Head: Dr. Stefan Schuppler
Electronic correlations in transition-metal oxides lead to a complex landscape of mutually competing structural, electronic, and magnetic phases. Their interplay can be varied by external parameters like chemical composition and/or doping, temperature, stress/strain, and dimensionality, often leading to novel and potentially useful functionalities. Examples include high-temperature superconductivity in cuprates as well as orbital ordering effects in manganites, ruthenates, and cobaltates. To gain a fundamental understanding of such phases and the associated transitions we study in depth their electronic and magnetic structure.
The experiments are performed at IFP′s soft x-ray analytics facility WERA at the synchrotron light source ANKA on site. With photoemission (PES) and angle-resolved PES (ARPES), the occupied electronic structure, band character, and Fermi surface are accessible; resonant PES (RESPES) is element specific. X-ray absorption (XAS), in particular near the absorption edge (NEXAFS) gives the unoccupied electronic structure and orbital character; magnetic dichroism (XMCD) sensitively probes spin states as well as spin and orbital components of the magnetization. Imaging with lateral resolution down to 100 nm and microspectroscopy is possible in a photoemission electron microscope (PEEM). All methods are (or will soon be) interconnected in ultrahigh vacuum (UHV), also for being mutually accessible from various in-situ preparation chambers for, e.g., epitaxial thin-film growth by pulsed-laser deposition (PLD) and by evaporation, or for carbon-based and other nanosystems. This ′integrated approach′ to instrumentation and preparation is at the core of WERA′s design. It allows the user to prepare his/her samples and to study them with a combination of complementing electron spectroscopies, each emphasizing a different aspect, yet all in one instrument and without having to expose the samples to air or other possible contamination.
With a bending magnet as the light source, the beamline covers photon energies between 100 and 1500 eV, providing high flux at high resolution as well as circular and linear polarization. In the near future an undulator can alternatively be utilized as the light source, further increasing flux density for demanding experiments.
Beamline and experimental stations are also user facilities embedded in ANKA′s beamtime application system. Calls for scientific external proposals are posted twice a year; applications are rated and beamtime is awarded by an independent review panel. More than 50% of the beamtime is awarded to external users. Vice versa, WERA′s wide experimental base is to a substantial extent due to long-term cooperations with strong external partners: for instance, the PEEM is provided through a cooperation contract with the manufacturer (Focus GmbH), while the XMCD chamber is contributed by the Max-Planck Institute Stuttgart (Prof. G. Schütz, PD E. Goering).
Soft x-ray analytics facility WERA at the Karlsruhe synchrotron light source ANKA:
1 - Refocusing mirror;
2 - Photoemission electron microscope (PEEM);
3 - Photoemission/NEXAFS chamber; 4 - Load lock;
5 - High resolution (AR)PES analyzer; 6 - Fluorescence yield for NEXAFS;
7 - Surface preparation chamber; 8 - Chamber for pulsed laser deposition (PLD)
Soft phonons reveal the nematic correlation length in Ba(Fe0.94Co0.06)2As2
F. Weber, D. Parshall, L. Pintschovius, J.-P. Castellan, M. Kauth, M. Merz,
Th. Wolf, M. Schütt, J. Schmalian, R. M. Fernandes, D. Reznik
Phys. Rev. B 98 (2018) 14516.
Spin State in Perfluorinated FePc films on Cu(111) and Ag(111) in Dependence on Film Thickness
A. Belser, R. Karstens, P. Grüninger, P. Nagel, M. Merz, S. Schuppler,
E. A. Suturina, A. Chassé, Th. Chassé, H. Peisert
J. Phys. Chem. C (2018) .
On the thermodynamic stability of (Eu,Y)-doped barium cerate
N. I. Matskevich, Th. Wolf, M. Merz, P. Adelmann, O. I. Anyfrieva,
M. Y. Matskevich
Mendeleev Comm. 28 (2018) 108.
Size-induced changes of structural and ferromagnetic properties in La1-xSrxMnO3 Nanoparticles
C. Hintze, D. Fuchs, M. Merz, M. Amari, C. Kuebel, M.-J. Huang, A. Powell,
J. Appl. Phys. 121 (2017) 214303.
Current research interests
IFP operates a number of in-house x-ray diffractometers for the characterization of samples of crystalline materials:
- For phase identification, powder diffractometers are available which can be operated with Cu or Mo radiation and position-sensitive detectors.
- A four-circle-diffractometer and an image-plate single-crystal diffractometer allow for a more detailed determination of lattice parameters, atomic positions, bond lengths, doping levels or structural/nematic phase transitions, the last-mentioned instrument even at temperatures down to 80 K and up to 350 K.
- A Laue apparatus with back-scattering geometry is used for high-precision alignment of single crystals.
For many transition-metal oxides, the intricate interplay between charge, spin (up or down), orbital, and lattice degrees of freedom leads to interesting and unusual electronic and magnetic phenomena such as high-temperature superconductivity, colossal magnetoresistance, and complex magnetic orbital ordering. In the case of the cobaltates, the spin state is an additional degree of freedom depending on the delicate balance between the crystal-field splitting, i.e., the energetic splitting between t2g and eg orbitals, and the exchange interaction associated with Hund’s rule coupling.
Comparison of the experimental Co L2,3 NEXAFS spectra of La2CoO4, La1.5Ca0.5CoO4, LaCaCoO4, and La0.5Ca1.5CoO4 taken at 300 K (left) with the results of Co 2p XAS multiplet calculations of Co for different valence and spin states (lower curves in the right panel; LS: low spin, IS: intermediate spin, HS: high spin): Co3+ LS, Co3+ HS, Co3+ IS, Co2+ HS, and Co4+ HS. Simulated spectra of La2CoO4, La1.5Ca0.5CoO4, LaCaCoO4, and La0.5Ca1.5CoO4 are shown in the upper part of the right panel. For clarity, the spectra for the different configurations are offset vertically.
- Double exchange via t2g orbitals and Jahn-Teller effect in ferromagnetic La0.7Sr0.3CoO3 probed by epitaxial strain
D. Fuchs, M. Merz, P. Nagel, R. Schneider, S. Schuppler, H. v. Löhneysen
Phys. Rev. Lett. 111 (2013) 257203.
- Spin and Orbital States in Single-Layered La2−xCaxCoO4 Studied by Doping- and Temperature-Dependent Near-Edge x-Ray Absorption Fine Structure
M. Merz, D. Fuchs, A. Assmann, S. Uebe, H. v. Löhneysen, P. Nagel, S. Schuppler
Phys. Rev. B 84 (2011) 14436.
- X-Ray Absorption and Magnetic Circular Dichroism of LaCoO3, La0.7Ce0.3CoO3, and La0.7Sr0.3CoO3 Films: Evidence for Cobalt-Valence-Dependent Magnetism
M. Merz, P. Nagel, C. Pinta, A. Samartsev, H. v. Löhneysen, M. Wissinger, S. Uebe,
A. Assmann, D. Fuchs, S. Schuppler
Phys. Rev. B 82 (2010) 174416.
- Suppression of spin-state transition in epitaxially strained LaCoO3
C. Pinta, D. Fuchs, M. Merz, M. Wissinger, E. Arac, H. v. Löhneysen, A. Samartsev,
P. Nagel, S. Schuppler
Phys. Rev. B 78 (2008) 174402.
The charge ordering (CO) phenomena in colossal magneto resistance (CMR) materials occurs in perovskite manganese oxides in which the on-site Coulomb interaction is stronger than the kinetic energy of the charge carriers. The antiferromagnetic insulating (AFMI)-ferromagnetic metallic (FMM) transition induced by charge carrier doping in many of the CMR compounds is suppressed by the formation of CO, especially when the carrier concentration equals a commensurate fraction such as 1/8, 1/3 or 1/2. This CO state is intimately related to the CMR properties.
- Electronic Structure of the Electron-Doped Ca0.86Pr0.14MnO3
M.K. Dalai, P. Pal, B.R. Sekhar, M. Merz, P. Nagel, S. Schuppler, C. Martin
Phys. Rev. B 85 (2012) 155128.
- Resonant Photoemission Spectroscopy Studies of the Magnetic Phase Transitions in Pr0.5Sr0.5MnO3
P. Pal, M.K. Dalai, B.R. Sekhar, I. Ulfat, M. Merz, P. Nagel, S. Schuppler
Physica B 406 (2011) 3519.
- Orbital degree of freedom in single-layered La1-xSr1+xMnO4: doping- and temperature-dependent rearrangement of orbital states
M. Merz, G. Roth, P. Reutler, B. Büchner, D. Arena, J. Dvorak, Y. U. Idzerda, S. Tokumitsu, S. Schuppler
Phys. Rev. B 74 (2006) 184414.
Superconductivity in iron-based
materials emerges - as in heavy-fermion systems and high-Tc cuprates - in the vicinity of a magnetic instability. Edge-sharing Fe(As/Se)4 tetrahedra are the structural key ingredient. Many studies strongly suggest that distinct nesting properties of the Fermi surfaces are important for the magnetic properties (development of a spin density wave and antiferromagnetic order at low temperature) as
well as for the superconducting characteristics.
Our investigation shows that charge carrier doping of the Fe 3d states is not crucial for superconductivity in Co-doped Fe pnictides. Rather, the change of the topology of the Fermi surface induced by the Co-substitution seems to be the key parameter.
Comparison of the (a) normal- and (b) grazing-incidence Fe L2,3 NEXAFS spectra of Sr(Fe1-xCox)2As2 (x = 0, 0.05, 0.11, 0.17, and 0.38; for clarity, the spectra are vertically offset) recorded at 300 K. The spectral shape of both edges is unaffected upon doping. The multiplet calculations show that the spectra can be described reasonably well for tetrahedrally coordinated Fe2+. In addition, the spectrum of a deteriorated iron-oxide-containing sample is included in (a) (topmost panel), thereby exhibiting the respective peak positions of the iron oxide.
- Of Substitution and Doping: Spatial and Electronic Structure in Fe Pnictides
M. Merz, P. Schweiss, P. Nagel, M.-J. Huang, R. Eder, T. Wolf, H.v.Löhneysen, S. Schuppler
J. Phys. Soc. Jpn. 85 (2016) 44707.
- Electronic Structure of Single-Crystalline Sr(Fe1-xcCox)2As2 Probed by x-Ray Absorption Spectroscopy: Evidence for Effectively Isovalent Substitution of Fe2+ by Co2+
M. Merz, F. Eilers, Th. Wolf, P. Nagel, H. v. Löhneysen, S. Schuppler
Phys. Rev. B 86 (2012) 104503.
Within the allocation of ANKA beam time to external users we collaborate with many external groups who perform experiments at WERA. Many projects in the field of magnetism and superconductivity are of common interest.
- Luminescence, patterned metallic regions, and photon-mediated electronic changes in single-sided fluorinated graphene sheets
A.L. Walter, H. Sahin, K.J. Jeon, A. Bostwick, S. Horzum, R. Koch, F. Speck, M. Ostler, P. Nagel, M. Merz, S. Schuppler, L. Moreschini, Y.J. Chang, T. Seyller, F.M. Peeters, K. Horn, E. Rotenberg
ACS Nano 8 (2014) 7801.
- Beyond the Heisenberg model: anisotropic exchange interaction between a Cu-tetraazaporphyrin monolayer and Fe3O4 (100)
J. Klanke, E. Rentschler, K. Medjanik, D. Kutnyakhov, G. Schönhense, S. Krasnikov, I. V. Shvets, S. Schuppler, P. Nagel, M. Merz, H. J. Elmers
Phys. Rev. Lett. 110 (2013) 137202.
- Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane-A
K. Medjanik. D. Chercka, P. Nagel, M. Merz, S. Schuppler, M. Baumgarten, K. Müllen,
S.A. Nepijko, H.-J. Elmers, G. Schönhense, H.O. Jeschke, R. Valenti
J. Am. Chem. Soc. 134 (2012) 4694.
- Multiplexed lipid dip-pen nanolithography on subcellular scales for the templating of functional proteins and cell culture
S. Sekula, J. Fuchs, S. Weg-Remers, P. Nagel, S. Schuppler, J. Fragala, N. Theilacker, M. Franzreb, C. Wingren, P. Ellmark, C.A.K. Borrebaeck, C.A. Mirkin, H. Fuchs, S. Lenhert
Small 4 (2008) 1785.
As a further particular example, intense collaborations exist in the field of multilayers and superlattices, in particular those compounds composed of transition-metal oxides.
- X-ray absorption spectroscopy study of the electronic and magnetic proximity effects in YBa2Cu3O7/La2/3Ca1/3MnO3 and La2−xSrxCuO4/La2/3Ca1/3MnO3 multilayers
M.A. Uribe-Laverde, S. Das, K. Sen, I. Marozau, E. Perret, A. Alberca, J. Heidler, C. Piamonteze, M. Merz, P. Nagel, S. Schuppler, D. Munzar, C. Bernhard
Phys. Rev. B 90 (2014) 205135.
- Magnetic Proximity Effect in YBa2Cu3O7/LaMnO3+δ Superlattices
D.K. Satapathy, M.A. Uribe-Laverde, I. Morazau, V.K. Malik, S. Das, Th. Wagner,
C. Marcelot, J. Stahn, S. Brück, A. Rühm. S. Macke, T. Tietze, E. Goering, A. Frano,
J.-H. Kim, M. Wu, E. Benckiser, B. Keimer, A. Devishvili, B.P. Toperverg, M. Merz, P. Nagel, S. Schuppler, C. Bernhard
Phys. Rev. Lett. 108 (2012) 197201.
- YBa2Cu3O7/La0.7Ca0.3MnO3 Bilayers: Interface Coupling and Electric Transport Properties
R. Werner, C. Raisch, A. Ruosi, B.A. Davidson, P. Nagel, M. Merz, S. Schuppler,
M. Glaser, J. Fujii, T. Chassé
Phys. Rev. B 82 (2010) 224509.
Current group members (in alphabetical order)
- Severin Adandogou
- Amir Ghiami
- Fatemeh Ghorbani
- Dr. Michael Merz
- Dr. Peter Nagel
- Dr. Stefan Schuppler
Former group membersDr. Daniel Ade, Dr. Johannes Armbruster, Dr. Andrea Assmann, Steffen Bender, Dr. Tom R. Cummins, Dr. Stefan Gerhold, Dr. C.S. Gopinath, Prof. Dr. Franz Ulrich Hillebrecht (1953-2006), Dr. Meng-Jie Huang, Daniel Kronmüller, Dr. Ingo Krug, Prof. Dr. Christine A. Kuntscher, Dr. Guodong Liu, Dr. Dong-Hui Lu, Dr. Nikolaus Nücker, Dr. Eric Pellegrin, Dr. Christian Pinta, M.Sc. Andreas Plog, Dr. Andrey Samartsev, Dipl.-Ing. (FH) Bernd Scheerer, Dr. Michael Schmidt, Prof. Dr. Biju Sekhar, Dr. Shuzo Tokumitsu, Dr. Stephan Uebe, et al.