Electron-doped high-Tc Superconductors Research Articles

63,65Cu, 139La-NMR study of Electron-doped High Temperature Cuprate Superconductor T’-La1.8Eu0.2CuO4–y F y

T’-La1.8Eu0.2CuO4–y F y (0 ≤ y ≥ 0.125) was measured by 63,6BCu and 139La NMR. This material is an electron-doped high-Tc cuprate superconductor which has Nd2CuO4-type structure (so-called T’-structure). The nuclear spin-lattice relaxation rate 1/T 1 revealed that pseudogap behavior exists in the lightly electron-doped region of y ≤ 0.075 and electron doping suppresses the antiferromagnetic spin flutuations. These behaviors in electron-doped high-T c cuprate superconductor are analogous with the optimum- and over-doped region in hole-doped type. 139 La NMR spectra shows almost no antiferromagnetic region in the samples of T’-La1.8Eu0.2CuO4–y F y .

Effect of Antiferromagnetic Order on Mixed States in Electron-Doped High-Tc Superconductors

The vortex states in electron-doped high-Tc superconductors are studied theoretically at the mean-field approach and based on the Bogoliubov-de Gennes equations. The local density of states is calculated, through which the in-gap bound states are revealed. They are robust at the sites around the vortex core. In the underdoped region where the antiferromagnetic order and the superconducting order coexist, the in-gap peaks are lying symmetric with respect to the Fermi energy, while the peak intensities at the negative energy are much stronger. A stripe-type modulation is revealed for the spin-resolved local density of states. As the doping density increases to the optimal or over-doped region, the long-range antiferromagnetic order disappears, and the two in-gap peaks merge to one single peak. The peak position is near the Fermi energy. Our results indicate that the vortex states may be used to detect the signature of the antiferromagnetic order in electron-doped high-Tc superconductors.

Suppression of Pseudogap Behavior and Antiferromagnetic Fluctuations by Electron Doping in T′-La1.8Eu0.2CuO4−yFy Studied by 63,65Cu NMR

We performed 63,65Cu NMR measurements of an electron-doped high-Tc cuprate superconductor T′-La1.8Eu0.2CuO4−yFy (0 ≤ y ≤ 0.125) having Nd2CuO4-type structure (so-called T′-structure). Pseudogap beh...

Pseudogap Behavior in T′-Pr1.3−xLa0.7CexCuO4 Revealed by 63,65Cu NMR

We performed 63,65Cu NMR measurements of electron-doped high-Tc cuprate superconductor T′-Pr1.3−xLa0.7CexCuO4 with the Nd2CuO4-type structure (so-called T′-structure) for 0.00 ≤ x ≤ 0.15. Pseudogap...

Study of Annealing Effect on Momentum Distribution Functions in Parent Compound of Electron-Doped High-Tc Superconductor by High Resolution Compton Scattering

Study of Annealing Effect on Momentum Distribution Functions in Parent Compound of Electron-Doped High-Tc Superconductor by High Resolution Compton Scattering

Impurity effect in electron-doped high-Tc superconductors

The quasiparticle states around a nonmagnetic impurity in electron-doped high-Tc superconductors are studied systematically based on the Bogoliubov–de Gennes equations. In the antiferromagnetic state, one in-gap impurity resonance state is revealed. As the impurity scattering potential increases, the resonance peak shifts to the gap edge and finally disappears for a strong impurity. The antiferromagnetic order and superconducting order coexist when the doping density increases. In this coexisting state, the in-gap resonance peaks are rather robust and appear in pairs that are lying symmetric with the Fermi energy. The peak positions and intensities strongly depend on the impurity potentials and the next-nearest-neighbour hopping constants. For a rather strong impurity, the resonance peaks shift to near the gap edge. When further increasing the doping density, the system is in the pure superconducting state. The resonance peaks still appear in pairs, with the peak intensities being weaker compared to those in the coexisting state. The two resonance peaks may occasionally merge into one zero-energy peak for both the coexisting state and the pure superconducting state. The spin-resolved LDOS are also investigated and may be used to detect possible antiferromagnetic order.

Electronic Homogeneity in the Overdoped Regime of the Electron-Doped High-Tc Superconductors

In order to investigate the electronic homogeneity in electron-doped high-Tc cuprates, magnetization and specific heat measurements have been performed for Pr1-xLaCexCuO4 single crystals with x = 0.13 – 0.17. It has been found from the magnetization curve that the hysteresis is relatively small and that the so-called second magnetization peak is absent for every x. Moreover, the Sommerfeld constant in the ground state has shown HlnH dependence on magnetic field H for every x, suggesting that the superconductivity is not pure but dirty. These results are in contrast to those observed in the overdoped regime of hole-doped La2-xSrxCuO4 where an inhomogeneous superconducting state is formed in spite of the superconductivity being pure. Therefore, it has been concluded that a rather homogeneous electronic state is realized in the electron-doped cuprates, suggesting different electronic states between hole- and electron-doped cuprates.

Superconducting anisotropy in the electron-doped high-Tc superconductors Pr2−xCexCuO4−y

We report superconducting anisotropy measurements in the electron-doped high-Tc superconductors (HTSCs) Pr2−xCexCuO4−y (PCCO, x = 0.15 and 0.17) with an applied magnetic field (H0) up to 28 T. Our results show that the upper critical field [Hc2(T)] of PCCO is highly anisotropic and as the temperature T → 0, the value of it at H0 ∥ c [Hc2,∥c(0)] is far less than the Pauli limit. The low temperature anisotropic character of PCCO is found to be rather similar to that of hole-doped cuprate HTSCs, but apparently larger than that of typical Fe-based superconductors. This study also proves a new sensitive probe of detecting rich properties of unconventional superconductors with the use of the resonant frequency of an NMR probe circuit.

Rectifying characteristic of perovskite oxide La1.89Ce.11CuO4/Ba.5Sr.5TiO3/La.67Sr.33MnO3 heterostructures

Heterostructure composed of electron-doped high-Tc superconductor La1.89Ce0.11CuO4 (LCCO), ferroelectric Ba0.5Sr0.5TiO3 (BST) and colossal magnetoresistance (CMR) La0.67Sr0.33MnO3 (LSMO) thin films were deposited on the (001) SrTiO3 (STO) substrate by pulsed laser deposition method. The current-voltage measurement shows that the LCCO/BST/LSMO structure has strong rectification comparing to the LCCO/LSMO, LCCO/BST and BST/LSMO bilayer structures. The experiment indicates that both the BST and LCCO layers play important roles in the rectifying effect in the oxide heterostructures. We discuss the mechanism of their rectification based on the energy band bending as well as the depletion and the diffusion of the charge carriers at the interfaces. The growth conditions and the multilayer features as well as the properties of individual material in the heterostructures were also investigated.

Spin fluctuation spectrum of electron-doped high-Tc superconductors

The magnetic excitation spectrum of electron-doped copper oxide superconductors is calculated using the Hubbard model on a square lattice. First, the on-site repulsion is treated with the random phase approximation. The spectrum of electron-doped systems in the superconducting state is compared with that of hole-doped systems, and the relationship between the frequency at which a peak grows in the spectrum and the superconducting energy gap at a hot spot (an intersection of the Fermi surface and the magnetic Brillouin zone boundary) is investigated. As compared with the hole-doped systems, the resonance condition is difficult to be satisfied in the electron-doped systems because of the small density of states around the hot spot. Moreover, the correlation effect in the Hubbard model is treated by the fluctuation-exchange approximation (FLEX), and the spin fluctuation spectra in the superconducting state in a wide region of the wave vector and frequency are calculated. We have found that the intensity of the magnetic spectrum at incommensurate wave vectors obtained with the FLEX is considerably weaker than that obtained with the RPA. The validity of the Fermi-liquid approach is also discussed.

Effect of strong correlations on the high energy anomaly in hole- and electron-doped high-Tc superconductors

Recently, angle-resolved photoemission spectroscopy (ARPES) has been used to highlight an anomalously large band renormalization at high binding energies in cuprate superconductors: the high energy ‘waterfall’ or high energy anomaly (HEA). This paper demonstrates, using a combination of new ARPES measurements and quantum Monte Carlo simulations, that the HEA is not simply the by-product of matrix element effects, but rather represents a cross-over from a quasi-particle band at low binding energies near the Fermi level to valence bands at higher binding energy, assumed to be of strong oxygen character, in both hole- and electron-doped cuprates. While photoemission matrix elements clearly play a role in changing the aesthetic appearance of the band dispersion, i.e. the ‘waterfall’-like behavior, they provide an inadequate description for the physics that underlies the strong band renormalization giving rise to the HEA. Model calculations of the single-band Hubbard Hamiltonian showcase the role played by correlations in the formation of the HEA and uncover significant differences in the HEA energy scale for hole- and electron-doped cuprates. In addition, this approach properly captures the transfer of spectral weight accompanying both hole and electron doping in a correlated material and provides a unifying description of the HEA across both sides of the cuprate phase diagram.

Effects of annealing on the electronic structure of the electron-doped high-Tc superconductor Nd1.85Ce0.15CuO4

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of as-grown Nd1.85Ce0.15CuO4 (ag-NCCO) and annealed Nd1.85Ce0.15CuO4 (an-NCCO) samples in order to elucidate how the superconductivity is induced by the annealing process in Nd2-xCexCuO4. In the ARPES spectra of ag-NCCO, a gap opens in the nodal region due to the effects of antiferromagnetism. This nodal gap is closed in an-NCCO, indicating that the effects of antiferromagnetism become weaker by annealing, similar to the previous ARPES results on Pr1.85Ce0.15CuO4. However, while in as-grown Pr1.85Ce0.15CuO4 the gap opens on the entire Fermi surface, in ag-NCCO the gap does not open around (π, 0).

Femtosecond time-resolved dynamics in electron-doped high-Tc superconductor La2-xCexCuO4

The quasiparticle dynamics in the electron-doped high-Tc superconductor La2-xCexCuO4 (LCCO) was investigated by ultrafast time-resolved optical pump-probe technique. We present results in three temperature regions: namely the low temperature (TTc) region, that around Tc(0.7Tc≤T≤Tc) and the high temperature (T>Tc) region. In general, there was a slowdown of the relaxation as the temperature decreases. However, the quasiparticle lifetime was found to exhibit a quasi-divergence as the temperature approached Tc from below. The relaxation time in normal state was much longer than what was expected for metallic relaxation, indicating the presence of a T-independent pseudogap. We have also observed picosecond rise component of photoinduced change in reflectivity ΔR/R in the electron-doped LCCO, which is attributed to the Cooper pair-breaking dynamics. The rise time became shorter as the temperature increased. The experimental results are analyzed by Rothwarf-Taylor model.

Quantum spin correlations through the superconducting-to-normal phase transition in electron-doped superconducting Pr 0.88 LaCe 0.12 CuO 4-δ

The quantum spin fluctuations of the S = 1/2 Cu ions are important in determining the physical properties of high-transition-temperature (high T(c)) copper oxide superconductors, but their possible role in the electron pairing of superconductivity remains an open question. The principal feature of the spin fluctuations in optimally doped high-T(c) superconductors is a well defined magnetic resonance whose energy (E(R)) tracks T(c) (as the composition is varied) and whose intensity develops like an order parameter in the superconducting state. We show that the suppression of superconductivity and its associated condensation energy by a magnetic field in the electron-doped high-T(c) superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) (T(c) = 24 K), is accompanied by the complete suppression of the resonance and the concomitant emergence of static antiferromagnetic order. Our results demonstrate that the resonance is intimately related to the superconducting condensation energy, and thus suggest that it plays a role in the electron pairing and superconductivity.

Lanthanide Substitution Effects in Electron-Doped High-T c Superconductors Studied by Angle-Resolved Photoemission Spectroscopy

We have performed an angle-resolved photoemission study of the electron-doped high-T c superconductors (HTSCs) Sm1.85Ce0.15CuO4 (SCCO) and Eu1.85Ce0.15CuO4 (ECCO). Around the nodal point $\vec{k}\sim(\pi /2,\pi /2)$ , we observed an energy gap at the Fermi level (E F) for both samples, resulting from the strong effects of antiferromagnetism. The magnitude of this gap in ECCO is larger than that in SCCO, implying that the effects of antiferromagnetism in ECCO are stronger than that in SCCO. Also, we quantitatively confirmed the effects of antiferromagnetism by the band dispersions to tight-binding calculation.

Pulsed neutron-scattering study of an entire spin excitations in the electron-doped high-Tc superconductor

We have investigated magnetic excitations of an electron-doped superconductor Pr0.89LaCe0.11CuO4 (Tc=26K) using pulsed neutron-scattering over a wide range of energy up to ∼180meV. Spin fluctuations were found to persist around (π,π) reciprocal position at least up to ∼180meV. With increasing the energy (ω) below 60meV, peak-width broadens rapidly, while above this energy the width weakly depends on the energy. On the other hand, the scattering intensity shows a gradual decrease as the ω increase. However, no well-defined magnetic intensity was observed, where the peak-width saturates. The observed spin excitations in Pr0.89LaCe0.11CuO4 is compared with those in the hole-doped system from the viewpoint of dual structure of the spectrum.

High-resolution ARPES study of many-body interactions in high-Tc cuprate superconductors

We report our recent ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) results on several hole- and electron-doped high-Tc cuprate superconductors. We have studied the momentum and temperature dependence of the kink in the dispersion near the Fermi level and its “magnetic isotope effect” in hole-doped superconductors. The momentum dependence of pseudogap and superconducting gap has been studied for electron-doped superconductors. All the experimental results unambiguously indicate that the magnetic interaction is essential in characterizing the electronic structure and the superconductivity.

Influence of structural disorder on low-temperature behavior of penetration depth in electron-doped high-TC thin films

To probe the influence of structural disorder on low-temperature behavior of magnetic penetration depth, λ(T), in electron-doped high-TC superconductors, a comparative study of high-quality Pr1.85Ce0.15CuO4 (PCCO) and Sm1.85Ce0.15CuO4 (SCCO) thin films is presented. The λ(T) profiles are extracted from conductance-voltage data using a highly-sensitive home-made mutual-inductance technique. The obtained results confirm a d-wave pairing mechanism in both samples (with nodal gap parameter Δ0/kBTC=2.0 and 2.1 for PCCO and SCCO films, respectively), substantially modified by impurity scattering (which is more noticeable in less homogeneous SCCO films) at the lowest temperatures. More precisely, Δλ(T)=λ(T)−λ(0) is found to follow the Goldenfeld–Hirschfeld interpolation formulae Δλ(T)/λ(0)=AT2/(T+T0) with T0=ln(2)kBΓ1/2Δ01/2 being the crossover temperature which demarcates pure and impure scattering processes (T0/TC=0.13 and 0.26 for PCCO and SCCO films, respectively). The value of the extracted impurity scattering rate Γ correlates with the quality of our samples and is found to be much higher in less homogeneous films with lower TC.

Direct Observation of a Nonmonotonic dx2-y2-Wave Superconducting Gap in the Electron-Doped High-Tc Superconductor Pr0.89LaCe0.11CuO4

We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4 to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the dx2-y2- wave symmetry, but obviously deviates from the monotonic dx2-y2- gap function. The maximum gap is observed not at the zone boundary, but at the hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. The present experimental results suggest the spin-mediated pairing mechanism in electron-doped high-Tc superconductors.

Momentum Dependence of Charge Excitations in the Electron-Doped SuperconductorNd1.85Ce0.15CuO4: A Resonant Inelastic X-Ray Scattering Study

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-T(c) superconductor Nd1.85 Ce0.15 CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model.