Spurious magnetism in high-T_c superconductor

Abstract

Mang et al. observe a cubic (Nd,Ce)₂O₃ impurity phase grown epitaxially in annealed samples of electron-doped Nd_2−xCe_xCuO₄ (NCCO). They claim that this impurity phase has long-range order parallel to the CuO₂ planes of NCCO but extending only about 4a_c perpendicular to the planes, thus forming a quasi-two-dimensional (Nd,Ce)₂O₃ lattice matched with the a–b plane of NCCO.

Kang et al. reply

Although we have confirmed the presence of such an impurity phase, (Nd,Ce)₂O₃ in our samples forms a three-dimensional long-range structural order¹ and is unrelated to the quasi-two-dimensional superlattice reflections^1,2. In the paramagnetic state of (Nd,Ce)₂O₃, a field will induce a net moment on magnetic Nd. By arbitrarily scaling the impurity scattering at (0,0,2.2) for fields less than 7 T to our c-axis field-induced scattering of NCCO at (1/2,1/2,0), Mang et al. argue that our observed magnetic scattering² is due entirely to (Nd,Ce)₂O₃. We disagree, however.

There are three ways to resolve this impurity problem. First, if the magnetic scattering at (1/2,1/2,0) (ref. 2) is due entirely to (Nd,Ce)₂O₃, one would expect the field-induced intensity to be identical when B is parallel to the c-axis and when it is parallel to the [1, −1,0] axis, as required by the cubic symmetry of (Nd,Ce)₂O₃. Experimentally, we find that the field-induced effect at (1/2,1/2,0) is much larger when B is parallel to the c-axis¹, which is inconsistent with the cubic symmetry of (Nd,Ce)₂O₃ but consistent with the upper critical field of NCCO being much smaller in this geometry^1,2.

Second, as the lattice parameter of (Nd,Ce)₂O₃ does not match the c-axis lattice parameter of NCCO (ref. 1), measurements at non-zero integer L cannot be contaminated by (Nd,Ce)₂O₃. Our experiments indicate that the (1/2,1/2,3) peak shows an induced antiferromagnetic component when the field is along the c-axis and hence superconductivity is strongly suppressed¹, but not when in the a–b plane and superconductivity is only weakly affected². This is direct proof of the connection between field-induced antiferromagnetic order and suppression of superconductivity in NCCO. We also note that the qualitatively different behaviour observed when B is perpendicular to c, in comparison with when it is parallel to c, directly violates the cubic symmetry of (Nd,Ce)₂O₃.

Finally, an independent report³ confirms our principal findings^1,2 in studies of annealed superconducting Pr_0.89LaCe_0.11CuO₄ (PLCCO), a similar electron-doped material in which the cubic impurity phase (Pr,La,Ce)₂O₃ has a non-magnetic ground state and no field dependence below 7 T (our unpublished observations). For fields up to 5 T, Fujita et al.³ find enhanced antiferromagnetic order at (1/2,3/2,0) with increasing field in PLCCO. Above 5 T, this order decreases with increasing field, which is consistent with the field dependence of (1/2,1/2,0) of NCCO (ref. 2). The agreement between two different electron-doped systems in two independent experiments^1,2,3 confirms the quantum phase transition from the superconducting to an antiferromagnetic state in electron-doped, high-T_c superconductors².