Coupling between spin, lattice, and charge at the surface of complex transition metal compounds

Abstract

Understanding and controlling the complexity that develops in complex transition metal compounds such as high-Tc superconductivity, "colossal" magnetoresistance in manganites, and heavy-fermion compounds, is one of the grand challenges of the 21st century. The exotic properties displayed by these compounds are closely related to the coexistence of nearly degenerate states, coupling simultaneously several active degrees of freedom such as the charge, lattice, orbital, and spin. In this work, we have focused on two systems, one is the newly discovered Fe-based superconducting compounds ((Ba, Ca)(Fe1-xCox)2As2, FeTe1-xSex) and the other one is the doped Ruddleden-Popper (RP) ruthenates (Sr3(Ru1-xMnx)2O7). The materials community was astonished by the discovery of superconductivity with a critical temperature exceeding 55 K in the iron-based superconductors in 2008. This new family of high Tc superconductors with layered structure without Cu has opened up a completely new venue for understanding not only high Tc superconductors but in general the coupling between lattice, charge, orbital and spin. While ruthenates is a prototype of strong correlated electron materials (CEMs) and Mn-doping in Sr3(Ru1-xMnx)2O7 have induced a rich coupled phase diagrams. We approach from the surface to study their geometric and electronic structure because the symmetry breaking offers great opportunities to tune the balance of the coupling. We applied Low energy electron diffraction (LEED) and its Intensity-voltage (I-V) analysis to quantitatively characterize the detail surface structure from momentum space. Then we used low and variable temperature scanning tunneling microscopy/spectroscopy (STM/S) to study surface electronic structure from real space. At last, spin-polarized density functional theory (DFT) calculations were utilized to enhance our understanding of the experimental data, thus providing a new prospective of our discovery. Our results on the domain surface of BaFe2As2 show that the strong spin-lattice coupling at the surface results in the coexistence of structure and spin antiphase domain boundaries with C2 symmetry. For the stripe surface of (Ba, Ca)(Fe1-xCox)2As2, we determined the surface structure which is proved to be stabilized by bulk spin ordering through spin-lattice-charge coupling. Superconductivity has also been observed on stripe surface indicating a spatial-resolved coexisting of anti-ferromagnetic and superconducting order. On FeTe1-xSex system, we observed a nano-scale chemical phase separation of Te and Se atoms thus the optimally doped superconductor is chemically inhomogeneous but electronically homogeneous, in contrast to many CEMs. However, using STM on a different system, Mn-dopants in Sr3(Ru1-xMnx)2O7 were shown to homogeneously (random) distribute on the surface in micro-scale but maybe phase separated