| Recent highlights in the research of high temperature superconductivity by Hugo Keller (UniZH) and Annette Bussmann-Holder (Max Planck Institut, Stuttgart in Germany) |
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| In collaboration with MaNEP the University of Zürich group has recently performed two decisive experiments on cuprate high temperature superconductors (HTS’s) which challenge theoretical models based on purely electronic mechanisms.
From muon spin rotation ( 
SR) experiments the temperature dependence of the superfluid density has been determined from which clear conclusions on the pairing symmetry can be extracted. The experimental results for three different cuprate families disclose generic trends for HTS’s, namely the coexistence of an s-wave and a d-wave gap in the CuO2 planes and a predominantly s-wave gap along the c-axis [1 – 3] (Fig. 1 shows representative results). These findings are not compatible with approaches that concentrate on the planes only, but consistent with early predictions where the 3D nature of high temperature superconductivity was suggested to be manifest in coupled gaps [4, 5]. In addition, the existence of the s-wave gap is in strong support of ideas that the lattice plays a crucial role in HTS’s.These latter conclusions are in accord with oxygen isotope effect (OIE) experiments where not only OIE’s on the superconducting transition temperature but also on the magnetic penetration depth [6], the superconducting gaps, the Néel temperature, and the spin glass temperature have been observed [7]. Throughout the whole phase diagram of cuprates OIE’s exist which are sign reversed for the superconducting properties as compared to the magnetic states. The strong doping dependence of these OIE’s has been shown to stem from renormalizations of the kinetic energy caused by polaron formation [8]. This interpretation is consistent with ideas that led to the discovery of high temperature superconductivity, namely |
Since both above mentioned experiments are direct, bulk sensitive, and unambiguous and have been carried through systematically for different cuprate families and as functions of doping, we conclude that the order parameter in cuprates is much more complex than just d-wave symmetry, the third dimension, i.e., physics involving the c-axis are of outermost importance, lattice effects in terms of polaron formation crucially dominate the whole phase diagram. |
 Figure 2 : the same as in Figure 1, but for YBa2Cu3O7-x  [2].1. R. Khasanov et al., Phys. Rev. Lett. 98, 057007 (2007). 2. R. Khasanov et al., Phys. Rev. Lett. 99, 237601 (2007). 3. R. Khasanov et al., J. Supercond. Nov. Magn. (online 20.12.2007). 4. K. A. Müller, Nature (London) 377, 133 (1995). 5. K. A. Müller and H. Keller, in High Tc Superconductivity 1996: Ten Years after the Discovery (Kluwer, Dordrecht, 1997) p. 7. 6. See e.g., H. Keller, in Superconductivity in Complex Systems, eds. K. A. Müller and A. Bussmann-Holder (Springer Series Structure and Bonding) 114, 143 (2005), and refs. therein. 7. R. Khasanov et al., arXiv:0711.2257. 8. A. Bussmann-Holder and H. Keller, in Polarons in Advanced Materials, ed. A. S. Alexandrov (Springer Series in Materials Science) 103, 599 (2007). 9. K. A. Müller, J. Phys.: Cond. Mat. 19, 1 (2007); and refs. therein. |
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of single crystals of YBa2Cu4O8 measured along the crystallographic directions a, b, c (ρa, ρb, and ρc are the corresponding superfluid densities) [3]. The insets to the figures show the individual contributions from the s- and d-wave gap. The full lines are results from a power law dependence.