V. Time-Resolved Spectroscopy of Correlated Electron Materials
Mapping the flow of energy among strongly-coupled degrees of freedom
- Correlated electron phases
- X-ray methods in correlated electron science
- The origin of the metal-insulator transition in TaS2
- Ultrafast investigations of the electron-phonon interaction
- Complexity in correlated electron materials
Multiferroic materials Order parameters that can be switched between and “up” and “down” states are called ferroic. If a material has simultaneously two ferroic order paramaters, then it is called multiferroic. This definition has been somewhat relaxed in the past few years, and it has now customary to call any material multiferroic that shows spontaneous magnetic order and ferroelectricity . An example is when a material has a spontaneous dipole moment and antiferromagnetic order. Because multiple order parameters are almost always coupled, multiferroic materials hold the promise that the electric dipole moment can be manipulated magnetically, or that ferromagnetic magnetization can be manipulated electrically, with exciting possibilities for novel device applications involving ultrafast switching.
There are different mechanisms that can lead to the simultaneous presence of ferroelectricy and magnetic order. One of the simplest is when ferroelectricity emerges directly from magnetic order. This can happen when magnetic order breaks the symmetry in such a way that a switchable electric polarization occurs. There are other mechanisms, such as geometric ferroelectrics and lone-pair ferroelectrics, which are as yet not fully understood. The most interesting and promising cases are materials in which ferroelectricity arises from charge frustration which is coupled with magnetism (see Fig. V.i1). This can lead to a large electric polarization and strong coupling effects at high temperature. There are only few such electronic ferroelectrics known to date, and their physics is presently under intense investigation.
V.i1.The multiferroic material RbFe(MoO4)2.