Abstract
In the interfacial superconductor Bi_{2}Te_{3}/Fe_{1+y}Te, two dimensional superconductivity occurs in direct vicinity to the surface state of a topological insulator. If this state were to become involved in superconductivity, under certain conditions a topological superconducting state could be formed, which is of high interest due to the possibility of creating Majorana fermionic states. We report directional pointcontact spectroscopy data on the novel Bi_{2}Te_{3}/Fe_{1+y}Te interfacial superconductor for a Bi_{2}Te_{3} thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe_{1+y}Te film at an atomically sharp interface. Our data show highly unconventional superconductivity, which appears as complex as in the cuprate high temperature superconductors. A very large superconducting twingap structure is replaced by a pseudogap above ~12 K which persists up to 40 K. While the larger gap shows unconventional order parameter symmetry and is attributed to a thin FeTe layer in proximity to the interface, the smaller gap is associated with superconductivity induced via the proximity effect in the topological insulator Bi_{2}Te_{3}.
Introduction
Tailoring a topological superconductor by combining the topologically protected surface states of a TI with a superconductor via the proximity effect is of enormous theoretical and technological interest, principally due to the possibility of finding the Majorana fermionic states which are predicted to exist in the vortex cores of topological superconductors^{1,2}. A novel Bi_{2}Te_{3}/Fe_{1+y}Te heterostructure^{3} may represent a promising material. Although both parent materials are nonsuperconducting, the interface becomes a 2D superconductor which undergoes a characteristic 2D BerezinskiKosterlitzThouless (BKT) superconducting transition^{4,5,6}. Bi_{2}Te_{3} is a 3D topological insulator (TI) whose surface states consist of a single Dirac cone at the Γ point^{7}, while Fe_{1+y}Te is the parent compound of the ‘11’ family of Febased superconductors. Despite the exact mechanism for superconductivity in Bi_{2}Te_{3}/Fe_{1+y}Te remaining unknown, it has been shown previously that any doping effect by O, Bi or Te impurities can be excluded^{3}. Evidence that TI surface states play a role in the emergence of superconductivity is found in the fact that a critical thickness of the Bi_{2}Te_{3} layer is required^{3}. As the number of Bi_{2}Te_{3} quintuple layers (QL) increases, T_{c} rises from ~1.2 K (1 QL) to 12 K (5 QL) and then saturates. This correlates with the 5 QL critical thickness required to form a fullydeveloped surface state in Bi_{2}Te_{3}^{8,9}. The heterostructure may thus represent a model system to study proximityinduced topological superconductivity in the Bi_{2}Te_{3} layer, and a highly complex superconducting mechanism is likely involved in the interfacial superconductivity.
In this article, we report directional pointcontact data measured for current injection (a) parallel to the interface into the edge of the heterostructure, simultaneously probing both layers and (b) perpendicular to the interface into the top Bi_{2}Te_{3} layer. We observe a pronounced twin gap structure with a large gap of unconventional pairing symmetry, corresponding to the interfacial superconductivity, and a smaller gap which opens below the characteristic BKT temperature and is associated with proximityinduced superconductivity in the topologically insulating Bi_{2}Te_{3} layer. In addition, a pseudogap is observed which extends up to 40 K.
Pointcontact spectroscopy is an energyresolved technique directly probing the amplitude, symmetry and temperaturedependence of the superconducting gap^{10}. Our data were acquired using bilayers of Bi_{2}Te_{3}(9QLs)/Fe_{1+y}Te, chosen for their high T_{c} = 12 K. The VanderWaals bonding between the materials results in extremely high quality atomicallysharp interfaces.
Figure 1 displays the temperaturedependent resistance of the heterostructure, as measured with 4 contacts established by silverloaded paint on top of the Bi_{2}Te_{3} surface. An insulatortometal transition is visible in the form of a broad maximum at 76 K, which we associate with the antiferromagnetic transition of the bulk Fe_{1+y}Te layer. At lower temperatures, the resistance passes through a minimum at 24 K before increasing steeply. The overall resistive behavior (including this increase) is typical for FeTe with a high content of interstitial excess Fe^{11}, and can therefore be attributed to the bulk FeTe layer of our heterostructure. Indeed, highresolution energydispersive Xray spectroscopy in a scanning transmission electron microscope indicates that y = 0.15 ± 0.02 in our Fe_{1+y}Te bulk layer. It should be noted that FeTe does not exist in stoichiometric form and always has some excess Fe in the form of individual interstitial ions^{12}. Superconductivity in the vicinity of the Bi_{2}Te_{3}/Fe_{1+y}Te interface reveals itself by a rapid resistance decrease below T_{c} = 12 K until zero resistance is reached at T_{0} = 8 K. From Fig. 1, the overall behavior of our heterostructure is in good agreement with our previous work^{3}, in which we have shown that the broadness of the resistance drop associated with the superconducting transition between 12 K and 8 K is intrinsic, and the transition falls into the 2DXY universality class of a BKT transition. It is important to note that the resistance is composed of 3 parallel components: the Bi_{2}Te_{3} layer, the bulk Fe_{1+y}Te layer and the intermediate thin interfacial layer. The resistance drop below 12 K is associated with the diverging superconducting correlation length in the interfacial layer upon approaching the BKT transition. However, 12 K is not necessarily the onset of the superconducting transition: the rapidly increasing resistance of the interface will cause the current to gradually move away from the interfacial region into the 140 nm thick bulk Fe_{1+y}Te layer, which then begins to shunt the interface resistance. Therefore, the normal state resistance of the interface will be largely hidden due to the large Fe_{1+y}Te thickness (140 nm) relative to that of the thin superconducting interfacial layer.
Figure 2a shows the temperature dependent pointcontact spectra for the nanocontact on the heterostructure edge. At the highest temperatures a smooth parabolic background is seen, with little difference in data acquired at 70, 50 and 40 K. Below 40 K a pseudogap develops symmetrically around V_{b} = 0, gradually deepening as the temperature falls. Until 15 K, the gap is rounded at low energy, but at 12 K the conductance flattens around V_{b} = 0 prior to the emergence of a zero bias conductance peak (ZBCP) below ~10 K. Concurrently, shoulderlike structures develop at ~10 mV and ~5 mV, which as we will now demonstrate correspond to a phasecoherent superconducting twingap structure^{13}.
To fit the temperature dependence of our data, we primarily employ a modified BlonderTinkhamKlapwijk (BTK) model for finitetransparency tunnel junctions^{14}, excluding the energy range of the ZBCP. Our fits are based on a 1D BTK model for simplicity, since higherdimensional models are equivalent to the 1D case except for small shifts in the barrier heights Z. In such a highly twodimensional superconductor, fluctuations are expected to significantly reduce the quasiparticle lifetime; we account for this in our model with an energydependent Dynes parameter^{15} of the form Γexp[(│V_{b}│−Δ)/W] where Γ and W are free parameters. The shoulders at ~5 and ~10 mV are modeled by an anisotropic twoband swave order parameter Δ(α+(1 − α) cos θ)^{16}, in which we determine the gap anisotropies α_{1,2} from a fit at our lowest achievable temperature 0.27 K, then fix α_{1,2} at these values for all other temperatures.
The fitting results are shown in Fig. 2b: we are able to accurately reproduce our experimental data – including the doublegap structure – across the entire energy range. Since our BTK fits indicate large barrier heights (Z_{1} ≥ 0.35 for Δ_{1} and Z_{2} ~ 1000 for Δ_{2}), we also attempt to model our data using a twoband Dynes model (with a metallic conduction component to compensate for the lower barrier Z_{1}). Both models yield similar results: at 0.27 K, Δ_{1} = 6 meV and Δ_{2} = 12 meV from the BTK model, while Δ_{1} = 4 meV and Δ_{2} = 13 meV from the Dynes fit. Δ_{1} exhibits a pronounced anisotropy α ~ 0.7, whereas Δ_{2} is approximately isotropic (α = 1). In each case, Δ_{2} provides the dominant contribution to the spectral weight: 60 ± 5% versus 40 ± 5% for Δ_{1} in the BTK model, compared with a Δ_{1}:Δ_{2}:metallic ratio of 8 ± 0.5%: 46 ± 4% : 46 ± 4% in the Dynes model. The smaller values for Δ_{1} and its spectral weight from the Dynes fit are due to nonnegligible Andreev reflections, which cannot be perfectly simulated by the metallic component within this model. To account for the presence of the ZBCP, we also attempted to reproduce this data using a twogap dwave model, but no improvement of the fit or significant variation of the gap values were observed compared to the anisotropic swave case. Δ_{1} closes at 8 K while Δ_{2} appears to close at 40 K. The magnitude of Δ_{2} abruptly increases above 8 K. As we will demonstrate later, Δ_{2} actually consists of a large superconducting gap which transforms continuously into a pseudogap in the temperature range between 8 and 12 K. The pseudogap then closes at 40 K.
In Fig. 3 we present the differential conductance upon injecting the current through a scanning probe tip on the Bi_{2}Te_{3} surface. The current was injected perpendicular to the film and hence only probes the Bi_{2}Te_{3} layer. A small 5.4 meV gap dominates the conductance at low temperature, in excellent agreement with the small gap Δ_{1} observed upon injecting the current into the edge of the interface. The conductance saturates for V_{b} > 6 mV and no signature of the larger gap Δ_{2} is observed. This suggests that the Bi_{2}Te_{3} layer becomes superconducting and is responsible for Δ_{1}, similar to what has been observed in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}/Bi_{2}Se_{3}^{17,18}, Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}/Bi_{2}Te_{3}^{18} or Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}/Bi_{2}Te_{2}Se^{19} junctions grown on a cuprate hightemperature superconductor, or in proximity contact with classical superconductors^{20}. Here it should be noted that certain other groups have reported the absence of a proximity effect in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}/Bi_{2}Se_{3} junctions^{21,22}. Additional point contact data can be found in the supplementary information.
In Fig. 4 we plot the pointcontact spectra in 0 and 15 T applied (a) perpendicular and (b) parallel (field perpendicular to the current injection direction) to the film plane. Note that for technical reasons, different edge contacts on the same sample have been used, and the spectra in Fig. 4b are somewhat broader but show qualitatively the same features. The gap structure and ZBCP are quite robust with respect to magnetic fields, irrespective of their orientation. The spectra hardly change in 15 T: the gap becomes marginally shallower, but the gap energies Δ_{1,2} do not shrink significantly. This resilience of the overall gap structure demonstrates that the Cooper pairing strength is almost impervious to strong fields, in direct contrast with the critical field H_{c2} = 17 T which has been estimated from resistance data^{3}. We deduce that H_{c2} merely corresponds to a fieldinduced loss of phase coherence. We observe reductions in the ZBCP height of 20% and 30% in 15 T applied inplane and perpendicular to the interface, respectively, measured with respect to the minimum in the dI/dV curve. The ZBCP does not experience any splitting, regardless of the magnetic field orientation. In Fig. 4c we show the effect of a 9T magnetic field on the pseudogap: a magnetic field has a weak suppressing effect at temperatures up to 40 K.
The large size of Δ_{2} ≥ 12 meV is a striking feature of our Bi_{2}Te_{3}/Fe_{1+y}Te heterostructure. It exceeds the superconducting gap in bulk FeSe^{23} or FeSe_{1−x}Te_{x}^{24} by at least a factor of 4, despite the T_{c} of our heterostructures being comparable to T_{c} in these bulk materials. Our data constitute a demonstration of the potential for strongcoupling superconductivity which could persist up to far higher temperatures than the critical temperatures observed in the bulk ironchalcogenides. The origin of this gap enhancement is unclear, but since the presence of the Bi_{2}Te_{3} layer is essential for the appearance of superconductivity^{3}, it is possible that the topological surface states of Bi_{2}Te_{3} play a certain role.
Multiband superconductivity with various gaps has been reported in various Febased superconductors^{10,25,26} and is usually attributed to multiple electronic bands crossing the Fermi level in the same material. However, as we are probing the properties of the FeTe and Bi_{2}Te_{3} layers in parallel, the twingap feature could also originate from two spatially separated regions each with its corresponding electronic bands involved. Superconductivity induced by the proximity effect in the Bi_{2}Te_{3} layer could therefore play a role in the opening of the second gap. A proximityinduced small gap has previously been observed in devices of Bi_{2}Se_{3} and Bi_{2}Te_{3} which were mechanically bonded to the cuprate highT_{c} Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}: these showed a similar reduction of the gap in the cuprate when the TI became superconducting^{18}.
The temperature dependence of the gaps extracted from the BTK and Dynes fits is shown in Fig. 2c,d: it is clear that both models yield qualitatively identical results. Below 2 K the smaller gap Δ_{1} is approximately constant; upon increasing the temperature its magnitude decreases rapidly, reaching zero at ~8 K. In contrast, the larger gap Δ_{2} is almost temperatureindependent between 10 K and 25 K, gradually decreasing towards zero when the temperature is increased further up to 40 K. Between 10 K and 8 K, Δ_{2} is slightly reduced, which clearly correlates with the opening of Δ_{1}. Below 8 K, the sample exhibits zero resistance^{3} and is hence globally phasecoherent. Although the pronounced coherence peaks, which are characteristic of superconducting tunneling spectra, are absent from our data due to the short quasiparticle lifetimes imposed by lowdimensional fluctuations, we may nevertheless infer the presence of coherence by the sharp falls in dI/dV close to Δ_{1,2}. Above 8 K, global phase coherence is lost, the resistance gradually increases until the normal state is reached at 12 K and Δ_{1} vanishes, while Δ_{2} becomes a pseudogap which persists up to 40 K. The Dynes parameter Γ_{2} describing the quasiparticle lifetime in Δ_{2} also rises steeply above 8 K (Fig. 2e), supporting our observed loss of phase coherence above this temperature.
The edge contact data does not allow us to judge whether Δ_{2} closes at 8 K and is replaced by a pseudogap due to a competing order, or if it transforms continuously into the hightemperature pseudogap, thus suggesting a phaseincoherent superconducting (i.e. pairing) origin. A pronounced pseudogap state above the superconducting critical temperature is most famous in the cuprate high temperature superconductors^{27}, and its origin is still widely debated. In addition, various Febased superconductors display signatures of a pseudogap well above T_{c}^{10,24,28,29,30,31} and its origin has been suggested to be due to fluctuations of a nematic electronic order^{31,32,33}.
The pseudogap opens well below the antiferromagnetic transition at ~80 K of the bulk Fe_{1+y}Te layer as seen in the resistivity (Fig. 1)^{34} and thus does not appear to be related to its magnetic ordering. Similar to Sedoped FeTe, it is likely that the magnetic order in the interface region is weakened by charge transfer across the interface from the ndoped Bi_{2}Te_{3} layer. In addition, the topological surface state may contribute electrons with a strong spinorbit coupling, thus further suppressing the interfacial magnetic order and paving the way for the emergence of superconductivity. In Ba_{0.85}K_{0.15}Fe_{2}As_{2} it has been shown by angle resolved photoemission spectroscopy that the SDW order is associated with a ~20 meV gap which forms below the SDW ordering temperature^{35}. At low temperature the SDW gap is reduced concurrently with the onset of superconducting order, in a very similar manner to what we observe for the larger gap Δ_{2} above 8 K in Fig. 2c,d. A normal state origin is thus compatible with the increase of Δ_{2} above 8 K, which is suggestive of a replacement of the superconducting gap by a normal state pseudogap. This suggests that the pseudogap which we observe has a competitive relationship with superconductivity and is presumably related to SDW and/or nematic order, which are therefore likely to develop below ~40 K in the interface region.
On the other hand, we have previously shown that the interfacial superconductivity in our heterostructures lies in the extreme 2D limit and the resistance drop and IV characteristics can be perfectly modelled by a BKT transition^{3}. This represents a pure 2D phaseordering transition of Cooper pairs which are already formed at higher temperature, where the phase of the superconducting order parameter is stabilized below a characteristic temperature T_{BKT} (lying just above T_{0}), at which thermallyinduced vortices and antivortices are bound into pairs^{4,5,6}. This naturally implies the existence of phaseincoherent Cooper pairs within a certain temperature range above T_{BKT}, creating a pseudogap in the density of states. The 10 meV magnitude of this superconducting gap indicates a potential for strong coupling superconductivity with stable Cooper pairing at temperatures well above 12 K. Superconducting fluctuations have been observed at temperatures many times higher than T_{c} in stronglyunderdoped layered cuprate HTSCs^{36,37}, where they contribute in part to the pseudogap formation. Furthermore, scanning tunneling spectroscopy on ultrathin titanium nitride films has shown that the strong phase fluctuations associated with twodimensionality can induce a pseudogap in conventional superconducting films at temperatures up to 14 times T_{c}^{38}. A superconducting origin for the pseudogap in the interfacial superconductor Bi_{2}Te_{3}(9QLs)/Fe_{1+y}Te is supported by the fact that the pseudogap is partially suppressed by a magnetic field of 9 T in temperatures up to 40 K (Fig. 4c), which could be a consequence of pairbreaking effects and is not expected for the SDW gap in an Fe based superconductor. If the pseudogap had an entirely phaseincoherent superconducting origin, then the reduction of the gap upon transformation into a real superconducting gap Δ_{2}below ~8 K could be caused by the opening of the proximityinduced gap Δ_{1} in the Bi_{2}Te_{3} layer, similar to the reduction of the superconducting gap observed in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} in proximity contact to a TI^{18}.
However, in a pseudogap which is caused entirely by fluctuations of the superconducting order parameter, it is expected that the zerobias conductance G_{N} should vary linearly with ln(ln(T/T_{0}))^{38,39}, where T_{0} is the phase coherence temperature, represented by the establishment of zero resistivity at 8 K. From Fig. 5 this trend is not observed in our data, thus suggesting that a competing normalstate pseudogap at least partially contributes to the spectra. The most likely explanation of the pseudogap is thus a mixture of a normal state pseudogap (e.g. caused by interfacial SDW or nematic order) and a phase incoherent superconducting pseudogap caused by the strong phase fluctuations of a 2D superconductor.
As demonstrated by our point contact data on the top of the Bi_{2}Te_{3} layer, interfacial contact with the Fe_{1+y}Te induces superconductivity in Bi_{2}Te_{3}, which is therefore a potential candidate to host a topological superconducting state^{40}. The combination of superconductivity with the nontrivial topological symmetry of the surface states in a TI naturally evokes the question whether the ZBCP could be caused by Majorana bound states^{41,42,43}. Care has been taken to eliminate any spurious origin for the ZBCP, e.g. heating or proximity effects^{44}: (1) our highresistance pointcontact lies comfortably within the ballistic spectroscopic tunneling regime, (2) the spectra were verified to be identical upon increasing and decreasing the tunnel current and (3) the ZBCP width remains roughly constant at all temperatures below 8 K (Fig. 2a), despite the gap energy Δ_{1} increasing from zero to 5 meV within this temperature range. The ZBCP is therefore of intrinsic origin. The proximity effect of an swave superconductor on a TI^{1} creates a fully gapped energy spectrum without any ingap states; in this case, Majorana modes would only be created in the vortex cores in an applied magnetic field. Since the pairing symmetry in Bi_{2}Te_{3}(9QLs)/Fe_{1+y}Te remains unclear, two possible mechanisms exist for the formation of Majorana edge states. One possibility is that the contact with FeTe drives the Bi_{2}Te_{3} layer to become an intrinsic topological superconductor with Majorana surface states^{40,41,45,46}, e.g. by a charge transfer effect, instead of a proximityinduced superconductor. This would explain the ZBCP observed in the point contact spectra of both the edge (Fig. 2) and the top contacts (Fig. 3). In a magnetic field the ZBCP should be suppressed due to broken timereversal symmetry: a reduction in the ZBCP height is indeed observed in high magnetic fields, but the effect is only ~20% in 14T. This could be a consequence of a particularly strong Rashba spinorbit coupling at the interface as well as the extremely high critical field for pairing. A detailed theoretical study of the possible pairing symmetries in Bi_{2}Te_{3} would be required to confirm this possibility. The alternative mechanism requires nodal d_{x}^{2}_{−}_{y}^{2} superconductivity (associated with the Fe_{1+y}Te) combined with Rashba spinorbit coupling (enhanced by the topological surface states of Bi_{2}Te_{3})^{47}. Although a dwave order parameter alone could create a fermionic ZBCP at the sample edge, a ZBCP composed of the fermionic edge states and Majorana fermions should form in the presence of strong spinorbit coupling at zero field. It has been predicted that an inplane applied field will split and shift the fermionic states to finite energy via the Zeeman effect, while the Majorana state remains at zero energy^{47}. However, no splitting of the ZBCP is observed up to 15 T for applied fields parallel or perpendicular to the interface (Fig. 4a,b), and its height is rather small, thus rendering such a dwave scenario unlikely. Furthermore, the ZBCP visible in our top contact (Fig. 3) is at odds with a dwave scenario.
A more conventional explanation for the ZBCP is related to the large Fe excess in the Fe_{1+y}Te layer: scanning probe measurements have recently shown that excess iron in Fe(Te,Se) superconductors causes pronounced local ingap states, which do not split, shift or vanish in applied magnetic fields^{48}. However, the presence of a ZBCP when tunneling directly into the Bi_{2}Te_{3} layer (in which Fe impurities are absent) discourages this interpretation and rather points to an unconventional pairing mechanism as its origin.
A similar ZBCP was observed in Bi2212/Bi_{2}Te_{2}Se junctions with a very similar temperature and magnetic field dependence as in our heterostructure^{19}. This suggests that such a ZBCP could be an universal feature of interfaces between unconventional superconductors and topological insulators. Its origin remains a mystery and requires further experiments such as angle resolved photoemission to clarify the exact electronic density of states in Bi_{2}Te_{3}/Fe_{1+y}Te heterostructures.
Our Bi_{2}Te_{3}(9QLs)/Fe_{1+y}Te heterostructures reveal a highly unusual superconducting state with an extraordinarily large superconducting gap, a pronounced pseudogap and compelling evidence for proximityinduced superconductivity in the topological insulating Bi_{2}Te_{3} top layer. Our experiments alone are not able to prove the topological nature of this superconductivity, nor provide an indisputable explanation of the origin of the interfacial superconductivity. Nevertheless, it is clear that Bi_{2}Te_{3}(9QLs)/Fe_{1+y}Te interfaces display a similarly rich behavior to the cuprate superconductors, which remain one of the major unsolved mysteries in physics.
Methods
Film growth
The heterostructure studied in this work was synthesized by a VGV80H MBE system. A 50 nm ZnSe buffer was first grown on a GaAs(100) semiinsulating substrate. A 140 nm thick FeTe layer is then deposited on the buffer layer, followed by a 9QL thick Bi_{2}Te_{3} film. Detailed information about the quality and characterization of this interface, including scanning transmission microscope micrographs and resistivity data may be found in Ref. 3. Our data were acquired using bilayers of Bi_{2}Te_{3}(9QLs)/Fe_{1+y}Te, chosen for their high T_{c} = 12 K.
Device fabrication and point contact spectroscopy
To fabricate a pointcontact device on the edge of the bilayer, a thin slab was attached to a silicon substrate with one edge facing upwards. Ordinary lowOhmic contacts were prepared by RS 186–3593 silver conductive paint on the Bi_{2}Te_{3} surface. The edge of the sample was finely polished, instantly covered by a thin layer of Au, and an isolated 100 nm wide Au strip was separated using a focusedionbeam. The maximum contact area is ~100 × 149 nm^{2} (from the width of the Au strip and the total thickness of the FeTe/Bi_{2}Te_{3} bilayer, respectively). A schematic drawing of the contact configuration is shown in Fig. 6. In order to reduce the effect of Ag pollution at the interface through our surface electrical contacts, we prepared all surface electrical contacts at a distance of at least 1 mm from the point contact junctions.
The edge contacts typically had resistances in the kΩ range and the data reproducibility was verified on 4 different devices. The pointcontact spectral shape is strongly dependent on the tunnel barrier height parameter Z^{10}, which is linked to the contact resistance. Z = 0 corresponds to pure Andreev reflection, while larger values represent the spectroscopic tunneling regime. In our contacts we consistently achieve Z ≥ 0.35 and our nanoscale contact area ensures that our experimental tunneling regime is ballistic and not thermal or diffusive, i.e. the applied bias voltage V_{b} corresponds to the electron injection energy. This is confirmed by the temperatureindependent value of the normalstate contact resistance. A point contact with similar properties was established on the Bi_{2}Te_{3} surface for perpendicular current injection with the help of a scanning probe device, by gently approaching a tungsten tip to the Bi_{2}Te_{3} surface. The differential conductance dI/dV vs V_{b} was measured at temperatures from 0.27 K to 70 K in magnetic fields up to 15 T with a quasifourprobe method, using a Keithley 6221 AC/DC current source to generate a small, constantamplitude (10 nA) AC current I_{AC} with frequency 5 Hz, superposed on a ramped DC bias current. A standard lockin technique in combination with a DC multimeter was used to measure dI/dV and V_{b} = V_{DC} across the junction.
Additional Information
How to cite this article: He, M. Q. et al. Pseudogap and proximity effect in the Bi_{2}Te_{3}/Fe_{1+y}Te interfacial superconductor. Sci. Rep. 6, 32508; doi: 10.1038/srep32508 (2016).
References
 1.
Fu, L. & Kane, C. L. Superconducting proximity effect and majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008).
 2.
Xu, J.P. et al. Experimental detection of a Majorana mode in the core of a magnetic vortex inside a topological insulatorsuperconductor Bi_{2}Te_{3}/NbSe_{2} heterostructure, Phys. Rev. Lett. 114, 017001 (2015).
 3.
He, Q. L. et al. Twodimensional superconductivity at the interface of a Bi_{2}Te_{3}/FeTe heterostructure. Nat. Commun. 5 4247 (2014).
 4.
Kosterlitz, J. M. & Thouless, D. M. Ordering, metastability and phase transitions in twodimensional systems. J. Phys. C 6, 1181–1203 (1973).
 5.
Kosterlitz, J. M. The critical properties of the twodimensional XY model. J. Phys. C 7, 1046–1060 (1974).
 6.
Berezinskii, V. L. Destruction of longrange order in onedimensional and twodimensional systems having a continuous symmetry group I. classical systems. Sov. Phys. JETP 32, 493–500 (1971).
 7.
Chen,Y. L. et al. Experimental realization of a threedimensional topological insulator, Bi_{2}Te_{3}. Science 325, 178–181 (2009).
 8.
Li, Y.Y. et al. Intrinsic topological insulator Bi_{2}Te_{3} thin films on Si and their thickness limit. Adv. Mater. 22, 4002–4007 (2010).
 9.
Taskin, A. A., Sasaki, S., Segawa, K. & Ando, Y. Manifestation of topological protection in transport properties of epitaxial Bi_{2}Se_{3} thin films. Phys. Rev. Lett. 109, 066803 (2012).
 10.
Daghero, D., Tortello, M., Ummarino, G. A. & Gonnelli, R. S. Directional pointcontact Andreevreflection spectroscopy of Febased superconductors: Fermi surface topology, gap symmetry, and electron–boson interaction. Rep. Prog. Phys. 74, 124509 (2011).
 11.
Koz, C., Rößler, S., Tsirlin, A. A., Wirth, S. & Schwarz, U. Lowtemperature phase diagram of Fe_{1+y}Te studied using xray diffraction. Phys. Rev. B 88, 094509 (2013).
 12.
Bao, W. et al. Tunable (δπ, δπ)Type Antiferromagnetic Order in αFe(Te,Se) Superconductors. Phys. Rev. Lett. 102, 247001 (2009).
 13.
Eskildsen, M. R. et al.Vortex imaging in magnesium diboride with H⊥c. Phys. Rev. B 68, 100508 (2003).
 14.
Blonder, G. E., Tinkham, M. & Klapwijk, T. M. Transition from metallic to tunneling regimes in superconducting microconstrictions: excess current, charge imbalance, and supercurrent conversion. Phys. Rev. B 25, 4515 (1982).
 15.
Dynes, C., Naraynamurti, V. & Garno, J. P. Direct measurement of quasiparticlelifetime broadening in a strongcoupled superconductor. Phys. Rev. Lett. 41, 1509 (1978).
 16.
Petrović, A. P. et al. Multiband superconductivity in the Chevrel phases SnMo_{6}S_{8} and PbMo_{6}S_{8}. Phys. Rev. Lett. 106, 017003 (2011).
 17.
Wang, E. et al. Fully gapped topological surface states in Bi_{2}Se_{3} films induced by a dwave hightemperature superconductor. Nat. Phys. 9, 621–625 (2013).
 18.
Zareapour, P. et al. Proximityinduced hightemperature superconductivity in the topological insulators Bi_{2}Se_{3} and Bi_{2}Te_{3}. Nat. Commun. 3, 1056 (2012).
 19.
Zareapour, P. et al. Evidence for a new excitation at the interface between a highT_{c} superconductor, and a topological insulator, Rev. B 90, 241106 (R) (2014).
 20.
Zhang, D. et al. Superconducting proximity effect and possible evidence for Pearl vortices in a candidate topological insulator. Phys. Rev. B 84, 165120 (2011).
 21.
Yilmaz, T. et al. Absence of a proximity effect for a thinfilms of a Bi_{2}Se_{3} topological insulator grown on top of a Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} cuprate superconductor. Phys. Rev. Lett. 113, 067003 (2014).
 22.
Xu, S.Y. et al. Fermilevel electronic structure of a topologicalinsulator/cupratesuperconductor based heterostructure in the superconducting proximity effect regime. Phys. Rev. B 90, 085128 (2014).
 23.
Song, C.L. et al. Suppression of superconductivity by twin boundaries in FeSe. Phys. Rev. Lett. 109, 137004 (2012).
 24.
Kato, T. et al. Local density of states and superconducting gap in the iron chalcogenide superconductor Fe_{1+δ}Se_{1−x}Te_{x} observed by scanning tunneling spectroscopy. Phys. Rev. B 80, 180507 (2009).
 25.
Hardy, F. et al. Calorimetric evidence of multiband superconductivity in Ba(Fe_{0.925}Co_{0.075})_{2}As_{2} single crystals. Phys. Rev. B 81, 060501 (R) (2010).
 26.
Tortello, M. et al. Multigap superconductivity and strong electronboson coupling in Febased superconductors: a pointcontact Andreevreflection study of Ba(Fe_{1−x}Co_{x})_{2}As_{2} single crystals. Phys. Rev. Lett. 105, 237002 (2010).
 27.
Timusk, T. & Statt, B. The pseudogap in hightemperature superconductors: an experimental survey. Rep. Prog. Phys. 62, 61–122 (1999).
 28.
Xu, Y.M. et al. Fermi surface dichotomy of the superconducting gap and pseudogap in underdoped pnictides. Nat. Commun. 2, 392 (2011).
 29.
Shimojima, T. et al. Pseudogap formation above the superconducting dome in iron pnictides. Phys. Rev. B 89, 045101 (2014).
 30.
Kwon, Y. S. et al. Evidence of a pseudogap for superconducting ironpnictide Ba_{0.6}K_{0.4}Fe_{2}As_{2} single crystals from optical conductivity measurements. New J. Phys. 14, 063009 (2012).
 31.
Arham, H. Z. et al. Detection of orbital fluctuations above the structural transition temperature in the iron pnictides and chalcogenides. Phys. Rev. B 85, 214515 (2012).
 32.
Chu, J.H. et al. Inplane resistivity anisotropy in an underdoped iron arsenide superconductor. Science 329, 824–826 (2010).
 33.
Harriger, L. W. et al. Nematic spin fluid in the tetragonal phase of BaFe_{2}As_{2}. Phys. Rev. B 84, 054544 (2011).
 34.
Karki, A. B. et al. Interplay between superconductivity and magnetism in Fe_{1−x}Pd_{x}Te. PNAS 110, 9283–9288 (2013).
 35.
Yi, M. et al. Dynamic competition between spindensity wave order and superconductivity in underdoped Ba_{1−x}K_{x}Fe_{2}As_{2}. Nat. Commun. 5, 3711 (2012).
 36.
Meingast, C. et al. Phase fluctuations and the pseudogap in YBa_{2}Cu_{3}O_{x}. Phys. Rev. Lett. 86, 1606 (2001).
 37.
Wang, Y. et al. The onset of the vortexlike Nernst signal above T_{c} in La_{2−x}Sr_{x}CuO_{4} and Bi_{2}Sr_{2−y}La_{y}CuO_{6}. Phys. Rev. B 64, 224519 (2001).
 38.
Sacépé, B. et al. Pseudogap in a thin film of a conventional superconductor. Nat. Commun. 1, 140 (2010).
 39.
Varlamov, A. A. & Dorin, V. V. Fluctuation resistance of Josephson junction, Soviet Phys. JETP 57, 1089–1096 (1983).
 40.
Schnyder, A. P., Ryu, S., Furusaki, A. & Ludwig, A. W. W. Classification of topological insulators and superconductors in three spatial dimensions. Phys. Rev. B 78, 195125 (2008).
 41.
Sasaki, S. et al. Topological Superconductivity in Cu_{x}Bi_{2}Se_{3}. Phys. Rev. Lett. 107, 217001 (2011).
 42.
Alicea, J. New directions in the pursuit of Majorana fermions in solid state systems. Rep. Prog. Phys. 75, 076501 (2012).
 43.
Beenakker, C. W. J. Search for Majorana fermions in superconductors. Annu. Rev. Con. Mat. Phys. 4, 113–136 (2013).
 44.
Sheet, G., Mukhopadhyay, S. & Raychaudhuri, P. Role of critical current on the pointcontact Andreev reflection spectra between a normal metal and a superconductor. Phys. Rev. B 69, 134507 (2004).
 45.
Fu, L. & Berg, E. Oddparity topological superconductors: theory and application to Cu_{x}Bi_{2}Se_{3}. Phys. Rev. Lett. 105, 097001 (2010).
 46.
Hsieh, T. H. & Fu, L. Majorana fermions and exotic surface Andreev bound states in topological superconductors: application to Cu_{x}Bi_{2}Se_{3}. Phys. Rev. Lett. 108, 107005 (2012).
 47.
Yuan, N. F. Q., Wong, C. L. M. & Law, K. T. Probing Majorana flat bands in nodal d_{x}^{2}_{−y}^{2}wave superconductors with Rashba spin–orbit coupling. Physica E 55, 30–36 (2014).
 48.
Yin, J.X. et al. Observation of a robust zeroenergy bound state in iron based superconductor Fe(Te,Se). Nat. Phys. 11, 543–546 (2015).
Acknowledgements
We thank M. L. Cohen, Y.R. Shen and S. G. Louie for stimulating discussions and U. Lampe for technical support. This work was supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (603010, 16304515, 604910, 602813, SEG_HKUST03, CRF3/HKUST/13G, FSGRF13SC23, FSGRF14SC25, SRFI11SC02).
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Affiliations
Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S.A.R., China
 M. Q. He
 , J. Y. Shen
 , Q. L. He
 , H. C. Liu
 , Y. Zheng
 , C. H. Wong
 , Q. H. Chen
 , J. N. Wang
 , K. T. Law
 , I. K. Sou
 & R. Lortz
CorreLab, Gerbang Institute for Complex Matter, 81560 Nusajaya, Johor, Malaysia
 A. P. Petrović
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Contributions
R.L. initiated this study; M.Q.H., R.L. and J.Y.S. designed the experiments; M.Q.H. and J.Y.S. conducted the experiment with contributions from Y.Z., C.H.W. and Q.H.C; Q.L.H. carried out the sample growth and structural characterization with contributions from H.C.L. and J.N.W.; A.P.P. performed the fitting analysis of the data with contributions from M.Q.H. and J.Y.S.; I.K.S. designed the material and initiated the sample growth; K.T.L. contributed the theoretical interpretation of the data and all authors contributed to the scientific planning and discussions. R.L. and M.Q.H. and A.P.P. wrote the manuscript with contributions from the other authors.
Competing interests
The authors declare no competing financial interests.
Corresponding author
Correspondence to R. Lortz.
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