Cooperative Effect of Electron Spin Polarization in Chiral Molecules Studied with Non-Spin-Polarized Scanning Tunneling Microscopy

Abstract

Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al 2 O 3 / Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered selfassembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

Spin Polarization of Polyalanine Molecules in 2D and Dimer-Row Assemblies Adsorbed on Magnetic Substrates: The Role of Coupling, Chirality, and Coordination

“…Thereby, the small peak at around +1.4 V is a fingerprint for a Au–S bond (originating from the SH group in cysteine), and it is characteristic for chemisorbed systems. , The spin polarization measured along the LUMO state of the PA molecule, located at a + 1.8 V bias voltage, is equal to P = 68% for the 26-mer molecules. In our recently published study we have measured a spin polarization of P = 75% after adsorption of entantiopure 31-mer cyst-LPA on the Au surface (shown by blue line in Figure c) . Apparently, the longer molecular helix gives rise to higher spin accumulation at the interface, in agreement with previous reports. ,, This supports the general picture of the CISS effect, that the spin polarization measured in transport experiments depends on the propagation along the potential of the backbone structure, and it is of molecular origin.…”

“…(c) I – V curves of cyst-DL-PA on Au/Co/Au substrate with M⃗ Co = false↓ for the L-PA molecule in the hexagonal and heterodimer phase areas. For comparison, we have plotted also the I – V curve for 31-mer cyst- LPA molecules (blue curve) . The chirality of the molecules, L or D, is labeled by a red color in the legend.…”

“…The deposition rates and growth temperatures were optimized; thus, atomically flat Au(111) terraces of at least 20 nm in width were obtained suitable for STM measurements. Further details are mentioned in ref and references therein.…”

“…The spin polarization is given by P = I false↑ − I false↓ I false↑ + I false↓ , where the arrows indicate the direction of the Co-layer magnetization. Further details are outlined in ref .…”

“…The mutual interplay between structure and spin polarization can be studied using local probe techniques, e.g., atomic force microscopy (AFM) or scanning tunneling microscopy (STM). − Using the latter technique, we have recently demonstrated molecular resolution and measured the spin polarization in enantiomerically pure 2D and cluster α-PA phases . Thus, this approach appears to be capable of investigating various defects of the surface and within the molecular adsorbate, as well as the role of cooperative effects.…”