Electrical spin polarization through spin-momentum locking in topological-insulator nanostructures

doi:10.1088/1674-1056/27/9/097307

Abstract

Recently, spin-momentum-locked topological surface states (SSs) have attracted significant attention in spintronics. Owing to spin-momentum locking, the direction of the spin is locked at right angles with respect to the carrier momentum. In this paper, we briefly review the exotic transport properties induced by topological SSs in topological-insulator (TI) nanostructures, which have larger surface-to-volume ratios than those of bulk TI materials. We discuss the electrical spin generation in TIs and its effect on the transport properties. A current flow can generate a pure in-plane spin polarization on the surface, leading to a current-direction-dependent magnetoresistance in spin valve devices based on TI nanostructures. A relative momentum shift of two coupled topological SSs also generates net spin polarization and induces an in-plane anisotropic negative magnetoresistance. Therefore, the spin-momentum locking can enable the broad tuning of the spin transport properties of topological devices for spintronic applications.

Keywords: spin-momentum locking electrical spin generation topological insulators topological devices spintronics

Received: 29 April 2018
Revised: 18 May 2018
Accepted manuscript online:

PACS:	73.50.Jt	(Galvanomagnetic and other magnetotransport effects)
	05.30.Rt	(Quantum phase transitions)

Fund:

Project supported by the National Key Basic Research Program of China (Grant Nos. 2014CB921103 and 2017YFA0206304), the National Natural Science Foundation of China (Grant Nos. 61822403, 11874203, U1732159, and U1732273), Fundamental Research Funds for the Central Universities, China (Grant No. 021014380080), and Collaborative Innovation Center of Solid-State Lighting and Energy-Saving Electronics, China.

Corresponding Authors: Xuefeng Wang, Rong Zhang
E-mail: xfwang@nju.edu.cn;rzhang@nju.edu.cn

Cite this article:

Minhao Zhang(张敏昊), Xuefeng Wang(王学锋), Fengqi Song(宋凤麒), Rong Zhang(张荣) Electrical spin polarization through spin-momentum locking in topological-insulator nanostructures 2018 Chin. Phys. B 27 097307

[1]	Fu L, Kane C L and Mele E J 2007 Phys. Rev. Lett. 98 106803
[2]	Moore J E and Balents L 2007 Phys. Rev. B 75 121306
[3]	Hasan M Z and Kane C L 2010 Rev. Mod. Phys. 82 3045
[4]	Qi X L and Zhang S C 2011 Rev. Mod. Phys. 83 1057
[5]	Burkov A A and Hawthorn D G 2010 Phys. Rev. Lett. 105 066802
[6]	Culcer D, Hwang E H, Stanescu T D and Sarma S D 2010 Phys. Rev. B 82 155457
[7]	Yazyev O, Moore J and Louie S 2010 Phys. Rev. Lett. 105 266806
[8]	Li X and Yang J 2016 Natl. Sci. Rev. 3 365
[9]	Hsieh D, Qian D, Wray L, Xia Y, Hor Y S, Cava R J and Hasan M Z 2008 Nature 452 970
[10]	Zhang H, Liu C X, Qi X L, Dai X, Fang Z and Zhang S C 2009 Nat. Phys. 5 438
[11]	Xia Y, Qian D, Hsieh D, Wray L, Pal A, Lin H, Bansil A, Grauer D, Hor Y S, Cava R J and Hasan M Z 2009 Nat. Phys. 5 398
[12]	Hsieh D, Xia Y, Qian D, Wray L, Dil J H, Meier F, Osterwalder J, Patthey L, Checkelsky J G, Ong N P, Fedorov A V, Lin H, Bansil A, Grauer D, Hor Y S, Cava R J and Hasan M Z 2009 Nature 460 1101
[13]	Chen Y L, Analytis J G, Chu J H, Liu Z K, Mo S K, Qi X L, Zhang H J, Lu D H, Dai X, Fang Z, Zhang S C, Fisher I R, Hussain Z and Shen Z X 2009 Science 325 178
[14]	Ren Z, Taskin A A, Sasaki S, Segawa K and Ando Y 2010 Phys. Rev. B 82 241306
[15]	Xiong J, Petersen A C, Qu D X, Hor Y S, Cava R J and Ong N P 2012 Physica E 44 917
[16]	Arakane T, Sato T, Souma S, Kosaka K, Nakayama K, Komatsu M, Takahashi T, Zhi R, Segawa K and Ando Y 2012 Nat. Commun. 3 636
[17]	Xia B, Ren P, Sulaev A, Liu P, Shen S Q and Wang L 2013 Phys. Rev. B 87 085442
[18]	Xu Y, Miotkowski I, Liu C, Tian J, Nam H, Alidoust N, Hu J, Shih C K, Hasan M Z and Chen Y P 2014 Nat. Phys. 10 956
[19]	Zhang S, Pi L, Wang R, Yu G, Pan X C, Wei Z, Zhang J, Xi C, Bai Z, Fei F, Wang M, Liao J, Li Y, Wang X, Song F, Zhang Y, Wang B, Xing D and Wang G 2017 Nat. Commun. 8 977
[20]	Chen T, Liu W, Zheng F, Gao M, Pan X, Van d L G, Wang X, Zhang Q, Song F, Wang B, Wang B, Xu Y, Wang G and Zhang R 2015 Adv. Mater. 27 4823
[21]	Chen T, Chen Q, Schouteden K, Huang W, Wang X, Li Z, Miao F, Wang X, Li Z, Zhao B, Li S, Song F, Wang J, Wang B, Van Haesendonck C and Wang G 2014 Nat. Commun. 5 5022
[22]	Zhang M, Li Y, Song F, Wang X and Zhang R 2017 Chin. Phys. B 26 127305
[23]	Wang X, Pan X, Gao M, Yu J, Jiang J, Zhang J, Zuo H, Zhang M, Wei Z, Niu W, Xia Z, Wan X, Chen Y, Song F, Xu Y, Wang B, Wang G and Zhang R 2016 Adv. Electron. Mater. 2 1600228
[24]	Pan X C, Chen X, Liu H, Feng Y, Wei Z, Zhou Y, Chi Z, Pi L, Yen F, Song F, Wan X, Yang Z, Wang B, Wang G and Zhang Y 2015 Nat. Commun. 6 7805
[25]	Nielsen H B and Ninomiya M 1981 Phys. Lett. B 105 219
[26]	Yoshimi R, Tsukazaki A, Kozuka Y, Falson J, Takahashi K S, Checkelsky J G, Nagaosa N, Kawasaki M and Tokura Y 2015 Nat. Commun. 6 6627
[27]	Yan Y, Liao Z M, Zhou Y B, Wu H C, Bie Y Q, Chen J J, Meng J, Wu X S and Yu D P 2013 Sci. Rep. 3 1264
[28]	Kong D, Randel J C, Peng H, Cha J J, Meister S, Lai K, Chen Y, Shen Z X, Manoharan H C and Cui Y 2010 Nano Lett. 10 329
[29]	Pan H, Zhang K, Wei Z, Wang J, Han M, Song F, Wang X, Wang B and Zhang R 2017 Appl. Phys. Lett. 110 053108
[30]	Zhang K, Pan H, Wei Z, Zhang M, Song F, Wang X and Zhang R 2017 Chin. Phys. B 26 324
[31]	Zhang K, Pan H, Zhang M, Wei Z, Gao M, Song F, Wang X and Zhang R 2017 Rsc Adv. 7 17689
[32]	Niu W, Du K, Wang S, Zhang M, Gao M, Chen Y, Liu H, Zhou W, Song F, Wang P, Xu Y, Wang X, Shen J and Zhang R 2017 Nanoscale 9 12372
[33]	Xiu F, He L, Wang Y, Cheng L, Chang L T, Lang M, Huang G, Kou X, Zhou Y, Jiang X, Chen Z, Zou J, Shailos A and Wang K 2011 Nat. Nanotech. 6 216
[34]	Jauregui L A, Pettes M T, Rokhinson L P, Shi L and Chen Y P 2016 Nat. Nanotech. 11 345
[35]	Kong D, Koski K J, Cha J J, Hong S S and Cui Y 2013 Nano Lett. 13 632
[36]	Fei F, Wei Z, Wang Q J, Lu P, Wang S, Qin Y, Pan D, Zhao B, Wang X, Sun J, Wang X, Wang P, Wan J, Zhou J, Han M, Song F, Wang B and Wang G 2015 Nano Lett. 15 5905
[37]	Baibich M N, Broto J M, Fert A, Van Dau F N, Petroff F, Etienne P, Creuzet G, Friederich A and Chazelas J 1988 Phys. Rev. Lett. 61 2472
[38]	Barnaś J, Fuss A, Camley R, Grünberg P and Zinn W 1990 Phys. Rev. B 42 8110
[39]	Zhou Y, Yang C S, Yu J Q, Zhao X L and Mao H P 2000 Chin. Phys. Lett. 17 835
[40]	Huang L, Liu L, Wang R and Hu L 2017 Chin. Phys. B 26 077201
[41]	Jedema F J, Filip A and Van Wees B 2001 Nature 410 345
[42]	Jedema F, Heersche H, Filip A, Baselmans J and Van Wees B 2002 Nature 416 713
[43]	Jonker B T, Kioseoglou G, Hanbicki A T, Li C H and Thompson P E 2007 Nat. Phys. 3 542
[44]	Dash S P, Sharma S, Patel R S, de Jong M P and Jansen R 2009 Nature 462 491
[45]	Zhou Y, Han W, Chang L T, Xiu F, Wang M, Oehme M, Fischer I A, Schulze J, Kawakami R K and Wang K L 2011 Phys. Rev. B 84 125323
[46]	Jeon K R, Min B C, Jo Y H, Lee H S, Shin I J, Park C Y, Park S Y and Shin S C 2011 Phys. Rev. B 84 165315
[47]	Lou X, Adelmann C, Crooker S A, Garlid E S, Zhang J, Reddy K M, Flexner S D, Palmstrom C J and Crowell P A 2007 Nat. Phys. 3 197
[48]	Zhu H, Ramsteiner M, Kostial H, Wassermeier M, Schönherr H P and Ploog K 2001 Phys. Rev. Lett. 87 016601
[49]	Tombros N, Jozsa C, Popinciuc M, Jonkman H T and Van Wees B J 2007 Nature 448 571
[50]	Han W, Pi K, McCreary K, Li Y, Wong J J, Swartz A and Kawakami R 2010 Phys. Rev. Lett. 105 167202
[51]	Kamalakar M V, Groenveld C, Dankert A and Dash S P 2015 Nat. Commun. 6 6766
[52]	Datta S and Das B 1990 Appl. Phys. Lett. 56 665
[53]	Johnson M 1993 Science 260 320
[54]	Sugahara S and Tanaka M 2004 Appl. Phys. Lett. 84 2307
[55]	Sasaki T, Ando Y, Kameno M, Tahara T, Koike H, Oikawa T, Suzuki T and Shiraishi M 2014 Phys. Rev. Appl. 2 034005
[56]	Xu S Y, Xia Y, Wray L A, Jia S, Meier F, Dil J H, Osterwalder J, Slomski B, Bansil A, Lin H, Cava R J and Hasan M Z 2011 Science 332 560
[57]	Mciver J W, Hsieh D, Steinberg H, Jarillo Herrero P and Gedik N 2012 Nat. Nanotech. 7 96
[58]	Park C H and Louie S G 2012 Phys. Rev. Lett. 109 097601
[59]	Li C, van't Erve O, Robinson J, Liu Y, Li L and Jonker B 2014 Nat. Nanotech. 9 218
[60]	Tian J, Childres I, Cao H, Shen T, Miotkowski I and Chen Y P 2014 Solid State Commun. 191 1
[61]	André D, Johannes G, M Venkata K, Sophie C and Dash S P 2015 Nano Lett. 15 7976
[62]	Ando Y, Hamasaki T, Kurokawa T, Ichiba K, Yang F, Novak M, Sasaki S, Segawa K, Ando Y and Shiraishi M 2014 Nano Lett. 14 6226
[63]	Tang J, Chang L T, Kou X, Murata K, Choi E S, Lang M, Fan Y, Jiang Y, Montazeri M, Jiang W, Wang Y, He L and Wang K 2014 Nano Lett. 14 5423
[64]	Lee J S, Richardella A, Hickey D R, Mkhoyan K A and Samarth N 2015 Phys. Rev. B 92 155312
[65]	Tian J, Miotkowski I, Hong S and Chen Y P 2015 Sci. Rep. 5 14293
[66]	Zhang M, Wang X, Zhang S, Gao Y, Yu Z, Zhang X, Gao M, Song F, Du J, Wang X, He L, Xu Y and Zhang R 2016 IEEE Electron. Dev. Lett. 37 1231
[67]	Vaklinova K, Hoyer A, Burghard M and Kern K 2016 Nano Lett. 16 2595
[68]	Zhao C J, Ding L, Huangfu J S, Zhang J Y and Yu G H 2013 Rare Met. 32 213
[69]	Xu Y and Cai J W 2011 Acta Phys. Sin. 60 117308 (in Chinese)
[70]	Xiong J W, Hu L B and Zhang Z X 2006 Chin. Phys. Lett. 23 1278
[71]	Sulaev A, Zeng M, Shen S Q, Cho S K, Wei G Z, Yuan P F, Eremeev S V, Kawazoe Y, Lei S and Lan W 2015 Nano Lett. 15 2061
[72]	Cha J J, Claassen M, Kong D, Hong S S, Koski K J, Qi X L and Yi C 2012 Nano Lett. 12 4355
[73]	Chen J, Qin H J, Yang F, Liu J, Guan T, Qu F M, Zhang G H, Shi J R, Xie X C, Yang C L, Wu K H, Li Y Q and Lu L 2010 Phys. Rev. Lett. 105 176602
[74]	Liu M, Zhang J, Chang C Z, Zhang Z, Feng X, Li K, He K, Wang L L, Chen X, Dai X, Fang Z, Xue Q K, Ma X and Wang Y 2012 Phys. Rev. Lett. 108 036805
[75]	Yong S K, Brahlek M, Bansal N, Edrey E, Kapilevich G A, Iida K, Tanimura M, Horibe Y, Cheong S W and Oh S 2011 Phys. Rev. B 84 073109
[76]	Brahlek M, Koirala N, Salehi M, Bansal N and Oh S 2014 Phys. Rev. Lett. 113 026801
[77]	Bansal N, Yong S K, Brahlek M, Edrey E and Oh S 2012 Phys. Rev. Lett. 109 116804
[78]	Lu H Z, Shi J and Shen S Q 2011 Phys. Rev. Lett. 107 076801
[79]	Liu M, Chang C Z, Zhang Z, Zhang Y, Wei R, He K, Wang L, Chen X, Jia J F, Zhang S C, Xue Q K, Ma X and Wang Y 2010 Phys. Rev. B 83 165440
[80]	Cha J J, Kong D, Hong S S, Analytis J G, Lai K and Cui Y 2012 Nano Lett. 12 1107
[81]	Lee J, Park J, Lee J H, Kim J S and Lee H J 2012 Phys. Rev. B 86 245321
[82]	Taskin A A, Sasaki S, Segawa K and Ando Y 2012 Phys. Rev. Lett. 109 066803
[83]	Steinberg H, Lalo J B, Fatemi V, Moodera J S and Jarilloherrero P 2011 Phys. Rev. B 84 233101
[84]	Li Z, Garate I, Pan J, Wan X, Chen T, Ning W, Zhang X, Song F, Meng Y, Hong X, Wang X, Pi L, Wang X, Wang B, Li S, Reed M A, Glazman L and Wang G 2015 Phys. Rev. B 91 041401
[85]	Sato T, Segawa K, Kosaka K, Souma S, Nakayama K, Eto K, Minami T, Ando Y and Takahashi T 2011 Nat. Phys. 7 840
[86]	Brahlek M, Bansal N, Koirala N, Xu S Y, Neupane M, Liu C, Hasan M Z and Oh S 2012 Phys. Rev. Lett. 109 186403
[87]	Xu S Y, Neupane M, Belopolski I, Liu C, Alidoust N, Bian G, Jia S, Landolt G, Slomski B, Dil J H, Shibayev P P, Basak S, Chang T R, Jeng H T, Cava R J, Lin H Bansil A and Hasan M Z 2015 Nat. Commun. 6 6870
[88]	Lou R, Liu Z, Jin W, Wang H, Han Z, Liu K, Wang X, Qian T, Kushnirenko Y, Cheong S W, Osgood R M, Ding H and Wang S 2015 Phys. Rev. B 92 115150
[89]	Salehi M, Shapourian H, Koirala N, Brahlek M J, Moon J and Oh S 2016 Nano Lett. 16 5528
[90]	Wu L, Brahlek M, Aguilar R V, Stier A V, Morris C M, Lubashevsky Y, Bilbro L S, Bansal N, Oh S and Armitage N P 2013 Nat. Phys. 9 410
[91]	Sim S, Koirala N, Brahlek M, Ji H S, Park J, Cha S, Jo M H, Oh S and Choi H 2015 Phys. Rev. B 91 235438
[92]	Linder J, Yokoyama T and Sudbo A 2009 Phys. Rev. B 80 205401
[93]	Zhang M, Wang H, Mu K, Wang P, Niu W, Zhang S, Xiao G, Chen Y, Tong T, Fu D, Wang X, Zhang H, Song F, Miao F, Sun Z, Xia Z, Wang X, Xu Y, Wang B, Xing D and Zhang R 2018 ACS Nano 12 1537
[94]	Yang J, Zhang X and Miao R D 2014 Acta Phys. Sin. 63 217202 (in Chinese)
[95]	Fan Y, Upadhyaya P, Kou X, Lang M, Takei S, Wang Z, Tang J, He L, Chang L T, Montazeri M, Yu G, Jiang W, Nie T, Schwartz R N, Tserkovnyak Y and Wang K 2014 Nat. Mater. 13 699
[96]	Fan Y, Kou X, Upadhyaya P, Shao Q, Pan L, Lang M, Che X, Tang J, Montazeri M, Murata K, Chang L T, Akyol M, Yu G, Nie T, Wong K, Lou J, Wang Y, Tserkovnyak Y and Wang K 2016 Nat. Nanotech. 11 352
[97]	Mellnik A R, Lee J S, Richardella A, Grab J L, Mintun P J, Fischer M H, Vaezi A, Manchon A, Kim E A, Samarth N and Ralph D C 2014 Nature 511 449
[98]	Wang Y, Deorani P, Banerjee K, Koirala N, Brahlek M, Oh S and Yang H 2015 Phys. Rev. Lett. 114 257202
[99]	Shiomi Y, Nomura K, Kajiwara Y, Eto K, Novak M, Segawa K, Ando Y and Saitoh E 2014 Phys. Rev. Lett. 113 196601
[100]	Deorani P, Son J, Banerjee K, Koirala N, Brahlek M, Oh S and Yang H 2014 Phys. Rev. B 90 094403
[101]	Jamali M, Lee J S, Jeong J S, Mahfouzi F, Lv Y, Zhao Z, Nikolić B K, Mkhoyan K A, Samarth N and Wang J P 2015 Nano Lett. 15 7126
[102]	Baker A A, Figueroa A I, Collinsmcintyre L J, Laan G V D and Hesjedal T 2015 Sci. Rep. 5 7907

[1]	Hall conductance of a non-Hermitian two-band system with k-dependent decay rates Junjie Wang(王俊杰), Fude Li(李福德), and Xuexi Yi(衣学喜). Chin. Phys. B, 2023, 32(2): 020305.
[2]	Magnetic triangular bubble lattices in bismuth-doped yttrium iron garnet Tao Lin(蔺涛), Chengxiang Wang(王承祥), Zhiyong Qiu(邱志勇), Chao Chen(陈超), Tao Xing(邢弢), Lu Sun(孙璐), Jianhui Liang(梁建辉), Yizheng Wu(吴义政), Zhong Shi(时钟), and Na Lei(雷娜). Chin. Phys. B, 2023, 32(2): 027505.
[3]	Magnetic van der Waals materials: Synthesis, structure, magnetism, and their potential applications Zhongchong Lin(林中冲), Yuxuan Peng(彭宇轩), Baochun Wu(吴葆春), Changsheng Wang(王常生), Zhaochu Luo(罗昭初), and Jinbo Yang(杨金波). Chin. Phys. B, 2022, 31(8): 087506.
[4]	Current spin polarization of a platform molecule with compression effect Zhi Yang(羊志), Feng Sun(孙峰), Deng-Hui Chen(陈登辉), Zi-Qun Wang(王子群), Chuan-Kui Wang(王传奎), Zong-Liang Li(李宗良), and Shuai Qiu(邱帅). Chin. Phys. B, 2022, 31(7): 077202.
[5]	The 50 nm-thick yttrium iron garnet films with perpendicular magnetic anisotropy Shuyao Chen(陈姝瑶), Yunfei Xie(谢云飞), Yucong Yang(杨玉聪), Dong Gao(高栋), Donghua Liu(刘冬华), Lin Qin(秦林), Wei Yan(严巍), Bi Tan(谭碧), Qiuli Chen(陈秋丽), Tao Gong(龚涛), En Li(李恩), Lei Bi(毕磊), Tao Liu(刘涛), and Longjiang Deng(邓龙江). Chin. Phys. B, 2022, 31(4): 048503.
[6]	Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions Pei-Sen Li(李裴森), Jun-Ping Peng(彭俊平), Yue-Guo Hu(胡悦国), Yan-Rui Guo(郭颜瑞), Wei-Cheng Qiu(邱伟成), Rui-Nan Wu(吴瑞楠), Meng-Chun Pan(潘孟春), Jia-Fei Hu(胡佳飞), Di-Xiang Chen(陈棣湘), and Qi Zhang(张琦). Chin. Phys. B, 2022, 31(3): 038502.
[7]	Skyrmion transport driven by pure voltage generated strain gradient Shan Qiu(邱珊), Jia-Hao Liu(刘嘉豪), Ya-Bo Chen(陈亚博), Yun-Ping Zhao(赵云平), Bo Wei(危波), and Liang Fang(方粮). Chin. Phys. B, 2022, 31(11): 117701.
[8]	Ultra-low Young's modulus and high super-exchange interactions in monolayer CrN: A promising candidate for flexible spintronic applications Yang Song(宋洋), Yan-Fang Zhang(张艳芳), Jinbo Pan(潘金波), and Shixuan Du(杜世萱). Chin. Phys. B, 2021, 30(4): 047105.
[9]	Progress on 2D topological insulators and potential applications in electronic devices Yanhui Hou(侯延辉), Teng Zhang(张腾), Jiatao Sun(孙家涛), Liwei Liu(刘立巍), Yugui Yao(姚裕贵), Yeliang Wang(王业亮). Chin. Phys. B, 2020, 29(9): 097304.
[10]	Exploring ferromagnetic half-metallic nature of Cs₂NpBr₆ via spin polarized density functional theory Malak Azmat Ali, G Murtaza, A Laref. Chin. Phys. B, 2020, 29(6): 066102.
[11]	Tunneling magnetoresistance in ferromagnet/organic-ferromagnet/metal junctions Yan-Qi Li(李彦琪), Hong-Jun Kan(阚洪君), Yuan-Yuan Miao(苗圆圆), Lei Yang(杨磊), Shuai Qiu(邱帅), Guang-Ping Zhang(张广平), Jun-Feng Ren(任俊峰), Chuan-Kui Wang(王传奎), Gui-Chao Hu(胡贵超). Chin. Phys. B, 2020, 29(1): 017303.
[12]	SymTopo:An automatic tool for calculating topological properties of nonmagnetic crystalline materials Yuqing He(贺雨晴), Yi Jiang(蒋毅), Tiantian Zhang(张田田), He Huang(黄荷), Chen Fang(方辰), Zhong Jin(金钟). Chin. Phys. B, 2019, 28(8): 087102.
[13]	Spin transport in antiferromagnetic insulators Zhiyong Qiu(邱志勇), Dazhi Hou(侯达之). Chin. Phys. B, 2019, 28(8): 088504.
[14]	Liquid phase epitaxy magnetic garnet films and their applications Yi-Heng Rao(饶毅恒), Huai-Wu Zhang(张怀武), Qing-Hui Yang(杨青慧), Dai-Nan Zhang(张岱南), Li-Chuan Jin(金立川), Bo Ma(马博), Yu-Juan Wu(吴玉娟). Chin. Phys. B, 2018, 27(8): 086701.
[15]	Recent spinterfacial studies targeted to spin manipulation in molecular spintronic devices Xian-Rong Gu(谷现荣), Li-Dan Guo(郭立丹), Xiang-Nan Sun(孙向南). Chin. Phys. B, 2018, 27(10): 107202.

No Suggested Reading articles found!

Viewed

Full text

Abstract

Cited

Metrics
Related Articles

Electrical spin polarization through spin-momentum locking in topological-insulator nanostructures

Cite this article:

Online attention