Macroscopic Quantum Tunneling in 1-Dimensional Al Nanowires and Quasi-Bound States in Quantum Point Contacts (QPCs) Albert M. Chang, Duke University, DMR (1)Superconductivity (SC) in ultra-narrow metallic nanowires is exciting from both fundamental and applications perspectives. When the width and height of a nanowire approach 40 atoms in each direction, one enters the 1-dimensional limit. We now have solid evidence that the switching current Is, at which an Al SC nanowire switches from an SC to a resistive normal state, is determined by the occurrence of a single phase-slip (PS). This was established by studying the statistics of Is upon repeated upsweep of the current I. Is characterized by the average, and a width of the distribution,  Is. Below 0.4 K, the width saturates, now observed for 3 different nanowires, signaling that an individual event of the Macroscopic Quantum Tunneling of the phase of the order parameter--a Quantum PS--triggers switching. (2)We discovered unusual resonances, indicative of the presence of quasi-bound states, in a new geometry in GaAs/AlGaAs heterostructure QPCs (quantum-point-contacts). Using a highly asym- metric geometry, we tune to the single-channel, 1- dimensional limit, in which GEOMETRY- (as opposed to impurity-) induced resonances exhibiting signs of non-fermi liquid interaction effects are observable. Fig. 1 Right panel--A scanning electron microscope) picture of an aluminum nanowire. Left panel– (a) Is distribution for nanowire S2 at different temperature T— right to left: 0.3 K to 1.2 K. (b) versus T. (c) Symbols—  Is versus T. Dashed lines indicate thermal-activated phase- slips, while the clear saturation in the data for nanowires S3, S4, and S4, observable below 0.4 K, is indicative of the macroscopic quantum- tunneling of phase-slips. Fig. 2 (a) Unequally gated conductance traces in the symmetric geometry. No resonances were observable. (b)-(d) In contrast, traces for asymmetric QPCs with lithographic gap widths of (b) 250 nm (c) 450 nm (d) 300 nm, respectively clearly show sharp resonances below the first conductance plateau at 2e 2 /h.

Macroscopic Quantum Tunneling in 1-Dimensional Al Nanowires and Quasi-Bound States in Quantum Point Contacts (QPCs) Albert M. Chang, Duke University, DMR )P.I. A.M. Chang has continued to actively engage in outreach. In May of 2011, he was part of a Duke Physics Department team, that hosted ~ 20 female high school students. He had the opportunity to talk to several different groups of students with differing interests and backgrounds, including African American students. (Unfortunately, we have not been able to locate the student, who took photos of the visit to PI A.M. Chang’s lab. However, other photos of the students’ visit to Duke are shown.) 2)A.M. Chang has taken his outreach effort overseas as well, to Taiwan. In a conference OCPA7 held in Kaohsiung, Aug. 1-5, 2011, he gave a plenary talk on science education to an audience, which included both professional physicists with keen interest in physics education, as well as talented high school students.