Yale Physicists Measure 'Persistent Current' 68
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."
Re:I'm no EE (Score:1, Informative)
They are already developing them.
googling it and picking a random one
http://www2.computer.org/portal/web/csdl/doi/10.1109/MM.2007.59
Re:I'm no EE (Score:4, Informative)
Re:Could this be used for memory? (Score:3, Informative)
Possibly. It was just measured they need time to figure out what the limits of it are.
Re:I'm no EE (Score:4, Informative)
It takes a heatsink the size of a small house to deal with current overclocked CPUs and that's on a single plane. The more layers you put between your heatsink and the bottom-most layer of your CPU, the poorer the conduction of heat away from it and the worse off you'll be.
He's quite right, without a heatsink the latest CPUs instantly rise to over 90C and then reset or throttle themselves down to unusable levels.
Re:Wait... (Score:5, Informative)
Conservation of energy is absolute, as far as we know, not statistical, even in QM. It would be a major revolution in physics if that weren't the case, as conservation is associated with important symmetries, such as the laws of physics not changing from past to future.
Entropy increasing *is* statistical, but no, you can't get around it. See Maxwell's Demon or the Brownian ratchet.
There are existing examples of persisting currents, in superconductors. No way to get energy out without reducing the current, of course, and you have to put energy in to get it back.
Re:And I have measured the.... (Score:2, Informative)
Yes, it just makes the theory stronger. Just like the theory of evolution. For all practical purposes it is fact. We just aren't so arrogant as to call it a law anymore.
It's conservation of energy (Score:3, Informative)
As long as you're not taking energy out of it, no, it's not. Well, actually, energy is perpetual; it's power that's not. Perpetual motion exists in a vacuum. It just doesn't on earth with all that friction that requires perpetual power to counteract.
You can also maintain a perpetual current in a supraconductor, as long as you're not messing with the magnetic field it generates. But just like a hard vacuum, it's not a natural state down here.
Re:It's the little things that impress (Score:5, Informative)
Induction isn't a quantum-scale effect, just plain old electromagnetics. Floating pins (that is, any pin configured as an input but not connected to a circuit) are notorious for causing strange effects that mess up both digital logic and analog sensing. Some pretty spooky behavior can result, like states that change when you wave your hand over the chip (due to the capacitance created by the proximity of the hand).
The input pin-state doesn't allow significant current to flow, and all microcontrollers use it as the default state on power-up since it negates the possibility of a short circuit. But if an input isn't connected to anything, it will generate spontaneous readings due to charge accumulation / current drift--some of which might be inductive but it can also be plain old resistive pathways since there are no perfect insulators (for example a PCB might look like a resistor of a few dozen megaohms, or even less if the board is dirty from handling).
This is easily dealt with by grounding the unused pins, either externally or internally (by switching it to an output low state). This comes up often enough that forgetting to assert disconnected pins to ground is what I'd call a classic "101" embedded hardware design bug. I've done it a few times myself and had various apparently inexplicable results followed by feeling stupidity when I realize whats actually happening.
Short explanation (Score:5, Informative)
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.
Re:Short explanation (Score:5, Informative)
I should also add to this that one must remember that electrons are as much waves as they are particles. Because of the circular geometry, electron wave functions around the loop acquire a phase in integer multiples.
The group is measuring the changes in magnetic moment that these currents produce.