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【TDLI】TDLI Made Important Progress in the Study of Strongly Correlated Two-Dimensional Electron Liquid

April 21, 2021      Author:

Tsung-Dao Lee Institute Fellow Chi Ming Yim et al. have made important progress in the study of strongly correlated 2D electron liquid, with the related research results published as a research article entitled "Quasi-particle interference and quantum confinement in a correlated Rashba spin-split 2D electron liquid" in Science Advances.

Exploiting inversion symmetry breaking (ISB) in systems with strong spin-orbit coupling promises control of spin through electric fields-crucial to achieve miniaturization in spintronic devices. Delivering on this promise requires a two-dimensional electron gas with a spin precession length shorter than the spin coherence length and a large spin splitting so that spin manipulation can be achieved over length scales of nanometers.

Layered structured correlated oxide metal delafossite PdCoO2 is one of the material candidates that can realize the above conditions.  PdCoO2 is composed of highly conductive layers of Pd ions sandwiched between layers of CoO2 octahedra, leading to considerably large transport anisotropy in this material.  Meanwhile, its layered triangular lattice also leads to a nearly perfect hexagonal Fermi surface with almost no warping, in turn resulting in highly-directional ballistic transport properties within the a-b plane in this material.  Regarding the electron energy structure, the CoO2 layers within the bulk are Mott-insulating.  However, exposing the CoO2 terminated surface to vacuum would lead to a giant spin-split Rashba-like surface states with whose formation dominated by inversion symmetry breaking.  Compared to the conventional Rashba systems where the spin-split surface states are dominated by spin-orbit coupling, the Fermi surface of the system here exhibits identical spin-texture, but totally different orbit texture (Figure 1).  This raises questions about quasiparticle interference (QPI) in this system.

Figure 1.  (A) Schematic Fermi surface of a Rashba system where SOC dominates over the ISB. The spin and orbital textures are directly coupled and have the same chirality on the two subbands. The selection rules for QPI are the same for the spin and orbital component-both result in the same dominant scattering wave vectors. Black arrows show the orbital angular momentum (OAM), color encodes the x-projection of the spin angular momentum (SAM). (B) Schematic Fermi surface of the CoO2-derived surface state where ISB dominates. When ISB dominates over SOC, the OAM exhibits the same chirality on both bands, whereas the spin texture shows opposite chirality.

To answer these questions, this work employed spectroscopic imaging technique in low temperature scanning tunneling microscopy to study QPI of the giant spin-split 2D electronic states of the CoO2 terminated surface.  Experimental results show that under this special condition, QPI obeys pure spin selection results instead of the spin-orbit selection rules obeyed by conventional Rashba systems.  The results also show that the spin coherence length of the quasiparticles is much longer than their spin precession length, suggesting that this system can be of potential use for spintronics.

Figure 2.  QPI of the CoO2 surface termination.  (A) Real-space map of the normalized conductance measured from the CoO2-terminated surface at a bias voltage of 1 mV (scale bar, 10 nm).  (B) Corresponding momentum-space map. Hexagons mark the Brillouin zone (BZ) in the kz=0Å-1 plane. A red arrow indicates the dominant scattering vector along the Γ → M direction. Pink arrows are the hypothesized scattering vectors assuming that intraband scattering also takes place. (C) Fermi surface of the CoO2 termination. Pockets of different colors have opposite spin texture in all directions, as a result of Rashba spin splitting. A red arrow marks the interband scattering vector as observed in the experiment; pink arrows mark the intraband scattering vectors not observed in the experiment. (D) Simulated QPI pattern at zero energy from the tight binding calculations.

Research collaborators of this work include Prof. Peter Wahl group in the University of St. Andrews (UK) and Prof. Andy Mackenzie group in the Max Planck Institute for Chemical Physics of Solids, Dresden (Germany).   T. D. Lee Fellow Prof. Chi Ming Yim is the first author and the co-corresponding author with Prof. Peter Wahl.

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Source: Tsung-Dao Lee Institute