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Publications (10 of 14) Show all publications
Bagschik, K., Froemter, R., Bach, J., Beyersdorff, B., Mueller, L., Schleitzer, S., . . . Oepen, H. P. (2016). Employing soft x-ray resonant magnetic scattering to study domain sizes and anisotropy in Co/Pd multilayers. Physical Review B, 94(13), Article ID 134413.
Open this publication in new window or tab >>Employing soft x-ray resonant magnetic scattering to study domain sizes and anisotropy in Co/Pd multilayers
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2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 13, article id 134413Article in journal (Refereed) Published
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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-196260 (URN)10.1103/PhysRevB.94.134413 (DOI)000385620300003 ()2-s2.0-84992143129 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20161114

Available from: 2016-11-14 Created: 2016-11-14 Last updated: 2017-11-29Bibliographically approved
Bagschik, K., Frömter, R., Müller, L., Roseker, W., Bach, J., Staeck, P., . . . Oepen, H. P. (2016). Spatial coherence determination from the Fourier analysis of a resonant soft X-ray magnetic speckle pattern. Optics Express, 24(20), 23162-23176
Open this publication in new window or tab >>Spatial coherence determination from the Fourier analysis of a resonant soft X-ray magnetic speckle pattern
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2016 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 24, no 20, p. 23162-23176Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Optical Society of America, 2016
Keywords
Critical currents, Ferromagnetic materials, Fourier analysis, Synchrotron radiation, Synchrotrons, Coherence properties, Ferromagnetic films, Magnetic speckles, Multi domains, Nano-structured, Spatial coherence, Test object, Transverse coherence, Speckle
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-195259 (URN)10.1364/OE.24.023162 (DOI)000387543000079 ()2-s2.0-84989895771 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20161115

Available from: 2016-11-15 Created: 2016-11-02 Last updated: 2017-11-29Bibliographically approved
Wojek, B. M., Hårdensson Berntsen, M., Jonsson, V., Szczerbakow, A., Dziawa, P., Kowalski, B. J., . . . Tjernberg, O. (2015). Direct observation and temperature control of the surface Dirac gap in a topological crystalline insulator. Nature Communications, 6, Article ID 8463.
Open this publication in new window or tab >>Direct observation and temperature control of the surface Dirac gap in a topological crystalline insulator
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2015 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, article id 8463Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Nature Publishing Group, 2015
Keywords
Phase-Transition, Pb1-Xsnxse, Semiconductors, System, SNTE
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-179283 (URN)10.1038/ncomms9463 (DOI)000364925900002 ()26458506 (PubMedID)2-s2.0-84944104336 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20151215

QC 20151216

Available from: 2015-12-15 Created: 2015-12-14 Last updated: 2017-12-01Bibliographically approved
Matt, C. E., Fatuzzo, C. G., Sassa, Y., Mansson, M., Fatale, S., Bitetta, V., . . . Chang, J. (2015). Electron scattering, charge order, and pseudogap physics in La1.6-xNd0.4SrxCuO4: An angle-resolved photoemission spectroscopy study. Physical Review B. Condensed Matter and Materials Physics, 92(13), Article ID 134524.
Open this publication in new window or tab >>Electron scattering, charge order, and pseudogap physics in La1.6-xNd0.4SrxCuO4: An angle-resolved photoemission spectroscopy study
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 13, article id 134524Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Physical Society, 2015
Keywords
High-T-C, Temperature Cuprate Superconductors, Fermi-Surface Reconstruction, Copper-Oxide Superconductors, Stripe-Phase Order, Quantum Oscillations, Symmetry-Breaking, Energy Gaps, Underdoped Bi2sr2cacu2o8+Delta, Transition-Temperature
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-177053 (URN)10.1103/PhysRevB.92.134524 (DOI)000363518800002 ()2-s2.0-84946950112 (Scopus ID)
Funder
Wenner-Gren FoundationsSwedish Research Council, INCA-2014-6426
Note

QC 20151217

Available from: 2015-12-17 Created: 2015-11-13 Last updated: 2017-12-01Bibliographically approved
Wojek, B. M., Dziawa, P., Kowalski, B. J., Szczerbakow, A., Black-Schaffer, A. M., Hårdensson Berntsen, M., . . . Tjernberg, O. (2014). Band inversion and the topological phase transition in (Pb,Sn)Se. Physical Review B. Condensed Matter and Materials Physics, 90(16), 161202
Open this publication in new window or tab >>Band inversion and the topological phase transition in (Pb,Sn)Se
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 16, p. 161202-Article in journal (Refereed) Published
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.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-156989 (URN)10.1103/PhysRevB.90.161202 (DOI)000344024300001 ()2-s2.0-84908042676 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20150720

Available from: 2014-12-05 Created: 2014-12-04 Last updated: 2017-12-05Bibliographically approved
Polley, C. M., Dziawa, P., Reszka, A., Szczerbakow, A., Minikayev, R., Domagala, J. Z., . . . Balasubramanian, T. (2014). Observation of topological crystalline insulator surface states on (111)-oriented Pb1-xSnxSe films. Physical Review B. Condensed Matter and Materials Physics, 89(7), 075317
Open this publication in new window or tab >>Observation of topological crystalline insulator surface states on (111)-oriented Pb1-xSnxSe films
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 7, p. 075317-Article in journal (Refereed) Published
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.

Keywords
Snte, Epitaxy, Phase
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-143704 (URN)10.1103/PhysRevB.89.075317 (DOI)000332421900004 ()2-s2.0-84897585096 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20150623

Available from: 2014-03-28 Created: 2014-03-27 Last updated: 2017-12-05Bibliographically approved
Berntsen, M. H. (2013). Consequences of a non-trivial band-structure topology in solids: Investigations of topological surface and interface states. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>Consequences of a non-trivial band-structure topology in solids: Investigations of topological surface and interface states
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. p. x, 82
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2013:02
Keywords
spin- and angle-resolved photoelectron spectroscopy, time-of-flight analyzer, laser based light source, topological insulator, topological crystalline insulator, thin films, surface state, interface state, Bi2Se3, Pb1-xSnxSe
National Category
Physical Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-121974 (URN)978-91-7501-735-8 (ISBN)
Public defence
2013-05-31, Sal D, KTH-Forum, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20130507

Available from: 2013-05-07 Created: 2013-05-06 Last updated: 2013-05-07Bibliographically approved
Berntsen, M. H., Götberg, O., Wojek, B. M. & Tjernberg, O. (2013). Direct observation of decoupled Dirac states at the interface between topological and normal insulators. Physical Review B. Condensed Matter and Materials Physics, 88(19), Article ID 195132.
Open this publication in new window or tab >>Direct observation of decoupled Dirac states at the interface between topological and normal insulators
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 19, article id 195132Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Physical Society, 2013
Keywords
Topological insulator, bismuth selenide, interface state, surface state
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-121940 (URN)10.1103/PhysRevB.88.195132 (DOI)000327158600003 ()2-s2.0-84888309073 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20131217

Available from: 2013-05-06 Created: 2013-05-06 Last updated: 2017-12-06Bibliographically approved
Wojek, B. M., Buczko, R., Safaei, S., Dziawa, P., Kowalski, B. J., Hårdensson Berntsen, M., . . . Tjernberg, O. (2013). Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se. Physical Review B. Condensed Matter and Materials Physics, 87(11), 115106
Open this publication in new window or tab >>Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 11, p. 115106-Article in journal (Refereed) Published
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.

Keywords
Pb1-Xsnxse, Inversion, Snte, Alloys, Phase, Bands
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-120162 (URN)10.1103/PhysRevB.87.115106 (DOI)000315731400002 ()2-s2.0-84874844135 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilEU, European Research Council
Note

QC 20150623

Available from: 2013-04-02 Created: 2013-04-02 Last updated: 2017-12-06Bibliographically approved
Wojek, B. M., Hårdensson Berntsen, M., Boseggia, S., Boothroyd, A. T., Prabhakaran, D., McMorrow, D. F., . . . Tjernberg, O. (2012). The J(eff)=1/2 insulator Sr3Ir2O7 studied by means of angle-resolved photoemission spectroscopy. Journal of Physics: Condensed Matter, 24(41), 415602
Open this publication in new window or tab >>The J(eff)=1/2 insulator Sr3Ir2O7 studied by means of angle-resolved photoemission spectroscopy
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2012 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 41, p. 415602-Article in journal (Refereed) Published
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.

Keywords
Crystal-Structure, Sr2iro4
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-104360 (URN)10.1088/0953-8984/24/41/415602 (DOI)000309512200014 ()2-s2.0-84866867149 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20121105

Available from: 2012-11-05 Created: 2012-11-01 Last updated: 2017-12-07Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-7833-3943

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