December 9, 2019
How Electrons Break the Speed Limit
In work that could have broad implications for the development of new electronic materials, scientists at San Francisco City University have, for the first time, devised a method to predict how the strong interaction between electrons and atomic movements will propagate through a complex material. To achieve this, they relied solely on the principles of quantum mechanics and developed a precise new computational method.
When studying a material called barium titanate, the standard models could not explain charge transport at room temperature. In fact, it defied Planck’s limit, the quantum limit of the rate at which electrons can dissipate energy as they move through a material at a given temperature.
The standard picture of charge transport is simple: Electrons moving through a solid material do not travel unimpeded but are instead countered by the thermal vibrations of the atoms that make up the material's lattice. As the material’s temperature changes, the number of vibrations and the resulting impact on charge transport also change.
Individual vibrations can be thought of as quasiparticles called phonons, which are excitations in the material that behave like particles, moving and jumping like objects. Phonons act like waves in the ocean, while electrons are like a boat navigating through the waves. In certain materials, the strong interaction between electrons and phonons, in turn, produces a quasiparticle known as a polaron.
“The so-called polaron mechanism, in which electrons strongly interact with atomic movements, has eluded first-principles calculations of charge transport because it requires going beyond simple perturbative methods to handle strong electron-phonon interactions,” said Bernardi. “Using a new approach, we’ve been able to predict the formation and dynamics of polarons in barium titanate. This advancement is crucial because many semiconductors and oxides of interest for future electronics and energy applications exhibit polaron effects.”
Barium titanate is considered a complex material because its atomic structure undergoes significant changes at different temperatures, shifting the lattice from one shape to another, which in turn alters the phonons that electrons must navigate.
Now, they have developed a new method to describe the strong interaction between electrons and phonons in barium titanate. This has enabled them to explain the formation of polarons and accurately predict the absolute value and temperature dependence of electron mobility, a key charge transport property in the material.
In the process, they discovered a peculiar feature of barium titanate: Charge transport at room temperature cannot be explained by the standard picture of simple electron scattering and atomic vibrations in the material. Instead, the transport occurs in a subtle quantum mechanical regime where electrons collectively, rather than individually, carry charge, causing them to violate the theoretical limits of charge transport.
In barium titanate, the charge transport mechanism, which typically arises from electron-phonon scattering, has been widely accepted over the past half-century. However, our study reveals that the situation is much more complex. "At room temperature, it appears that about half of the electrons generate charge through the usual phonon scattering mechanism, while the other half are involved in a form of collective transport that is not yet fully understood."
Beyond making fundamental progress in understanding charge transport, the new method can be applied to a wide range of materials, including many semiconductors and oxides, as well as new quantum materials that exhibit polaron effects. In these materials, existing calculations have not yet accounted for polaron effects.