Effects of strontium on the characteristics of Pb(Zr1/2Ti1/2)O3--Pb(Zn1/3Nb2/3)O3 ceramics

The dielectric and piezoelectric properties of pyrochlore-free lead zirconate titanate-lead zinc niobate ceramics were investigated systematically as a function of Sr doping. The powders of Pb_{(1? x )}Sr _x [0.7(Zr_{1 / 2}Ti_{1 / 2})–0.3(Zn_{1 / 3}Nb_{2 / 3})]O₃, where x?=?0–0.06 were prepared using the columbite-(wolframite) precursor method. The ceramic materials were characterized using X-ray diffraction, dielectric spectra, hysteresis and electromechanical measurements. The phase-pure perovskite phase of Sr-doped PZN--PZT ceramics was obtained over a wide compositional range. The results showed that the optimized electrical properties were also achieved at composition x?=?0.0, which were K _P?=?0.69, d ₃₃?=?670?pC?N^?1, P _r?=?31.9?µC?cm^?2 and ε_rmax?=?18600. Maximum dielectric constant values of the systems decreased rapidly with increasing Sr concentration. Moreover, with increasing Sr concentration dielectric constant versus temperature curves become gradually broader. The diffuseness parameter increased significantly with Sr doping. Furthermore, Sr doping has been shown to produce a linear reduction in the transition temperature (T _m)?=?294.1–12.7x°C with concentration (x). Sr shifts the transition temperature of this system at a rate of 12.7°C?mol^?1%.     

Effects of doping Na on the structure and physical properties of La2/3Ca1/3MnO3

Effects of doping Na on the structure, electrical and magnetic properties of La_2/3Ca_1/3MnO₃ are investigated. A structural phase transition from orthorhombic to rhombohedral structure takes place at y=0.375. All samples show metal-insulator (M-I) transition at the transition temperature and undergo the transition from paramagnetism to ferromagnetism at the Curie temperature T_C. and T_C increase monotonically with increasing Na content. However the Na-doped samples have a shoulder in their electrical transport curves found below and shows a widened magnetic transition process. On the other hand, intrinsic colossal magnetoresistance (CMR) peaks are observed in all the samples, but samples with y around 0.25 show two MR peaks which can be attributed to magnetic inhomogeneity induced by the doped Na⁺ ions. Here we propose a method to broaden the CMR peak of perovskite manganite, which is beneficial for practical applications.     

Effects of the surface layer of La2/3Ca1/3MnO3 thin films

The surface layer effects on transport in epitaxial La_2/3Ca_1/3MnO₃ thin films are studied. It was found that the two-probe resistance is nonlinear which is enhanced with decreasing temperature. Similar to the resistance of intrinsic La_2/3Ca_1/3MnO₃ thin films reported in the literature, the apparent dynamic contact resistance behaves semiconducting at high temperatures, passes through a peak, and displays a metallic behavior. At lowest temperatures, the curve of the contact resistance versus temperature shows a little upturn. The temperature dependent work function difference between the surface layer and the thin film underneath, together with the tunneling process across either the resulting charge depleted layer or the semiconducting surface layer is used to explain our observations.     

Photoemission study of glycine adsorption on Cu/Au(1 1 1) interfaces

The adsorption of glycine on Au(1 1 1) pre-deposited with different amounts of Cu was investigated with both conventional X-ray photoelectron spectroscopy (XPS) and synchrotron-based photoemission. In the Cu submonolayer range, glycine physically adsorbs on the Cu/Au(1 1 1) surfaces in its zwitterionic form and completely desorbs at 350 K. The C 1s, O 1s and N 1s core level binding energies monotonically increase with Cu coverage. This indicates that, in the Cu submonolayer range, the admetal is alloyed with Au rather than forming overlayers on the Au(1 1 1) substrate, consistent with our recent experimental and theoretical results [X. Zhao, P. Liu, J. Hrbek, J.A. Rodriguez, M. Pérez, Surf. Sci. 592 (2005) 25]. Upon increasing the amount of deposited Cu over 1 ML, part of the glycine overlayer transforms from the zwitterionic form to the anionic form (NH₂CH₂COO⁻) and adsorbs chemically on the Cu/Au(1 1 1) surface with the N 1s binding energy shifted by −2.3 eV. When the amount of deposited Cu is at 3.0 or 6.0 ML, the intensity of the N 1s chemisorption peak increases with aging time at 300 K. It indicates that glycine adsorption induces Cu segregation from the subsurface region onto the top layer of the substrate. Judging from the initial N 1s peak intensities, it is concluded that 64% and 36% of the top layer are still occupied by Au atoms before glycine adsorption even when the amounts of deposited Cu are 3.0 and 6.0 ML, respectively. On the Au(1 1 1) surface pre-dosed with 6.0 ML of Cu, part of the chemisorbed glycine will desorb and part will decompose upon heating to 450-500 K. In addition, about 20% of the glycine exists in the neutral form when the glycine overlayer was dosed on Cu/Au(1 1 1) held at 100 K.     

Colossal room-temperature magnetoresistance of La1/3Nd1/3Sr1/3MnO3 single crystals

Replacement of one half of the neodymium ions by lanthanum in Nd_2/3Sr_1/3MnO₃ is shown to result in a considerable increase in the Curie temperature. The single-crystal La_1/3Nd_1/3Sr_1/3MnO₃, whose Curie point lies at 315 K, has been found to exhibit a record-high magnetoresistance of 27% in a weak magnetic field of 8.4 kOe in the temperature range above room temperature.     

Effects of Si surficial structure on transport properties of La_(2/3)Sr_(1/3)MnO_3 films

La_(2/3)Sr_(1/3)MnO_3 films are deposited on(001) silicon substrates,in which the silicon surfaces have artificially been treated into the scallops-like,pyramid-like,and smooth polishing structure,by pulsed laser deposition.The magnetoresistances of the films on etched substrates under low applied field are very sensitive to the applied field,and much larger(14.3% for acid-etched,and 42.9% for alkali-etched) than that on the polished Si at 5 K.Zero-field-cooled and field-cooled magnetization behaviors are measured and analyzed.Remarkable upturn behaviors in temperature-dependent resistivity for all samples are observed at low temperature,which follows the Efros-Shkloskii variable range hopping law and the Arrhenius law.We believe that the rough surface may be useful in device design.     

Acceptor Concentration Effects on Photovoltaic Response in the La1-xSrxMnO3/SrNbyTi1-yO3 Heterojunction

Photovoltaic response in the heterojunction of La1-x SrxMnO3/SrNby Ti1-yO3 （LSMO/SNTO） is analyzed theoretically based on the drift-diffusion model. It is found that the decrease of acceptor concentration in the La1-xSrxMnO3 layer of hereto junction can increase the peak value of photovoltaic signal and the speed of photovoltaic response, whereas the changing of donor concentration in the SrNby Ti1-yO3 layer has no such evident effect. Furthermore, the result also indicates that the modulation of Sr doping in La1-xSrxMnO3 is an effective method to accommodate the sensitivity and the speed of photovoltaic response for LSMO/SNTO photoelectric devices.     

Pressure-induced phase transition of B-type Y_2O_3

The synthesized monoclinic(B-type) phase of Y_2O_3 has been investigated by in situ angle-dispersive x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. A phase transition occurs from monoclinic(B-type) to hexagonal(A-type) phase at 23.5 GPa and these two phases coexist even at the highest pressure. Parameters of isothermal equation of state are V_0= 69.0(1) ~3, K_0= 159(3) GPa, K_0= 4(fixed) for the B-type phase and V_0= 67.8(2) ~3, K_0= 156(3) GPa,K'_0= 4(fixed) for the A-type phase. The structural anisotropy increases with increasing pressure for both phases.     

Effects of growing conditions on the electric and magnetic properties of strained La2/3Sr1/3MnO3 thin films

We investigate the growing condition dependences of magnetic and electric properties of the La_2/3Sr_1/3MnO₃ thin films grown on SrTiO₃（001） substrates.With reducing the film thickness and growth pressure,the Curie temperature（T_c）drops off,and the magnetism and metallicity are suppressed.At an appropriate deposition temperature,we can obtain the best texture and remarkably enhance the magnetic and electrical properties.However,the resistivity of film cannot be modulated by changing the dc current and green light intensity.This result may be induced by the coherent strains in the epitaxially grown film due to its lattice mismatching that of the SrTiO₃ substrate.Furthermore,we show that the relations between the magnetism and the resistivity for the typical films with different thickness values.For the 13.4-nm-thick film,the R-T curve presents two transition behaviors:insulator-to-metal and metal-to-insulator in the cooling process:the former corresponds to magnetic transition,and the later correlates with thermal excitation conduction.     

Exchange interaction between ferromagnetic and antiferromagnetic phases in La1/3Nd1/3Ca1/3MnO3

Magnetization measurements of a La_1/3Nd_1/3Ca_1/3MnO₃ perovskite at magnetic field up to 6 T have revealed an anomalous behaviour – above 130 K the material exhibits a loop displacement about a field of 2 T. We assume that this is the result of an exchange interaction between ferromagnetic and antiferromagnetic phases in a magnetically inhomogeneous compound. At about 115 K a transition from a semiconducting to a metallic-like state has been observed.     

异质界面对Ca(Mg1/3Nb2/3)O3/CaTiO3叠层薄膜结构和介电性能的影响

采用异质叠层方式制备出一定厚度的Ca(Mg_1/3Nb_2/3)O₃/CaTiO₃(CMN/CT)叠层薄膜,研究了异质界面对薄膜结构、微观形貌及介电性能的影响及其规律.根据实验测试结果,提出CMN/CT叠层薄膜的模拟等效电路,建立介电常数和介电损耗的理论计算公式.结果表明:CMN/CT异质叠层薄膜具有完全正交钙钛矿结构,结构致密,厚度均匀,薄膜中存在独立的CMN和CT相.异质界面处存在过渡层,随着薄膜中异质界面个数增加,介电常数增大,介电损耗减小.减小界面过渡层的厚度,有利于提高CMN/CT叠层薄膜的介电性能.     

点缺陷浓度对非化学计量比L12型结构的A13Sc弹性性能的影响

采用密度泛函理论与Wagner-Schottky热力学模型计算了金属间化合物L1₂-A1₃Sc中点缺陷浓度与温度、成分间的关系. 结果表明: 在考察的温度区间(T=300-1200 K), 理想化学计量比L1₂-A1₃Sc中的点缺陷主要为Al空位和Sc空位, 且缺陷浓度较低(在1200 K时仅约为10^-6). 当L1₂-A1₃Sc偏离化学计量比成分时, 富Al成分端的点缺陷主要为Al反位与Sc空位, 且两种缺陷的浓度相当; 富Sc成分端的点缺陷则主要为Sc反位. 利用超胞模型进一步计算了含点缺陷L1₂-A1₃Sc晶体的弹性常数, 并计算预测了点缺陷形式和浓度对其弹性性能的影响. 结果表明: 在理想化学计量比成分附近, 点缺陷的引入均会降低非化学计量比L1₂-Al₃Sc晶体的杨氏、剪切和体积弹性模量, 增加非化学计量比L1₂-Al₃Sc弹性性能的各向异性, 但是对其脆-韧性的影响不大.     

A Quantum Monte Carlo Study on Mixed-Spin Chains of 1/2-1/2-1-1 and 3/2-3/2 -1-1

The ground-state and thermodynamic properties of quantum mixed-spin chains of1/2-1/2-1-1and 3/2-3/2-1-1are investigated by a quantum Monte Carlo simulation with the loop-cluster algorithm. For 1/2-1/2-1-1 chain, we find it has two phases separated by an energy-gap vanishing point in the ground-state. For 3/2-3/2-1-1 chain, the numerical results show two energy-gap vanishing points isolated by different phases in its ground-state. Our calculations indicate that all these ground state phases can be understood by means of valence-bond-solid picture, and the thermodynamic behavior at finite temperatures is continuous as a function of parameterα=J2/J1.     

A Quantum Monte Carlo Study on Mixed-Spin Chains of 1／2-1／2-1-1 and 3／2-3／2-1-1

The ground-state and thermodynamic properties of quantum mixed-spin chains of 1/2-1/2-1-1 and 3/2-3/2-1-1 are investigated by a quantum Monte Carlo simulation with the loop-cluster algorithm. For 1/2-1/2-1-1 chain, we find it hastwo phases separated by an energy-gap vanishing point in the ground-state. For 3/2-3/2-1-1 chain,the numerical results show two energy-gap vanishing points isolated by different phases in its ground-state. Our calculations indicate that all these ground state phases can be understood by means of valence-bond-solid picture, and the thermodynamic behavior at finite temperatures is continuous as a function of parameter α=J2/J1.     

Characterization of Na-substituted LiNi1/3Co1/3Mn1/3O2 cathode materials for lithium-ion battery

Highly crystalline layered Li_1?xNa_xNi_1/3Co_1/3Mn_1/3O₂ (x?=?0, 0.001, 0.01, 0.03, 0.05) materials are synthesized by molten salts method and characterized by scanning electron microscopy, inductively coupled plasma (ICP), X-ray diffraction, Rietveld refinement, and electrochemical measurement, respectively. ICP, SEM, and EDS results show that Na ions are incorporated in LiNi_1/3Co_1/3Mn_1/3O₂. Rietveld refinement results show that suitable Na substitution leads to stable layered structure by full Na occupying in Li layer and further attributes to low cation mixing. Electrochemical studies demonstrate that the Na-substituted LiNi_1/3Co_1/3Mn_1/3O₂ shows improved rate capability and cycling performance compared to that of pure LiNi_1/3Co_1/3Mn_1/3O₂.     

Synthesis of spherical LiNi1/3Co1/3Mn1/3O2 cathode materials for Li-ion batteries

Spherical LiNi_1/3Co_1/3Mn_1/3O₂ was successfully prepared by controlled crystallization. The preparation started with the spherical coprecipitate of Ni_1/3Co_1/3Mn_1/3CO₃ from NiSO₄, CoSO₄, MnSO₄, NH₄HCO₃, and NH₃·H₂O, followed by pyrolysis of Ni_1/3Co_1/3Mn_1/3CO₃ at 600°C for 3 h. The X-ray diffraction analysis showed that the homogeneous cubic (Ni_1/3Co_1/3Mn_1/3)₃O₄ was obtained after the pyrolysis. Spherical LiNi_1/3Co_1/3Mn_1/3O₂ was obtained by sintering of the mixture of as-obtained (Ni_1/3Co_1/3Mn_1/3)₃O₄ and LiOH·H₂O at 900°C for 6 h in air. As-prepared spherical LiNi_1/3Co_1/3Mn_1/3O₂ presented initial discharge capacity of 162.9 mA h g⁻¹ and capacity retention of 98% at 50th cycle.     

制备压力对La2/3Sr1/3MnO3的磁电阻效应的影响

 研究了制备压力对纳米块状样品La_2/3Sr_1/3MnO₃的结构、磁学和电学性质的影响。结果表明：样品的晶粒尺寸随制备压力的增加而变小，且不同样品的磁电阻效应不同。低场磁电阻效应在整个实验温区都随制备压力的升高而变弱，这主要是由制备压力使样品晶粒界面连接更紧密所导致；T<200 K时，高场磁电阻效应随制备压力的升高而变强，T>200 K时，高场磁电阻效应随制备压力的升高反而变弱，这主要是由制备压力改变样品的晶粒尺寸所引起。     

Effects of Interfacial Polarization on Voltage Tunability of Pb(Fe1/2Nb1/2)1-xTixO3 Single Crystals

The voltage tunability of Pb(Fe_1/2Nb_1/2)_1-xTi_xO₃ single crystals is investigated at a low electric field (<130 V/cm) in a low frequency range (<1 MHz). The results show that the capacitance is strongly suppressed by the applied dc biases for both the rhombohedral sample and the tetragonal sample. A negative voltage tunability is only detected in the tetragonal sample. The origin of the giant tunability and the negative tunability is discussed based on the multipolarization-mechanism model and the equivalent circuit model, respectively. It is ascribed to the interfacial polarization at the interface of electrode/sample.     

Effects of synaptic depression and adaptation on spatiotemporal dynamics of an excitatory neuronal network

We analyze the spatiotemporal dynamics of a system of integro-differential equations that describes a one-dimensional excitatory neuronal network with synaptic depression and spike frequency adaptation. Physiologically suggestive forms are used for both types of negative feedback. We also consider the effects of employing two different types of firing rate function, a Heaviside step function and a piecewise linear function. We first derive conditions for the existence of traveling fronts and pulses in the case of a Heaviside step firing rate, and show that adaptation plays a relatively minor role in determining the characteristics of traveling waves. We then derive conditions for the existence and stability of stationary pulses or bumps, and show that a purely excitatory network with synaptic depression cannot support stable bumps. However, bumps do not exist in the presence of adaptation. Finally, in the case of a piecewise linear firing rate function, we show numerically that the network also supports self-sustained oscillations between an Up state and a Down state, in which a spatially localized oscillating core periodically emits pulses at each cycle.