Jim Valles: Insulator to superconductor transitions come in multiple flavors in quench condensed films

Jim Valles describes that superconductor to insulators transitions (SIT) come in 3 flavors in quenched condensed films: granular, uniform and nano-honecomb (NHC) structures. The NHC films usually have the thickness about the same as the uniform ones and the variation in their thickness is about a couple of angstroms.

Jim Valles discussed the possible ways to kill the superconducting order parameter through SIT. In weakly localized system, the transition happens in an amplitude reduction fashion and cooper pairs (CPs) are de-paired. Another way to kill superconductivity is phase fluctuation which leads to localized CPs. In granular films, quasi-particle (qp) tunneling dominates while for NHC films, CPs tunneling takes charge.

For quenched condensed Bi films, with a-Sb underlayer, those films on glass or AAO (honeycomb structure) substrates are homogeneous, while the films are granular when the underlayer is missing. Most of the Bi measurements have to be done in situ.

He explained how resistance evolves in granular films through SIT and those films tend not to be conducting unless one has 2 layers of grains. Granular films usually have localized CPs and phase fluctuations near SIT. The transport effects are dominated by interisland qp tunneling. He showed data for granular lead which has giant negative magnetoresistance(MR) and can be fitted to SIS model with magnetic field induced pair-breaking.

For uniform amorphous Bi film, he thinks they are in the Fermi insulator phase. For weak insulating normal state, the change in conductance is proportional to log(T). They have positive but not large MR. Superconducting gap also disappears near SIT and the energy gap can be fit roughly with BCS form. Reference Valles, Dynes, Garno PRL 1992.

For NHC Bi films, perpendicular penetration length is about 1 mm and coherence length is about 10 to 20 nm. AFM images of this type of samples show “regular” height +variation. Thickness tuned SIT shows re-entrant behavior. NHC insulating films has a hard gap R~R0 e^(T0/T) and the activation energy T0 goes to 0 for thicker films. They exhibit large flux oscillation under perpendicular field and also giant positive MR. Transport measurements are dominated by incoherent CP tunneling. The size of the orbit of CP is dictated by the size of the unit cell. He believes that for NHC Bi films, they have phase fluctuations driven SIT and localized CPs do exist.

He raised a couple of questions about the origin of the activation energy, origin of giant MR, the mechanism causing localized CP in NHC films and also why qp/CP tunneling dominated for granular/NHC films.

He answered some of the above questions by introducing a local Tc0 variation which is caused by thickness variations in NHC films. He mapped out Tc as a function of position near SIT and introduced a weak link model. By including a circuit model, he got R^(link) at the critical point to be R_Q.

Blogged by Wei Liu (JHU)