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用g(G)和δ(G)分别表示一个图G的围长和顶点最小度. ζ(G)为图G的Betii亏数,主要证明了以下2个结果1)设G为k-边连通简单图,若对G中任意圈C,存在点x∈C满足dG(x)>|V(G)|/(k-1)2+2)+k-g(G)+2,k=1,2,3,则G是上可嵌入的.且不等式的下界是最好的;2)设G为k-边连通简单图,则ζ(G)≤{max{1,m},k=1,max{1,1/(k-1)m -1}K=2,3 其中m= |V(G)|g(G)-6/g(G)2+(δ(G)-2)g(G)-4'且不等式的上界是可达的.进而得到了最大亏格一个比较好的下界. 相似文献
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Ling-Mei Zhang 《中国物理 B》2022,31(5):57303-057303
Based on first-principles calculations, the bias-induced evolutions of hybrid interface states in π-conjugated tricene and in insulating octane magnetic molecular junctions are investigated. Obvious bias-induced splitting and energy shift of the spin-resolved hybrid interface states are observed in the two junctions. The recombination of the shifted hybrid interface states from different interfaces makes the spin polarization around the Fermi energy strongly bias-dependent. The transport calculations demonstrate that in the π -conjugated tricene junction, the bias-dependent hybrid interface states work efficiently for large current, current spin polarization, and distinct tunneling magnetoresistance. But in the insulating octane junction, the spin-dependent transport via the hybrid interface states is inhibited, which is only slightly disturbed by the bias. This work reveals the phenomenon of bias-induced reconstruction of hybrid interface states in molecular spinterface devices, and the underlying role of conjugated molecular orbitals in the transport ability of hybrid interface states. 相似文献
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The effect of lattice dimerization on the magnetoresistance (MR) in organic spin valves is investigated based on the Su-Schrieffer-Heeger (SSH) model and the Green's function method. By comparing with the results for a uniform chain, we find that the dimerization of the molecular chain modifies the monotonic dependence of the MR on the bias to an oscillatory one. A sign inversion of the MR is observed when the amplitude of the dimerization is adjusted. The results also show that at a low bias, the MR through a dimerized chain decreases with the increasing bias as well as the increasing chain length, which is consistent with the experimental reports. A further understanding can be achieved by analyzing the electronic states and the spin-dependent transmission spectrum with the parallel and antiparallel magnetization orientations of the two ferromagnetic electrodes. 相似文献
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理论研究了铁磁/有机半导体肖特基接触时的电流自旋极化注入,并讨论了电流自旋极化率随界面处肖特基势垒高度、有机半导体层中特殊载流子及其迁移率、界面附近掺杂浓度的变化关系.通过计算发现,寻找在势垒区中载流子迁移率比较大的有机半导体材料对实现有效的自旋注入是必要的;同时还发现,由于铁磁/有机半导体接触而形成的肖特基势垒不利于自旋注入.因此要想实现有效的自旋注入,界面附近必须采用重掺杂来有效减少势垒区的宽度,且势垒的高度要限制在一定的范围内. 相似文献
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The rectification behaviours in organic magnetic/nonmagnetic co-oligomer spin rectifiers are investigated theoretically.It is found that both the charge current and the spin current through the device are rectified at the same time.By adjusting the proportion between the magnetic and nonmagnetic components,the threshold voltage and the rectification ratio of the rectifier are modulated.A large rectification ratio is obtained when the two components are equal in length.The intrinsic mechanism is analysed in terms of the asymmetric localization of molecular orbitals under biases.The effect of molecular length on the rectification is also discussed.These results will be helpful in the future design of organic spin diodes. 相似文献
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The effects of electric and magnetic fields on the current spin polarization and magnetoresistance in a ferromagnetic/organic semiconductor/ferromagnetic(FM/OSC/FM) system 下载免费PDF全文
From experimental results of spin polarized injection and transport in organic semiconductors(OSCs),we theoretically study the current spin polarization and magnetoresistance under an electric and a magnetic field in a ferromagnetic/organic semiconductor/ferromagnetic(FM/OSC/FM) sandwich structure according to the spin drift-diffusion theory and Ohm’s law.From the calculations,it is found that the interfacial current spin polarization is enhanced by several orders of magnitude through tuning the magnetic and electric fields by taking into account the specific characteristics of OSC.Furthermore,the effects of the electric and magnetic fields on the magnetoresistance are also discussed in the sandwich structure. 相似文献
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Spin-excited states in an asymmetric magnetic organic co-oligomer diode are investigated theoretically. The results demonstrate that the structural asymmetry of the co-oligomer is modulated by the spin-excited states, which is embodied in the wave functions of the eigenstates as well as the spin density wave. By calculating the transport property, a robust spin-current rectification concomitant with a charge-current rectification is observed in all spin-excited states. However, the current through the diode is suppressed distinctly by the spin-excited states, while the rectification ratios may be reduced or enhanced depending on the bias and the excited spins. The intrinsic mechanism is analyzed from the spin-dependent trans- mission combined with the change of molecular eigenstates under bias. Finally, the temperature-induced spin excitation is simulated. Significant rectification behavior is obtained even at room temperature. 相似文献
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