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Supercurrents Flex Cantilever to Reveal VorticesIRG 1: Y. Wang, L. Li, M.J. Naughton, G.D. Gu, S. Uchida, S. Ono, Y. Ando, and N. P. Ong Variation of the torque versus applied magnetic field H at temperatures T indicated in Kelvin (Tc = 50 K). Curves with deep minima (40-45 K) reveal a large negative torque caused by strong supercurrents flowing in the layers. These supercurrents remain observable to T high above 50 K. The inset shows the cantilever beam (blue) with the cuprate crystal (red) glued to its tip [from Ref. 1]. A soft cantilever beam ("diving board") is capable of detecting a
very weak force. In a car, for example, the release of an air-bag is triggered by an
accelerometer based on a cantilever. A similar cantilever (inset) has been used by Ong
and collaborators1,2 to uncover a novel, striking property of cuprate superconductors. When
a superconductor is exposed to a magnetic field, magnetic flux enters the sample as vortices.
In the cuprates, evidence suggest that vortices exist at temperatures above the transition
temperature Tc. Despite the loss of superconductivity, trace supercurrents around the vortices
should produce a magnetic moment m. The layered structure of the cuprate crystal provides an
elegant way to detect this weak moment. If the field is tilted at an angle to the layers, a
force (torque) acts to rotate the crystal. The torque is easily resolved with a soft cantilever
(curves in figure)1. In addition to confirming the existence of vortices above Tc, the results
reveal that electrons drawn together as "Cooper pairs" are very strongly bound. The field
needed to unbind them (called the upper critical field) is 60-100 times stronger than that
inside a clinical MRI machine.
1. Y. Wang, Lu Li, M.J. Naughton, G.D. Gu, S. Uchida,
and N.P. Ong, Phys. Rev. Lett. 95, 247002 (2005).
2. L. Li, Y. Wang, M.J. Naughton, S. Ono, Y. Ando, and N.P. Ong, Europhys. Lett. 72, 451 (2005).
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