Role of Disorder in Systems Possessing Strong Coupling
between Spin, Charge and Lattice Degrees of Freedom

Robert H. Heffner

MST-10, MS K764

Los Alamos National Laboratory

Los Alamos, NM 87545

(505) 667-0397

(505) 665-7652

heffner_robert_h@lanl.gov

This research involves the study of systems with strongly coupled and often competing interactions involving the spin, charge and lattice degrees of freedom in condensed matter. The interactions involve magnetic exchange, long- and short-ranged Coulomb correlations and local lattice distortions. The coupling and competition between these interactions results in a variety of ground states (magnetic/paramagnetic, insulating/conducting, charge-ordered/disordered) which are easily perturbed by relatively small changes in extrinsic conditions, such as pressure, applied magnetic field, temperature, and disorder induced by chemical substitution. Two very different classes of materials are under study: transition metal oxides and heavy fermion intermetallics.

Transition metal oxides:

The magnetoresistive manganites, rediscovered in the early 1990ís for their very large negative magnetoresistance (and possible use as magnetic field sensors), are archetypes for the kinds of materials described above. The manganites consist of n layers of corner-sharing Mn-O octahedra separated by (La, A)O insulating layers: (La_1-x A_xMnO₃)_nAO, A = Ca, Sr and Ba. Both the n = infinity systems (with nearly cubic perovskite structure) and the n =2 systems (2D in nature) are under vigorous worldwide investigation. The undoped LaMnO₃ exhibits both Jahn-Teller and d-spin orbital ordering and is insulating. As divalent alkaline earths are substituted for La³⁺, the doped holes induce a conducting ferromagnetic ground state, which exhibits large negative magnetoresistance when 0.2 < x < 0.5. At larger doping (x ³ 0.5) charge and orbital ordering is found, accompanied by antiferromagnetic ground states. Recent attention has focused on understanding the relative importance of magnetic exchange, local (Jahn-Teller) lattice distortions and Coulomb correlations for both the charge and spin behavior. The discovery of charge ordering for x = 1/8, 4/8, 5/8 in the manganites correlates with that found in nickelates and with the phase separation of holes in Sr-doped La₂CuO₄. The striped phases in the cuprates, and their associated magnetic correlations, are currently being investigated theoretically for their possible association with the phenomenon of high-temperature superconductivity.

Our research involves the use of local magnetic (m SR and neutron scattering), structural (neutron diffraction and XAFS), thermodynamic and transport probes to study the interplay of the spin, charge and lattice degrees of freedom in these highly perturbable systems. One important finding is that the spin dynamics in the ferromagnetic phase of the manganites is highly disordered and thus very unconventional. We believe that this phenomenon is related to disorder-induced phase separation into hole-rich ferromagnetic/conducting regions and hole-depleted/less-conducting regions, resulting in percolating ferromagnetic networks. This work has impacted theoretical models involving phase separation and percolation in these materials.

Heavy Fermion magnets:

In systems like UCu₄Pd competition between the Kondo and RKKY magnetic couplings can lead to a non-magnetic ground state which exhibits non-Fermi-liquid transport and thermodynamic properties. In systems like CeCu_5.9Au_0.1 the non-Fermi liquid behavior appears to be associated with a zero-temperature quantum critical point. Our m SR and XAFS experiments have shown that disorder between the Cu and Pd sites apparently induces the non-Fermi liquid behavior in UCu₄Pd, however, by producing a distribution of Kondo temperatures and the possible formation of ëGriffith-phases,í or a tendency to form spin clusters. Thus disorder plays a fundamental role in altering the magnetic, transport and thermodynamic properties of this system by tipping slightly the balance between the competing magnetic interactions.

Superconductors:

We are also investigating the effects of small quantities of impurity atoms and of the application of magnetic fields on the symmetry of the superconducting order parameter in heavy fermion and oxide superconductors. A key question here concerns the different response of a d-wave superconducting order parameter to magnetic and non-magnetic impurity atoms.