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تسجيل مستخدم جديدSymmetry aspects of spin-filtering in molecular junctions: hybridization and quantum interference effects
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Control and manipulation of electric current and, especially, its degree of spin polarization (spin filtering) across single molecules are currently of great interest in the field of molecular spintronics. We explore one possible strategy based on the modification of nanojunction symmetry which can be realized, for example, by a mechanical strain. Such modification can activate new molecular orbitals which were inactive before due to their orbital mismatch with the electrodes conduction states. This can result in several important consequences such as (i) quantum interference effects appearing as Fano-like features in electron transmission and (ii) the change in molecular level hybridization with the electrodes states. We argue that the symmetry change can affect very differently two majority- and minority-spin conductances and thus alter significantly the resulting spin-filtering ratio as the junction symmetry is modified. We illustrate the idea for two basic molecular junctions: Ni/benzene/Ni (perpendicular vs tilted orientations) and Ni/Si chain/Ni (zigzag vs linear chains). In both cases, one highest occupied molecular orbital (HOMO) and one lowest unoccupied molecular orbital (LUMO) (out of HOMO and LUMO doublets) are important. In particular, their destructive interference with other orbitals leads to dramatic suppression of majority-spin conductance in low-symmetry configurations. For a minority-spin channel, on the contrary, the conductance is strongly enhanced when the symmetry is lowered due to an increase in hybridization strength. We believe that our results may offer a potential route for creating molecular devices with a large on-off ratio of spin polarization via quantum interference effects.
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One of the important issues of molecular spintronics is the control and manipulation of charge transport and, in particular, its spin polarization through single-molecule junctions. Using $ab$ $initio$ calculations, we explore spin-polarized electron
transport across single benzene derivatives attached with six different anchoring groups (S, CH$_3$S, COOH, CNH$_2$NH, NC and NO$_2$) to Ni(111) electrodes. We find that molecule-electrode coupling, conductance and spin polarization (SP) of electric current can be modified significantly by anchoring groups. In particular, a high spin polarization (SP $>$ 80%) and a giant magnetoresistance (MR $>$ 140%) can be achieved for NO$_2$ terminations and, more interestingly, SP can be further enhanced (up to 90%) by a small voltage. The S and CH$_3$S systems, on the contrary, exhibit rather low SP while intermediate values are found for COOH and CNH$_2$NH groups. The results are analyzed in detail and explained by orbital symmetry arguments, hybridization and spatial localization of frontier molecular orbitals. We hope that our comparative and systematic studies will provide valuable quantitative information for future experimental measurements on that kind of systems and will be useful for designing high-performance spintronics devices.
We investigate within a coarse-grained model the conditions leading to the appearance of Fano resonances or anti-resonances in the conductance spectrum of a generic molecular junction with a side group (T-junction). By introducing a simple graphical
representation (parabolic diagram), we can easily visualize the relation between the different electronic parameters determining the regimes where Fano resonances or anti-resonances in the low-energy conductance spectrum can be expected. The results obtained within the coarse-grained model are validated using density-functional based quantum transport calculations in realistic T-shaped molecular junctions.
Introduction (2) Experimental background: Test beds (8) Theoretical approaches: A microscopic model(10) The electron-phonon coupling(14)Time and energy scales(15) Theoretical methods(19)Numerical calculations(28) Incoherent vs. coherent transpo
rt (28) Inelastic tunneling spectroscopy: Experimental background(31) Theoretical considerations:the weak coupling limit(36) Theoretical considerations: moderately strong coupling(41)Comparison of approximation schemes(48)Asymmetry in IETS(51)The origin of dips in IETS signals(53)Computational approaches (56) Effects of electron-electron(e-e)interactions (63) Noise (66) Non-linear conductance phenomena (73) Heating and heat conduction: General considerations(77) Heat generation(81) Heat conduction(85) Junction temperature(88) Current induced reactions (91) Summary and outlook (91)
We investigate the influence of gauge fields induced by strain on the supercurrent passing through the graphene-based Josephson junctions. We show in the presence of a constant pseudomagnetic field ${bf B}_S$ originated from an arc-shape elastic defo
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