Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl_0.5Pb_0.5)Sr₂Ca (Cu_2−xZn_x)O₇ system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_x²−y² state due to the full d band of Zn ions.     

FAR-INFRARED AND INFRARED REFLECTIONS OF Tl2Ba2Ca2Cu3O10 FILM

The far-infrared and infrared reflection spectra of Tl₂Ba₂Ca₂Cu₃O₁₀ superconducting film are reported in this paper. It is found there are three sharp reflection edges in both the superconducting and normal states, their positions being near ≈ 150, ≈ 350 and ≈600 cm^-1. We attribute these reflection spectra to the Drude free carriers and a wide mid-infrared absorption band. When the incident angle increases from 7° to 30°, some additional stractures appear in the refiection spectra, which are the A_2u phonon modes vibrating along c-axis.     

A low temperature internal friction study of Y0.85Ca0.15Ba2Cu3O7−δ with different oxygen content

The ceramic sample of Y_0.85Ca_0.15Ba₂Cu₃O_7−δ was prepared by standard solid-state reaction method, and samples with different oxygen concentration were obtained by quenching from high temperature. The internal friction was measured using the vibrating reed method from liquid-nitrogen temperature to room temperature at kilohertz frequency. An internal friction peak was observed around 250 K in Y_0.85Ca_0.15Ba₂Cu₃O_7−δ quenched from 1023 K. The peak is related to the one observed around 220 K (labeled as P3 peak) in undoped YBa₂Cu₃O_7−δ (Y123). This result shows the dependence of P3 peak on carriers density and P3 peak has a strong correlation to the abnormal behavior of Y123 in the underdoped range. The variation of two low temperature thermal activated relaxation peaks (P1 and P2) on oxygen content were also investigated. And consistent explanations were given based on all recent researches.     

Characterization of a thin CeO2-ZrO2-Y2O3 films electrochemical deposited on stainless steel

In this paper, we report for the first time formation of a thin CeO₂-ZrO₂-Y₂O₃ films electrodeposited on a stainless steel substrate. The samples have been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD and XPS data indicate formation of a solid solution and additional existence of Ce³⁺ states near the surface. After annealing, SEM examination has shown a microstructure formed by dispersed spherical agglomerates having a size between 20 and 60 nm.     

The degradation process in Tl2Ba2Ca1Cu2O10+x

Degradation processes in 2212 thallium-based superconductors have been investigated using X-ray diffraction and electron microscopy techniques. It has been shown that the formation of amorphous or microcrystalline material by decomposition of the superconducting phase is followed by the crystallization of BaCo₃ and other oxide compounds. The influence of preexisting BaCO₃ on the rate of this degradation process has also been studied, and the results indicate that the removal of this stable precursor compound is an important step in improving the long term stability of the 2212 phase.     

Surface atomic imaging of La2CuO4 and superconducting Tl2Ba2Ca2Cu3O10

Surface structures of the high T_c oxide superconductors Tl₂Ba₂Ca₂Cu₃O₁₀ and La₂CuO₄ have been studied by high resolution electron microscopy. In addition to the single atomic layer of La on the (113) surface observed previously, several atomic layers of the C-form of La₂O₃ were found to coat the (001) surfaces of La₂CuO₄. Surface reconstruction with the incorporation of excess Ba on the (110) surface of Tl₂Ba₂Ca₂Cu₃O₁₀ has been observed together with a stepped (11l) surface produced by a combination of the (110) and (001) planes. The (001) surface of Tl₂Ba₂Ca₂Cu₃O₁₀ was also found to terminate with a Ba-O layer. Possible mechanisms for the formation of these surface structures are discussed.     

Calcium and oxygen doping in Y Ba2Cu3Oy

Both oxygen and calcium play important roles in inducing superconductivity in Y Ba₂Cu₃O_y (YBCO), which is an antiferromagnetic insulator at low O and Ca content. O induces superconductivity in Ca-free YBCO, while Ca does similarly in oxygen-deficient YBCO. For doping oxygen HgO was used as it decomposes at 476 ^°C into Hg, which escapes from the matrix leaving the crystal unaltered, and O, which provide a way to dope O in YBCO. Considering these facts, polycrystalline samples of Y _1−xCa_xBa₂Cu₃O_y with x=0, 0.1 and 0.2 with and without HgO addition were prepared through a solid-state reaction method. The samples were sintered at 950 ^°C in open atmosphere. These synthesized samples were characterized through using the X-ray diffraction technique (XRD) for phase evaluation, scanning electron microscopy (SEM) for grain morphology, energy dispersive X-ray analysis (EDX) for compositional analysis and the four-contact measurement technique for determining the superconducting transition temperature.     

NMR study of local hole distribution, spin fluctuation and superconductivity in Tl2Ba2Ca2Cu3O10

⁶³Cu, ¹⁷O and ²⁰⁵Tl NMR have been performed in the high-T_c superconductor Tl₂Ba₂Ca₂Cu₃O₁₀ whose T_c(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO₂ plane have been extracted from the nuclear quadrupole frequency ν_Q. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T₁ and T_2G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa₂Cu₃O₇, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher T_c is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, T_c would go up to 132 K which is quite close to the record T_c realized in the Hg based compound.     

Superconductivity with transition temperature up to 80 K for TbSr2Cu2.7Mo0.3O7+δ

A Tb-123 phase with the composition, TbSr₂Cu_2.7Mo_0.3O_7+δ, has been synthesized in single-phase form by the solid-state reaction route. Its phase purity has been confirmed from neutron powder diffraction studies. The as-synthesized Tb-123 sample does not show superconductivity down to 5 K. On the other hand, an unusually high antiferromagnetic ordering temperature (T_N) of around 7 K is seen for the Tb moments. After 120-atm-O₂ post-annealing, bulk superconductivity is achieved in this compound with a superconducting transition temperature (T_c) of about 30 K, without any significant effect on T_N. To achieve higher oxygen content and higher T_c, the as-synthesized sample was subjected to high-pressure oxygenation, carried out in a closed cell, at 5 GPa and 400 °C in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction larger than 10% at 5 K. Our observation of superconductivity at 80 K is the highest T_c to-date for the Tb-123 type compounds.     

Surface and transport studies on La0.7Ba0.3MnO3:SnO2 bilayer

We report on study of morphology, optical contrast and transport characteristics of La_0.7Ba_0.3MnO₃ (LBMO) manganite thin films bilayered with SnO₂ on Si (0 0 1) substrate, synthesized using pulsed laser deposition system. X-ray diffraction study reveals that both LBMO and SnO₂ show polycrystalline growth over the substrate. Atomic force microscopy shows interesting pyramidal structures of LBMO of size ∼2 μm × 1 μm × 0.1 μm. On the other hand, SnO₂ grows in the form of close packed cylindrical clusters of ∼200 nm radius. Near-field optical microscopy (NSOM) study using 532 nm laser reveal that optical NSOM output intensity in LBMO is four times less than SnO₂ signal. Transport characterizations show that this bilayer configuration exhibit non-linear current-voltage characteristics from 300 upto 50 K. The nature becomes linear below this temperature. The results project the system as a promising candidate in non-conventional device category in the area of spintronics.     

High-throughput characterization of local conductivity of Nd0.9Ca0.1Ba2Cu3O7−δ thin film by the low-temperature scanning microwave microscope

We developed a scanning microwave microscope (SμM) designed for high-throughput electric-property screening as well as for rapid construction of electronic phase diagrams at low temperatures. As a sensor probe, we used a high-Qλ/4 coaxial cavity resonator to which a thin needle with ball-tip end was attached. The sensor module was mounted on the low-temperature XYZ stage, which allowed us to map out the change of resonance frequency and quality factor due to the local tip-sample interaction at low temperatures. From the measurements of combinatorial thin films, such as Ti_1−xCo_xO_2−δ and Nd_0.9Ca_0.1Ba₂Cu₃O_7−δ (NCBCO), it was demonstrated that this SμM system has enough performance for the high-throughput characterization of sample conductance under variable temperature conditions.     

Preparation of transparent conductive TiO2:Nb thin films by pulsed laser deposition

This study investigated the optical and electrical properties of Nb-doped TiO₂ thin films prepared by pulsed laser deposition (PLD). The PLD conditions were optimized to fabricate Nb-doped TiO₂ thin films with an improved electrical conductivity and crystalline structure. XRD analyses revealed that the deposition at room temperature in 0.92 Pa O₂ was suitable to produce anatase-type TiO₂. A Nb-doped TiO₂ thin film attained a resistivity as low as 6.7 × 10⁻⁴ Ω cm after annealing at 350 °C in vacuum (<10⁻⁵ Pa), thereby maintaining the transmittance as high as 60% in the UV-vis region.     

Frequency dependence of the resistive switching effect in Bi2Sr2CaCu2O8+y/Ag film heterocontacts

In this work we have performed the relaxation studies “in situ” of the electron instability effect (EIE) in the heterostructures based on BSCCO single crystals. The new effect of suppression of EIE or colossal electroresistance via application of an alternating low frequency electric field to the heterojunctions in the BSCCO-based single crystals has been found. It has been shown that the top possible frequencies for observation of the effect are of the order of 10³ Hz. This fact is interpreted as accumulation of the oxygen ions driven by the electric field to the interface. On the other hand, it has been shown that the switching events are limited by two time processes: t≈1 ms and about ten seconds. The first ones are caused by rearrangement of a charge net in the degraded surface at the electric field switching. The latter are caused by oxygen diffusion to vacancies under electric field above some threshold value. The considered experimental data confirm the correlation character of the HTSC properties as Mott systems, which appears in extreme sensitivity to the doping level, in the tendency to phase separation under external actions, in the hysteresis character of the metal-insulator transition.     

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

The annealing characteristics and the superconducting properties of Tl₂Ca₂Ba₂Cu₃O₁₀ thin films sputter-deposited onto yttrium- stabilized ZrO₂ substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T_c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T_c achieved tends to degrade with increasing substrate temperatures, a zero resistance T_c of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest T_c phase (Tl₂Ca₂Ba₂Cu₃O₁₀) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T_c phases such as Tl₂Ca₁Ba₂Cu₂O₈.     

Microstructure and dielectric properties of La2O3 doped amorphous SiO2 films as gate dielectric material

As a potential gate dielectric material, the La₂O₃ doped SiO₂ (LSO, the mole ratio is about 1:5) films were fabricated on n-Si (0 0 1) substrates by using pulsed laser deposition technique. By virtue of several measurements, the microstructure and electrical properties of the LSO films were characterized. The LSO films keep the amorphous state up to a high annealing temperature of 800 °C. From HRTEM and XPS results, these La atoms of the LSO films do not react with silicon substrate to form any La-compound at interfacial layer. However, these O atoms of the LSO films diffuse from the film toward the silicon substrate so as to form a SiO₂ interfacial layer. The thickness of SiO₂ layer is only about two atomic layers. A possible explanation for interfacial reaction has been proposed. The scanning electron microscope image shows the surface of the amorphous LSO film very flat. The LSO film shows a dielectric constant of 12.8 at 1 MHz. For the LSO film with thickness of 3 nm, a small equivalent oxide thickness of 1.2 nm is obtained. The leakage current density of the LSO film is 1.54 × 10⁻⁴ A/cm² at a gate bias voltage of 1 V.     

c-Axis orientation control of YBa2Cu3O7−x films grown on inclined-substrate-deposited MgO-buffered metallic substrates

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on non-textured metal substrates with inclined-substrate-deposited (ISD) MgO as template. The biaxial texture feature of the films was examined by X-ray pole-figure analysis, φ-scan, and 2θ-scan. A tilt angle of 32° of the MgO[001] with respect to the substrate normal was observed. Epitaxial growth of YBCO films with c-axis tilt angle of 32° with respect to the substrate normal was obtained on these substrates with SrTiO₃(STO) as buffer layer. Whereas, by choosing yttria-stabilized ZrO₂ and CeO₂ instead of STO as buffer layer, a c-axis untilted YBCO film was obtained. Higher values of T_c=91 K and J_c=5.5×10⁵ A/cm² were obtained on the c-axis untilted YBCO films with 0.46 μm thickness at 77 K in zero field. Comparative studies revealed a unique role of CeO₂ in controlling the orientation of the YBCO films grown on ISD-MgO buffered metal substrates.     

Effects of excess oxygen content on the hole-carrying CuO2-layers in Tl2(Ba1−xSrx)2Ca2Cu3Oy superconducting oxides

The effects of oxygen doping on the hole-carrying CuO₂-layers in Tl₂(Ba_1−xSr_x)₂Ca₂Cu₃O_y were studied by combined chemical and valence analysis, T_c measurements and neutron diffraction. The highest T_c is characterized by an optimal excess oxygen content, Δy, dichotomizing the under- and over-doped regions for each Sr concentration. While the average Tl valence is close to 3.0 and independent of Δy, the average Cu valence shows a linear dependence with Δy. An increase of the flatness of the CuO₂ plane, characterized by the O(2)-Cu(2)-O(2) bond angle of ∼176°, was observed at the optimal Δy.     

Tl2Ba2Ca2Cu3Oy超导体低场钉扎力的性质

测量了Tl₂Ba₂Ca₂Cu₃O_y超导体多晶样品在不同外磁场和不同温度下的磁滞性质,利用临界态方程求出有效钉扎力随外磁场及温度的变化,有效钉扎力包括晶粒界面处及晶粒内的贡献,发现有效钉扎力有峰效应,峰位置随温度降低向较高磁场移动,峰效应被解释为两种钉扎机制产生的,即两种机制的贡献与外磁场的关系不同,还讨论与这些结果有关的问题。 关键词：     

Reversible magnetization and irreversibility line of tri-layer superconductor Ba2Ca2Cu3O6(O,F)2 with Tc∼108 K

We report magnetization measurements of grain-aligned Ba₂Ca₂Cu₃O₆(O,F)₂ with T_c?108 K. The interlayer distance of the material is the shortest among known tri-layer superconductors. Unexpectedly, the magnetization data show that the coupling strength between CuO₂ layers is rather weak. A direct reflection of the weak coupling is highly suppressed irreversibility line, i.e. a broad reversible region in H-T plane. The decoupling field obtained from the irreversibility line is less than 0.1 T, which is comparable with that of quasi two-dimensional superconductor Bi₂Sr₂CaCu₂O_8+δ. Comparison of data with the Hao-Clem model gives characteristic parameters [ξ_ab(0) and λ_ab(0)] and the critical fields [H_c(0) and H_c2^c(0)]. A large value of penetration depth, λ_ab(0)=240 nm reflects a small carrier concentration in CuO₂ planes, and explains the reason of the weak interlayer coupling.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.