Josephson scanning tunneling microscopy: A local and direct probe of the superconducting order parameter

TL;DR

This paper demonstrates the use of Josephson scanning tunneling microscopy to directly and locally probe the superconducting order parameter in high-temperature superconductors like BSCCO, revealing inhomogeneities and relationships with the quasiparticle density of states.

Contribution

It introduces a novel application of Josephson STM for high-resolution, local measurements of the superconducting order parameter in complex materials.

Findings

Successful measurement of Josephson tunneling in BSCCO

Identification of local inhomogeneities in the superconducting gap

Determination of measurement limits to avoid altering the sample

Abstract

Direct measurements of the superconducting superfluid on the surface of vacuum-cleaved Bi2Sr2CaCu2O8(BSCCO) samples are reported. These measurements are accomplished via Josephson tunneling into the sample using a scanning tunneling microscope (STM) equipped with a superconducting tip. The spatial resolution of the STM of lateral distances less than the superconducting coherence length allows it to reveal local inhomogeneities in the pair wavefunction of the BSCCO. Instrument performance is demonstrated first with Josephson measurements of Pb films followed by the layered superconductor NbSe2. The relevant measurement parameter, the Josephson ICRN product, is discussed within the context of both BCS superconductors and the high transition temperature superconductors. The local relationship between the ICRN product and the quasiparticle density of states (DOS) gap are presented within the context of phase diagrams for BSCCO. Excessive current densities can be produced with these measurements and have been found to alter the local DOS in the BSCCO. Systematic studies of this effect were performed to determine the practical measurement limits for these experiments. Alternative methods for preparation of the BSCCO surface are also discussed.