Berlin 2005 – wissenschaftliches Programm

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DF: Dielektrische Festkörper

DF 5: Dielektrische und ferroelektrische dünne Schichten und Nanostrukturen I

DF 5.1: Hauptvortrag

Montag, 7. März 2005, 10:00–10:40, TU TC6

Physics of Ultra-Thin Dielectrics — •James F. Scott — Symetrix Centre for Ferroics, Department of Earth Science, University of Cambridge, Cambridge CB2 3EQ, UK

A combined Cambridge-Belfast collaboration has prepared and characterized high-dielectric single crystals, emphasizing barium titanate, down to 35 nm thickness. Rather surprisingly these materials exhibit perfectly bulk characteristics, with sharp first-order phase transitions at the known bulk Curie temperatures and dielectric constants peaking over 25 000. This shows that previously reported effects of broadening or conversion to second-order thermodynamics in ceramic films are due to extrinsic causes, such as oxygen vacancy gradients. With these data as a clue, we have studied the broad phase transitions in equally thin fine-grained (40 nm) ceramic films of barium strontium titanate (BST) and find that if the samples are cooled to T=50 K, the sharp bulk transitions are observed as reentrant upon warming; this shows clearly that transitions in such thin films are limited by sluggish kinetics and not by thermal equilibrium. Finally, we measure leakage currents up to E=1.4 GV/m (5 x the highest previous values) and show that at high fields these are space charge limited and not variable range hopping.