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In a number of high Tc superconductors, small distortions of the lattice structure results in surprisingly large symmetry breaking of the electronic states and macroscopic properties, an effect often referred to as nematicity. This nematicity has been studied extensively on materials with orthorhombic crystal structure, where the lattice symmetry is already reduced from four-fold (C4) to two-fold (C2) [1]. To directly study the impact of symmetry breaking lattice distortions on the electronic structure, using scanning tunnelling microscopy (STM) we image at the atomic scale the influence of strain tuned lattice distortions on the correlated electronic states of two different Fe-based materials: Lithium iron arsenide (LiFeAs) and Iron telluride (FeTe).
LiFeAs is a Fe-based superconductor that is tetragonal in its ground state, with C4 symmetry. By applying uniaxial strain to this material, our experiments uncover a new strain-stabilised nematic phase comprised of a unidirectional charge density wave (CDW), an electronic state which not only breaks rotational symmetry but also reduces translational symmetry of LiFeAs. We follow the evolution of the superconducting gap from the unstrained material with C4 symmetry through the new nematic phase with C2 symmetry, to a state where superconductivity is completely suppressed [2].
Using the same methodology, we have also studied the influence of lattice distortions by strain tuning on the ground state of Iron telluride (FeTe), the non-superconducting parent compound of the Fe chalcogenide materials. FeTe exhibits a peculiar (π, 0) magnetic order not seen in other iron-based superconductors [3], which show (π, π) magnetic order. Using STM to image the strained samples, we study the impact of small lattice distortions on the magnetic order of Fe1+xTe (x~0.06). Straining the material along the [110] direction, we have observed a new ordered phase that is characterized by a (π, π) order. I will discuss the origin of this new phase, and show measurements of its electronic and magnetic properties.
[1] Chu J- H, Kuo H-H, Analytis J. G. and Fisher I. R., “Divergent Nematic Susceptibility in an Iron Arsenide Superconductor”, Science 337, 710-712 (2012).
[2] Yim C. M. et al., “Discovery of a strain-stabilised smectic electronic order in LiFeAs”, Nature Communications 9, 2602 (2018).
[3] Enayat M et al., “Real-space imaging of the atomic-scale magnetic structure of Fe1+yTe”, Science 345, 653-656 (2014).
Dr Chi Ming Yim is a condensed matter physicist who specialises in the use of low temperature scanning tunnelling microscopy to study the electronic and magnetic properties of strongly correlated electron materials. Chi Ming obtained his PhD degree in Chemistry at University College London in 2011, under the supervision of Professor Geoff Thornton. Then, Chi Ming continued his research in Thornton group as a post-doc. where he studied the polaronic behaviour of excess electrons in TiO2 (110) single crystals and thin films, as well as the adsorption properties of model oxide-supported Pd nanoparticles.
Chi Ming then joined Peter Wahl group at University of St Andrews in 2016. In Wahl group, he employed low temperature scanning tunnelling microscopy to study the surface electronic and magnetic properties of different types of quantum materials, especially unconventional superconductors. Chi Ming also pioneers in studying material ground states under uniaxial strain in STM.
Chi Ming’s work has been published in internationally recognised journals including Nature Communications, Science Advances, PNAS, and Physical Review Letters.