Electron-doped Superconductor Nd2 Research Articles

The Interplay of the Charge and Vortex Subsystems in Anisotropic Electron-Doped Superconductor Nd2 – xCexCuO4

The Interplay of the Charge and Vortex Subsystems in Anisotropic Electron-Doped Superconductor Nd2 – xCexCuO4

Anisotropy of the critical current density in a layered electron-doped superconductor Nd2–xCexCuO4+δ

The results of studying the conductivity and critical current density in monocrystalline Nd2–xCexCuO4+δ/SrTiO3 (x = 0.15; 0.17) films with c-axis perpendicular or parallel to the substrate plane are presented. It is found that in stoichiometrically annealed films with the optimum cerium content (x = 0.15) the resistance anisotropy is maximal and the anisotropy of the critical current density is jcab/jcc ≅ 3 · 103 at T = 4.2 K. The strong anisotropy of the critical current density is considered in the framework of natural superlattices, with alternating conductive CuO2 and non-conducting buffer Nd(Ce)O layers. It is shown that the high density of the critical current along the conductive CuO2 planes is caused by the pinning of vortices on the buffer layers, whereas the strong anisotropy of the critical current is caused by the anisotropic motion of the vortex lattice in the layered superconductor.

Electrical transport properties of sputtered Nd2−xCexCuO4±δ thin films

Thin films of the electron-doped high-temperature superconductor Nd2−xCexCuO4±δ have been deposited by dc sputtering technique on (100) SrTiO3 substrates. A tuning of the oxygen content in the as-grown non-superconducting samples has been achieved by changing the oxygen partial pressure during the growth in the Argon sputtering atmosphere. All samples show the superconducting transition after a suitable two-step thermal treatment in an oxygen-reducing environment. Structural and electrical transport properties on the as-grown as well as on the superconducting samples have been investigated. We find that the structural properties are consistent with a deficiency of the oxygen content with respect to optimally annealed samples, and that the transition to the superconducting phase is always accompanied by an increase of the c-axis lattice parameter. Measurements of the Hall coefficient RH as a function of temperature and in the normal state of our epitaxial films are presented and discussed. RH results negative for all the films regardless of the oxygen content and it decreases with the temperature. In particular, the Hall coefficient is only about 10% lower than the value measured in the as-grown oxygen-deficient phase, in contrast to the results reported in literature. The removal of the excess oxygen in as-grown samples seems not to be the only requirement for triggering the superconducting transition in electron-doped compounds. The microstructural change associated with the increase of the c-axis parameter in our deoxygenated samples could help in understanding the microscopic mechanism underlying the reduction process of n-type superconductors, which is still under debate.

Magnetoresistance and hall effect in electron-doped superconductor Nd2 − x Ce x CuO4+δ with different degrees of nonstoichiometric disorder: A two-band model

Investigations have been performed on the magnetic-field dependences of the longitudinal and Hall resistivity of the electron-doped Nd2 − xCexCuO4+δ system with an optimum content of cerium (x = 0.15) and different degree of the nonstoichiometric disorder in magnetic fields of up to 8 T (B ‖ c; J ‖ ab) in a temperature range of 0.4–4.2 K. The observed anomalous dependences of the Hall coefficient R(B) along with the dependences of the electrical resistivity ρ(B) have been interpreted based on the Drude-Lorentz model in the case of the normal state and in the Bardeen-Stephen model in the mixed state modified with allowance for the coexistence of two types of charge carriers with the opposite charge signs (electrons and holes).

Pseudogap behavior in the emery model for the electron-doped superconductor Nd2 − x Ce x CuO4: Multiband LDA + DMFT + Σk approach

We propose a generalization of the LDA + DMFT + Σk approach to the multiband case, in which correlated and uncorrelated states are present in the model simultaneously. Using the multiband version of the LDA + DMFT + Σk approach, we calculate the density of states and spectral functions for the Emery model in a wide energy interval around the Fermi level. We also obtain the Fermi surfaces for the electron-doped high-temperature superconductor Nd2 − xCexCuO4 in the pseudogap phase. The self-energy part Σk introduced additionally to take into account pseudogap fluctuations describes the nonlocal interaction of correlated electrons with collective Heisenberg short-range spin fluctuations. To solve the effective impurity model, the numerical renorm-group (NRG) method is used for the DMFT equations. Good qualitative agreement of the Fermi surfaces calculated using the LDA + DMFT + Σk approach and experimental angle-resolved photoemission spectroscopic data is attained. The stability of the dielectric solution with charge transfer in the Emery model with correction for double counting is analyzed in the Appendix.

Upper critical field in electron-doped cuprate superconductor Nd2−xCexCuO4+δ: Two-gap model

We present resistivity measurements of the upper critical field (Hc2) phase diagram as a function of temperature (T) for Nd1.85Ce0.15CuO4+δ/SrTiO3 single crystal films with different degree of disorder (δ) in magnetic fields up to 90 kOe at temperatures down to 0.4K. The data are well described by a two-band/two-gap model for a superconductor in the dirty limit.

Doping effect on the anomalous behavior of the Hall effect in electron-doped superconductor Nd2−xCexCuO4+δ

Transport properties of Nd2−xCexCuO4+δ/SrTiO3 single crystal films (B‖c,J‖ab) are investigated in magnetic fields B up to 9T at T=(0.4–4.2)K. An analysis of normal state (at B>Bc2) Hall coefficient RHn dependence on Ce doping takes us to a conclusion about the existence both of electron-like and hole-like contributions to transport in nominally electron-doped system. In accordance with RHn(x) analysis an anomalous sign reversal of Hall effect in mixed state at B<Bc2 may be ascribed to a flux-flow regime for two types of carriers with opposite charges.

Fermi surface of the electron-doped cuprate superconductor Nd2−xCexCuO4 probed by high-field magnetotransport

We report on the study of the Fermi surface of the electron-doped cuprate superconductor Nd2−xCexCuO4 by measuring the interlayer magnetoresistance as a function of the strength and orientation of the applied magnetic field. We performed experiments in both steady and pulsed magnetic fields on high-quality single crystals with Ce concentrations of x=0.13–0.17. In the overdoped regime of x>0.15, we found both semiclassical angle-dependent magnetoresistance oscillations (AMROs) and Shubnikov–de Haas (SdH) oscillations. The combined AMROs and SdH data clearly show that the appearance of fast SdH oscillations in strongly overdoped samples is caused by magnetic breakdown. This observation provides clear evidence for a reconstructed multiply connected Fermi surface up to the very end of the overdoped regime at x≃0.17. The strength of the superlattice potential responsible for the reconstructed Fermi surface is found to decrease with increasing doping level and likely vanishes at the same carrier concentration as superconductivity, suggesting a close relation between translational symmetry breaking and superconducting pairing. A detailed analysis of the high-resolution SdH data allowed us to determine the effective cyclotron mass and Dingle temperature, as well as to estimate the magnetic breakdown field in the overdoped regime.

Local dynamic deformation of the superconducting CuO2 plane in the Nd2 − x Ce x CuO4 + δ compound

The local structure of the electron-doped high-temperature superconductor Nd2 − x Ce x CuO4 + δ has been investigated by EXAFS spectroscopy using synchrotron radiation in the temperature range 5–300 K. The radii, coordination numbers, and Debye-Waller factors of the nearest coordination spheres of the copper, neodymium, and cerium local environment have been obtained. The Einstein temperatures, characterizing the strength of Cu-O, Nd-O, and Ce-O interatomic bonds have been determined. The dynamic local deformation of the superconducting CuO2 plane in the form of vibrations of some part of oxygen ions in a double-well potential due to the difference in the electronic filling of neighboring CuO4 complexes has been established.

Giant magnetoresistance in Ce-doped manganite systems

The fascinating properties like giant magnetoresistance (GMR) effect, metal-insulator transition, charge ordering phenomenon etc. have made the divalent ion doped RMnO3 (R = rare-earth elements) an attractive system for investigation. Resistivity of these compounds shows a peak near the ferromagnetic transition temperature (T c ). The application of magnetic field inhibits the spin-disorder scattering and the resistivity decreases drastically. Keeping electrondoped superconductor Nd2−x Ce x CuO4 in mind we have doped RMnO3 (R = La, Pr, Nd) with tetravalent Ce ion. These compounds are very susceptible to the annealing treatment and belong to the orthorhombic perovskite phase. They show a very high value of resistivity at the peak and under the magnetic field the GMR effect is observed. For La0.7Ce0.3MnO3 and Pr0.7Ce0.3MnO3 the magnetoresistance ratio reaches about 54% and 82.5% respectively at 7.7 T. With the increase of the temperature the magnetic state changes from ferromagnetic to paramagnetic regime. This magnetic transition is not very sharp and the resistivity peak appears at a temperature higher than T c .

Tunneling measurements of the superconductor Nd1.85C0.15CuO4??

We report tunneling measurements of the electron-doped superconductor Nd1.85Ce0.15Cu04−gd using break junctions. The observed finest gap structure is well expressed by the BCS density of states with the energy gap of Δ = 6.1 meV and a very small broadening parameter, having the ratio 2Δ/kBTc = 8.2 with Tc = 17.3 K. These features are similar to our tunneling results for the hole-doped cuprate superconductors. Therefore we find no essential difference in the gap structure, in spite of a common believing of the pairingsymmetry difference between them.

Improved laser-ablated thin films of NdCeCuO by use of N2O

Thin films of the electron-doped superconductor Nd1.85Ce0.15CuO4−δ were prepared by laser ablation in O2 and N2O ambients. Although films prepared in an O2 ambient need to be reduced to become superconducting, the use of a lower oxygen pressure during deposition produces inferior films. A more metallic character can be obtained by increasing the O2 pressure during deposition. The substitution of N2O for O2 results in films that are superconducting without a separate reduction step, with a sharp transition at about 16 K. A short vacuum anneal at the deposition temperature yields reproducible films with Tc=20 K, without change in the transition width.

Oxygen nonstoichiometry of the (Nd, Ce)2CuO4−δ system

Abstract Oxygen nonstoichiometry of the electron-doped superconductor Nd1.85Ce0.15CuO4−δ and nondoped Nd2CuO4−δ was determined using thermogravimetry and chemical titration analysis. The oxygen deficiency of Nd2CuO4−δ at 900°C and PO2=10-3 atm was approximately 0.04, while that of Nd1.85Ce0.15CuO4−δ was about 0.008, both being substantially smaller than that of Ba2YCu3O7−δ. In this system, however, a small amount of oxygen defect seems to play an essential role in the manifestations of superconductivity.

Deposition of epitaxial thin films of Nd1.85Ce0.15CuO4−y by laser ablation

Thin films of the electron-doped superconductor Nd1.85 Ce0.15 CuO4−y have been deposited on (100) SrTiO3 substrates at 780 °C using the laser ablation technique. The deposited films are very smooth and show epitaxial growth with the c axis normal to the substrate. The transport properties of the films are very sensitive to the concentration of oxygen vacancies. Films deposited and cooled in the presence of 150 mTorr O2 exhibit localization behavior with no evidence of superconductivity down to 5 K. Superconductivity is observed on vacuum annealing the films in situ after deposition. Films with optimum concentration of oxygen vacancies show a superconducting onset temperature of 21 K and Tc (R=0) of 20 K, with a critical current density of 2×105 A/cm2 at 5.5 K in zero magnetic field.

Anomalous absence of pressure effect on transition temperature in the electron-doped superconductor Nd1.85Ce0.15CuO4–δ

THE recent discovery of the electron-doped superconductors1 added fresh fuel to controversies about the mechanism of high-temperature superconductivity in copper oxide compounds. The superconducting transition temperature (Tc) of almost all the hole-doped copper oxide compounds increases with increasing pressure2. The pressure coefficient has a large positive value compared with a typical conventional (BCS) superconductor. A systematic survey of the TcS of Y–Ba–Cu–O and La–Sr–Cu–O compounds has shown that Tc is strongly correlated with hole concentration3–5, but the Hall coefficient is only weakly dependent on pressure6,7. These results indicate that the large change in Tc with pressure is caused, not by a change in hole concentration, but by other factors related to changes in the interatomic distances. Here we report that the newly discovered electron-doped superconductor Nd,1.85Ce0.15CuO4–δ exhibits almost no pressure effect on Tc up to 2.5 GPa, in remarkable contrast to the hole-doped superconductor Nd1.3Ce0.3Sr0.5CuO4–δ. The Cu–O pyramids characteristic of the hole-doped compounds lose their apical oxygens to become square planes in the electron-doped materials. If the pressure effect on Tc does arise from changes in bond lengths, the difference in behaviour of the two compounds points to the involvement of the bond between copper and apical oxygen, which is missing in the electron-doped material.

Neutron and electron diffraction study of the electron-doped superconductor Nd1.845Ce0.155CuO4 − y

The crystal structure and non-stoichiometry of Nd1.845Ce0.155CuO4 − y were investigated by the combined use of neutron powder diffraction and selected-area electron diffraction. It is isomorphous with Nd2CuO4 (tetragonal, I4/mmm), but a minor portion of a superconducting sample had a pseudo-tetragonal superlattice with A = 2√2 a. This superstructure results from the partial ordering of oxygen vacancies and/or the two lanthanoid metals. The distance of an equatorial Cu-O bond, 0.1973 nm, in Nd1.845Ce0.155CuO4 − y is somewhat longer than those in hole-doped superconductors containing Cu, which reflects higher electron densities along the Cu-O bond in the former, compared with the latter. An oxygen site between two Nd(Ce) sheets was 3–4% deficient. Furthermore, another oxygen position on a two-dimensional [CuO2]∞ sheet may also become very slightly vacant on thermal treatment under a low partial pressure of O2. Electrons are, therefore, doped into the [CuO2]∞ sheets by not only substitution of Ce4+ ions for Nd3+ ions but creation of the oxygen vacancies.