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Register a new userBand inversion and topology of the bulk electronic structure in FeSe${}_{0.45}$Te${}_{0.55}$
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FeSe${}_{0.45}$Te${}_{0.55}$ (FeSeTe) has recently emerged as a promising candidate to host topological superconductivity, with a Dirac surface state and signatures of Majorana bound states in vortex cores. However, correlations strongly renormalize the bands compared to electronic structure calculations, and there is no evidence for the expected bulk band inversion. We present here a comprehensive angle resolved photoemission (ARPES) study of FeSeTe as function of photon energies ranging from 15 - 100 eV. We find that although the top of bulk valence band shows essentially no $k_z$ dispersion, its normalized intensity exhibits a periodic variation with $k_z$. We show, using ARPES selection rules, that the intensity oscillation is a signature of band inversion indicating a change in the parity going from $Gamma$ to Z. Thus we provide the first direct evidence for a topologically non-trivial bulk band structure that supports protected surface states.
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The engineering of Majorana zero modes in topological superconductors, a new paradigm for the realization of topological quantum computing and topology-based devices, has been hampered by the absence of materials with sufficiently large superconducting gaps. Recent experiments, however, have provided enthralling evidence for the existence of topological surface superconductivity in the iron-based superconductor FeSe$_{0.45}$Te$_{0.55}$ possessing a full $s_pm$-wave gap of a few meV. Here, we propose a mechanism for the emergence of topological superconductivity on the surface of FeSe$_{0.45}$Te$_{0.55}$ by demonstrating that the interplay between the $s_pm$-wave symmetry of the superconducting gap, recently observed surface magnetism, and a Rashba spin-orbit interaction gives rise to several topological superconducting phases. Moreover, the proposed mechanism explains a series of experimentally observed hallmarks of topological superconductivity, such as the emergence of Majorana zero modes in the center of vortex cores and at the end of line defects, as well as of chiral Majorana edge modes along certain types of domain walls. We also propose that the spatial distribution of supercurrents near a domain wall is a characteristic signature measurable via a scanning superconducting quantum interference device that can distinguish between chiral Majorana edge modes and trivial in-gap states.
Hexagonal FeSe thin films were grown on SrTiO3 substrates and the temperature and thickness dependence of their electronic structures were studied. The hexagonal FeSe is found to be metallic and electron doped, whose Fermi surface consists of six elliptical electron pockets. With decreased temperature, parts of the bands shift downward to high binding energy while some bands shift upwards to EF. The shifts of these bands begin around 300 K and saturate at low temperature, indicating a magnetic phase transition temperature of about 300 K. With increased film thickness, the Fermi surface topology and band structure show no obvious change except some minor quantum size effect. Our paper reports the first electronic structure of hexagonal FeSe, and shows that the possible magnetic transition is driven by large scale electronic structure reconstruction.
PtBi2 with a layered trigonal crystal structure was recently reported to exhibit an unconventional large linear magnetoresistance, while the mechanism involved is still elusive. Using high resolution angle-resolved photoemission spectroscopy, we present a systematic study on its bulk and surface electronic structure. Through careful comparison with first-principle calculations, our experiment distinguishes the low-lying bulk bands from entangled surface states, allowing the estimation of the real stoichiometry of samples. We find significant electron doping in PtBi2, implying a substantial Bi deficiency induced disorder therein. We discover a Dirac-cone-like surface state on the boundary of the Brillouin zone, which is identified as an accidental Dirac band without topological protection. Our findings exclude quantum-limit-induced linear band dispersion as the cause of the unconventional large linear magnetoresistance.
We study the Atiyah-Hirzebruch spectral sequence (AHSS) for equivariant K-theory in the context of band theory. Various notions in the band theory such as irreducible representations at high-symmetric points, the compatibility relation, topological gapless and singular points naturally fits into the AHSS. As an application of the AHSS, we get the complete list of topological invariants for 230 space groups without time-reversal or particle-hole invariance. We find that a lot of torsion topological invariants appear even for symmorphic space groups.
We demonstrate that the differential conductance, $dI/dV$, measured via spectroscopic imaging scanning tunneling microscopy in the doped iron chalcogenide FeSe$_{0.45}$Te$_{0.55}$, possesses a series of characteristic features that allow one to extract the orbital structure of the superconducting gaps. This yields nearly isotropic superconducting gaps on the two hole-like Fermi surfaces, and a strongly anisotropic gap on the electron-like Fermi surface. Moreover, we show that the pinning of nematic fluctuations by defects can give rise to a dumbbell-like spatial structure of the induced impurity bound states, and explains the related $C_2$-symmetry in the Fourier transformed differential conductance.
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