Quantum Critical Behavior in Deeply Underdoped Cuprate Films and Pairing Symmetry in Iron Pnictide Superconductors Probed by Penetration Depth Measurements

Superfluid density n_s(T) [∝ λ^-2(T) and λ is the penetration depth] is a fundamental parameter in superconductivity. In my Ph. D. work, by measuring the complex sheet conductivity of thin superconducting films in a two-coil mutual inductance apparatus, both the amplitude and the temperature dependence of superfluid density are determined in various superconducting systems. This is then used to understand various physics like pairing symmetry, and thermal and quantum critical fluctuations in low dimensions, in the following materials:

(1)Bi₂Sr₂CaCu₂O_8+x (Bi-2212) films: A wide range of doping is achieved by pulsed laser deposition (PLD) and sputtering with transition temperatures (T_c) ranging from 80K near optimal doping to 6K for severely underdoped ones. In moderately underdoped Bi-2212 films with T_c > 50K, Beresinskii-Kosterlitz-Thouless (BKT) transition, an evidence of 2-D thermal critical behavior, is observed by a sharp downturn of superfluid density near T_c. This is consistent with the extremely anisotropic nature of Bi-2212. However, this thermal critical behavior disappears for severely underdoped Bi-2212 with T_c < 45K. At this regime, T_c scales linearly with the superfluid density λ^-2(0). These two results are the evidence of the 2-D quantum critical behavior in deeply underdoped Bi-2212 near a quantum critical point (QCP).

(2) Iron-based Ba(Co_xFe_1-x)₂As₂ films near optimal doping: At low temperatures, superfluid density varies like λ^-2(T) ∝ T^2.55, consistent with a novel s± pairing symmetry with significant interband scattering between two different Fermi surfaces. Overall, λ^-2(T) can be fit by a dominant small BCS-like superconducting gap 2Δ/k_BT_c ~2.1. The possible bigger gap vanishes and this may due to its strong intraband scattering.

(3) Ultrathin NbN and Pb films: sharp downturns, which clearly deviate λ^-2(T) from its standard BCS theory predictions, are observed in these ultrathin films near T_c. They are associated with two dimensional thermal critical fluctuations and are clear evidences of Beresinskii-Kosterlitz-Thouless (BKT) transitions. However, it seems that downturns always occur earlier than what XY model predicts. This means that the vortices are energetically easier to create, and the reason behind this finding is still unclear.