Activity Report

Transcription

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3 ctivity Report Edited by U. Johansson,. Nyberg, R. Nyholm, H. Ullman

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5 Preface The present MX-lab ctivity Report summarizes the activities at MX-lab for the period July 2005 to December The expansion of the laboratory continues in terms of users, beamlines and other experimental facilities. Furthermore, the Swedish Research Council (VR) has decided to continue the ramping up of the operating budget also for the coming two years. The 0.7 GeV MX III ring has been commissioned during this period and the handling of the MX IV proposal has taken further steps. The MX II operation has been characterized by increasing reliability, increasing maximum beam current and further improved beam lifetime. The first three beamlines of the Cassiopeia system for protein crystallography, including the MD beamline, are in full use. new Small-ngle X-ray Scattering (SXS) activity has started at L I711 and has attracted a large number of new users. mong other expansions we note the SPELEEM system at L I311 that has added most interesting new research opportunities in the field of spectromicroscopy and a new magnetism beamline (I1011) based on an EPU insertion device that is presently under commissioning. number of synchrotron radiation beamlines are still in use at MX I. The new IR microscope has been set up at the existing IR beamline at MX I and will later be relocated to a new beamline at MX III. The very successful beamline 33 from MX I is presently being relocated to MX III. new NIM beamline is also being set up at this ring. This beamline will have one branch line for the investigation of solids. Finnish-Estonian consortium has financed a second branch line that will be used for atomic and molecular research and for luminescence work. The photonuclear research at MX I has been restarted after the injector upgrade, and there have been a number of successful runs during the reporting period. One important aspect of the MX III project is also to carefully investigate the magnet design and other design concepts that are central to the MX IV project. The results have been very positive and they have brought the MX IV design forward by another important step. Free Electron Laser (FEL) test facility, based on the 500 MeV linac injector, is being assembled. The project is part of the EU-financed EUROFEL project and it will be used for seeding experiments. This work is done in collaboration with other European partners, mainly ESSY. MX-lab and the Lund Laser Centre are also setting up new collaborative research projects in this field. The MX IV CDR report has been submitted to VR and has been evaluated by two international evaluation panels. The evaluations are very positive and the concluding recommendation is: MX IV should be funded to the level requested, and the funding should commence as soon as possible. The board of VR is supporting the project and has recommended the Government to investigate how to find ways to finance the project. I want to take this opportunity to thank the MX-lab staff and the users for creating such an excellent and pleasant research environment at MX-lab. I also want to thank the Swedish Research Council, the Knut and lice Wallenberg foundation, the Foundation for Strategic Research, Lund University as well as all other agencies that contribute to the financing of the laboratory. Lund 10 June 2007 Nils Mårtensson Director MX-lab

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7 Synchrotron Radiation eamline I511 Resonant inelastic x-ray scattering at the NiO O K-resonance: non-local charge-transfer and double singlet excitations Resonant L.-C. inelastic Duda, T. Schmitt x-ray, scattering M. Magnuson, atj. the Forsberg, NiO. O Olsson, K-resonance: and J. Nordgren non-local charge-transfer and double singlet excitations L.-C. Duda, T. Schmitt, M. Magnuson, J. Forsberg,. Olsson, and J. Nordgren Department of Physics, Uppsala University, P. O. ox 530, S Uppsala, Sweden and present L.-C. address: Duda, Swiss T. Schmitt Light Source,, M. Magnuson, Paul Scherrer J. Forsberg, Institute, CH Olsson, Villigen and J. PSI, Nordgren Resonant Department inelastic of Physics, x-ray Uppsala scattering University, P. ato. the oxnio 530, S-751 O K-resonance: 21 Uppsala, Sweden Switzerland non-local and present address: charge-transfer Swiss Light Source, and Paul K. Okada, Scherrer doubleinstitute, singlet and. Kotani C,D CH-5232 excitations Villigen PSI, Switzerland The Graduate School of Natural Science and Technology, Okayama University, Okayama , Japan L.-C. C RIKEN/Spring8,1-1-1 Duda, T. Schmitt Kouto,, K. M. Okada, Magnuson, Mikazuki-cho, andj.. Saya-gun, Forsberg, Kotani C,D Hyogo. Olsson, , and Japan J. Nordgren and D Photon The Factory, Graduate Department IMSS, School of High Physics, of Energy Natural Uppsala ccelerator ScienceUniversity, andresearch Technology, P. Organization, O. ox Okayama 530, 1-1 S-751 University, Oho 21Tsukuba, Uppsala, Okayama Ibaragi Sweden , , and Japan Japan present C RIKEN/Spring8,1-1-1 address: Swiss Light Kouto, Source, (Dated: Mikazuki-cho, Paul March Scherrer 30, Saya-gun, Institute, 2007) Hyogo CH , VilligenJapan PSI, Switzerland D Photon Factory, IMSS, High Energy ccelerator Research Organization, 1-1 Oho Tsukuba, Ibaragi , Japan K. Okada, (Dated: March and. 30, Kotani 2007) C,D The Graduate School of Natural Science and Technology, Okayama University, Okayama , Japan C RIKEN/Spring8,1-1-1 Kouto, Mikazuki-cho, Saya-gun, Hyogo , Japan and NiO D Photon is onefactory, of the prototypical IMSS, High Energy compounds ccelerator that has Research highlighted Organization, the importance 1-1 Oho Tsukuba, of correlation Ibaragi effects , in transition Japan metal oxides. Core level spectroscopies bear evidence (Dated: for March the 30, highly 2007) correlated nature of low energy excitations. For instance, the asymmetry of the Ni 2p-line shape has been attributed to non-local charge transfer excitations and multi-site cluster calculations show that solid state effects generally are appreciable for correlated materials, such as cuprates NiO is and one high of the T c prototypical -compounds compounds [1]. that has highlighted the importance of correlation effects in transition metal We have oxides. performed Core level high-resolution spectroscopies angle bear dependent evidence RIXS for thexperiments highly correlated the Onature K-resonance of low energy of NiOexcitations. and comparefor to cluster instance, model the calculations asymmetry of using theani Ni 6 2p-line O 19 cluster[2]. shape has The been O K-RIXS attributed measurements to non-localhave charge beentransfer performed excitations beamline and I511-3 multi-site at MX cluster IIcalculations which is based show on that a modified solid state SX-700 effects monochromator generally arelayout[3]. appreciable Thefor detection correlated system materials, was a grazing such as incidence cuprates and grating highspectrometer T c -compounds in [1]. the Rowland geometry [4]. The spectrometer resolution was set to about 0.5 ev and We the have monochromator performed high-resolution spectral bandangle width dependent was chosen RIXS to experiments have a somewhat thesmaller O K-resonance value. of NiO and compare to cluster We observe, model calculations apart from using the maina Niband 6 O 19 cluster[2]. with a highthe energy O K-RIXS shoulder measurements (HES), previously have been undetected performed dd- at and beamline double singlet I511-3 excitations. MX II which We clarify is based the onorigin a modified of thesx-700 HES which monochromator is found layout[3]. be due tothe non-local detection charge system transfer was a(nlct). grazing incidence grating spectrometer in the Rowland geometry [4]. The spectrometer resolution was set to about 0.5 ev and the monochromator spectral band width was chosen to have a somewhat smaller value. We observe, apart from the main band with a high energy shoulder (HES), previously undetected dd- and double NiO singlet excitations. We clarify the origin of the HES which is found O1s-absorption to be due to non-local charge transfer (NLCT). NiO O1s-absorption Incident x-ray energy NiO O1s-absorption Incident x-ray energy NiO O1s-RIXS DSP Intensity Intensity (arb. Intensity units) (arb. units) (arb. units) bsorption bsorption bsorption depolarized 530 NiO polarized O1s-RIXS depolarized NiO polarized O1s-RIXS depolarized polarized 540 Incident x-ray energy 1.75 ev dd 0.8 ev 1.75 ev dd 0.8 ev 1.75 ev Energy Loss (ev) FIG. 1: Top panel: O K-absorption of NiO. The lettered Energy arrows Loss (ev) mark the chosen excitation energies for the RIXS spectra. ottom panel: O K-RIXS at the first absorption resonance -10 of NiO -5() and 0.5 ev 0 below (). The inset gives a magnified view of the excitations below the NLCT energy of spectra atenergy excitation Loss energy (ev) (the heavy lines represent a three-point average of the data). DSP 550 DSP dd 0.8 ev FIG. 1: Top panel: O K-absorption of NiO. The lettered arrows mark the chosen excitation energies for the RIXS spectra. ottom panel: O K-RIXS at the first absorption resonance of NiO () and 0.5 ev below (). The inset gives a magnified view of the excitations below the NLCT energy of spectra at excitation energy (the heavy lines represent a three-point average of the data). 270 MX-lab ctivity Report

8 Synchrotron Radiation eamline I511 2 depolarized S zx polarized S xx +S zx P2 P3 NiO O1s-RIXS cluster model calculation Intensity [arb. units] P1 P4 P Raman shift [ev] FIG. 2: The line spectra at the bottom show the polarized RIXS components S xx and S zx. The curves represent the theoretical O K-RIXS spectra for NiO in the depolarized geometry and the polarized geometry as labeled. We applied a variable final state lifetime broadening and a gaussian broadening to simulate the instrumental resolution of the RIXS spectra. S xx S zx Moreover, our study highlights more generally the applicability of RIXS for investigating non-local magnetic excitations such as double singlet creation (DSC) states. The upper panel of Fig. 1 shows an O K-absorption scan where the lettered arrows indicate the respective excitation energies for the RIXS spectra. The lower panel of Fig. 1 shows the O K-RIXS emission spectra excited () on the maximum of the first O K-resonance and () at 0.5 ev below the maximum on an energy loss scale. When tuning the x-ray energy to the first NiO O K-absorption peak, the O 1s electron is excited into empty O 2p-states strongly hybridized with the Ni 3d-states. Two different detection geometries are compared: depolarized (polarized) geometry means that the scattered x-rays are detected along (perpendicular to) the direction of the electric field vector of the incident x-rays. The spectra in Fig. 1 are dominated by an intense broad peak (maximum about 7eV energy loss) with a high energy shoulder (4-5eV energy loss). Moreover, we observe excitations at lower energies (< 2eV) which are shown in detail in the inset of Fig. 1. The main contribution of this part of the spectrum is attributed to dd-excitations at about 1 ev which are mediated by the O 1s-core hole state. Note also the extra intensity at about 1.75 ev energy loss that is observed in the polarized geometry but absent in the depolarized geometry. This is attributed to a nonlocal spin-excitation state which we call double-singlet creation (DSC). The calculation reproduces the observations quite well in detail. Four structures (P 1 P 4) are found in both of the polarization specific spectra S zx and S xx and indicates that the overall polarization dependence of O K-RIXS in NiO is weak, as expected, due to the high symmetry in the electronic state of the cluster. On the other hand, we find an extra peak P 5 at 1.9 ev in S xx. This loss energy is roughly twice the intra-atomic exchange interaction strength (Hund coupling energy J H ), which indicates that the peak is caused by a DSC excitation. Local spin-flip excitations have been predicted earlier for RIXS at the L- and M-edges of Cu 2+ and Ni 2+ [5]. In that case a single spin-flip leads to an loss peak at relatively low energy which is presently difficult to resolve instrumentally. In contrast, the DSC excitation is a non-local-type excitation that occurs as a result of exchanging two holes between neighboring Ni sites. In other words, a double singlet state is created as a result of double inter-site CT. The DSC excitation energy is characterized by 2J H but offset somewhat by the strong covalence energy present in NiO. Our results demonstrate that solid state effects of correlated oxides, such as the non-local excitations in NiO, can be identified and the corresponding energies can be accurately determined by combining O K-RIXS experiments and multi-site cluster calculations. [1] M.. van Veenendaal and G.. Sawatzky, Phys. Rev. Lett. 70, 2459 (1993). [2] L.-C. Duda, T. Schmitt, M. Magnuson, J. Forsberg,. Olsson, J. Nordgren, K. Okada, and. Kotani, Phys. Rev. Lett. 96, (2006);L.-C. Duda, T. Schmitt, M. Magnuson, J. Forsberg,. Olsson, J. Nordgren, K. Okada, and. Kotani, Phys. Rev. Lett. 97, (2006). [3] R. Denecke et al., J. Electron Spectrosc. Relat. Phenom , 971 (1999). [4] J. Nordgren et al., Rev. Sci. Instrum. 60, 1690 (1989). [5] F. M. F. de Groot, P. Kuiper, and G.. Sawatzky, Phys. Rev. 57, (1998). MX-lab ctivity Report