Ordinary solids are complicated enough with around 10²³/cm³ electrons whose every move not only affects but is influenced by the motion of all their neighbors, an insoluble problem. Remarkably, theorists are now able to transform this confusion of strongly interacting electrons into a mathematically tractable collection of noninteracting quasiparticles whose properties match the results of experiments. For example, one can visualize a quasielectron as an electron surrounded by a cloud of displaced neighboring electrons. So far, so good, but when the electrons interact too strongly, the quasiparticle model breaks down, and so far no tractable alternative has arisen.

CMR oxides undergo a transition from a paramagnetic insulator to a ferromagnetic "poor" metal as the temperature is lowered. An externally applied magnetic field can also drive this transition, which results in a "colossal" thousandfold decrease in the resistivity.

Now You See Them, Now You Don't!

Now they're here, now they're there! Fluctuations comprising rapidly changing entities are one of the themes of modern physical science, sometimes occurring during a transition from one state of matter to another. Chuang et al. have invoked fluctuations to understand one of the current hot topics in solid-state science, the "colossal" magnetoresistance (CMR) effect in certain compounds containing manganese and oxygen atoms (manganites). Application of a magnetic field causes the electrical resistivity of a CMR material to decrease by a factor of 1000 or more, a phenomenon both for physicists to explain and technologists someday to convert into useful devices, such as high-density magnetic data-storage devices. In this case, x-ray (technically, angle-resolved photoelectron spectroscopy) measurements of the electronic structure of a CMR manganite also containing the elements lanthanum and strontium provided a chain of evidence culminating in fluctuations that involve minute (nanometer-sized) regions of electrically conducting regions that coexist with insulating regions in the lower resistivity state but cease to exist in the insulating state.

CMR oxides undergo a transition from a paramagnetic insulator to a ferromagnetic "poor" metal as the temperature is lowered. Poor means the electrical resistivity is relatively high. An externally applied magnetic field can also drive this transition, which results in a "colossal" thousandfold decrease in the resistivity. The American-Japanese team studied the manganite compound La_1.2Sr_1.8Mn₂0₇, whose crystal structure is built around double planes of manganese and oxygen atoms separated by lanthanum and strontium atoms. The CMR effect is thought to take place in these planes.

At ALS Beamline 10.0.1, the investigators were able to make angle-resolved photoemission spectroscopy (ARPES) measurements with much higher angular and energy resolution than before, which allowed them to conduct a comprehensive examination of the electronic structure of the material at low temperature in the conducting state. For example, they were able to resolve an energy band associated with the manganese-oxygen layers that exhibited the classic step-like drop in photoemission intensity as it crossed the Fermi energy above which electron quantum states are unoccupied, a telltale signature of quasiparticles with well-defined energies and momenta. They were also able to map the Fermi surface (contour in momentum space of the Fermi energy).

Left: A quasielectron band extends to the Fermi surface in the low-temperature metallic state, but the low intensity (spectral weight) is indicative of a pseudogap that pushes most of the electrons to higher binding energies. Right: The change in the spectral weight near the Fermi energy with temperature shows that the pseudogap grows stronger at higher temperatures, resulting in an insulating state.

Here, the story becomes too complicated to tell in detail. In brief, calculation of the electrical resistivity from parameters extracted from the Fermi surface, the energy band, and other details of the photoemission spectra resulted in a value 10 times lower than measured experimentally. The shape of the Fermi surface, which has parallel straight lines, provided a clue why. Such lines are associated with electronic and structural instabilities (charge/orbital density waves cooperating with a Jahn-Teller distortion), for which there is evidence in La_1.2Sr_1.8Mn₂0₇ from neutron and x-ray scattering experiments by other groups.

The long parallel sections of the experimentally measured Fermi surface, which well matches that calculated by theorists, are associated with instabilities that drive nanoscale fluctuating phase separation of conducting and insulating regions.

These instabilities give rise to a "pseudogap" (seen in the present, as well as past, ARPES measurements) in which the electron energy bands are pushed well below the Fermi energy and hence do not contribute to the conductivity. In the low-temperature conducting state, the pseudogap is not total, however, because there are nanometer-sized conducting regions with no pseudogap (where the quasiparticles are seen) as well as insulating regions with charge/orbital ordering. Moreover, competition between these regions causes fluctuations in their size and location with time. The net result is a poor metal at low temperature and an insulator at high temperature when the conducting regions disappear.

Research conducted by Y.-D. Chuang (University of Colorado and ALS); A.D. Gromko and D.S. Dessau (University of Colorado); T. Kimura (University of Tokyo); and Y. Tokura (University of Tokyo and Joint Research Center for Atom Technology, Tsukuba).

Research funding: National Science Foundation and U. S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: Y.-D. Chuang, A.D. Gromko, D.S. Dessau, T. Kimura, and Y. Tokura, "Fermi Surface Nesting and Nanoscale Fluctuating Charge/Orbital Ordering in Colossal Magnetoresistive Oxides," Science 292, 1509 (2001).

ALSNews Vol. 180, July 18, 2001