Heat capacity jump at Tc and pressure derivatives of superconducting transition temperature in the Ba1−x</ma... (original) (raw)
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2009
We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe 2 As 2 , BaFe 2 As 2 , and (Ba 0.55 K 0.45)Fe 2 As 2 , by means of measurements of electrical resistivity (ρ) in the 1.8-300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe 2 As 2 and BaFe 2 As 2 (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 (Ba 0.55 K 0.45)Fe 2 As 2 is superconducting for T < 30 K. The effect of pressure on BaFe 2 As 2 is to shift the onset of the crystallographic transformation down in temperature at the rate of ~-1.04 K/kbar, while shifting the whole ρ(T) curves downward, whereas its effect on superconducting (Ba 0.55 K 0.45)Fe 2 As 2 is to shift the onset of superconductivity to lower temperatures at the rate of ~-0.21 K/kbar. The effect of pressure on CaFe 2 As 2 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 (H c2) data in (Ba 0.55 K 0.45)Fe 2 As 2 and CaFe 2 As 2 are discussed.
Pressure-induced superconductivity in BaFe2As2 single crystal
The evolution of pressure-induced superconductivity in single crystal as well as polycrystalline samples of BaFe2As2 have been investigated through temperature-dependent electrical resistivity studies in the 0-7 GPa pressure range. While the superconducting transition remains incomplete in the polycrystalline sample, a clear pressure-induced superconductivity with zero resistivity at the expense of magnetic transition, associated with spin density wave (SDW), is observed in the single-crystal sample. The superconducting transition temperature (TC) is seen to increase upto a moderate pressure of about ∼ 1.5 GPa and decreases monotonically beyond this pressure. The SDW transition temperature TSDW decreases rapidly with increasing pressure and vanishes above ∼ 1.5GPa.
Collapsed tetragonal phase and superconductivity ofBaFe2As2under high pressure
Physical Review B, 2010
High pressure x-ray diffraction and electrical resistance measurements have been carried out on BaFe 2 As 2 to a pressure of 35 GPa and temperature of 10 K using a synchrotron source and designer diamond anvils. At ambient temperature, a phase transition from the tetragonal phase to a collapsed tetragonal ͑CT͒ phase is observed at 17 GPa under nonhydrostatic conditions as compared to 22 GPa under hydrostatic conditions. The superconducting transition temperature increases rapidly with pressure up to 34 K at 1 GPa and decreases gradually with a further increase in pressure. Our results suggest that T C falls below 10 K in the pressure range of 16-30 GPa, where CT phase is expected to be stable under high-pressure and low-temperature conditions.
Physical Review B, 2011
We investigate the electronic properties and the superconducting gap characteristics of a single crystal of hole-doped 122 Fe-pnictide Ba0.65Na0.35Fe2As2 by means of specific heat measurements. The specific heat exhibits a pronounced anomaly around the superconducting transition temperature Tc = 29.4 K, and a small residual part at low temperature. In a magnetic field of 90 kOe, the transition is broadened and Tc is lowered insignificantly by an amount ∼ 1.5 K. We estimate a high electronic coefficient in the normal state with a value 57.5 mJ mol −1 K 2 , being consistent with holedoped 122 compounds. The temperature-dependent superconducting electronic specific heat cannot be described with single-gap BCS theory under weak coupling approach. Instead, our analysis implies a presence of two s-wave like gaps with magnitudes ∆1(0)/kBTc = 1.06 and ∆2(0)/kBTc = 2.08 with their respective weights of 48% and 52%. While our results have qualitative similarities with other hole-doped 122 materials, the gap's magnitude and their ratio are quite different.
High-resolution measurements of the thermal expansion of superconducting Co-doped BaFe2As2
Physical Review B, 2009
High-resolution thermal expansion measurements of single crystalline BaFe 1.84 Co 0.16 As 2 and BaFe 1.77 Co 0.23 As 2 in the temperature range 5 Ͻ T Ͻ 300 K are reported. The thermal expansion is highly anisotropic, with the largest expansion along the c axis. Distinct anomalies are present at the normal-tosuperconducting phase-transition temperature T c ; the phase transition appears to be continuous. No structural transitions are observed over the temperature range of our measurements. The thermal expansion data and heat-capacity data acquired on the same specimens are used to estimate the volumetric pressure derivative of T c using the Ehrenfest relation.
Pressure-induced superconductivity and structural transitions in Ba(Fe0.9Ru0.1)2As2
The European Physical Journal B, 2014
High-pressure electrical resistance and x-ray diffraction measurements have been performed on ruthenium-doped Ba(Fe 0.9 Ru 0.1) 2 As 2 , up to pressures of 32 GPa and down to temperatures of 10 K, using designer diamond anvils under quasi-hydrostatic conditions. At 3.9 GPa, there is an evidence of pressure-induced superconductivity with T C onset of 24 K and zero resistance at T C zero 14.5 K. The superconducting transition temperature reaches maximum at ~ 5.5 GPa and decreases gradually with increase in pressure before completely disappearing above 11.5 GPa. Upon increasing pressure at 200 K, an isostructural phase transition from a tetragonal (I4/mmm) phase to a collapsed tetragonal phase is observed at 14 ± 1 GPa and the collapsed phase persists up to at least 30 GPa. The changes in the unit cell dimensions are highly anisotropic across the phase transition and are qualitatively similar to those observed in undoped BaFe 2 As 2 parent.
Thermodynamics of the Superconducting Phase Transition in Ba0.6K0.4BiO3
Physical Review Letters, 1999
We suggest that the transition to superconductivity in a single crystal of Ba0.6K0.4BiO3 with a Tc = 32K, and having critical fields with anomalous temperature dependencies and vanishing discontinuities in specific heat and magnetic susceptibility, may well be an example of a fourth order (in Ehrenfest's sense) phase transition. We have derived a free energy functional for a fourth order transition and calculated (for the temperature range Tc/2 < T ≃ Tc) the temperature dependence of the critical fields. We find Hc1(T ) ∝ (1 − T /Tc) 3 , H0(T ) ∝ (1 − T /Tc) 2 and Hc2(T ) ∝ (1 − T /Tc) 1 in general agreement with experiments.
Pressure-induced superconductivity in Ba 0.5 Sr 0.5 Fe 2 As 2
2012
High-pressure electrical resistance measurements have been performed on single crystal Ba 0.5 Sr 0.5 Fe 2 As 2 platelets to pressures of 16 GPa and temperatures down to 10 K using designer diamond anvils under quasi-hydrostatic conditions with an insulating steatite pressure medium. The resistance measurements show evidence of pressure-induced superconductivity with an onset transition temperature at ~31 K and zero resistance at ~22 K for a pressure of 3.3 GPa. The transition temperature decreases gradually with increasing in pressure before completely disappearing for pressures above 12 GPa. The present results provide experimental evidence that a solid solution of two 122-type materials, e.g., Ba 1-x. Sr x Fe 2 As 2 (0 < x <1), can also exhibit superconductivity under high pressure.