Strongly disordered superconductors

We study how strong disorder reshapes superconductivity and drives the superconductor–insulator transition (SIT). Our central question is how pairing amplitude, phase coherence, and electronic inhomogeneity evolve from the superconducting state to insulating behavior—and what universal laws govern this evolution. Using ultrathin films (granular Al, InOx_xx​, TiN and related materials), we probe the SIT from the nanoscale to microwave frequencies.

Microscopically, scanning tunneling spectroscopy reveals spatially inhomogeneous gaps and pseudogapped states, showing that local pairing can persist even when long-range phase coherence collapses. In transport, we map phase-dominated regimes, vortex-glass behavior, and signatures of localized Cooper pairs on the insulating side. At microwave frequencies, we quantify how superfluid stiffness and dissipation change with disorder, extracting robust trends that link the internal quality factor of resonators to intrinsic material scales—clarifying practical coherence limits for superconducting circuits.

Methodologically, we combine advanced nanofabrication, ultra-low-noise DC transport under extreme conditions (mK temperatures, high magnetic fields), scanning tunneling microscopy/spectroscopy, and broadband microwave electrodynamics. Together, these tools let us connect real-space inhomogeneity to phase stiffness, critical currents, and high-frequency loss.

The broader goal is a predictive framework for engineering disordered superconductors as functional elements—superinductors, protected resonators, and materials testbeds for quantum technologies—while establishing falsifiable benchmarks for theories of the SIT. Along the way, we emphasize open, reproducible measurements and cross-checks across techniques so that results generalize beyond a single material system.