Investigation of superconductivity and metamagnetism in single and double layered ruthenates using angle resolved photoelectron spectroscopy
Final Report Abstract
The aim of the project was to study single-layerd (Sr2 RuO4 ) and double-layered (Sr3 Ru2 O7 ) ruthenates using angle resolved photoemission spectroscopy, as this is the method that provides the most direct insight into the low-energy electronic structure, which in turn determines numerous properties, like unconventional superconductivity, unusual heat capacity and meta-magnetism that these materials are famous for. Earlier studies of Sr2 RuO4 by surface sensitive methods like photoemission or scanning tunneling microscopy were confronted with difficulties due to the well known issue with surface reconstruction. In the project is was shown that circularly polarized light can be used to disentangle the signals from the bulk and the surface in a photoemission experiment, thus opening new possibilities for studying effects of many-body interactions both in the bulk and surface layer. The proposed procedure, as compared to a previously used surface degradation, results in an improved effective momentum resolution. Owing to the minimized surface degradation we were able to observe bulk α, β , γ bands and their surface counterparts along with an additional new feature. According to its dichroic pattern, the new feature must be yet another surface counterpart of the β band. Since there are numerous examples where the surface state undergoes splitting due to the spin–orbit interaction we suggested that fully relativistic calculations might be needed to understand the origin of the new state. Comparing obtained data to earlier dHvA parameterization of band structure of the Sr2 RuO4 we concluded that this dHvA parametrization can be improved by inclusion of higher cylindrical harmonics, to better reproduce the form of the Fermi surface. As a next step, relying on our extensive band mapping of Sr2 RuO4, an effective tight-binding fit to the low energy quasiparticle dispersion in Sr2 RuO4 has been developed. In a certain sense, the model is comparable to the famous fit to the de Haas-van Alphen data by Bergeman et al., but owing to specifics of ARPES it benefits from three advantages: (1) unlike the dHvA, the form of the Fermi surface does not have to be reconstructed from a set of cross-sectional areas, but rather each momentum point (kx , ky) is probed directly; similarly, the dispersion (hence the Fermi velocity) of the low energy quasiparticles for each k-point is captured directly in ARPES energy–momentum cuts, thus is does not have to be inferred from fits to Landau-Kosevich formula, which gives only an averaged over the Fermi surface estimate for the effective mass or the Fermi velocity; (3) instead of doing a parametric fit in polar coordinates that would capture only the form of the FS, we rely on a TB-Hamiltonian, which allows easy estimates for the the orbital character of the quasiparticles and their momentum dependent velocities. The developed model will be of value for a more realistic input to compute the orbital dependent magnetic properties in order to test, for example, the relevance of the ferromagnetic or antiferormagnetic fluctuations in settling the spin-triplet pairing in the superconducting phase of Sr2 RuO4. The code developed within the frames of the project to fit the experimental data to tight-binding models was general enough to be applied to other hot-topic systems in the field, namely the unconventional superconductors LiFeAs and YBCO, which resulted in additional external collaboration and publications. As regards the momentum dependence of superconducting gap in Sr2 RuO4 , the observed shift of the leading edge gap when lowering temperature below Tc (from 3 K to 1 K) would be compatible with opening of the superconducting gap in the energy spectrum of Sr2 RuO4. However mapping of the gap was found to be technically unfeasible. It was concluded that such a challenging measurement has to wait for it’s turn, when the development of experimental hardware used in ARPES experiment will offer energy resolution better than 0.5 meV and temperatures as low as 500mK. While it was possible to proceed with the experimental bandstructure mapping of Sr3 Ru2 O7, the unfavorable interplay of the sample quality, quality of the cleaved sample surfaces and the level of complexity of the band structure became a blocking factor preventing establishment of a unique TB model and, hence, hindered further analysis. Microphotographs of the cleaved sample surfaces showed that highly uneven profiles may partially account for the loss of resolution in k-space. One cold still elaborate in this direction of improving sample quality and quality of the cleaved surfaces, however this would require much higher load on the part of our collaborators involved in crystal grows as originally estimated. One possible way to tackle this problem in the future could be nano-ARPES currently developing at several different locations. The I-05 beam-line at DIAMOND synchrotron, UK would be one possibility. Part of the experimental results of the project were published in the Journal of Visualized Experiments, which is the world’s first peer reviewed scientific journal devoted to publishing scientific research in a visual format to increase the accessibility of scientific research. The publication will serve to popularization of the current results as well as the 13 ARPES end-station at Helmholtz-Zentrum Berlin.
Publications
- J. Vis. Exp. 68, e50129, (2012)
S. V. Borisenko, V. B. Zabolotnyy, A. A. Kordyuk, D.V. Evtushinsky, et al.
(See online at https://doi.org/10.3791/50129) - New J. Phys. 14, 063039 (2012)
V. B. Zabolotnyy, E. Carleschi, T. K. Kim, A. A. Kordyuk, et al.
- Phys. Rev. B 85, 064507 (2012)
V. B. Zabolotnyy, A. A. Kordyuk, D. Evtushinsky, V. N. Strocov, et al.
- Phys. Rev. B 86, 174519 (2012)
J. Knolle, V. B. Zabolotnyy, I. Eremin, S. V. Borisenko, et al.
- J. Electron. Spectrosc. Relat. Phenom. 191, 48 (2013)
V. B. Zabolotnyy, D. V. Evtushinsky, A. A. Kordyuk, T. K. Kim, et al.
- Phys. Rev. B 88, 174516 (2013)
Y.Wang, A. Kreisel, V. B. Zabolotnyy, S. V. Borisenko, et al.
- Phys. Rev. B 89, 064514 (2014)
D. V. Evtushinsky, V. B. Zabolotnyy, T. K. Kim, A. A. Kordyuk, et al.
(See online at https://doi.org/10.1103/PhysRevB.89.064514) - Phys. Rev. B 89, 144513 (2014)
F. Ahn, I. Eremin, J. Knolle, V. B. Zabolotnyy, et al.
(See online at https://doi.org/10.1103/PhysRevB.89.144513)
