Journal
PHYSICAL REVIEW B
Volume 78, Issue 14, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.78.140510
Keywords
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Funding
- U.S. Department of Energy-Basic Energy Science [DEAC02-06CH11357]
- Ministry of Science and Technology of China [2006CB60100, 2006CB921802, 2006CB921107]
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We present heat-capacity measurements of the upper critical fields of single-crystal NdFeAsO1-xFx. In zero-magnetic field a clear step in the heat capacity is observed at T-c approximate to 47 K. In fields applied perpendicular to the FeAs layers the step broadens significantly whereas for the in-plane orientation the field effects are small. This behavior is reminiscent of the CuO2-high-T-c superconductors and is a manifestation of pronounced fluctuation effects. Using an entropy conserving construction we determine the transition temperatures in applied fields and the upper critical-field slopes of partial derivative H-c2(c)/partial derivative T=-0.72 T/K and partial derivative H-c2(ab)/partial derivative T=-3.1 T/K. Zero-temperature coherence lengths of xi(ab)approximate to 3.7 nm and xi(c)approximate to 0.9 nm and a modest superconducting anisotropy of lambda similar to 4 can be deduced in a single-band model.
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