Superconducting Tunnel Junctions

Rubens

Just want to confirm if my understanding is correct regarding the detection method of terahertz radiation using superconducting tunnel junctions (a Superconductor-Insulator-Superconductor device). For a Niobium superconductor material (having Eg = 3meV corresponding to about 0.7 THz frequency from E=hv, where v is the frequency) all I need is about 0.7 THz of electromagnetic wave to incident on this superconducting tunnel junction detector in order for the electrons (Cooper pairs breaking up) to tunnel through the insulator and in to the other superconductor. Is my understanding correct?

Please help!

Thank you!

ZapperZ

I haven't done work on radiation detection, but here's what I remember.

First of all, Nb energy gap is about 1.3 - 1.5 meV. If you do a superconducting-insulator-normal metal (SIN) junction, this is the minimum energy that you would need to start seeing current across your junction, because this is the energy you need to break a cooper pair. However, in a SIS junction, although 1.5 meV is enough to break the cooper pair, the cooper pair has no states to tunnel to on the other side of the junction because it will be tunneling right into the gap on the other superconducting side. So you need to double the energy, and that's why the tunneling gap is around 3.0 meV.

I not sure if you still need to apply a minimal potential across the junction to help with the current, but in many cases, you need to apply a small amount of magnetic field to the junction (well below Hc) to suppress supercurrent tunneling. This current has nothing to do with the presence of the radiation, so you don't want that to overwhelm your measurement.

Zz.

Rubens

Zz, thank you so much. Yup, I've read in an article that a magnetic field was applied to the device to suppress the supercurrent (dc Josephson effect).

One more thing, for a SIS device having a very thin insulator (approx. 10 Angstrom), the wave functions of the superconductors are coupled together right? So their wavefunctions and thus phase are the same. So any photon excitation onto the SIS device will alter the wavefunction of one of the superoconductors, resulting to a phase difference, thus, by de Broglie relation, there will be momentum (velocity) of electrons. Resulting to a current that will tunnel through the junction. Is my understanding correct?

It's really hard to do self-studying.

Thanks!

Rubens

By the way, I don't know why the magnetic field can suppress the supercurrent? Is it just the typical magnetic force to deflect the movement of the charges?

Thanks again.

ZapperZ

The magnetic field adds just enough disruption to destroy the phase coherence of the supercurrent across the tunneling junction.

In SIN tunneling, what is tunneling across is the "electron", not the pair. The magnetic field doesn't destroy the electron in terms of its ability to tunnel through, at least not for low fields. In SIS tunneling, both electrons in the pair must maintain coherence with each other while they're tunneling across. The magnetic field, even a weak one, is enough to cause a slight "mis-step" during this dance.

This, btw, is why a device such as a SQUID is so sensitive to magnetic fields. Even one single magnetic quantum flux across a SQUID loop can cause a slight phase shift in the supercurrent and be detected.

Zz.

Rubens

Ah! now I understand. Thanks so much Zz.

Rubens.