CZUŁOŚĆ SPEKTROSKOPII MÖSSBAUEROWSKIEJ NA PRZEJŚCIE DO NADPRZEWODNICTWA W Ba 0.6 K 0.4 Fe 2 As 2

1. A. K. Jasek1, K. Komędera1, A. Błachowski1, K. Ruebenbauer1, Z. Bukowski2, J. G. Storey3,4, J. Karpinski5,6 CZUŁOŚĆ SPEKTROSKOPII MÖSSBAUEROWSKIEJ NA PRZEJŚCIE DO NADPRZEWODNICTWA W Ba0.6K0.4Fe2As2 1Zakład Spektroskopii Mössbauerowskiej, Instytut Fizyki, Uniwersytet Pedagogiczny, Kraków, Polska 2Instytut Niskich Temperatur i Badań Strukturalnych, Polska Akademia Nauk, Wrocław, Polska 3Cavendish Laboratory, University of Cambrige, United Kingdom 4School of Chemical and Physical Sciences, Victoria University, Wellington, New Zealand 5Laboratory of Solid State Physics, ETH Zurich, Switzerland 6Institute of Condensed Matter Physics, EPFL, Lausanne, Switzerland ------------------------------------------------------------------------------------------------------ X Ogólnopolskie Seminarium Spektroskopii Mössbauerowskiej OSSM’2014 Wrocław, 15-18 czerwca 2014

2. Iron-based superconductor families 111112211111 LnO(F)FeAs AFe2As2AFeAs FeTe(Se,S) Ln = La, Ce, Pr, Nd, Sm, Gd … A = Ca, Sr, Ba, Eu, K A = Li , Na Tscmax = 56 K 47 K 18 K 15 K The aim of the experiment was to check whether the Mӧssbauer spectroscopy is sensitive to the superconducting transition in Ba0.6K0.4Fe2As2.

3. Ba1-xKxFe2As2 parentcompoundBaFe2As2 dopingK superconductivityTsc=38K Tetragonal unit cell of BaFe2As2 and phase diagram of Ba1-xKxFe2As2

4. Fig. 2. Resistivity of Ba1-xKxFe2As2(x = 0.1-0.3) Fig. 3. Lattice parameters of the series Ba1-xKxFe2As2 (x = 0–0.3) Fig. 1. Resistivity of Ba0.6K0.4Fe2As2 plotted vs. temperature

5. BaFe2As2 vs. Ba0.6K0.4Fe2As2 Fig. 3. The difference in total molar specific heat coefficients between superconductor (s) and parent compound (p) versus temperature γtot=Ctot/T Ctot-the total molar heat capacity The inset shows the electronic specific heat coefficient of the superconductor versus temperature Cel- the electronic molar heat capacity Figs. 1,2. 57Fe Mössbauer spectra versus temperature forthe parent compound BaFe2As2 and the Ba0.6K0.4Fe2As2

6. Ba0.6K0.4Fe2As2 Tsc = 38 K Selected Mössbauer spectra of the Ba0.6K0.4Fe2As2 across the transitionto the superconducting state. Note the abrupt changesin the regions 40 K - 38 K and 28 K - 24 K A. K. Jasek et al., J. Alloys Comp. 609, 150 (2014)

7. Parameters derived from the Mössbauer spectra ofBa0.6K0.4Fe2As2 plotted versus temperature. S– total spectrum shift versus roomtemperature α-Fe Δ0– constant component of the quadrupole splitting Γ– absorber line width tA– dimensionless absorber resonant thickness Ratio of the recoilless fractions f/f0and dispersion of CDW Δρ versus temperature

8. Electric field gradient wave (EFGW)in Ba0.6K0.4Fe2As2 Shape of EFGW  - quadrupole coupling constant A - amplitude of EFGW β - shape parameter of EFGW

9. 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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