Twisted phonons produce giant magnetic moments

An archetypical perovskite compound, SrTiO 3 (STO), never ceases to amaze.Originally viewed as a potential synthetic replacement of diamond in the '50s, * STO was the first doped insulator in which superconductivity was discovered in 1967, [1] but the mechanism of superconductivity still remains the subject of depbate.In the '70s and '80s, STO received a new wave of attention after Müller and Burkard [2] identified it as a quantum paraelectric: a material balancing on the verge of ferroelectricity but never quite making it due to disruptive zero-point motion of light oxygen atoms (later on, it was realized that a structural antiferrodistortive transition at 105 K also plays a role in arresting ferroelectricity [3,4]).From 2015 and on, STO is again on the stage thanks to finely crafted transport measurements, which unveiled a T 2 resistivity scaling in doped STO [5] † and the strongest thermal Hall effect among all non-magnetic insulators [6].Paraelectricity is STO (and related compounds, such as KTaO 3 , EuTiO 3 , etc.) is linked to the softening of transverse optical phonon modes.In STO, there are two such modes at the Γ point of the Brillouin zone, with E u and A 2u symmetries respectively.The frequencies of these modes drops precipitously as temperature decreases; namely, the frequency of the softest mode, A 2u , drops from about 12 meV at room temperature to the saturation value of 0.8 meV (0.2 THz) at liquid Helium temperatures.Concomitantly, the dielectric constant increases from about 300 at room temperature to 25,000 at Helium, making STO the highest-K insulator known.The soft mode had long been suspected to be responsible for superconductivity in and has been recently proposed to be the reason for the T 2 scaling of its resistivity [12].It is this soft mode that is the main subject of recent experimental study by Basini et al. [13].Common wisdom says that STO may be many things, but magnetic it is not.Basini et al. defy this notion by demonstrating that intense circularly polarized THz pulses induce * It did not work out.† If you think that this is just the usual Fermi-liquid T 2 scaling, note that in STO it extends well beyond the Fermi temperature.