Equilibrium nonlinear phononics by electric field fluctuations of terahertz cavities

Selective excitation of vibrational modes using strong laser pulses has emerged as a powerful material engineering paradigm. However, to have deterministic control over material properties for device applications, an analogous scheme without drive, operating in thermal equilibrium, is desirable. We here propose such an equilibrium analog of the light-driven paradigm, leveraging the strong coupling between lattice degrees of freedom and the quantum fluctuations of the electric field of a THz microcavity. We demonstrate this approach by showing, using data, how electric field fluctuations can induce a subdominant ferromagnetic order, on top of the dominant zigzag antiferromagnet order, in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>FePS</a:mi><a:mn>3</a:mn></a:msub></a:math> close to its Néel temperature.