Quantum atmospherics

Why the library suggests it

Park the probe a few nanometres above the surface and read what the empty space there has inherited from the matter below. The whole diagnostic is in the disagreement between two detectors: a spin probe reads the shift easily, and a SQUID, held at the same height over the same sample, registers nothing at all.

The quantum atmosphere, read by two detectors

A material does not stop at its surface. If the matter inside treats left and right, or forward and backward in time, differently, the quantum fluctuations just outside inherit that lopsidedness — and a probe parked a few nanometres above can read the hidden order without touching the sample. Above a topological insulator the effect pulls on an electron's spin like a magnetic field of about ten gauss at ten nanometres, falling as the inverse square of the height. The diagnostic is not that number. It is that the same reading turns up on one detector and not on the other.

What is under the probe

A topological insulatorThe same surface, with an electric field appliedA surface that breaks no symmetry

Raise and lower the probe

Probe height above the surface

10

Back to the published height

• A topological insulator
• The same surface, with an electric field applied
• A surface that breaks no symmetry

A nitrogen-vacancy centre reads:

1E1 gauss

A SQUID reads:

0 gauss

What the two detectors say: This shift is not true magnetic flux. A spin probe reads it; a SQUID registers nothing at all. A reading on one and silence on the other is the signature — and it is why this is a diagnostic rather than a magnetometry measurement.

The case with no published magnitude

• A time-reversal-breaking superconductor

The paper gives this surface a signature and no number: the atmosphere splits states of opposite angular momentum, which is a discovery signature that does not rely on the Meissner effect and therefore survives in small samples and in the presence of magnetism. With no magnitude stated there is nothing to draw, so it is named here instead of being placed on the axis.

This level is what an applied electric field of ten thousand volts per centimetre induces on such a surface — a Hall-like current sheet or a surface magnetic charge, either producing a real magnetic field of order ten gauss, aligned or antialigned with the field applied.

No detection floor is drawn on this axis. The paper places the effect many orders of magnitude above nitrogen-vacancy magnetometry's sensitivity and gives no figure for that sensitivity, so the read-out says which detector answers and never how close to its limit the answer sits.

The sheet every figure here is stated in

Qing-Dong Jiang and Frank Wilczek work one case out in full: the surface of a topological insulator carries the extra term of axion electrodynamics, and the atmosphere above it pulls on an electron's spin like a magnetic field of about ten gauss at ten nanometres — far above what diamond magnetometers detect. They give the same treatment to time-reversal-breaking superconductors and then classify every operator such an atmosphere can be built from (Quantum atmospherics for materials diagnosis, 2019). The same author is a co-author on two of the cavity-superconductivity measurements in section 1, which is not a coincidence: the atmosphere and the cavity are the same physics read in two directions.

The experiment or build

Mount a nitrogen-vacancy centre in a diamond tip on a scanning stage and raster it a few nanometres above a topological insulator, reading the spin resonance shift. Confirm the predicted magnitude and its distance dependence, then turn it on the samples this field actually needs identified: a superconductor too small or too magnetic for a Meissner-effect test, or the surface of a hydrogen-loaded lattice. The settling measurement is the spectral shift in gauss-equivalent units against tip height, matched to the predicted power law, on a material with known broken symmetry and on a control with none. Once that curve is reproduced, the instrument can be pointed anywhere.