Superconductivity can be produced locally in various materials by placing them in direct contact with a superconductor1, resulting in unique combinations of distinct properties that do not coexist otherwise2. This proximity effect has been proposed as a powerful resource for practical applications in optoelectronics3 and has been predicted to produce the elusive Majorana fermion4 by combining a superconductor with a topological insulator (TI)5,6,7,8,9,10. These goals have been pursued actively, including the recent demonstrations of novel semiconductor light sources11 and induced superconductivity in TIs12,13,14,15,16 such as Bi2Se3 and Bi2Te3, which have been shown to have topologically protected surface states17,18,19. Nonetheless, all such experiments performed to date have employed low critical-temperature (low-Tc) materials that require extreme cooling, and the important ratio of the superconducting gap to the Fermi energy in such systems is very small. Inducing superconductivity with the larger energy scales and higher temperatures possible in high critical-temperature (high-Tc) superconductors20 can enable future realizations of more practical devices, as well as greatly expand the ability to study the underlying physical phenomena21.

A widely used approach for the demonstration of proximity-induced superconductivity in larger-scale devices is based on conductance spectra at superconducting–normal (S–N) interfaces22. The proximity-induced superconductivity was established in low-Tc junctions with a metal23,24, or a semiconductor25,26 as the N-contact. Specifically, Andreev reflection features with the width corresponding to the induced gap in the normal material, along with a reduced gap in the superconductor, were shown to manifest the signature of the proximity effect23,24,27,28. Low-Tc superconductivity has been observed recently in CuxBi2Se329,30,31, and a corresponding theory of unconventional superconductivity has been developed32,33. These low-Tc studies provide the evidence for existence of interactions in TIs, enabling the possibility of inducing a superconducting gap in TIs in proximity with high-Tc materials as well.

Here we generate proximity-induced superconductivity in Bi2Se3 and Bi2Te3 by combining them with a high-Tc superconductor, Bi2Sr2CaCu2O8+δ (Bi-2212), and characterize the induced superconductivity by studying the resulting current and differential conductance across the interface. Andreev reflection is observed as an excess current and an increase in differential conductance when cooling below Tc. The widths of the central features in the Andreev spectra do not reflect the full gap of Bi-2212, but rather the induced gap in the Bi2Se3 and Bi2Te3 due to the proximity effect. Furthermore, we observe additional features in the Andreev spectrum due to the reduced gap in the Bi-2212 at the interface—consistent with previous studies of the proximity effect with low-Tc superconductors23,24,27,28. This is a natural consequence of the extension of the superconducting order parameter into the Bi2Se3 and Bi2Te3, and confirms the existence of the proximity effect in our junctions. The Andreev scattering spectrum we observe shows good agreement with a recently developed d-wave theory34, modified to account for the high-Tc order parameter profile in the proximity region.


Sample characteristics

High-quality bulk Bi2Se3, Bi2Te3 and optimally doped Bi-2212 crystals were cleaved, resulting in atomically flat surfaces over large areas (Fig. 1a and b), and were subsequently mechanically bonded (Fig. 1c-f) in a dry atmosphere. These low-resistance tunnel junctions were then probed by conductance spectroscopy measurements, including the current and differential conductance versus voltage (Fig. 1g). The conductance spectroscopy experiments were performed using four-point probe measurements in a liquid He flow cryostat at different temperatures ranging from 295 to 4.5 K. Our mechanically bonded tunnel junction method was also verified to perform successfully on a Bi-2212/graphite junction (Supplementary Fig. S1), resulting in a typical S–N tunnelling spectra seen previously for Bi-2212, with point contact or scanning tunnelling microscopy35.

Figure 1: Device fabrication and measurement set-up.
figure 1

(a) Atomic force microscope (AFM) image of the crystal surface of Bi2Se3. The scale bar corresponds to 2 μm. (b) Larger area AFM scan of the Bi2Te3 sample, demonstrating the vertical inhomogeneity of the surface is limited to ±2 unit cells. The scale bar corresponds to 3 μm. Junction fabrication technique: (c) Bi-2212 (BSCCO) crystal is cleaved using scotch tape. (d) Bi2Se3 (or Bi2Te3) is sandwiched between glass slides with double-sided tapes and the top glass slide is lifted off, cleaving a flat surface. (e) The Bi2Se3 (or Bi2Te3) is transferred to a Cu sample holder, and the cleaved Bi-2212 crystal is applied to Bi2Se3 or Bi2Te3 using GE varnish at the corners. (f) Contacts are made with Ag epoxy or evaporated Au/Ti. (g) Experimental set-up: four-point DC current-voltage and AC differential conductance measurements performed down to 4.5 K, using a liquid He-flow cryostat with lock-in amplifiers. DC bias from the power supply is combined with the AC signal from the voltage lock-in amplifier in a transformer-based adder.

Bi-2212/Bi2Se3 experiments

The first set of experiments was performed on Bi-2212/Bi2Se3 junctions. DC current along the c axis versus voltage (I–V) characteristics (Fig. 2a) reveal excess current below the Bi-2212 Tc (~85 K) due to the Andreev reflection at the S–N interfaces36, consistent with the model developed for anisotropic superconductors34. The observed Andreev reflection indicates that we have achieved a surprisingly low barrier between Bi-2212 and Bi2Se3. Furthermore, this is the key mechanism for the superconducting proximity effect37, suggesting the existence of a proximity-induced superconducting region at the Bi-2212/Bi2Se3 interface. Just below Tc, the excess current, Ie, reaches the maximal value similar to the normal–normal (N–N) interface current (measured for T >Tc), resulting in total current nearly twice that of the N–N interface due to the contribution of the Cooper pairs when Andreev reflection occurs. A different measurement—AC differential conductance below Tc, (dI/dV)S, divided by the normal state conductance(dI/dV)N measured at 105 K, confirms the DC I–V measurement. Specifically, the AC differential conductance reveals a zero-bias conductance peak due to Andreev reflection below Tc (Fig. 2b). At lower temperatures (around 60 K), reduction of higher-bias current was observed, and at the same temperatures, additional higher-bias features appeared in the differential conductance measurement. These features result from the gap reduction in Bi-2212 and the induced gap in Bi2Se3. However, the detailed structure of these spectral features could be studied only at lower temperatures as discussed in detail below.

Figure 2: Bi-2212/Bi2Se3 junction measurements.
figure 2

(a) DC current–voltage characteristics of the low-resistance Bi-2212/Bi2Se3 junction for different temperatures above and below Tc. Below Tc, excess current typical of Andreev reflection at an S–N interface is exhibited. The 70 K I–V curve shows the maximal excess current Ie ~4.85 μA—similar to the value of the normal junction (above Tc) at the voltage where the superconducting state current becomes linear (black dashed line). The total excess current is smaller for lower temperatures due to additional gap features. (b) AC differential conductance(dI/dV)S, normalized by the normal state conductance (dI/dV)N at 105 K, for the low-resistance Bi-2212/Bi2Se3 junction at different temperatures above and below Tc. A zero-bias conductance Andreev peak is clearly seen below Tc, with additional features in the spectrum appearing at lower temperatures. The curves are shifted for clarity. (c) DC current–voltage characteristics of the low-resistance Bi-2212/Bi2Se3 junction for different temperatures well below Tc. In addition to the excess current, the low-temperature curves exhibit two distinct steps corresponding to the dips in the differential conductance measured for AC bias. The black arrow shows the induced Bi2Se3 gap, Δi, with a current twice that of the normal state at the same voltage (a), and the red arrow indicates the reduced Bi-2212 gap, Δr, whereas the intrinsic Bi-2212 gap, Δ0, is shown by purple arrows. The inset shows the DC and the AC differential conductance at 4.5 K, indicating the good correspondence between the different measurements of Δ0 (purple arrow), Δr (red arrow) and Δi (black arrow); AU, arbitrary units. (d) AC differential conductance (dI/dV)S, normalized by the normal state conductance (dI/dV)N at 105 K, for a low-resistance Bi-2212/Bi2Se3 junction for different temperatures well below Tc. The curves are shifted for clarity. The zero-bias conductance feature is due to the Andreev reflection between the normal and proximity-induced superconducting regions in Bi2Se3, where the width of the peak is nearly 2Δi. The two additional peaks indicate the reduced and the intrinsic Bi-2212 gaps.

To demonstrate the spectral features more clearly, we performed both DC and AC measurements at temperatures well below Tc. DC I–V characteristics of the junction (Fig. 2c) show excess current at lower voltages up to about 13 mV with a correspondingly smaller value IS ~2.7 μA. However, two additional step-like features appear near 27 and 45 mV. These features are confirmed by an AC differential conductance measurement (Fig. 2d), showing a wide conductance feature between −13 and +13 mV, as well as peaks at ±27 and ±45 mV, in good quantitative agreement with the DC measurement (Fig. 2c inset). Similar results were obtained with Bi-2212/Bi2Te3 junctions as described below. This conductance spectrum is a clear signature of a proximity-induced superconducting region in the Bi2Se3.

When the proximity effect occurs (Fig. 3a), a superconducting gap is induced in the normal material, Δi, and near the interface, the gap in the superconducting material is reduced from the intrinsic value, Δ0 (Fig. 3b), to a smaller one, Δr1,23,24,27,28. Generally, the superconducting gap is position-dependent along the axis normal to the interface plane, Δ(x), and Andreev-scattering probability is finite in the whole proximity region. However, the scattering can be divided into two main energy ranges: electrons (holes) with energy |E|<Δi will be mainly Andreev-reflected inside the proximity-induced superconducting region of the normal material, whereas electrons (holes) with Δi<|E|<Δr will continue to the S–N interface and then Andreev-reflect, mainly near the material interface. Thus, conductance spectrum taken on an interface exhibiting the proximity effect will show Andreev-scattering features corresponding to both gaps23,24,27,28. In our experiments, therefore, the central conductance peak is a manifestation of Andreev reflection from the normal region to the proximity-induced superconducting region of Bi2Se3 with a gap Δi~13 mV. The reduced gap in Bi-2212 due to the proximity effect appears as a conductance peak at Δr~27 mV. The change in the zero-bias conductance with temperature is an additional manifestation of the Andreev scattering. Indeed, just below Tc, the zero-bias conductance increases to nearly twice its value above Tc (Fig. 3c). This temperature dependence, as well as the spectral shape of the conductance at various temperatures (Fig. 3c inset), completely rules out any possible heating-related effects, whereas a junction with an increased barrier exhibits no proximity (Fig. 3d). Andreev reflection between Bi-2212 and normal Bi2Se3 (or Bi2Te3) with no proximity would have appeared as a single Andreev feature with a width corresponding to the full gap of Bi-2212, which is completely different from our observations. The full gap appears in our experiment as additional peaks at Δ0~45 mV, whose magnitude and temperature dependence are quantitatively consistent with previous tunnelling studies of Bi-2212 (ref. 35) and our high-barrier junctions (Fig. 3d).

Figure 3: Proximity induced superconductivity.
figure 3

A schematic drawing of the junction in two regimes: (a) low-resistance with proximity induced superconductivity in Bi2Se3 (or Bi2Te3). Andreev scattering takes place in the whole proximity region with lower energy particles reflected mainly in Bi2Se3 (or Bi2Te3), and higher energy ones mainly at the interface with Bi-2212; (b) high-resistance with no proximity-induced superconductivity in Bi2Se3 (or Bi2Te3)—quasiparticle scattering occurs at the interface. (c) Temperature dependence of the zero-bias differential conductance for Bi-2212/Bi2Se3. Below Tc, the Andreev process enhances the conductance by almost a factor of 2. The inset shows the temperature dependence of the reduced Bi-2212 gap Δr blue circles), and the intrinsic Bi-2212 gap Δ0 in the low-resistance junction (green squares) and in the high-resistance junction (red diamonds). (d) Differential conductance (dI/dV)S, normalized by the normal state conductance (dI/dV)N at 105 K, for a high-resistance Bi-2212/Bi2Se3 junction for different temperatures, demonstrating the typical quasiparticle tunnelling differential conductance, resulting in a conductance dip in the gapped region. The curves are shifted for clarity. The arrows indicate the intrinsic superconducting gap of Bi-2212, ±Δ0, around 45 mV. (e) Measured (solid red line) and calculated (dashed blue line) 15 K differential conductance (dI/dV)S, normalized by the normal state conductance (dI/dV)N at 105 K, for a low-resistance Bi-2212/Bi2Se3.

The zero-bias conductance feature has a number of characteristics consistent with Andreev reflection in a proximity-induced region. The height of the Bi2Se3 Andreev zero-bias conductance feature is nearly twice the normal conductance value due to the Cooper pair contribution (Figs 2d and 3c), whereas the width is determined by the induced superconducting gap, 2Δi. The width of the Andreev feature is also manifested in the excess current up to a negative bias Δi in the DC I–V characteristic (Fig. 2c)—nearly twice as high as the current in the normal state (Fig. 2a). The probability of Andreev scattering at the interface with the Bi-2212 is reduced by the scattering in Bi2Se3, resulting in slightly smaller contribution to the conductance at Δr. At low temperatures, both the induced gap and the reduced gap features in differential conductance result in a step-like I–V curve (Fig. 2c). With increasing temperature, the features in the differential conductance merge into one wide central peak (Fig. 2b) and the step-like structure in I–V disappears (Fig. 2a). Therefore, at certain higher temperatures, the total excess current can be larger than at lower temperatures.

Theoretical modelling

For the quantitative theoretical modelling of the effect, we calculated the c axis Bi-2212/Bi2Se3 and Bi-2212/Bi2Te3 conductance spectra using the S–N transport formalism developed for anisotropic superconductors34. The differential conductance below Tc (dI/dV)S, divided by the normal state conductance (dI/dV)N is given by the half-sphere integration over solid angle38 Ω:

where E is the quasiparticle energy and θN is the incidence angle (relative to the interface normal) in the normal material, σN is the conductance from normal to normal material with the same geometry, and

where —electron-like and hole-like quasiparticle effective pair potentials with the corresponding phases iφ±.

In case of c axis tunnelling, the hole-like and the electron-like quasiparticles transmitted into the superconductor experience the same effective pair potentials, which have similar dependence on the azimuthal angle α in the ab plane . The total Andreev reflection spectrum is obtained by calculating the reflection and the transmission in the proximity region, followed by reflection at the interface between the two materials. The calculated spectra in this two-stage scattering model with the modified gaps as fit parameters show good agreement with the experimental conductance measurements (Fig. 3e).

Bi-2212/Bi2Te3 experiments

To demonstrate the wide applicability of our mechanical bonding technique, we have constructed similar S–N junctions combining Bi-2212 from a different batch of crystals and Bi2Te3 grown by a different group (Princeton University) than that producing the Bi2Se3 (Rutgers University). Conductance measurements of low-barrier Bi-2212/Bi2Te3 junctions also clearly show the proximity-induced gap in Bi2Te3, as well as the reduced gap in Bi-2212 (Fig. 4a). The central Andreev feature due to the induced gap in the spectrum also appears immediately below Tc similar to Bi2Se3, with a conductance increase of nearly twice the normal value (Fig. 4b), and a width consistent with Andreev reflection in the proximity-induced region. We have constructed several Bi-2212/Bi2Te3 junctions with various induced gap sizes, and the measured spectra agree well with the calculations (Fig. 4b inset). One of the Bi-2212/Bi2Te3 junctions exhibited particularly large proximity-induced features in differential conductance (Fig. 4c), and the corresponding current-voltage DC measurement shows excess current as high as 90 μA (Fig. 4d). The proximity-related Andreev features and the excess current were observed repeatedly in several Bi-2212/Bi2Se3 (Fig. 4e) and Bi-2212/Bi2Te3 (Fig. 4f) devices.

Figure 4: Bi-2212/Bi2Te3 junction measurements.
figure 4

(a) AC differential conductance(dI/dV)S, normalized by the normal state conductance (dI/dV)N at 100 K, for a low-resistance Bi-2212/Bi2Te3 junction at different temperatures above and below Tc. The curves are shifted for clarity. (b) Temperature dependence of the zero-bias differential conductance for Bi-2212/Bi2Te3. Below Tc, the Andreev process enhances the conductance by almost a factor of 2. The inset shows 10 K measured (solid lines) and calculated (dashed lines) normalized differential conductance for several Bi-2212/Bi2Te3 devices with different values of the induced gap size. (c) AC differential conductance(dI/dV)S, normalized by the normal state conductance (dI/dV)N at 100 K, for a large-induced-gap Bi-2212/Bi2Te3 junction at different temperatures. The curves are shifted for clarity. (d) DC current–voltage characteristics of the large-induced-gap Bi-2212/Bi2Te3 junction, for different temperatures above and below Tc. (e) Measured normalized differential conductance for two Bi-2212/Bi2Se3 devices with different values of the induced gap size. The curves are shifted for clarity (f) Measured normalized differential conductance for two Bi-2212/Bi2Te3 devices with different values of the induced gap size. The curves are shifted for clarity.


To rule out alternative interpretations of our results, we considered various complications caused by the planar geometries of our devices, which are different from those based on point contacts that guarantee ballistic transport. In planar junctions, electron scattering can slightly affect the ballistic nature of the transport. This could have some effect on our measurements; nonetheless, it can only result in the observation of features at voltages that are a little higher than the true electron energies. However, this cannot produce an Andreev feature that is narrower than the true gap of the system. Furthermore, this effect cannot produce additional features in the spectra that we observe, which are consistent with the reduced gap at the interface. Moreover, to the extent that this effect is relevant, it could result in a measured bulk Bi-2212 gap that is bigger than the known gap size from photoemission and tunnelling studies. However, we observe a bulk gap in accord with previous measurements and that is fairly insensitive to the details of the contact (see Supplementary Discussion and Supplementary Fig. S2).

Previous low-Tc S–N planar-junction conductance experiments25 showed a narrow and small zero-bias conductance peak (several percent increase) due to pair currents. Later, a more detailed interpretation was suggested with a more complex Andreev reflection process including an electron-hole phase conjugation39. This phase conjugation model, however, predicts a rapid monotonic decrease of conductance at non-zero bias due to the different phases accumulated by quasiparticles. Therefore, this model is not applicable to the wide spectral features with complex shapes corresponding to the Andreev reflection at the induced gap interface observed in our experiments and in previous low-Tc work23,24,27,28. Moreover, our experimental results are also different from the low-Tc results25,39. Our measurements show a proximity-induced gap feature with a wide region, appearing immediately below Tc with almost twice the normal conductance (Figs 3c and 4b). Furthermore, our measured Andreev spectra show several distinct features related to the induced gap in the normal material and the reduced gap in the superconductor (similar to ref. 28). At lower temperatures, the central feature in our measured spectra is almost independent of the bias, which is consistent with the proximity-induced gap and different from the phase-conjugation model39.

Moreover, any possible ab-plane tunnelling in our devices can be ruled out for a number of experimental reasons. First, the areal coverage is much greater for c axis tunnelling then for ab-plane tunnelling. In fact, when the TI surfaces were rougher, we found no proximity effects, confirming that additional area for any possible ab-plane tunnelling did not change the spectra significantly. Second, in bulk Bi-2122, the c axis resistivity is very high only in the normal state, whereas in the superconducting state, all directions reveal zero resistance. Therefore, as the area perpendicular to the c axis is much larger than the exposed area normal to the ab plane, the overall current is along the c axis, eliminating any possible ab-plane effects. Last, our c axis tunnelling conductance measurements are very different from an ab-plane Andreev-bound state peak40. Specifically, our differential conductance measurements reveal a very wide central Andreev feature, which is almost independent of the bias. This is consistent with Andreev reflection in the normal material from an induced gap and is completely different from an Andreev-bound state. Furthermore, an Andreev-bound state-related peak in conductance should have a height that is independent of the barrier strength—Z. However, in our case, increasing Z results in a very different conductance spectrum shape. This is consistent with elimination of a proximity region at high Z and is inconsistent with a bound state.

To further verify that the Andreev scattering is due to the proximity-induced superconductivity in Bi2Se3, and dismiss any possible processes occurring in Bi-2212 alone, the Bi-2212/Bi2Se3 junction barrier was increased mechanically. Rapidly heating the junction to 300 K and cooling it back to 4 K in less than 30 min made it possible to achieve this. As expected, the differential conductance measurement performed on the high-resistance junction revealed only spectra typical of a c axis tunnelling measurement on Bi-2212 (ref. 35), indicating the lack of a proximity effect. Specifically, the superconducting gap of Bi-2212 was observed; however, no Andreev peak appeared at the Bi-2212 reduced gap Δr, and no conductance features corresponding to the proximity-induced gap in the Bi2Se3 were observed (Fig. 3d). The differential conductance spectrum for the high-resistance junction showed the superconducting gap of Bi-2212, Δ0, and its temperature dependence agrees with the values from the low-resistance junction measurement (Fig. 3c inset) as well as for the graphite junction (Supplementary Fig. S1), confirming the origin of the conductance peaks.

Our results present clear evidence of proximity-induced high-Tc superconductivity in Bi2Se3 and Bi2Te3. We achieved this via our newly developed technique of mechanically bonded junctions and confirmed it by various experiments. Specifically below Tc, we observed Andreev features corresponding to a reduced gap of Bi-2212 appearing concurrently with an induced gap in Bi2Se3 or Bi2Te3. The disappearance of these features in the same junctions when the barrier is increased further confirms that the proximity effect is the origin of these features. The proximity-induced superconductivity in Bi2Se3 and Bi2Te3 is demonstrated at temperatures at least an order of magnitude higher than previously reported results. The developed mechanical bonding technique may render various future experiments on novel materials, including high-Tc superconductors and TIs feasible. Furthermore, our proximity demonstration paves the way for practical realization of TI-based devices involving superconductivity, including Majorana fermion-based topological quantum computing.


Device fabrication

High-quality bulk Bi2Se3 and Bi2Te3 crystals were prepared as described elsewhere41. In addition, optimally doped Bi2Sr2CaCu2O8+δ (Bi-2212) crystals (Tc ≈85 K) were grown by the floating-zone method42. Bi-2212/Bi2Se3 and Bi-2212/Bi2Te3 junctions were fabricated by using these crystals as the starting materials. The formation of atomically smooth surfaces via cleaving is well established for Bi-2212, Bi2Te3 and Bi2Se3. Bi-2212 was cleaved in a dry nitrogen-purged box with adhesive tape (Fig. 1c), producing a clean, flat piece of Bi-2212 a few hundred micrometres thick. However, we have found the Bi2Te3 and Bi2Se3 surfaces were not smooth enough when simply cleaved using scotch tape. Thus, we cleaved a bulk Bi2Se3 or Bi2Te3 crystal in the dry box by sandwiching the crystal between two glass slides with double-sided tape. After applying slight pressure to the glass slides, the top glass slide was lifted off (Fig. 1d). This approach was taken due to the difficulty of producing flat surfaces by cleaving Bi2Se3 or Bi2Te3 with scotch tape, as was done for Bi-2212. Next, the Bi2Se3 or Bi2Te3 was transferred to a Cu sample holder by attaching a double-sided tape to the Cu and placing one of the glass slides with Bi2Se3 or Bi2Te3 on the sample holder. By lifting off the glass slide, a smooth and fresh surface of Bi2Se3 or Bi2Te3 was left on the sample holder. The Bi-2212 was then attached to the Bi2Se3 or Bi2Te3 by placing the Bi-2212 on the Bi2Se3 or Bi2Te3, applying GE varnish on the corners of Bi-2212 (Fig. 1e). In the fabrication process, no pressure is required and the junction is formed spontaneously. The important point is cleaving the materials in a dry and inert atmosphere (a nitrogen-purged glove box in our case) with a technique that ensures large, atomically smooth surfaces. Contacts were made on the sample using Cu wires and Ag epoxy or Ti/Au (Fig. 1f). This method was inspired by the mechanical exfoliation technique; however, no exfoliation is needed here, and bulk-cleaved crystals are attached mechanically using GE varnish on the corners. Low-barrier junctions were only revealed when the Bi2Se3 and Bi2Te3 were cleaved using our glass-slide technique. When simply cleaving the samples with scotch tape, all junctions revealed high-barrier behaviour, specifically the resistance across the junction was high (greater than tens of kiloOhms at room temperature) and no Andreev features were observed upon cooling.

Measurement set-up description

In our set-up, four-point probe measurements were performed in a liquid He-flow cryostat at different temperatures ranging from 295 to 4.5 K. Bias-dependent differential conductance as well as current versus voltage were measured on the junctions using two lock-in amplifiers (Stanford Research Systems SR810), a DC voltage source (BK Precision 1787B), two DC multi-meters (Hewlett Packard 3457A and Agilent 34401A) and a home-built, shielded AC+DC adder box (Fig. 1g). One of the lock-in amplifiers was used to produce a small AC voltage output at a frequency near 1 kHz. The AC voltage was added to the DC output of a DC power supply using a transformer-based adder. A voltage was applied to the sample, with the resulting voltage measured with a lock-in (the AC part) and a multimetre (the DC part). At the same time, the current was converted to voltage, amplified with a pre-amplifier (SRS 570) and measured with another lock-in (AC) and a multimetre (DC).

Additional information

How to cite this article: Zareapour, P. et al. Proximity-induced high-temperature superconductivity in the topological insulators Bi2Se3 and Bi2Te3. Nat. Commun. 3:1056 doi: 10.1038/ncomms2042 (2012).