Charge Inhomogeneities in the Colossal Magnetoresistant Manganites From the Local Atomic Structure
- PDF / 968,428 Bytes
- 6 Pages / 415.8 x 637.2 pts Page_size
- 46 Downloads / 174 Views
S. J. L. BILLINGE*, V. PETKOV*, TH. PROFFEN*, G. H. KWEI**, J. L. SARRAO**, S. D. SHASTRI*** and S. KYCIA.... *Center for Fundamental Materials Research and Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824. **Los Alamos National Laboratory, Los Alamos, NM 87545 ***Advanced Photon Source, Argonne National Laboratory, Argonne, IL *...CornellHigh Energy Synchrotron Source, Ithaca, NY
ABSTRACT We have measured atomic pair distribution functions (PDF) of Lal-,CaxMnO 3 using high energy x-ray diffraction. This approach yields accurate PDFs with very high real-space resolution. It also avoids potential pitfalls from the more usual neutron measurements that magnetic scattering is present in the measurement, that the neutron scattering length of manganese is negative leading to partial cancellation of PDF peaks, and that inelasticity effects might distort the resulting PDF We have used this to address the following questions which do not have a satisfactory answer: (1) What are the amplitudes and natures of the local Jahn-Teller and polaronic distortions in the CMR region. (2) Is the ground-state of the ferromagnetic metallic phase delocalized or polaronic. (3) As one moves away from the ground-state, by raising temperature or decreasing doping, towards the metal insulator transition, how does the state of the material evolve? INTRODUCTION A very large magnetoresistance is observed in the, so-called, colossal magnetoresistant manganites [1]. This coupling of magnetic field to transport is qualitatively understood using the concept of double exchange [2]. That this description underlies the CMR phenomenon appears not to be in question [3]. However, the DE model has difficulty to account quantitatively for the observed Tc's and the magnitude of the resistivity change at Tc, and is totally incapable of explaining the complex phenomenology away from the ideal CMR compositions [4,5]. A number of authors [4,6-10] have suggested that the extra contribution to the model should be an electronlattice coupling term. Extensive experimental evidence exists for lattice effects in these materials. For example, anomalous increases in crystallographic Debye-Waller factors [I I] and, equivalently, PDF peak-widths [12] and XAFS Debye-Waller factors [13,14] at the MI transition; an oxygen isotope dependence to T, [15,16]; thermal conductivity [17,18] and transport measurements above T, interpreted in terms of polaron dynamics [ 19,20]. These results can be consistently interpreted if charges localize as small polarons above T, Early work using a number of different techniques [11-13,15,21] established the importance of the lattice to the colossal magnetoresistance phenomenon. More recently attention has been focussed on understanding the precise nature of the polaronic state and the related Jahn-Teller distortions. We have concerned ourselves with a number of questions: (1) What does the polaronic distortion look like? (2) What is the ground-state of the ferromagnetic metallic (FM) phase? (3) How do the p
Data Loading...
