[Paper Review] The Ferroelectric Photo-Groundstate of SrTiO$_3$: Cavity Materials Engineering

Simone Latini, Dongbin Shin+5 more|arXiv (Cornell University)|Jan 27, 2021

Mechanical and Optical Resonators1 references15 citations

TL;DR

This paper demonstrates, through first-principles calculations, that strong light-matter coupling in an optical cavity can induce a ferroelectric ground state in SrTiO3, a material that is normally quantum parapiezoelectric. The coupling to vacuum fluctuations of photons suppresses nuclear quantum fluctuations, effectively increasing ion effective mass and stabilizing a macroscopic polarization, thus engineering a novel photo-groundstate that is ferroelectric at equilibrium—offering a new route to control quantum phases via cavity engineering.

ABSTRACT

Optical cavities confine light on a small region in space which can result in a strong coupling of light with materials inside the cavity. This gives rise to new states where quantum fluctuations of light and matter can alter the properties of the material altogether. Here we demonstrate, based on first principles calculations, that such light-matter coupling induces a change of the collective phase from quantum paraelectric to ferroelectric in the SrTiO$_3$ groundstate, which has thus far only been achieved in out-of-equilibrium strongly excited conditions[1, 2]. This is a light-matter-hybrid groundstate which can only exist because of the coupling to the vacuum fluctuations of light, a "photo-groundstate". The phase transition is accompanied by changes in the crystal structure, showing that fundamental groundstate properties of materials can be controlled via strong light-matter coupling. Such a control of quantum states enables the tailoring of materials properties or even the design of novel materials purely by exposing them to confined light.

Motivation & Objective

• To explore whether strong light-matter coupling in an optical cavity can stabilize a ferroelectric phase in SrTiO3, which is normally quantum paraelectric due to strong nuclear quantum fluctuations.
• To investigate if vacuum fluctuations of cavity photons can suppress quantum fluctuations in the lattice, thereby enabling a phase transition to a macroscopic ferroelectric state at equilibrium.
• To demonstrate that the ground state of a quantum material can be fundamentally altered by coupling to the quantum vacuum, leading to a new class of hybrid light-matter states.
• To establish a revised phase diagram for SrTiO3 with cavity coupling strength as a new tunable parameter, extending beyond conventional thermal and quantum phase boundaries.

Proposed method

• Employed time-dependent density functional theory (TDDFT) with the Tamm-Dancoff approximation to model the coupled electron-photon system in SrTiO3 under cavity confinement.
• Used a 2D potential energy surface based on the PBE functional to map the ferroelectric soft mode (FES) and its coupling to cavity photons.
• Applied Kubo’s linear response theory to calculate the frequency response of the FES mode under varying cavity coupling and temperature, using an external electric field as a probe for ferroelectricity.
• Tracked the evolution of the FES mode frequency minimum as a signature of the paraelectric-to-ferroelectric phase transition, identifying the transition temperature via softening and stiffening behavior.
• Simulated the orbital-resolved electron density for ground and first excited states inside and outside the cavity to visualize the impact of cavity dressing on electronic structure.
• Constructed a revised phase diagram by introducing cavity coupling strength as a new dimension, extending the conventional temperature-driven phase boundary.

Experimental results

Research questions

• RQ1 Can strong light-matter coupling in an optical cavity induce a ferroelectric ground state in SrTiO3, a material that is normally quantum paraelectric?
• RQ2 How do vacuum fluctuations of cavity photons suppress nuclear quantum fluctuations in the lattice to stabilize a macroscopic polarization?
• RQ3 To what extent does cavity coupling alter the effective mass of ions and thereby influence the stability of the ferroelectric phase?
• RQ4 Can the phase transition from paraelectric to ferroelectric be induced at equilibrium, rather than through transient, non-equilibrium excitation?
• RQ5 How does the inclusion of cavity coupling strength as a new parameter reshape the phase diagram of SrTiO3, especially at finite temperatures?

Key findings

• The coupling of the ferroelectric soft mode in SrTiO3 to cavity photons suppresses nuclear quantum fluctuations by enhancing the effective mass of the Ti and O ions, effectively localizing them and stabilizing a ferroelectric state.
• A new photo-groundstate emerges that is ferroelectric at equilibrium, induced solely by coupling to vacuum fluctuations of light, without external excitation.
• The phase transition from paraelectric to ferroelectric is signaled by a minimum in the frequency of the ferroelectric soft mode, which shifts with increasing cavity coupling strength.
• The transition temperature is lowered with increasing cavity coupling, indicating that the quantum paraelectric state is suppressed and the ferroelectric phase becomes stable at higher temperatures.
• The phase diagram of SrTiO3 is extended by introducing cavity coupling strength as a new tunable dimension, enabling control of the ground state via optical cavity engineering.
• The effect persists at finite temperatures, where thermal fluctuations dominate over quantum ones, demonstrating robustness of the cavity-induced ferroelectricity.