|Influence of Topological Spin Fluctuations on Charge Transport|
Layered transition metal oxides are the focus of intense research efforts because they might clarify the superconducting mechanism of cuprate high-temperature superconductors (HTSCs). A case in point is NaxCoO2 with x = 0.7, which is a parent compound for a family of cobaltites that exhibits superconductivity. This class of materials is also thought to be ideal for detecting the long-sought resonating valence bond (RVB) state of matter proposed by Philip Anderson of Princeton University in 1973. Researchers from Princeton University and ALS are the first to use angle-resolved photoemission spectroscopy (ARPES) to demonstrate the strongly electron correlated nature of this material and to provide evidence that charge transport is strongly influenced by topological spin frustration.
The family of sodium cobalt oxides or oxyhydrates (cobaltites) NaxCoO2 (with variable x) is similar to cuprate HTSCs. The parent compounds are Mott insulators, in which a strong electrostatic repulsion blocks charge transport; they have layered crystal structures with electronic two-dimensionality; and they can be chemically doped by altering the sodium concentration, thereby changing the electron concentration in the electronically active cobalt-oxygen layers. There is one very important difference, however. While the electronically active copper-oxygen planes in HTSCs have square symmetry, in cobaltites, the symmetry is triangular, a configuration that geometrically frustrates formation of the antiferromagnetically ordered Néel state present in HTSCs and results in a less well ordered "quantum spin liquid" state.
The interplay of charge and spin degrees of freedom is indeed exciting in cobaltites and leads to a bouquet of changing electronic properties with doping. With increasing sodium concentration, the material progresses from a paramagnetic metal that is superconducting at low-temperature for 1/4 < x 3/4.
The Princeton/ALS group performed a detailed investigation of low-energy electronic structure and charge dynamics of the parent cobaltite compound Na0.7CoO2 at ALS Beamlines 7.0.1 and 12.0.1. This technique is sensitive to an electron’s quantum correlations because it directly probes the electron distribution function over a complete Brillouin zone (unit cell in momentum space) with good resolution.
ARPES spectra taken in the energy range of valence (loosely bound) electrons are in good agreement with the results of mean-field (first principles) calculations and map the dispersion (energy–momentum relationship) of several distinct bands. Most exciting, ARPES spectra also reveal an additional satellite feature centered at a much higher binding energy near 11 eV. Separation of this correlation satellite from the valence band gives an estimate of a strong on-site Coulomb repulsion (Hubbard U) between electrons of about 5 eV, which is in the same range as the Hubbard U for the cuprates and provides strong evidence for the highly correlated nature of electrons.
In addition, the researchers discovered a tiny feature (quasielectron) adjacent to the Fermi energy (zero binding energy) that was dwarfed by the presence of the valence band. This feature is essentially flat in momentum space with a dispersion less than 100 meV, which is a factor of 5 smaller than in cuprates and an order of magnitude below the mean-field (first principles) calculations. Accompanying the small bandwidth is a very weak nearest-neighbor single-particle hopping energy t of about 10 meV. That t is of the same order of magnitude as the spin exchange coupling J for this family implies that the charge dynamics are strongly perturbed by spin fluctuations.
Temperature-dependent ARPES measurements revealed that quantum coherent quasielectrons exist only at low temperatures and linearly disappear as their motion becomes incoherent beyond 150 K. This value, which is well below the decoherence temperature of tens of thousands of degrees in conventional metals, is of the order of t and J, making geometrical frustration of antiferromagnetic interactions in triangular cobalt lattice planes the leading cause of quasielectron’s quantum decoherence.
Research conducted by M.Z. Hasan (Princeton University, Princeton Materials Institute, and ALS); Y.-D. Chuang (Princeton University and ALS); D. Qian, Y.W. Li, and Y. Kong (Princeton University); A. Kuprin, A.V. Fedorov, R. Kimmerling, E. Rotenberg, K. Rossnagel, Z. Hussain, and H. Koh, (ALS); and N.S. Rogado, M.L. Foo, and R.J. Cava (Princeton Materials Institute and Princeton University).
Research funding: National Science Foundation, Princeton University, and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.
Publication about this research: M.Z. Hasan, Y.-D. Chuang, D. Qian, Y.W. Li, Y. Kong, A. Kuprin, A.V. Fedorov, R. Kimmerling, E. Rotenberg, K. Rossnagel, Z. Hussain, H. Koh, N.S. Rogado, M.L. Foo, and R.J. Cava, “Fermi surface and quasiparticle dynamics of Na0.7CoO2 investigated by angle-resolved photoemission spectroscopy,” Phys. Rev. Lett. 92, 246402 (2004).