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Interface charge transfer and electron–phonon coupling have been suggested to play a crucial role in the recently discovered high-temperature superconductivity of single unit-cell FeSe films on SrTiO3. However, their origin remains elusive. Here, using ultraviolet photoemission spectroscopy and element-sensitive X-ray photoemission spectroscopy, we identify the strengthened Ti–O bond that contributes to the interface enhanced electron–phonon coupling and unveil the band bending at the FeSe/SrTiO3 interface that leads to the charge transfer from SrTiO3 to FeSe films. We also observe band renormalization that accompanies the onset of superconductivity. Our results not only provide valuable insights into the mechanism of the interface-enhanced superconductivity, but also point out a promising route toward designing novel superconductors in heterostructures with band bending-induced charge transfer and interfacial enhanced electron–phonon coupling.

Using a cryogenic scanning tunneling microscopy, we report the signature of topologically nontrivial superconductivity on a single material of β-Bi2Pd films grown by molecular beam epitaxy. The superconducting gap associated with spinless odd-parity pairing opens on the surface and appears much larger than the bulk one due to the Dirac-fermion enhanced parity mixing of surface pair potential. Zero bias conductance peaks, probably from Majorana zero modes (MZMs) supported by such superconducting states, are identified at magnetic vortices. The superconductivity exhibits resistance to nonmagnetic defects, characteristic of time-reversal-invariant topological superconductors. Our study reveals β-Bi2Pd as a prime candidate for topological superconductor. PACS numbers: 74.55.+v, 68.65.-k, 74.25.Ha, 74.25.Jb Topological superconductors (TSCs) are a novel quantum phase of matter characterized by a fully gapped bulk state and gapless boundary states hosting exotic Majorana fermions that are their own anti-particles [1]. The Majorana fermions obey non-Abelian braiding statistics and could be useful for fault-tolerant quantum computors [2, 3]. Following theoretical proposals[4-8], several experiments have disclosed their signatures in semiconductor nanowires [9, 10], iron atomic chains [11] and topological insulators [12, 13] by proximity to superconductors, all sharing complex hybrid heterostructures. Alternatively, the newly discovered single-component superconductors, such as Cu/Sr/Nb-doped Bi 2 Se 3 [14-16], In-doped SnTe [17] and PbTaSe 2 [18], have been suggested as potential TSC candidates, but far from a final conclusion [19].Tetragonal Bi 2 Pd (hereafter, β-Bi 2 Pd) crystallizes into a simple CuZr 2 -type (I4/mmm) structure [ Fig. 1(a)], and exhibits classical s-wave bulk superconductivity with a transition temperature (T c ) close to 5.4 K [20]. Intriguingly, it was recently demonstrated from angleresolved photoemission spectroscopy (ARPES) that β-Bi 2 Pd holds several topologically protected surface bands cross the Fermi level (E F ) [21]. The nontrivial surface states of β-Bi 2 Pd are subject to a classical s-wave bulk pairing, which naturally satisfies the key ingredients of proximity-induced two-dimensional (2D) topological superconductivity near the surface [4]. Here the proximityinduced electron pairing on the spin-momentum-locked topological surface has a nontrivial topology and is obliged to be effectively spinless p-wave so as to guarantee the pair wave function antisymmetric [4,22,23]. Such superconducting states are anticipated to carry Majorana zero mode (MZMs) at the end of magnetic vortex lines, and thus reignite numerous research interests in β-Bi 2 Pd. However, the subsequent studies consistently reveal a conventional s-wave superconductivity [24][25][26] and no MZM at vortices of β-Bi 2 Pd single crystals [27]. In this work, we used a state-of-the-art molecular beam epitaxy (MBE) in ultrahigh vacuum (UHV) to prepare β-Bi 2 Pd thin films on SrTiO 3 (001) substrate and characterized their supercond...