The group performs x-ray absorption spectroscopy (XAS) with linear (XLD) and circular polarized light (XMCD) as well as resonant x-ray diffraction (RXD), resonant (RIXS) and nonresonant inelastic x-ray scattering (NIXS), plus angle resolved  (ARPES) and hard x-ray photo emission spectroscopy (HAXPES) on bulk and thin film materials of 3d, 4d transition metal and rare earth compounds.

Hu, Chang, Haverkort, Tjeng

1. Correlation between grounds state and orbital anisotropy in heavy fermion materials [Willers et al. PNAS 112, 2384 (2015), doi: 10.1073/pnas.1415657112]

The interplay of structural, orbital, charge, and spin degrees of freedom is at the heart of many physical phenomena, including superconductivity. Unravelling the underlying forces requires the understanding of each of these degrees of freedom as well as the interplay between them. Cerium-based heavy fermion compounds are an ideal playground for investigating these interdependencies, and we recently were able to show a correlation between the orbital anisotropy and the ground states in a representative family of materials.

The 4f crystal-electric field ground-state wave functions of the strongly correlated materials CeRh1xIrxIn5 was measured with great accuracy using linear polarization-dependent soft X-ray absorption spectroscopy (XAS) and it turned out these wave functions correlate with the ground-state properties of the substitution series. This material was chosen because the phase diagram of CeMIn5, with M= Co, Rh, and Ir, covers long-range antiferromagnetic order, unconventional superconductivity, and coexistence of these two states (see Figure) as well as Fermi surface changes when substituting one M element for another, thus making it an important model system for searching parameters that correlate with ground state properties.  We found that the 4f ground state orbital elongates increasingly in c-direction (long tetragonal axis) when going from the antiferromagnetic to the superconducting region, but not in a linear manner (see red dots in figure below). The purely magnetic (purely superconducting) regions clearly prefer the shorter (elongated) orbitals.  We could rule out trivial lattice effects because the orbital extension anticorrelates with the lattice expansion when substituting Rh with Ir in CeRh1-xIrIn5. The conclusion drawn from this study is that orbital anisotropy and/or hybridization are important aspects that should be taken into account when modelling these materials.

Traditionally crystal-field wave functions in rare earth compounds are determined with inelastic neutron scattering. However, in the presence of strong correlation the magnetic excitation are strongly broadened and the magnetic intensities are hard to determine accurately. Consequently the resulting wave functions are not very reliable. We therefore started some years ago to use soft XAS at the cerium M-edge to target specifically the 4f ground state wave function [see Hansmann et al., doi: 10.1103/PhysRevLett.100.066405]. Here we take advantage of the dipole selection rules for linear polarized light that give sensitivity to the initial state ground state symmetry.



2. Absence of orbital rotation in superconducting CeCu2Ge2 [J.-P- Rueff et al., PRB 91, 201108 (R), doi: 10.1103/PhysRevB.91.201108]

Non-resonant inelastic x-ray scattering (NIXS) with hard x-rays at the rare earth N-edge is a new, state-of-the-art approach for determining ground state wave functions in rare earth compounds. Here we exploit the direction dependence of the scattering function S(q,w) in analogy to the polarization dependence in an XAS experiment.


The use of a scattering experiment with hard x-rays has several major advantages: 1) it is truly bulk sensitive, 2) large momentum transfers (≈10Å-1) can be reached so that scattering from terms higher than dipole occur in the scattering function S(q,w), and 3) it can be modelled quantitatively due to the absence of an intermediate state. The method is particularly useful for measuring charge densities or anisotropies with a higher than two fold symmetry (cubic, tetragonal basal plane, trigonal, etc) where a dipole technique is insensitive. The beyond-dipol-limit gives rise to extra excitations from the higher order scattering terms that are not observable at low momentum transfers (dipole limit) and these extra excitations are direction sensitive even in higher rotational symmetry. We had shown the feasibility of such an experiment at the prototypical heavy fermion compound CeCu2Si2 [see Willers et al.,  doi: 10.1103/PhysRevLett.109.046401].

In the present study the recently proposed orbital transition scenario [Pourovski et al., doi: 10.1103/PhysRevLett.112.106407] in heavy fermion compounds exhibiting two superconducting domes was addressed. The charge densities of CeCu2Ge2, a model system of this class, were probed with NIXS and modelled quantitatively with the full mulitplet code Quanty by M.W. Haverkort. Our NIXS results are well described by considering the same sequence of orbitals at all temperatures, thus putting into question whether orbital transitions are related to the formation of the second superconducting dome.


3. A complete high-to-low spin state transition of Co3+ ion in SrCo0.5Ru0.5O3-d. [J.-M. Chen et al., J. Am. Chem. Soc. 136, 1514 (2014), doi: 10.1021/ja4114006]


The complex metal oxide SrCo0.5Ru0.5O3δ possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution, and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy (XAS). We have discovered that Co3+ ion is purely high-spin at room temperature, which is unique for a Co3+ in an octahedral oxygen surrounding. We attribute this to the crystal-field interaction being weaker than the Hund’s rule exchange due to relatively large mean Co−O distances of 1.98(2) Å. In the FIgure the evolution of the Co Kβ x-ray emission line is shown as a function of pressure from ambient up to 39.6 GPa at 300 K. The intensity of the Kβ satellite peak is widely applied to estimate the local 3d spin moment of transition metal ions. The inset shows the so-called integrated absolute difference (IAD) as a function of pressure, which is proportional to the spin moment of the Co ion. A gradual high-to-low spin state transition is completed by applying hydrostatic pressure as high as 40 GPa. Across this spin state transition, the Co Kβ emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. The highly debated intermediate spin state of Co3+ is absent in this material.


4. Spin-state order/disorder and metal–insulator transition in GdBaCo2O5.5: experimental determination of the underlying electronic structure [see Z. Hu et al., NJP 14, 123025 (2012), doi:10.1088/1367-2630/14/12/123025]

The layered cobaltates RBaCo2O5.5 (R = rare earth) composed of an equal number of Co3+O6 octahedra and Co3+O5 pyramids have attracted considerable attention of the scientific community in recent years because of their peculiar magnetic and transport properties, showing giant magneto-resistance as well as metal–insulator (MIT) and antiferro–ferro-paramagnetic transition phenomena. This MIT is commonly attributed to a sudden spin-state switch of Co3+ at octahedral site, but its modality is controversially discussed with scenarios including full or partial LS-IS or LS®HS state transitions without direct experimental evidence for any of the claimed spin-state transitions.

We have utilized O-K and Co-L2,3 XAS spectroscopy to reveal that half of the Co3+ ions at the octahedral sites are in the low spin (LS) and the other half in the high spin (HS) state, while the Co3+ ions at the pyramidal sites are in the HS configuration. Upon increasing the temperature across the MIT, part of the LS octahedral Co3+ undergoes a spin-state transition into the HS configuration. We infer that this destroys the spin-state ordering and thus explains the decrease in resistivity. We observed that the band gap is reduced but not closed in the high-temperature phase.


5. Coupled valence and spin state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3 [see F. Guillou et al., Phys. Rev. B 87, 115114 (2013), doi: 10.1103/PhysRevB.87.115114]

The coupled valence and spin state transition (VSST) taking place at T*∼89.3 K in (Pr0.7Sm0.3)0.7Ca0.3CoO3 was investigated by XAS experiments carried out at the Pr-M4,5, Co-L2,3, and O-1s edges. At T <T* , we found that the praseodymium displays a mixed valence Pr3+ /Pr4+ with about 0.13 Pr4+/f.u., while all the Co3+ ions are in the LS state. At T T*, the sharp valence transition converts all the Pr4+ to Pr3+ with a corresponding Co3+ to Co4+ compensation. This is accompanied by an equally sharp spin state transition of the Co3+ from the low to an incoherent mixture of low and HS states. An involvement of IS state can be discarded for the Co3+. While above T* and at high temperatures the system shares rather similar properties as Sr-doped LaCoO3, at low temperatures, it behaves much more like EuCoO3 with its highly stable LS configuration for the Co3+. Apparently, the mechanism responsible for the formation of Pr4+ at low temperatures also helps to stabilize the Co3+ in the LS configuration despite the presence of Co4+ ions. We also found out that the Co4+ is in an IS state over the entire temperature range investigated in this study (10–290 K). The presence of Co3+ HS and Co4+ IS at elevated temperatures facilitates the conductivity of the material.


6. Electronic and spin states of SrRuO3 thin films: An x-ray magnetic circular dichroism study [S. Agrestini et al., Phys. Rev. B 91, 075127 (2015),]

SrRuO3 is one of the few known 4d transition-metal oxide ferromagnets with a Curie temperature as high as 160 K. Its rare physical properties have raised the interest of the scientific community during the last fifty years and recently also of the applied science sector due to its capacity for being used as an electrically conducting layer within magnetic heterostructure devices as used in storage technologies.

The systematic growth of thin films on (111) oriented SrTiO3 substrates is very recent and first SQUID measurements suggest the surprising possibility of the stabilization of the high spin state (HS) S = 2 for Ru4+ ion. In this study, we have investigated strained SrRuO3 thin layers on (001)- and (111)- oriented SrTiO3 substrates and compared them with unstrained SrRuO3 single crystals.

Using synchrotron light from the BOREAS beamline at the synchrotron ALBA, we wanted to determine if the compressive strain from the SrTiO3 substrate can indeed induce a spin-state transition of the Ru4+ cations and if the Ru orbital magnetic moment is quenched (i.e., close to zero). To this end, X-ray magnetic circular dichroism (XMCD) studies of the Ru L2,3 absorption edges were performed.

Results show that for the strained as well as the unstrained samples the Ru orbital moment is close to zero and that the Ru spin (and thus the associated spin magnetic moment) is close to the low-spin value of S=1, as could be verified using theoretical calculations on the shape of the Ru L2,3 absorption spectra (see Figure).

From a comparison of the experimental with simulated spectra we could determine the effective crystal field. The hypothesis of a compressive strain-induced spin-state transition, as proposed in literature on the basis of SQUID measurements, can be ruled out as the stabilization of the high spin state with S = 2 would be too costly in energy.


7. Spectroscopic evidence for exceptionally high orbital moment induced by local distortions in α-CoV2O6 [N. Hollmann et al., Phys. Rev. B 89, 20110 (R) (2014), doi 10.1103/PhysRevB.89.201101]

We have examined the local magnetism of the Co spin chain compounds CoV2O6, which crystallizes in two different allotropic phases, α- and γ-CoV2O6 with x-ray magnetic circular dichroism. We have found an exceptionally high and a moderate orbital contribution to the magnetism in α-CoV2O6 and γ -CoV2O6, respectively. Full-multiplet calculations indicate that the differences in the magnetic behavior of α- and γ-CoV2O6 phases originate from different local distortions of the CoO6 octahedra. In particular, the strong compression of the CoO6 octahedra together with the unusually small octahedral crystal-field splitting in α-CoV2O6 lead to a strong mixture of t2g and eg orbitals which, via the local atomic Coulomb and exchange interactions, results in an exceptionally large orbital moment close to 2 µB.


8. Orthorhombic BiFeO3 [J.C. Yang, et al., Phys. Rev. Lett. 109, 247606 (2012), doi: 10.1103/PhysRevLett.109.247606]

Multiferroic BiFeO3 (BFO) with its high ferroelectric Curie temperature of 820 Cº and Neel temperature of 370 Cº has received great research interest from a fundamental point of view but also because of its potential for applications to next-generation logic, memory, and high-frequency devices. Up to now, most studies focus on the ferroelectricity. The antiferromagnetism (AFM) is still far less studied because of the limited tools for investigating the orientation of the AFM axis in    thin films. Here using strain should give more insight into the rich spin physics of the AFM response in BFO thin.

Bulk BiFeO3 has cycloidal spin structure and epitaxial BiFeO3 films exhibit four AFM axis. The spin orientation in the BiFeO3 film films can be determined by XLD at the Fe-L2,3 edges. Figure (a) and (b) show the experimental Fe L2,3 edge XAS spectra for linear polarized light with perpendicular ad parallel to the c-axis [E^c (red) and E||c (black)] for two different samples; (a) for BFO thin films on DyScO3 (DSO c/a = 1.005) with very weak compressive strain and (b) for BFO thin films on orthorhombic NdScO3(110) (NSO c/a = 0.981) with in-plane tensile strain. There is distinct polarization dependence. The A and C intensities are larger for E^c and the intensities at B and D are larger for E||c in the BFO/DSO thin films while it is the opposite in BFO/NSO.

To extract the orientations of theses AFM axes, we have simulated the experimental spectra using configuration interaction cluster calculations. Figure (c)–(g) show the theoretical Fe-L2,3 E^c (red line and E//c (black line) XAS spectra of BFO thin films as a function of the angle θ between the AFM axis and the c axis. We found that the best agreement between theoretical and experimental spectra occurs at θ = 34º for BFO/NSO and θ = 66º for BFO/DSO. Our results indicate that under tensile strain and compressive strain the AFM axis is close to c axis and ab-plane, respectively.


9. k=0 magnetic structure and absence of ferroelectricity in SmFeO3 [C.-Y. Kuo et al., Phys. Rev. Lett. 113, 217203 (2014), doi: 10.1103/PhysRevLett.113.217203]

It is hard to rotate the spin orientation in antiferromagnetic materials by applying a magnetic field because there is no net magnetic moment. However, for a high spin Fe3+ ion with half-filled 3d orbitals, the orbital moment is strongly quenched resulting in a very weak magnetic anisotropy. Hence the spin direction could be altered by slightly modifying the crystal structure via changing temperatures. The antiferromagnetic SmFeO3 is a nice example for the spin reorientation by change of temperature.

The Figure shows a considerable size of the XMLD signals between the electric field E//b and E//c in (a), between E//a and E//c in (b), but nearly no difference between E//a and E//b (c). The sign of the XMLD signals is reversed when going from 440 K to 490K in (a-b). The experimental spectra are nicely reproduced by the calculated spectra shown below experimental data with spins parallel to c- and a-axis at 440 K and 490 K, respectively.



10. Analysis of charge and orbital order in Fe3O4 by Fe L2,3 resonant x-ray diffraction [A. Tanaka et al., Phys. Rev. B 88, 195110 (2013), doi: 10.1103/PhysRevB.88.195110]

Resonant X-ray Diffraction (RXD) is a useful tool to investigate systems with multiple sites, because of its site selectivity. Particularly, RXD at the transition-metal L2,3 edge is expected to provide direct information on the 3d electronic state of transition-metal compounds, since the 2p to 3d dipole excitation occurs to its intermediate state. Here we exploit the azimuth angle, incident photon polarization, and energy dependence of the (001/2)c and (001)c reflection intensities to investigate the orbital and charge order below the Verwey transition temperature (Tv).

Magnetite (Fe3O4) is a magnetic mineral so abundant even known as a loadstone in the Greek era. Nowadays, in addition to its magnetic properties, considerable attention is drawn to a mysterious transition accompanied by a lattice deformation at temperature Tv ~125 K, where an abrupt increase of resistivity by two orders of magnitude on cooling was discovered by Verwey in 1939. Although extensive studies have been made both on experimental and theoretical sides even after more than 70 years, the mechanism of this Verwey transition still remains a highly controversial problem.

To elucidate charge and orbital order below the Verwey transition temperature, a thin layer of magnetite partially detwined by growth on the stepped MgO (001) substrates has been studied by means of soft RXD at the Fe L2,3 resonance. The azimuth angle, incident photon polarization, and energy dependence of the (001/2)c and (001)c reflection intensities have been measured, and analysed using a configuration-interaction FeO6 cluster model. The azimuth dependence of the (001/2)c reflection intensities directly represents the space-group symmetry of the orbital order in the initial state rather than indirectly through the intermediate-state level shifts caused by the order-induced lattice distortions. From the analysis of the (001/2)c reflection intensities, the orbital order in the t2g orbitals of B sites below Tv is proved to have a large monoclinic deformation with the value of Re[Fxy]/Re[Fyz] ~2. This finding extremely limits possible theories on the Verwey transition, since the majority of them so far proposed do not assume charge or orbital orders with such a large monoclinic deformation. Furthermore, the incident photon polarization and energy dependence of the (001/2)c reflection intensities cannot be explained by real-number orbital and charge orders predicted by the LDA + U and GGA + U band structure theories [Jeng et al., doi: 10.1103/PhysRevLett.93.156403; Leonov et al., doi: 10.1103/PhysRevLett.93.146404], but by the complex-number orbital order (COO) with a large monoclinic deformation [H. Uzu and A. Tanaka, doi: 10.1143/JPSJ.77.074711].

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