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"Hot" Surprises - Electronic ordering above room temperature in the presence of frustration Speaker: Prof. Paolo G. Radaelli - Clarendon Laboratory, UK Date: 2011-03-08 Time: 15:30 Place: Physics Dep. Hall B, Lund Abstract: Charge localisation, whereby itinerant electrons become “frozen” into an insulating state, is one of the central and most studied phenomena in condensed-matter research, and one that underpins many of physics’ unanswered questions, such as the mystery of high-temperature superconductivity. In metals, charges usually localise due to Fermi surface instabilities, giving rise to inhomogeneous and continuously modulated states known as “charge density waves” (CDWs). Conversely, in more ionic systems like the metal oxides, charges tend to localise into a small number of well-defined “valence” states – an effect known as charge ordering or charge disproportionation [1]. Although the resulting ordered structures are often complex, they tend to be in register with the original lattice, whereas true CDW are almost always incommensurate. Charge and orbital ordering acquire a particularly interesting character when they occur in the presence of geometrical frustration, since this condition tends to disfavour simple arrangements such as the chequer-board pattern and may lead to the development of unconventional patterns, often with propagation vectors in the interior of the Brillouin zone. I will discuss in particular the case of two oxides with simple stoichiometries, AgNiO2 (delafossite 2H structure) and YbFe2O4, but with rather extraordinary electronic ordering properties developing above room temperature. In the case of AgNiO2 [2-4], one would expect a cooperative Jahn-Teller distortion (Ni3+ is a Jahn-Teller- active ion), which would reduce the symmetry down from the hexagonal, high-temperature structure due to orbital ordering. Instead, orbital degeneracy in AgNiO2 is lifted on cooling below Ts=365 K through charge disproportionation and charge ordering rather than orbital ordering, in sharp contrast with some closely- related nickelates like NaNiO2. YbFe2O4 provides another extremely interesting example of electronic ordering on a triangular lattice. In this case, the average high-temperature Fe valence is 2.5+, and electronic ordering occurs below Ts= 320 K. A number of models [5] have been proposed in the past, especially for isostructural LuFe2O4, to explain how two valence states (Fe2+ and Fe3+) can order on a tripartite lattice – some of them involving the development of ferroelectric polarisation. We will show that electronic ordering in YbFe2O4 is in fact of an altogether different character, never previously observed, to our knowledge, in any oxide. Most extraordinarily, above Ts YbFe2O4 retain very long range structural correlations, but develops into a unique topological phase, in which electronic ordering with continuously degenerate propagation vectors coexist [6]. [1] The classic case is magnetite: E.J.W. Verwey, Nature 144, 327 (1939). [2] E.Wawrzynska et al., Phys. Rev. Lett. 99, 157204 (2007); [3] E.Wawrzynska et al., et al., Phys. Rev. B 77, 094439 (2008). [4] L. Pascut et al., arXiv:1009.2053v1 [cond-mat.str-el] [5] For example, Angst et al, PRL 227601 (2008) [6] A. Hearmon et al., arXiv:1101.5600 [cond-mat.str-el] Contact person, ESS seminars:
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