Effects of annealing on the physical properties of BaFe2As2

Costel R Rotundu (Lawrence Berkeley National Lab)

The discovery of the high temperature superconductivity in iron-pnictides is one of the most important discoveries of the last years in the field of condensed matter. One remarkable system is the 122 series, in which superconductivity is produced by doping a parent compound such as BaFe2As2 with electrons (e.g., Co on the Fe sites), holes (e.g., K on the Ba sites), or by substitution of isovalent elements, for example, P for As or Ru for Fe. In the parent compound the magnetic and structural transitions are coincident. A common feature of the phase diagram of these systems is the emergence of superconductivity with suppression of both magnetic and structural phase transitions. Here I present a combination of neutron and heat capacity measurements of a high quality as-grown BaFe2As2 crystal, and X-ray diffraction, heat capacity, and resistivity measurements after various annealing periods. After a 30-day anneal the resistivity in the ab plane decreases by more than an order of magnitude to 12 µ.cm, with a residual resistance ratio .36; the heat capacity anomaly at the magneto-structural transition sharpens to an overall width of .1 K, and shifts from 135.4 to 140.2 K. The heat-capacity anomaly in both the as-grown sample and after the 30-day anneal shows a hysteresis of .0.15 K, and is unchanged in a magnetic field µ0H = 14 T. Both the X-ray and heat-capacity data confirm the first order nature of the transition.

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