Mohammad Hamidian – Condensed Matter Experiment Candidate Seminar

Directly Imaging the Super States of Nature

A superconductor is a homogeneous quantum condensate of Cooper pairs, each formed by binding two electrons into a zero-spin, zero-momentum eigenstate. In 1964 FuldeFerrel-Larkin-Ovchinnikov (FFLO) proposed an alternative ground state wavefunction of Cooper pairs formed under a magnetic field, thus arriving at a new super state of electronic matter. The resulting pairs carry momentum Q requiring the superfluid density to modulate with wavevector Q. The last 40 years have seen a proliferation of novel and exotic superconductors. D espite decades of effort, however, FFLO-like states remained unobserved. The challenge to detect modulated superfluids has become particularly urgent because of implications for the theory of high temperature superconductivity, and in particular for the class of materials with some of the highest transition temperatures. The presence of a spontaneously generated modulated super state in the cuprate compounds, referred to as a Pair Density Wave (PDW), provides a possible missing link to unify our understanding of the cuprate phase diagram and ultimately the mechanism for high temperature superconductivity. Over the last decade we have engineered novel instruments for electronic structure imaging to discover exotic phases of quantum matter. I will describe the development of nanometer-resolution scanned Josephson tunneling microscopy (SJTM) to directly image Cooper-pair density and the first visualization of a modulated superfluid. I will go on to describe how we are also converging on the existence of PDW state in the high field regime of the cuprates. By using sub-unit-cell magnetic field subtraction imaging we uncovered eight unit-cell density modulations in superconducting vortex halos, the long predicted PDW signature in cuprates. Our new visualization techniques for pair condensates, furthermore, opens the prospect of condensate visualization in other unconventional superconductors , and will be especially advantageous in the study of topological superconductors.