Quantum Matter Seminar
Quantum mechanics allows for a wide variety of materials which often exhibit exotic and potential useful properties such as magnetism, orbital order, ferroelectricity, and more. A key goal of condensed matter physics is finding ways to control these properties. In this talk we will explore two possible systems where this control may be established through optical means, either by using intense coherent light, or by carefully designing the radiative environment.
First, I will focus on a recent experiment  which demonstrated nonequilibrium optical enhancement of magnetism in the strongly fluctuating ferromagnetic insulator YTiO3. In this experiment coherent terahertz radiation was used to resonantly drive various phonon modes, leading to large coherent lattice vibrations. The magnetization was then monitored using time-resolved Kerr rotation measurements, which revealed the emergence of long-lasting pump-induced magnetic order. Taking into account the spin and orbital degrees of freedom, we argue that the pump induces a dynamical modulation of the superexchange interaction, so as to suppress competing magnetic orders. We then go further and show how the coherent drive can modify the coupling of the spins to the orbital bath, allowing for a nonequilibrium acceleration of the magnetization dynamics only during the duration of the drive, explaining the large asymmetry between the growth and decay rate of the magnetization.
In the second part of the talk, we will switch gears to explore how it may be possible to exert control over ferroelectric correlations in a quantum paraelectric insulator by using a resonant cavity to control the equilibrium fluctuations. In particular, we show how the fluctuations of a soft polar phonon mode may be tuned by appropriate engineering of the cavity geometry. In general, the electromagnetic field helps suppress fluctuations of the polar mode, encouraging ferroelectric order. Near a metallic interface, the electromagnetic field is screened, leading to a "proximity effect" locally enhancing polar mode fluctuations near the surface. We argue this may be observable in thin films of SrTiO3, and make connections to the study of Casimir and Van der Waals forces
 Disa, A., et. al., "Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO3." arXiv: 2111.13622 (2021)