Biography
Biography: Pushan Ayyub
Abstract
The study of superconductivity in nanostructured systems is particularly fascinating due to the existence of a multitude of length scales, such as the coherence length (x) and the penetration depth (lL). Here, we focus on quasi-zero dimensional superconductors, such as isolated nanoparticles or nanocrystalline solids. In such systems, superconductivity usually persists down to length scales much smaller than x and lL. Ultimately, the lower size limit for superconducting order to exist is set by the ‘Anderson criterion’, which arises from quantum confinement and is believed to be remarkably accurate and universal. We report, however, a recent result that questions the validity of the Anderson criterion. We show that phase-pure, nanocrystalline bcc-Ta remains superconducting (with, TC»0.9K) down to sizes 40% below the conventional estimate of the Anderson limit for Ta (4.0nm). Further, both the TC and HC exhibit unusual, non-monotonic size dependences, which we explain in terms of a complex interplay of quantum size effects, surface phonon softening and lattice expansion. An estimation of TC within first-principles density functional theory shows that even a moderate lattice expansion allows superconductivity in Ta to persist down to sizes much below the Anderson limit. This indicates the possibility of bypassing the Anderson criterion by suitable crystal engineering and obtaining superconductivity at arbitrarily small sizes, an obviously exciting prospect for futuristic quantum technologies. We take a critical look at how lattice expansion modifies the Anderson limit, an issue of fundamental interest to nanoscale superconductivity.