Proximity-effect-induced Superconducting Gap in Topological Surface States – A Point Contact Spectroscopy Study of NbSe2/Bi2Se3 Superconductor-Topological Insulator Heterostructures

Abstract

Proximity-effect-induced superconductivity was studied in epitaxial topological insulator Bi2Se3 thin films grown on superconducting NbSe2 single crystals. A point contact spectroscopy (PCS) method was used at low temperatures down to 40 mK. An induced superconducting gap in Bi2Se3 was observed in the spectra, which decreased with increasing Bi2Se3 layer thickness, consistent with the proximity effect in the bulk states of Bi2Se3 induced by NbSe2. At very low temperatures, an extra point contact feature which may correspond to a second energy gap appeared in the spectrum. For a 16 quintuple layer Bi2Se3 on NbSe2 sample, the bulk state gap value near the top surface is ~159 μeV, while the second gap value is ~120 μeV at 40 mK. The second gap value decreased with increasing Bi2Se3 layer thickness, but the ratio between the second gap and the bulk state gap remained about the same for different Bi2Se3 thicknesses. It is plausible that this is due to superconductivity in Bi2Se3 topological surface states induced through the bulk states. The two induced gaps in the PCS measurement are consistent with the three-dimensional bulk state and the two-dimensional surface state superconducting gaps observed in the angle-resolved photoemission spectroscopy (ARPES) measurement.

Proximity-Induced Superconducting States of Magnetically Doped 3D Topological Insulators with High Bulk Insulation

“…However, the observed spectra resembled a V-shape. Similar V-shape dips have been reported in other TI/superconductor (TI/S) junctions [39,40], while the zero-bias conductance peaks have been also detected in TI/S junctions [17,37,41]. As mentioned in the introduction, TI/S junctions are expected to behave as a p-wave superconductor, and both the dip and the peak in the spectra possibly reflect this p-wave nature.…”

Observation of zero-bias conductance peak in topologically-trivial hybrid superconducting interfaces

“…As shown in figure 1(a), this induced condensate pair potential, Δ i , then decay with a characteristic length determined by the strength of decoherence or depairing properties of the induced layer. Experimentally, similar observations at hybrid interfaces involving TMs are confirmed using angle resolved photoemission spectroscopy (ARPES) and point contact spectroscopy [5,6]. Secondly, in some cases, the microscopic nature of the induced condensate pairs is found to differ from the singlet s-wave pairing observed in the bulk of SC films [7].…”

“…Additionally, we also propose that by simulating device B (TM/TB/NM2 in figure 1(b)), our model as a fitting routine can also help identify the dominant depairing mechanisms that strongly suppress induced superconductivity in the TM layer. This interest is driven by the fact that in many systems, these PIS states are observed to disappear at relatively small magnetic fields [5,6]. Here, the role of the atomic SO interaction to the depairing effects may be investigated.…”

“…Often, Blonder-Tinkham-Klapwijk (BTK) [15] formalism that considers anisotropic pairing potential with different orbital symmetries is widely used to model the quasiparticle density of states and to fit the differential conductance measurements using tunneling and point contact spectroscopy [5,6,11]. Here, the BTK model uses a phenomenological Dyne's parameter [16], g, to incorporate depairing effects caused by finite lifetime of the quasiparticle excitations.…”

Interface Characterization and Control of 2D Materials and Heterostructures

“…Through additional interfacial coupling with a superconductor, artificial topological superconductor heterostructures are predicted to host Majorana fermions for fault‐tolerant quantum computation . Toward this end, 2D heterostructures based on NbSe 2 /Bi 2 Se 3 and FeTe/Bi 2 Te 3 are being actively explored.…”