Complex electronic order in pnictide superconductors studied by nuclear probe spectroscopy – Hans-Henning Klauss, TU Dresden, Germany

Hans-Henning Klauss, TU Dresden, Institut für Festkörperphysik, Helmholtzstraße 10
01069 Dresden, Deutschland.

The interplay of itinerant magnetism and unconventional superconductivity in Fe based superconductors and other intermetallic systems with complex Fermi surfaces is a fascinating topic in contemporary correlated electron physics.

In my talk I will discuss Mössbauer spectroscopy and muon spin relaxation experiments on several series of hole doped 122 pnictides like Ba_1−xK_xFe₂As₂ [1]. I will present new data on Ca_1−xNa_xFe₂As₂ (x = 0.35, 0.50 and 0.67). The parent compound CaFe₂As₂ shows a spin density wave order below T_N = 165 K [2,3]. Our microscopic study proves that with increasing Na-substitution level, the magnetic order parameter as well as the magneto-structural phase transition is suppressed. For x = 0.50 we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature T_c. A systematic comparison with other 122 pnictides reveals a linear correlation between the magnetic order parameter reduction and the ratio of the transition temperatures, T_c/T_N, which can be understood in the framework of a Landau-theory.  In the optimally doped specimen with T_c ≈ 34 K, the temperature dependence of the penetration depth and superfluid density were obtained, which proves the presence of two superconducting s-wave gaps.

I will also discuss multigap superconductivity with broken time reversal symmetry in locally non-centrosymmetric SrPtAs.  µSR experiments prove the development of a small static spontaneous internal field just below Tc, evidencing the time reversal symmetry (TRS) breaking SrPtAs [4].  ⁷⁵As-NMR, and NQR-nvestigations clearly reveal a two step behaviour in the spin-lattice relaxation rate evidencing multigap superconductivity with very weak inter-band coupling and constraints for the possible pairing symmetries. Unconventional chiral d-wave or f-wave order parameters are consistent with our data [5].

1.  E. Wiesenmayer et al., Phys. Rev. Lett. 107, 237001 (2011).
2.  K. Zhao et al., Phys. Rev. B 84, 184534 (2011)
3. G. Wu et al., Journal of Physics: Condensed Matter 20, 422201 (2008).
4. F. Brückner et al., Phys. Rev. B 90, 220503 (2014).

Voici le lien vers la page web du groupe de recherche du Pr Klauss

Cette conférence est présentée par le RQMP Versant Nord du Département de physique de l'Université de Montréal et le Département de génie physique de Polytechnique Montréal.