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.

The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators like Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin-and angle-resolved photoelectron spectroscopy (ARPES) experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.The hallmark of a topological insulator is a single spinpolarized Dirac cone at the surface which is protected by time reversal-symmetry and originates from a band inversion in the bulk [1,2]. Notably, breaking time-reversal symmetry by magnetic order does not necessarily destroy the non-trivial topology but instead may drive the system into another topological phase. One example is the quantum anomalous Hall (QAH) state that has been observed in magnetically doped topological insulators [3]. The QAH state, in turn, may form the basis for yet more exotic electronic states, such as axion insulators [4,5] and chiral Majorana fermions [6]. Another example is the antiferromagnetic topological insulator state which is protected by a combination of time-reversal and lattice translational symmetries [7].Magnetic order in a topological insulator has mainly been achieved by doping with 3d impurities [3,8], which however inevitably gives rise to increased disorder. By contrast, the layered van der Waals material MnBi 2 Te 4 [9, 10] has recently been proposed to realize an intrinsic magnetic topological insulator [11][12][13][14], i.e. a compound that features magnetic order and a topologically non-trivial bulk band structure at the arXiv:1903.11826v2 [cond-mat.str-el]

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Ribonuclease A (RNase A) is immobilized on silver surfaces in oriented and random form via self-assembled monolayers (SAMs) of alkanethiols. The immobilization process is characterized step-by-step using chemically selective near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the C, N, and S K-edges. Causes of imperfect immobilization are pinpointed, such as oxidation and partial desorption of the alkanethiol SAMs and incomplete coverage. The orientation of the protein layer manifests itself in an 18% polarization dependence of the NEXAFS signal from the N 1s to pi* transition of the peptide bond, which is not seen for a random orientation. The S 1s to C-S sigma* transition exhibits an even larger polarization dependence of 41%, which is reduced to 5% for a random orientation. A quantitative model is developed that explains the sign and magnitude of the polarization dependence at both edges. The results demonstrate that NEXAFS is able to characterize surface reactions during the immobilization of proteins and to provide insight into their orientations on surfaces.

Static charge-density wave (CDW) and spin-density wave (SDW) order has been convincingly observed in La-based cuprates for some time. However, more recently it has been suggested by quantum oscillation, transport and thermodynamic measurements that density wave order is generic to underdoped cuprates and plays a significant role in YBa2Cu3O 6+δ (YBCO). We use resonant soft x-ray scattering at the Cu L and O K edges to search for evidence of density wave order in Ortho-II and Ortho-VIII oxygen-ordered YBCO. We report a null result -no evidence for static CDW order -in both Ortho-II and Ortho-VIII ordered YBCO. While this does not rule out static CDW order in the CuO2 planes of YBCO, these measurements place limits on the parameter space (temperature, magnetic field, scattering vector) in which static CDW order may exist. In addition, we present a detailed analysis of the energy and polarization dependence of the Ortho-II superstructure Bragg reflection [0.5 0 0] at the Cu L edge. The intensity of this peak, which is due to the valence modulations of Cu in the chain layer, is compared with calculations using atomic scattering form factors deduced from x-ray absorption measurements. The calculated energy and polarization dependence of the scattering intensity is shown to agree very well with the measurement, validating the approach and providing a framework for analyzing future resonant soft x-ray scattering measurements.PACS numbers: 74.72. Gh,61.05.cp,71.45.Lr,78.70.Dm In the cuprate superconductors, it has long been recognized that incommensurate spin density wave (SDW) and charge density wave (CDW) order co-exists or competes with the superconducting phase.1 This CDW/SDW order is most clearly manifested in 1/8-doped La-based cuprates where CDW and SDW are stabilized and made static by a low temperature tetragonal (LTT) lattice distortion.2-4 In other cuprates, checkerboardlike static density wave order, different from the stripe ordering in La-based cuprates, has been observed with surface-sensitive scanning tunnelling microscopy measurements. 5,6 More recently, static SDW order has also been observed in low-doped (below p = 1/8) YBa 2 Cu 3 O 6+δ (YBCO) by neutron scattering at low temperatures and in high magnetic fields. 7,8 In addition to these direct observations of density wave order, other indirect measurements have suggested that density wave order is more generic to the cuprates than was previously believed. These include recent quantum oscillation measurements detecting the presence of unexplained electron pockets in underdoped YBCO 9-13 that may result from density wave order causing a Fermi surface reconstruction, 14 a striking similarity in the Hallcoefficient between YBCO and stripe-ordered LSCO,15 and anisotropy in the Nernst co-efficient suggestive of unidirectional order.16 Despite this evidence, it is not yet apparent how generic static CDW and/or SDW order is to the cuprates and ultimately what role these density wave orders play in the superconductivity. 17-19Furthermore, it is still open ...