Superconductivity is a macroscopic phenomenon characterized by zero electrical resistance and complete expulsion of magnetic fields below a certain temperature, the superconducting transition temperature Tc. Superconductivity involves the formation of so-called Cooper pairs of electrons within a crystal, which often is mediated by the quantized vibrations of the atoms (phonons). In conventional theories of superconductivity, the phonon frequency of needs to be very slow compared to the electron motion. SrTiO3 is a system in which this condition is violated: the characteristic frequency of phonon vibrations is comparable to that of the electron motion, but the system still exhibits superconductivity, which has been a puzzle for more than five decades.
The application of reversible, elastic uniaxial strain has recently emerged as a powerful means to study and manipulate quantum materials. The effects of uniaxial stress beyond the elastic regime, however, are not widely studied in single-crystalline materials, with notable exceptions in the field of geophysics. The resultant plastic deformation fundamentally differs from elastic strain, as it creates extended defects – dislocations – and induces their self-organization into structures spanning many length scales. The local atomic arrangement near dislocation cores is very different from bulk, significantly modifying nearby electronic properties. Such effects are expected to be amplified when dislocations assemble into larger structures.
In this work, a team of scientists from the University of Minnesota Center for Quantum Materials, in collaboration with scientists at Argonne National Laboratory (ANL) and Oak Ridge National Laboratory (ORNL), uncovered inhomogeneously-enhanced superconductivity in plastically-deformed SrTiO3 crystals, with superconducting transition temperatures significantly greater than for undeformed samples. Through an extensive collaboration between theoretical and experimental scientists within the center, this enhanced superconductivity was found to be associated with self-organized dislocation structures formed as a result of plastic deformation, which decreases some of the phonon frequencies locally, and hence favors superconductivity. The study demonstrates the promise of plastic deformation as a tool to manipulate quantum materials to obtain exciting electronic properties.