molecular dynamics simulation of lithium niobate under a time varying electric field

The image shows a molecular dynamics simulation of lithium niobate under a time varying electric field, which changes the sign of the polarization. Red is niobium, green is oxygen, and lithium shows a range of colors for different time steps. The niobium and oxygen are shown only for one time step for clarity. The image shows a small part of the actual simulation.

Researchers at the Carnegie Institution recently discovered a new efficient way to pump heat using crystals. The crystals  pump or extract heat, even on the nanoscale, which means they could be used to overcome current constraints to higher computer speeds by providing a solution to chip overheating.

Ronald Cohen, staff scientist at Carnegie’s Geophysical Laboratory and Maimon Rose, led the team that carried out the research. They performed simulations on ferroelectric crystals—materials that have electrical polarization in the absence of an electric field. The electrical polarization is reversed by applying an external electrical field. Introducing an electric field causes a giant temperature change in the material, the “electrocaloric effect”, far above a temperature to achieve a so-called paraelectric state.

“The electrocaloric effect pumps heat through changing temperature by way of an applied electric field,” explained Cohen. “The effect has been known since the 1930s, but has not been exploited because people were using materials with high transition temperatures. We found that the effect is larger if the ambient temperature is well above the transition temperature, so low transition temperature materials are preferred.”

Ferroelectrics become paraelectric—that is, have no polarization under zero electric field above their transition temperature, which is the temperature at which a material changes its state from ferroelectric to paraelectric.

Rose and Cohen used atomic-scale molecular dynamics simulations, where they followed the behavior of atoms in the ferroelectric lithium niobate as functions of temperature and an electrical field. Rose remarked, “Lithium niobate had not been studied before like this. We were pretty surprised to see such a huge temperature change.”