Superconductivity in New Pb-Based 1222 Layered Cuprates of (Pb0.75W0.25)Sr2(Eu2.0−x Ce x )Cu2O z

New Pb-based 1212 cuprates of (Pb_0.8W_0.2)Sr₂(Nd_1?x Ca _x )Cu₂O_7?δ were synthesized. The sample with x=0.6 was reported to show superconductivity with an onset temperature up to 82 K. Interestingly, in order to obtain superconductivity of the sample, synthesis in N₂ was necessary, then synthesis in air or post-treatment in O₂ led to the disappearance of superconductivity. Recently, we have successfully synthesized new Pb-based 1222 cuprates for the composition range of 0.3≤x≤1.0 in the (Pb_0.75W_0.25)Sr₂(Eu_2.0?x Ce _x )Cu₂O _z system. The samples have a tetragonal crystal structure; the typical lattice parameters are a=0.3850 nm and c=2.889 nm. Among them, a sample with x=0.3 is discovered to show superconductivity with an onset of about 15 K. These cuprates have the same (Pb, W)O monolayers as the Pb-based 1212 cuprates described above. In these new cuprates, superconductivity appears when the samples are annealed not in air and under He atmosphere, but under high-pressure O₂.     

Synthesis of New Pb-Based 1222 Cuprates Containing Phosphorus, (Pb0.75P0.25)Sr2(RE2–x–y Ce x Sr y )Cu2O z (RE = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y)

New Pb-based 1222 cuprates containing phosphorus have been synthesized in the (Pb_0.75P_0.25)Sr₂- (RE_2–x–y Ce _x Sr _y )Cu₂O _z (RE = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y) systems. The almost-single phase samples are obtained for 0.5≤x≤1.0 and y=0.1 in the cases of RE = Nd, Dy, Ho, Er, Tm, Yb and Y, while for 0.3≤x≤1.0 and y=0.1 in the cases of RE = Sm, Eu and Gd. The samples crystallize in a tetragonal lattice, the lattice parameters a and c are decreasing with the decrease of the ionic radius of the RE element. Even after annealing under 143 atm O₂ atmosphere at 400 °C, almost all the samples with the common values x=0.5 and y=0.1 are semiconductors with a transport process followed by three-dimensional variable range hopping. However, the samples of RE = Sm, Eu and Gd, which are of almost single 1222 phase, even for x=0.3 and y=0.1, show superconductivity with the onsets at about 25 K, 20 K and 22 K, respectively. Through this study, we find very important procedure for discovery of new superconducting 1222 compound.     

New Members of the Pb-based 1222 Superconductor Containing Phosphorus: (Pb0.75P0.25)Sr2(Ln1.9?x Ce x Sr0.1)Cu2O z (Ln=Sm and Gd)

We previously reported on the synthesis of new Pb-based 1222 superconductor containing phosphorus in the (Pb_0.75P_0.25)Sr₂(Eu_1.9−x Ce _x Sr_0.1)Cu₂O _z system. Recently, we have discovered new members of the Pb-based 1222 superconductor containing phosphorus in the (Pb_0.75P_0.25)Sr₂(Ln_1.9−x Ce _x Sr_0.1)Cu₂O _z (Ln=Sm and Gd) systems. In each system, it is found that the samples of almost the single 1222 phase can be obtained for a wide composition area of 0.3≤x≤1.0. After annealing under 143 atm O₂ atmosphere at 400 °C, each sample of Ln=Sm and Gd with x=0.3 is found to show an onset of resistivity-drop at about 28 K and at about 24 K, respectively. Moreover, each samples of Ln=Sm and Gd is found to show an onset of diamagnetic signal at about 25 K and about 22 K, respectively. From these facts, these samples of Ln=Sm and Gd are found to be new members of the Pb-based 1222 superconductor containing phosphorus.     

Synthesis of New Superconducting Nb-Based 1212 Cuprates, (Nb1−x Cd x )Sr2EuCu2O8−δ

New superconducting layered cuprates with 1212 structure have been synthesized in the (Nb_1?x Cd _x )Sr₂EuCu₂O_8?δ system. X-ray powder diffraction measurements showed that almost-single phase samples were obtained in a range of 0≤x≤ 0.2. The superconducting onset transition temperature of ～ 43 K was obtained for the sample with x=0.2. The crystal structure of this sample has a tetragonal symmetry with lattice parameters a=3.874(1) Å and c=11.629(4) Å.     

New Pb-Based 1222 Cuprates, (Pb0.5B0.5)Sr2(RE2−x−y Ce x Sr y )Cu2O z (RE = Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y)

New Pb-based layered cuprates with the 1222 structure have been synthesized in the (Pb_0.5B_0.5)Sr₂ (RE_2−x−y Ce _x Sr _y )Cu₂O _z (RE = Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y) systems. The almost-single-phase samples in the systems can be obtained for a nominal composition of x=0.7 and y=0.1. The crystal structure of the samples has a tetragonal symmetry, the lattice parameters of a and c are increasing with increasing the ionic radius of RE element. Despite treatment under high O₂ pressure of 100 atm, the samples are semiconductors with the transport process characteristic of three-dimensional variable range hopping conduction.     

Synthesis of new members of Pb-based 1222 superconductors containing sulfur: (Pb0.75S0.25)Sr2(RE1.6Ce0.4)Cu2O z (RE = Sm,Gd, Dy,and Ho)

We have discovered new members of the Pb-based 1222 layered cuprates containing sulfur of (Pb_0.75S_0.25)Sr₂(RE_1.6Ce_0.4)Cu₂O _z with RE = Sm, Gd, Dy, and Ho. All the samples are of almost-single 1222 phase. From the ρ–T and χ–T measurements, they are found to be superconductors after annealing under 125 atm O₂ atmosphere at 400 °C. Among them, the sample with RE = Gd is found to show superconductivity at the highest temperature of 30 K.     

Oxygen Content and Valence of Ru in RuSr2(Gd0.75Ce0.25)2Cu2O10−δ (Ru-1222) Magnetosuperconductor

The valence of Ru was analyzed for two RuSr₂(Gd_0.75Ce_0.25)₂Cu₂O_10– samples with different oxygen contents by Ru L _III-edge x-ray absorption near-edge structure (XANES) spectroscopy. For the sample as-synthesized in 1 atm O₂ the DC magnetization data measured in an applied field of 5 Oe showed a clear branching of the zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, an up-turn for both around 100 K and a cusp at 85 K and a diamagnetic transition around 20 K in the ZFC part. A further confirmation for the superconductivity at 28 K was obtained from a resistance vs. temperature measurement. Annealing the as-synthesized sample in 100-atm O₂ atmosphere at 420°C increased the diamagnetic transition temperature from 20 K to 40 K. According to a thermogravimetric analysis, the oxygen content increased accordingly by ca. 0.1 oxygen atoms per formula unit. Quantitative analysis of the XANES spectra using Sr₂RuO₄ (Ru^IV) and Sr₂GdRuO₆ (Ru^V) as reference materials revealed a valence value of +4.74 and +4.81 for Ru in the as-synthesized and the 100-atm O₂-annealed sample, respectively. The obtained result suggests that the valence of Ru in Ru-1222 is affected by the change in oxygen content.     

Bond Angle Study of Self-compensating Y1−x Ca x Ba2−x La x Cu3O z System

Single-phase samples of a self-compensating Y_1?x Ca _x Ba_2?x La _x Cu₃O _z system were synthesized through a solid-state reaction method with x<0.4. The structure of all samples were characterized by X-ray diffraction and refined by the Rietveld method. Superconducting properties have been investigated by the DC magnetization measurement. The critical temperature (T _c ) decreases evidently with the increment of x although the carrier concentration remains constant in the samples for different doping level. Careful study of the chemical bonds in the crystalline lattice demonstrates that the T _c is closely correlated to four pairs of bond angles in the unit cell. The analysis indicates that crystalline structure is one of the important factors to high-T _c superconductivity, and its influence is independent of the carrier concentration.     

Magnetic Properties of the Ce(Pd1−x Cu x )3 System

CePd₃ is an intermediate valence compound with a variety of interesting electronic properties resulting from the partially filled f-shell electrons of cerium atom. Substitution on cerium site with Cu causes a dramatic change in the electronic behavior of the compound. In this work, Ce(Pd_1?x Cu _x )₃ system, 0 ≤ x ≤ 0.133, with x = 0, 0.017, 0.033, and 0.133, has been synthesized by arc melting method. The powder X-ray diffraction reveals that the compounds crystallize in AuCu₃-type structure where copper enters the lattice substitutionally. Magnetic susceptibility measurements as a function of temperature have been carried out on these alloys which show that with increasing copper content x, the alloys Ce(Pd_1?x Cu _x )₃ show a smooth crossover from the weak temperature dependence of CePd₃ to a Curie-Weiss behavior for higher concentrations of x. The value of μ _eff increases with x, while the paramagnetic Curie temperature shows a large reduction with increase in x; this suggests a very rapid decrease in the Kondo temperature with x.     

Superconducting and Magnetic Properties of Polycrystalline [(Bi,Pb)2Sr2Ca2Cu3O x ]1−x (La0.7Sr0.3MnO3) x Thick Films Grown Using MQA Method

Polycrystalline thick films with the nominal composition [(Bi,Pb)₂Sr₂Ca₂Cu₃O _x ]_1?x (La_0.7Sr_0.3MnO₃) _x (x=0, 0.01, 0.03, 0.05) were prepared by the melting-quenching-annealing (MQA) method on (001)-oriented LaAlO₃ (LAO) substrates. The XRD patterns show that the samples are composites consisting of (Bi,Pb)₂Sr₂Ca₂Cu₃O _x and La_0.7Sr_0.3MnO₃ particles with average grain sizes of ～60 and ～30 nm, respectively. From electric transport measurements, the superconducting onset temperature, T _onset, and superconducting critical temperature, T _c, ended up being ～80 and ～60 K, respectively. The depression of T _c may be attributed to the proximity effect between ferromagnetism and superconductivity. The M–H hysteresis loops indicate the coexistence of ferromagnetism and superconductivity in the films. The magnetic field dependence of the superconducting current density and the volume pinning force F _p=J _c×B were obtained by applying Bean’s model to the isothermal M–H loops.     

Mechanical Properties of Bi1.6Pb0.4Sr1.8Ba0.2Ca2Cu3?x Ni x O10+δ Superconducting System

Samples of Bi_1.6Pb_0.4Sr_1.8Ba_0.2Ca₂Cu_3?x Ni _x O_10+?? were prepared by solid-state reaction methods. Mechanical properties (Vickers microhardness, Young modulus, yield strength, fracture toughness and surface energy) by Vickers microhardness measurements have been carried out to examine the effects of Ni substitution. The results showed a deterioration of the mechanical properties with the enhancement of Ni content.     

Magnetic Ordering in the Mixed Ruthenium-Copper Oxide Ba2Pr(Ru1−x Cu x )O6 System

We have synthesized a series of mixed ruthenium-copper oxides of general formulae Ba₂PrRu_1–x Cu _x O₆ with x=0, 0.05, 0.1, 0.15 by high temperature solid-state reaction process. From the SQUID measurements, we observed a strong diamagnetic response on zero-field-cooling (ZFC), meanwhile a ferromagnetic-like transition under field cooling, both at about 11 K. In addition, we also observed a broad transition at about 125 K by SQUID measurements. Electrical resistivity measurements also show a sharp resistive drop at about 150 K, suggesting the presence of a magnetic transition. High intensity and high resolution neutron diffraction studies further confirm the magnetic phase transition at 100 K into a Type I antiferromagnetic structure.     

Synthesis and IR-excited luminescence of (Y1 − x Tm x )2O2S solid solutions

We have studied the key features of the luminescence spectra and kinetics of (Y_{1 ? x} Tm _x )₂O₂S solid solutions in the range 400–2000 nm under laser excitation at 790 and 810 nm. The results have been used to develop a series of IR phosphors “invisible” under laser excitation in the range 790–810 nm and possessing tunable and reproducible relative intensities of three groups of IR luminescence bands in the ranges 770–840, 1360–1520, and 1650–1980 nm, respectively.     

Synthesis and Excess Conductivity Analyses of Ce-Doped Tl1−x Ce x Ba2Ca2Cu3O10−δ Superconductors

The role of mobile carriers density supplied by TlBa ₂ O _4?δ charge reservoir layer is investigated by doping Ce in TlBa ₂Ca ₂Cu ₃ O _10?δ superconductors. The T _c(onset) and T _c(R= 0) versus Ce contents have shown almost a parabolic-shaped phase diagram which is interpreted in terms of hole’s doping. The magnetic measurements showed that magnitude of diamagnetism is gradually suppressed with Ce doping at Tl site. Fourier transform infrared (FTIR) revealed that the phonon modes associated with apical oxygen atoms type Cu(2)–O _A –Tl are softened, whereas the planar oxygen modes are hardened with the doping of Ce the unit cell. In the excess conductivity, analyses of conductivity data of Tl _1?x Ce _x Ba ₂Ca ₂Cu ₃ O _10?δ samples have shown a suppression of T _{c m f} (K) and the width of 2D conductivity regimes, whereas the coherence length along the c-axis ξ _c(0) and the interlayer coupling J are enhanced with the doping of Ce ⁺⁴ in the charge reservoir. The increase in ξ _c(0) is interpreted in the form of homogenization of the carrier’s density of carriers the CuO ₂ planes. The process of homogenization is accomplished with the reduction of density of carriers in the outer CuO ₂ that consequently promotes increase in the phase relaxation time, τ _? , of the carriers, their Fermi velocity, and the energy required to break apart the Cooper pairs.     

Disorder Tuned Superconductor Insulator Transition in La2−x (Sr/Ce) x CuO4 & NbN Superconducting Thin Films

In studying the magneto-resistivity ρ(B) in high pulsed magnetic fields up to 55 T, it is often observed that the ρ(B) curves for different temperatures in the vicinity of the critical temperature cross at the same field value, B=B _CP . We show how the crossing field changes as a function of the hole (or electron) doping x in La_2?x (Sr/Ce) _x CuO₄. The resistivity ρ and the magnetic field B at different temperatures may be scaled as R/R _CP and |B?B _CP|/T ^γ , respectively. This kind of scaling resulted in a conventional critical exponent γ=1/ν z=1.35 (ν z～0.74) in our La_2?x (Sr/Ce) _x CuO₄ thin films, and a much higher value of γ=4.35 (ν z～0.23) in the case of our disordered NbN films.     

Synthesis of monodisperse Ce x Zr1−x O2 nanocrystals and the size-dependent enhancement of their properties

Monodisperse Ce_1−x Zr _x O₂ nanocrystals have been synthesized using a simple two-phase approach; adjusting the ratio of precursors used, amount of capping agent used, reaction time and temperature affords precise control over their composition, structure and size. Size-dependent enhancement of oxygen-storage capacity and kinetics of oxygen storage and release were observed. Systematic studies were conducted in order to understand the size-dependent enhancement of these properties. This work provides important insights into the synthesis and fundamental understanding of multi-component nanocrystals with a large variety of applications.     

Synthesis and properties of dielectric (HfO2)1 − x (Sc2O3) x films

(HfO₂)_{1 ? x} (Sc₂O₃) _x films have been grown by chemical vapor deposition (CVD) using the volatile complexes hafnium 2,2,6,6-tetramethyl-3,5-heptanedionate (Hf(thd)₄) and scandium 2,2,6,6-tetramethyl-3,5-heptanedionate (Sc(thd)₃) as precursors. The composition and crystal structure of the films containing 1 to 36 at % Sc have been determined. The results demonstrate that, in the composition range 9 to 14 at % scandium, the films are nanocrystalline and consist of an orthorhombic three-component phase, which has not been reported previously. Using Al/(HfO₂)_{1 ? x} (Sc₂O₃) _x /Si test structures, we have determined the dielectric permittivity of the films and the leakage current through the insulator as functions of scandium concentration. The permittivity of the films with the orthorhombic structure reaches k = 42–44, with a leakage current density no higher than ～10^?8 A/cm².     

Effect of Ce Substitution on the Magnetoresistivity and Flux Pinning Energy of the Bi2Sr2Ca1−x Ce x Cu2O8+δ Superconductors

In this study, the effect of Ce doping on the properties of Bi₂Sr₂Ca_1?x Ce _x Cu₂O_8+δ ceramic superconductors, with x=0.0, 0.01, 0.05, 0.1, and 0.25, has been investigated. Samples’ precursors were prepared using the conventional solid state method and subsequently textured using the Laser Floating Zone technique. The magnetoresistance measurements were studied under various applied magnetic fields. The activation energies, irreversibility fields (H _irr ), upper critical fields (H _c2) and coherence lengths at 0 K (ξ(0)) were calculated from the resistivity versus temperature (ρ–T) curves, under DC magnetic fields up to 5 T. The thermally activated flux flow model has been applied in order to calculate the flux pinning energies. The results indicated that H _c2(0) varied from 416.19 to 115 T and the flux pinning energies varied from 1.46 to 0.042 eV at 0 T.