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

T sc max = 56 K 47 K 18 K 15 K Fe-based Superconducting Families pnictogens: P, As, Sb chalcogens: S, Se, Te LnO(F)FeAs AFe 2 As 2 AFeAs FeTe(Se,S) Ln = La, Ce, Pr, Nd, Sm, Gd … A = Ca, Sr, Ba, Eu, K A = Li, Na

Layered Structure of Fe-based Superconductors Parent Compounds Doped Compounds  Superconductors BaFe 2 As 2 Ba 1-x K x Fe 2 As 2 BaFe 2-x Co x As 2 BaFe 2 As 2-x P x Phase Diagram Holes, electrons or isovalent doping Spin density wave (SDW) magnetic order SDW

Spin density wave (SDW) – simple non-interlaced picture h 2n-1 – amplitudes of subsequent harmonics q – wave number of SDW x – relative position of the resonant nucleus along propagation direction of the stationary SDW perpendicular longitudinal commensurate or incommensurate

Spin density wave (SDW) seen by Mössbauer Spectroscopy h 2n-1 – amplitudes of subsequent harmonics q – wave number of SDW x – relative position of the resonant nucleus along propagation direction of SDW SDW hyperfine field distribution 57 Fe Mössbauer spectrum

”122” family of Fe-based superconductors

BaFe 2 As 2 (parent) T SDW = 136 K Ba 0.7 Rb 0.3 Fe 2 As 2 (superconductor) T sc = 37 K 57 Fe Mössbauer spectra NM non-magnetic Shape of SDW SDW is suppressed by doping

CaFe 2 As 2 (parent) T SDW = 175 K CaFe 1.92 Co 0.08 As 2 (superconductor) T sc = 20 K Resistivity measurements: It seems that magnetism and superconductivity coexist ( ? ). Mössbauer measurements: Superconductivity has filamentary character and occurs in the regions free of 3d magnetic moments.

EuFe 2 As 2 critical exponent  0 ≈  universality class (1, 2) ↓ one dimension in the spin space (Ising model) and two dimensions in the real space (magnetic planes) Root mean square amplitude of SDW

EuFe 2-x Co x As 2 57 Fe Mössbauer spectra T SDW = 190 K T SDW = 150 K T SDW = 100 K traces of SDW at 80 K lack of SDW T N (Eu) = 19 K Eu 2+ Transferred Field on 57 Fe filamentary superconductivity superconductor

EuFe 2-x Co x As Eu Mössbauer spectra  Eu(3+) Eu(2+)  EuFe 2 As 2 T SDW (Fe) = 190 K T N (Eu) = 19 K  Parent  Superconductor T sc = 9.5 K  Over-doped 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 x = 0.06, 0.10, 0.14, 0.18 Magnetic-crystallographic phase diagram S. Rö  ler et al., Phys. Rev. B (2011) x in Fe 1+x Te

Parent Compound Fe 1+y Te Doped Compound → Superconductor y ≈ 0 Fe 1+y Te 1-x Se x Fe 1+y Te 1-x S x K. Katayama et al., J. Phys. Soc. Japan (2010)

Fe 1.06 Te 57 Fe Mössbauer spectrum SDW field distribution shape of SDW  regular (tetrahedral) Fe excess (interstitial) Fe SDW

Fe 1.14 Te 57 Fe Mössbauer spectrum SDW field distribution shape of SDW    Three different kinds (surroundings) of excess (interstitial) Fe. Magnetism of the excess Fe and SDW disappear at the same transition temperature. regular Fe - SDW 

Fe 1+x Te x=0.06 x=0.10 x=0.14 x= K 4.2 K shape of SDW at 4.2 K 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. regular Fe (SDW) excess Fe    

Fe 1.01 Se T sc = 8 K High (external) magnetic field Mössbauer spectroscopy Hyperfine magnetic field is equal to applied external magnetic field - it means that there is no magnetic moment on the Fe atoms tetragonal orthorhombic and superconductor orthorhombic structural distortion

sharp magnetic transition paramagnetic region magnetic region SPIN SPIRAL FeA s Crystal structure Pnma or Pna2 1 ? Arrows show Pna2 1 – like distortion 83 E.E. Rodriguez et al., PRB 83, (2011)

Anisotropy of the hyperfine magnetic fields (spiral projections onto a-b plane) in FeAs Left column shows [0 k+1/2 0] iron, right column shows [0 k 0] iron. B a and B b - 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 for [0 k+1/2 0] and [0 k 0] irons. T c - transition temperature  - static critical exponent A. Błachowski et al., JALCOM 582, 167 (2014)

FeAs Spectral shift S and quadrupole coupling constant A Q 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 / 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 Thank you very much for your attention! AFe 2 As 2 - parents The SDW magnetic order with universality class (1, 2) and with almost rectangular shape at saturation. Ba 1-x Rb x Fe 2 As 2 The SDW vanishes upon doping leading to superconductivity. CaFe 2-x Co x As 2 Superconductivity has filamentary character and occurs in the regions free of 3d magnetic moments. EuFe 2-x Co x As 2 Localized 4f 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. FeSe There is no magnetic moment on iron in superconducting FeSe and it is PRESUMABLY the feature of all iron-based superconductors. FeAs 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 3d 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.