Phonon drag thermal Hall effect in metallic strontium titanate

SrTiO3, a quantum paralectric, displays a detectable phonon thermal Hall effect (THE).Here, we show that the amplitude of the THE is extremely sensitive to stoichiometry. Itdrastically decreases upon substitution of a tiny fraction of Sr atoms with Ca, which sta-bilizes the ferroelectric order. It drastically increases by an even lower density of oxygenvacancies, which turn the system to a dilute metal. The enhancement in the metallic stateexceeds by far the sum of the electronic and the phononic contributions. We explain thisobservation as an outcome of three features : 1) Heat is mostly transported by phonons ;2) the electronic Hall angle is extremely large ; and 3) there is substantial momentumexchange between electrons and phonons. Starting from Herring’s picture of phonondrag, we arrive to a quantitative account of the enhanced THE. Thus, phonon drag,hitherto detected as an amplifier of thermoelectric coefficients, can generate a purelythermal transverse response in a dilute metal with a large Hall angle. Our results reveal ahitherto-unknown consequence of momentum-conserving collisions between electronsand phonons

Shan Jiang, Xiaokang Li, Benoît Fauqué and Kamran Behnia
PNAS 119, 35 (2022)

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