Influence of the reaction conditions on the formation of Tl(Re)-Ba-Ca-Cu-O superconducting thin films by thallination in open system

Rhenium avoids air degradation of the target as well as of precursor thin films during the preparation of high temperature Tl-based superconductors. Addition of Re has been used during synthesis of oriented Tl(Re)-Ba-Ca-Cu-O films. High temperature superconducting Tl-based thin films were prepared by a two step method combining RF magnetron sputtering and ex-situ thallination. Precursors with a composition of Re_0.1Ba₂Ca₂Cu₃O_x were deposited on a CeO₂ buffered R-plane sapphire substrate. The sputtered films were prepared at room temperature in an Ar atmosphere. The thallination of the precursor films was performed in a one zone configuration, where both the pellet and precursor film were kept at the same temperature. The thallination temperature varied in the range of 850–880°C and samples were held for 30, 45 and 60 min at this temperature in a flowing oxygen atmosphere. Besides Tl-2212, X-ray diffraction reveals the possibility of also preparing the Tl-2223 phase, which was until now not reported in a Re doped form. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia     

高温退火对蓝宝石基片的表面形貌和对CeO2缓冲层以及Tl-2212超导薄膜生长的影响

研究了蓝宝石(1102)基片在不同温度和时间下退火时表面形貌和表面相结构的变化,以及它对CeO₂缓冲层和Tl-2212超导薄膜生长的影响.原子力显微镜(AFM)研究表明,在流动氧环境中1000℃温度下退火,蓝宝石(1102)的表面首先局部区域形成台阶结构,然后表面形成叠层台阶结构,随着退火时间的延长,表面发生了台阶合并现象,表面形貌最终演化为稳定的具有光滑平台的宽台阶结构.XRD测试表明,通过高温热处理可以大幅度提高蓝宝石基片表面结构的完整性.在1000℃温度下热处理20 h的蓝宝石 关键词： Tl-2212超导薄膜 蓝宝石 缓冲层     

Thermodynamic conditions for formation of single phase in Bi-superconductor thin films

High quality BSCCO thin films have been fabricated by means of an ion beam sputtering at various substrate temperatures, T_sub, and ozone gas pressures, pO₃. The correlation diagrams of the BSCCO phases appearing against T_sub and pO₃ have been established in the 2212 and 2223 compositional films. In spite of 2212 compositional sputtering, Bi2201 and Bi2223 phases as well as Bi2212 have emerged as stable phases depending on T_sub and pO₃. From these results, the thermodynamic evaluations of ΔH and ΔS, which are related with Gibbs’ free energy change for single Bi2212 or Bi2223 phase have been performed.     

Specific heat and thermal expansion of the Bi and Tl high temperature superconductors nearT c

We present measurements of the specific heat and of the thermal expansion of the most prominent phases of the Bi and Tl high temperature superconductors (Bi-2212, Bi-2223, Tl-2212, Tl-2223) in the neighbourhood ofT _c . In all these systems we observe small but sharp anomalies which have very little similarity with the mean field jump found in the conventional superconductors. The analysis of these anomalies shows clear evidence for the presence of strong fluctuations of the order parameter. For temperatures more than 5 K away fromT _c ,2D Gaussian fluctuations are found, while within ±5 K ofT _c the fluctuation contribution is best fitted by critical fluctuations. The shape of the thermal expansion anomalies is similar to that of the specific heat. Combination of both measurements predicts dT _c /dp+0.2 K/kbar for Bi- and Tl-2223 systems.     

Anharmonic oxygen vibration and mode softening in Tl2Ba2Ca2Cu3O10 superconductor

The softening of the IR active 300 cm⁻¹ phonon mode in Tl-2223 superconductor, aroundT _c has been explained by considering a sixth order polarization potential at the off-center oxygen ion site in the Tl-O plane, and by using a nonlinear lattice dynamical theory. The present theory explains, more or less satisfactorily, the unusual temperature dependence of oxygen ion vibration and frequency shifts at higher temperature. The existence of strong nonlinear electron-phonon interaction atT _c has been emphasized.     

采用快速升温烧结方法生长Tl-1223超导薄膜的研究

报道了在铝酸镧(00l)衬底上生长Tl-1223超导薄膜的快速升温烧结方法以及铊(Tl)源陪烧靶的配比对Tl-1223薄膜晶体结构的影响.扫描电子显微镜观测表明,采用快速升温烧结方法生长的Tl-1223超导薄膜具有致密的晶体结构.X-射线衍射等测试表明,采用合适配比的陪烧靶在氩气环境下可以制备出纯c轴取向的Tl-1223超导薄膜,充氧退火后的薄膜具有较好的电学性能,其临界转变温度T_(c onset)达到116K,临界电流密度达到1.5MA/cm~2(77 K,0 T).实验结果表明,采用这一新的烧结方法制备Tl系超导薄膜具有升降温时间和恒温时间短、生产成本低等特点.     

Synthesis and characterization of electron beam evaporated LiCoO<Subscript>2</Subscript> thin films

LiCoO₂ thin films were prepared by electron beam evaporation technique using LiCoO₂ target with Li/Co ratio 1.1 in an oxygen partial pressure of 5 × 10⁻⁴ mbar. The films prepared at substrate temperature T _s < 573 K were amorphous in nature, and the films prepared at T _s > 573 K exhibited well defined (104), (101), and (003) peaks among which the (104) orientation predominates. The X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) data revealed that the films prepared in the substrate temperature range 673–773 K are nearly stoichiometric. The grain size increases with an increase of substrate temperature. The Co–e_g absorption bands, are empty and their peak position lies at around 1.7 eV above the top to the Co–t_2g bands. The fundamental absorption edge was observed at 2.32 eV. The films annealed at 1,023 K in a controlled oxygen environment exhibit (104) out plane texture with large grains. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006     

蓝宝石基片上制备大面积Tl2Ba2CaCu2O8超导薄膜

在2英寸双面蓝宝石基片上采用CeO₂作为缓冲层制备了高质量Tl₂Ba₂CaCu₂O₈(Tl-2212)超导薄膜.以金属铈作为溅射靶材,采用射频磁控反应溅射法生长了c轴织构的CeO₂缓冲薄膜,并研究了不同生长条件对于CeO₂缓冲层的晶体结构及表面形貌的影响.超导薄膜采用直流磁控溅射和后热处理的方法制备.扫描电子显微镜(SEM)图像显示,超     

Studies on fabrication of Ag/Tl2Ba2Ca2Cu3O10/CdSe heterostructures with electrochemical technique

The Ag/Tl₂Ba₂Ca₂Cu₃O₁₀/CdSe heterostructure was fabricated at room temperature by soft electrochemical processing technique for the first time. The formation of the heterostructure with non-diffusive interfaces was confirmed by X-ray diffraction. The crystallite sizes determined for Tl-2223 and CdSe films were 33 nm and 25 nm, respectively. The Tl₂Ba₂Ca₂Cu₃O₁₀ film electrodeposited onto Ag-substrate has shown the superconducting transition temperature T_c at 116.5 K and J_c = 2.1 × 10³ A/cm². These values were found to improve after the deposition of CdSe onto Ag/Tl-2223 films. The effect of red He-Ne laser irradiation on the superconducting properties of heterostructure are studied and discussed at length in this paper.     

TlBaCaCuO superconducting thin films via metal-organic chemical vapour deposition (MOCVD) and thallium vapour diffusion

Summary Superconducting thin films of the TlBaCaCuO family have been prepared through a combined approach of MOCVD and thallium vapour diffusion. A three-step process has been adopted involving i) the synthesis of Ba−Ca−Cu−O films by MOCVD using ?second-generation? barium and calcium sources, ii) the hydrolysis of the fluoride phases by annealing in water vapour-saturated oxygen and iii) the annealing in the presence of a thallium source. Reproducible results, in terms of phase formation, are discussed depending on the different conditions of the preannealing process. The superconducting films consist predominantly of single-phase Tl₂Ba₂Ca₂Cu₃O _x (2223). Preferential orientation of the crystallitec-axes perpendicular to the substrate surface has been observed. The films exhibit superconducting onset temperatures of 115–120 K with zero resistance higher than 100 K. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Effects of low-level La doping and ageing on TlPb-1223 high Tc superconductors

Bulk samples of the nominal composition of (Tl_0.6Pb_0.5)(Sr_0.8Ba_0.2)₂Ca₂Cu₃O_8+δ−xLaO_1.5 (x = 0–0.1) were prepared by using two-step process and their microstructure, T _c values, and magnetization were studied. The samples consist of the Tl-1223 dominant phase with small Tl-1212 admixture, which increases with a rise of La content. Five years ageing and following oxygen annealing at 450°C and subsequently at 750°C have only a modest effect on T _c values of the studied samples. Low-level La doping (x = 0:04) leads to an increase of T _c values by about 2 K in comparison with undoped samples. Oxygen annealing at 750°C results in an increase in the volume magnetization hysteresis in low applied magnetic fields and rise of critical current density at zero magnetic field and 77 K. This effect is most pronounced for the low La doped sample with x = 0.04. Changes of the induced voltage, U originating in the Meissner effect and of its harmonics in dependence on temperature were measured and used for characterization of the temperature distribution of inter-grain junctions.     

Superparamagnetic Ni:SiO<Subscript>2</Subscript>–C nanocomposites films synthesized by a polymeric precursor method

In this work, we report on the magnetic properties of nickel nanoparticles (NP) in a SiO₂–C thin film matrix, prepared by a polymeric precursor method, with Ni content x in the 0–10 wt% range. Microstructural analyses of the films showed that the Ni NP are homogenously distributed in the SiO₂–C matrix and have spherical shape with average diameter of ~10 nm. The magnetic properties reveal features of superparamagnetism with blocking temperatures T _B ~ 10 K. The average diameter of the Ni NP, estimated from magnetization measurements, was found to be ~4 nm for the x = 3 wt% Ni sample, in excellent agreement with X-ray diffraction data. M versus H hysteresis loops indicated that the Ni NP are free from a surrounding oxide layer. We have also observed that coercivity (H _C) develops appreciably below T _B, and follows the H _C ∝ [1 – (T/T _B)^0.5] relationship, a feature expected for randomly oriented and non-interacting nanoparticles. The extrapolation of H _C to 0 K indicates that coercivity decreases with increasing x, suggesting that dipolar interactions may be relevant in films with x > 3 wt% Ni.     

Photoconductivity of poly-<Emphasis Type="Italic">p</Emphasis>-xylylene + CdS nanocomposite films over a wide temperature range

For poly-p-xylylene + CdS (PPX + CdS) nanocomposite films, the dependences of the photo-conductivity σ _ph (T) on the concentration C of CdS nanoparticles, intensity and wavelength of exciting light, and temperature T within 15–300 K are examined. An appreciable photocurrent appears at C ≥ 10 vol %, when a large percolation cluster of CdS nanoparticles is formed. The photocurrent spectrum is compared to the absorption spectrum of the film. The photocurrent I _ph (P) increases with the intensity of light flux P in a wavelength range near 435 nm according to the I _ph (P) ∼ P ⁿ power law, where n < 1. At 15 K, the photoconductivity of films with C ≈ 11.5 and 13.5 vol % is higher than that of a pure CdS film (C = 100 vol %) by factors of ≈100 and ≈30, respectively. For films with C > 11.5 vol %, the σ _ph (T) dependence at low T exhibits a metal-like character (σ _ph (T) decreases with increasing temperature). Atomic force microscopy is used to examine the surface topography of PPX + CdS films, which is found to be strongly dependent on the concentration of nanoparticles. The dark conductivity and photoconductivity of nanocomposite films arise due to the thermo- and photoexcitation transfer of electrons from the CdS nanoparticles to the PPX matrix with the formation of an electronic double layer at the PPX matrix-large percolation CdS cluster interface, a process that populates the phenyl rings of the adjacent PPX layer with excess electrons. As a result, various mechanisms of electron transfer in the polymer matrix can be realized: Mott’s hopping conduction mechanism with variable-range hopping in the matrix between CdS clusters and the metal-like behavior of the conductivity in the polymer shell of the large cluster at low temperatures. The polymer shell contains excess electrons on the phenyl rings -C₆H₄- in the composition of anion-resonances -C₆H₄⁻-.     

Effect of Li doping on the critical temperature and glass formation in the Bi-Sr-Ca-Cu-O system

A study has been made of the glass-forming ability, structure, and superconducting properties of Bi_2.2Sr_1.8Ca_1.05Cu_2.15Li_xO_y and Bi_2.2Sr_1.8Ca_1.05Cu_2.15−x Li_xO_y (x=0;0.3;0.5;0.7). The compounds were melted by rf at T=1300–1500 °C. Rapid quenching produces glassy alloys whose glass-forming ability is the highest when lithium is substituted for copper. Glass annealing at 700–800 °C results in the formation of the HTSC phase 2212 with a critical temperature of up to 91 K. In lithium-doped samples the HTSC phase forms at lower temperatures and shorter anneals and it depends on the cooling rate following the anneal. The composition and properties of the 2212 phase depend nonmonotonically on the anneal time. The lattice parameter C of the 2212 phase increases with increasing lithium content. Fiz. Tverd. Tela (St. Petersburg) 41, 18–21 (January 1999)     

Magnetic, electrical, and crystallographic properties of thin La1−x SrxMnO3 films

A study is reported of the magnetic, electrical, and crystallographic properties of La_1−x Sr_xMnO₃ (0.15⩽x⩽0.23) epitaxial films grown on single-crystal substrates of (001)ZrO₂(Y₂O₃) having the fluorite structure and (001)LaAlO₃ having the perovskite structure. It was found that films with close compositions for x=0.15 and 0.16, grown on different substrates, have different properties, namely, the film on a fluorite substrate is semiconducting and has a coercive strength 30 times that of the film on a perovskite substrate; the temperature dependence of electrical resistance of the latter film has a maximum around the Curie point T _C and follows metallic behavior for T<T _C. These differences are explained as due to different structures of the films. The x=0.23 film on the perovskite substrate has been found to exhibit a combination of giant magnetoresistance at room temperature with a resistance of ≈300 Ω which is useful for applications. The maxima in resistance and absolute value of negative magnetoresistance are accounted for by the existence of two-phase magnetic states in these films. Fiz. Tverd. Tela (St. Petersburg) 40, 290–294 (February 1998)     

Intrinsic Josephson effects in 60-K phase YBCO thin film mesas

We fabricatedc-axis oriented meas on YBa₂Cu₃O_7−δ thin films. Columnar structures with an area of 20×20 μm² and a depth of 0.1 μm were formed on oxygen deficient films with a critical temperature (T _c ) of 64 K or less. The devices showed hysteretic I–V curves without any branches. The dependence of the critical current density on the temperature nearT _c is explained by the Ambegaokar-Baratoff relation.     

Low-temperature behavior of specific heat curves in ruthenocuprate RuSr<Subscript>2</Subscript>Gd<Subscript>1.5</Subscript>Ge<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10 − δ</Subscript>: Superconducting,magnetic and crossing-point effects

Magnetic and superconducting properties of polycrystalline samples of RuSr₂Gd_1.5Ce_0.5Cu₂O_{(10 − δ)}, asprepared (by solid-state reaction) and annealed in pure oxygen at different pressure are presented. Specific heat and magnetization were investigated in the temperature range 1.8–300 K with a magnetic field up to 8 T. Specific heat, C (T), shows a jump at the superconducting transition (with onset at T ≈ 37.5 K) and a Schottky-type anomaly below 20 K. It is found that curves C(T) taken for different values of magnetic field have the same crossing point (at T _* ≈ 2.7 K) for all samples studied. At the same time, C(H) curves taken for different temperatures have a crossing point at a characteristic field H _* ≈ 3.7 T. These effects are manifestations of the crossing-point phenomenon, which is supposed to be inherent for strongly correlated electron systems.     

Tl系超导薄膜金属涂层的生长与特性研究

我们研究了Tl-2212超导薄膜在带有YSZ/CeO2缓冲层和带有CeO2/YSZ/CeO2缓冲层的Ni金属RABiTS基带上的生长情况.基带上的缓冲层是采用PLD方法制备的,Tl-2212薄膜的制备采用了磁控溅射和后热处理两步方法.XRD 实验结果表明,Tl-2212薄膜都具有很好的C轴垂直于膜面的织构,并具有双向外延生长特性.在CeO2/YSZ/CeO2/Ni基带上制作的Tl-2212薄膜的Tc达到102.8 K,Jc(77 K,0 T)达到2.6 MA/cm2;在YSZ/CeO2/Ni基带上薄膜Tc可达97.7 K,Jc(77 K,0 T)也可以达到0.45 MA/cm2.     

Effect of oxygen on the electrophysical properties of some carbon allotropes (graphite and C60 fullerene)

The electrophysical properties of powder samples of graphite and C₆₀ fullerene have been studied in vacuum, oxygen ambient, and air. Room-temperature conduction of the samples has a percolative character. An increase of temperature brings about an increase of sample conductivity caused by oxygen desorption from the bulk. A peak-shaped feature not observed before has been found in the temperature behavior of the resistance of finely dispersed C₆₀ and graphite powder samples in the T=310–340 K interval. Oxygen atoms were found to be involved in charge transport in powder samples of graphite and C₆₀ fullerene through their electrical activity on the surface of grains differing in size. Fiz. Tverd. Tela (St. Petersburg) 39, 1703–1705 (September 1997)     

Analysis of the temperature dependence of the heat capacity of solid tin in the vicinity of its melting temperature

Summary The experimental temperature dependence of the heat capacityC _p(T) of solid tin (Sn) in its premelting region 402.07–502.22 K was investigated and two regions with different temperature dependences ofC _p(T) were found. In the far (from the melting temperatureT _m) region 402.07–485.88 K the experimentalC _p(T) of Sn is described by the standard vacancy model. In the close region 485.88–502.22 K it is described by the formation of complicated volumetric defects in the crystalline lattice of solid Sn near itsT _m.