Trapped photons produce ferroelectric strontium titanate


Photons trapped in a cavity can ferroelect a crystal known as strontium titanate (SrTiO₃), according to a new study from the MPSD Theoretical Group. Photons randomly created and destroyed in a cavity vacuum – as determined by the laws of quantum mechanics – can dramatically alter the behavior of electrons and atoms in the material, as in the case of SrTiO₃. These theoretical predictions, now published in PNAS, demonstrate the great potential of confined light for the toolset of materials engineering.

The team’s discoveries pave the way for the exploration of new mixed light-matter phases, called “photo-fund states”. These are phases where the atoms and electrons of the material have reached a new stable state after mixing strongly with the confined light.

The phases of materials, such as magnetic, superconducting or ferroelectric phases, among others, are determined by the collective behavior of the atoms and electrons in the material. When such microscopic components are forced to strongly interact with light, their collective behavior can change completely and the material can reach a new stable phase. In this sense, light can now be considered as an additional tool for designing the phases of materials and adapting their properties to specific technological applications, such as information processing, detection or light harvesting.

The MPSD team predicts a new photo-background state for the SrTiO₃ crystal once it is placed inside an optical cavity (see illustration), where the two metal plates of the cavity press light into a region narrow space. This reinforces the interaction between the photons and the particles of the encrusted material. In SrTiO₃, the positively charged titanium atoms can vibrate relative to the negatively charged oxygen atoms, which generates oscillating dipoles. In the usual ground state, such vibrations occur only as random movements of ions, called quantum fluctuations, which cancel each other out normally and have no observable effect.

However, the optical cavity causes fundamentally different behavior in SrTiO₃. The atomistic description of the MPSD theory team shows that fluctuations in the photon vacuum in the cavity can in turn collectively alter the quantum fluctuations of the nuclei in the material. As a result, the oscillating dipoles start to oscillate together instead of moving randomly and generate a non-zero macroscopic electric field. Now the ferroelectric phase – the phase with the finite macroscopic electric field – becomes the privileged ground state instead of the quantum paraelectric phase where no macroscopic electric field is present.

Thus, the complex properties of the SrTiO₃ crystal, such as its crystal structure and vibrational frequencies, are changed by adding trapped photons to the image. This requires an extension of the standard phase diagram to include the new dimension of light-matter coupling. However, the phase of a material can not only be controlled by pressure and temperature, but also through the coupling of the material to light.

“Our discovery is based on the fundamental nature of quantum mechanics of matter and light and on how the two can jointly alter the properties of a material”, explains lead author Simone Latini, post-doctoral researcher and Humboldt Fellow at MPSD. “This study has already motivated a number of experimental collaborators around the world to prove the existence of the proposed ground state.”

The team is now looking for the next photo ground state to open up innovative new avenues for engineering cavity materials.


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