CZUŁOŚĆ SPEKTROSKOPII MÖSSBAUEROWSKIEJ NA PRZEJŚCIE DO NADPRZEWODNICTWA W Ba 0.6 K 0.4 Fe 2 As 2 1 Zakład Spektroskopii Mössbauerowskiej, Instytut Fizyki, Uniwersytet Pedagogiczny, Kraków, Polska 2 Instytut Niskich Temperatur i Badań Strukturalnych, Polska Akademia Nauk, Wrocław, Polska 3 Cavendish Laboratory, University of Cambrige, United Kingdom 4 School of Chemical and Physical Sciences, Victoria University, Wellington, New Zealand 5 Laboratory of Solid State Physics, ETH Zurich, Switzerland 6 Institute of Condensed Matter Physics, EPFL, Lausanne, Switzerland A. K. Jasek 1, K. Komędera 1, A. Błachowski 1, K. Ruebenbauer 1, Z. Bukowski 2, J. G. Storey 3,4, J. Karpinski 5, X Ogólnopolskie Seminarium Spektroskopii Mössbauerowskiej OSSM’2014 Wrocław, czerwca 2014

T sc max = 56 K 47 K 18 K 15 K Iron-based superconductor families 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 The aim of the experiment was to check whether the M ӧ ssbauer spectroscopy is sensitive to the superconducting transition in Ba 0.6 K 0.4 Fe 2 As 2.

Ba 1-x K x Fe 2 As 2 parent compound BaFe 2 As 2 doping K superconductivity T sc =38K Tetragonal unit cell of BaFe 2 As 2 and phase diagram of Ba 1-x K x Fe 2 As 2

Fig. 3. Lattice parameters of the series Ba 1-x K x Fe 2 As 2 (x = 0–0.3) Fig. 2. Resistivity of Ba 1-x K x Fe 2 As 2 (x = ) Fig. 1. Resistivity of Ba 0.6 K 0.4 Fe 2 As 2 plotted vs. temperature

BaFe 2 As 2 vs. Ba 0.6 K 0.4 Fe 2 As 2 Figs. 1,2. 57 Fe Mössbauer spectra versus temperature for the parent compound BaFe 2 As 2 and the Ba 0.6 K 0.4 Fe 2 As 2 Fig. 3. The difference in total molar specific heat coefficients between superconductor (s) and parent compound (p) versus temperature γ tot =C tot /T C tot - the total molar heat capacity The inset shows the electronic specific heat coefficient of the superconductor versus temperature C el - the electronic molar heat capacity

Ba 0.6 K 0.4 Fe 2 As 2 T sc = 38 K Selected Mössbauer spectra of the Ba 0.6 K 0.4 Fe 2 As 2 across the transition to the superconducting state. Note the abrupt changes in the regions 40 K - 38 K and 28 K - 24 K A. K. Jasek et al., J. Alloys Comp. 609, 150 (2014)

P arameters derived from the Mössbauer spectra of Ba 0.6 K 0.4 Fe 2 As 2 plotted versus temperature. S – total spectrum shift versus room temperature α-Fe Δ 0 – constant component of the quadrupole splitting Γ – absorber line width t A – dimensionless absorber resonant thickness Ratio of the recoilless fractions f/f 0 and dispersion of CDW Δρ versus temperature

Electric field gradient wave (EFGW) in Ba 0.6 K 0.4 Fe 2 As 2 Shape of EFGW A - amplitude of EFGW β - shape parameter of EFGW  - quadrupole coupling constant

Conclusions CDW and modulation of the EFG on the iron nuclei develop within this system. The new type of hyperfine interaction modulation called electric field gradient wave (EFGW) is seen on the iron nuclei. The charge modulation is sensitive to the transition between normal and superconducting state. CDW and EFGW strongly vary at the superconducting gap opening. A distribution of the “covalent” electrons is strongly perturbed by the itinerant electrons forming Cooper pairs. Dynamic properties of the iron nuclei seem unaffected by a transition to the superconducting state.

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