We studied the valency and spin behavior of M = Mn, Fe, Co, Li, and Al in the high-temperature superconducting compound Bi 2.15 Sr 1.85 Ca(Cu 1−z M z) 2 O 8+δ (Bi-2212) for small values of z. Mn, Fe, and Co retain their magnetic moments, and our thermopower and magnetic susceptibility data imply ionization states Mn 3+ , Fe 2+ , and Co 2+ , while Li and Al are accommodated in the charge reservoir layers. Single-crystal studies show that the susceptibility of Co 2+ ions in Bi-2212 is strongly anisotropic, with a weak anisotropy detected for Mn 3+ and none for Fe 2+. Fits to a pseudogap formula for a pure Bi-2212 crystal suggest that the spin susceptibility of the host compound is more anisotropic than previously realized. Data in the superconducting state allow us to compare the pair-breaking properties of the different impurities. Several aspects of the data, including the stronger suppression of the superconducting transition temperature T c by Co compared with Fe for underdoped and optimally doped samples, show that the d-level structure of the magnetic ions and multiorbital effects are important. We also find that the temperatures of the magnetization crossing points are equal to the low-field T c values to within 1% or 2%. This agrees with a 2D thermodynamic fluctuation argument given by Junod et al.
We have observed a magnetic vortex lattice (VL) in BaFe 2 (As 0.67 P 0.33 ) 2 (BFAP) single crystals by small-angle neutron scattering. With the field along the c axis, a nearly isotropic hexagonal VL was formed in the field range from 1 to 16 T, and no symmetry changes in the VL were observed. The temperature dependence of the VL signal was measured and confirms the presence of (non-d-wave) nodes in the superconducting gap structure for measurements at 5 T and below. The nodal effects were suppressed at high fields. At low fields, a VL reorientation transition was observed between 1 and 3 T, with the VL orientation changing by 45 • . Below 1 T, the VL structure was strongly affected by pinning and the diffraction pattern had a fourfold symmetry. We suggest that this (and possibly also the VL reorientation) is due to pinning to defects aligned with the crystal structure, rather than being intrinsic. The temperature dependence of the scaled intensity suggests that BFAP possesses at least one full gap and one nodal gap with circular symmetry. Judging from the symmetry, the node structure should take the form of an "accidental" circular line node, which is consistent with recent angle-resolved photoemission spectroscopy
We report on local magnetization, tunnel diode oscillators, and specific-heat measurements in a series of Ba(Ni x Fe 1−x ) 2 As 2 single crystals (0.26 x 0.74). We show that the London penetration depth λ(T ) = λ(0) + λ(T ) scales as λ (0) in both underdoped and overdoped samples. Moreover, the slope of the upper critical field [H c2 = −(dH c2 /dT ) |T →Tc ] decreases with T c in overdoped samples but increases with decreasing T c in underdoped samples. The remarkable variation of λ(0) with T c and the nonexponential temperature dependence of λ clearly indicates that pair-breaking effects are important in this system. We show that the observed scalings strongly suggest that those pair-breaking effects could be associated with quantum fluctuations near three-dimensional superconducting critical points.
We report small-angle neutron scattering measurements of the vortex lattice (VL) structure in single crystals of the lightly underdoped cuprate superconductor YBa 2 Cu 3 O 6.85 . At 2 K, and for fields of up to 16 T applied parallel to the crystal c-axis, we observe a sequence of field-driven and first-order transitions between different VL structures. By rotating the field away from the c-axis, we observe each structure transition to shift to either higher or lower field dependent on whether the field is rotated towards the [100] or [010] direction. We use this latter observation to argue that the Fermi surface morphology must play a key role in the mechanisms that drive the VL structure transitions. Furthermore, we show this interpretation is compatible with analogous results obtained previously on lightly overdoped YBa 2 Cu 3 O 7 . In that material, it has long-been suggested that the high field VL structure transition is driven by the nodal gap anisotropy. In contrast, the results and discussion presented here bring into question the role, if any, of a nodal gap anisotropy on the VL structure transitions in both YBa 2 Cu 3 O 6.85 and YBa 2 Cu 3 O 7 .
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