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Register a new userPair weakening in mixed rare-earth nickel borocarbides (R,R)Ni2B2C
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In this study pseudoquaternary rare-earth nickel borocarbide superconductors RxR1-xNi2B2C have been investigated predominantly in the diluted limit x << 1 or (1 x) << 1. In all of these materials structural disorder results in a reduction of the superconducting transition temperature Tc . Depending on the selection of the rare earth elements R and R this disorder induced deterioration of superconductivity is combined with magnetic pair breaking of Abrikosov-Gorkov type or pair breaking of non-magnetic impurities in antiferromagnetic superconductors (Morozov-type of pair breaking).
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The temperature and magnetic field dependence of the specific heat cp(T,H) in the superconducting mixed state as well as the upper critical field Hc2(T) have been measured for polycrystalline Y_xLu_{1-x}Ni_2B_2C and Y(Ni_{1-y}Pt_y)_2B_2C samples. The linear-in-T electronic specific heat contribution gamma(H)T exhibits significant deviations from the usual linear-in-H law for all x and y the transition metal site (T) resulting in a disorder dependent negative curvature of gamma(H). The deviations from that linear behaviour of our unsubstituted samples are the largest reported so far for any superconductor. The H_c2(T) data point to the quasi-clean limit for (Y,Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni,Pt)-substitutions. The gamma(H) dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of gamma(H) data only, d-wave pairing cannot be ruled out.
Temperature and applied magnetic field dependent magnetization measurements on 34 single crystalline samples of (R, R,R...)Ni2Ge2 compounds (R, R, R, etc. being primarily Gd-Lu, Y), were made. These measurements reveal that, despite extremes in local moment anisotropy, the average de Gennes parameter is a remarkably good predictor of the paramagnetic to antiferromagnetic ordering temperature. In addition, the pronounced metamagnetic phase transitions seen in the low temperature phase of TbNi2Ge2 are found to be remarkably robust to high substitution levels of Gd and 25% substitutions of other heavy rare earths.
The properties of the MoSr2RCu2O8 (R=rare earth) system are found to systematically change with the contraction of the R ions. For the light R ions (La-Nd) the samples are paramagnetic down to 5 K, whereas in the intermediate range (Sm-Tb), the Mo sublattice orders antiferromagnetically at TN, ranging from 11 to 24 K. For the heavy R ions, Ho-Tm and Y, superconductivity appears at TC in the range 19-27 K and antiferromagnetism sets in at TN < TC. This latter behavior resembles most of the magneto-superconductors, but is in sharp contrast to the iso-structural RuSr2RCu2O8 system where TN > TC.
The superconducting energy gap and quasiparticle density of states (DOS) is measured by means of Andreev-reflection spectroscopy in normal metal - superconductor point contacts. It is found that both in paramagnetic and in antiferromagnetically ordered states (Tm- and Dy-compounds) the DOS has the BCS-like form while in Er- the dip in the temperature dependence of the gap is observed at the Neel temperature. In Ho-compound, the non-BCS-like form of DOS is observed in the temperature region where the incommensurate spin density waves exist along the axes a and c . Inelastic point-contact spectroscopy performed on these compounds shows that the low-frequency phonons are important for the superconducting state which are strongly mixed with the magnetic excitation branches in magnetic superconductors.
We report inelastic neutron scattering measurements of crystal field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal field excitations, providing evidence that there are two distinct charge environments around the rare earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have non-magnetic and magnetic ground states, respectively, indicating that the enhancement of Tc compared to LaFeAsO1-xFx is not due to rare earth magnetism.
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