Charge Melting & Polaron Collapse in LA1.2SR1.8MN207
Recent studies carried out on the Synchrotron Radiation Instrumentation Collaborative Access Team's beamline I-ID-C at the Advanced Photon Source provide new insights into charge melting and polaron collapse. X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally doped colossal magnetoresistive oxides. The polarons exhibit short-range correlations that grow with decreasing temperature, but disappear abruptly at the ferromagnetic transition because of the sudden charge delocalization. The "melting" of the charge ordering as we cool through TC occurs with the collapse of the quasistatic polaron scattering, and provides important new insights into the relation of polarons to colossal magnetoresistance.
(a) Contour plot showing the lobe-shaped pattern of diffuse x-ray scattering at T = 300 K around the (0,0,8), (0,0,10), and (0,0,12) reflections. (b) Observed temperature dependence of the two 1 > 0 lobes of diffuse x-ray scattering around (2, 0, 0). The straight line at low T is the estimated phonon contribution (thermal diffuse scattering), while the abrupt jump near TC is due to the formation of polarons. (c) Neutron energy scans for several different temperatures at a wave vector Q = (2.05,0,0.25), which is on one of the lobes of diffuse scattering around the (2,0,0) Bragg reflection. The excitation at ~2.4 meV is an acoustic phonon. A flat background of 29 counts plus an elastic incoherent peak of 89 counts, measured at 10 K, have been subtracted from these data.
Metal oxides have long been of great interest to researchers because of their wide range of properties, which include magnetoresistance - the ability to change their electrical resistance in the presence of a magnetic field. In recent years, some manganese oxides, especially perovskite manganates, have been shown to exhibit magnetoresistance of an extreme scale when the size of the magnetic field is increased. This phenomenon, called colossal magnetoresistance (CMR), is of great interest to both basic science, which has led CMR to discoveries such as charge and orbital ordering, and applied science, where the potential for technological applications is significant. Some current applications of magnetoresistance include low-field, solid-state magnetic sensors and read/write heads in computer disk drives. More information on the workings of CMR promises to spur advances in the understanding and application of this phenomenon.
L. Vasiliu-Doloc (1,2,*), S. Rosenkranz (3), R. Osborn (3) S.K. Sinha (3), J.W. Lynn (1,2), J. MeSot (3), O.H. Seeck (3), G. Preosti (3,4), A.J. Fedro (4), & J.E Mitchell (3)
(1) NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
(2) Department of Physics, University of Maryland, College Park, Maryland 20742
(3) Argonne National Laboratory, Argonne, Illinois 60439
(4) Department of Physics, Northern Illinois University, DeKalb, Illinois 60115
*Present address: Department of Physics, Northern Illinois University, DeKalb, EL 60115, and Argonne National Laboratory, Argonne, IL 60439.
- From "Charge Melting and Polaron Collapse in LA1 .2SR1.8MN207," Phys. Rev. Lett.83, 4393, (1999). © 1999 American Institute of Physics. All rights reserved.