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Yale Physicists Measure 'Persistent Current'
eldavojohn writes "Modern processors rely on wires mere nanometers wide, and now Yale physicists have successfully measured a theoretical 'persistent current' that flows through them when they are formed into rings. The researchers predict this will help us understand how electrons behave in metals — more specifically, the quantum mechanical effect that influences how these electrons move through the metals. Hopefully, this work will shed new light on what dangers (or uses) quantum effects could have on classical processors as the inner workings shrink in size. The breakthrough involved rethinking how to measure this theoretical effect, as they previously relied on superconducting quantum interference devices to measure the magnetic field such a current would create — complicated devices that gave incorrect and inconsistent measurements. Instead, they turned to nothing but mechanical devices, known as cantilevers ('little floppy diving boards with the nanometer rings sitting on top'), that yielded measurements with a full order of magnitude more precision."
I am a solid state physics Ph.d. student. There seems to be a lot of confusion on how these things work, which is unsurprising given the lack of details in this slightly sensationalist story published by Yale about work done at Yale. Hopefully this helps a bit.
First, these currents don't spontaneously arise out of the blue. There is an external applied magnetic field, so every metal ring has at least 1 flux line passing through it. As most should know, a changing magnetic field induces an electric current. Normally, in non-superconducting metals, inelastic scattering of electrons causes the current to dissipate (ie there is resistance).
The unique thing about these metal rings is that they are smaller than the electron's phase coherence length, or the distance the electron travels before it is scattered inelastically. Electrons will scatter elastically off of impurities, but those collisions are not dissipative.
This Yale group by no means discovered this phenomenon, nor are they the first to measure it. What they did was measure it with greater accuracy. The things that have been unclear for awhile are the direction the current travels in and the magnitude. Hopefully these new measurements will shed some light on the matter.
P.S. I hate Slashdot's comment system. Every time I clicked off this typing box, it refused to accept any input until I clicked randomly around the screen for at least 15 seconds.