A topological insulator protected by time-reversal symmetry is realized via spinorbit interaction driven band inversion. The topological phase in the Bi 1−x Sb x system is due to an odd number of band inversions. A related spin-orbit system, the Pb 1−x Sn x Te, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry protected topological crystalline insulator phase in the Pb 1−x Sn x Te class of materials which has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite-Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi 1−x Sb x . Our observation of the spin-polarized Dirac surface states in the inverted Pb 1−x Sn x Te and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.

Understanding and control of spin degrees of freedom on the surfaces of topological materials are the key to future applications as well as for realizing novel physics such as the axion electrodynamics associated with time-reversal symmetry breaking on the surface. We experimentally demonstrate magnetically induced spin reorientation phenomena simultaneous with a Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped Bi 2 Se 3 thin films. The resulting electronic groundstate exhibits unique hedgehog-like spin textures at low energies which directly demonstrates the mechanics of timereversal symmetry breaking on the surface. We further show that an insulating gap induced by quantum tunneling between surfaces exhibits spin texture modulation at low energies but respects time-reversal invariance. These spin phenomena and the control of their Fermi surface geometrical phase first demonstrated in our experiments pave the way for future realization of many predicted exotic magnetic phenomena of topological origin.Since the discovery of three dimensional topological insulators [1][2][3][4][5], topological order proximity to ferromagnetism has been considered as one of the core interests of the field [6][7][8][9][10][11][12][13][14][15][16]. Such interest is strongly motivated by the proposed time-reversal (TR) breaking topological physics such as quantized anomalous chiral Hall current, spin current, axion electrodynamics, and inverse spin-galvanic effect [9][10][11][12], all of which critically rely on finding a way to break TR symmetry on the surface and utilize the unique TR broken spin texture for applications. Since quantum coherence is essential in many of these applications, devices need to be engineered into thin films in order to enhance or de-enhance surface-tosurface coupling or the quantum tunneling of the electrons. The experimental spin behavior of surface states under the two extreme limits, namely the doped magnetic groundstate and ultra-thin film quantum tunneling groundstate, is thus of central importance to the entire field. However, surprisingly, it is not known what happens to the spin configuration under these extreme conditions relevant for device fabrications. Fundamentally, TR symmetry is inherently connected to the Kramers' degeneracy theorem which states that when TR symmetry is preserved, the electronic states at the TR invariant momenta have to remain doubly spin degenerate. Therefore, the establishment of TR breaking effect fundamentally requires measurements of electronic groundstate with a spin-sensitive probe. Here we utilize spin-3 resolved angle-resolved photoemission spectroscopy to measure the momentum space spin configurations in systematically magnetically doped, non-magnetically doped, and ultra-thin quantum coherent topological insulator films [17], in order to understand the nature of electronic groundstates under two extreme limits vital for magnetic topological devices. These measurements allow us to make definitive conclusions regarding magnetism on to...

We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi(2)Se(3) from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.