Specific heat and phase diagrams of single crystal iron pnictide superconductors.
We present specific heat measurements on single crystals of the pnictide superconductors NdFeAsO{sub 1-x}F{sub x} and Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2}. Low-temperature measurements on Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2} reveal that the Sommerfeld coefficient of the quasiparticle specific heat increases linearly with applied magnetic field. This is the signature of a fully gapped superconducting state as is expected for instance in the extended s-wave scenario. The large value of the normal state Sommerfeld coefficient indicates a large electronic density of states and significant mass enhancements. We determine the phase diagram using an entropy conserving construction, which does not require the choice of a resistivity criterion to define the transition temperature. Both materials display clear mean-field steps in the specific heat at T{sub c} of 47 K and 34.6 K, respectively, proving bulk superconductivity. They are characterized by a low superconducting anisotropy near T{sub c} of {Gamma} {approx} 4 for NdFeAsO{sub 1-x}F{sub x} and 2.6 for Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2}, which is promising for potential applications. We observe extraordinarily high upper critical field slopes of {mu}{sub 0}{partial_derivative}H{sub c2}{sup e}/{partial_derivative}T = -6.5 T/K and {mu}{sub 0}{partial_derivative}H{sub c2}{sup ab}/{partial_derivative}T = -17.4 T/K for Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2} which points to the emergence of paramagnetic limiting effects at low temperatures. A thermodynamic analysis reveals that this material is extreme type-II with K{sub c} {approx} 100 and k{sub ab} {approx} 260.
- Research Organization:
- Argonne National Laboratory (ANL)
- Sponsoring Organization:
- SC; Natural Science Foundation of China; Ministry of Science and Technology of China; Chinese Academy of Science
- DOE Contract Number:
- AC02-06CH11357
- OSTI ID:
- 1006267
- Report Number(s):
- ANL/MSD/JA-68809
- Journal Information:
- Physica C, Journal Name: Physica C Journal Issue: Mar. 2009 Vol. 469; ISSN 0921-4534
- Country of Publication:
- United States
- Language:
- ENGLISH
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