Interplay between topological insulators and superconductors

Jian Wang, Cui-Zu Chang, Handong Li, Ke He, Duming Zhang, Meenakshi Singh, Xu-Cun Ma, Nitin Samarth, Maohai Xie, Qi-Kun Xue, and M. H. W. Chan

Phys. Rev. B 85, 045415 – Published 10 January 2012

Abstract

Topological insulators are insulating in the bulk but possess metallic surface states protected by time-reversal symmetry. Here, we report on a detailed electronic transport study in high-quality Bi $_{2}$ Se $_{3}$ topological insulator thin films contacted by superconducting (In, Al, and W) electrodes. The resistance of the film shows an abrupt and significant upturn when the electrodes become superconducting. In turn, the Bi $_{2}$ Se $_{3}$ film greatly weakens the superconductivity of the electrodes, significantly reducing both their transition temperatures and their critical fields. A possible interpretation of these results is that the superconducting electrodes are accessing the surface states and the experimental results are consequences of the interplay between the Cooper pairs of the electrodes and the spin-polarized current of the surface states in Bi $_{2}$ Se $_{3}$ .

Received 14 December 2011

DOI:https://doi.org/10.1103/PhysRevB.85.045415

Authors & Affiliations

Jian Wang^1,2,*, Cui-Zu Chang^3,4, Handong Li^5,6, Ke He³, Duming Zhang¹, Meenakshi Singh¹, Xu-Cun Ma³, Nitin Samarth¹, Maohai Xie⁵, Qi-Kun Xue^3,4, and M. H. W. Chan^1,†

¹Center for Nanoscale Science and Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802-6300, USA
²International Center for Quantum Materials and State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing, 100871, China
³Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁴Department of Physics, Tsinghua University, Beijing 100084, China
⁵Physics Department, University of Hong Kong, Pokfulam Road, Hong Kong, China
⁶State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054, China

^*jianwangphysics@pku.edu.cn
^†chan@phys.psu.edu

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Issue

Vol. 85, Iss. 4 — 15 January 2012

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Images

Figure 1
$R$ – $T$ behavior of the 5-nm-thick Bi $_{2}$ Se $_{3}$ film contacted by two superconducting In dots. (a) $R$ vs $T$ of the 5-QL Bi $_{2}$ Se $_{3}$ film from room temperature to low temperature. The left inset is a STM image of the Bi $_{2}$ Se $_{3}$ film. The right inset is the measurement structure. (b) $R$ vs $T$ at different perpendicular fields. The curves at 0.2 and 0.3 kOe are superimposed.Reuse & Permissions
Figure 2
$R$ – $H$ scans of the 5-nm-thick Bi $_{2}$ Se $_{3}$ film contacted by In electrodes. (a) Resistance as a function of perpendicular magnetic field at 4 and 1.8 K. (b) Magnified MR for several temperatures. (c) MR peaks near the zero field show terrace structure and hysteresis for scans made below $T_{C}$ . The → arrow indicates the scan was made from a negative to a positive field.Reuse & Permissions
Figure 3
(a) 3D image of resistance as a function of the perpendicular field and temperature measured with a bias current of 500 nA for the 5-QL-thick Bi $_{2}$ Se $_{3}$ film contacted by bulk In electrodes (In-Bi $_{2}$ Se $_{3}$ -In). A sharp resistance enhancement induced by the interaction between superconducting electrodes and Bi $_{2}$ Se $_{3}$ film is found. (b) Color contour map of resistance along the temperature and perpendicular magnetic field axes of the 5-nm-thick Bi $_{2}$ Se $_{3}$ film contacted by In electrodes. The colors represent resistance from 410 Ω (deep purple, the shade at the top of the color scale) to 510 Ω (deep red, the shade at the bottom of the color scale). White means the resistance is larger than 510 Ω.Reuse & Permissions
Figure 4
Transport behaviors of 200-nm-thick Bi $_{2}$ Se $_{3}$ films contacted by superconducting Al electrodes. (a) A sharp resistance enhancement is seen at 0.95 K, which saturates below 0.85 K. An applied magnetic field of 100 Oe suppresses the enhancement. The inset is a SEM image of the Al contacts on the surface of the Bi $_{2}$ Se $_{3}$ film. (b) The details of the MR in a small field. The resistance peak at 0.65 K is suppressed under a field of less than 100 Oe, which is much smaller than the $H_{C}$ (800 Oe) of a 50-nm-thick Al film not contacting a Bi $_{2}$ Se $_{3}$ film. The $T_{C}$ of such an “isolated” Al film is 1.4 K.Reuse & Permissions
Figure 5
Transport behaviors of a 200-nm-thick Bi $_{2}$ Se $_{3}$ film contacted by superconducting W electrodes. (a) $R$ vs $T$ scans under different magnetic fields. The inset is a SEM image of the W contacts on the surface of the Bi $_{2}$ Se $_{3}$ film. (b) MR at different temperatures. When the W electrodes become superconducting, the MR shows a large peak around the zero field. This behavior disappears when the temperature is larger than the $T_{C}$ . The resistance peak is suppressed under a field of ∼10 kOe at 2.2 K, much smaller than the $H_{C}$ of the W strips (80 kOe).Reuse & Permissions
Figure 6
(a) $I$ vs $V$ at different perpendicular magnetic fields for the W-Bi $_{2}$ Se $_{3}$ -W sample at 0.5 K. (b) $d I / d$ $V$ vs $V$ at different magnetic fields at 0.5 K. (c) $d I / d$ $V$ as a function of current $I$ and field $H$ at $T$ = 0.5 K.Reuse & Permissions

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