Effects of dimensionality, sample topology, and disorder on superconductivity

by 1977- Wang, Haohua

iii The primary goal of this thesis work is to explore various phenomena related to the effects of dimensionality, sample topology, and disorder on superconductivity, motivated by theoretical questions and potential applications of superconductivity. First, doubly connected ultrathin superconducting cylinders of Al with a diameter down to 100 nm were studied. Following the previous discovery of the destructive regime, the suppression of superconductivity at zero temperature around half-integer flux quanta, in these cylinders, we have carried out detailed resistance vs. temperature measurements, revealing the possible existence of phase separation near the quantum phase transition (QPT) at the onset of the destructive regime. Resistance vs. magnetic field measurements suggested that the QPT might be continuous, and the normal state inside the destructive regime was possibly characterized by quantum fluctuations of superconductivity. Other interesting phenomena, such as a new phase diagram not considered in the original theory of de Gennes and the thermally activated phase slips were also observed in these ultrathin cylinders. Destructive regime physics of the Al cylinders relies on the doubly connected geometry. To gain further insight into the effect of sample topology on superconductivity in reduced geometry, we also studied the singly connected nanowires grown by electrochemical deposition in a porous host medium as a comparison to the doubly connected cylinders. Ru, with its oxide conducting, was chosen in this study for its potential to circumvent the oxidation problem encountered in measurements on individual nanowires iv released from their host medium. Electrical transport measurements on arrays of Ru nanowires as well as on individual nanowires were attempted down to 0.3 K in a 3He refrigerator, with a particular sample measured down to 50 mK in a dilution refrigerator. These Ru nanowires had a diameter ? 50 nm depending on the pore size of the host medium. However, no superconductivity was found. The loss of superconductivity was attributed to the ultrasmall grain size (? 2 nm) in these nanowires as revealed by high-resolution transmission electron microscopy. Finally, analysis of our previous measurements on mesoscopic superconducting Au0.7In0.3 rings is included in this thesis as an example of effects of strong disorder on mesoscopic superconductivity.