Electron transport along chiral polypeptides studied by scanning tunneling microscopy

Prof. Dr. Christoph Tegenkamp, TU Chemmitz

The transmission properties of electrons through chiral systems are currently attracting significant attention. In DNA and polypeptides, the so-called chiral-induced spin selectivity (CISS) effect describes the emergence of extraordinarily high spin polarizations during long-range electron transfer. Moreover, strong spin polarizations have also been reported in chiral carbon nanotubes. This is surprising, as heavy atoms are absent in these carbon-based systems. It has been shown that even in systems with weak spin–orbit coupling, spin transmission can be strongly amplified by molecular helicity.
We performed local transport experiments using scanning tunneling microscopy (STM) on lysine-doped and cysteine-terminated single helical polyalanine (PA) molecules adsorbed on magnetic Al₂O₃/Pt/Au/Co/Au nanostructures with perpendicular anisotropy. The highest CISS magnetoresistance values were observed for ordered self-assembled monolayers with well-defined chemical coupling between the molecules and the magnetic substrate.
In recent experiments, the same molecules were inverted, for example by adsorption onto the Au tip or by functionalization of the opposite terminus. Surprisingly, high electron transmission required opposite orientations of the Co layer magnetization. This result clearly demonstrates that the molecular orientation plays a decisive role. The approximately point-symmetric behavior of the I–V curves can be understood in terms of a “spinterface,” defined by the helicity and electric dipole orientation of the molecule at the interface. In this sense, the helical system does not act as a simple spin filter or polarizer.