Topological insulators are a new class of material 1,2 , that exhibit robust gapless surface states protected by time-reversal symmetry 3,4 . The interplay of such symmetry-protected topological surface states and symmetry-broken states (for example, superconductivity) provides a platform for exploring new quantum phenomena and functionalities, such as one-dimensional chiral or helical gapless Majorana fermions 5 , and Majorana zero modes 6 that may find application in faulttolerant quantum computation 7,8 . Inducing superconductivity on the topological surface states is a prerequisite for their experimental realization 1,2 . Here, by growing high-quality topological insulator Bi 2 Se 3 films on a d-wave superconductor Bi 2 Sr 2 CaCu 2 O 8+δ using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi 2 Se 3 films with a large pairing gap up to 15 meV. Interestingly, distinct from the d-wave pairing of Bi 2 Sr 2 CaCu 2 O 8+δ , the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes.The search for exotic quantum phenomena and new functionalities has been among the most tremendous driving forces for the fields of condensed-matter physics and materials science. Majorana zero modes, that is, Majorana fermions that are their own antiparticles and occur at exactly zero energy, are particularly fascinating not only because of their intriguing physics obeying robust non-Abelian statistics, but also owing to their potential application as building blocks for topological quantum computers 7,8 . Although significant progress has been made recently in one-dimensional semiconductor quantum wires coupled with conventional superconductors 9-12 , decisive evidence of Majorana zero modes has been lacking and many puzzles remain 13 . Topological insulators, whose hallmark is time-reversal-symmetryprotected surface states, may offer less restrictive experimental conditions for realizing Majorana zero modes 1,2 . Theoretically, Majorana zero modes are predicted to occur in vortex cores of three-dimensional topological insulators when they are in close proximity to conventional s-wave superconductors 6 ; however,

The pairing mechanism of high-temperature superconductivity in cuprates remains the biggest unresolved mystery in condensed matter physics. To solve the problem, one of the most effective approaches is to investigate directly the superconducting CuO 2 layers. Here, by growing CuO 2 monolayer films on Bi 2 Sr 2 CaCu 2 O 8+δ substrates, we identify two distinct and spatially separated energy gaps centered at the Fermi energy, a smaller U-like gap and a larger V-like gap on the films, and study their interactions with alien atoms by low-temperature scanning tunneling microscopy. The newly discovered U-like gap exhibits strong phase coherence and is immune to scattering by K, Cs and Ag atoms, suggesting its nature as a nodeless superconducting gap in the CuO 2 layers, whereas the V-like gap agrees with the well-known pseudogap state in the underdoped regime. Our results support an s-wave superconductivity in Bi 2 Sr 2 CaCu 2 O 8+δ , which, we propose, originates from the modulation-doping resultant two-dimensional hole liquid confined in the CuO 2 layers.

We report on the observation of high-temperature (Tc) superconductivity and magnetic vortices in single-unit-cell FeSe films on anatase TiO2(001) substrate by using scanning tunneling microscopy. A systematic study and engineering of interfacial properties has clarified the essential roles of substrate in realizing the high-Tc superconductivity, probably via interface-induced electron-phonon coupling enhancement and charge transfer. By visualizing and tuning the oxygen vacancies at the interface, we find their very limited effect on the superconductivity, which excludes interfacial oxygen vacancies as the primary source for charge transfer between the substrate and FeSe films. Our findings have placed severe constraints on any microscopic model for the high-Tc superconductivity in FeSe-related heterostructures.PACS numbers: 74.70. Xa, 68.37.Ef, 74.62.Dh, 74.25.Uv The recent discovery of superconductivity with an exceptionally high critical temperature (T c ) over 65 K in single-unit-cell (SUC) FeSe films on SrTiO 3 has received extensive attention [1][2][3][4][5][6][7][8][9][10]. Distinct from iron pnictide and bulk FeSe compounds [11,12], the superconducting SUC FeSe films prepared on SrTiO 3 substrate not only possess a rather simple Fermi surface topology-only having electron pockets around the zone corner (M point) of Brillouin zone Fermi surface (E F ) [2,4,7], but also reach a record of T c values among all iron-based superconductors (Fe-SCs) [1][2][3][4]. In order to understand the enhancement in T c , several different scenarios invoking interface effects, such as interfacial electron-phonon coupling [1, 7, 9, 10], charge transfer prompted by interfacial oxygen vacancies [2][3][4]6], tensile strain effect induced by the lattice mismatch between FeSe and SrTiO 3 [4,13], screening effect by SrTiO 3 ferroelectric phonons [5], have been proposed. However, a consensus on which factors play the primary roles in enhancing the superconductivity of FeSe has not yet be achieved. The situation is further complicated by the observation of high-T c superconductivity in anisotropic SrTiO 3 (110) substrates [14,15] and heavily electron-doped FeSe-derived superconductors involving little interfacial effect [16][17][18][19][20].Attempt to separate the effects of substrate by preparing FeSe films on graphitized SiC(0001) leads to the discovery of two disconnected superconducting domes upon alkali-metal potassium doping [19]: a low-T c phase in undoped parent FeSe and a high-T c phase in heavily electron-doped regime. This points out a different pairing mechanism of high-T c phase from that in other Fe-SCs. Most importantly, the observed T c of ∼ 48 K and ∆ of ∼ 14 meV in these systems [16,17,19,20] imply that, to boost the higher T c in FeSe [1][2][3][4], the SrTiO 3 must impose additional effect(s) other than the electron doping.In order to clarify the above-mentioned controversies, it is highly desirable to find an alternative substrate that can host high-T c superconductivity and meanwhile allow a straightforward co...

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...

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