The properties of complex materials – those having many competing degrees of freedom – are highly controllable with external ‘handles', such as epitaxial strain, pressure and chemical substitution, because their ground states can be tuned to the vicinity of phase boundaries. For example, the electrical resistance of some perovskite manganites (a classic family of complex materials that can be readily tuned with chemical substitution) becomes very sensitive to magnetic fields at phase boundaries, where competing electronic, spin, orbital and structural orders give rise to colossal magnetoresistance. In contrast with conventional materials, which generally exhibit small changes in their properties that are difficult to tune, the properties of complex materials are controlled not just by the crystal topology generally, they also depend sensitively on geometry and small structural distortions. Controlling these individual distortions to produce collective functional responses has proven a remarkably successful materials design strategy.
In this talk, I will discuss progress (by my own group and others) in the discovery and understanding of complex materials in which the lattice, spin, and orbital degrees of freedom are coupled and controllable with either electric fields or light. I will first focus on a particular class of materials that undergo inversion symmetry-breaking transitions through a so-called ‘trilinear coupling' mechanism, in which a combination of different structural distortions – which were long thought to compete with and suppress each other – cooperate to give rise to a polar structure. I will then describe how we are leveraging the insights gained from this work to understand and predict how ultrafast optical pulses can be used to dynamically stabilize the properties of complex materials by selective excitation of particular phonon modes. Our work demonstrates how elucidating the interplay between the lattice structure and chemical composition of a material can form the foundation for progress across several areas of condensed matter science.
Join via Zoom:
Meeting ID: 858 1199 4621
The colloquium is held in Feynman Lecture Hall, 201 E. Bridge.
In person is open to those with a valid Caltech ID.