Mssbauer study of ironbased superconductors A Bachowski 1
Mössbauer study of iron-based superconductors A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2 1 Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University, Cracow, Poland 2 Department of Solid State Physics, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow, Poland ----------------------------------------------------------------------------- ICAME 2013 International Conference on the Applications of the Mössbauer Effect 1 -6 September 2013, Opatija, Croatia
Superconductivity in the non-magnetic state of iron under pressure K. Shimizu et al. Nature 412, 316 (2001) hcp Fe becomes superconductor at temperatures below 2 K and at pressures between 15 and 30 GPa
Journal of American Chemical Society Received January 2008, Published online February 2008 ----------------------------------------------------------------------------------------------------- Up to now the maximum superconducting critical temperature of iron-based superconductors is 56 K
Fe-based Superconducting Families pnictogens: P, As, Sb chalcogens: S, Se, Te 1111 122 111 11 Ln. O(F)Fe. As AFe 2 As 2 AFe. As Fe. Te(Se, S) A = Ca, Sr, Ba, Eu, K A = Li , Na Ln = La, Ce, Pr, Nd, Sm, Gd … Tsc max = 56 K 47 K 18 K 15 K
Layered Structure of Fe-based Superconductors Spin density wave (SDW) magnetic order ----------------------------------------------------------------------------------------------------- Phase Diagram Ba. Fe 2 As 2 Holes, electrons or isovalent doping Ba 1 -x. Kx. Fe 2 As 2 Parent Compounds Ba. Fe 2 -x. Cox. As 2 SDW Ba. Fe 2 As 2 -x. Px Doped Compounds Superconductors
Spin density wave (SDW) – simple non-interlaced picture perpendicular longitudinal commensurate or incommensurate h 2 n-1 – amplitudes of subsequent harmonics q – wave number of SDW x – relative position of the resonant nucleus along propagation direction of the stationary SDW
Spin density wave (SDW) seen by Mössbauer Spectroscopy h 2 n-1 – amplitudes of subsequent harmonics q – wave number of SDW x – relative position of the resonant nucleus along propagation direction of SDW hyperfine field distribution 57 Fe Mössbauer spectrum
” 122” family of Fe-based superconductors
Ba. Fe 2 As 2 (parent) 57 Fe Mössbauer spectra TSDW = 136 K Ba 0. 7 Rb 0. 3 Fe 2 As 2 Shape of SDW (superconductor) Tsc = 37 K NM non-magnetic SDW is suppressed by doping
Ca. Fe 2 As 2 Ca. Fe 1. 92 Co 0. 08 As 2 (parent) TSDW = 175 K (superconductor) Resistivity measurements: It seems that magnetism and superconductivity coexist (? ). Mössbauer measurements: Superconductivity has filamentary character and occurs in the regions free of 3 d magnetic moments. Tsc = 20 K
Eu. Fe 2 As 2 Root mean square amplitude of SDW critical exponent 0 ≈ 0. 125 universality class (1, 2) ↓ one dimension in the spin space (Ising model) and two dimensions in the real space (magnetic planes)
Eu. Fe 2 -x. Cox. As 2 57 Fe Mössbauer spectra TN (Eu) = 19 K TSDW = 190 K TSDW = 150 K superconductor TSDW = 100 K superconductor traces of SDW at 80 K superconductor lack of SDW filamentary superconductivity Eu 2+ Transferred Field on 57 Fe
Eu(2+) Eu. Fe 2 -x. Cox. As 2 151 Eu Mössbauer spectra Eu. Fe 2 As 2 TSDW (Fe) = 190 K TN (Eu) = 19 K Parent Superconductor Tsc = 9. 5 K Over-doped Eu(3+) Eu 2+ orders magnetically regardless of the Co-substitution level. Eu 2+ moments rotate from a-axis to c-axis. Eu 2+ magnetism and superconductivity coexist.
Fe 1+x. Te x = 0. 04 – 0. 18 Magnetic-crystallographic phase diagram x = 0. 06 , 0. 10 , 0. 14 , 0. 18 x in Fe 1+x. Te S. Rö ler et al. , Phys. Rev. B 84 174506 (2011)
Parent Compound Fe 1+y. Te Doped Compound → Superconductor y ≈ 0 Fe 1+y. Te 1 -x. Sex Fe 1+y. Te 1 -x. Sx K. Katayama et al. , J. Phys. Soc. Japan 79 113702 (2010)
57 Fe Mössbauer spectrum SDW field distribution shape of SDW Fe 1. 06 Te regular (tetrahedral) Fe excess (interstitial) Fe SDW
57 Fe Mössbauer spectrum SDW field distribution shape of SDW Fe 1. 14 Te regular Fe - SDW Three different kinds (surroundings) of excess (interstitial) Fe. Magnetism of the excess Fe and SDW disappear at the same transition temperature.
Fe 1+x. Te 65 K regular Fe (SDW) excess Fe 4. 2 K shape of SDW at 4. 2 K x=0. 06 x=0. 10 x=0. 14 x=0. 18 SDW is very sensitive to concentration of interstitial iron with relatively large localized magnetic moments. Localized iron moments prevent superconductivity, so interstitial iron must be removed by doping and/or deintercalation to get superconducting material.
Fe 1. 01 Se Tsc = 8 K High (external) magnetic field Mössbauer spectroscopy tetragonal structural distortion orthorhombic and superconductor Hyperfine magnetic field is equal to applied external magnetic field - it means that there is no magnetic moment on the Fe atoms
Fe. As paramagnetic region sharp magnetic transition Crystal structure Pnma or Pna 21 ? magnetic region SPIN SPIRAL Arrows show Pna 21 – like distortion E. E. Rodriguez et al. , PRB 83, 134438 (2011) 83
Anisotropy of the hyperfine magnetic fields (spiral projections onto a-b plane) in Fe. As Left column shows [0 k+1/2 0] iron, right column shows [0 k 0] iron. Ba and Bb - iron hyperfine field components along the a-axis and b-axis, respectively. Orientation of the EFG and hyperfine magnetic field in the main crystal axes Average hyperfine fields <B> for [0 k+1/2 0] and [0 k 0] irons. Tc - transition temperature - static critical exponent A. Błachowski et al. , JALCOM 582, 167 (2014)
Fe. As Spectral shift S and quadrupole coupling constant AQ versus temperature for [0 k+1/2 0] iron and [0 k 0] iron. Line at 72 K separate magnetically ordered region from paramagnetic region. Relative recoilless fraction <f>/<f 0> versus temperature Green points correspond to magnetically ordered region. Red point is the normalization point. Inset shows relative spectral area RSA plotted versus temperature.
Conclusions AFe 2 As 2 - parents The SDW magnetic order with universality class (1, 2) and with almost rectangular shape at saturation. Ba 1 -x. Rbx. Fe 2 As 2 The SDW vanishes upon doping leading to superconductivity. Ca. Fe 2 -x. Cox. As 2 Superconductivity has filamentary character and occurs in the regions free of 3 d magnetic moments. Eu. Fe 2 -x. Cox. As 2 Localized 4 f magnetic moments could order within the superconducting phase. Fe 1+x. Te Excess (interstitial) iron with relatively large localized magnetic moment strongly influence on the ordering temperature, shape and amplitude of the SDW. Fe. Se There is no magnetic moment on iron in superconducting Fe. Se and it is PRESUMABLY the feature of all iron-based superconductors. Fe. As Spin spiral leads to the complex variation of the hyperfine field amplitude with the spin orientation (local magnetic moment) varying in the a-b plane. Pattern express symmetry of 3 d electrons in the a-b plane with the significant distortion caused by the arsenic bonding p electrons. Strong coupling between magnetism and lattice dynamics i. e. strong phonon-magnon interaction. Thank you very much for your attention!
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