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
_“Electron correlation” is a dominant theme in condensed matter science, manifesting itself in, e.g., “metal-insulator transitions” (MIT), “high-temperature superconductivity” (HTS) and “colossal magneto-resistance” (CMR). On the microscopic scale, one speaks of the charge (C), spin (S) and orbital (O) degrees of freedom, each of which may show short- or long-range order, and each of which may exchange energy with the others and with the crystal lattice (L) (see Fig. V.1). Important correlation effects can occur in systems with partially-filled electron shells, such as those of 3d-transition metal ions, with anisotropic, ­quasi-localized character. Vast amounts of experimental and theoretical work have been published on electron correlation, triggered largely by the discovery of HTS in 1986 [2]. Phase ­diagrams of many interesting materials have been investigated in detail, and numerous theories of the microscopic charge-spin-orbital-lattice interactions have been proposed. Much has been achieved, but much is still unclear. There is increasing ­evidence of the ­importance of nanoscale inhomogeneities and fast fluctuations in correlated electron materials – indicating the important role that the SwissFEL will play. ­Furthermore, it has been suggested that the chicken-or-egg problem, of determining the cause and effect relationships among the C, S, O and L subsystems, may best be approached with pump-probe time-resolved spectroscopy: one pumps energy into a particular degree of freedom and measures the time required for a response to appear in the others.

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 [24]. 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.