After thermodynamics and quantum theory came together, physicists quickly realized that things like electrons can never really stand still. In a roundabout way, this has led to the notion that a small ring of metal should have a small, persistent current that flows without dissipating energy, even in the absence of a driving voltage. Of course, environmental fluctuations wash out such effects, so low temperatures and highly sensitive measurements are required to observe these persistent currents. Until now, though, the results of these highly sensitive measurements have resulted in controversy rather than enlightenment.
The idea that electrons have something like thermal motion, even in the absence of an applied voltage is not new, nor startling. It is, in fact, one of the sources of noise in many experiments, and can be measured in the lab with a sufficiently good experimental set up. These currents are random in direction, random in their duration, and random in their magnitude.
The idea that a small ring of metal would contain persistent currents that flow around the ring without dissipating any energy—despite the fact that the ring might be a resistive—is quite startling. It is also, as it turns out, quite difficult to measure.
Trying to measure them has led to a raft of results that, when placed in a Venn diagram, don't intercept with the theory circle and, often, don't intercept with each other, either. Clearly, what most experiments are measuring is noise. In large part, this is due to the way that these currents are measured—or not measured, as the case may be. Experimenters use superconducting quantum interference devices (SQUIDs) to try and detect the tiny magnetic field associated with the persistent current. Unfortunately, the SQUIDs themselves have a current and magnetic field, which drives a current in the ring, swamping any persistent current.
In this case, the researchers gave up on the idea of using SQUIDs and tried micro-mechanics instead. They fabricated an array of about 1700 aluminum rings on a silicon cantilever. This was cooled to below 3K and set in a magnetic field. The persistent current in the rings interacts with the magnetic field to induce a force on the lever, bending it slightly. By measuring this torque, the researchers were able to detect the size and direction of the persistent currents.
The researchers measured the behavior of persistent currents over a wide range of magnetic field strengths and orientations. In addition, they were able to measure the temperature dependence between 0.2 and 2.7K. At the end of it all is a graph with near data points lying in near perfect agreement with theory.
As a side note, since the direction of current flow in any particular ring is random, the force that is actually measured is proportional to the square root of the number of rings. However, the sensitivity of the cantilever was sufficient to detect the current in a single ring.
I think we can now say that the Venn diagram shows a much better overlap between theory and experiment. But what I found really cool about this experiment was the elegant methodology.
Science, 2009, DOI: 10.1126/science.1178139
Listing image by Harris Lab, Yale University