Strongly correlated electron systems
HZB studies of superconductors and related transition metal oxides have produced important results in the past few years. The study of charge order in La1.8−xEu0.2SrxCuO4 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 long-range incommensurate charge fluctuations in (Y,Nd)Ba2Cu3O6+x proving the concept that superconductivity competes with other orders in 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.
A different research topic is the complex charge ordering and associated magnetic ordering in CeRuSn. This compound has been extensively studied combining neutron and synchrotron x-ray diffraction with x-ray absorption spectroscopy.
In the following we present a few examples of research carried out within the last few years.
For a full list of publications of our institute klick here.
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, and 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
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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.
Charge order and its connection with Fermi-liquid charge transport in a pristine high-Tc cuprate
Electronic inhomogeneity appears to be an inherent characteristic of the enigmatic cupratesuperconductors. Here we report the observation of charge–density–wave correlations in the model cuprate superconductor HgBa2CuO4+δ (Tc=72 K) via bulk Cu L3-edge-resonant X-ray scattering. At the measured hole-doping level, both the short-range charge modulations and Fermi-liquid transport appear below the same temperature of about 200 K. Our result points to a unifying picture in which these two phenomena are preceded at the higher pseudogap temperature by q=0 magnetic order and the build-up of significant dynamic antiferromagnetic correlations. The magnitude of the charge modulation wave vector is consistent with the size of the electron pocket implied by quantum oscillation and Hall effect measurements for HgBa2CuO4+δ and with corresponding results for YBa2Cu3O6+δ, which indicates that charge–density–wave correlations are universally responsible for the low-temperature quantum oscillation phenomenon.
W Tabis, Y Li, M Le Tacon, L Braicovich, A Kreyssig, M Minola, G Dellea, E Weschke, MJ Veit, M Ramazanoglu, AI Goldman, T Schmitt, G Ghiringhelli, N Barišic, MK Chan, CJ Dorow, G Yu , X Zhao, B Keimer, M Greven, Nature Communications 5, 5875 (2014).
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Ubiquitous Interplay Between Charge Ordering and High-Temperature Superconductivity in Cuprates
Besides superconductivity, copper-oxide high-temperature superconductors are susceptible to other types of ordering. We used scanning tunneling microscopy and resonant elastic x-ray scattering measurements to establish the formation of charge ordering in the high-temperature superconductor Bi2Sr2CaCu2O8+δ. Depending on the hole concentration, the charge ordering in this system occurs with the same period as those found in Y-based or La-based cuprates and displays the analogous competition with superconductivity. These results indicate the similarity of charge organization competing with superconductivity across different families of cuprates. We observed this charge ordering to leave a distinct electron-hole asymmetric signature (and a broad resonance centered at +20 millielectron volts) in spectroscopic measurements, indicating that it is likely related to the organization of holes in a doped Mott insulator.
EH da Silva Neto, P Aynajian, A Frano, R Comin, E Schierle, E Weschke, A Gyenis, J Wen, J Schneeloch, Z Xu,S Ono, G Gu, M Le Tacon, A Yazdani, Science 343, 393 (2014)
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Discrete Fourier transform of the conductance map; the corners of the image represent the atomic Bragg peaks at (+/-2π/a, 0) and (0, (+/-2π/b). The discrete Fourier transform is mirror-symmetrized and normalized to its average value. The central peak (d 0.2) corresponds to long
wavelength inhomogeneity of the conductance map. © AAAS
Charge Order Driven by Fermi-Arc Instability in Bi2Sr2-xLaxCuO6+δ
The understanding of the origin of superconductivity in cuprates has been hindered by the apparent diversity of intertwining electronic orders in these materials. We combined resonant x-ray scattering (REXS), scanning-tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to observe a charge order that appears consistently in surface and bulk, and in momentum and real space within one cuprate family, Bi2Sr2-xLaxCuO6+δ. The observed wave vectors rule out simple antinodal nesting in the single-particle limit but match well with a phenomenological model of a many-body instability of the Fermi arcs. Combined with earlier observations of electronic order in other cuprate families, these findings suggest the existence of a generic charge-ordered state in underdoped cuprates and uncover its intimate connection to the pseudogap regime.
R Comin, A Frano, MM Yee, Y Yoshida, H Eisaki, E Schierle, E Weschke, R Sutarto, F He, A Soumyanarayanan, Y He, M Le Tacon, IS Elfimov, JE Hoffman, GA Sawatzky, B Keimer, A Damascelli, Science 343, 390 (2014)
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Momentum-dependent charge correlations in YBa2Cu3O6+x superconductors probed by resonant X-ray scattering: Evidence for three competing phases
We use resonant x-ray scattering to determine the momentum-dependent charge correlations in YBa2Cu3O6.55 samples with highly ordered chain arrays of oxygen acceptors (ortho-II structure). The results reveal nearly critical, biaxial charge density wave (CDW) correlations at in-plane wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length are enhanced as superconductivity is weakened by an external magnetic field. Analogous experiments are carried out on a YBa2Cu3O6.6 crystal with a dilute concentration of spinless (Zn) impurities, which had earlier been shown to nucleate incommensurate magnetic order. Compared to pristine crystals with the same doping level, the CDW amplitude and correlation length are found to be strongly reduced. These results indicate a three-phase competition between spinmodulated, charge-modulated, and superconducting states in underdoped YBa2Cu3O6+x.
S Blanco-Canosa, A Frano, T Loew, Y Lu, J Porras, G Ghiringhelli, M Minola, C Mazzoli, L Braicovich, E Schierle, E Weschke, M Le Tacon, B Keimer, Phys. Rev. Lett. 110, 187001 (2013)
S Blanco-Canosa, A Frano, E Schierle, J Porras, T Loew, M Minola, M Bluschke, E Weschke, B Keimer, M Le Tacon, Phys. Rev. B 90, 054513 (2014)
Long-Range Incommensurate Charge Fluctuations in (Y,Nd)Ba2Cu3O6+x
The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of ~3.2 lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba2Cu3O6+x, with hole concentrations of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature (Tc); further cooling below Tc abruptly reverses the divergence of the charge correlations. In combination with earlier observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge density wave instability that competes with superconductivity.
G Ghiringhelli, M Le Tacon, M Minola, S Blanco-Canosa, C Mazzoli, NB Brookes, GM De Luca, A Frano, DG Hawthorn, F He, T Loew, M Moretti Sala, DC Peets, M Salluzzo, E Schierle, R Sutarto, GA Sawatzky, E Weschke, B Keimer, L Braicovich, Science 337, 821 (2012).
Valence modulations and magnetic ordering in CeRuSn
HZB research on 4f and 5f based electron systems focuses on Ce and U based compounds. Recent synchrotron x-ray diffraction experiments , combined with x-ray absorption spectroscopy , revealed that CeRuSn exhibits both thermally and x-ray induced reversible transformations of the Ce3+/Ce(4-x)+ valence modulation pattern, a behavior which appears to be unique among any know intermetallic compound. Correspondingly, the Ce magnetic moments exhibit a complex modulated ordering below TN = 2.8K, that could be revealed by neutron diffraction .
 R Feyerherm, E Dudzik, S Valencia, JA Mydosh, Y-K Huang, W Hermes, R Pöttgen, Phys Rev B 85, 085120 (2012)
 R Feyerherm, E Dudzik, K Prokes, JA Mydosh, Y-K Huang, R Pöttgen, Physical Review B 90, 041104(R) (2014)
 K Prokes, V Petricek, E Ressouche, S Hartwig, B Ouladdiaf, JA Mydosh, RD Hoffmann, Y-K Huang, R Pöttgen, Journal of Physics: Condensed Matter 26, 122201 (2014)