Morphology and electrical properties of pulsed-laser deposited Bi2Sr2CaCu2O8+δ films on vicinal substrates

Thin films of Bi₂Sr₂CaCu₂O_8+δ have been grown on vicinal (001) SrTiO₃ substrates by pulsed-laser deposition. Transmission electron microscopy (TEM) and X-ray diffraction reveal well ordered films with the c axis of the film parallel with the c axis of the substrate for miscut angles up to θ_S≈15°. TEM also reveals the step-like film morphology due to step-flow growth. The in-plane and out-of-plane resistivities are independent of film thickness within the range 20–300 nm and agree quite well with Bi₂Sr₂CaCu₂O_8+δ single-crystal data. Received:15 May 2000 / Accepted:17 May 2000 / Published online: 9 August 2000     

Growth evolution of Bi2Sr2CaCu2O8+δ thin films deposited by infrared (1064 nm) pulsed laser deposition

Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) thin films were grown on MgO (1 0 0) using 1064 nm infrared pulsed laser deposition with post heat treatment. The material arrives at the substrate surface with characteristic spheroidal morphology. Smooth, homogeneous and highly c-axis oriented films were obtained after heat treatment. Using infrared laser as excitation, the stoichiometry of the target is preserved on the film. However, the films were apparently under-doped.     

Particle-free superconducting Bi2Sr2CaCu2Ox ultrathin films prepared by atomic-layer-controlled molecular beam epitaxy technique

Atomic-layer-controlled molecular beam epitaxy (ALC-MBE) technique is applied for preparing Bi₂Sr₂CaCu₂O_x ultrathin films. Precise control of the composition by ALC-MBE enables successful growth of particle-free ultrathin films. Superconducting zero-resistance transitions are observed for the films more than 3.0 nm in thickness. It is also shown that growth of a Bi₂Sr₂CuO_y buffer layer between the film and SrTiO₃(100) substrate contributes to preparing such particle-free ultrathin films with good superconducting properties.     

Step-like growth of Bi2Sr2CaCu2O8 films on off axis oriented (001) SrTiO3

High-quality Bi₂Sr₂CaCu₂O₈ (BSCCO) films were grown by means of pulsed-laser deposition on 10° off-axis oriented (001) SrTiO₃ substrates. The films exhibit a step-like morphology and they are properly oriented along the (001) direction. Their normal-state resistivity measured along the tilt (off-axis (100) direction of the substrate) was, typically, 260 times higher than that along the (010) direction. Such films reveal a transverse Seebeck-effect which significantly exceeds that observed with YBa₂Cu₃O₇ (YBCO).     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

Thermodynamic parameters of Bi2Sr2CaCu2O8+δ thin film prepared by molecular beam epitaxy

Bi₂Sr₂CaCu₂O_8+δ (Bi2212) thin film is an important superconducting material ascribed to its high transition temperatures and low toxicity, but the application of Bi2212 thin films is limited due to the appearances of intergrowth and impurity phases. To achieve high-quality Bi2212 thin film by molecular beam epitaxy (MBE), the oxidizing gas pressure and substrate temperature are the key parameters. Here, the phase formation in Bi-based thin films grown by MBE was studied as a function of oxidizing gas pressure and substrate temperature. Furthermore, the thermodynamic parameters have been investigated through enthalpy change. This study indicates that the substrate temperature was increased with the enhancing of ozone partial pressure when the composition ratio is fixed, Bi2212 single phase can be formed. Moreover, the chemical stability decreases in the order of Bi₂Sr₂CuO_6+δ > Bi2212 > Bi₂Sr₂Ca₂Cu₃O_10+δ.     

Low-temperature formation of thermal and electrodynamic states in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8</Subscript>

The formation of thermal and electrodynamic states in Bi₂Sr₂CaCu₂O₈ under the condition of current input is studied. The analysis is carried out for partial and complete current penetration under the assumption that the superconductor is cooled down to liquid helium temperature at the zero time. When the current input is continuous, the temperature dependence of the Bi₂Sr₂CaCu₂O₈ specific heat influences the form of the I-V and I-T characteristics of the superconductor. This effect is observed at high electric fields when both stable and unstable states form. As a result, the nonstationary I-V characteristic of Bi₂Sr₂CaCu₂O₈ has the only branch the slope of which is positive and decreases with increasing temperature. Therefore, the higher the rate of current input, the more pronounced the decrease in the slope. It is concluded that one cannot find the current above which instability develops from the Bi₂Sr₂CaCu₂O₈ I-V characteristic if the current input is continuous.     

Lock-in transition in Bi2Sr2CaCu2O8+δ observed through 10GHz dissipation

We observe the lock-in transition in Bi₂Sr₂CaCu₂O_8+δ single crystals, by measuring the 10 GHz dissipation for various orientations of the magnetic field; various vortex lattice structures are revealed by different dissipation regimes.     

Laser–MBE growth of high-quality ZnO thin films on Al2O3(0001) and SiO2/Si(100) using the third harmonics of a Nd:YAG laser

High-quality ZnO thin films were grown on single-crystalline Al₂O₃(0001) and amorphous SiO₂/Si(100) substrates at 400–640 °C using laser molecular beam epitaxy. For film growth, the third harmonics of a pulsed Nd:YAG laser were illuminated on a ZnO target. The ZnO films were epitaxially grown on Al₂O₃(0001) with the narrow X-ray diffraction full width at half maximum (FWHM) of 0.04° and the films on SiO₂/Si(100) exhibited a preferred c-axis orientation. Furthermore, the films exhibited excellent optical properties in photoluminescence (PL) measurements with very sharp excitonic and weak deep-level emission peaks. At 15 K, PL FWHM values of the films grown on Al₂O₃(0001) and SiO₂/Si(100) were 3 and 18 meV, respectively. Received: 8 May 2001 / Accepted: 18 September 2001 / Published online: 20 December 2001     

在H2气氛中Bi2Sr2CaCu2O8+δ的热处理效应

对高品质单相多晶Bi₂Sr₂CaCu₂O_8+δ样品作真空或H₂气氛中不同温度下的热处理。由X射线结构分析,其正常态和超导态性质(特别是载流子浓度)的系统测量结果表明:Bi₂Sr₂CaCu₂0_8+δ样品在H₂气氛中热处理,其氧的释放可分为两个阶段:一是Bi-O层之间的过量氧原子的逸出,降低系统的载流子浓度,从而调节T_c;二是CuO₂面内氧原子的迁移,产生氧空位,使CuO₂面内Cu²⁺的短程二维反铁磁(2D-AFM)有序背景遭到破坏。可以认为:以往文献中报道的Bi₂Sr₂CaCu₂O_8+δ超导体的吸氢效应导致T_c的变化,很可能主要的并不是氢原子的电子掺杂行为,而是与吸氢过程中氧含量的变化相联系。 关键词：     

Doping-dependent electronic structure of cuprates studied using angle-scanned photoemission

Full k -maps of the electronic structure near the Fermi level of differently doped cuprates measured with angle-scanned photoelectron spectroscopy are presented. The valence band maximum of the antiferromagnetic insulator Sr₂CuO₂Cl₂, which is taken as a representative of an undoped cuprate, and the Fermi surfaces of overdoped, optimally doped and underdoped Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductors are mapped in the normal state. The results confirm the existence of large Luttinger Fermi surfaces at high doping with a Fermi surface volume proportional to (1+x), where x is the hole concentration. At very low doping, however, we find that this assumption based on Luttinger's theorem is not fulfilled. This implies a change in the topology of the Fermi surface. Furthermore the intensity of the shadow bands observed on the Fermi surface of Bi₂Sr₂CaCu₂O_8+δ as a function of the doping is discussed. Received 12 October 1999 and Received in final form 12 April 2000     

Film structure and ferroelectric properties of in situ grown SrBi2Ta2O9 films

Ferroelectric SrBi₂Ta₂O₉ (SBT) films were grown by pulsed-laser deposition (PLD) at different substrate temperatures and fluences. A correlation between film structure and ferroelectric properties is established. The dielectric function ε^′ of thin SBT films shows a Curie–Weiss behavior well below the peak temperature T_max and relaxor-like behavior in the vicinity of T_max. Domain walls have a strong influence on the dielectric and ferroelectric properties and on the polarization fatigue of SBT films below 100 °C. The formation of ferroelectric phases is favored at lower substrate temperatures by incorporating Bi₂O₃ template layers into the structure. Received: 18 March 1999 / Accepted: 19 March 1999 / Published online: 5 May 1999     

Evidence for superconductivity in the Bi-O layers and not in the cuprate planes of Bi2Sr2CaCu2O8

The superconductivity in Bi₂Sr₂CaCu₂O₈occurs in Bi-O slabs, with the CuO₂planes being insulating.     

Polarized X-ray absorption study of Bi2Sr2CuO6 and Bi2Sr2CaCu2O8

Polarization dependent X-ray absorption measurements on single crystal Bi₂Sr₂CuO₆ (T _c =9 K) and Bi₂Sr₂CaCu₂O₈ (T _c =80 K) with one and two CuO₂ layers, respectively, show no energy shift of the Cu 2p main peak, and no relation between the amount of 3d(m=0, ±1) character and the critical temperatureT _c . At grazing incidence a structure in Bi₂Sr₂CaCu₂O₈ is found at 937 eV, which can be ascribed to a composite state of Cu 3d(z ²), Cu 4s ando–2p(z).     

Effect of microstructure on thermoelectric properties of CSD-grown Bi2Sr2Co2Oy thin films

Three Bi₂Sr₂Co₂O_y thin films with different microstructures have been prepared by chemical solution deposition on LaAlO₃(001) through varying the annealing temperature. With the decrease in the annealing temperature, both the size and c-axis alignment degree of grains in the film decrease as well, leading to an increase in the film resistivity. In addition, the decrease in the annealing temperature also results in a slight increase in the seebeck coefficient due to the enhanced energy filtering effect of small-grain film. The nanostructured Bi₂Sr₂Co₂O_y film with the average grain size of about 100 nm shows a power factor comparable to that of the films with larger grains. Since the thermal conductivity of the nanostrcutured films can be depressed due to the enhanced phonon scattering by grain boundary, a higher figure of merit is expected in Bi₂Sr₂Co₂O_y thin film with grains in nanometer size.     

High Q-factor superconductor VHF antennae

Two versions of elementary antennae fabricated of high temperature superconductors (HTS) are presented. The ceramic YBa₂Cu₃O₇ strip as well as Bi₂Sr₂CaCu₂O₈ thick film have been used as radiators operating in the wavelength ranges of 5 cm and 8 mm respectively. Substantial gain and Q-factor enhancement if compared with similar metallic ones have been achieved for the HTS antennas.     

Current instability mechanisms in liquid-coolant-cooled high-temperature superconductors

The stable current distribution in Bi₂Sr₂CaCu₂O₈, Bi₂Sr₂Ca₂Cu₃O₈, and YBa₂Cu₃O₇ high-temperature superconductors depending on conditions of cooling thereof by liquid cryocoolants—helium, hydrogen, and nitrogen, respectively—is studied. It is shown that the current instability mechanism may change in going from one coolant to another. Consequently, stable states may be disturbed, first, when the conditions of cooling the superconductor surface change from nucleate boiling to film boiling. Such a thermal mechanism of stable current state disturbance is observed largely when a superconductor is cooled by liquid helium. Second, even for the nucleate boiling of a liquid coolant, current instability may result from the stable formation of the voltage-current characteristic of the superconductor. This type of injected current stability disturbance is most likely when a superconductor is cooled by liquid nitrogen. Criteria for determining a current instability mechanism in relation to the properties of the superconductor and coolant are given.     

Measurements of the London penetration depth in Bi-based high-T c compounds

The muon spin relaxation was measured in the high-T _c superconductors Bi₂Sr_1.3Ca_0.7 CuO_6.15, Bi₂Sr₂CaCu₂O₈ and Bi₂Pb _x Sr₂Ca₂Cu₃O₁₀. We present our method to determine the London penetration depth perpendicular toc-axis λ_⊥ (0) taking also contributions from the anisotropic magnetization and nuclear dipoles to the muon frequency spectrum into account.     

Growth of Bi2Sr2Cu1Ox films by laser ablation

The formation and the properties of thin BiSrCuO films grown on MgO single crystal substrates by the pulsed laser ablation of a Bi₂Sr₂Cu₁O₆ target have been studied. The precise influence of the oxygen pressure and substrate temperature on the atomic composition, nature and structure of the grown phases, crystalline quality and superconductivity of the films has been analyzed. In the 600–750°C temperature range and 0.05–0.5 mbar oxygen pressure range, highly textured films of the 2201 phase (Bi₂Sr₂Cu₁O₆ phase) were formed despite large composition deviations (Bi enrichment and Sr depletion) with respect to the ideal composition. A high crystalline quality was evidenced for films grown at low and intermediate pressure and high temperature. χ_min values in the films deduced from channeling experiments were similar to the case of single crystal material. Despite this high crystalline quality, incomplete or very low T_c superconducting transitions were observed in the resistivity measurements.     

Magnetization anomaly and anisotropy of Bi2Sr2CaCu2−xNixO8 single crystals

We have observed the anomalous magnetization of Bi₂Sr₂CaCu_2−xNi_xO₈ (x = 0 and 0.02) single crystals. Anisotropy decreases with iodine intercalation although it expands the space between CuO₂ layers. Iodine intercalation seems to suppress the magnetization anomaly for Ni = 1% crystals, but not for Ni = 1% substituted crystals. We have discussed these results in terms of the increase of anisotropy by Ni substitution and the dimensional crossover of flux lines. Effects of both oxygen concentration and substitution of a magnetic element for the Cu site on the anisotropy of Bi₂Sr₂CaCu₂O₈ crystals show the same tendency as the case of the YBa₂Cu₃O₇ superconductor.