Superconductivity on a Charge Diet

Abstract

Viewpoint: Superconductivity on a Charge Diet Lu Li, Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA Published April 15, 2013 | Physics 6, 45 (2013) | DOI: 10.1103/Physics.6.45 A study of the thermoelectric properties of the doped insulator, strontium titanate, shows that it superconducts with the lowest charge density ever observed. Fermi Surface of the Most Dilute Superconductor Xiao Lin, Zengwei Zhu, Benoît Fauqué, and Kamran Behnia Phys. Rev. X 3, 021002 (2013) Published April 15, 2013 | PDF (free) +Enlarge image Figure 1 APS/Alan Stonebraker Figure 1 The Nernst effect occurs when a material is exposed to a temperature gradient and a magnetic field. Mobile charges (referred to as quasiparticles) will tend to move from the warm to the cold side (x direction), but this motion will induce a force coming from the magnetic field (z direction). Charge collection in the y direction results in an electric field, constituting the quasiparticle Nernst signal. For decades researchers have searched for dilute superconductors in insulators and semiconductors. Unlike superconducting metals, in which the charge density is fixed, superconducting semiconductors can be doped with different amounts of carriers, allowing physicists to explore the dependence of superconductivity on carrier density. A well-known example is the doped-band insulator strontium titanate (SrTiO3). Below its critical temperature of 0.3 kelvin, it is the most dilute superconductor known. Previous observations have shown that the number of charge carriers in SrTiO3—coming from oxygen vacancies or niobium doping—is comparable to the metal bismuth, which has no known superconducting state. New observations, reported in Physical Review X, show that SrTiO3 superconductivity occurs at even lower carrier densities than previously assumed. As observed in thermoelectric measurements, the superconducting phase occurs in samples doped with one carrier per 105 unit cells. This “charge-starved” density is too low to be explained by the conventional superconducting model, the BCS (Bardeen-Cooper-Schrieffer) theory, in which electrons form pairs through a phonon-mediated mechanism.