Construction of a beam line at the advanced photon source for the study of quantum frustration in condensed matter

One of the great intellectual challenges facing condensed matter physics is to understand the physics of materials that exhibit quantum frustration. Such materials have many exotic and potentially useful properties, including high-temperature superconductivity, but it is not yet possible to describe their ground states and elementary excitations within a single theory.

One example of quantum frustration is the doped Mott insulator. Conceptually, this system consists of a two-dimensional array of electron spins on a square lattice. Each spin sits on a lattice site and interacts with its neighbors via the exchange interaction, J. If J is positive, each spin will attempt to lower its energy by pointing in a direction opposite that of each of its neighbors and/or moving to an empty lattice site. Each spin is “quantum” in the sense that it need not have purely up or down spin but could be in a superposition of the two states.

This system faces the problem of quantum frustration. It is energetically favorable for an electron to move because, as a quantum object, its kinetic energy is higher if it is forced to remain localized on one site. But there are constraints on its motion: an up spin can hop to an empty site whose neighboring spins are down; but if it hops to a site whose neighbors are up, this results in frustrated bonds. Doped Mott insulators exist in real life as high-temperature superconductors, and physicists have been unable for several decades to determine the mechanism by which these systems superconduct.

Professor Abbamonte, with Professor Juan-Carlos Campuzano at the University of Illinois-Chicago, is co-leading a research effort to better understand phenomena associated with quantum frustration. His group will construct a new beam line at the Advanced Photo Source, Argonne National Laboratory. The facility will support two experimental techniques: resonant soft x-ray scattering, for detecting ordered ground states; and angle-resolved photoemission spectroscopy, for studying elementary excitations. During his Center appointment, Professor Abbamonte will oversee the critical early stages of construction and equipment calibration.