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Journal Articles

Flux pinning in PrFeAsO$$_{0.9}$$ and NdFeAsO$$_{0.9}$$F$$_{0.1}$$ superconducting crystals

Van der Beek, C. J.*; Rizza, G.*; Konczykowski, M.*; Fertey, P.*; Monnet, I.*; Klein, T.*; Okazaki, Ryuji*; Ishikado, Motoyuki; Kito, Hijiri*; Iyo, Akira*; et al.

Physical Review B, 81(17), p.174517_1 - 174517_11, 2010/05

 Times Cited Count:97 Percentile:93.8(Materials Science, Multidisciplinary)

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO$$_{1-y}$$ and NdFeAs(O,F)superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasi-particle scattering.

Oral presentation

Quantum bi-critical point in YbAgGe

Tokiwa, Yoshifumi; Garst, M.*; Gegenwart, P.*; Bud'ko, S. L.*; Canfield, P. C.*

no journal, , 

Bicritical points, at which two distinct symmetry-broken phases become simultaneously unstable, are typical for spin-flop metamagnetism. Interestingly, the heavy-fermion compound YbAgGe also possesses such a bicritical point (BCP) with a low temperature 0.3K at a magnetic field of 4.5T. In its vicinity, YbAgGe exhibits anomalous behavior that we attribute to the influence of a quantum bicritical point that is close in parameter space yet can be reached by tuning $$T_{BCP}$$ further to zero. Using high-resolution measurements of the magnetocaloric effect, we demonstrate that the magnetic Gruneisen parameter $$Gamma$$ indeed both changes sign and diverges as required for quantum criticality. Moreover, $$Gamma$$ displays a characteristic scaling behavior but only on the low-field side $$H_{BCP}$$, indicating a pronounced asymmetry with respect to the critical field. We speculate that the small value of $$T_{BCP}$$ is related to the geometric frustration of the Kondo lattice of YbAgGe.

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