The doping dependence of electronic states in an electron-doped high-temperature superconductor ͑HTSC͒ Nd 2−x Ce x CuO 4 was studied by high-resolution angle-resolved photoemission spectroscopy. We observed that the high-energy pseudogap around the hot spot shows an abrupt filling-in at the magnet-superconductor phase boundary, resulting in the unusual reconstruction of the Fermi surface. The magnitude ͑⌬ PG ͒ and the temperature ͑T * ͒ at which the pseudogap is filled-in show a close relation to the effective hyperfine coupling energy ͑J eff ͒ and the spin-correlation length ͑ AF ͒, respectively. These results suggest the magnetic origin of the pseudogap and the unconventional nature of the magnet-superconductor phase transition in electron-doped HTSC.

We report neutron scattering studies on single crystals of the electron-doped (n-type) superconducting cuprate Nd2−x Cex CuO4 (x=0.15) with Tc = 18 K and 25 K. Unlike the hole-doped (p-type) superconducting cuprates where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the superconducting and in the normal state. A spin gap opens up when the n-type cuprate becomes superconducting, as in the optimally doped p-type La2−x Srx CuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from Tc to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the superconducting state of p-type cuprates. High-T c superconductivity emerges when charge carriers, holes or electrons, are doped into an antiferromagnetic Mott insulator [1,2,3]. Mechanism of the superconductivity lies on their common two-dimensional CuO 2 planes into which the charge carriers go. One of the issues in understanding the mechanism is question of the electron-hole symmetry. Electronic structure of the optimally doped cuprates shows evidence for the electron-hole symmetry. [4,5] Their phase diagrams over doping concentrations, however, are asymmetric.[6] For hole doping, antiferromagnetism rapidly weakens and is replaced by a spin-glass-like phase with characteristics of incommensurate spin correlations and pseudo-gap in transport measurements. The pseudo-gap temperature, T * , is well defined in the underdoped region and decreases with doping. The system becomes superconducting (SC) over a wide range of the hole concentration, x, around the optimal x = 0.15. The SC state has incommensurate spin correlations with a T-independent spin gap. The normal state of the underdoped and optimally doped SC region also shows unusual non-Fermiliquid (FL) behaviors. There are increasing evidence for a Quantum Critical Point (QCP) around the optimal doping which is responsible for the unusual properties of the SC and the normal phase [7,8,9,10]. For the electrondoped (n-type) cuprates, on the other hand, antiferromagnetism survives until the superconductivity appears over a narrow range of x around the optimal x ∼ 0.15. The normal state of the n-type cuprates shows FermiLiquid T 2 behavior in resistivity rather than the linear behavior of the hole-doped (p-type) cuprates. Therefore, investigating similarities and differences of the n-type and p-type cuprates would be crucial to understanding physics of the high-T c superconductivity. Compared to a large number of studies on the hole-doped cuprates using various techniques, however, only a small number of key experiments have been done on electron-doped cuprates [4,5,11,12] mainly because it is difficult to grow large single crystals and to prepare homogeneous superconducting samples by post-growth heat treatment.In this paper, we report neutron scattering measurements on single crystals of the electron-doped (n-type) superconducting cuprate N...