dispersive bands at EF suggests a Fermi surface similar to that obtained from a band calculation. These results, together with a BCS-like energy gap, are not inconsistent with the notion of a Fermi liquid consisting of hybridized p-d bands.