Bernd Buechner

Publications (3)0 Total impact

• Article: Intrinsic pinning and the critical current scaling of clean epitaxial Fe(Se,Te) thin films

  Kazumasa Iida, Jens Haenisch, Elke Reich, Fritz Kurth, Ruben Huehne, Ludwig Schultz, Bernhard Holzapfel, Ataru Ichinose, Masafumi Hanawa, Ichiro Tsukada, Michael Schulze, Saicharan Aswartham, Sabine Wurmehl, Bernd Buechner
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  ABSTRACT: We report on the transport properties of clean, epitaxial Fe(Se,Te) thin films prepared on Fe-buffered MgO (001) single crystalline substrates by pulsed laser deposition. Near Tc a steep slope of the upper critical field for H||ab was observed (74.1 T/K), leading to a very short out-of-plane coherence length, \xi c, of 0.2 nm, yielding 2\xi c(0) approximately 0.4 nm. This value is shorter than the interlayer distance (0.605 nm) between Fe-Se(Te) planes, indicative of modulation of the superconducting order parameter along the c-axis. An inverse correlation between the power law exponent N of the electric field-current density (E-J) curve and the critical current density, Jc, has been observed at 4 K, when the orientation of H was close to the ab-plane. These results prove the presence of intrinsic pinning in Fe(Se,Te). A successful scaling of the angular dependent Jc and the corresponding exponent N can be realized by the anisotropic Ginzburg Landau approach with appropriate \Gamma values 2~3.5. The temperature dependence of \Gamma behaves almost identically to that of the penetration depth anisotropy.
  03/2013;
• Source

  Available from: Jens Hänisch

  Article: Generic Fe buffer layers for Fe-based superconductors: Epitaxial FeSe1-xTex thin films

  Kazumasa Iida, Jens Haenisch, Michael Schulze, Saicharan Aswartham, Sabine Wurmehl, Bernd Buechner, Ludwig Schultz, Bernhard Holzapfel
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  ABSTRACT: Biaxially textured FeSe1-xTex films have been realized on Fe-buffered MgO substrates by pulsed laser deposition. Similar to the Fe/BaFe2As2 bilayers, the crystalline quality of FeSe1-xTex films exhibit a sharp out-of-plane and in-plane texture less than 0.9{\deg}. The Fe/FeSe1-xTex bilayers showed high superconducting transition temperatures of over 17 K. The angular-dependent critical current densities exhibit peaks positioned at H // ab similar to other pnictide thin films. The volume pinning force of FeSe1-xTex in this direction is very strong compared with that of Co-doped BaFe2As2, due to a good matching between the interlayer distance in the c direction and the out-of-plane coherence length.
  11/2011;
• Source

  Available from: Jeroen van den Brink

  Article: As-vacancies, local moments, and Pauli limiting in LaO_0.9F_0.1FeAs_(1-delta) superconductors

  Vadim Grinenko, Konstantin Kikoin, Stefan-Ludwig Drechsler, Guenter Fuchs, Konstantin Nenkov, Sabine Wurmehl, Franziska Hammerath, Guillaume Lang, Hans-Joachim Grafe, Bernhard Holzapfel, Jeroen van den Brink, Bernd Buechner, Ludwig Schultz
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  ABSTRACT: We report magnetization measurements of As-deficient LaO_0.9F_0.1FeAs_1-delta (delta about 0.06) samples with improved superconducting properties as compared with As-stoichiometric optimally doped La-1111 samples. In this As-deficient system with almost homogeneously distributed As-vacancies (AV), as suggested by the (75)As-nuclear quadrupole resonance (NQR) measurements,we observe a strong enhancement of the spin-susceptibility by a factor of 3-7. This observation is attributed to the presence of an electronically localized state around each AV, carrying a magnetic moment of about 3.2 mu_Bohr per AV or 0.8 mu_Bohr/Fe atom. From theoretical considerations we find that the formation of a local moment on neighboring iron sites of an AV sets in when the local Coulomb interaction exceeds a critical value of about 1.0 eV in the dilute limit. Its estimated value amounts to ~ 2.5 eV and implies an upper bound of ~ 2 eV for the Coulomb repulsion at Fe sites beyond the first neighbor-shell of an AV. Electronic correlations are thus moderate/weak in doped La-1111. The strongly enhanced spin susceptibility is responsible for the Pauli limiting behavior of the superconductivity that we observe in As-deficient LaO_0.9F_0.1FeAs_1-delta. In contrast, no Pauli limiting behavior is found for the optimally doped, As-stoichiometric LaO_0.9F_0.1FeAs superconductor in accord with its low spin susceptibility.
  05/2011;