Do any types of superconductors maintain zero electrical resistance as the AC frequency goes up? I was thinking about applications using superconducting plasmonic diodes like the ones on this page for solar panels:
formatting link
Would it be possible for a superconductor to efficiently conduct THz and higher frequencies, I guess the current density will go up as the electrons get compressed on the surface, is that the limiting factor?
Also is there a type of superconductor that only is superconductive on the surface, similar to how a topological insulator only conducts electricity on its surface?
The particle accelerator people use superconducting resonators in their linacs and some synchrotrons and storage rings. Some of these run at GHz frequencies, others in the tens to hundreds of MHz.
On a sunny day (Sun, 24 Feb 2013 10:02:41 -0800) it happened Jamie M wrote in :
Some superconductors emit THz freqwuencies when irradiated by laser light.
I have a several hundred MHz superconduction filter (from a cellphone tower).
There is a paper on THz frequency emission from super conductors irradiated by laser light, they use it to find where exactly the material superconducts and where not (isles).
I dunno about THz superconducting filters, mine is around 800MHz IIRC, but why not.
Not really. Some linacs use superconductive resonators (resonant microwave cavities) that consist of thin lead plate on copper, that have very high Q at microwave frequencies, but it isn't infinite. Superconductors have some AC resistivity (or equivalent, there's energy losses).
The niobium resonators at Jefferson Labs, the electron accelerator, have unloaded Qs around 1e8. Their limit on RF power is arcing, from electrons and ions being ripped out of the metal.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Yes, they typically run about 100 kV peak on the resonator, and do all kinds of chemical super-polishing and exotic cleaning to try to keep electric fields as uniform as possible. The slightest speck of dirt, and pow!
No, there is a loss component at any frequency above dc. As a rule of thumb, the resistivity approaches the ballpark of the non-superconductive metal resistivity at the 'gap frequency', which for niobium is roughly 700 GHz. Below the gap frequency the conductivity increases IIRC ~ 1/f^2, which means that the losses become pretty damn small quite quickly as the frequency is lowered.
There are materials with higher gap frequency, e.g. NbTiN is used in SIS mixers at >1 THz.
More exotic materials such as YBCO show even higher gap frequency, but they are a bit messy. They don't obey the BCS theory, so they are not understood as well as the ordinary superconductors. Their gap is typically not isotropic so that the gap frequency is different depending on the direction of the field components w.r.t the lattice planes.
All superconductors carry the current on their surfaces only, that is the distribution which minimizes the magnetic energy. But this is not the same thing as "being superconducive only at the surface". The effect has the same mechanism behind it as the ordinary skin effect, except that (i) it kicks in already at zero frequency, because the material conductivity is infinite; and (ii) the thickness of the current-carrying layer is not zero, but rather it equals the so-called London penetration depth (~90nm in Nb).
No, they are extremely reflective only up to the gap frequency, which is below 1 THz for most ordinary superconductors. If someone in interested in details, the SC behaviour in the vicinity of the gap frequency is described by the Bardeen-Mattis theory.
Is that analogous to the plasma frequency in ordinary metals? Which I've heard is about where they start absorbing (most in the UV, except copper among others, which, as we know, looks pink because it's not such a great conductor up around blue). Eyeballing, is it coincidence that it's on the order of kT/q, I suppose taking T as the critical temperature?
Interesting (in your other post) that you say it's roughly 1/f^2 (two pole lowpass :) ), whereas from gap theory (again, with ordinary materials) one might expect, say, an exponential response or something. Actually, the cutoff of ordinary materials isn't exponential with respect to frequency, in fact I've measured it for two semiconductors... I'll dig through my notes when I get back...
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
The plasma frequency is due to the 'slowness' of the Thomas- Fermi screening, whereas the SC gap frequency is caused by photons starting to break the electron pairs. I don't see a connection offhand, but then again, maybe there is a way to visualize the pairing mechanism in such a way that an analogy can be recognized.
the
It is not a coincidence. The supercurrent is carried by paired electrons, and you can break the pairs either by kicking them with strong-enough photons or by thermal agitation. The required energy is roughly the same in both cases.
e
e
I think the 1/f^2 dependence comes from the two-fluid model, in case you want to look it up.
Hmm.. so to draw that analogy, one would have to know more about Thomas-Fermi screening. Which I don't. I'll have to read up!
Now, by "break", does that actually mean a superconductor, say a superconducting film, becomes less conductive and can be "broken" by sufficiently intense light?
That would make sense from a different standpoint, namely: a sufficiently intense B field is known to cause disruption (and, I would suppose, E as well, though a sufficiently strong surface E field might simply not be achievable in vacuum, i.e., it sparks first). If Bmax of "critical light flux" coincides with critical field strength, it would be very interesting. But then, such a classical explanation wouldn't depend on frequency, either.
Ahh, physics...
And, naturally, most stuff doesn't superconduct, because it's bathed in...at least that much energy!
Ah, the cutoff I was referring to:
formatting link
x axis is octaves in photon energy (i.e., log2(Ephoton), with E in eV), y axis is attenuation in dB. Passband attenuation is due to reflection and whatnot (0dB = no wafer in the apparatus). GaAs appears much sharper, which I'm sure is something to do with density of states around the bandgap or something like that. Evidently the silicon is ~100dB/octave, or around 16 poles, a pretty sharp cutoff (not related to 1/f^2). A pretty reasonable Butterworth filter, or something thereabouts.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.