2012, Zabolotnyy, V.B., Carleschi, E., Kim, T.K., Kordyuk, A.A., Trinckauf, J., Geck, J., Evtushinsky, D., Doyle, B.P., Fittipaldi, R., Cuoco, M., Vecchione, A., Büchner, B., Borisenko, S.V.

We present an angle-resolved photoemission study of the surface and bulk electronic structure of the single layer ruthenate Sr2RuO4. As the early studies by photoemission and scanning tunneling microscopy were confronted with a problem of surface reconstruction, surface ageing was previously proposed as a possible remedy to access the bulk states. Here, we suggest an alternative way by demonstrating that, in the case of Sr2RuO4, circularly polarized light can be used to disentangle the signals from the bulk and surface layers, thus opening the possibility to investigate many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting of the surface β band. We discuss the origin of the splitting of the β band and the possible connection with the Rashba effect at the surface.

2015, Zwiebler, M., Hamann-Borrero, J.E., Vafaee, M., Komissinskiy, P., Macke, S., Sutarto, R., He, F., Büchner, B., Sawatzky, G.A., Alff, L., Geck, J.

The analysis of x-ray reflectivity data from artificial heterostructures usually relies on the homogeneity of optical properties of the constituent materials. However, when the x-ray energy is tuned to the absorption edge of a particular resonant site, this assumption may no longer be appropriate. For samples realizing lattice planes with and without resonant sites, the corresponding regions containing the sites at resonance will have optical properties very different from regions without those sites. In this situation, models assuming homogeneous optical properties throughout the material can fail to describe the reflectivity adequately. As we show here, resonant soft x-ray reflectivity is sensitive to these variations, even though the wavelength is typically large as compared to the atomic distances over which the optical properties vary. We have therefore developed a scheme for analyzing resonant soft x-ray reflectivity data, which takes the atomic structure of a material into account by 'slicing' it into atomic planes with characteristic optical properties. Using LaSrMnO4 as an example, we discuss both the theoretical and experimental implications of this approach. Our analysis not only allows to determine important structural information such as interface terminations and stacking of atomic layers, but also enables to extract depth-resolved spectroscopic information with atomic resolution, thus enhancing the capability of the technique to study emergent phenomena at surfaces and interfaces.

2018, Bastien, G., Garbarino, G., Yadav, R., Martinez-Casado, F.J., Beltrán, Rodríguez, R., Stahl, Q., Kusch, M., Limandri, S.P., Ray, R., Lampen-Kelley, P., Mandrus, D.G., Nagler, S.E., Roslova, M., Isaeva, A., Doert, T., Hozoi, L., Wolter, A.U.B., Büchner, B., Geck, J., Van Den Brink, J.

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material α-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p∼0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in α-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid.

2015, Zhu, Pengfei, Zhu, Y., Hidaka, Y., Wu, L., Cao, J., Berger, H., Geck, J., Kraus, R., Pjerov, S., Shen, Y., Tobey, R.I., Hill, J.P., Wang, X.J.

We report the experimental demonstration of femtosecond electron diffraction using high-brightness MeV electron beams. High-quality, single-shot electron diffraction patterns for both polycrystalline aluminum and single-crystal 1T-TaS2 are obtained utilizing a 5 fC (~3 × 104 electrons) pulse of electrons at 2.8 MeV. The high quality of the electron diffraction patterns confirms that electron beam has a normalized emittance of ~50 nm rad. The transverse and longitudinal coherence length is ~11 and ~2.5 nm, respectively. The timing jitter between the pump laser and probe electron beam was found to be ~100 fs (rms). The temporal resolution is demonstrated by observing the evolution of Bragg and superlattice peaks of 1T-TaS2 following an 800 nm optical pump and was found to be 130 fs. Our results demonstrate the advantages of MeV electrons, including large elastic differential scattering cross-section and access to high-order reflections, and the feasibility of ultimately realizing below 10 fs time-resolved electron diffraction.

2004, Geck, J., Wochner, P., Kiele, S., Klingeler, R., Revcolevschi, A., v. Zimmermann, M., Büchner, B., Reutler, P.

At temperatures below the metal-insulator transition of La 1-xSrxMnO3 with 0.1 < x < 0.15, a peculiar ferromagnetic and insulating phase is observed which has been intensively discussed over the last few years. We present a detailed investigation of this phase by means of resonant and high energy x-ray scattering along with measurements of the electrical resistivity, thermal expansion, magnetization, and specific heat. Interestingly, the data show that the metal-insulator transition of lightly doped manganites is accompanied by an orbital rearrangement. The microscopic information provided by the x-ray scattering studies together with the analysis of the macroscopic properties implies that the orbital reordering maximizes the gain of double exchange energy and, at the same time, induces an insulating behaviour. The relevance of the double-exchange mechanism for the stabilization of the ferromagnetic insulating phase is further substantiated by studies of (La1-yPr y)7/8Sr1/8MnO3: with increasing praseodymium content, the metal-insulator transition is dramatically suppressed which can naturally be explained by a reduction of the band width upon praseodymium doping.

2020, Zwiebler, M., Di Gennaro, E., Hamann-Borrero, J.E., Ritschel, T., Green, R.J., Sawatzky, G.A., Schierle, E., Weschke, E., Leo, A., Granozio, F. Miletto, Geck, J.

We present a combined resonant soft X-ray reflectivity and electric transport study of LaAlO 3/SrTiO 3 field effect devices. The depth profiles with atomic layer resolution that are obtained from the resonant reflectivity reveal a pronounced temperature dependence of the two-dimensional electron liquid at the LaAlO 3/SrTiO 3 interface. At room temperature the corresponding electrons are located close to the interface, extending down to 4 unit cells into the SrTiO 3 substrate. Upon cooling, however, these interface electrons assume a bimodal depth distribution: They spread out deeper into the SrTiO 3 and split into two distinct parts, namely one close to the interface with a thickness of about 4 unit cells and another centered around 9 unit cells from the interface. The results are consistent with theoretical predictions based on oxygen vacancies at the surface of the LaAlO 3 film and support the notion of a complex interplay between structural and electronic degrees of freedom. © 2020, The Author(s).

2018, Kuzian, R.O., Klingeler, R., Lorenz, W.E.A., Wizent, N., Nishimoto, S., Nitzsche, U., Rosner, H., Milosavljevic, D., Hozoi, L., Yadav, R., Richter, J., Hauser, A., Geck, J., Hayn, R., Yushankhai, V., Siurakshina, L., Monney, C., Schmitt, T., Schmitt, T., Roth, G., Ito, T., Yamaguchi, H., Matsuda, M., Johnston, S., Málek, J., Drechsler, S.-L.

In a recent work devoted to the magnetism of Li2CuO2, Shu et al (2017 New J. Phys. 19, 023026) have proposed a 'simplified' unfrustrated microscopic model that differs considerably from the models refined through decades of prior work. We show that the proposed model is at odds with known experimental data, including the reported magnetic susceptibility χ(T) data up to 550 K. Using an 8th order high-temperature expansion for χ(T), we show that the experimental data for Li2CuO2 are consistent with the prior model derived from inelastic neutron scattering studies. We also establish the T-range of validity for a Curie–Weiss law for the real frustrated magnetic system. We argue that the knowledge of the long-range ordered magnetic structure for T < T N and of χ(T) in a restricted T-range provides insufficient information to extract all of the relevant couplings in frustrated magnets; the saturation field and INS data must also be used to determine several exchange couplings, including the weak but decisive frustrating antiferromagnetic interchain couplings.

2014, Wadati, H., Geck, J., Schierle, E., Sutarto, R., He, F.-J., Hawthorn, D.G., Nakamura, M., Kawasaki, M., Tokura, Y., Sawatzky, G.A.

Coherent epitaxial growth allows us to produce strained crystalline films with structures that are unstable in the bulk. Thereby, the overlayer lattice parameters in the interface plane, (a, b), determine theminimum-energy out-of-plane lattice parameter, cmin (a, b).We showbymeans of density-functional total energy calculations that this dependence can be discontinuous and predict related firstorder phase transitions in strained tetragonal films of the elements V, Nb, Ru, La, Os, and Ir. The abrupt change of cmin can be exploited to switch properties specific to the overlayer material. This is demonstrated for the example of the superconducting critical temperature of a vanadium film which we predict to jump by 20% at a discontinuity of cmin.