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Publications (10 of 15) Show all publications
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
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
Morenzoni, E., Prokscha, T., Saadaoui, H., Salman, Z., Suter, A., Wojek, B. M., . . . Ofer, O. (2014). Low-Energy Muons at PSI: Examples of Investigations of Superconducting Properties in Near-Surface Regions and Heterostuctures. In: Proceedings of the International Symposium on Science Explored by Ultra Slow Muon (USM2013): . Paper presented at International Symposium on Science Explored by Ultra Slow Muon (USM2013); Matsue, Shimane, Japan, August 9-12, 2013. The Physical Society of Japan, 2, Article ID 010201.
Open this publication in new window or tab >>Low-Energy Muons at PSI: Examples of Investigations of Superconducting Properties in Near-Surface Regions and Heterostuctures
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2014 (English)In: Proceedings of the International Symposium on Science Explored by Ultra Slow Muon (USM2013), The Physical Society of Japan , 2014, Vol. 2, article id 010201Conference paper, Published paper (Refereed)
Abstract [en]

With the advent of polarized low-energy muons, with tunable energy in the kiloelectronvolt range, it is possible to use the sensitivity and the local-probe character of µSR to perform depth-dependent investigations on the nanometer scale of magnetic and superconducting properties of materials. Here, after a brief summary of the present status of LE-µSR at PSI, we give some examples of investigations of superconducting properties in the near-surface regions of single crystals and thin-film materials.

Place, publisher, year, edition, pages
The Physical Society of Japan, 2014
Series
JPS Conference Proceedings ; 010201
Keywords
Low-energy muons, superconductivity, thin films, heterostructures
National Category
Accelerator Physics and Instrumentation Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-163172 (URN)10.7566/JPSCP.2.010201 (DOI)
Conference
International Symposium on Science Explored by Ultra Slow Muon (USM2013); Matsue, Shimane, Japan, August 9-12, 2013
Note

QC 20150331

Available from: 2015-03-28 Created: 2015-03-28 Last updated: 2015-03-31Bibliographically 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
Ahmadi, S., Agnarsson, B., Bidermane, I., Wojek, B. M., Noël, Q., Sun, C. & Göthelid, M. (2014). Site-dependent charge transfer at the Pt(111)-ZnPc interface and the effect of iodine. Journal of Chemical Physics, 140(17), 174702
Open this publication in new window or tab >>Site-dependent charge transfer at the Pt(111)-ZnPc interface and the effect of iodine
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2014 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 17, p. 174702-Article in journal (Refereed) Published
Abstract [en]

The electronic structure of ZnPc, from sub-monolayers to thick films, on bare and iodated Pt(111) is studied by means of X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and scanning tunneling microscopy. Our results suggest that at low coverage ZnPc lies almost parallel to the Pt(111) substrate, in a non-planar configuration induced by Zn-Pt attraction, leading to an inhomogeneous charge distribution within the molecule and an inhomogeneous charge transfer to the molecule. ZnPc does not form a complete monolayer on the Pt surface, due to a surface-mediated intermolecular repulsion. At higher coverage ZnPc adopts a tilted geometry, due to a reduced molecule-substrate interaction. Our photoemission results illustrate that ZnPc is practically decoupled from Pt, already from the second layer. Pre-deposition of iodine on Pt hinders the Zn-Pt attraction, leading to a non-distorted first layer ZnPc in contact with Pt(111)-I(root 3x root 3) or Pt(111)-I(root 7x root 7), and a more homogeneous charge distribution and charge transfer at the interface. On increased ZnPc thickness iodine is dissolved in the organic film where it acts as an electron acceptor dopant.

Keywords
Charge distribution, Charge transfer, Electronic structure, Iodine, Molecules, Monolayers, Photoelectrons, Scanning tunneling microscopy, Thick films, X ray absorption spectroscopy, X ray photoelectron spectroscopy, Zinc, Electron acceptor, Intermolecular repulsions, Nonplanar configuration, Organic films, Pre-deposition, Second layer, Sub-monolayers, Tilted geometry
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-147044 (URN)10.1063/1.4870762 (DOI)000336048000034 ()2-s2.0-84900031581 (Scopus ID)
Funder
Swedish Energy AgencySwedish Research CouncilCarl Tryggers foundation
Note

QC 20150623

Available from: 2014-06-25 Created: 2014-06-23 Last updated: 2017-12-05Bibliographically approved
Suter, A., Morenzoni, E., Prokscha, T., Wojek, B. M., Luetkens, H., Gozar, A., . . . Božović, I. (2014). Two-Dimensional Magnetic and Superconducting Phases in Metal-Insulator La2−xSrxCuO4 Superlattices Studied by Low-Energy Muon-Spin Rotation. In: Proceedings of the International Symposium on Science Explored by Ultra Slow Muon (USM2013): . Paper presented at International Symposium on Science Explored by Ultra Slow Muon (USM2013), Matsue, Shimane, Japan, August 9-12, 2013 (pp. 010204-010204). Physical society of Japan, 2, Article ID 010204.
Open this publication in new window or tab >>Two-Dimensional Magnetic and Superconducting Phases in Metal-Insulator La2−xSrxCuO4 Superlattices Studied by Low-Energy Muon-Spin Rotation
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2014 (English)In: Proceedings of the International Symposium on Science Explored by Ultra Slow Muon (USM2013), Physical society of Japan, 2014, Vol. 2, p. 010204-010204, article id 010204Conference paper, Published paper (Refereed)
Abstract [en]

We show, by means of low-energy muon-spin-rotation measurements, that few-unit-cells thick La2CuO4 layers synthesized digitally by molecular beam epitaxy are antiferromagnetically ordered. Below a thickness of about 5 CuO2 layers the long-range ordered state breaks down, and a magnetic state appears with enhanced quantum fluctuations and a reduced spin stiffness. This magnetic state can exist in close proximity (few Å) to high-temperature superconducting layers, without transmitting supercurrents. Charge-transfer effects and potential softening of the antiferromagnetic state due to the proximity to the superconducting state are discussed.

Place, publisher, year, edition, pages
Physical society of Japan, 2014
Series
JPS Conference Proceedings
Keywords
low energy muons, magnetism, superconductivity, superlattices
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-163173 (URN)10.7566/JPSCP.2.010204 (DOI)
Conference
International Symposium on Science Explored by Ultra Slow Muon (USM2013), Matsue, Shimane, Japan, August 9-12, 2013
Note

QC 20150330

Available from: 2015-03-28 Created: 2015-03-28 Last updated: 2015-03-30Bibliographically 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
Stilp, E., Suter, A., Prokscha, T., Morenzoni, E., Keller, H., Wojek, B. M., . . . Božović, I. (2013). Magnetic phase diagram of low-doped La2-xSrxCuO4 thin films studied by low-energy muon-spin rotation. Physical Review B. Condensed Matter and Materials Physics, 88(6), 064419-1-064419-11, Article ID 064419.
Open this publication in new window or tab >>Magnetic phase diagram of low-doped La2-xSrxCuO4 thin films studied by low-energy muon-spin rotation
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 6, p. 064419-1-064419-11, article id 064419Article in journal (Refereed) Published
Abstract [en]

The magnetic phase diagram of La2-xSrxCuO4 thin films grown on single-crystal LaSrAlO4 substrates has been determined by low-energy muon-spin rotation. The diagram shows the same features as the one of bulk La2-xSrxCuO4, but the transition temperatures between distinct magnetic states are significantly different. In the antiferromagnetic phase the Neel temperature T-N is strongly reduced, and no hole spin freezing is observed at low temperatures. In the disordered magnetic phase (x greater than or similar to 0.02) the transition temperature to the cluster spin-glass state T-g is enhanced. Possible reasons for the pronounced differences between the magnetic phase diagrams of thin-film and bulk samples are discussed.

Place, publisher, year, edition, pages
American Physical Society, 2013
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-136681 (URN)10.1103/PhysRevB.88.064419 (DOI)000323571000002 ()2-s2.0-84883317845 (Scopus ID)
Note

QC 20140602

Available from: 2013-12-06 Created: 2013-12-06 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
Ofer, O., Baglo, J. C., Hossain, M. D., Kiefl, R. F., Hardy, W. N., Thaler, A., . . . Salman, Z. (2012). Absolute value and temperature dependence of the magnetic penetration depth in Ba(Co 0.074Fe 0.926) 2As 2. Physical Review B. Condensed Matter and Materials Physics, 85(6), 060506
Open this publication in new window or tab >>Absolute value and temperature dependence of the magnetic penetration depth in Ba(Co 0.074Fe 0.926) 2As 2
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2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 6, p. 060506-Article in journal (Refereed) Published
Abstract [en]

The absolute value and temperature dependence of the in-plane magnetic penetration depth lambda have been measured on a single crystal of Ba(Co0.074Fe0.926)(2)As-2 using low-energy muon-spin rotation and microwave cavity perturbation. The magnetic field profiles in the Meissner state are consistent with a local London model beyond a depth of 15 nm. We determine the gap symmetry through measurements of the temperature dependence of the superfluid density which follows a two-gap s-wave model over the entire temperature range below T-c. While the intermediate to high temperature data is well fit by an energy gap model in the BCS-like (weak-coupling) limit, a second smaller gap becomes apparent at low temperatures.

Keywords
Mu-Sr; Superconductor; Yba2cu3o6.95; Surface; Psi
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-99606 (URN)10.1103/PhysRevB.85.060506 (DOI)000300830500001 ()2-s2.0-84863260853 (Scopus ID)
Note

QC 20150626

Available from: 2012-08-01 Created: 2012-08-01 Last updated: 2017-12-07Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-8216-5321

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