Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero energy end states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

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Bi 2 Se 3 is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the Bi 2 Se 3 layers, yielding an electron concentration of ~ 2 x 10 20

. Potential quantum computing and spintronic applications using these states require manipulation of their electronic properties at the Dirac energy of their band structure by inducing magnetism or superconductivity through doping and the proximity effect [6][7][8][9] . Yet, the response of these states near the Dirac energy in their band structure to various perturbations has remained unexplored. Here we use spectroscopic mapping with the scanning tunnelling microscope to study their response to magnetic and non-magnetic bulk dopants in Bi 2 Te 3 and Bi 2 Se 3 . Far from the Dirac energy, helicity provides remarkable resilience to backscattering even in the presence of ferromagnetism. However, approaching the Dirac point, where the surface states' wavelength diverges, bulk doping results in pronounced nanoscale spatial fluctuations of energy, momentum and helicity. Our results and their connection with similar studies of Dirac electrons in graphene [10][11][12][13] demonstrate that although backscattering and localization are absent for Dirac topological surface states, reducing charge defects is required for both tuning the chemical potential to the Dirac energy and achieving high electrical mobility for these novel states.Since the recent discovery of topological insulators, modifications of the bulk chemical compositions or deposition of surface adsorbates have been commonly used to tune their chemical potential into the bulk gap and close to the Dirac energy 3,[14][15][16][17][18] . Such efforts are motivated not only by attempts to maximize the ratio of surface to bulk conductivity, but also by the theoretical proposals for realizing yet more exotic states, such as Majorana fermions, based on topological insulators 6,8,9 . Simultaneous proximity to magnetism or superconductivity as well as tuning the chemical potential to the Dirac point of the surface state band structure are required to test these important predictions. It is often assumed that any chemical doping that is used to manipulate these states would not disrupt transport of topological surface states because of their topological protection against backscattering. However, experiments on graphene, which is by now a well-studied electronic Dirac system, have demonstrated that nearby defects (adsorbates or those in the substrate) can result in random local gating of such two-dimensional systems. This behaviour results in a residual concentration of induced carriers in graphene, hence preventing the chemical potential from uniformly reaching the Dirac energy throughout the system, and scattering that also limits the mobility of Dirac electrons in that system 19 . Whether such phenomena are also relevant for topological Dirac surface states that are protected by helical spin texture (as opposed to sub-lattice pseudospin texture 1 Joseph Henry Laboratory, Department of Physics, Princeton University, Princeton, New Jersey 08544, USA, 2 Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA. *e-mail: yazdani@princeton.edu.in gr...

The development of ferromagnetism in Mn-doped Bi 2 Te 3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi 2−x Mn x Te 3 , and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi 1.96 Mn 0.04 Te 3 and Bi 1.91 Mn 0.09 Te 3 , a second-order ferromagnetic transition is observed, with T C ϳ 9 -12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi 2 Te 3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi 2 Te 3 crystals are p-type. Angle-resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi 2 Te 3 are also present at 15 K in ferromagnetic Mn-doped Bi 2−x Mn x Te 3 and that the dispersion relations of the surface states are changed in a subtle fashion.