arXiv subjects

A. Kwadrin

Publications and source records attributed to A. Kwadrin.

3 recordsLinked to original sources

arXiv2009

Interplay of Rare Earth and Iron magnetism in RFeAsO with R = La, Ce, Pr, and Sm: A muon spin relaxation study and symmetry analysis

We report zero field muon spin relaxation (muSR) measurements on RFeAsO with R = La, Ce, Pr, and Sm. We study the interaction of the FeAs and R (rare earth) electronic systems in the non superconducting magnetically ordered parent compounds of RFeAsO{1-x}Fx superconductors via a detailed comparison of the local hyperfine fields at the muon site with available Moessbauer spectroscopy and neutron scattering data. These studies provide microscopic evidence of long range commensurate magnetic Fe order with the Fe moments not varying by more than 15 % within the series RFeAsO with R = La, Ce, Pr, and Sm. At low temperatures, long range R magnetic order is also observed. Different combined Fe and R magnetic structures are proposed for all compounds using the muon site in the crystal structure obtained by electronic potential calculations. Our data point to a strong effect of R order on the iron subsystem in the case of different symmetry of Fe and R order parameters resulting in a Fe spin reorientation in the R ordered phase in PrFeAsO. Our symmetry analysis proves the absence of collinear Fe--R Heisenberg interactions in RFeAsO. A strong Fe--Ce coupling due to non--Heisenberg anisotropic exchange is found in CeFeAsO which results in a large staggered Ce magnetization induced by the magnetically ordered Fe sublattice far above T_N{Ce}. Finally, we argue that the magnetic R--Fe interaction is probably not crucial for the observed enhanced superconductivity in RFeAsO{1-x}Fx with a magnetic R ion.

cond-mat.supr-con↗

arXiv2009

Orbital and spin effects for the upper critical field in As deficient disordered Fe pnictide superconductors

We report B_c2 data for LaO_{0.9}F_{0.1}FeAs_{1-delta} in a wide T and field range up to 60 Tesla. The large slope of B_c2 approx ~ -6 Tesla/K near an improved T_c = 28.5 K of the in-plane B_c2(T) contrasts with a flattening starting at 23 K above 30 Tesla we regard as the onset of Pauli-limited behavior (PLB) with B_c2(0) about 65 Tesla. We interpret a similar hitherto unexplained flattening of the B_c2(T) curves reported for at least three other disordered closely related systems as also as a manifestation of PLB. Their Maki parameters have been estimated analyzing their B_c2(T) data within the WHH approach. The pronounced PLB of (Ba,K)Fe_2As_2 single crystals from a tin-flux is attributed also to a significant As deficiency. Consequences of our results are discussed in terms of disorder effects within conventional (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), can be discarded for our samples. The increase of dB_c2/dT|_{T_c} by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity driven transition from s_{+-} USC to CSC of an extended s_{++}-wave state or to a stabilized s_{+-}-state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with B_c2 data from the literature with no PLB for fields below 60 to 70 Tesla probed so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.

cond-mat.supr-con↗

arXiv2009

Superconductivity and Field-Induced Magnetism in SrFe_1.75Co_0.25As_2

Using muon-spin rotation, we studied the in-plane (λ_ab) and the out of plane (λ_c) magnetic field penetration depth in SrFe_1.75Co_0.25As_2 (T_c=13.3 K). Both λ_ab(T) and λ_c(T) are consistent with the presence of two superconducting gaps with the gap to T_c ratios 2Δ/k_BT_c=7.2 and 2.7. The penetration depth anisotropy γ_λ=λ_c/λ_ab increases from γ_λ=2.1 at T_c to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.

cond-mat.supr-con↗