In a pot of boiling water, steam bubbles appear and disappear — a similar struggle takes place in two-dimensional materials. Here, electrons both create magnetism and try to extinguish it. This phenomenon is called a Kondo lattice: the magnetic moments of atoms (tiny arrows) either align into antiferromagnetic order, neighboring arrows point in opposite directions, or become screened by a cloud of conduction electrons, forming "heavy" particles with a large effective mass. Which state wins depends on the strength of the coupling between them.
The authors placed such a two-dimensional film in an optical cavity — two parallel mirrors with a gap of fractions of a micron. Inside, even in complete darkness, there is no absolute silence: virtual photons constantly appear and disappear — vacuum fluctuations of the electromagnetic field. They become the third player. Calculations showed that fluctuations polarized along the film (like light in a slit of hexagonal boron nitride) act as a glue for electrons and magnetic moments: they enhance screening and expand the region of "heavy" particles. If the polarization is transverse, the effect is opposite, and magnetism gains the upper hand.
So, the very geometry of the light trap and the direction of field polarization act as a tuning knob. By turning it, you can switch the material from a magnetic to a non-magnetic state and back without ever touching it. This is reminiscent of how the same piece of dough becomes either airy bread or a dense cake, depending on how vigorously it is kneaded. Only here, the "kneading" is done not by a baker, but by the quantum vacuum.
In a cavity a few microns wide, the pressure of vacuum fluctuations can reach noticeable magnitudes — enough to shift a microscopic mirror.
The idea of controlling the properties of matter with nothing but vacuum echoes the "zero field" from Isaac Asimov's novels, where emptiness became a source of infinite energy.