Calibration of shielded microwave probes using bulk dielectrics

A stripline-type near-field microwave probe is microfabricated for microwave impedance microscopy. Unlike the poorly shielded coplanar probe that senses the sample tens of microns away, the stripline structure removes the stray fields from the cantilever body and localizes the interaction only around the focused-ion beam deposited Pt tip. The approaching curve of an oscillating tip toward bulk dielectrics can be quantitatively simulated and fitted to the finite-element analysis result. The peak signal of the approaching curve is a measure of the sample dielectric constant and can be used to study unknown bulk materials.Comment: 10 pages, 3 figure

Doping evolution of the electronic structure in the single-layer cuprates Bi2_2Sr2−x_{2-x}Lax_xCuO6+δ_{6+\delta}: Comparison with other single-layer cuprates

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly-doped to optimally-doped regions. We have observed the formation of the dispersive quasi-particle band, evolution of the Fermi ``arc'' into the Fermi surface and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_2$ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc/surface are formed around the Fermi level already in the lightly-doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral $t'$ also confirm the similarity to Na-CCOC and the difference from LSCO

Symmetry of the Gap in Bi2212 from Photoemission Spectroscopy

In a recent Letter, Shen et al have detected a large anisotropy of the superconducting gap in Bi2212, consistent with d-wave symmetry, from photoemission spectroscopy. Moreover, they claim that the change in their spectra as a function of aging is also consistent with such an intrepretation. In this Comment, I show that the latter statement is not entirely correct, in that the data as a function of aging are inconsistent with a d-wave gap but are consistent with an anisotropic s-wave gap.Comment: 3 pages (Plain TeX with macros), plus 1 postscript figur