Atomic dynamics investigations with neutrons, x-rays, and optical spectroscopy combined with first-principles simulations

Overview of our lab's major work areas:

Introduction

Our research is driven by outstanding questions in microscopic dynamics at the nano-scale, revealing novel properties of materials for improved energy conversion. We investigate key aspects of transport properties and thermodynamics in important energy materials, including thermoelectrics, ferroelectrics/multiferroics, and superconductors. We also have an interest in photovoltaics and ionic conductors. To this end, we use state-of-the-art experimental techniques ranging from neutron and x-ray scattering at large-scale facilities in the National Laboratories, as well as optical spectroscopy, synthesis, and thermodynamic and transport measurements. In parallel, we perform first-principles simulations leveraging large-scale  computing to identify the fundamental origins of the effects observed in our measurements. We also develop advanced modeling techniques based on statistical analysis and supercomputing. This combined experimental and computational approach enables us to gain deeper understanding and ultimately to improve the performance of energy materials.

 

Phonons in Thermoelectrics

Understanding the microscopic processes involved in the transport and conversion of energy from the atomic scale to the mesoscale is key for the development of next-generation materials for energy sustainability. For example, the interplay between phonons (atomic vibrations) and charge transport is crucial in thermoelectrics. This enables practical devices for “waste-heat harvesting” and heat pumping. At a microscopic level, such couplings result from interactions of phonons with other phonons (phonon-phonon interaction from anharmonicity), or with charge carriers (electron-phonon interaction). In particular, it is key to probe and control phonon scattering mechanisms in thermoelectrics, in order improve the thermoelectric conversion efficiency. The novel approaches we develop provide hitherto unavailable information about mode-resolved phonon scattering rates, leading to a microscopic understanding of thermal transport and guiding rational material design.

 

Lattice instabilities and anharmonic phonons in ferroelectrics

The phonon-phonon interaction is also at the heart of soft-mode lattice distortions central to ferroelectrics, and underlies atomic diffusion in fast ion conductors.

 

Coupling of spin and lattice degrees of freedom in multiferroics

An additional and exciting dimension to these interactions has emerged in the recent discovery of coupled spin/phonon transport, whereby the interplay of phonons and magnons can link thermal and magnetic properties in new devices. The magnon-phonon interactions are also central for multiferroics.

 

Anharmonic Phonons and Thermodynamic Stability

Short Summary