The Interplay Between Magnetism and Superconductivity in Strongly Correlated Materials

We performed out-of-plane angular dependent magnetoresistivity measurements on La_0.7Ca_0.3MnO₃/YBa₂Cu₃O_7-δ(LCMO/YBCO) trilayer heterostructure. Our results show that the conductance of the LCMO layers of the trilayer increases by two orders of magnitude below the superconducting transition temperature T_c of the trilayer. We found that triplet correlations are induced in LCMO over an effective penetration depth ξ_F resulting in this significant increase in conductance. We estimated the dissipation present in the LCMO layers is ξ_F≅4.7nm at 10 K. We concluded that triplet superconducting pairs are induced in the LCMO layers.

In-plane resistance and magnetization measurements were performed on LCMO/YBCO trilayers below the T_c of the YBCO layer. We found two magnetoresistance (MR) peaks. The first MR peak is consistent with the suppression of superconductivity due to the stray fields generated by the domain walls and the second one is most likely the result of the antiparallel orientation of the net magnetic moment of the top and bottom LCMO layers. We concluded that stray field and spin imbalance effects coexist and compete, hence modulate the superconductivity of heterostructures.

We performed in-plane angular dependent resistivity measurements in the non-Fermi liquid regime of CeCoIn₅ single crystals. We found the presence of two different scaling behaviors in the low-field region where resistivity shows a linear temperature dependence, separated by a critical angle related with the anisotropy of CeCoIn₅ . We explained this anomalous angular dependent resistivity in terms of d-wave density waves with Landau quantization of the quasiparticle spectrum.

Current-voltage (I-V) measurements were performed on CeCoIn₅ in the mixed state. We observed that the flux-flow resistivity ρ_ff obtained from I-V measurements is abnormal. Specifically, ρ_ff increases sharply with decreasing magnetic field and temperature. Our result revealed the abnormal insulating core of the vortex line structure. We found that vortex core becomes normal when the system is tuned away from quantum critical point. We concluded that the abnormal insulating core is a result of quantum criticality.