Materials Science Research Lecture
NOTE: At this time, in-person Materials Research Lectures are open to all Caltech students/staff/faculty/visitors with a valid Caltech ID. Outside community members are welcome to join our online webinar.
Webinar ID: 832 7665 2110
Many problems in materials cannot be solved solely with experiments, and first-principles theory opens a window to understanding material behavior and predicting new materials. A range of materials problems from ferroelectrics to Earth's core and beyond will be discussed, as will a range of techniques from static total energies, structure searching, phonons from density functional perturbation theory (DFPT), and first-principles molecular dynamics (FPMD). Ferroelectrics are active dielectric materials that respond to applied electric fields—they change shape and interconvert energy from one form to another. Most importantly, they have a switchable polarization—net dipole per volume and are not centrosymmetric—they are polar. Surprisingly, experiments show that Hafnia (HfO2) is ferroelectric in thin films. Most ferroelectrics lose their ferroelectricity in nanoscale samples due to depolarization fields. Also, the structure for bulk hafnia is baddeleyite, a centrosymmetric, non-ferroelectric structure. Ferroelectric hafnia is of great technological interest for electronic since it is compatible with silicon. Many experimental and theoretical studies have failed to demonstrate conclusively why and how nanoscale hafnia is ferroelectric. I will show that it can be understood by epitaxial strain. Secondly the issue of high-pressure ferroelectricity will be discussed. For many years it was thought that pressure squeezed out ferroelectricity, until theorists predicted high pressure ferroelectricity. But our experiments on PbTiO3 contradict this. New computations that show why. Moving to high pressure research important in the Earth and exoplanets, I will discuss experiments and theory for a fascinating disordered germanate, a structure that may form as a silicate in superEarths. Finally, I will show how DFT (and beyond DFT) has been used to better understand Earth's dynamo through transport properties of high pressure iron alloys.
More about the Speaker:
Ronald Cohen uses fundamental physics to understand natural and technological materials. Some of his work focuses on understanding the behavior of high-technology materials called ferroelectrics—non-conducting crystals with an electric dipole moment similar to the opposite poles found in a common bar magnet. He also looks at minerals in Earth's deep interior and of materials that display interesting physical and chemical properties. Ferroelectrics are the key component in medical ultrasound and sonar, and he is working on developing them as new energy materials for energy scavenging and solid state refrigeration. Currently he is working on understanding Earth's core and the cores of giant planets and exoplanets, and on developing new classes of useful active materials. Cohen obtained a B.Sc. in Geology from Indiana University in 1979 and a Ph.D. in Geology from Harvard University in 1985. Before coming to Carnegie as a staff scientist in 1990, he was a research associate at the National Research Council from 1985-1987 and a research physicist at the Naval Research Laboratory from 1987 to 1990. He held an ERC Advanced grant from 2013-2018 and has been an honorary professor at University College London and LMU Munich.