各向异性自旋散射界面的Andreev反射谱

电流的自旋极化率是自旋电子学的核心参数，Giant Magnetoresistance (GMR)，Tunnel Magnetoresistance (TMR)和Spin-transfer Torque(STT)等自旋电子器件的性能主要与电流的极化率大小有关，而自旋极化率的符号直接导致这些自旋电子器件信号反向。自旋极化电流通常由磁性材料产生，近年来理论上提出很多量子材料包括拓扑材料都能提供自旋极化电流，但迄今缺乏实验证据。实验上测量自旋极化率并不容易，Andreev反射谱基于修改型BTK(modified Blonder-Tinkham-Klapwijk)模型，把一般非极化电流(P = 0,BTK模型)与半金属电流(P = 100%,Mazin模型)线性叠加，从而测量电流的极化率，但也不能解决自旋极化率的符号问题。本论文进一步改进修订的BTK模型，使得其可以包含具有自旋各向异性散射的界面。我们发现，Andreev反射的发生必须比较进入超导的透射率，而不仅仅是像改进型BTK模型那样比较自旋不同的入射电子个数。
本论文的重要结果包括：即使在一般金属上，自旋各向异性散射界面也可导致非常高的自旋极化电流，这可实现基于非磁性材料的抗干扰自旋电子器件(基于磁性材料的自旋电子器件易受外磁场干扰);在磁性材料上，自旋各向异性散射界面可增加或者减少电流的极化率，这可解释文献中某些材料的少数原子在界面上可增加TMR或者GMR;此外，我们提出一种可确定材料自旋极化率的符号的方法。
关键词：自旋电子学；超导；自旋极化率；Andreev反射；自旋散射
Abstract
Spin polarization is the core parameter of spintronics. The performance of spintronics devices such as Giant Magnetoresistance (GMR), Tunnel Magnetoresistance (TMR) and Spin-transfer Torque effects (STT) is mainly related to the polarization of the current. The sign of spin polarizability directly causes the signal of these spintronic devices to reverse. Spin-polarized currents are usually generated by magnetic materials. In recent years, many quantum materials including topological materials have been proposed to provide spin-polarized currents, but there is no experimental evidence so far. It is not easy to measure the spin polarizability experimentally. Andreev reflection spectrum is based on the Modified Blonder-Tinkham-Klapwijk (BTK) model, which combines the general non-polarized current (P = 0, BTK model) with the semi-metallic current (P = 100%, Mazin model) to measure the polarizability of the current. But it does not solve the sign problem of spin polarizability. In this paper, the BTK model is further modified to include interfaces with spin anisotropic scattering. We find that the Andreev reflection must occur by comparing the transmissibility into superconductivity, not just the number of electrons with different spins, as in the improved BTK model.
The important results of this thesis: even on normal metals, the spin anisotropic scattering interface can lead to a very high spin polarization current, which enables anti-jamming spintronic devices of non-magnetic materials (spintronic devices based on magnetic materials are susceptible to external magnetic field interference). Furthermore, in magnetic materials, the spin anisotropic scattering interface can increase or decrease the polarizability of the current, which may explain the increase of TMR or GMR at the interface of a few atoms in some materials reported in the literature. Finally, based on known spin anisotropic scattering materials, our results provide a new method to determine the sign of spin polarizability of materials.
Keywords: Spintronics;Superconductivity;Spin polarizability;Andreev reflection; Spin scattering

[1]BAIBICH, BROTO, FERT, et al. Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices [J]. Physical review letters, 1988, 61 21: 2472-5.
[2]MOODERA, KINDER, WONG, et al. Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions [J]. Physical review letters, 1995, 74 16: 3273-6.
[3]WANG L M, UMEMOTO K, WENTZCOVITCH R M, et al. Co1-xFexS2: a tunable source of highly spin-polarized electrons [J]. Physical review letters, 2005, 94 5: 056602.
[4]CAI H, KANG W, WANG Y, et al. High Performance MRAM with Spin-Transfer-Torque and Voltage-Controlled Magnetic Anisotropy Effects [J]. Applied Sciences, 2017, 7: 929.
[5]LI P, WU W, WEN Y, et al. Spin-momentum locking and spin-orbit torques in magnetic nano-heterojunctions composed of Weyl semimetal WTe2 [J]. Nature Communications, 2018, 9.
[6]ANDREEV A F. THERMAL CONDUCTIVITY OF THE INTERMEDIATE STATE OF SUPERCONDUCTORS. PART II, F, 1964 [C].
[7]CHEN T Y, TESANOVIC Z, CHIEN C L. Unified Formalism of Andreev Reflection at a Ferromagnet/Superconductor Interface [J]. Physical Review Letters, 2012, 109(14): 146602.
[8]BLONDER G E, TINKHAM M, KLAPWIJK T M. Transition from metallic to tunneling regimes in superconducting microconstrictions: Excess current, charge imbalance, and supercurrent conversion [J]. Physical Review B, 1982, 25(7): 4515-32.
[9]WOODS G T, SOULEN R J, MAZIN I I, et al. Analysis of point-contact Andreev reflection spectra in spin polarization measurements [J]. Physical Review B, 2004, 70: 054416.
[10]KASHIWAYA S, TANAKA Y, YOSHIDA N, et al. SPIN CURRENT IN FERROMAGNET-INSULATOR-SUPERCONDUCTOR JUNCTIONS [J]. Physical Review B, 1999, 60: 3572-80.
[11]WANG L, CHEN T, CHIEN C L, et al. Sulfur stoichiometry effects in highly spin polarized CoS2 single crystals [J]. Applied Physics Letters, 2006, 88: 232509.
[12]LU W-T, SUN Q. Electrical control of crossed Andreev reflection and spin-valley switch in antiferromagnet/superconductor junctions [J]. Physical Review B, 2021.
[13]KAREL J, BOUMA D S, MARTINEZ J M, et al. Enhanced spin polarization of amorphous FexSi1 x thin films revealed by Andreev reflection spectroscopy [J]. Physical Review Materials, 2018.
[14]ZHANG K, ZENG J, DONG X, et al. Spin dependence of crossed Andreev reflection and electron tunneling induced by Majorana fermions [J]. Journal of physics Condensed matter : an Institute of Physics journal, 2018, 30 50: 505302.
[15]SHIGA M, NISHIMURA N, INAGAKI Y, et al. Spin polarization measurements in ferromagnetic SrRuO3 using point-contact Andreev reflection technique, F, 2017 [C].
[16]VISANI C, SEFRIOUI Z, TORNOS J, et al. Equal-spin Andreev reflection and long-range coherent transport in high-temperature superconductor/half-metallic ferromagnet junctions [J]. Nature Physics, 2012, 8: 539-43.
[17]WILKEN F, BROUWER P W. Impurity-assisted Andreev reflection at a spin-active half metal-superconductor interface [J]. Physical Review B, 2012, 85.
[18]NAIDYUK Y G, KVITNITSKAYA O E, GAMAYUNOVA N V, et al. Superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy [J]. Physical Review B, 2017, 96: 094517.
[19]FANG J-X, DUAN W, LIU J, et al. Pairing-dependent superconductivity gap and nonholonomic Andreev reflection in Weyl semimetal/Weyl superconductor heterojunctions [J]. Physical Review B, 2018, 97.
[20]BASHLAKOV D L, GAMAYUNOVA N V, TYUTRINA L V, et al. Sub-kelvin Andreev reflection spectroscopy of superconducting gaps in FeSe [J]. Low Temperature Physics, 2019.
[21]KUZMICHEVA T E, KUZMICHEV S A, PERVAKOV K S, et al. Superconducting order parameters in overdoped BaFe1.86Ni0.14As2 revealed by multiple Andreev reflection spectroscopy of planar break junctions [J]. Physical Review B, 2021.
[22]NICHELE F, PORTOLéS E, FORNIERI A, et al. Relating Andreev Bound States and Supercurrents in Hybrid Josephson Junctions [J]. Physical review letters, 2020, 124 22: 226801.
[23]LAI Y-H, SAU J D, DAS SARMA S. Presence versus absence of end-to-end nonlocal conductance correlations in Majorana nanowires: Majorana bound states versus Andreev bound states [J]. Physical Review B, 2019.
[24]FREAMAT M V, NG K W. Andreev bound states in normal and ferromagnet/high-Tc superconducting tunnel junctions [J]. Physica C-superconductivity and Its Applications, 2003, 400: 1-6.
[25]RIGATO F, PIANO S, FOERSTER M, et al. Andreev reflection in ferrimagnetic CoFe2O4 spin filters [J]. Physical Review B, 2010, 81.
[26]DITTMANN N, SPLETTSTOESSER J, GIAZOTTO F. Clocked single-spin source based on a spin-split superconductor [J]. New Journal of Physics, 2016, 18: 083019.
[27]LUO K J, ZHOU T, CHEN W. Probing the valley filtering effect by Andreev reflection in a zigzag graphene nanoribbon with a ballistic point contact [J]. Physical Review B, 2017, 96.
[28]WANG J, LIU S. Crossed Andreev reflection in a zigzag graphene nanoribbon-superconductor junction [J]. Physical Review B, 2012, 85: 035402.
[29]BAI C, YANG Y, JIANG Y, et al. Andreev reflection in a patterned graphene nanoribbon superconducting heterojunction [J]. Physics Letters A, 2019.
[30]SEKERA T, BRUDER C, TIWARI R P. Spin transport in a graphene normal-superconductor junction in the quantum Hall regime [J]. Physical Review B, 2018.
[31]ABENDROTH J M, STEMER D M, BLOOM B P, et al. Spin Selectivity in Photoinduced Charge-Transfer Mediated by Chiral Molecules [J]. ACS nano, 2019, 13 5: 4928-46.
[32]MB, DOMBDSKI T. Polarization of the Majorana quasiparticles in the Rashba chain [J]. Scientific Reports, 2017, 7.
[33]BASS J M, PRATT W P. Version 7/21/01 current-perpendicular-to-plane (CPP) magnetoresistance [J]. Physica B-condensed Matter, 2002, 321: 1-8.
[34]WIGGER G A, BEELI C, FELDER E C, et al. Percolation and the colossal magnetoresistance of Eu-based hexaboride [J]. Physical review letters, 2004, 93 14: 147203.
[35]PRATT W P, LEE S-W, HOLODY P R, et al. Giant magnetoresistance with current perpendicular to the multilayer planes [J]. Journal of Magnetism and Magnetic Materials, 1993, 126: 406-9.
[36]VALET T, FERT A. Theory of the perpendicular magnetoresistance in magnetic multilayers [J]. Physical Review B, 1993, 48: 7099-113.
[37]COOPER L N. Bound Electron Pairs in a Degenerate Fermi Gas [J]. Physical Review, 1956, 104(4): 1189-90.
[38]曾谨言.量子力学Ⅱ[M].北京：科学出版社，2007：978-7-03-018139-8.
[39]CHARLES P POOLE JR,HORACIO A FARACH,RICHARD J CRESWICK.Superco-nductivity[M].London:Academic Press,2007:978-0-12-088761-3.
[40]MICHAEL THIKHAM.Introduction to superconductivity[M].New York:McGraw Hi-ll,Inc.,1996:0-07-064878-6.
[41]B ANDREI BERNEVIG.Topological Insulators and Topological Superconductors[M] Princeton:PRINCETON UNIVERSITY PRESS,2013:978-0-691-15175-5.
[42]光的偏振_百度百科 (baidu.com)