The discovery of high temperature superconductors in 1986 was one of the most exciting breakthroughs in condensed matter physics. The critical temperature, the temperature below which electrons pair and there is no resistance, of many materials in this family was substantially higher than all other know superconductors. However, almost 30 years later we still don't know why these materials superconduct. To become a superconductor the electrons in a material must overcome their coulomb repulsion and form pairs. One way to measure this pairing mechanism is to measure the density of superconducting pairs (superfluid density) and track how it changes with temperature. In this paper we measured the superfluid density using a magnetic force microscope in a series of samples with different chemical doping. Our measurements show that proximity to a magnetic state may contribute to the electron pairing mechanism.

The image shows a sketch of a magnetic force microscope tip. This is the sensor that was used to make these measurements.

Lan Luan, Thomas M. Lippman, Clifford W. Hicks, Julie A. Bert, Ophir M. Auslaender, Jiun-Haw Chu, James G. Analytis, Ian R. Fisher, and Kathryn A. Moler, Physical Review Letters 106 067001 (2011). Full Text