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Measuring Macroscopic Quantum Phase Slips in One-Dimensional Superconductors
Jonathan Wilson, University of Illinois at Urbana-Champaign
Introduction
Results
Sample Fabrication and Measurement
Goals/Motivation
1
• Understand the factors that stabilize superconductivity in one dimension
• Obtain experimental evidence to test a controversial theory
Using a dc Superconducting Quantum Interference
Device (SQUID) to Measure MQPS
1. A multilayer chip of Si,
SiO2, and SiN with a
trench is cleaned and
prepared
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
2. Carbon nanotubes are
deposited in solution on
the wafer
It is believed that macroscopic quantum (MQPS) and thermally activated
(TAPS) phase slips maintain the stability of the superconducting state
Particle in a Washboard Potential
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)
3
Thermally activated
phase slips (TAPS))
5
Combining TAPS and MQPS
A theory that incorporates both TAPS
and MQPS fits the experimental data
better than a theory using only TAPS
41 nm
72 nm
Preliminary Results Using the dc SQUID
3. MoGe is sputtered on
the surface
4. Bridging nanowires
are located with an SEM
Macroscopic
quantum phase
slips (MQPS)
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
However, it is controversial whether
MQPS have ever been observed
experimentally
Images 1, 2, and 3 courtesy of Robert Colby
2
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.
4
300 nm
SQUID Pick-up Loop Only
6
11-nm MoGe Sample
Acknowledgments
Professor Alexey Bezryadin, Mitrabhanu Sahu, the Bezryadin
Research Group, the Fredrick Seitz Materials Research Laboratory
Funding Provided By: Anthony Undergraduate Research
Fellowship 2005