Nature of the Reversible Insulator–to-Metal Transition in

Nature of the Reversible Insulator–to-Metal Transition in the Surface Layer of
SrTiO3 and BaTiO3.
Krzysztof Szot1,2
1)
Peter Grünberg Institut, Forschungszentrum Jülich, 52-428 Jülich, Germany
2)
Institute of Physics, University of Silesia, 40-007 Katowice, Poland
The discovery of the resistive switching phenomenon
in multinary oxides (especially in ternary oxides with perovskite structure) has raised
fundamental questions concerning the nature of the reversible insulator-to-metal (I/M) transition
[1,2]. How can a band insulator (e.g. SrTiO3 or BaTiO3) be switched into
a metallic conductor under the influence of different stimuli such as the chemical potential
gradient or an electrical current flow. In our opinion, for the description of the nature of this
transition a paradigm shift is needed in relation with the semiconducting-like concepts of
electrical transport traditionally applied to this class of oxides. Instead of only considering a
current flow through the whole bulk we should also accept the possibility of the creation of a
network of highly conducting nano-filaments through the matrix of binary and ternary oxides.
Reduction or electrodegradation of the crystals transform such a network into metallic nanowires
and short-circuit the rest of the non-conducting (insulating part) of the crystal. Without accepting
the special role of such filaments (due to the available extended defects in the crystal and their
high concentration in the surface layer) in electrical transport or as a sink for oxygen vacancies it
seems impossible to explain the I/M transition in, for instance, SrTiO3. This is due to the fact that
the actual concentration measured for defects and corresponding charge carriers, which are made
responsible for the I/M transition, is 4 to 5 orders of magnitude lower than predicted by the Mott
criterion [3]. For the apparently oxymoronic statement that BaTiO3 is a ferroelectric metal in the
surface [4] we need this new composite-like interpretation with an insulating bulk and a network
of conducting filaments [3], according to which the reversible I/M transition takes place only
along extended defects (here, edge dislocations) while the rest of the material remains a perfect
ferroelectric. In my lecture, I will argue that using a nanosensitive method (e.g. local
conductivity AFM (LCAFM), Kelvin probe, AFM- potentiometry, PFM) we can reveal the
“hidden aspects" of the I/M transition on the atomic scale in both models of ternary oxides with
perovskite structure
1) K.Szot et al, Nature Materials 5, 312, (2006)
2) Waser et al, Advanced Materials 21, 2632 (2009)
3) K.Szot et al, Solid State Physics 65, Chapt.4, (2014)
4) Y.Watanabe et al, Physical Review Letters 86, 332 (2001)
Prof. Dr. habil. Krzysztof Szot
Forschungszentrum Jülich,
Peter Grünberg Institut (PGI-7)
52425-Jülich, Germany
University professor (full professor) at Institute for Physics-University of
Silesia,
40-007 Katowice Poland
Education and career
1977
Diploma in Physics (Mgr.), University of Silesia in Katowice,
1977-1981
Assistant at Faculty of Mathematic-Physics and Chemistry (University of Silesia)
1982–1983
DAAD Scholarship, Kernforschungsanlage Jülich (Prof. M. Campagna und Prof.
J. Fuggle)
1985
Dr. rer. nat., University of Silesia
1985-1989
Scientist, Institute for Physics, University of Silesia in Katowice
1989–1996
Visiting scientist at Forschungszentrum Jülich
(with Prof. W.Eberhardt, Prof.H.Lüth, Prof.H.Wagner)
1997–
Scientist (permanent position), Institut für Festkörperforschung,
Forschungszentrum Jülich
1997-
Habilitation, University of Silesia in Katowice
2002–
University Professor at University of Silesia in Katowice, Institute for Physics
2008-
Nomination to titular professor by President L. Kaczynski
Publications and Patents
• Papers in scientific journals ~114, citations ~4200
• Books: 6 (coauthor)
• Patents: 14
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