Hydrostatic pressure study of the structural phase transitions and superconductivity in single crystals of (Ba 1− x K x )Fe 2 As 2 ( x =0 and 0.45) and CaFe 2 As 2
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Abstract
We studied the effect of hydrostatic pressure (P) on the structural phase
transitions and superconductivity in the ternary and pseudo-ternary iron
arsenides CaFe2As2, BaFe2As2, and (Ba0.55K0.45)Fe2As2, by means of measurements
of electrical resistivity (rho) in the 1.8 - 300 K temperature (T) range,
pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe2As2 and BaFe2As2
(lightly doped with Sn) display structural phase transitions near 170 K and 85
K, respectively, and do not exhibit superconductivity in ambient pressure,
while K-doped (Ba0.55K0.45)Fe2As2 is superconducting for T < 30 K. The effect
of pressure on BaFe2As2 is to shift the onset of the crystallographic
transformation down in temperature at the rate of about -1.04 K/kbar, while
shifting the whole rho(T) curves downward, whereas its effect on
superconducting (Ba0.55K0.45)Fe2As2 is to shift the onset of superconductivity
to lower temperatures at the rate of about -0.21 K/kbar. The effect of pressure
on CaFe2As2 is first to suppress the crystallographic transformation and induce
superconductivity with onset near 12 K very rapidly, i.e., for P < 5 kbar.
However, higher pressures bring about another phase transformation
characterized by reduced resistivity, and the suppression of superconductivity,
confining superconductivity to a narrow pressure dome centered near 5 kbar.
Upper critical field (Hc2) data in (Ba0.55K0.45)Fe2As2 and CaFe2As2 are
discussed.