Strain-engineered interaction of quantum polar and superconducting phases

The pairing mechanism of unconventional superconductivity in strontium titanate is hotly debated. Here, using a multisensor experimental apparatus with a mechanical strain cell, an optical microscope, and with transport and magnetic probes all contained in a closed-cycle dilution refrigerator, we determined that the superconducting transition temperature of strontium titanate increases dramatically even for very small strains induced by application of uniaxial tension. These results imply that superconductivity is controlled by very small atomic shifts; the only strain-sensitive pairing channel candidate is the one linked to quantum ferroelectric (polar) instability. This investigation, therefore, uncovers additional constraints on the debated theories of superconductivity in this low carrier concentration material near the ferroelectric quantum phase transition.