Anomalous electronic structure and pseudogap effects in Nd1.85Ce0.15CuO4

We report a high-resolution angle-resolved photoemission spectroscopic study of the electron-doped ( n-type) cuprate superconductor Nd1.85Ce0.15CuO4. We observe regions along the Fermi surface where the near- E(F) intensity is suppressed and the spectral features are broad in a manner reminiscent of the high-energy "pseudogap" in the underdoped p-type (hole doped) cuprates. However, instead of occurring near the (pi,0) region, as in the p-type materials, this pseudogap falls near the intersection of the underlying Fermi surface with the antiferromagnetic Brillouin zone boundary.     

电子型超导体Nd1.85Ce0.15CuO4单晶输运性质研究

我们测量了最佳掺杂的Nd1．85Ce0．15CuO4单晶的电阻率和ab面以及c方向热电势．ρ-T曲线在高温下表现为非常好的直线，超导转变比较尖锐Tc,mid=20．6K，说明我们的样品品质很好．低温下的电阻率拟合结果表明频率大约100K的软声子模与载流子有强烈相互作用，这是极化子模型有力的证据．对于热电势结果，令人惊奇的是，无论数值还是曲线形状，包括低温曳引峰的位置，c方向的热电势Sc与ab面内的热电势Sab非常接近．这可能是某种共同的机制导致的结果．比较Nd1．85Ce0．15CuO4和Sm1.85Ce0.15CuO4的热电势曲线可以发现，二的数值和曲线形状基本相同，而与La1．85Sr0．15CuO4差异较大，这进一步说明了顶点氧在这些体系中的作用．     

Experimental observation of localization-delocalization of Cooper pairs in Nd1.85Ce0.15CuO4

Localization—delocalization of cooper pairs on ⁶⁷Zn impurity centers in the copper sublattice in the high-temperature superconductor Nd_1.85Ce_0.15CuO₄ was observed by emission Mössbauer spectroscopy on ⁶⁷Cu (⁶⁷Zn).     

Superconducting gap anisotropy in Nd1.85Ce0.15CuO4: results from photoemission

We have performed angle resolved photoelectron spectroscopy on the electron doped cuprate superconductor Nd1.85Ce0.15CuO4. A comparison of the leading edge midpoints between the superconducting and normal states reveals a small, but finite shift of 1.5-2 meV near the ( pi,0) position, but no observable shift along the zone diagonal near ( pi/2, pi/2). This is interpreted as evidence for an anisotropic superconducting gap in the electron doped materials, which is consistent with the presence of d-wave superconducting order in this cuprate superconductor.     

Neutron-spin resonance in the optimally electron-doped superconductor Nd1.85Ce0.15CuO4-delta

We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped superconductor Nd1.85Ce0.15CuO4-delta above and below its superconducting transition temperature Tc=25 K. In addition to gradually opening a spin pseudogap at the antiferromagnetic ordering wave vector Q=(1/2,1/2,0), the effect of superconductivity is to form a resonance centered also at Q=(1/2,1/2,0) but at energies above the spin pseudogap. The intensity of the resonance develops like a superconducting order parameter, similar to those for hole-doped superconductors and electron-doped Pr0.88LaCe0.12CuO4. The resonance is therefore a general phenomenon of cuprate superconductors, and must be fundamental to the mechanism of high-Tc superconductivity.     

Magnetic field effect on the superconducting magnetic gap of Nd1.85Ce0.15CuO4

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.     

电子型最佳掺杂高温超导体Nd1.85Ce0.15CuO4-δ中的磁激发谱

文章作者用非弹性中子散射探测了电子型最佳掺杂高温超导体Nd1.85Ce0.15CuO4-δ(Tc=25K)的磁激发谱.当样品的温度从正常态逐渐降低至超导态时,在以波矢Q=(π,,π0)为中心的区域,系统打开了一个小于4m eV的自旋隙,与此同时,在9.5m eV附近出现了一个"中子-自旋磁共振峰",并且"共振峰"的强度随温度的变化关系类似于超导序参量在超导相变发生时的变化,这一点与以前发现的各种空穴型超导体以及电子型超导体Pr0.88LaCe0.12CuO4中的"共振峰"的性质极为相似.因此可以说明"共振峰"是所有高温超导体的共性,并且对高温超导电性的形成有着非常关键的作用.     

NMR evidence for a two-step phase separation in Nd1.85Ce0.15CuO4-delta

By Cu NMR we studied the spin and charge structure in Nd(2-x)Ce(x)CuO(4-delta). For x=0.15, starting from a superconducting sample, the low temperature magnetic order in the sample reoxygenated under 1 bar oxygen at 900 degrees C reveals a peculiar modulation of the internal field, indicative of a phase characterized by large charge droplets ("blob" phase). By prolonged reoxygenation at 4 bars the blobs break up and the spin structure changes to that of an ordered antiferromagnet. We conclude that the superconductivity in the n-type systems competes with a genuine type I Mott-insulating state.