Properties of Transition Metal Oxides

Magnetism and Charge Inhomogeneity in Two Dimensional Cuprates and Nickelates

H A M M E L  G R O U P


Condensed Matter and Thermal Physics
MS K764, Los Alamos National Laboratory, 
Los Alamos, NM 87545  USA
Contact:  Chris Hammel  + 505 665 0759

Understanding the doped transition metal oxides, in particular the copper, nickel and manganese oxides is one of the most important and widely studied problems in condensed matter physics. This is motivated by the wide range of properties exhibited by these materials. These unusual behaviors arise from the complex interplay between electronic, magnetic and lattice degrees of freedom and lead to very important technological properties extending from high temperature superconductivity to a dramatic sensitivity of electronic properties to the presence of a magnetic field. The fundamental electronic character of these materials remains poorly understood; it is now becoming clear that this is due in part to the fact that these materials are intrinsically inhomogeneous. While minimizing the carrier kinetic energy plays a central role in driving the tendency toward charge inhomogeneity, the fact that the interactions of charge carriers involving spin, charge and lattice degrees of freedom are all of roughly comparable strength is also important. In the nickel based oxides, the charge-rich regions form into one dimensional structures, or "stripes." We have studied this inhomogeneous state using magnetic resonance to better understand magnetic dynamics of these structures and their influence on the underlying magnetism. It is now recognized that similar charged stripes are also present in the cuprates. We are studying the magnetism of charge-striped lanthanum cuprate, focusing at present on europium co-doped lanthanum cuprate, a close analog of the material in which static stripes have been observed in elastic neutron scattering experiments. These studies are revealing an interesting and broad array of complex behaviors. We believe that obtaining a detailed microscopic understanding of the magnetic properties of these materials is crucial to understanding the normal state of the cuprates and the transition metal oxides in general.

A second, and related thrust of research is to employ high magnetic fields to better understand the suppression of susceptibility observed in the cuprates, a phenomenon known as the spin pseudo gap. We have recently found that the copper spin-lattice relaxation rate is independent of applied magnetic field up to 14.8 Tesla. This is significant because it shows that the onset temperature of the spin gap is not affected by the application of a magnetic field sufficiently large to decrease the superconducting Tc by 8 K. This shows that the spin gap is not a manifestation of conventional fluctuation superconductivity and demonstrates that the spin gap and superconductivity are distinct phenomena.

P U B L I C A T I O N S
Here is a sampling of our recent publications on NMR studies of correlated electron systems:

"Inhomogeneous Low Frequency Spin Dynamics in La1.65Eu0.2Sr0.15CuO0.4"
       Phys. Rev. Lett. 85, 642 (2000)       (LAUR 99-5452)

"Local Magnetic and Structural Properties of the LTO -> LTT Transition:
A 139La NQR Study in Lightly Hole-Doped La1.8-xEu0.2SrxCuO0.4"
       Phys. Rev. B 59, R3952 (1999)       (LAUR 98-4013)

"Spin dynamics in the low-temperature tetragonal phase of ~1/8 doped single crystal La1.67Eu0.2Sr0.13CuO4"
       Phys. Rev. B 61, R9265 (2000)       (LAUR 98-4014)

"Magnetism of stripe-ordered La5/3Sr1/3NiO4"
      Phys. Rev. Lett., 82, 3536 (1999)       (LAUR 98-730)

"Magnetic Field Independence of the Spin Gap in YBa2Cu3O7 – delta"
      Phys. Rev. Lett., 82, 177 (1999)       (LAUR-98-4640)

"Suppression of Antiferromagnetic Order by Light Hole Doping in La2Cu1 – xLixO4 :
A 139La NQR Study"
      Phys. Rev. Lett., 81, 2791 (1998)      (LAUR 97-3040)

"Magnetic Excitations of the Doped-Hole State in Diamagnetic La2Cu0.5Li0.5O4"
      Phys. Rev. Lett., 77, 2069 (1996)       (LAUR-96-890)

"NMR study of local structure in metallic La2CuO4+ delta"
      Phys. Rev. Lett., 71, 440 (1993)       (LAUR 93-1254)

NMR study of U(Be, B)13 in the normal and superconducting states
       Phys. Rev. B 59, 1432 (1999)       (LAUR-98-1603)

"Temperature dependence of the anisotropy of the planar
oxygen nuclear spin-lattice relaxation rate in YBa2Cu3Oy"
      Phys. Rev. B 57, 11769 (1998)       (LAUR-95-4104)

"Localized holes in superconducting lanthanum cuprate"
      Phys. Rev. B 57, R712 (1998)       (LAUR-96-3366)

"Oxygen ordering and phase separation in La2CuO4+ delta "
      Phys. Rev. B 52, 15575 (1995)       (LAUR-95-3186)
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