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Knut, Ronny
Publications (10 of 33) Show all publications
Stefanuik, R., Knut, R., Jana, S., Terschlüsen, J. A., Sandell, A. & Söderström, J. (2018). Developments and enhancements to the HELIOS pump probe system. Paper presented at The Future of X-Ray and Electron Spectroscopies, Uppsala, Sweden, June 15-17, 2016. Journal of Electron Spectroscopy and Related Phenomena, 224, 33-37
Open this publication in new window or tab >>Developments and enhancements to the HELIOS pump probe system
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2018 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 224, p. 33-37Article in journal, Editorial material (Refereed) Published
Abstract [en]

In this progress report we describe several design improvements that have been implemented at the HELIOS laboratory, as well as presenting the output characteristics that have been measured as a result. The main focus will be on the redesign of the gas cell, which has enhanced the photon flux of the XUV probe beam. Also, a frequency trippler utilizing sum frequency generation has been installed at the end of the pump line, which increases the photon flux available for both 3.1 eV (400 nm) and 4.66 eV (266 nm) applications without significant increment in the pulse width of the pump.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
High Harmonic Generation, Pump-Probe, Gas cell
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-330775 (URN)10.1016/j.elspec.2017.09.004 (DOI)000428825400007 ()
Conference
The Future of X-Ray and Electron Spectroscopies, Uppsala, Sweden, June 15-17, 2016
Funder
Knut and Alice Wallenberg FoundationCarl Tryggers foundation
Available from: 2017-10-03 Created: 2017-10-03 Last updated: 2018-06-07Bibliographically approved
Paul, S., Iusan, D., Thunström, P., Kvashnin, Y., Hellsvik, J., Pereiro, M., . . . Eriksson, O. (2018). Investigation of the spectral properties and magnetism of BiFeO3 by dynamical mean-field theory. Physical Review B, 97(12), Article ID 125120.
Open this publication in new window or tab >>Investigation of the spectral properties and magnetism of BiFeO3 by dynamical mean-field theory
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 12, article id 125120Article in journal (Refereed) Published
Abstract [en]

Using the local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence-band photoelectron spectra and magnetic excitation spectra of BiFeO3, one of the most studied multiferroics. Within the DMFT approach, the local impurity problem is tackled by the exact diagonalization solver. The solution of the impurity problem within the LDA+DMFT method for the paramagnetic and magnetically ordered phases produces result in agreement with the experimental data on electronic and magnetic structures. For comparison, we also present results obtained by the LDA+U approach which is commonly used to compute the physical properties of this compound. Our LDA+DMFT derived electronic spectra match adequately with the experimental hard x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy for Fe 3d states, whereas the LDA+U method fails to capture the general features of the measured spectra. This indicates the importance of accurately incorporating the dynamical aspect of electronic correlation among Fe 3d orbitals to reproduce the experimental excitation spectra. Specifically, the LDA+DMFT derived density of states exhibits a significant amount of Fe 3d states at the position of Bi lone pairs, implying that the latter are not alone in the spectral scenario. This fact might modify our interpretation about the origin of ferroelectric polarization in this material. Our study demonstrates that the combination of orbital cross sections for the constituent elements and broadening schemes for the spectral functions are crucial to explain the detailed structures of the experimental electronic spectra. Our magnetic excitation spectra computed from the LDA+DMFT result conform well with the inelastic neutron scattering data.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-351431 (URN)10.1103/PhysRevB.97.125120 (DOI)000427602000002 ()
Funder
Swedish Research CouncilCarl Tryggers foundation
Available from: 2018-06-01 Created: 2018-06-01 Last updated: 2018-06-01Bibliographically approved
Jana, S., Terschlüsen, J. A., Stefanuik, R., Plogmaker, S., Troisi, S., Malik, R. S., . . . Karis, O. (2017). A setup for element specific magnetization dynamics using the transverse magneto-optic Kerr effect in the energy range of 30-72 eV. Review of Scientific Instruments, 88(3), Article ID 033113.
Open this publication in new window or tab >>A setup for element specific magnetization dynamics using the transverse magneto-optic Kerr effect in the energy range of 30-72 eV
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2017 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 88, no 3, article id 033113Article in journal (Refereed) Published
Abstract [en]

In this paper, we present a spectrometer that is designed for element-specific and time-resolved transverse magneto-optic Kerr effect experiments at the high-harmonic generation pump-probe facility High Energy Laser Induced Overtone Source (HELIOS) laboratory. HELIOS delivers photons with energies between 30 eV and 72 eV with an overall time resolution of less than 40 fs. The spectrometer is based on a Rowland-circle geometry and allows for simultaneous measurements of all magnetic transition-metal elements. The setup also features easy sample transfer and alignment, and it combines high photon throughput, optimized data acquisition, and a fast switching of the magnetic field at the sample. The spectrometer performance is demonstrated by measuring the ultrafast demagnetization of permalloy. Our data are, for all practical purposes, identical to what have been reported in the earlier high-order harmonic generation work of a similar sample by Mathias et al. [Proc. Natl. Acad. Sci. U.S.A. 109, 4792-4797 (2012)], however, obtained within 15% of the acquisition time compared to their study. Furthermore, our data show a shift of the demagnetization curve of Ni relative to Fe, which has previously been interpreted as a delay of the Ni demagnetization to that of Fe [S. Mathias et al., Proc. Natl. Acad. Sci. U. S. A. 109, 4792-4797 (2012)].

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2017
National Category
Accelerator Physics and Instrumentation Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-320922 (URN)10.1063/1.4978907 (DOI)000397871400014 ()28372391 (PubMedID)
Available from: 2017-05-30 Created: 2017-05-30 Last updated: 2018-05-16Bibliographically approved
Mazumdar, D., Knut, R., Thöle, F., Gorgoi, M., Faleev, S., Mryasov, O. N., . . . Karis, O. (2016). The valence band electronic structure of rhombohedral-like and tetragonal-like BiFeO3 thin films from hard X-ray photoelectron spectroscopy and first-principles theory. Journal of Electron Spectroscopy and Related Phenomena, 208, 63-66
Open this publication in new window or tab >>The valence band electronic structure of rhombohedral-like and tetragonal-like BiFeO3 thin films from hard X-ray photoelectron spectroscopy and first-principles theory
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2016 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 208, p. 63-66Article in journal (Refereed) Published
Abstract [en]

Abstract We investigate the electronic structure of rhombohedral-like (R) and tetragonal-like (T) BiFeO3 thin films using high energy X-ray photoelectron spectroscopy and first-principles electronic structure calculations. By exploiting the relative elemental cross sections to selectively probe the elemental composition of the valence band, we identify a strong Bi 6p contribution at the top of the valence band in both phases, overlapping in energy range with the O 2p states; this assignment is confirmed by our electronic structure calculations. We find that the measured occupied Bi 6p signal lies closer to the top of the valence band in the T phase than in the R phase, which we attribute, using our electronic structure calculations, to lower Bi–O hybridization in the T phase. We note, however, that our calculations of the corresponding densities of states underestimate the difference between the phases, suggesting that matrix element effects resulting from the different effective symmetries also contribute. Our results shed light on the chemical nature of the stereochemically active Bi lone pairs, which are responsible for the large ferroelectric polarization of BiFeO3.

Keywords
Hard X-ray photoelectron spectroscopy, Multifunctional materials, Multiferroics, Electronic structure, Density functional theory, Bismuth ferrite
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-285250 (URN)10.1016/j.elspec.2015.10.002 (DOI)000375738900011 ()
Funder
Swedish Research CouncilEU, European Research Council
Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2017-11-30Bibliographically approved
Chen, C., Tao, Z., Hernández-García, C., Matyba, P., Carr, A., Knut, R., . . . Murnane, M. (2016). Tomographic reconstruction of circularly polarized high-harmonic fields: 3D attosecond metrology.. Science advances, 2(2)
Open this publication in new window or tab >>Tomographic reconstruction of circularly polarized high-harmonic fields: 3D attosecond metrology.
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2016 (English)In: Science advances, ISSN 2375-2548, Vol. 2, no 2Article in journal (Refereed) Published
Abstract [en]

Bright, circularly polarized, extreme ultraviolet (EUV) and soft x-ray high-harmonic beams can now be produced using counter-rotating circularly polarized driving laser fields. Although the resulting circularly polarized harmonics consist of relatively simple pairs of peaks in the spectral domain, in the time domain, the field is predicted to emerge as a complex series of rotating linearly polarized bursts, varying rapidly in amplitude, frequency, and polarization. We extend attosecond metrology techniques to circularly polarized light by simultaneously irradiating a copper surface with circularly polarized high-harmonic and linearly polarized infrared laser fields. The resulting temporal modulation of the photoelectron spectra carries essential phase information about the EUV field. Utilizing the polarization selectivity of the solid surface and by rotating the circularly polarized EUV field in space, we fully retrieve the amplitude and phase of the circularly polarized harmonics, allowing us to reconstruct one of the most complex coherent light fields produced to date.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-285220 (URN)10.1126/sciadv.1501333 (DOI)26989782 (PubMedID)
Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2016-05-04
Kfir, O., Grychtol, P., Turgut, E., Knut, R., Zusin, D., Popmintchev, D., . . . Cohen, O. (2015). Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics. Nature Photonics, 9(2), 99-105
Open this publication in new window or tab >>Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics
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2015 (English)In: Nature Photonics, ISSN 1749-4885, E-ISSN 1749-4893, Vol. 9, no 2, p. 99-105Article in journal (Refereed) Published
Abstract [en]

Circularly-polarized extreme ultraviolet and X-ray radiation is useful for analysing the structural, electronic and magnetic properties of materials. To date, such radiation has only been available at large-scale X-ray facilities such as synchrotrons. Here, we demonstrate the first bright, phase-matched, extreme ultraviolet circularly-polarized high harmonics source. The harmonics are emitted when bi-chromatic counter-rotating circularly-polarized laser pulses field-ionize a gas in a hollow-core waveguide. We use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase-matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to linearly polarized high harmonic sources. This work represents a critical advance towards the development of table-top systems for element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-285243 (URN)
Note

Article

Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2017-11-30
Hickstein, D. D., Dollar, F. J., Grychtol, P., Ellis, J. L., Knut, R., Hernández-García, C., . . . Durfee, C. G. (2015). Non-collinear generation of angularly isolated circularly polarized high harmonics. Nature Photonics, 9(11), 743-750
Open this publication in new window or tab >>Non-collinear generation of angularly isolated circularly polarized high harmonics
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2015 (English)In: Nature Photonics, ISSN 1749-4885, E-ISSN 1749-4893, Vol. 9, no 11, p. 743-750Article in journal (Refereed) Published
Abstract [en]

We generate angularly isolated beams of circularly polarized extreme ultraviolet light through the first implementation of non-collinear high harmonic generation with circularly polarized driving lasers. This non-collinear technique offers numerous advantages over previous methods, including the generation of higher photon energies, the separation of the harmonics from the pump beam, the production of both left and right circularly polarized harmonics at the same wavelength and the capability of separating the harmonics without using a spectrometer. To confirm the circular polarization of the beams and to demonstrate the practicality of this new light source, we measure the magnetic circular dichroism of a 20 nm iron film. Furthermore, we explain the mechanisms of non-collinear high harmonic generation using analytical descriptions in both the photon and wave models. Advanced numerical simulations indicate that this non-collinear mixing enables the generation of isolated attosecond pulses with circular polarization.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-285230 (URN)
Note

Article

Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2017-11-30
Knut, R., Lagerqvist, U., Palmgren, P., Pal, P., Svedlindh, P., Pohl, A. & Karis, O. (2015). Photoinduced reduction of surface states in Fe:ZnO. Journal of Chemical Physics, 142(20), Article ID 204703.
Open this publication in new window or tab >>Photoinduced reduction of surface states in Fe:ZnO
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2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 20, article id 204703Article in journal (Refereed) Published
Abstract [en]

We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been found to be an efficient photocatalyst. Using resonant photoemission spectroscopy, we determine the binding energy of Fe 3d states corresponding to different valencies and coordination of the Fe atoms. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material due to the formation of oxygen vacancies. Electronic states corresponding to low-spin Fe2+ are observed and attributed to crystal field modification at the surface. These states are potentially important for the photocatalytic sensitivity to visible light due to their location deep in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is limiting the photocatalytic functionality of ZnO, where the most efficient Fe doping concentration has been shown to be 1%-4%.

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-258046 (URN)10.1063/1.4921570 (DOI)000355919300030 ()26026457 (PubMedID)
Available from: 2015-07-13 Created: 2015-07-10 Last updated: 2017-12-04
Mukherjee, S., Knut, R., Mohseni, S. M., Anh Nguyen, T. N., Chung, S., Tuan Le, Q., . . . Sarma, D. D. (2015). Role of boron diffusion in CoFeB/MgO magnetic tunnel junctions. Physical Review B. Condensed Matter and Materials Physics, 91(8), Article ID 085311.
Open this publication in new window or tab >>Role of boron diffusion in CoFeB/MgO magnetic tunnel junctions
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, ISSN 1098-0121, Vol. 91, no 8, article id 085311Article in journal (Refereed) Published
Abstract [en]

Several scientific issues concerning the latest generation read heads for magnetic storage devices, based on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) are known to be controversial, including such fundamental questions as to the behavior and the role of B in optimizing the physical properties of these devices. Quantitatively establishing the internal structures of several such devices with different annealing conditions using hard x-ray photoelectron spectroscopy, we resolve these controversies and establish that the B diffusion is controlled by the capping Ta layer, though Ta is physically separated from the layer with B by several nanometers. While explaining this unusual phenomenon, we also provide insight into why the tunneling magnetoresistance (TMR) is optimized at an intermediate annealing temperature, relating it to B diffusion, coupled with our studies based on x-ray diffraction and magnetic studies.

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-247784 (URN)10.1103/PhysRevB.91.085311 (DOI)000350319200013 ()2-s2.0-84924081350 (Scopus ID)
Available from: 2015-03-23 Created: 2015-03-23 Last updated: 2017-12-04Bibliographically approved
Mancuso, C. A., Hickstein, D. D., Grychtol, P., Knut, R., Kfir, O., Tong, X.-M., . . . Murnane, M. M. (2015). Strong-field ionization with two-color circularly polarized laser fields. Physical Review A, 91(3)
Open this publication in new window or tab >>Strong-field ionization with two-color circularly polarized laser fields
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2015 (English)In: Physical Review A, Vol. 91, no 3Article in journal (Refereed) Published
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-285240 (URN)
Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2016-05-04
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