Seminar By Yasuo Yoshida, University of Tokyo
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Title: Atomic-scale visualization of surface-assisted orbital order in the heavy fermion compound CeCoIn5
Abstract:The newfangled orbital-mediated quantum phenomena have proved over the past decade to be far-reaching and complex as exemplified in exotic orbital orders, nontrivial orbital-fluctuation-mediated superconductivity, orbital Kondo effect, and multipole-moment ordering. To understand electronic, spin, and orbital correlations in these phenomena, it is crucial to have a direct access to real-space orbital texture, but so far orbital-sensitive probes have shown rather limited functionality. Recent progress of a scanning tunneling microscope (STM) has enabled orbital-selective tunneling by fine-tuning the tip-sample distance (TSD). We exploit the orbital sensitivity of STM to unveil a surface-assisted cobalt d- orbital order in the heavy fermion compound CeCoIn5. We find that at a small TSD, cobalt atoms in STM topographies take on dumbbell shapes alternatingly aligned in the  and  directions on a cleaved (001) surface. A domain boundary of this ordered structure, which is localized within a terrace, denotes two-dimensionality of the ordered structure. First-principles calculations show that the structure is a consequence of a staggered dxz-dyz orbital order assisted by surface termination. This novel surface-assisted orbital ordering seems to be ubiquitous in transition metal oxides, heavy fermion superconductors and other materials, but has been overlooked until now.