Professor Stephen Blundell

Abstract:

Substantial control of the interlayer spacing in Bi-based high temperature superconductors has been achieved through the intercalation of guest molecules between the superconducting layers. Measurements using implanted muons reveal that the penetration depth increases with increasing layer separation while T_{c} does not vary appreciably, demonstrating that the bulk superfluid density is not the determining factor controlling T_{c}. Our results strongly suggest that for Bi-based high temperature superconductors the superfluid density appearing in the Uemura scaling relation rho_{s} proportional, variantT_{c} should be interpreted as the two-dimensional density within the superconducting layers, which we find to be constant for each class of system investigated.

Abstract:

Transverse-field muon-spin rotation measurements performed on two samples of LiFeAs demonstrate that the superfluid stiffness of the superconducting condensate in relation to its superconducting transition temperature is enhanced compared to other pnictide superconductors. Evidence is seen for a field-induced magnetic state in a sample with a significantly suppressed superconducting transition temperature. The results in this system highlight the role of direct Fe-Fe interactions in frustrating pairing mediated by antiferromagnetic fluctuations and indicate that, in common with other pnictide superconductors, the system is close to a magnetic instability. © 2009 The American Physical Society.

Abstract:

We report muon-spin relaxation measurements on SrFeAsF, which is the parent compound of an iron-arsenic- fluoride based series of superconducting materials. We find that this material has very similar magnetic properties to LaFeAsO, such as separated magnetic and structural transitions (TN =122 K, Ts =175 K), contrasting with SrFe2 As2 where they are coincident. The muon oscillation frequencies fall away very sharply at TN [β=0.22 (3)], which suggests that the magnetic exchange between the layers is weaker than in comparable oxypnictide compounds. Our specific-heat measurements show that a small but distinct feature occurs at Ts. There is no anomaly evident at TN as is typical for quasi-two-dimensional magnets. The magnetic entropy change between 100 and 185 K is ΔS=0.5 J mol-1 K-1. © 2009 The American Physical Society.