0701948_Chang

1. 1-Dimensional Aluminum Nanowires and Possible Wigner Crystals Albert M. Chang, Duke University, DMR 0701948 • Superconductivity (SC) in ultra-narrow metallic wires is exciting from both fundamental and applications-technology perspectives. When the width and height of a nanowire approach 40 atoms in each direction, one enters the 1-dimensional limit. We have found 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 varies, and is characterized by the average Is_ave, and a width of the distribution, DIs. In Fig. 1(d) below 0.4 K, the width saturates, signaling that an individual event of the macroscopic tunneling of the phase of the order parameter, called a quantum PS, triggers switching. • We discovered a new geometry in GaAs/AlGaAs heterostructure QPC (quantum-point-contacts). Using a simple but highly asymmetric geometry shown in Fig. 2, we succeeded in tuning to a regime of the single-channel, 1-dimensional limit, in which the lowest quantized conductance plateau is completely suppressed. This may be indicative of the formation of a quantum Wigner crystal! Fig. 1 (a) Is distribution for sample S2 at different temperatures, in 0.1 K increments (right-most, 0.3 K). (b) The distribution of S2 at 0.3 K fitted with Gumbel distribution. (c) DIs and Is_ave for S1 and S2 versus temperature, T. (d) DIs and Is_ave of S3 versus T. Fig. 2 Evolution of the 2e2/h plateau in an asymmetric QPC at 4.2 K. At Vw=-2.35 V (red) while Vf is swept, the 2e2/h plateau becomes suppressed. At -2.4 V (green), a straight drop is observed. Top inset: similar behavior for a second sample at 300 mK. Bottom inset: asymmetric geometry . Scale bar is 500 nm.

2. 1-Dimensional Superconducting Nanowires and Possible Wigner CrystalsAlbert M. Chang, Duke University, DMR 0701948 P.I. A.M. Chang has continued to actively engage in outreach. In May of 2010, he again brought his lecture/demonstration to several Advanced-Placement classes in local, Durham area high schools, including Riverside and Jordan High, where AP physics programs are offered. The outreach involves introducing magnetic fields and magnetic phenomena, such induction and eddy currents. Increasing the magnitude of eddy currents by the cooling of metallic rings to liquid nitrogen temperatures (77 K) is always a hit. Photos show students involvement in Mr. Vincent’s class at Jordan High School.