Superconductor Nd2 Research Articles

On the interpretation of the phase diagram of La2−xMxCuO4

The difference between the phase diagrams of La2−x(Ba,Sr)xCuO4 and Nd2−xCexCuO4 is discussed. It is proposed that the discrepancy of x-values corresponding to the transition from antiferromagnetic dielectric state to conducting one (respectively, x≈0.06 in La2−x(Ba,Sr)xCuO4 and x≈0.125 in Nd2−xCexCuO4) results from non-homogeneous doping of La2−x(Ba,Sr)xCuO4 over the range 0.06<x<0.125, when localized holes are added to each second CuO2 layer. Therefore the actual phase diagram of the “holedoped” superconductor La2−x(Ba,Sr)xCuO4 coincides with one for the “electron-doped” superconductor Nd2−xCexCuO4. It is shown that all features observed in La2−x(Ba,Sr)xCuO4 around x=0.125 are clarified on this basis.

Orthorhombic to tetragonal transition and appearance of superconductivity in Nd 0.7Pr 0.3Ba 2 − kSr kCu 3O 6+z (0≤k≤1)

Preparation, X-ray diffraction and ac susceptibility of Nd 0.7Pr 0.3Ba 2 − kSr kCu 3O 6+z (0≤k≤1) are reported. An orthorhombic to tetragonal transition was observed for k≥0.75. Superconductivity occurred only for k≥0.25. When the samples were heated in argon prior to oxygen annealing both the orthorhombicity (for 0≤k ≤0.75) and T c increased (by 10 K for k = 1). Though a weakening of Pr-f hybridization with CuO bands due to Sr substitution may explain the data, a detailed knowledge of oxygen defect chemistry will be required to understand the behavior of Pr in LnBa 2Cu 3O 6+z(Ln = rare earths).

Determination of the dielectric tensor in anisotropic materials

We demonstrate a new approach to determine the dielectric tensor of anisotropic materials using a rotating polarizer (or analyzer) ellipsometer. The dependence of the ellipsometric parameters as a function of azimuthal angle shows characteristic patterns very sensitive to the magnitude of the dielectric tensor and its orientation with respect to the sample surface. These patterns are in general nonsinusoidal and depend strongly on the angle setting of the fixed analyzer (or polarizer). We illustrate the method with results obtained on a generic crystallographic face of a uniaxial material, namely, a single crystal of the electron-doped high-temperature superconductor Nd1.85Ce0.15CuO4−δ .

Anomalous optical response of Nd1.85Ce0.15CuO4−y superconducting thin films

A study of the normal state optical response of epitaxial thin films of electron doped superconductor Nd1.85Ce0.15CuO4−y is reported. The normal state optical response of these films were not proportional to the temperature derivative of the resistance of the films as expected by a bolometric model and as seen in p-type superconductors such as YBa2Cu3O7−δ. The normal state optical response of the films with different transition temperatures were also remarkably different from each other. The observation of anomalous optical response is attributed to the possible presence of photoconductivity in Nd1.85Ce0.15CuO4−y.

An investigation about the improvement ofTc for the n-type superconductor Nd2−xCexCuO4

Nd2-xCexCuO4(x = 0.12 to 0.18)with T(onset) = 17 to 26 K is synthesized from the nominal composition of Nd2-xCexCuO4 + nSrCuO2 (+ nCaCuO2) (n = 1, 2) under standard conditions without high pressure treatment. The sintering temperature, atmosphere, and synthetic procedure are important for the superconductivity of Nd2-xCexCuO4. It is difficult to build up n blocks of SrCuO2, CaCuO2, or CdCuO2 on Nd2-xCexCuO4 so as to add CuO2 layers. In addition, the crystal structure and electrical resistance of Nd2-xCexCuO4 are studied.

The influence of lithium intercalation and iodine on the electrical properties of the superconductor Nd1.82Ce0.18CuOz

As-prepared Nd 1.82 Ce 0.18 CuO z pellets treated by electrochemical lithium intercalation and iodine vapour were studied. It was found that the sample Li 0.06 Nd 1.82 Ce 0.18 CuO z intercalated by lithium at 705 °C, using LiF/KF melt electrolyte, became superconductive at T rmc =23.5 K. The mechanism is considered to be that lithium has been inserted into the interstitial site near the CuO square and has donated its electron to the CuO 2 plane, while the extra oxygen atom has been removed. The insertion of lithium using LiClO 4 polypropylene carbonate electrolyte cells at room temperature was very slow and no superconductivity was observed. For these samples with lithium inserted at room temperature and then annealed at 900 °C in air, the defect oxygen atoms can be introduced into the structure along with lithium as shown by the increase of resistivity with the lithium amount. The samples treated with iodine at 200 °C showed a resistivity drop between 9.9 and 13.9 K, which might indicate superconductivity in a small portion with possible composition Nd 1.82 Ce 0.18 CuO z−y I y . Combined with the results of the luminescence spectra, the mechanism of deoxygenation can be considered as the delocalization of the doped electrons in the crystal.

Thermoelectric power studies of a Nd1.82−xSrxCe0.18CuOy superconducting system (abstract)

We have reported recently on the revival of superconductivity in Nd1.82Ce0.18CuOy by doping with a divalent alkaline earth element. The doped superconductor Nd1.82−xSrxCe0.18CuOy has a Nd2CuO4 (T′-type) structure. This system possesses a normal state metallic resistivity [ρ(T)] in polycrystalline form. The ρ(T) versus temperature (T) plots are, in general, linear. We report here the results of our studies of ρ(T) and thermopower S(T) and their dependence on the lattice oxygen content (y) that is a function of the Sr-content x. The dρ(T)/dT improves and then deteriorates with an increase in y. However, the thermopower behavior is a more complicated function of y (or x) and its results indicate that the majority of the charge carriers are electronlike. The thermopower (in absolute value) of the superconducting Nd1.73Sr0.09Ce0.18CuO3.97 compound gradually decreases from 250 to 150 K, remains almost constant from 150 to 75 K, decreases very fast below 75 K, and shows negative dS/dT. The overall feature of the S vs T plot suggests that the main contribution to S is from diffusion. Unlike reduced Nd1.82Ce0.18CuO4.01 and other electron-doped superconductors, we have not observed a peak near the transition temperature (Tc) on the higher temperature side of the S vs T plot of the present sample. An attempt is made to explain the salient features of the thermopower.

HTSC anomalies as result of the special electron spectrum. III. Doping influence on transport and superconducting properties

The influence of doping-induced variations of electron structure on transport and superconducting properties of La 2−xSr xCuO 4 and Nd 2−xCe xCuO 4 is considered. In the frames of proposed model the dependence of Hall resistance on concentration x of doping element is obtained. It is shown that in Nd 2−xCe xCuO 4 just at x=0.15 the “negative-U centers” arises at the top of valence band and the dielectric-metal transition takes place. The superconductivity in Nd 2−xCe xCuO 4 occurs in hole-conductive phase. The pressure dependence of T c in HTSC is considered.

Hole filling, charge transfer and superconductivity in Nd 1+ xBa 2− xCu 3O 7+δ

Unlike YBa 2Cu 3O 7−δ (Y123), the Nd-Ba-Cu-O system exhibits a solid solution, Nd 1+ x Ba 2− x Cu 3O 7+δ (Nd123ss), for 0.04⩽ x⩽0.6. However, the superconducting properties decrease nonlinearly for increasing x, and T c can also be varied by lowering the annealing temperature without changing the low-temperature oxygen soak. The changes in microstructure and T c with increasing x are analogous with Y123 with increasing δ, yet for fully oxygenated Nd123ss samples, electron energy loss spectroscopy (EELS) demonstrates that the total unoccupied density of states below the unfilled band is constant for all x. The neutron diffraction and the inert gas fusion analysis support an overall charge balance for all x. T c is modeled in terms of a forced oxygen disorder resulting from Nd 3+ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in T c as a function of x and heat treatments can be explained by the number of four-fold coordinated Cu's on the chains which can be varied by differing the amounts of paired and unpaired Nd 3+ substituting for Ba 2+, resulting in more or less charge transfer from the planes to the chains.

Ultrafast nonequilibrium carrier relaxation in single-crystal Nd1.85Ce0.15CuO4−y

We present a temperature-dependent investigation of the femtosecond relaxation dynamics of the n-type superconductor Nd1.85Ce0.15CuO4−y using ultrashort pulses from a Ti-sapphire laser in a transient absorption, pump-probe configuration. The energy relaxation time of the single crystals in the normal state was observed to increase by over an order of magnitude as the critical temperature was approached. This behavior has been modeled by the scattering of electrons with two-dimensional phonons. In the superconducting state, an increase and subsequent saturation of the relaxation time has been discerned. A possible reason for this is a complex interplay among the characteristic relaxation times, including scattering and recombination of quasiparticles optically excited by ultrashort laser pulses and order parameter relaxation close to Tc.

The phase stability diagrams for the systems Nd 2CuO 4-δ and Nd 1.85Ce 0.15CuO 4-δ

The dependences of the degrees of oxygen nonstoichiometry in Nd 2CuO 4-δ and Nd 1.85Ce 0.15CuO 4-δ on temperature and oxygen pressure have been determined by thermogravimetric measurements in the range of temperature 350–950°C and the range of oxygen pressure 10 −5−1 atm. The nature of the decomposition of these oxides in the range of oxygen pressure 10 −5−1 atm has been determined by thermogravimetric analysis and X-ray diffraction. In the solid state, decreasing oxygen pressure causes Nd 2CuO 4-δ to decompose to NdCuO 2 + Nd 2 O 3 and then to Nd 2 O 3 + Cu 2O. The solid solution Nd 1.85Ce 0.15CuO 4δ decomposes to Nd 2 O 3 + NdCeO 3.5 + Cu 2 O. The solidus line of incongruent melting has been determined in the range of oxygen partial pressure 10 −6−1 atm by differential thermal analysis and the phases formed on solidification have been identified by X-ray diffraction. The phase stability diagrams for Nd 2CuO 4-δ and Nd 1.85Ce 0.15CuO 4-δ have been constructed. The dependence of the onset of superconductivity in Nd 1.85Ce 0.15CuO 4-δ on the oxygen pressure and temperature at which the specimen was annealed, and hence on the oxygen nonstoichiometry, has been determined.

Effect of Ni and Zn substitution on magnetic ordering in Gd 2CuO 4

Ceramic samples of Gd 2Cu 1−xNi xO 4 with x=0, 0.03, 0.05, 0.07, 0.09 and 0.11, and Gd 2Cu 1−xZn xO 4 with x= 0.01, 0.03 and 0.05, were prepared. Magnetic measurements showed that the antiferromagnetic ordering temperature of Cu decreased due to the substitution of Cu by Ni or Zn. The effect of Ni on suppressing antiferromagnetic ordering is larger than that of Zn. The results are compatible with the substitution effect on superconductivity in Nd 1.85Ce 0.15CuO 4.

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.

Charge transfer and superconductivity in Nd(Ba 1− xNd x) 2Cu 3O 7+ x prepared in high pressure oxygen

By annealing in oxygen at high pressure around 600 ≈ 1000 atm, we have been able to make a series of the single-phase solid solution system Nd(Ba 1− x Nd x ) 2Cu 3O 7+ x i.e. σ = x in Nd(Ba 1− x Nd x ) 2Cu 3O 7+ x . x-dependences of both hole concentration and superconductivity have been investigated in this system. In this series of samples, the hole concentration deduced from the Hall effect measurement changes monotonically with x while the average charge of a [CuO] unit, i.e. the total hole concentration, deduced from a chemical analysis is not changed for all samples. This clearly demonstrates that the mobile hole concentration in this system is determined by a charge transfer between CuO 2 sheet-layer and CuO 1+ x chain-layer. This charge transfer is understood by consideration of the electrostatic energy based on a simple model. x-dependence of the superconductivity transition temperature T c is interpreted as the dependence of T c on the mobile hole concentration of the CuO 2 plane, which is expressed by an almost linear relation up to the highest T c of 95 K.

The role of of oxygen defects in superconductivity of Nd 2−xCe xCuO 4−y studied by x-ray absorption fine structure

The role of oxygen defects in superconductivity and their effect on the electron states has been studied by X-ray absorption fine structure on the Cu K and Ce L 3 edges, for Nd 2−xCe xCuO 4−y (0.0 <x<0.20) annealed at various oxygen partial pressures ( pO 2 = 1−10 −4 atm). The results indicate that the reduction enhances the electron density at the copper sites, without changing the oxygen deficiency y. A possible mechanism of carrier enhancement through the removal of a small number of excess oxygen atoms which would suppress n-type doping in the CuO 2 plane is presented.

Growth and superconductivity of Nd 2−xCe xCuO 4 single crystals

Nd 1.85Ce 0.15CuO 4 single crystals were grown by the traveling solvent floating zone (TSFZ) method. The grown single crystals were 6 mm in diameter and 50 mm in length with a metallic luster, and had no inclusions and cracks. The as-grown crystals were semiconductors. When annealed in nitrogen, the crystals became superconductive with T c=22 K and Δ T c=2 K, showing a significant anisotropy in resistivities above T c.

Temperature dependence of activation energy due to intrinsic pinning for flux lines in (Nd1−xCex)2CuO4−δ films

Using electron superconductor (Nd1−xCex)2CuO4−δ films prepared by a laser ablation technique, we measured the temperature dependence of the resistivity in magnetic fields and the magnetic field-direction dependence of the critical current. The temperature dependence of the resistivity in magnetic fields was analyzed using an activation type of formula. The temperature dependence of the activation energy for some oxide superconductors for H∥ the basal plane was calculated using the intrinsic pinning model proposed by Tachiki and Takahashi. The temperature dependence changed systematically depending on the ratio of the coherence length along the c-axis to the inter-distance between CuO conductive planes. The magnetic field-direction dependence of the critical current was also explained using the model. From these analyses, the origin of the temperature dependence of the activation energy is discussed.

Phonon density of states and superconductivity in Nd1.85Ce0.15CuO4.

Inelastic-neutron-scattering techniques have been used to measure the generalized phonon density of states in superconducting ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$. There is reasonable agreement at low energies between our measurements and recent point-contact-tunneling density-of-states measurements reported by Huang et al., which suggests that phonons play a major role in pairing in the electron-superconductor systems. The correspondence at higher energies, however, is not particularly good, but probably reflects the difficulty in obtaining quantitative tunneling data.

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.

In Situ Pulsed Laser Deposition of Nd1.85Ce0.15Cu04−y

ABSTRACTWe have deposited thin films of the electron-doped, high temperature superconductor Nd1.85Ce0.15CuO4−y by pulsed laser deposition. Films were deposited from a stoichiometric target using a KrF excimer laser (248 nm, 250 mJ/pulse, −2 J/cm2) as a function of substrate temperature, oxygen pressure and vacuum anneal conditions. The film composition and structure, as determined by RBS and XRD, were very sensitive to the deposition and subsequent anneal conditions. Stoichiometric films were deposited at low substrate temperatures (740 °C) but contained other orientations and phases. Predominantly c-axis oriented films were formed at high substrate temperatures (900 °C) and high background pressures of oxygen (200 mtorr). These films were semiconducting when quenched on oxygen or nitrogen following deposition. Slow cooling in a vacuum, yielded superconducting films with a maximum Tc-(onset) of 15 K and Tc(R=0) of 11 K. A variation of the deposition and anneal conditions indicated that loss of copper competed with the optimization of the carrier concentration.