University of Minnesota
School of Physics & Astronomy

Condensed Matter Seminar

Thursday, April 17th 2008
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: John Martinis, UCSB
Subject: Magnetism in Superconducting Devices at Millikelvin Temperatures

I will discuss a recent experiment where the flux threading a dc SQUID was measured to change with a 1/T temperature dependence, characteristic of the paramagnetic response of electron spins. With the magnitude of the effect being proportional to the density of trapped vortices, this data is compatible with the thermal polarization of surface spins in the trapped fields of superconducting vortices. In the absence of trapped flux, the surface spins also show evidence of 2-D spin-glass freezing at a temperature below about 100 mK. These results suggest that surface spins are present on metals with a density of about 1/nm^2; however, the microscopic origin of the spin states are currently unknown. It also suggests an explanation for the "universal" 1/f flux noise observed in SQUIDs that has not been understood for 20 years.
This experiment was performed at the University of Wisconsin in
collaboration with Robert McDermott.

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