Nd2 xCexCuO4 Research Articles

Theoretical investigation of the impact of intra‐ and inter‐layer coupling of high TC superconductor Nd2‐xCexCuO4

AbstractThe aim of the current study is to investigate the impact of layer coupling on the characteristics of high TC superconductor Nd2‐xCexCuO4 (NCCO). In this study, the intra‐ and inter‐layer couplings are taken into consideration. By applying the double time Green's function (GF) technique and formulating a model Hamiltonian, the transition temperature (TC) of superconductor Nd2‐xCexCuO4 is computed quantitatively by considering the intra‐ and inter‐layer interactions as independent variables. By increasing the layer interactions, the increase in TC is observed. Furthermore, TC is calculated by taking into consideration different number of interlayer interactions as independent variables which subsequently resulted in the enhancement of TC. Besides, TC has been computed as a function of electron doping as a result of which a dome shaped figure has been perceived in the temperature versus doping phase diagram. The superconducting order parameter (Δ) has been also reckoned as a function of temperature and is observed to gradually decrease as temperature increases and eventually vanishes at TC. A slight increment in TC is observed as the doping values of charge carriers increase. Our findings are discussed in connection with the experimental studies of superconductivity (SC) in Nd2CuO4 based layered compounds.

Normal state interlayer conductivity in epitaxial Nd2–xCexCuO4 films deposited on SrTiO3 (110) single crystal substrates

In this paper we present the experimental results on the temperature dependences of c-axis resistivity, (T), for epitaxial films of n-type superconductor Nd2–xCexCuO4+δ deposited on (110) SrTiO3 single crystal substrate in a wide range of Ce content from lightly to heavily substitution regions () under an optimal annealing. Alternative empirical models were used for a description of the observed semiconductor–like dependence. An excellent quantitative depiction of the experimental dependences turned out to be possible for all investigated samples in a model of natural superlattice with temperature - dependent barrier height.

Fermi surface reconstruction in electron-doped cuprates without antiferromagnetic long-range order

Fermi surface (FS) topology is a fundamental property of metals and superconductors. In electron-doped cuprate Nd2-x Ce x CuO4 (NCCO), an unexpected FS reconstruction has been observed in optimal- and overdoped regime (x = 0.15-0.17) by quantum oscillation measurements (QOM). This is all the more puzzling because neutron scattering suggests that the antiferromagnetic (AFM) long-range order, which is believed to reconstruct the FS, vanishes before x = 0.14. To reconcile the conflict, a widely discussed external magnetic-field-induced AFM long-range order in QOM explains the FS reconstruction as an extrinsic property. Here, we report angle-resolved photoemission (ARPES) evidence of FS reconstruction in optimal- and overdoped NCCO. The observed FSs are in quantitative agreement with QOM, suggesting an intrinsic FS reconstruction without field. This reconstructed FS, despite its importance as a basis to understand electron-doped cuprates, cannot be explained under the traditional scheme. Furthermore, the energy gap of the reconstruction decreases rapidly near x = 0.17 like an order parameter, echoing the quantum critical doping in transport. The totality of the data points to a mysterious order between x = 0.14 and 0.17, whose appearance favors the FS reconstruction and disappearance defines the quantum critical doping. A recent topological proposal provides an ansatz for its origin.

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.

Resistive switching in mesoscopic heterostructures based on Nd2–x Ce x CuO4–y epitaxial films

Reversible and stable bistable resistive switching were observed in planar-type junctions Nd2–x Ce x CuO4–y /Nd2–x Ce x O1.75/Ag. It was shown that current transport in such junctions has a diode character with Schottky-like barriers in highly doped semiconductors. It was revealed that the key factor for switching is the presence of the second phase Nd2–x Ce x O1.75, deficient in oxygen, epitaxially germinated at the Nd2–x Ce x CuO4–y surface.

The mixed-state Hall conductivity of single-crystal films Nd2–xCexCuO4+δ (x = 0.14)

The magnetic-field dependencies of the longitudinal and Hall resistivity of the electron-doped compounds Nd2–xCexCuO4+δ in underdoped region (x = 0.14) were investigated. It was established experimentally a strong magnetic field dependence of the Hall conductivity, σxy(B) = C – b/B, in the region of magnetic fields corresponding to a transition from superconducting to resistive state. The observed feature can be explained with the sum of contributions of the quasiparticles and moving Abrikosov vortices into Hall effect in a mixed state of type-II superconductor.

Resistive switchings and diode properties of heterostructures based on epitaxial superconducting Nd2–x Ce x CuO4–y films

Reversible and reliable bistable resistive switchings are observed in planar-type Nd2–xCexCuO4–y/Nd2–xCexO1.75/Ag junctions based on the Nd2–xCexCuO4–y superconducting films. Investigation of electronic structure of surface of the Nd0.5Ce0.5O1.75/Nd2–xCexCuO4–y via high-resolution X-ray photoelectron spectroscopy and atomic force microscopy (AFM) show that the key factor for the switchings is the presence of an oxygen-deficient Nd2-xCexO1.75 phase, epitaxially intergrown on surfaces of Nd2–xCexCuO4–y.

Charge Transfers and Band Dispersions in Magnetic Nd2−x Ce x CuO4

The electronic structure of Ce-doped Nd2−xCex CuO4 (NCCO) is investigated through ab-initio density functional calculations for supercells. The effective electron doping, h, i.e., the additional electronic charge per Cu atom due to Ce doping, is found to be significantly smaller than x. The explanation of this result is to be found in the hybridization of the CuO band with the Nd-f bands. This hybridization makes the Fermi surface break to occur at a larger k-vector than what is expected from the unhybridized band. The true band dispersion of the CuO band near EF is different from what is seen in ARPES, which partly can be understood from calculations of relaxation energies in photoemission.

Non-BCS Temperature Dependence of Energy Gap in Thin Film Electron-Doped Cuprates

We investigate the dependence of the energy gap ($G$) on the temperature ($T$) for the electron-doped high-temperature superconductors. The following compounds, in the form of the thin films, have been taken into consideration: $\rm La_{2-x}Ce_xCuO_{4}$ (LCCO), $\rm Pr_{2-x}Ce_xCuO_{4}$ (PCCO), and $\rm Nd_{2-x}Ce_xCuO_4$ (NCCO). It was found that $G\left(T\right)$ deviates from the BCS prediction more, if a concentration of cerium assumes the lower values. For the lowest concentration (in the case of LCCO and NCCO), the function $G\left(T\right)$ is not quite like the BCS curve, which is connected with the existence of the residual Nernst region. Next, it has been pointed out that the NCCO superconductor becomes structurally unstable for the maximum concentration of cerium, which is leading to the anomalous dependence of the energy gap on the temperature and the induction of the wide Nernst region.

Study of Nd2–x Ce x CuO4 ± δ (x = 0.1–0.25) as cathode material for intermediate-temperature solid oxide fuel cells

The solid solubility limit of Ce in Nd2–xCexCuO4 ± δ, prepared by sol–gel process, is established up to x = 0.2. The transition from negative temperature coefficient to positive temperature coefficient, within the solid solubility region, is observed at 620 °C. The area-specific-resistance (ASR) is optimized for electrochemical cell sintered at 800 °C. ASR enhances with increase in sintering temperature of cell. ASR value of 0.93 ohm cm2 at 700 °C, determined by electrochemical impedance spectroscopy is comparable against that by voltage versus current (V–I) characteristics at 0.98 ohm cm2 at the same temperature. Electrochemical performance and ASR of Nd1.8Ce0.2CuO4 ± δ is improved when prepared by sol–gel route over solid-state reaction, which is attributed to uniform size and shape of nanocrystalline grains.

Pseudogap phase of high-Tc compounds described within the LDA+DMFT+[formula omitted] approach

LDA+DMFT+Σk approach was applied to describe pseudogap phase of several prototype high-Tc compounds e.g. hole doped Bi2Sr2CaCu2O8−δ (Bi2212) and La2−xSrxCuO4 (LSCO) systems and electron doped Nd2−xCexCuO4 (NCCO) and Pr2−xCexCuO4 (PCCO), demonstrating qualitative difference of the Fermi surfaces (FS) for these systems. Namely for Bi2212 and LSCO the so called “hot-spots” (intersection of a bare FS and AFM Brillouin zone (BZ) boundary), where scattering on pseudogap fluctuations is most intensive were not observed. Instead here we have Fermi arcs with smeared FS close to the BZ boundary. However, for NCCO and PCCO “hot-spots” are clearly visible. This qualitative difference is shown to have material specific origin. Good agreement with known ARPES data was demonstrated not only for FS maps but also for spectral function maps (quasiparticle bands including lifetime and interaction broadening).

Correlation of the local and the macroscopic properties of high-temperature superconductors

Temperature dependent comparative EXAFS studies of Ba1–xKxBiO3 superconducting oxides and copper based superconductors with hole (La2–xSrxCuO4) and electron (Nd2–xCexCuO4–δ) doping showed the common dynamical local structure nonuniformity observed as the double-well potential of oxygen ion oscillations. Obtained data evidence that these local structure nonuniformity arise from the local dynamic charge ordering due to different electronic structure of the neighbouring BiO6 or CuOn (n = 4, 6) complexes in perovskite-like lattice. Based on the experimental results, the phenomenological description was proposed which allowed us to establish the correlations between the local and the macroscopic properties and to explain the insulator-metal phase transition and the appearance of the superconductive state with both hole and electron doping of parent BaBiO3, La2CuO4 and Nd2CuO4 insulators. The presented results makes clear the role of electron-phonon coupling in the mechanism of high temperature superconductivity.

Destruction of Néel order in the cuprates by electron doping

Motivated by the evidence in ${\text{Pr}}_{2\ensuremath{-}x}{\text{Ce}}_{x}{\text{CuO}}_{4\ensuremath{-}y}$ and ${\text{Nd}}_{2\ensuremath{-}x}{\text{Ce}}_{x}{\text{CuO}}_{4\ensuremath{-}y}$ of a magnetic quantum critical point at which N\'eel order is destroyed, we study the evolution with doping of the $T=0$ quantum phases of the electron-doped cuprates. At low doping, there is a metallic N\'eel state with small electron Fermi pockets, and this yields a fully gapped ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ superconductor with coexisting N\'eel order at low temperatures. We analyze the routes by which the spin-rotation symmetry can be restored in these metallic and superconducting states. In the metal, the loss of N\'eel order leads to a topologically ordered ``doublon metal'' across a deconfined critical point with global O(4) symmetry. In the superconductor, in addition to the conventional spin-density wave transition, we find a variety of unconventional possibilities, including transitions to a nematic superconductor and to valence-bond supersolids. Measurements of the spin-correlation length and of the anomalous dimension of the N\'eel order by neutron scattering or NMR should discriminate these unconventional transitions from spin-density wave theory.

Apical oxygen, 3D‐2D cross over and superconductivity in Sm2–xCexCuO4–δ

AbstractIn spite of the vast amount of experimental and theoretical articles accumulated in HTSC, the mechanism of the interaction driving charge carriers to form Cooper pairs below Tc is still unknown. The comparison of the normal state transport properties of YBa2Cu3O7–δ and the Sm2–xCexCuO4–δ [1, 2] might shed some light on the microscopic origin of HTSC. In comparison to the YBCO, the apical oxygen in Sm2–xCexCuO4–δ [3] destroys the superconductivity via the vertical ionic bonding which localizes the charge in the Cu‐O squares, however the hole transfer by moving O(4) towards the CuO2 planes, leads to the optimization of YBCO properties. The behaviour of C axis parameter vs the oxygen content cannot be explained by a BSC mechanism. The high amount of anisotropy ratio [4] is explained by the sheer square planes in NCCO system, i.e. without apical oxygen (SC with Tc maximum). From the data of the resistivity in the normal state, we conclude the observation of a 3D‐2D cross over only in Sm2–xCexCuO4–δ [2] and Nd2–xCexCuO4–δ [5] which is also related to its high anisotropy. The competition between anisotropy and superconductivity destroys the superconducting state in the 2D limit even in the ground state. In this material the superconductivity cannot be enhanced at high temperature because the compound is a quasi 2D system (sheer square planes of CuO2) and the cuprate superconductors is a genuine three‐dimensional (3D) phenomenon [6]. The Josephson coupling between the different layers is S‐I‐S for NCCO and S‐N‐S for YBCO, thus the Lawrence and Doniach model (LD) [7] with neighbouring layers coupled by the Josephson tunnelling is appropriate. In summary the behaviour of apical oxygen is intrinsically different in the two kinds of cuprates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of site selectivity, dimensionality, and strong correlations in angle-resolved photoemission from cuprate superconductors

We have carried out extensive first-principles computations of angle-resolved photoemission (ARPES) spectra from the cuprate superconductors within the general framework of the local density approximation (LDA). Selected results on Bi2Sr2CaCu2O8 (Bi2212), La2−xSrxCuO4 (LSCO) and Nd2−xCexCuO4 (NCCO) are presented and discussed. Our focus is on understanding how the underlying electronic structure is mapped via the complex process of photoexcitation into the observed ARPES intensities. Effects of the ARPES matrix element and its remarkable selectivity properties with respect to the energy and polarization of the incident photons in exciting a specific state and/or electrons from a particular site in the lattice are clarified. The importance of deviations from perfect two-dimensionality and the associated interlayer couplings in shaping the ARPES spectra of the cuprates is delineated. Our computations explain many salient features of the experimental spectra. Surprisingly, this agreement extends in some cases to the underdoped regime where strong electron correlations are obviously important. We discuss how the LDA-inspired tight-binding parameters can serve as a useful starting point for the treatment of strong coupling effects in the cuprates.

Phase decomposition and chemical inhomogeneity inNd2−xCexCuO4±δ

Extensive X-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd2-xCexCuO4 (NCCO) in a low-oxygen-fugacity environment such as that typically realized during the annealing process required to create a superconducting state. Unlike a typical situation in which a disordered secondary phase results in diffuse powder scattering, a serendipitous match between the in-plane lattice constant of NCCO and the lattice constant of one of the decomposition products, (Nd,Ce)2O3, causes the secondary phase to form an oriented, quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from the secondary phase appear at rational positions (H,K,0) in the reciprocal space of NCCO. Additionally, because of neodymium paramagnetism, the application of a magnetic field increases the low-temperature intensity observed at these positions via neutron scattering. Such effects may mimic the formation of a structural superlattice or the strengthening of antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified through careful and systematic experimentation. For typical reduction conditions, the (Nd,Ce)2O3 volume fraction is ~1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50-100 angstromsthick. The presence of the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements.

Superconductivity in NCCO thin films and effect of Gd and Ni doping

We have prepared thin films of Nd2−xCexCuO4 (NCCO), Nd1.8−xGd0.2CexCuO4 and Nd2−xCexCu0.998Ni0.002O4 superconductors with Ce concentration of 0.095⩽x⩽0.14 by pulsed laser deposition technique. Superconductor–insulator transition point in NCCO thin film is observed below x=0.11. The presence of the underdoped superconducting region is shown in NCCO thin film.

Doping Dependence of Superconducting Energy Gap of NCCO Observed by Tunneling Spectroscopy

In order to clarify the electron-hole symmetry in cuprates, Nd2−xCe x CuO4 (NCCO) thin films were fabricated for various Ce concentrations and conductance spectra were measured by tunneling spectroscopy. Since the superconductivity appears even at x=0.095, thin film samples have an advantage over single crystals for studying the electronic states in underdoped region. It is shown that the amplitude of superconducting energy gap was almost 7–11 meV in the whole superconducting range. Comparing with hole-doped cases, the energy gap of NCCO is relatively insensitive to the doping concentration.

Heavy‐fermion systems

AbstractDifferent scenarios of heavy‐fermion behaviour are pointed out. They include Kondo lattices, the Zeeman effect and special forms of charge ordering. Specific examples for the three cases are discussed, i.e., CeRu2Si2, Nd2—xCexCuO4 and Yb4As3.

Heavy-fermion systems

Different scenarios of heavy-fermion behaviour are pointed out. They include Kondo lattices, the Zeeman effect and special forms of charge ordering. Specific examples for the three cases are discussed, i.e., CeRu2Si2, Nd2—xCexCuO4 and Yb4As3.