Measuring Macroscopic Quantum Phase Slips in One-Dimensional Superconductors

1. Measuring Macroscopic Quantum Phase Slips in One-Dimensional Superconductors Jonathan Wilson, University of Illinois at Urbana-Champaign Thermally activated phase slips (TAPS)) 1 2 Macroscopic quantum phase slips (MQPS) 3 4 6 300 nm 5 41 nm 72 nm Results Introduction Sample Fabrication and Measurement Using a dc Superconducting Quantum Interference Device (SQUID) to Measure MQPS Goals/Motivation 1. A multilayer chip of Si, SiO2, and SiN with a trench is cleaned and prepared 2. Carbon nanotubes are deposited in solution on the wafer 3. MoGe is sputtered on the surface 4. Bridging nanowires are located with an SEM 5. The MoGe is removed from specific places to form the desired pattern on the chip 6. The sample is mounted on a dipstick and inserted in a helium dewar for measurement • Understand the factors that stabilize superconductivity in one dimension • Obtain experimental evidence to test a controversial theory The SQUID acts as a voltage transducer by transforming the magnetic flux through the sample to an output voltage that can be read into the data acquisition software What is a Phase Slip? A phase slip occurs when the magnitude of the superconducting order parameter fluctuates to zero at a point, allowing the phase of the order parameter to slip by 2π before the magnitude returns to its original value It is believed that macroscopic quantum (MQPS) and thermally activated (TAPS) phase slips maintain the stability of the superconducting state Preliminary Results Using the dc SQUID Particle in a Washboard Potential The SQUID measures no flux without a sample present, but shows many flux spikes in the presence of a sample. These spikes may be signatures of MQPS taking place in the sample. • The phase of the superconducting order parameter can be modeled as a particle in a washboard potential • The particle can roll over the hill (TAPS) • The particle can quantum mechanically tunnel through the hill (MQPS) Combining TAPS and MQPS SQUID Pick-up Loop Only 11-nm MoGe Sample A theory that incorporates both TAPS and MQPS fits the experimental data better than a theory using only TAPS However, it is controversial whether MQPS have ever been observed experimentally Acknowledgments Professor Alexey Bezryadin, Mitrabhanu Sahu, the Bezryadin Research Group, the Fredrick Seitz Materials Research Laboratory Funding Provided By: Anthony Undergraduate Research Fellowship 2005 Images 1, 2, and 3 courtesy of Robert Colby