USTC obtains giant Rashba–Dresselhaus spin splitting in 2D chiral metal-organic frameworks

A research team led by Prof. LI Xingxing and academician YANG Jinlong from University of Science and Technology of China (USTC), has developed two-dimensional (2D) chiral metal-organic frameworks as Rashba-Dresselhaus(R-D) semiconductors with large spin splitting. The study was published in Chemical Science.

The R-D effect is a spontaneous spin splitting phenomenon caused by spin-orbit coupling in a space-inversion symmetry-breaking environment. The effect does not require the material to be inherently magnetic, and thus avoids the problem that the Curie temperature of low-dimensional magnetic materials is usually well below room temperature. Semiconductors with large R-D spin splitting are promising for the fabrication of electric field-controlled spintronic devices. However, the currently reported 2D R-D semiconductors are mainly inorganic materials and limited in quantity. In addition, the potential factors affecting spin splitting and general methods to realize large spin splitting have yet to be explored.

In recent years the academic community has begun to focus on 2D chiral metal–organic frameworks (CMOFs). Two-dimensional CMOFs are an important subclass of the MOF family and have received extensive attention in asymmetric catalysis and enantioselective applications. Since one of the basic conditions for the emergence of R-D spin splitting is the breaking of spatial inversion symmetry, CMOFs lacking inversion and mirror symmetry happen to be a natural design platform. The questions that urgently need to be addressed by the researchers in this study are whether significant R-D spin splitting can be achieved in 2D CMOFs, how it can be achieved, and the correlation between chirality and R-D effects.

Researchers constructed a series of CMOFs materials by utilizing inorganic ligands (-I, -Br, -Cl, -F, -CN, -H) coordinated heavy metal atoms (Sr–Sn, Ba–Pb) as nodes, and an axially chiral ligand, a 4,4'-bipyridine derivative, as a linker. Based on first-principles calculations, a series of two-dimensional R-D semiconductors with large spin splitting and large R-D coupling constants in the valence band were theoretically obtained by a three-step screening strategy.

Interestingly, the spin texture in the valence band is tunable by changing the chirality of the metal-organic backbone. Finally, the researchers identified five key elements for obtaining large R-D spin splitting in 2D COMFs: (i) chirality, (ii) large spin–orbit coupling, (iii) narrow band gap, (iv) valence and conduction bands having the same symmetry at the Г point, and (v) strong ligand field.

The study reveals the underlying factors that control the R–D spin splitting, which can benefit the future development of 2D R–D semiconductors with giant spin splitting.