Berlin 2005 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 2: Correlated Electrons - Metal Insulator Transition

TT 2.10: Vortrag

Freitag, 4. März 2005, 12:30–12:45, TU H2053

Unconventional temperature-dependence of charge order in La_1.8Sr_0.2NiO₄ — •J. Schlappa¹, C. Schüßler-Langeheine¹, C.-F. Chang¹, Z. Hu¹, M. Benomar¹, E. Schierle², H. Ott^1,2, E. Weschke², G. Kaindl², A. Tanaka³, M. Braden¹, and L. H. Tjeng¹ — ¹II. Physikalisches Institut, Universität zu Köln — ²Institut für Experimentalphysik, Freie Universität Berlin — ³ADSM, Hiroshima University, Japan

We report on a detailed study of charge order in Sr-doped La₂NiO₄, using the new technique of resonant soft x-ray diffraction at the Ni L_2,3 and La M_4,5 resonances. For La_2−xSr_xNiO₄ we observe a decrease of the coherence length of charge order, as determined from the width of the corresponding superstructure peak, not only when the system is heated up, but also when it is cooled below ≈ 65 K. While the superstructure-peak height decreases towards higher temperatures in a way resembling usual 2nd-order phase transitions, the change in the integrated intensity is much smaller, thus showing a behavior that is significantly different from that of the order parameter of other phase transitions. This means that upon heating the charge order breaks into smaller and smaller domains, while inside these domains the character of the order, as given by its period length and the energy dependence of the superstructure peak across the resonance, is conserved. A similar behavior is found upon cooling. A possible reason for this loss of coherence at low-temperatures is the influence of the disorder potential formed by the dopant atoms; a frustration caused by a competing magnetic order appears unlikely considering the observed temperature dependence.