Keimer's department > Research > Superconductivity > Fe-based superconductors

Fe-based superconductors

Until 2008, all known high-Tc superconductors were a number of copper oxides which are conceptually difficult to comprehend, while the well-understood, conventional materials have much lower Tc. In many aspects, the recent discovery of a new class of superconductors with superconductivity occurring in iron arsenide planes, instead of those made up of copper oxide, has spanned a bridge over this "gap". Iron pnictides appear to be characterized by moderate electron correlations and intermediate pairing strengths that yield Tc up to 55 K — well above the Tc of conventional superconductors but significantly below that of the copper oxides.

Early on, the crystal growth service group synthesized single crystals of hole-doped Ba1-xKxFe2As2 in tin flux. They were systematically investigated by Raman scattering [1] in our department (Fig. 1), by transport measurements [2,3], and by ARPES [3-5] in collaboration with IFW Dresden. We further investigated these samples by neutron & X-ray diffraction [6,7], magnetic force microscopy (MFM) [6,8] and µSR [6,9,10], in collaboration with the MPI-MF, the IFW and the PSI in Switzerland. As the quality of such samples can be affected by the presence of tin inclusions, we have switched the focus of our research to self-flux grown compounds.

Using this improved technique, large electron-doped single crystals of BaFe2-xCoxAs2 and BaFe2-xNixAs2 with various doping levels were grown by our crystal growth group with a mass up to 4 g [11], suitable for inelastic neutron scattering (INS). In optimally doped BFCA, an intense spectrum of spin fluctuations has been measured in a wide range of energies [12] in collaboration with the FRM-II (Munich) and the LLB (Saclay, France). The spin resonance mode appears at the antiferromagnetic wavevector below the superconducting transition temperature, as shown in Fig. 2. In these experiments, we managed to obtain the dynamic spin susceptibility in absolute units — a very valuable information for theory and the quantification of the coupling strength between electrons and the observed spin fluctuations.

Hole-doped arsenide samples of high quality have also been grown by the self-flux method, with an optimal Tc of 38.5 K and a high superconducting volume fraction of ~98% [13], but with much smaller typical sizes. Magnetic field penetration has been studied by Bitter decoration [14]. The mass of these samples, being so far insufficient for INS measurements, is suitable for spectroscopic ellipsometry. Therefore, the present emphasis is being put on studying the optical properties of these hole-doped compounds.

Earlier, we have reported a comprehensive spectroscopic ellipsometry study on LaFeAsO1-xFx and Ba(Sr)1-xKxFe2As2 [15], where we have found that the optical conductivity spectra are dominated by a sequence of interband transitions, as shown in Fig. 3(a), which agree very well with those predicted by LDA calculations. The free charge carrier response is, however, heavily damped, as depicted in Fig. 3(b,c). Our results imply that the electronic states near the Fermi surface are strongly renormalized, with the possible renormalization factors being electron-electron correlation effects, electron-phonon coupling, and dynamic charge and spin ordering.

Another family of oxypnictides, SmFeAsO1-xFx, has been recently investigated by the inelastic photon scattering group [16]. In this study, the dispersion of the three optical phonon modes along the (100) crystallographic direction revealed a strong renormalization of two of these modes upon fluorine doping, shedding light on the spin-phonon coupling.

References

  1. M. Rahlenbeck, G. L. Sun, D. L. Sun, C. T. Lin, B. Keimer, C. Ulrich, Phys. Rev. B 80 (2009), 064509
  2. V. N. Zverev, A. V. Korobenko, G. L. Sun, D. L. Sun, C. T. Lin, A. V. Boris, JETP Lett. 90, 140 (2009)
  3. A. Koitzsch, D. S. Inosov, D. V. Evtushinsky, V. B. Zabolotnyy, A. A. Kordyuk, A. Kondrat, C. Hess, M. Knupfer, B. Büchner, G. L. Sun, V. Hinkov, C. T. Lin, A. Varykhalov, S. V. Borisenko, Phys. Rev. Lett. 102 (2009), 167001
  4. V. B. Zabolotnyy, D. S. Inosov, D. V. Evtushinsky, A. Koitzsch, A. A. Kordyuk, G. L. Sun, J. T. Park, D. Haug, V. Hinkov, A. V. Boris, C. T. Lin, M. Knupfer, A. N. Yaresko, B. Büchner, A. Varykhalov, R. Follath, S. V. Borisenko, Nature 457 (2009), 569
  5. D. V. Evtushinsky, D. S. Inosov, V. B. Zabolotnyy, A. Koitzsch, M. Knupfer, B. Büchner, G. L. Sun, V. Hinkov, A. V. Boris, C. T. Lin, B. Keimer, A. Varykhalov, A. A. Kordyuk, S. V. Borisenko, Phys. Rev. B 79 (2009), 054517
  6. J. T. Park, D. S. Inosov, Ch. Niedermayer, G. L. Sun, D. Haug, N. B. Christensen, R. Dinnebier, A. V. Boris, A. J. Drew, L. Schulz, T. Shapoval, U. Wolff, V. Neu, Xiaoping Yang, C. T. Lin, B. Keimer, V. Hinkov, Phys. Rev. Lett. 102 (2009), 117006
  7. D. S. Inosov, A. Leineweber, Xiaoping Yang, J. T. Park, N. B. Christensen, R. Dinnebier, G. L. Sun, Ch. Niedermayer, D. Haug, P. W. Stephens, J. Stahn, O. Khvostikova, C. T. Lin, O. K. Andersen, B. Keimer, V. Hinkov, Phys. Rev. B 79 (2009), 224503
  8. D. S. Inosov, T. Shapoval, V. Neu, U. Wolff, J. S. White, S. Haindl, J. T. Park, D. L. Sun, C. T. Lin, E. M. Forgan, M. S. Viazovska, J. H. Kim, M. Laver, K. Nenkov, O. Khvostikova, S. Kühnemann, V. Hinkov, Phys. Rev. B 81 (2010), 014513
  9. R. Khasanov, D. V. Evtushinsky, A. Amato, H.-H. Klauss, H. Luetkens, Ch. Niedermayer, B. Büchner, G. L. Sun, C. T. Lin, J. T. Park, D. S. Inosov, V. Hinkov, Phys. Rev. Lett. 102 (2009), 187005
  10. D. V. Evtushinsky, D. S. Inosov, V. B. Zabolotnyy, M. S. Viazovska, R. Khasanov, A. Amato, H.-H. Klauss, H. Luetkens, Ch. Niedermayer, G. L. Sun, V. Hinkov, C. T. Lin, A. Varykhalov, A. Koitzsch, M. Knupfer, B. Büchner, A. A. Kordyuk, S. V. Borisenko, New J. Phys. 11 (2009), 055069
  11. D. L. Sun, Y. Liu, J. T. Park, C. T. Lin, Supercond. Sci. Technol. 22 (2009), 105006
  12. D. S. Inosov, J. T. Park, P. Bourges, D. L. Sun, Y. Sidis, A. Schneidewind, K. Hradil, D. Haug, C. T. Lin, B. Keimer, V. Hinkov, Nature Physics 6 (2010), 178–181
  13. P. Popovich, A. V. Boris, O. V. Dolgov, A. A. Golubov, D. L. Sun, C. T. Lin, R. K. Kremer, B. Keimer, preprint: arXiv:1001.1074 (2010)
  14. L. Ya. Vinnikov, T. M. Artemova, I.S. Veshchunov, N.D. Zhigadlo, J. Karpinski, P. Popovich, D. L. Sun, C. T. Lin, A. V. Boris, JETP Letters 90 (2009), 299
  15. A. V. Boris, N. N. Kovaleva, S. S. A. Seo, J. S. Kim, P. Popovich, Y. Matiks, R. K. Kremer, B. Keimer, Phys. Rev. Lett. 102 (2009), 027001
  16. M. Le Tacon, T. R. Forrest, Ch. Rüegg, A. Bosak, A. C. Walters, R. Mittal, H. M. Rønnow, N. D. Zhigadlo, S. Katrych, J. Karpinski, J. P. Hill, M. Krisch, D. F. McMorrow, Phys. Rev. B 80 (2009), 220504(R)
Figures
Fig. 1. Raman spectra of underdoped
Ba1-xKxFe2As2 at 300 K in y(xu)y and y(zu)y polarization configurations. The insets show a comparison of the Raman spectra at 5 and 300 K. The displacement patterns show the Raman-active A1g and B1g modes as obtained from shell-model calculations. 
M. Rahlenbeck et al., Phys. Rev. B, 2009
Temperature dependence of the spin-resonance mode in BFCA
Fig. 2. The spectrum of spin excitations at the antiferromagnetic wavevector in optimally doped BaFe1.85Co0.15As2 (Tc = 25 K) over a wide range of temperatures and energies in absolute units. 
D. Inosov et al., Nature Physics, 2009
Fig. 3. Imaginary part of the dielectric function of LaFeAsO0.9F0.1 measured at T = 300 K (blue heavy lines) and represented by the total contribution of separate Lorenzian bands determined by the dispersion analysis. The low-energy part of the optical conductivity spectra is shown in (b) for the undoped and in (c) for superconducting F-doped samples at 300 K (red) and 10 K (blue). The dark gray lines and shaded areas show the low-energy tail from interband transitions and the remaining intraband charge carrier contribution, respectively. Arrows mark partial gaps opening at ωPG = 0.65 eV (ω*PG = 0.16 eV). 
A. V. Boris et al., Phys. Rev. Lett., 2009