Cottrell College Science Awards - 2015
Neutron Scattering and Muon Spin Relaxation Studies of Geometrically Frustrated Double Perovskites
Three electrons walk into a bar. One mutters “Gimme a beer,” the second one loudly declaims, “A scotch, if you please, good sir!” And the third one just spins around and around on his stool.
Needless to say, the atom-sized bartender might experience a moment of frustration with these errant subatomic particles. A physicist, however, would probably be very interested in why the electrons, which seem more or less identical, are nevertheless behaving so differently.
Drop the bar metaphor and substitute a crystalline lattice of atoms found in certain synthetic and naturally occurring minerals such as perovskite, and physicists even have a term for one cause of varying magnetic behavior of electrons in lattices: geometric frustration.
Jeremy P. Carlo, assistant professor of physics at Villanova University, says frustration occurs when magnetic moments are arranged such that “competing interactions [of electrons and associated forces] inhibit the development of magnetic order, yielding exotic magnetic and non-magnetic ground states,” among other phenomena. (The “magnetic moment” of an electron arises from its spin, i.e. its intrinsic orbital angular momentum, and it contributes to whether regions in a lattice can align magnetically or remain disordered. The “ground state” of a material is the lowest energy state achieved by the cooperative alignment of magnetic ions in the lattice.)
“As a window to exotic physics, frustration has been of substantial interest to the research community,” Carlo notes. In other words, geometric frustration may be opening new horizons for development of future electronics, including high-temperature superconductivity and the frictionless transmission of electricity.
Carlo has received a Cottrell College Science Award from Research Corporation for Science Advancement to study geometric frustration in perovskites, materials with crystal structures similar to their namesake, a calcium titanium oxide mineral with the chemical formula CaTiO₃. In perovskites, atoms are arranged in a crystalline lattice that has been described as “octahedra which share corners infinitely in all three dimensions, making for a very nice and symmetric structure.”
Because of their geometry and chemical versatility (perovskites can be made with nearly every element in the periodic table), perovskites have startling electronic properties. They have attracted the interest of researchers seeking to boost the efficiency of solar panels, which convert sunlight into electricity.
Carlo, however, seeks to work with materials in an even more exotic molecular structure called “double perovskite.” In double perovskites, magnetic ions are arranged at the corners of tetrahedra, which prevents the development of ordered magnetic structures. He and his students will employ advanced techniques including neutron scattering and muon spin relaxation, performed using particle beams at national laboratories in the U.S. and abroad, to probe the structure and subatomic behavior of this geometrically frustrated material.
“This project will support the development of a sustained research program at Villanova University, and will provide an opportunity for undergraduates to engage both in hands-on research in the on-campus laboratory environment, and in collaborative experiments at large-scale domestic and international facilities,” Carlo said.