Organic spintronics is a multidisciplinary field which controls the electron spin transport in organic molecular systems and is now attracting extensive interest. It was reported in 2011 [Science 331, 894 (2011)] that the electrons transmitted through the DNA molecule are highly spin polarized, i.e., the double-stranded DNA (dsDNA) can discern the spin-up electron and the spin-down one. But no spin polarization could be observed for single-stranded DNA (ssDNA). Then, Prof. Qing-Feng Sun of International Center of Quantum Materials of Peking University and Dr. Ai-Min Guo of Institute of Physics, Chinese Academy of Sciences proposed a model Hamiltonian to investigate the quantum spin transport through the DNA molecule and interpreted the above experiment. They found that the dsDNA can be a very efficient spin filter and the spin polarization increases with the length of the dsDNA, whereas the ssDNA cannot act as a spin filter. The work of Prof. Sun and Dr. Guo provided a clear physical mechanism for the large spin polarization observed in the DNA molecule and was published in Physical Review Letters 108, 218102 (2012). Besides, they studied the spin transport properties of the dsDNA molecule by considering the gate voltage, the DNA sequence, the mutation, and the molecule-electrode contact [see Physical Review B 89, 205434 (2014); 86, 115441 (2012); 86, 035424 (2012)].

Very recently, an important progress is made in molecular spintronics by Prof. Sun and Dr. Guo. They propose a model Hamiltonian, including the long-range hopping, to explore the spin transport through single-helical molecules connected by two nonmagnetic electrodes, and provide an unambiguous physical mechanism for large spin polarization observed in the protein and for the contrary experimental results between the protein and the ssDNA. Their results reveal that the alpha-helical protein is an efficient spin filter, whereas the ssDNA exhibits extremely small spin filtration efficiency with the order of magnitude being 10^{-5}, although both molecules possess single-helical structure. When the long-range hopping, such as the second nearest-neighbor (NN) hopping and the third one, is comparable to the NN hopping, the single-helical molecule may behave as an efficient spin filter, like the alpha-helical protein. When the long-range hopping is much weaker than the NN one, the single-helical molecule may not polarize the electron spin. These results are in excellent agreement with recent experiments [PNAS 110, 14872 (2013); Science 331, 894 (2011)] and may facilitate engineering of chiral-based spintronic devices.

This work was supported by National Basic Research Program of China, National Natural Science Foundation of China, and Postdoctoral Science Foundation of China. It has been published online in PNAS [PNAS 111, 11658(2014), doi: 10.1073/pnas.1407716111].