半导体/磁性d波超导隧道结中的散粒噪声

考虑到磁性d波超导体中的能隙与磁交换能的依赖关系,通过求解Bogoliubov-de Gennes(BdG)方程,利用Blonder-Tinkham-Klapwijk理论方法研究半导体/磁性d波超导隧道结中的散粒噪声.计算结果表明:磁性d波超导结中的磁交换能h0可导致散粒噪声在零偏压处的双峰和能隙处的峰出现劈裂,劈裂的宽度为2h0;磁交换能h0同时对散粒噪声及噪声功率与平均电流的比值有抑制作用.     

磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森效应

通过求解磁性d波超导中的能隙和磁交换能的自洽方程, 研究磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森电流. 计算结果表明: 1)临界电流随中间的铁磁层厚度呈现出两种不同周期的振荡混合, 通过增强铁磁层中的磁交换能q₀和铁磁/磁性d波超导界面处的势垒强度z₀, 短周期分量可从长周期中分离出来, 反之, 通过降低q₀和z₀, 长周期分量可从短周期中分离出来; 2)在两边磁性d波超导的磁化方向取平行时, 在取一些特定的铁磁层厚度下, 磁性d波超导中的磁交换能可增强系统的临界电流. 关键词： 磁性d波超导体 铁磁体 约瑟夫森电流     

铁磁半导体/d波超导结的自旋极化输运

考虑到铁磁半导体和d波超导体中空穴的有效质量和费米速度错配,运用推广了的B londer-Tinkham-K lapw ijk(BTK)理论模型,研究了铁磁半导体/d波超导隧道结的电导谱。研究表明:(1)铁磁半导体和d波超导体中空穴的有效质量和费米速度错配对系统的微分电导影响显著;(2)铁磁半导体的磁交换能对Andreev反射有抑制作用。     

含有磁性杂质双AB环的自旋极化输运

我们研究了由电子通过含有磁性杂质双AB环结构的透射系数来表示的AB干涉振荡。在这里磁性杂质提供了自旋翻转机制。磁性杂质可以导致AB干涉的部分退相干。而且我们发现在磁通反向的情况下自旋向上和向下的透射系数关于磁通是不对称的，但反射系数和总的透射系数关于磁通却是对称的     

影响磁性pn结自旋极化输运特性的因素

利用漂移扩散理论研究了磁性pn结中自旋的输运特性.探讨了外加电压、平衡自旋极化率、外加自旋注入和自旋寿命对磁性pn结电流密度和电阻的影响,讨论了磁性pn结自旋伏特效应与pn结宽度的关系.发现平衡自旋极化率使得不同自旋方向电子具有不同的势垒高度从而能有效调制电流;而外加自旋注入则为磁性pn结提供了非平衡自旋极化电子从而达到对电流的调制作用,同时发现自旋伏特电流随准中性p区宽度减小而增大. 关键词： 磁性pn结 自旋极化率 自旋寿命 自旋伏特效应     

铁磁-p波超导结中的自旋极化隧道谱与散粒噪声

考虑到界面的势垒散射和铁磁层中的磁交换作用，依照Sr2RuO4超导体具有自旋三重配对态的p波对称结构，研究铁磁-p波超导结中的自旋极化隧道谱与散粒噪声，研究表明：1.对于幺正配对态，随着铁磁层中磁交换劈裂增强，隧道谱与散粒噪声都减少；2.对于非幺正配对态，隧道谱与散粒噪声都依赖于铁磁层的磁化轴方向，当磁化轴平行于非幺正配对态的自旋轴时，在低偏压下磁交换作用能使隧谱与散粒噪声增强，而当磁化轴反平行于自旋转轴时，其结果相反。     

钙钛矿氧化物异质结自旋极化输运机制研究

文章介绍了一个基于弱Hund耦合规则以及载流子漂移扩散机制所提出的关于钙钛矿氧化物p-n异质结构的自旋极化输运机制的物理模型．该理论不仅可以很好地解释由具有负磁阻效应的La0.9Sr0．1MnO3（LSMO）与非磁性的SrNb0.01Ti0.99O3（SNTO）所组成的异质结中所存在的正磁电阻效应，同时揭示了该体系中LSMO在界面区域的载流子与远离界面区域的载流子具有不同的自旋极化方向．这一结果将为理解钙钛矿氧化物异质结及多层膜的自旋极化输运机制开辟了一条新的途径．     

钙钛矿氧化物异质结自旋极化输运机制研究

文章介绍了一个基于弱Hund耦合规则以及载流子漂移扩散机制所提出的关于钙钛矿氧化物p-n异质结构的自旋极化输运机制的物理模型.该理论不仅可以很好地解释由具有负磁阻效应的La0.9Sr0.1MnO3（LSMO）与非磁性的SrNb0.01Ti0.99O3（SNTO）所组成的异质结中所存在的正磁电阻效应,同时揭示了该体系中LSMO在界面区域的载流子与远离界面区域的载流子具有不同的自旋极化方向.这一结果将为理解钙钛矿氧化物异质结及多层膜的自旋极化输运机制开辟了一条新的途径.     

有机自旋器件磁性渗透层中双极化子对自旋极化输运的影响

根据实验发现的有机器件如Co/有机半导体/La_0.7Sr_0.3MnO₃中磁性原子渗透现象, 利用自旋漂移-扩散方程, 理论研究了磁性渗透层中极化子-双极化子的转化对自旋极化输运的影响. 研究发现: 磁性渗透层具有不同于纯净有机层的迁移率和自旋反转时间, 都将影响极化子-双极化子的转化, 进而影响自旋极化的输运; 在磁性渗透层中极化子自旋反转时间的劈裂是引起自旋弛豫的主要因素, 而极化子和双极化子之间的转化是重要因素.     

光场辐照下稀磁半导体/半导体超晶格中自旋电子输运特性研究

基于单电子有效质量近似理论和传递矩阵方法,理论研究了稀磁半导体/半导体超晶格结构中电子的自旋极化输运特性.主要讨论了光场和磁场联合调制对自旋极化输运的影响,以及不同自旋电子在该超晶格结构中的隧穿时间.理论和数值计算结果表明,由于导带电子与掺杂Mn离子之间的sp-d电子相互作用引起巨塞曼劈裂,因此在磁场调制下,不同自旋电子在该结构中感受到的势函数不同而呈现出自旋过滤效应,不同自旋电子的共振透射能带的位置和宽度可以通过磁场进行调制.同时在该结构中考虑光场时,自旋依赖的透射谱会因为吸收和发射光子而呈现出对光场的强度和频率响应;最后,通过不同自旋电子的高斯波包在该结构中随时间的演化给出了不同自旋电子的隧穿时间.本文研究结果对研究和设计基于稀磁半导体/半导体超晶格结构的高速量子器件具有一定的指导意义.     

Zeeman effects on d-wave superconductor and tunneling spectrum in normal-metal/d-wave superconductor tunnel junction

We study the Zeeman effect on the d-wave superconductor and tunneling spectrum in normal-metal(N)/d-wave superconductor(S) junction by applying a Zeeman magnetic field to the S. It is shown that: (1) the Zeeman magnetic field can lead to the S gap decreasing, and with the increase in Zeeman energy, the superconducting state is changed to the normal state, exhibiting a first-order phase transition; (2) the Zeeman energy difference between the two splitting peaks in the conductance spectrum is equal to2h0 (h0 is the Zeeman energy); (3) both the barrier strength of interface scattering and the temperature can lower the magnitudes of splitting peaks, of which the barrier strength can lead to the splitting peaks becoming sharp and the temperature can smear out the peaks,however, neither of them can influence the Zeeman effect.     

Zeeman effects on Josephson current in d-wave superconductor/d-wave superconductor junctions

This paper solves a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, and studies the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor(S)/insulating layer(I)/d-wave S junctions are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction to a great extent depend on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field under certain conditions can enhance the Josephson critical current in a d-wave S/I/d-wave S junction.     

Differential conductance in normal-metal/insulator/metal/d-wave superconductor junction carrying a supercurrent

This paper applies the Bogoliubov--de Gennes equation and the Blonder--Tinkham--Klapwijk approach to study the oscillatory behaviour of differential conductance in a normal metal/insulator/metal/d-wave superconductor junction carrying a supercurrent I_s. We find that (i) a three-humped structure appears at a nearly critical supercurrent I_s and z ≈ 0.5 for the normal metal/insulator/metal/d_x² + y²-wave superconductor junction; (ii) the zero-bias conductance peak splits into two peaks with sufficiently large applied current for the normal metal/insulator/metal/d_xy-wave superconductor junction; (iii) the conductance spectrum exhibits oscillating behaviour with the bias voltage and the peaks of the resonances are suppressed by increasing supercurrent I_s.     

Spin-polarized quasiparticle transport in ferromagnet/insulator/p-wave superconductor junction

We have studied the tunneling conductance in ferromagnet/insulator/p-wave superconductor junctions, taking into account the rough interface scattering effect. We find that there exist zero-bias conductance peaks and single-minimum structure in tunneling spectroscopy. As the exchange energy increases, the Andreev reflection is always suppressed and the differential conductance decreases. The differential conductance depends on the barrier strength and the roughness at the interface. Supported by the Natural Science Foundation of Jiangsu Higher Education Institutions, China (Grant No. 06KJB140009)     

Spin dependent Andreev reflection in ferromagnetic semiconductor/d-wave superconductor ballistic junction

The Andreev reflection (AR) probability and transmission of quasiparticles in ferromagnetic semiconductor/d-wave superconductor (FS/DS) ballistic junctions are studied based on an extended Blonder–Tinkham–Klapwijk (BTK) theory. It is shown that the dependence of AR probability and pair potential on the spin orientation of incident quasiparticles for the heavy holes is much different from that for light holes due to the different mismatches in the effective mass and Fermi velocity between FS and DS. The junction conductance is dominated by the quasiparticles which undergo AR processes with the largest probability, and this provides a method for measuring the spin polarization in FS.     

Spin-polarized quasiparticle transport in ferromagnet/d-wave superconductor junctions

Within a scattering framework, a theoretical study is presented for the spin-polarized quasiparticle transport in ferromagnet/d-wave superconductor junctions. We find that the subgap conductance behavior is qualitatively different from a nonmagnetic junction, and can also be significantly different from those of a ferromagnet/s-wave junction. For a ballistic ferromagnet/d-wave superconductor junction, under appropriate conditions, a zero-bias conductance minimum could be achieved. In addition, a conductance maximum at finite bias could be evolved by interfacial scattering. For a normal-metal/ferromagnet/d-wave superconductor junction, conductance resonances are predicted.     

Spin-polarized transport in carbon nanotubes

We present transport measurements of ferromagnetically contacted carbon nanotubes. In both single- and multi-walled nanotube devices, a spin valve effect is observed due to spin-polarized transport. In one single-walled nanotube device, the spin-valve effect is suppressed as the influence of Coulomb charging is observed at around 10 K. To help understand the interplay between the Coulomb charging and the spin-polarized transport we investigated the temperature dependence of the carbon nanotube magnetoresistance.