Recent years have
seen significant developments in the study of
strong light–matter coupling including the control of chemical
reactions by altering the vibrational normal modes of molecules. In
the vibrational strong coupling regime, the normal modes of the system
become hybrid modes which mix nuclear, electronic, and photonic degrees
of freedom. First-principles methods capable of treating light and
matter degrees of freedom on the same level of theory are an important
tool in understanding such systems. In this work, we develop and apply
a generalized force constant matrix approach to the study of mixed
vibration-photon (vibro-polariton) states of molecules based on the
cavity Born–Oppenheimer approximation and quantum-electrodynamical
density-functional theory. With this method, vibro-polariton modes
and infrared spectra can be computed via linear-response techniques
analogous to those widely used for conventional vibrations and phonons.
We also develop an accurate model that highlights the consistent treatment
of cavity-coupled electrons in the vibrational strong coupling regime.
These electronic effects appear as new terms previously disregarded
by simpler models. This effective model also allows for an accurate
extrapolation of single and two molecule calculations to the collective
strong coupling limit of hundreds of molecules. We benchmark these
approaches for single and many CO
2
molecules coupled to
a single photon mode and the iron pentacarbonyl Fe(CO)
5
molecule coupled to a few photon modes. Our results are the first
ab initio results for collective vibrational strong coupling effects.
This framework for efficient computations of vibro-polaritons paves
the way to a systematic description and improved understanding of
the behavior of chemical systems in vibrational strong coupling.
Nonequilibrium atomic structures can host exotic and technologically relevant properties in otherwise conventional materials. Oxygen octahedral rotation forms a fundamental atomic distortion in perovskite oxides, but only a few patterns are predominantly present at equilibrium. This has restricted the range of possible properties and functions of perovskite oxides, necessitating the utilization of nonequilibrium patterns of octahedral rotation. Here, we report that a designed metastable pattern of octahedral rotation leads to robust room-temperature ferroelectricity in CaTiO3, which is otherwise nonpolar down to 0 K. Guided by density-functional theory, we selectively stabilize the metastable pattern, distinct from the equilibrium pattern and cooperative with ferroelectricity, in heteroepitaxial films of CaTiO3. Atomic-scale imaging combined with deep neural network analysis confirms a close correlation between the metastable pattern and ferroelectricity. This work reveals a hidden but functional pattern of oxygen octahedral rotation and opens avenues for designing multifunctional materials.
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