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  • 1. Bagschik, K.
    et al.
    Frömter, R.
    Müller, L.
    Roseker, W.
    Bach, J.
    Staeck, P.
    Thönnißen, C.
    Schleitzer, S.
    Hårdensson Berntsen, Magnus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Weier, C.
    Adam, R.
    Viefhaus, J.
    Schneider, C. M.
    Grübel, G.
    Oepen, H. P.
    Spatial coherence determination from the Fourier analysis of a resonant soft X-ray magnetic speckle pattern2016Inngår i: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 24, nr 20, s. 23162-23176Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a method to determine the two-dimensional spatial coherence of synchrotron radiation in the soft X-ray regime by analyzing the Fourier transform of the magnetic speckle pattern from a ferromagnetic film in a multidomain state. To corroborate the results, a Young's double-pinhole experiment has been performed. The transverse coherence lengths in vertical and horizontal direction of both approaches are in a good agreement. The method presented here is simple and gives a direct access to the coherence properties of synchrotron radiation without nanostructured test objects.

  • 2. Bagschik, Kai
    et al.
    Froemter, Robert
    Bach, Judith
    Beyersdorff, Bjoern
    Mueller, Leonard
    Schleitzer, Stefan
    Berntsen, Magnus H.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Deutsch Elekt Synchrotron DESY.
    Weier, Christian
    Adam, Roman
    Viefhaus, Jens
    Schneider, Claus Michael
    Gruebel, Gerhard
    Oepen, Hans Peter
    Employing soft x-ray resonant magnetic scattering to study domain sizes and anisotropy in Co/Pd multilayers2016Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 13, artikkel-id 134413Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    It is demonstrated that themagnetic diffraction pattern of the isotropic disorderedmaze pattern is well described utilizing a gamma distribution of domain sizes in a one-dimensional model. From the analysis, the mean domain size and the shape parameter of the distribution are obtained. The model reveals an average domain size that is significantly different from the value that is determined from the peak position of the structure factor in reciprocal space. As a proof of principle, a wedge-shaped (Co-t angstrom/Pd-10 angstrom) 8 multilayer film, that covers the thickness range of the spin-reorientation transition, has been used. By means of soft x-ray resonant magnetic scattering (XRMS) and imaging techniques the thickness-driven evolution of the magnetic properties of the cobalt layers is explored. It is shown that minute changes of the domain pattern concerning domain size and geometry can be investigated and analyzed due to the high sensitivity and lateral resolution of the XRMS technique. The latter allows for the determination of the magnetic anisotropies of the cobalt layers within a thickness range of a few angstroms.

  • 3.
    Berntsen, Magnus H.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Consequences of a non-trivial band-structure topology in solids: Investigations of topological surface and interface states2013Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The development and characterization of experimental setups for angle-resolved photoelectron spectroscopy (ARPES) and spin- and angle-resolved photoelectron spectroscopy (SARPES) is described. Subsequently, the two techniques are applied to studies of the electronic band structure in topologically non-trivial materials.

    The laser-based ARPES setup works at a photon energy of 10.5 eV and a typical repetition rate in the range 200 kHz to 800 kHz. By using a time-of-flight electron energy analyzer electrons emitted from the sample within a solid angle of up to ±15 degrees can be collected and analyzed simultaneously. The SARPES setup is equipped with a traditional hemispherical electron energy analyzer in combination with a mini-Mott electron polarimeter. The system enables software-controlled switching between angle-resolved spin-integrated and spin-resolved measurements, thus providing the possibility to orient the sample by mapping out the electronic band structure using ARPES before performing spin-resolved measurements at selected points in the Brillouin zone.

    Thin films of the topological insulators (TIs) Bi2Se3, Bi2Te3 and Sb2Te3 are grown using e-beam evaporation and their surface states are observed by means of ARPES. By using a combination of low photon energies and cryogenic sample temperatures the topological states originating from both the vacuum interface (surface) and the substrate interface are observed in Bi2Se3 films and Bi2Se3/Bi2Te3 heterostructures, with total thicknesses in the ultra-thin limit (six to eight quintuple layers), grown on Bi-terminated Si(111) substrates. Band alignment between Si and Bi2Se3 at the interface creates a band bending through the films. The band bending is found to be independent of the Fermi level (EF) position in the bulk of the substrate, suggesting that the surface pinning of EF in the Si(111) substrate remains unaltered after deposition of the TI films. Therefore, the type and level of doping of the substrate does not show any large influence on the size of the band bending.

    Further, we provide experimental evidence for the realization of a topological crystalline insulator (TCI) phase in the narrow-band semiconductor Pb1−xSnxSe. The TCI phase exists for temperatures below the transition temperature Tc and is characterized by an inverted bulk band gap accompanied by the existence of non-gapped surface states crossing the band gap. Above Tc the material is in a topologically trivial phase where the surface states are gapped. Thus, when lowering the sample temperature across Tc a topological phase transition from a trivial insulator to a TCI is observed. SARPES studies indicate a helical spin structure of the surface states both in the topologically trivial and the TCI phase.

  • 4.
    Berntsen, Magnus H.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Götberg, Olof
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    An experimental setup for high resolution 10.5 eV laser-based angle-resolved photoelectron spectroscopy using a time-of-flight electron analyzer2011Inngår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 82, nr 9, s. 095113-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present an experimental setup for laser-based angle-resolved time-of-flight photoemission. Using a picosecond pulsed laser, photons of energy 10.5 eV are generated through higher harmonic generation in xenon. The high repetition rate of the light source, variable between 0.2 and 8 MHz, enables high photoelectron count rates and short acquisition times. By using a time-of-flight analyzer with angle-resolving capabilities, electrons emitted from the sample within a circular cone of up to +/- 15 degrees can be collected. Hence, simultaneous acquisition of photoemission data for a complete area of the Brillouin zone is possible. The current photon energy enables bulk sensitive measurements, high angular resolution, and the resulting covered momentum space is large enough to enclose the entire Brillouin zone in cuprate high-T(c) superconductors. Fermi edge measurements on polycrystalline Au shows an energy resolution better than 5 meV. Data from a test measurement of the Au(111) surface state are presented along with measurements of the Fermi surface of the high-T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212).

  • 5.
    Berntsen, Magnus H.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Götberg, Olof
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Wojek, Bastian M.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Direct observation of decoupled Dirac states at the interface between topological and normal insulators2013Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, nr 19, artikkel-id 195132Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Several proposed applications and exotic effects in topological insulators rely on the presence of helical Dirac states at the interface between a topological insulator and a normal insulator. In the present work, we have used low-energy angle-resolved photoelectron spectroscopy to uncover and characterize the interface states of Bi2Se3 thin films and Bi2Te3/Bi2Se3 heterostructures grown on Si(111). The results establish that Dirac fermions are indeed present at the topological-normal-insulator boundary and absent at the topological-topological-insulator interface. Moreover, it is demonstrated that band bending present within the topological-insulator films leads to a substantial separation of the interface and surface states in energy. These results pave the way for further studies and the realization of interface-related phenomena in topological-insulator thin-film heterostructures.

  • 6.
    Berntsen, Magnus H.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Palmgren, P.
    Leandersson, M.
    Hahlin, A.
    Åhlund, J.
    Wannberg, B.
    Mansson, M.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    A spin- and angle-resolving photoelectron spectrometer2010Inngår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 81, nr 3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A new type of hemispherical electron energy analyzer that permits angle and spin resolved photoelectron spectroscopy has been developed. The analyzer permits standard angle resolved spectra to be recorded with a two-dimensional detector in parallel with spin detection using a mini-Mott polarimeter. General design considerations as well as technical solutions are discussed and test results from the Au(111) surface state are presented.

  • 7.
    Duan, Yu-Xia
    et al.
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Zhang, Cheng
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Rusz, Jan
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden..
    Oppeneer, Peter M.
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden..
    Durakiewicz, Tomasz
    Marie Curie Sklodowska Univ, Lnstitute Phys, PL-20031 Lublin, Poland..
    Sassa, Yasmine
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden.;Chalmers Univ Technol, Dept Phys, S-41296 Gothenburg, Sweden..
    Tjernberg, Oscar
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Månsson, Martin
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Berntsen, Magnus H.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Wu, Fan-Ying
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Zhao, Yin-Zou
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Song, Jiao-Jiao
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Wu, Qi-Yi
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Luo, Yang
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Bauer, Eric D.
    Los Alamos Natl Lab, Condensed Matter & Magnet Sci Grp, Los Alamos, NM 87545 USA..
    Thompson, Joe D.
    Los Alamos Natl Lab, Condensed Matter & Magnet Sci Grp, Los Alamos, NM 87545 USA..
    Meng, Jian-Qiao
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China.;Hunan Normal Univ, SICQEA, Changsha 410081, Hunan, Peoples R China..
    Crystal electric field splitting and f-electron hybridization in heavy-fermion CePt2In72019Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, nr 8, artikkel-id 085141Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use high-resolution angle-resolved photoemission spectroscopy to investigate the electronic structure of the antiferromagnetic heavy fermion compound CePt2In7, which is amember of the CeIn3-derived heavy fermion material family. Weak hybridization among 4f electron states and conduction bands was identified in CePt2In7 at low temperature much weaker than that in the other heavy fermion compounds like CeIrIn5 and CeRhIn5. The Ce 4f spectrum shows fine structures near the Fermi energy, reflecting the crystal electric field splitting of the 4f(5/2)(1) and 4f(7/2)(1) states. Also, we find that the Fermi surface has a strongly three-dimensional topology, in agreement with density-functional theory calculations.

  • 8. Dziawa, P.
    et al.
    Kowalski, B. J.
    Dybko, K.
    Buczko, R.
    Szczerbakow, A.
    Szot, M.
    Lusakowska, E.
    Balasubramanian, T.
    Wojek, Bastian M.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Hårdensson Berntsen, Magnus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Story, T.
    Topological crystalline insulator states in Pb1-xSnxSe2012Inngår i: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 11, nr 12, s. 1023-1027Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection(1,2). In this study we show that the narrow-gap semiconductor Pb1-xSnxSe is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.

  • 9. Matt, C. E.
    et al.
    Fatuzzo, C. G.
    Sassa, Y.
    Mansson, M.
    Fatale, S.
    Bitetta, V.
    Shi, X.
    Pailhes, S.
    Hårdensson Berntsen, Magnus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Kurosawa, T.
    Oda, M.
    Momono, N.
    Lipscombe, O. J.
    Hayden, S. M.
    Yan, J. -Q
    Zhou, J. -S
    Goodenough, J. B.
    Pyon, S.
    Takayama, T.
    Takagi, H.
    Patthey, L.
    Bendounan, A.
    Razzoli, E.
    Shi, M.
    Plumb, N. C.
    Radovic, M.
    Grioni, M.
    Mesot, J.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Stockholm University, Sweden.
    Chang, J.
    Electron scattering, charge order, and pseudogap physics in La1.6-xNd0.4SrxCuO4: An angle-resolved photoemission spectroscopy study2015Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, nr 13, artikkel-id 134524Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report an angle-resolved photoemission study of the charge stripe ordered La1.6-xNd0.4SrxCuO4 (Nd-LSCO) system. A comparative and quantitative line-shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side (x = 0.20), a normal-state antinodal spectral gap opens upon cooling below 80 K. In this process, spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different line shape is observed in the antinodal region of charge ordered Nd-LSCO x = 1/8. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral-weight redistribution and gapping originating from charge stripe ordering.

  • 10. Polley, C. M.
    et al.
    Dziawa, P.
    Reszka, A.
    Szczerbakow, A.
    Minikayev, R.
    Domagala, J. Z.
    Safaei, S.
    Kacman, P.
    Buczko, R.
    Adell, J.
    Hårdensson Berntsen, Magnus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Wojek, Bastian M.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Kowalski, B. J.
    Story, T.
    Balasubramanian, T.
    Observation of topological crystalline insulator surface states on (111)-oriented Pb1-xSnxSe films2014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, nr 7, s. 075317-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present angle-resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb1-xSnxSe, a three-dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at (Gamma) over bar and (M) over bar in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality, and alternative surface orientations in (Pb,Sn)Se solid solutions.

  • 11. Razzoli, E.
    et al.
    Sassa, Y.
    Drachuck, G.
    Mansson, M.
    Keren, A.
    Shay, M.
    Berntsen, Magnus H.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Radovic, M.
    Chang, J.
    Pailhes, S.
    Momono, N.
    Oda, M.
    Ido, M.
    Lipscombe, O. J.
    Hayden, S. M.
    Patthey, L.
    Mesot, J.
    Shi, M.
    The Fermi surface and band folding in La2-xSrxCuO4, probed by angle-resolved photoemission2010Inngår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12, s. 125003-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A systematic angle-resolved photoemission study of the electronic structure of La2-xSrxCuO4 in a wide doping range is presented in this paper. In addition to the main energy band, we observed a weaker additional band, the (pi, pi) folded band, which shows unusual doping dependence. The appearance of the folded band suggests that a Fermi surface reconstruction is doping dependent and could already occur at zero magnetic field.

  • 12.
    Wojek, Bastian M.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Buczko, R.
    Safaei, S.
    Dziawa, P.
    Kowalski, B. J.
    Hårdensson Berntsen, Magnus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Balasubramanian, T.
    Leandersson, M.
    Szczerbakow, A.
    Kacman, P.
    Story, T.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se2013Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 11, s. 115106-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the nature of (001) surface states in Pb0.73Sn0.27Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above the band-gap-inversion temperature. Our calculations predict not only metallic surface states with a nontrivial chiral spin structure for the TCI case, but also nonmetallic (gapped) surface states with nonzero spin polarization when the system is a normal insulator. For both phases, angle- and spin-resolved photoelectron spectroscopy measurements provide conclusive evidence for the formation of these (001) surface states in Pb0.73Sn0.27Se, as well as for their chiral spin structure.

  • 13.
    Wojek, Bastian M.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Dziawa, P.
    Kowalski, B. J.
    Szczerbakow, A.
    Black-Schaffer, A. M.
    Hårdensson Berntsen, Magnus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Balasubramanian, T.
    Story, T.
    Tjernberg, Oskar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Band inversion and the topological phase transition in (Pb,Sn)Se2014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 16, s. 161202-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The recent discovery of a topological phase transition in IV-VI narrow-gap semiconductors has revitalized the decades-old interest in the bulk band inversion occurring in these materials. Here we systematically study the (001) surface states of Pb1-xSnxSe mixed crystals by means of angle-resolved photoelectron spectroscopy in the parameter space 0 <= x <= 0.37 and 300 K >= T >= 9 K. Using the surface-state observations, we monitor directly the topological phase transition in this solid solution and gain valuable information on the evolution of the underlying fundamental band gap of the system. In contrast to common model expectations, the band-gap evolution appears to be nonlinear as a function of the studied parameters, resulting in the measuring of a discontinuous band-inversion process. This finding signifies that the anticipated gapless bulk state is in fact not a stable configuration and that the topological phase transition therefore exhibits features akin to a first-order transition.

  • 14.
    Wojek, Bastian M.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Hårdensson Berntsen, M.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Jonsson, Viktor
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Szczerbakow, A.
    Dziawa, P.
    Kowalski, B. J.
    Story, T.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Direct observation and temperature control of the surface Dirac gap in a topological crystalline insulator2015Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, artikkel-id 8463Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Since the advent of topological insulators hosting Dirac surface states, efforts have been made to gap these states in a controllable way. A new route to accomplish this was opened up by the discovery of topological crystalline insulators where the topological states are protected by crystal symmetries and thus prone to gap formation by structural changes of the lattice. Here we show a temperature-driven gap opening in Dirac surface states within the topological crystalline insulator phase in (Pb,Sn) Se. By using angle-resolved photoelectron spectroscopy, the gap formation and mass acquisition is studied as a function of composition and temperature. The resulting observations lead to the addition of a temperature-and composition-dependent boundary between massless and massive Dirac states in the topological phase diagram for (Pb,Sn) Se (001). Overall, our results experimentally establish the possibility to tune between massless and massive topological states on the surface of a topological system.

  • 15.
    Wojek, Bastian M.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Hårdensson Berntsen, Magnus
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Boseggia, S.
    Boothroyd, A. T.
    Prabhakaran, D.
    McMorrow, D. F.
    Ronnow, H. M.
    Chang, J.
    Tjernberg, Oscar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    The J(eff)=1/2 insulator Sr3Ir2O7 studied by means of angle-resolved photoemission spectroscopy2012Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, nr 41, s. 415602-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The low-energy electronic structure of the J(eff) = 1/2 spin-orbit insulator Sr3Ir2O7 has been studied by means of angle-resolved photoemission spectroscopy. A comparison of the results for bilayer Sr3Ir2O7 with available literature data for the related single-layer compound Sr2IrO4 reveals qualitative similarities and similar J(eff) = 1/2 bandwidths for the two materials, but also pronounced differences in the distribution of the spectral weight. In particular, photoemission from the J(eff) = 1/2 states appears to be suppressed. Yet, it is found that the Sr3Ir2O7 data are in overall better agreement with band-structure calculations than the data for Sr2IrO4.

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