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Colloquium: Experimental realization of strongly correlated electron systems in the ultraclean limit

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Roman Engel-Herbert, The Pennsylvania State University
When
12 January 2017 from 3:45 PM to 5:00 PM
Where
117 Osmond Laboratory
Contact Name
Moses Chan
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The novel properties of strongly correlated electron systems are driven by a sizable Coulombic repulsion between electrons. They entail a range of fascinating phenomena that are beyond the description of a free electron gas. Examples include macroscopic quantum states like superconductivity, or electronic phase transitions between metastable states with dramatically different electronic properties (e.g. metal-to-insulator transitions). These properties render them very attractive for fundamental studies as well as for applications. An important material class in this context are the transition metal oxides whose electronic and optical properties are dominated by a partially filled narrow d band with high carrier effective masses and high carrier concentration. Studying these materials in low dimensional thin films is important both for the exploration of correlated electron systems in the ultraclean limit and for their utilization in electronic and photonic devices. While the structural perfection of conventionally synthesized films is high, evidenced by atomically abrupt interfaces, well defined heterostructures, and the demonstration of artificially layered oxide compounds, the level of perfection needed to minimize unintentional extrinsic defect concentration to approach an ‘electronically clean’ material has been found extremely challenging.

In this talk, I will show how the application of an epitaxial thin film synthesis technique, dubbed hybrid molecular beam epitaxy (MBE), overcomes the conventional challenges for the growth of transition metal oxide films [1]. We combine conventional MBE and chemical beam epitaxy (CBE) for the growth of SrVO3 and LaVO3, a correlated metal and a Mott insulator with perovskite structure. By tuning the composition between the two end members in the solid solution La1-xSrxVO3 , we find a temperature-phase diagram similar to that of the well-studied cuprate system, only providing access to the quantum critical point in La1-xSrxVO3 in the ultraclean limit. The synthesis of these films also allows us to exploit correlated metals as transparent conductors [2]. The strong electron interaction leads to a large, renormalized carrier effective mass, thus striking a new balance between the mutually exclusive demands of a high electrical conductivity with metal-like carrier concentration and high optical transparency in the visible range. These novel transparent conductors have a figure of merit comparable to the industry standard, tin-doped indium oxide, but at much thinner film thicknesses, and could thus provide a very cost effective solution of technological relevance.

1. H.-T. Zhang et al., Nature Communications 6, 8475 (2015).

2. L. Zhang et al., Nature Materials 15, 204 (2016). 

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