Two-dimensional quantum interference contributions to the magnetoresistance of Nd2−x CexCuO4−δ single crystals

Two-dimensional (2D) weak localization effects at low temperatures T=0.2–4.2 K are investigated in a nonsuperconducting sample Nd_1.88Ce_0.12CuO_4−δ and in the normal state of a superconducting sample Nd_1.82Ce_0.18CuO_4−δ for B>B _c2≃3 T. The phase coherence time τφ (≃5×10⁻¹¹ s at 1.9 K) and the effective thickness d of a conducting CuO₂ layer (≃1.5 Å) are estimated by fitting the expressions from the 2D weak localization theory to the magnetoresistivity data for the normal-to-plane and in-plane magnetic fields. The estimated value of the parameter d ensures the condition of strong carrier confinement and justifies a model of almost decoupled 2D metallic sheets for the Nd_2−x Ce_xCuO_4−δ single crystals. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 93–99 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Pairing type change upon an increase in the cerium doping level in the Nd<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4 + δ</Subscript> electronic superconductor

We report on the results of analyzing the temperature dependences of the resistivity of the Nd_{2 − x} Ce _x CuO_{4 + δ} electronic superconductor with x = 0.14 (underdoped range), x = 0.15 (optimal doping), and x = 0.18 (overdoped range) and with various degrees of annealing in an oxygen-free atmosphere in magnetic fields up to H = 90 kOe (H ‖ c, J ‖ ab) in the temperature range T = (0.4–300) K. It is shown that the observed differences in the dependences of the slope of the upper critical field (dH_c2 /dT)|_Tc (dH_{c2} /dT)|_{T_c } on the degree of disorder in the Nd_{2 − x} Ce _x CuO_{4 + δ} system upon a change in the cerium doping level indicate a change in the symmetry of d-type pairing to anisotropic s pairing.     

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd_2−x Ce _x CuO_4−y and a large single crystal of Nd₂CuO_4−y . We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La_2−x Sr _x CuO_4−y , although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd₂CuO_4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.     

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2?xLaxCuO6+δ and Nd2?xCexCuO4?δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi₂Sr_2−x La _x CuO_6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd_2−x Ce _x CuO_4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO₂ plane and superconductivity fluctuations in the insulating phase.     

Resistive switching effect in thin-film heterojunctions based on electron-doped Nd2 ? xCexCuO4 ? y superconductor

Thin-film heterojunctions Nd_{2 − x} Ce _x CuO_{4 − y} /Ag were obtained. The bipolar effect of resistive switching in these heterostructures was detected and investigated. X-ray diffraction data indicate the presence of a second phase in thin films; along with the basic phase Nd_{2 − x} Ce _x CuO_{4 − y} , it affects the behavior of the interface of investigated heterojunctions and leads to an alteration of the type of conductivity. The threshold frequency of alternating voltage at which the resistive switching effect is observed in heterojunctions was detected.     

Relation between T c and the defect structure of neodymium cerium cuprite Nd2−x CexCuO4−y

Neutron powder diffraction has been used to study the defect structure of neodymium cerium cuprite Nd_2−x Ce_xCuO_4±y (x=0.15). It has been shown that in addition to oxygen vacancies, O2 sites in superconducting samples may also contain a small quantity of implanted oxygen atoms positioned between copper ions and neodymium/cerium, which control the electrical charge in the Cu-O planes. The oxygen distribution among crystal lattice sites in Nd_2−x Ce_xCuO_4±y (x=0.15) as determined, the average charge of the copper ions was calculated by the method of valence sums, and a correlation was established between the charge of the copper-oxygen plane and T _c. Fiz. Tverd. Tela (St. Petersburg) 40, 177–183 (February 1998)     

μ− SR studies of oxide-superconductorsSR studies of oxide-superconductors

The negative muon spin rotation method (μ⁻ SR) has been applied to studies of electronic states at oxygen sites of oxide superconductors YBa₂Cu₃O₇, Nd_2−x Ce _x CuO_4−δ (x=0.15, oxygen reduced), LiTi₂O₄ and related oxide-insulators La₂CuO_4−δ, CuO, Cu₂O. The paramagnetic shifts of μ⁻ trapped at oxygen nuclei in these polycrystalline powder samples have been measured at 300 K. All the measured shifts are positive. In copper-oxides the paramagnetic shifts are of the order 10⁻³, while in LiTi₂O₄ is very small (8.4±3.34×10⁻⁵). In YBa₂Cu₃O₇, a fast μ⁻ spin relaxation timeT ₂ ^* (∼ 200 ns) has been observed; the reason for this is unknown and further studies are now in progress.     

Two-dimensional weak localization effects in high temperature superconductor Nd2−x CexCuO4−δ

A systematic study of the resistivity and Hall effect in single-crystal films Nd_2−x Ce_xCuO_4−δ (0.12⩽x⩽0.20) is presented, with special emphasis on the low-temperature dependence of the normal state conductance. Two-dimensional weak localization effects are found both in a normally conducting underdoped sample (x=0.12) and in situ superconducting optimally doped (x=0.15) or overdoped (x=0.18) samples in a high magnetic field B>B _c2. The phase coherence time τ _ϒ (5.4 · 10⁻¹¹ s at 2 K) and the effective thickness of a CuO₂ conducting layer d (≃1.5Å) have been estimated by fitting 2D weak localization theory expressions to magnetoresistivity data for magnetic fields perpendicular to the ab plane and in plane. Estimates of the parameter d ensure strong carrier confinement and justify a model consisting of almost decoupled 2D metallic sheets for the Nd_2−x Ce_xCuO_4−δ single crystal. Zh. éksp. Teor. Fiz. 116, 1723–1734 (November 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Mixed conductivity of zircon-type Ce1−xAxVO4±δ (A=Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0−0.2) was found to significantly increase the p-type electronic conductivity and to decrease the Seebeck coefficient, which becomes negative at x≥0.1. The oxygen ionic conductivity is essentially unaffected by doping. The ion transference numbers of Ce_1−xA_xVO_4+δ in air, determined by the faradaic efficiency measurements, are in the range from 2×10⁻⁴ to 6×10⁻³ at 973–1223 K, increasing when temperature increases or alkaline-earth cation content decreases. The results on the partial conductivities and Seebeck coefficient suggest the presence of hyperstoichiometric oxygen, responsible for ionic transport, in the lattice of doped cerium vanadates. The activation energies for the electron-hole and ionic conduction both decrease on doping and vary in the ranges 39–45 kJ/mol and 87–112 kJ/mol, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Magnetic ordering in Nd2-xCexCuO4-y

The n-type cuprate superconductor and closely related samples of the general form Nd_2−x Ce _x CuO_4−y have been studied with the muon spin relaxation technique. Non-super-conducting samples display antiferromagnetism (AFM) withT _N below 150 K, the value being dependent upon both Ce concentration and oxygen depletion. The results are interpreted in terms of carrier concentration.     

Structural phase transition in substituted La2CuO4 and Pr2CuO4−y

The structural phase transition from orthorhombic (T) phase to tetragonal (T′) phase in substituted La_2−x R _x CuO₄ (R = Pr, Nd, Sm, Eu and Gd) and T′ to T-phase in Pr_2−x M _x CuO_4−y (M = Sr, Ca) has been studied by X-ray diffraction technique. The T-phase of La₂CuO₄ is transferred to T′ phase abruptly atx=0.8, 0.4, 0.4, 0.3 and 0.4 respectively for substitution of Pr, Nd, Sm, Eu and Gd for La in La₂CuO₄ without evidence of the T* phase. The T′ structure of Pr₂CuO₄ (x = 0.0) gets transformed to the T* structure at 30% Ca doping (x=0.6) and then to the T structure at 50% Ca doping (x=1.0), while for Sr-contentx=0.0, 0.4 and 1.0 it shows T′, T* and T structure respectively.     

Observation of standing x-rays in Bragg diffraction on high-T c superconducting crystals Nd1.85Ce0.15CuO4−δ

Curves of the secondary-fluorescence yield from the constituent elements of high-T _c superconducting crystals Nd_1.85Ce_0.15CuO_4−δ are measured in the dynamic x-ray diffraction regime. The form of the curves attests to the appearance of a standing x-ray wave in the crystal. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 703–706 (10 May 1997)     

Effects of <Emphasis Type="Italic">d</Emphasis>-wave pairing in <Emphasis Type="Italic">n</Emphasis>-type high-temperature superconductors with anisotropic impurity scattering

The temperature dependences of the resistivity of single-crystal films of the Nd_{2 − x} Ce _x CuO_{4 + δ} n-type superconductors with x = 0.14 (underdoped region) and x = 0.15 (optimal doping region) and different degrees of disorder δ have been investigated in various magnetic fields (B ‖ c, J ‖ ab) in the temperature range 0.4–300 K. It has been demonstrated that there are differences in the behavior of the dependences of the slope of the upper critical field $ (dB_{c2} /dT)_{T \to T_c } $ (dB_{c2} /dT)_{T \to T_c } on the disorder parameter for the underdoped samples (x = 0.14) and the samples with the optimal doping (x = 0.15). The study of the dependence of the slope of the upper critical field on the degree of disorder has made it possible to discriminate experimentally between the superconductors with the d pairing and anisotropic s pairing. It has been revealed that the relative stability of the n-type superconductor with the optimal doping with respect to disordering is possibly due to the strong anisotropy of impurity scattering with symmetry of the d type.     

On the interpretation of the phase diagram of La<Subscript>2−x</Subscript>M<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

The difference between the phase diagrams of La_2?x(Ba,Sr)_xCuO₄ and Nd_2?xCe_xCuO₄ 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 La_2?x(Ba,Sr)_xCuO₄ and x≈0.125 in Nd_2?xCe_xCuO₄) results from non-homogeneous doping of La_2?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 La_2?x(Ba,Sr)_xCuO₄ coincides with one for the “electron-doped” superconductor Nd_2?xCe_xCuO₄. It is shown that all features observed in La_2?x(Ba,Sr)_xCuO₄ around x=0.125 are clarified on this basis.     

An EIS study of La2 − x Sr x NiO4 + δ SOFC cathodes

La_2 − x Sr _x NiO_{4 +  δ} materials were investigated as cathodes for the electrochemical reduction of oxygen on a Ce_1.9Gd_0.1O_1.95 (CGO10) electrolyte using cone-shaped electrodes together with electrochemical impedance spectroscopy. The area-specific resistance (ASR) of the La_2 − x Sr _x NiO_{4 +  δ} nickelates towards the reduction of oxygen is equal to the ASR of perovskites; however, it is not as low as for the best Fe–Co-based perovskites. The lowest ASR is found for the compound La_1.75Sr_0.25NiO_{4 +  δ} with an ASR of 23.8 Ωcm² measured on a cone-shaped electrode in air at 600 °C. It is suggested that difference in oxide ionic conductivity of the nickelates is the main cause for the different activity of the nickelates towards the electrochemical reduction of oxygen.     

Comparing the physical properties of Pr/Gd and Pr/Ce substitutions in Ru(Gd<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>)Sr<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10−<Emphasis Type="Italic">δ</Emphasis></Subscript>

We have compared the electrical and magnetic properties of Ru(Gd_1.5−x Pr _x )Ce_0.5Sr₂Cu₂O_10−δ (Pr/Gd samples) with x = 0.0, 0.01, 0.03, 0.033, 0.035, 0.04, 0.05, 0.06, 0.1 and RuGd_1.5(Ce_0.5−x Pr _x ) Sr₂ Cu₂O_10−δ (Pr/Ce samples) with x = 0.0, 0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2 prepared by the standard solid-state reaction technique. We obtained the XRD patterns for different samples with various x. The lattice parameters versus x for different substitutions have been obtained from Rietveld analysis. To determine how the magnetic and superconducting properties of these layered cuprate systems can be affected by Pr substitution, the resistivity, and magnetoresistivity, with H _ext varying from 0.0 to 15 kOe, have been measured at various temperatures. Superconducting transition temperature T _c and magnetic transition T _irr , have been obtained through resistivity and ac susceptibility measurements. The T _c suppression due to Pr/Gd and Pr/Ce substitutions show competition between pair breaking by magnetic impurities, hole doping due to different valances of ions, difference in ionic radii, and oxygen stoichiometry. Pr/Gd substitution suppresses superconductivity more rapidly than for Pr/Ce, showing that the effect of hole doping and magnetic impurity pair breaking is stronger than the difference in ionic radii. In Pr/Gd substitution, the small difference between the ionic radii of Pr^3+,4+ and Gd³⁺, and absorption of more oxygen due to the higher valence of Pr with respect to Gd, decreases the mean Ru-Ru distance, and as a result, the magnetic exchange interaction becomes stronger with the increase of x. However, Pr/Ce substitution has the opposite effect. The magnetic parameters such as H _c , obtained through magnetization measurements versus applied magnetic field isotherm at 77 K and room temperatures, become stronger with x in Pr/Gd and weaker with x in Pr/Ce substitution.     

Anomaly in the upper critical magnetic field common to YBa2Cu3O7−δ , HoBa2Cu3O7−δ , and Nd1.85Ce0.15CuO4−δ irradiated by helium ions

The high-temperature superconductors YBa₂Cu₃O_7−δ , HoBa₂Cu₃O_7−δ , and Nd_1.85Ce_0.15CuO_4−δ are found to possess a common anomaly in the variation of the upper critical magnetic field owing to irradiation by helium ions. While the resistivity ρ increases by many times, the increase in H _c2 from scattering by radiation defects typical of ordinary superconductors does not occur. In terms of the ordinary mechanism, the anomaly may be explained by a significant reduction in the conduction electron density resulting from a loss of oxygen, which causes a significant rise in ρ with a small change in the scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 1372–1376 (August 1999)     

Comment on “Is Nd2−xCexCuO4 an electron-superconductor?”

A critical comment was made to the electron energy-loss spectroscopy [Z. Phys. B-Condensed Matter75, 421 (1989)] which questionedn-type superconductivity in Nd_2–xCe_xCuO₄. Comparative oxygen-K absorption and oxygen 1s photoemission spectroscopies for Nd_2–xCe_xCuO₄ and Bi₂Sr₂CaCu₂O₈ have shown that these two superconductors are classified into two different categories.     

Is Nd2−xCexCuO4 an electron-superconductor?

We present results from electron energy-loss spectroscopy on Nd_2–xCe_xCuO₄. There is no clear evidence for Cu¹⁺ but for holes at the oxygen sites. The valency of Ce is found to be close to 3.4. Thus our results do not corroboraten-type superconductivity in this compound.     

Electrical properties of heavily doped fluorite-structured BaF2:RF3 (R=La, Pr, Nd, Gd, Tb, Y, Sc) single crystals

The superionic conductivity and dielectric response of heavily doped fluorite-structured Ba_1−xR_xF_2+x (R=La, Pr, Nd, Gd, Tb, Y, Sc; x=0.005–0.45) crystals are reported. The highest ionic conductivity is found for R=Sc and x=0.1. Upon ScF₃ doping, small Sc³⁺ ions rearrange their surroundings, create excessive fluoride interstitial ions and bring about a high ionic conductivity. For each dopant, the concentration dependence of the ionic conductivity is non-linear. A monotonous concentration dependence of the ionic conductivity is found only for La³⁺ doping. Upon doping with Nd³⁺, Gd³⁺, Tb³⁺, Y³⁺ and Sc³⁺ ions, a conductivity maximum is observed at x=0.1–0.2. Upon Pr³⁺ doping, this maximum is split. The influence of defect clustering on the concentration dependence of the conductivity is discussed. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.