Tuesday, July 03, 2018

A metal superconducting transistor (?!)

A paper was published yesterday in Nature Nanotechnology that is quite surprising, at least to me, and I thought I should point it out.

The authors make superconducting wires (e-beam evaporated Ti in the main text, Al in the supporting information) that appear to be reasonably "good metals" in the normal state.  [For the Ti case, for example, their electrical resistance is about 10 Ohms per square, very far from the "quantum of resistance" \(h/2e^{2}\approx 12.9~\mathrm{k}\Omega\).  This suggests that the metal is electrically pretty homogeneous (as opposed to being a bunch of loosely connected grains).  Similarly, the inferred resistivity of around 30 \(\mu\Omega\)-cm) is comparable to expectations for bulk Ti (which is actually a bit surprising to me).]

The really surprising thing is that the application of a large voltage between a back-gate (the underlying Si wafer, separated from the wire by 300 nm of SiO2) and the wire can suppress the superconductivity, dialing the critical current all the way down to zero.  This effect happens symmetrically with either polarity of bias voltage. 

This is potentially exciting because having some field-effect way to manipulate superconductivity could let you do very neat things with superconducting circuitry. 

The reason this is startling is that ordinarily field-effect modulation of metals has almost no effect.  In a typical metal, a dc electric field only penetrates a fraction of an atomic diameter into the material - the gas of mobile electrons in the metal has such a high density that it can shift itself by a fraction of a nanometer and self-consistently screen out that electric field. 

Here, the authors argue (in a model in the supplemental information that I need to read carefully) that the relevant physical scale for the gating of the superconductivity is, empirically, the London penetration depth, a much longer spatial scale (hundreds of nm in typical low temperature superconductors).    I need to think about whether this makes sense to me physically.


Anonymous said...

Surprising indeed!

The Nature link is down right now. Alternate one:


Anonymous said...

How can one gate a metal (Ti) with a semiconductor (p-type Si)?! It usually goes the other way around. There may be an electric field in the SiO2, but an electron few Angstroms below the surface in Ti won't see any electric field. And incidentally, the magnitude of the electric field applied by a 30 V gate voltage through a 300 nm-thick SiO2 is comparable to typical electric fields that are present at various interfaces simply because of different workfunctions.

Instead of falling for a fancy physical effect, the question to ask here is how uniform are the Ti and Al films, and what prevents their oxidation? Titanium is known as a gatherer, and the films seem to be exposed. Is this Ti, TiOx or a mix. Note that there is no thickness dependence. Is the 30 nm Ti thickness the right thickness, where the film is not fully oxidized, but not a good metal either?

Douglas Natelson said...

Anon1, thanks for the arxiv link. Somehow I was incompetent at finding that yesterday.

Anon2, yeah, it's bizarre. Using degenerately doped Si as a backgate is fine, but dc screening in a superconductor should basically be the same as in a conventional metal, as far as I can reason. Your thinking about oxidation is where I first went, too, but if their resistivity numbers are right, it's hard to see how that's the issue here. There are a huge number of people in a position to test this experimentally in basically a day or two, if they're so inclined. Heck, someone like Charlie Marcus could test this with single-crystal aluminum and an ALD gate dielectric and remove any concern about granularity.