Growth-induced anisotropy in Ba0.6K0.4BiO3 superconducting single crystal

0.6 K_0.4BiO₃ single crystal have been performed to study the nature of the growth-induced anisotropy revealed using chemical etching technique. It was found that this anisotropy is responsible for differences in low-field susceptibility and in hysteresis loops observed for different magnetic field orientations. Received: 9 December 1997/Accepted: 28 May 1998     

Microwave impedance of Ba0.6K0.4BiO3 crystals: Comparison with Nb

The surface impedance Z _s=R _s+iX _s of samples of Ba_0.6K_0.4BiO₃ in the temperature range 4<T<50 K is measured at 9.42 GHz. The BCS theory completely describes the electrodynamic properties of Nb in the dirty limit, and its application to Ba_0.6K_0.4BiO₃ allows determination of the London penetration depth λ _L(0)=3100±100 Å. Zh. éksp. Teor. Fiz. 111, 696–704 (February 1997)     

Linear low-temperature dependence of the surface impedance of a Ba0.6K0.4BiO3 single crystal

The temperature dependences of the real part R _s and the imaginary part X _s of the surface impedance Z _s =R _s +iX _s of the superconductor Ba_0.6K_0.4BiO₃ (T _c ≃30 K) are measured at a frequency of 9.4 GHz. Its temperature dependence _{Z s} (T) and that of the complex conductivity σ _s (T) can be described on the basis of a two-fluid model under two assumptions: The density of superconducting carriers increases linearly, and the relaxation time increases as a power law (∝1/T ⁵), with decreasing temperature T<T _c . This model also describes well the curves Z _s (T) and σs (T) recently measured for YBa₂Cu₃O_6.95 and Bi₂Sr₂CaCu₂O₈ single crystals. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 783–788 (10 December 1996)     

Increase in the magnetization loop width in the Ba0.6K0.4BiO3 superconductor: Possible manifestation of phase separation

The magnetization of Ba_0.6K_0.4BiO₃ samples in fields up to 90 kOe in the temperature range from 2 to 30 K is investigated. It is shown that the observed increase in the width of the magnetization loop can be explained by a decrease in the phase nonuniformity upon an increase in the magnetic field. The asymmetric hysteretic dependence of magnetization with the secondary peak was successfully described by the extended critical state model taking into account the phase separation in the superconductor.     

Anisotropic enhancement of superconductivity in heavy-ion irradiated (K,Ba)BiO3

We have measured the specific heat, resistivity, and ac susceptibility of (K,Ba)BiO3 single crystals before and after introduction of either point or columnar defects by electron (EI) or heavy-ion irradiation (HII). While the magnetic field dependence of these properties remains mainly unaffected by EI, the irreversibility line and the location of the specific heat anomaly are both shifted up in temperature after HII. The shift is apparent only if the magnetic field is applied parallel to the ion tracks. For perpendicularly applied fields, both lines lie at the same field as in the pristine sample. These experiments call the nature of the vortex liquid state into question.     

氧压对Ba0.6Sr0.4TiO3薄膜晶格常数的影响及BaTiO3/Ba0.6Sr0.4TiO3超晶格的制备

采用脉冲激光沉积(PLD)技术,经一系列的优化实验成功地制备了BaTiO₃(BT)和Ba_0.6Sr_0.4TiO₃(BST)单层膜.X射线衍射分析表明,在LaAlO₃(001)单晶平衬底上生长的BT和BST薄膜都是沿[001]取向的近外延生长.且随着氧压在10^-3—25 Pa范围内逐渐增大,BST薄膜的晶格常数与氧压之间近似满足Boltzmann函数关系.其次,在此优化条件下还 关键词： 超晶格 晶格常数 激光感生热电电压 脉冲激光沉积     

Soft phonons and structural phase transition in superconducting Ba0.59K0.41BiO3

We have observed a softening of phonons and a structural phase transition in a superconducting Ba_0.59K_0.41BiO₃ (T_c = 31 K) single crystal using elastic and inelastic neutron scattering measurements. The soft phonon occurs for the [1 1 1] transverse acoustic mode at the zone boundary. The phonon energies in this vicinity are found to continuously decrease with decreasing temperature from above room temperature. This softening stops at a temperature close to T_s, where a structural phase transition from cubic to tetragonal symmetry occurs. The overall results are consistent with previous data that reported phonon softening and a (0.5, 0.5, 0.5) type superstructure in several Ba_1?xK_xBiO₃ systems. However, we also find weaker (0.5, 0.5, 0) type superstructure peaks that reveal an additional component to the modulation. No significant change related to the superconductivity was observed for these soft phonon energies or linewidths.     

Impedance spectroscopy and conduction mechanism of multiferroic (Bi0.6K0.4)(Fe0.6Nb0.4)O3

Polycrystalline (Bi_0.6K_0.4) (Fe_0.6Nb_0.4)O₃ material has been prepared using a mixed-oxide route at 950 °C. It was shown by XRD that at room temperature structure of the compound is of single-phase with hexagonal symmetry. Some electrical characteristics (impedance, modulus, conductivity etc.) were studied over a wide frequency (1 kHz–1 MHz) and temperature (25–500 °C) ranges. The Nyquist plot (i.e., imaginary vs real component of complex impedance) of the material exhibit the existence and magnitude of grain interior and grain boundary contributions in the complex electrical parameters of the material depending on frequency, input energy and temperature. The nature of frequency dependence of ac conductivity follows Joncher׳s power law, and dc conductivity follows the Arrhenius behavior. The appearance of P–E hysteresis loop confirms the ferroelectric properties of the material with remnant polarization (2P_r) of 1.027 µC/cm² and coercive field (2E_c) of 16.633 kV/cm. The material shows very weak ferromagnetism at room temperature with remnant magnetization (2M_r) of 0.035 emu/gm and coercive field (2H_c) of 0.211 kOe.     

Low-Voltage Depletion-Mode Indium-Tin-Oxide Thin-Film Transistors Gated by Ba0.4Sr0.6TiO3 Dielectric

We report on the fabrication and characterization of low-voltage indium-tin-oxide （ITO） thin-film transistors （TFTs） gated by a0.4Sr0.6TiO3 （BST） gate dielectric deposited at room temperature. The 400-nm-thick BST film shows a low leakage current density of 6 × 10^-8 A/cm^2 and a high specific capacitance of 83 nF/cm^2 （corresponding εr= 37）. The ITO TFTs gated by such BST dielectric operate in a depletion mode with an operation voltage of 5.0 V. The device exhibits a threshold voltage of -3.7 V, a subthreshold swing of 0.5 V/decade, a field effect mobility of 3.2cm^2/Vs and a current on/off ratio of 1.4× 10^4.     

Ba1-xSrxTiO3及Ba0.6-xPbxSr0.4TiO3陶瓷的制备和介温特性研究

用固相反应法制备了Ba_1-xSr_xTiO₃(BST)及Ba_0.6-xPb_xSr_0.4TiO₃(BPST)陶瓷,通过XRD,FESEM和拉曼谱分析了Pb掺杂对Ba_0.6Sr_0.4TiO₃样品的晶格、相变及表面形貌的影响.测试了BST及B 关键词： BST BPST 弥散相 介温特性     

Mechanisms of asymmetric leakage current in Pt/Ba0.6Sr0.4TiO3/Nb-SrTiO3 capacitor

Mechanisms of leakage current have been investigated in the capacitor consisting of a Ba_0.6Sr_0.4TiO₃ thin film, a Pt top electrode, and a Nb-doped SrTiO₃ (STON) bottom electrode. The leakage current shows asymmetric behavior for different bias voltage. For the Pt electrode negatively biased, the leakage current can be explained by modified Schottky emission mechanism, and the barrier height is obtained as 0.44 eV. For the Pt electrode positively biased, the leakage current shows a space-charge-limited current behavior. The trap in dielectric film is regarded as deep traps, and the density of trapped carrier is estimated as about 3.2×10²³/m³. PACS 77     

Evolution of the 251 cm-1 temperature in infrared phonon mode with Ba0.6K0.4Fe2As2

The far-infrared optical reflectivity of an optimaUy doped Ba1-xKxFe2As2 （x = 0.4） single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 em-1. This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continu- ously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observa- tions indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Bao.6Ko.4Fe2As2 compound.     

PbZr0.4Ti0.6O3 and Ba0.9Sr0.1TiO3 reflectors derived from chemical solutions containing polymers

Ba(0.9)Sr(0.1)TiO(3) (BST)-based and PbZr(0.4)Ti(0.6)O(3)-based quasi-periodic multilayers consisting of dense and porous ferroelectric layers have been fabricated by solgel technique using chemical solutions containing polyethylene glycol (PEG) or polyvinylpyrrolidone k30 (PVP). All multilayers exhibit good performance as dielectric mirrors. For each multilayer, the maximum peak reflectivity is over 90% and the photonic stopband width is no less than 30 nm at room temperature. The reflection-band position can be easily tuned by varying the thickness of the bilayer. With the same processing conditions and number of periods, the Bragg reflection performance is almost the same for quasi-periodic PZT multilayers derived from two precursors containing different polymers. The BST multilayers deposited by using a PVP-containing precursor are superior in optical properties, including peak reflectivities and stop-band width, to those deposited by using the PEG-containing solution.     

Nonlinear ac response of the vortex system in the cubic (K, Ba)BiO3 superconductor

We present ac susceptibility measurements performed on single crystals with different geometries: thick films, bars and hollow cylinders. We show that the dependence of the real () and imaginary () parts of the ac susceptibility is in very good agreement with Brandt's numerical calculations (Phys. Rev. B 58, 6523 (1998)) in the modified Bean critical state. Creep effects (at ) are investigated by studying the frequency dependence of the current density deduced from the temperature scans of the ac susceptibility over a large frequency range (). The relaxation rate is temperature independent and very similar to the one usually obtained in high cuprates. Received 17 March 1999     

Direct transition from Bose glass to normal state in the (K,Ba)BiO3 superconductor

The introduction of columnar defects in (K,Ba )Bi O3 single crystals shifts both the irreversibility and thermodynamic transition lines, respectively, deduced from ac susceptibility (and/or transport) and specific heat measurements, upwards. This shift can be attributed to the defect-induced decrease of the difference (Delta F) between the free energies in the superconducting and the normal states, assuming that the position of the superconducting transition is given by the condition absolute value Delta F approximately k(B )T/xi(3 ). This criterion also perfectly reproduces the influence of the angle between the tracks and the external field. This result suggests that no vortex liquid phase exists in this system.