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Register a new userSuperconductivity induced by structural reorganization in electron-doped cuprate NCCO
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Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content only by a tiny amount, but has a dramatic impact on the electronic properties of the sample. Here, we report the occurrence of superconductivity in oxygen-deficient Nd$_{2-x}$Ce$_x$CuO$_4$ thin films grown in oxygen-free environment, after annealing in pure argon flow. As verified by x-ray diffraction, annealing induces an increase of the interlayer distance between CuO$_2$ planes in the crystal structure. Since this distance is correlated to the concentration of oxygens in apical positions, and since oxygen content cannot substantially increase during annealing, our experiments indicate that the superconducting phase transition has to be ascribed to a migration of oxygen ions to apical positions during annealing. Moreover, as we confirm via first-principles density functional theory calculations, the changes in the structural and transport properties of the films can be theoretically described by a specific redistribution of the existing oxygens ions at apical positions with respect to CuO$_2$ planes, which remodulates the electronic band structure and suppresses the antiferromagnetic order, allowing the emergence of hole superconductivity.
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We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth, lambda(T), and conductivity, sigma(T) of Pr_{2-x}Ce_{x}CuO_{4-delta} (PCCO) crystals, as well as parallel-plate resonator measurements of lambda(T) in PCCO thin films. Penetration depth measurements are also presented for a Nd_{2-x}Ce_{x}CuO_{4-delta} (NCCO) crystal. We find that delta-lambda(T) has a power-law behavior for T<T_c/3, and conclude that the electron-doped cuprate superconductors have nodes in the superconducting gap. Furthermore, using the surface impedance, we have derived the real part of the conductivity, sigma_1(T), below T_c and found a behavior similar to that observed in hole-doped cuprates.
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.
Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2 single-crystals is used to fine-tune structural, magnetic and electronic properties of this iron-based superconducting system. Neutron and single crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of $180times 10^{-6}$ K$^{-1}$. Electrical transport and magnetic susceptibility measurements reveal abrupt changes in the physical properties through the collapse as a function of temperature, including a reconstruction of the electronic structure. Superconductivity with onset transition temperatures as high as 47 K is stabilized by the suppression of antiferromagnetic order via chemical pressure, electron doping or a combination of both. Extensive investigations are performed to understand the observations of partial volume-fraction diamagnetic screening, ruling out extrinsic sources such as strain mechanisms, surface states or foreign phases as the cause of this superconducting phase that appears to be stable in both collapsed and uncollapsed structures.
We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using a variational cluster-perturbation theory approach based upon the self-energy-functional theory, the spectral function of the electron-doped two-dimensional Hubbard model is calculated. The model includes the next-nearest neighbor electronic hopping amplitude $t$ and a fixed on-site interaction $U=8t$ at half filling and doping levels ranging from $x=0.077$ to $x=0.20$. Our results support the fact that a comprehensive description of the single-particle spectrum of electron-doped cuprates requires a proper treatment of strong electronic correlations. In contrast to previous weak-coupling approaches, we obtain a consistent description of the ARPES experiments without the need to introduce a doping-dependent on-site interaction $U$.
Superconductivity research is like running a marathon. Three decades after the discovery of high-Tc cuprates, there have been mass data generated from transport measurements, which bring fruitful information. In this review, we give a brief summary of the intriguing phenomena reported in electron-doped cuprates from the aspect of electrical transport as well as the complementary thermal transport. We attempt to sort out common features of the electron-doped family, e.g. the strange metal, negative magnetoresistance, multiple sign reversals of Hall in mixed state, abnormal Nernst signal, complex quantum criticality. Most of them have been challenging the existing theories, nevertheless, a unified diagram certainly helps to approach the nature of electron-doped cuprates.
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