Inverse proximity effect in topological insulator films contacted by superconducting electrodes Jian Wang1,2*, Cui-Zu Chang3,4, Handong Li5,6, Ke He3, Meenakshi Singh1, Xu-Cun Ma3, Maohai Xie5, Qi-Kun Xue3,4 and M. H. W. Chan1* 1The Center for Nanoscale Science and Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, USA, 2International Center for Quantum Materials, 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, The University of Hong Kong, Pokfulam Road, Hong Kong, China, 6Deparment of Physics, Beijing Jiaotong University, Beijing 100044, China. *E-mail: [email protected]; [email protected] Topological insulators are insulating in the bulk but possess metallic surface states protected by time-reversal symmetry. The existence of these surface states has been confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements. Here, we report a detailed electronic transport study in high quality Bi2Se3 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 becomes superconducting. In turn, the Bi2Se3 film weakens the superconductivity of the electrodes, reducing both their transition temperatures and critical fields. A sensible interpretation of these results is that the superconducting electrodes are accessing the surface states and the experimental results are the consequence of the interplay of the cooper pairs and the spin polarized current. 1 Bismuth-based materials have long been studied for their thermoelectric properties1-3. Recently, bismuth selenide (Bi2Se3), bismuth antimonide (Bi1-xSbx), bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) have been predicted theoretically and confirmed experimentally by angle-resolved photoemission spectroscopy (ARPES) experiments to be three dimensional (3D) topological insulators (TIs) due to the strong spin-orbit 1-11 interactions . Of these, Bi2Se3, with a simple surface state structure (a single Dirac cone) and relatively large band-gap (0.3 eV) has become a reference material in 3D TIs. In transport measurements of TIs12-18, quantum magneto-resistance (MR) oscillations have been observed and interpreted as evidence of a topologically protected surface state in 12-16 Bi2Se3 . A key feature of the surface state is that the spin and momentum of the conduction electrons are locked4, which has not been revealed in transport experiments. In this paper, we report the transport behavior of crystalline Bi2Se3 films contacted by three different superconducting electrodes to study the interplay between the superconductivity and the TI surface state. We use superconducting bulk indium (In) electrodes and mesocopic aluminum (Al) and tungsten (W) electrodes to study the transport property of the Bi2Se3 films with thickness of 5 and 200 quintuple layers (QLs). Every quintuple layer (QL) is one nanometer thick. The distance between superconducting electrodes is 1 μm and 1 mm respectively. Irrespective of the material of the electrodes, the thickness of the Bi2Se3 film and the separation of electrodes, the resistance shows a large 2 and abrupt increase near the superconducting transition temperature (TC) of the superconducting electrodes. Below TC, surprising negative magneto-resistance (MR) at fields larger than the critical field (HC) of the superconducting electrodes is observed. Most interestingly, we observe an inverse proximity effect where the Bi2Se3 films dramatically reduce both the TC and HC of the superconducting electrodes. Recent progress in thin film growth of TIs by molecular beam epitaxy (MBE) has made 19-23 planar TI devices possible . Our high quality Bi2Se3 films were grown under Se-rich conditions on sapphire and high resistivity silicon substrates in ultrahigh-vacuum (UHV) MBE systems. A scanning tunneling microscope (STM) image of a typical MBE-grown Bi2Se3 film with a thickness of 5 QL is shown in the left inset of Fig. 1a. The atomically flat morphology demonstrates the high crystal quality of the film. The inset of Fig. 1b is the ARPES band map of the 5 QL Bi2Se3 film. The data indicate that our sample is a nearly intrinsic topological insulator with a single Dirac point and surface state. The carrier density and the mobility of the 5QL film at 2 K are around 4×1018 cm-3 and 3320 cm2/Vs by Hall measurement. With decreasing thickness of the film, the surface to volume ratio increases and surface properties should become more prominent. However, it has been shown by ARPES that in films with thickness less than 5 QL, the interaction between top and bottom surfaces may destroy the topologically protected surface state20. 3 The right inset of Fig.1a is a schematic diagram of our transport measurement structure. Superconducting In dots ~ 0.5 mm in diameter and ~ 0.2 mm thick are directly pressed onto the top surface of the Bi2Se3 film. The distance between the two electrodes is ~1 mm. Figure 1a shows resistance as a function of temperature (R-T) results for the 5 QL Bi2Se3 film. In this paper, unless noted otherwise, the magnetic field is always applied perpendicular to the film and the excitation current for the measurement is 50 nA. From 300 K (room temperature) to 45 K, the R-T curve shows linear metallic behavior. A resistance minimum is found at 13.3 K. The residual resistance ratio (RRR) between 300 K and 13.3 K is 2.1. Below 13.3 K, the resistance increases gradually with decreasing temperature. However, at 3.29 K (slightly below the TC of bulk In (3.4 K)) the resistance shows an abrupt increase. This resistance enhancement is shown in more details in Fig. 1b. The resistance at 1.8 K (967.23 Ω) is 2.34 times the resistance when the In electrodes are normal at T=3.4 K. With increasing field, this resistance enhancement decreases rapidly. When the field is 0.2 kOe (slightly below the critical field of 211 Oe at 1.8 K), the enhancement behavior is totally suppressed. We interpret the enhancement in R to be a consequence of the onset of superconductivity of the In electrodes, however, it appears the transition temperature and critical field of the In electrodes when contacting the Bi2Se3 film are slightly below the natural values. 4 Resistance as a function of the magnetic field (R-H) for the 5 QL Bi2Se3 film are shown in Fig. 2a. At 4 K (above TC of In), the R-H curve shows linear magnetoresistance (MR) from 26 kOe to 80 kOe. Such a linear MR has been attributed to the surface states with the 23,24 linear energy-momentum correlation . However, at 1.8 K (below TC of In), near zero field the sample exhibits a striking MR peak. Between 0.2 and 9 kOe, well above the critical field of the electrodes, the resistance decreases unexpectedly with field. Upon further increase in the magnetic field, the MR shows the same positive linear behavior as the R-H curve at 4 K. Figure 2b shows MR in small field at different temperatures in more details. Above TC of the In electrodes (at 3.4 K and 4.0 K) positive MR is observed. In addition, there is a small MR dip around zero field, which has been studied carefully and attributed to the weak 23,25 anti-localization in TIs . At temperatures below TC of the In electrodes, the MR dip disappears and a sharp MR peak comes into being. With decreasing temperature, the peak value increases rapidly, consistent with the R-T curves of Fig.1. At 1.8 K and 2.4 K, besides the sharp resistance peak around zero field, an additional negative MR is observed from 200 Oe to several kOe. At lower temperature, the negative MR is more robust. This result is unexpected. TI films contacted with normal metal electrodes show positive MR in perpendicular field23,25, hence the negative MR cannot be from the TI film itself. On the other hand, if the observed negative MR is due to the superconductivity of indium 5 electrodes, one would not expect this behavior for fields larger than the critical field (HC) of the indium electrodes (~200 Oe at 1.8 K). Interestingly, this negative MR behavior extends up 9 kOe, but only at temperatures below TC of the electrodes. Figure 2c shows the details of the MR peak shown in Figures 2a and b. Under higher field resolution, the MR ‘peak’ appears as a platform with terraces. When we scan magnetic field from negative to positive values and then from positive to back to negative values, the sample exhibits hysteretic behavior at 2.6 K and 1.8 K. The terrace structure of MR peak and the hysteresis is suggestive of a ferromagnetic response in the conduction electrons. We note that there is no possibility of magnetic contamination in the process of sample preparation. Fig 3 shows a resistance contour map along the T-H axes constructed from all the data we have obtained on this sample. In addition to bulk In electrodes measurements with mesoscopic superconducting Al and W electrodes were patterned on the TI films to test the universality of the observed phenomena. The inset of Fig. 4a is a scanning electron microscope (SEM) image of our measurement structure. The Bi2Se3 film for this sample is 200 nm thick and grown on high resistivity silicon substrate. The substrate is completely insulating below 150 K. The carrier density and the mobility of the film at 1.8 K are around 2.76×1018 cm-3 and 2800 cm2/Vs by Hall measurement. The superconducting Al electrodes are 50 nm thick and directly deposited on the top surface of the film by electron beam lithography (EBL) followed by 6 e-beam assisted evaporation.