ABSTRACT
We use pump-probe spectroscopy to measure the photoinduced reflectivity ΔR of the electron-doped cuprate superconductor Nd(2-x)Ce(x)CuO(4+δ) at a value of x near optimal doping, as a function of time, temperature, and laser fluence. We observe the onset of a negative ΔR signal at T(*)≈75 K, above the superconducting transition temperature, T(c), of 23 K. The relatively slow decay of ΔR, compared to the analogous signal in hole doped compounds, allows us to resolve time-temperature scaling consistent with critical fluctuations. A positive ΔR signal onsets at T(c) that we associate with superconducting order. We find that the two signals are strongly coupled below T(c), in a manner that suggests a repulsive interaction between superconductivity and another fluctuating order.
ABSTRACT
High-transition-temperature (high-T(c)) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd2-xCexCuO4+/-delta (NCCO) shows bulk superconductivity above x approximately 0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x approximately 0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies, arises from a build-up of spin correlations, in agreement with recent theoretical proposals.
ABSTRACT
Inelastic neutron-scattering measurements on the archetypical electron-doped material Nd1.85Ce0.15CuO4 up to a high relative magnetic-field strength, H/H(c2) approximately 50%, reveal a simple linear magnetic-field effect on the superconducting magnetic gap and the absence of field-induced in-gap states. The extrapolated gap-closing field value is consistent with the upper critical field H(c2), and the high-field response resembles that of the paramagnetic normal state.
