Superconductivity and magnetism in the phase diagram of bulk La2-xCexCuO4+-delta investigated by muon-spin spectroscopy and neutron scattering

• Applicants: Professorin Dr. Barbara Albert; Professor Dr. Lambert Alff
• Co-Applicant: Dr. Hubertus Luetkens
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2009 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 108195470

• Final Report Year: 2015

Final Report Abstract

The most important result of the project is the availability of bulk samples of the metastable compound T'-La2CuO4. This modification has a more ideal CuO-plane structure since the apical oxygen present of the stable modification of T-La2CuO4 is missing. Muon-spin rotation measurements showed that T'-La2CuO4 is more two-dimensional than T-La2CuO4. Stable antiferromagnetic order is only achieved at about 115 K (as compared to about 300 K for T-La2CuO4). Between 115 K and 200 K magnetic fluctuations are dominant due to the reduced coupling of the CuO-planes in the material. Theoretical results obtained for idealized CuO-planes should be tested using T'-La2CuO4 since it is closer to this idealized structure. We believe that the comparison of T-La2CuO4 and T'-La2CuO4 samples is a promising route to advance the still unresolved field of high-temperature superconductivity. Another important issue that has been solved in this project is the stoichiometry problem in electron-doped cuprates. Standard samples have to be reduced before the desired stoichiometry is achieved. In our case, the low-temperature route immediately leads to stoichiometric samples as shown by neutron powder diffraction. Our results confirm that the undoped compound is an antiferromagnetic insulator. The experimentally observed superconductivity in nominally undoped samples as reported in literature, therefore, is a result of hidden doping effects.

Publications

• Enhanced two dimensionality in the true parent compound of electron doped cuprates T'-La2CuO4. Phys. Rev. B 82, 180508(R) (2010)
  R. Hord, H. Luetkens, G. Pascua, A. Buckow, K. Hofmann, Y. Krockenberger, J. Kurian, H. Maeter, H.-H. Klauss, V. Pomjakushin, A. Suter, B. Albert, L. Alff
  
• Transitions between lanthanum cuprates: crystal structures of T', orthorhombic and K2NiF4-type La2CuO4. Z. Anorg. Allg. Chem. 637, 1114 (2011)
  R. Hord, G. Cordier, K. Hofmann, A. Buckow, G.Pascua, H. Luetkens, L. Alff, B. Albert
  
• Oxygen stoichiometry of low-temperature synthesized metastable T'-La2CuO4. Supercond. Sci. Technol. 26, 105026 (2013)
  R. Hord, G. Pascua, K. Hofmann, G. Cordier, J. Kurian, H. Luetkens, V. Pomjakushin, M. Reehuis, B. Albert, L. Alff
  
• Thermodynamic study of orthorhombic Tx and tetragonal T' lanthanum cuprate, La2CuO4. J. Solid State Chem. 204, 91 (2013)
  K. I. Lilova, R. Hord, L. Alff, B. Albert, and A. Navrotsky
  
• Synthesis of microcrystalline Ce2O3 and formation of solid solutions in the systems Ce2O3-La2O3 and CeO2-La2O3. Z. Anorg. Allg. Chem. 640, 1050 (2014)
  C. Hamm, L. Alff, B. Albert