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MgO衬底上的YBa2Cu3O7–δ (YBCO)台阶边沿型约瑟夫森结(台阶结)在高灵敏度高温超导量子干涉器(superconducting quantum interference device, SQUID)等超导器件研制方面具有重要的应用价值和前景. 本文对此类YBCO台阶结的制备和特性进行了研究. 首先利用离子束刻蚀技术和两步刻蚀法在MgO (100)衬底上制备陡度合适、边沿整齐的台阶, 然后利用脉冲激光沉积法在衬底上生长YBCO超导薄膜, 进而利用紫外光刻制备出YBCO台阶结. 在结样品的电阻-温度转变曲线中, 观测到低于超导转变温度时的电阻拖尾现象, 与约瑟夫森结的热激活相位滑移理论一致. 伏安特性曲线测量表明结的行为符合电阻分路结模型, 在超导转变温度TC附近结的约瑟夫森临界电流密度JC随温度T呈现出
$ (T_{\rm C}-T)^2 $ 的变化规律, 77 K时JC值为1.4 × 105 A/cm2. 利用制备的台阶结, 初步制备了YBCO射频高温超导SQUID, 器件测试观察到良好的三角波电压调制曲线, 温度77 K、频率1 kHz时的磁通噪声为250$\text{μ}\Phi_0/{\rm Hz}^{1/2}$ . 本文结果为进一步利用MgO衬底YBCO台阶结研制高性能的高温超导SQUID等超导器件奠定了基础.-
关键词:
- YBa2Cu3O7–δ /
- 台阶结 /
- 超导量子干涉器
The YBa2Cu3O7–δ (YBCO) step-edge Josephson junction on MgO substrate has recently been shown to have important applications in making advanced high-transition temperature (high-TC) superconducting devices such as high-sensitivity superconducting quantum interference device (SQUID), superconducting quantum interference filter, and THz detector. In this paper, we investigate the fabrication and transport properties of YBCO step-edge junction on MgO substrate. By optimizing the two-stage ion beam etching process, steps on MgO (100) substrates are prepared with an edge angle θ of about 34°. The YBCO step-edge junctions are then fabricated by growing the YBCO thin films with a pulsed laser deposition technique and subsequent traditional photolithography. The resistive transition of the junction shows typical foot structure which is well described by the Ambegaokar-Halperin theory of thermally-activated phase slippage for overdamped Josephson junctions. The voltage-current curves with temperature dropping down to 77 K exhibit resistively shunted junction behavior, and the Josephson critical current density JC is shown to follow the$(T_{\rm C}-T)^2$ dependence. At 77 K, the JC of the junction reaches 1.4 × 105 A/cm2, significantly higher than the range of 103–104 A/cm2 as presented by other investigators for YBCO step-edge junctions on MgO substrate with comparable θ of 35°–45°. This indicates a rather strong Josephson coupling of the junction, and by invoking the results of YBCO bicrystal junctions showing similar values of JC, it is tentatively proposed that the presently fabricated junction might be described as an S-s′-S junction with s′ denoting the superconducting region of depressed TC in the vicinity of the step edge or as an S-N-S junction with N denoting a very thin non-superconducting layer. By incorporating the MgO-based YBCO step-edge junction, high-TC radio frequency (RF) SQUID is made. The device shows decent voltage-flux curve and magnetic flux sensitivity of 250$ \text{μ}\Phi_0/{\rm Hz}^{1/2} $ at 1 kHz and 77 K, comparable to the values reported in the literature. To further improve the RF SQUID performance, efforts could be devoted to optimizing the junction parameters such as the junction JC. By using the YBCO step-edge junction on MgO substrate, high-TC direct current SQUID could also be developed, as reported recently by other investigators, to demonstrate the potential of MgO-based step-edge junction in making such a kind of device with superior magnetic flux sensitivity.[1] Clarke J, Braginski A I 2004 The SQUID Handbook (Vol. 1) (Weinheim: Wiley-VCH) pp1−395
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图 1 MgO (100)衬底上YBCO薄膜的表征 (a) SEM图; (b)电阻-温度(R-T)曲线, 插图为超导转变区域的放大; (c) 77 K时的伏安(V-I)特性曲线
Fig. 1. Characterization of YBCO film on MgO (100) substrate: (a) SEM image; (b) R-T curve with the inset showing a magnified view of the superconducting transition; (c) V-I curve at 77 K.
图 2 MgO衬底上台阶的两步法离子束刻蚀制备 (a)第一步刻蚀示意图; (b)第二步刻蚀示意图; (c)第一步刻蚀后测得的台阶轮廓图; (d)第二步刻蚀后测得的台阶轮廓图
Fig. 2. Fabrication of step on MgO substrate by using two-stage ion beam etching: (a) Schematic of the first ion beam etching; (b) schematic of the second ion beam etching; (c) step profile after the first etching measured by a stylus profiler; (d) step profile after the second ion beam etching.
图 3 台阶的SEM形貌图 (a)台阶断面图; (b)台阶斜视图; (c)刻蚀区域与台阶相对的另外一边衬底的断面图; (d)刻蚀区域与台阶相对的另外一边衬底的斜视图
Fig. 3. SEM images of the step: (a) Cross section of the step; (b) oblique view of the step; (c) cross section of the substrate at the other side (opposite to the step) of the defined etching area; (d) oblique view of the substrate at the other side of the defined etching area.
图 4 YBCO台阶结的R-T曲线(插图为拖尾区的放大, 其中红线代表A-H理论拟合)
Fig. 4. R-T curve of the YBCO step-edge junction on MgO substrate. The inset shows a magnified view of the foot-structure region with the red line being a fit to the A-H theory.
图 5 (a) YBCO台阶结的V-I特性曲线; (b)台阶结的JC随温度的变化, 红线代表S-N-S型约瑟夫森结的理论拟合, 内插图为ICRn随温度的变化, 其中菱形所代表的数据取自文献[17]
Fig. 5. (a) V-I curves of the YBCO step-edge junction on MgO substrate; (b) temperature dependence of JC, with the red line being a fit to the
$ {\left(1-T/{T}_{\rm{C}}\right)}^{2} $ dependence according to the theory of S-N-S Josephson junction. The inset shows ICRn, with data points in diamond taken from the Ref. [17].
图 6 YBCO RF SQUID器件的表征 (a)电压-磁通曲线; (b)噪声谱(频率1 kHz时磁通噪声250
$ \text{μ}\Phi_0/{\rm Hz}^{1/2} $ , 对应的磁场噪声800 fT/Hz1/2, 如蓝色虚线所示)Fig. 6. Characterization of the YBCO RF SQUID: (a) Voltage-flux curve; (b) noise spectra, with blue dashed line denoting 800 fT/Hz1/2 at 1 kHz.
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[1] Clarke J, Braginski A I 2004 The SQUID Handbook (Vol. 1) (Weinheim: Wiley-VCH) pp1−395
[2] 韩昊轩, 张国峰, 张雪, 梁恬恬, 应利良, 王永良, 彭炜, 王镇 2019 68 138501
Google Scholar
Han H X, Zhang G F, Zhang X, Liang T T, Ying L L, Wang Y L, Peng W, Wang Z 2019 Acta Phys. Sin. 68 138501
Google Scholar
[3] Song C, Xu K, Li H K, Zhang Y R, Zhang X, Liu W X, Guo Q J, Wang Z, Ren W H, Hao J, Feng H, Fan H, Zheng D N, Wang D W, Wang H, Zhu S Y 2019 Science 365 574
Google Scholar
[4] Xu K, Sun Z H, Liu W X, Zhang Y R, Li H K, Dong H, Ren W H, Zhang P F, Nori F, Zheng D N, Fan H, Wang H 2020 Sci. Adv. 6 eaba4935
Google Scholar
[5] 曹文会, 李劲劲, 钟青, 郭小玮, 贺青, 迟宗涛 2012 61 170304
Google Scholar
Cao W H, Li J J, Zhong Q, Guo X W, He Q, Chi Z T 2012 Acta Phys. Sin. 61 170304
Google Scholar
[6] Du J, Smart K, Li L, Leslie K E, Hanham S M, Wang D H C, Foley C P, Ji F, Li X D, Zeng D Z 2015 Supercond. Sci. Technol. 28 084001
Google Scholar
[7] Pfeiffer C, Ruffieux S, Jönsson L, Chukharkin M L, Kalaboukhov A, Xie M S, Winkler D, Schneiderman J F 2020 IEEE Trans. Biomed. Eng. 67 1483
Google Scholar
[8] Hilgenkamp H, Mannhart J 2002 Rev. Mod. Phys. 74 485
Google Scholar
[9] Hua T, Yu M, Geng H F, Xu W W, Lu Y P, Shi J X, Shen W K, Wu J B, Wang H B, Chen J, Wu P H 2018 Supercond. Sci. Technol. 31 085009
Google Scholar
[10] Kleiner R, Wang H B 2019 J. Appl. Phys. 126 171101
Google Scholar
[11] Liu Z H, Wei Y K, Wang D, Zhang C, Ma P, Wang Y 2014 Chin. Phys. B 23 097401
Google Scholar
[12] 郑鹏, 刘政豪, 魏玉科, 张辰, 张炎, 王越, 马平 2014 63 198501
Google Scholar
Zheng P, Liu Z H, Wei Y K, Zhang C, Zhang Y, Wang Y, Ma P 2014 Acta Phys. Sin. 63 198501
Google Scholar
[13] Chambers S A 2000 Surf. Sci. Rep. 39 105
Google Scholar
[14] Tanaka S, Kado H, Matsuura T, Itozaki H 1993 IEEE Trans. Appl. Supercond. 3 2365
Google Scholar
[15] Foley C P, Mitchell E E, Lam S K H, Sankrithyan B, Wilson Y M, Tilbrook D L, Morris S J 1999 IEEE Trans. Appl. Supercond. 9 4281
Google Scholar
[16] Mitchell E E, Foley C P 2010 Supercond. Sci. Technol. 23 065007
Google Scholar
[17] Du J, Hellicar A D, Li L, Hanham S M, Macfarlane J C, Leslie K E, Nikolic N, Foley C P, Greene K J 2009 Supercond. Sci. Technol. 22 114001
Google Scholar
[18] Kaczmarek L L, Ijsselsteijn R, Zakosarenko V, Chwala A, Meyer H G, Meyer M, Stolz R 2018 IEEE Trans. Appl. Supercond. 28 1601805
Google Scholar
[19] Faley M I, Poppe U, Dunin-Borkowski R E, Schiek M, Boers F, Chocholacs H, Dammers J, Eich E, Shah N J, Ermakov A B, Slobodchikov V Y, Maslennikov Y V, Koshelets V P 2013 IEEE Trans. Appl. Supercond. 23 1600705
Google Scholar
[20] Wang H Z, Li Y L, Wang Y, Gan Z Z 2020 Physica C 569 1353587
Google Scholar
[21] 高吉, 杨涛, 马平, 戴远东 2010 59 5044
Google Scholar
Gao J, Yang T, Ma P, Dai Y D 2010 Acta Phys. Sin. 59 5044
Google Scholar
[22] Proyer S, Stangl E, Borz M, Hellebrand B, Bäuerle D 1996 Physica C 257 1
Google Scholar
[23] Ando Y, Segawa K 2002 Phys. Rev. Lett. 88 167005
Google Scholar
[24] Faley M I, Slobodchikov V Y, Maslennikov Y V, Koshelets V P, Dunin-Borkowski R E 2016 IEEE Trans. Appl. Supercond. 26 1600404
Google Scholar
[25] Mitsuzuka T, Yamaguchi K, Yoshikawa S, Hayashi K, Konishi M, Enomoto Y 1993 Physica C 218 229
Google Scholar
[26] Foley C P, Lam S, Sankrithyan B, Wilson Y, Macfarlane J C, Hao L 1997 IEEE Trans. Appl. Supercond. 7 3185
Google Scholar
[27] Yamaguchi K, Yoshikawa S, Takenaka T, Fujino S, Hayashi K, Mitsuzuka T, Suzuki K, Enomoto Y 1994 IEICE Trans. Electron. E77-C 1218
[28] Gross R, Chaudhari P, Dimos D, Gupta A, Koren G 1990 Phys. Rev. Lett. 64 228
Google Scholar
[29] 张骏, 张辰, 张焱, 马平, 王越 2015 低温 37 423
Google Scholar
Zhang J, Zhang C, Zhang Y, Ma P, Wang Y 2015 Chin. J. Low Temp. Phys. 37 423
Google Scholar
[30] Ambegaokar V, Halperin B I 1969 Phys. Rev. Lett. 22 1364
Google Scholar
[31] Barone A, Paterno G 1982 Physics and Applications of the Josephson Effect (New York: Wiley) pp121−197
[32] Dimos D, Chaudhari P, Mannhart J 1990 Phys. Rev. B 41 4038
Google Scholar
[33] Siegel M, Herrmann K, Copetti C, Jia C L, Kabius B, Schubert J, Zander W, Braginski A I, Seidel P 1993 IEEE Trans. Appl. Supercond. 3 2369
Google Scholar
[34] Herrmann K, Kunkel G, Siegel M, Schubert J, Zander W, Braginski A I, Jia C L, Kabius B, Urban K 1995 J. Appl. Phys. 78 1131
Google Scholar
[35] Redwing R D, Hinaus B M, Rzchowski M S, Heinig N F, Davidson B A, Nordman J E 1999 Appl. Phys. Lett. 75 3171
Google Scholar
[36] Steel D G, Hettinger J D, Yuan F, Miller D J, Gray K E, Kang J H, Talvacchio J 1996 Appl. Phys. Lett. 68 120
Google Scholar
[37] Kupriyanov M Yu, Likharev K K 1991 IEEE Trans. Magn. 27 2460
Google Scholar
[38] Du J 2003 IEEE Trans. Appl. Supercond. 13 865
Google Scholar
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