Strongly correlated electron systems
HZB studies of superconductors and related transition metal oxides have produced important results in the past years. The study of charge order in La1.8−xEu0.2SrxCuO4 (J Fink el al., Phys. Rev. B 83, 092503 (2011)) revealed a first example of static charge order independent of a structural phase transition and in the absence of long-range magnetic order. While charge order is not very well developed in this material, resonant soft x-ray diffraction provided the required sensitivity to observe the subtle ordering phenomenon. Similar is true for the recent observation of doping dependent charge order correlations in electron-doped cuprates proving the concept that superconductivity competes with other orders not only in almost all known classes of hole-doped cuprate superconductors. The main goal in this field is to achieve a final general understanding of the interplay of charge ordering and high-Tc superconductivity with a present focus on manipulating charge order in thin film heterostructures and by external stimulus.
HZB studies on 5f-electron based magnetism focused on the determination of magnetic-field driven phase transitions in uranium compounds such as U(Ru0.92Rh0.08)2Si2 and U2Pd2In, mainly using neutron scattering in extremeley high magnetic fields at the unique High-Field-Magnet at BER II.
In the following we present a few examples of research carried out in the field of strongly correlated electron systems within the last few years.
For a full list of publications of our institute click here.
Noncollinear magnetic structure in U2Pd2In at high magnetic fields
We report an unexpected magnetic-field-driven magnetic structure in the 5f-electron Shastry-Sutherland system U2Pd2In. This phase develops at low temperatures from a noncollinear antiferromagnetic ground state above the critical field of 25.8 T applied along the a-axis. All U moments have a net magnetic moment in the direction of the applied field, described by a ferromagnetic propagation vector qF = (0 0 0) and an antiferromagnetic component described by a propagation vector qAF = (0 0.30 1/2) due to a modulation in the direction perpendicular to the applied field. This noncollinear magnetic structure is due to a complex competition between the single-ion anisotropy, the Dzyaloshinskii-Moryia interaction, and the external magnetic field.
K Prokeš, M Bartkowiak, DI Gorbunov, O Prokhnenko, O Rivin, P Smeibidl, Phys. Rev. Research 2 (2020) 013137
see also the associated Highlight Report
Stabilization of three-dimensional charge order in YBa2Cu3O6+x via epitaxial growth
Incommensurate charge order (CO) has been identified as the leading competitor of high-temperature superconductivity in all major families of layered copper oxides, but the perplexing variety of CO states in different cuprates has confounded investigations of its impact on the transport and thermodynamic properties. The three-dimensional (3D) CO observed in YBa2Cu3O6+x in high magnetic fields is of particular interest, because quantum transport measurements have revealed detailed information about the corresponding Fermi surface. Here we use resonant X-ray scattering to demonstrate 3D-CO in underdoped YBa2Cu3O6+x films grown epitaxially on SrTiO3 in the absence of magnetic fields. The resonance profiles indicate that Cu sites in the charge-reservoir layers participate in the CO state, and thus efficiently transmit CO correlations between adjacent CuO2 bilayer units. The results offer fresh perspectives for experiments elucidating the influence of 3D-CO on the electronic properties of cuprates without the need to apply high magnetic fields.
M Bluschke, A Frano, E Schierle, D Putzky, F Ghorbani, R Ortiz, H Suzuki, G Christiani, G Logvenov, E Weschke, RJ Birgeneau, ED da Silva Neto, M Minola, S Blanco-Canosa, B Keimer, Nature Commun. 9 (2018) 2978
Magnetic structure of U(Ru0.92Rh0.08)2Si2 in static magnetic fields up to 24 T
Neutron diffraction in high magnetic fields reveals a field-induced magnetic phase in a single crystal of U(Ru0.92Rh0.08)2Si2. Our neutron study, combined with high-field magnetization, shows that the magnetic phase above the first metamagnetic transition at μ0Hc1=21.6 T has an uncompensated commensurate antiferromagnetic structure with a propagation vector Q2=(2/3, 0, 0) possessing two single-Q domains. U moments of 1.45(9)μB directed along the c axis are arranged in an up-up-down sequence propagating along the a axis, in agreement with bulk measurements. The U magnetic form factor at high fields is consistent with both the U3+ and U4+ types. The low-field short-range order that emerges from pure URu2Si2 due to Rh doping is initially strengthened by the field but disappears in the field-induced phase. The tetragonal symmetry is preserved across the transition, but the a-axis lattice parameter increases already at low fields. Our results are in agreement with an itinerant electron model with 5f states forming bands pinned in the vicinity of the Fermi surface that is significantly reconstructed by the applied magnetic field.
K Prokeš, M Bartkowiak, O Rivin, O Prokhnenko, T Förster, S Gerischer, R Wahle, Y.-K. Huang, J. A. Mydosh, Phys. Rev B 96(R) (2017) 121117
Transfer of Magnetic Order and Anisotropy through Epitaxial Integration of 3d and 4f Spin Systems
Resonant x-ray scattering at the Dy M5 and Ni L3 absorption edges was used to probe the temperature and magnetic field dependence of magnetic order in epitaxial LaNiO3 − DyScO3 superlattices. For superlattices with 2 unit cell thick LaNiO3 layers, a commensurate spiral state develops in the Ni spin system below 100 K. Upon cooling below Tind = 18 K, Dy-Ni exchange interactions across the LaNiO3 − DyScO3 interfaces induce collinear magnetic order of interfacial Dy moments as well as a reorientation of the Ni spins to a direction dictated by the strong magnetocrystalline anisotropy of Dy. This transition is reversible by an external magnetic field of 3 T. Tailored exchange interactions between rare-earth and transition-metal ions thus open up new perspectives for the manipulation of spin structures in metal-oxide heterostructures and devices.
M Bluschke, A Frano, E Schierle, M Minola, M Hepting, G Christiani, G Logvenov, E Weschke, E Benckiser, B Keimer, Phys. Rev. Lett. 118 (2017) 207203
Doping dependent charge order correlations in electron-doped cuprates
Whereas charge order has been shown to be present in almost all known classes of hole-doped cuprate superconductors, evidence for this phenomenon in the case of electron doping was lacking. This gap in our understanding has now been closed by the observation of charge order in two electron-doped cuprate families, La2-xCexCuO4 (LCCO) and Nd2-xCexCuO4 (NCCO). The Figure displays a phase diagram that includes the region of charge order (data points), very much resembling the case of the hole-doped cuprates. These apparent similarities suggest a common origin for this type of instability in the Cu-O planes. Differences do however exist, application of an external magnetic field was not found to yield the substantial enhancement of charge order in electron doped cuprates in contrast to the enhancement typically found for hole-doped cuprates.
E. H. da Silva Neto, B. Yu, M. Minola, R. Sutarto, E. Schierle, F. Boschini, M. Zonno, M. Bluschke, J. Higgins, Y. Li, G. Yu, E. Weschke, F. He, M. Le Tacon, R. L. Greene, M. Greven, G. A. Sawatzky, B. Keimer, A. Damascelli, Sci. Adv. 2 (2016) e1600782
Long-range charge-density-wave proximity effect at cuprate/manganate interfaces
We used resonant X-ray scattering in zero magnetic field to show that interfaces with the metallic ferromagnet La2/3Ca1/3MnO3 greatly enhance CDW formation in the optimally doped high-temperature superconductor YBa2Cu3O6+x (x~1), and that this effect persists over several tens of nanometres. The wavevector of the incommensurate CDW serves as an internal calibration standard of the charge carrier concentration, which allows us to rule out any significant influence of oxygen non-stoichiometry, and to attribute the observed phenomenon to a genuine electronic proximity effect. Long-range proximity effects induced by heterointerfaces thus offer a powerful method to stabilize the charge-density-wave state in the cuprates and, more generally, to manipulate the interplay between different collective phenomena in metal oxides.
A Frano, S Blanco-Canosa, E Schierle, Y Lu, M Wu, M Bluschke, M Minola, G Christiani, HU Habermeier, G Logvenov, Y Wang, PA van Aken, E Benckiser, E Weschke, M Le Tacon, B Keimer, Nat. Mater. 15 (2016) 831
See also the associated highlight report
Scanning electron microscopy in combination with EELS electron spectroscopy permits to visualise atomic positions of the individual atoms in the heterostructure: Superconducting regions of YBaCuO are identified by yttrium (blue) and copper (pink), the ferromagnetic layers by manganese (green) and lanthanum (red). Courtesy MPI Stuttgart.