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Register a new userA close look at antiferromagnetism in multidimensional phase diagram of electron-doped copper oxide
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Emergency of superconductivity at the instabilities of antiferromagnetism (AFM), spin/charge density waves has been widely recognized in unconventional superconductors. In copper-oxide superconductors, spin fluctuations play a predominant role in electron pairing with electron dopants yet composite orders veil the nature of superconductivity for hole-doped family. However, in electron-doped ones the ending point of AFM is still in controversy for different probes or its sensitivity to oxygen content. Here, by carefully tuning the oxygen content, a systematic study of Hall signal and magnetoresistivity up to 58 Tesla on optimally doped La2-xCexCuO4+-{delta} (x = 0.10) thin films identifies two characteristic temperatures at 62.5+-7.5 K and 25+-5 K. The former is quite robust whereas the latter becomes flexible with increasing magnetic field, thereby linked to two- and three-dimensional AFM, evident from the multidimensional phase diagram as a function of oxygen as well as Ce dopants. Consequently, the observation of extended AFM phase in contrast to {mu}SR probe corroborates an elevated critical doping in field, providing an unambiguous picture to understand the interactions between AFM and superconductivity.
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The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron- and hole-dopings is still fragmentary. Here we combine x-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L3 resonant inelastic x-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone center and deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the {Gamma}-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin-excitations (paramagnons) recently observed in hole-doped compounds even at high doping-levels.
High-temperature (high-Tc) superconductivity in the copper oxides arises from electron or hole doping of their antiferromagnetic (AF) insulating parent compounds. The evolution of the AF phase with doping and its spatial coexistence with superconductivity are governed by the nature of charge and spin correlations and provide clues to the mechanism of high-Tc superconductivity. Here we use a combined neutron scattering and scanning tunneling spectroscopy (STS) to study the Tc evolution of electron-doped superconducting Pr0.88LaCe0.12CuO4-delta obtained through the oxygen annealing process. We find that spin excitations detected by neutron scattering have two distinct modes that evolve with Tc in a remarkably similar fashion to the electron tunneling modes in STS. These results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometer length scales, and the dominant electron-boson coupling at low energies originates from the electron-spin excitations.
The elementary CuO2 plane sustaining cuprate high-temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO5 pyramids (Fig 1a). Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate $t/{hbar}$ and across the charge-transfer energy gap E, generate superexchange spin-spin interactions of energy $Japprox4t^4/E^3$ in an antiferromagnetic correlated-insulator state1. Hole doping the CuO2 plane disrupts this magnetic order while perhaps retaining superexchange interactions, thus motivating a hypothesis of spin-singlet electron-pair formation at energy scale J as the mechanism of high-temperature superconductivity. Although the response of the superconductors electron-pair wavefunction $Psiequiv<c_uparrow c_downarrow>$ to alterations in E should provide a direct test of such hypotheses, measurements have proven impracticable. Focus has turned instead to the distance ${delta}$ between each Cu atom and the O atom at the apex of its CuO5 pyramid. Varying ${delta}$ should alter the Coulomb potential at the planar Cu and O atoms, modifying E and thus J, and thereby controlling ${Psi}$ in a predictable manner. Here we implement atomic-scale imaging of E and ${Psi}$, both as a function of the periodic modulation in ${delta}$ that occurs naturally in $Bi_2Sr_2CaCu_2O_{8+x}$. We demonstrate that the responses of E and ${Psi}$ to varying ${delta}$, and crucially those of ${Psi}$ to the varying E, conform to theoretical predictions. These data provide direct atomic-scale verification that charge-transfer superexchange is key to the electron-pairing mechanism in the hole-doped cuprate superconductor ${Bi_2Sr_2CaCu_2O_{8+x}}$.
We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-reduced Pr$_{2-x}$Ce$_x$CuO$_4$ and Pr$_{1-x}$LaCe$_{x}$CuO$_4$ electron-doped cuprates. In contrast to the common belief, neither the band filling nor the band parameters are significantly affected by the oxygen reduction process. Instead, we show that the main electronic role of the reduction process is to remove an anisotropic leading edge gap around the Fermi surface. While the nodal leading edge gap is induced by long-range antiferomagnetic order, the origin of the antinodal one remains unclear.
Superconductors are a striking example of a quantum phenomenon in which electrons move coherently over macroscopic distances without scattering. The high-temperature superconducting oxides(cuprates) are the most studied class of superconductors, composed of two-dimensional CuO2 planes separated by other layers which control the electron concentration in the planes. A key unresolved issue in cuprates is the relationship between superconductivity and magnetism. In this paper, we report a sharp phase boundary of static three-dimensional magnetic order in the electron-doped superconductor La2-xCexCuO4-d where small changes in doping or depth from the surface switch the material from superconducting to magnetic. Using low-energy spin polarized muons, we find static magnetism disappears close to where superconductivity begins and well below the doping where dramatic changes in the transport properties are reported. These results indicate a higher degree of symmetry between the electron and hole-doped cuprates than previously thought.
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