L. Z. Zhao

Thickness and dielectric constant of dead layer in Pt/(Ba0.7Sr0.3)TiO3/YBa2Cu3O7−x capacitor

Pt/(Ba0.7Sr0.3)TiO3 (BST)/YBa2Cu3O7−x capacitors were prepared and investigated for the dead-layer (DL) thickness (td) and the DL dielectric constant (ed). Based on the series capacitor model, the td/ed ratio of 0.066 nm and the bulk BST ferroelectric-layer dielectric constant of 1370 were obtained through the measurements of the capacitance–voltage characteristics. The td×ed value of 120 nm was obtained through the measurements of the current–voltage characteristics. Combining these data, the DL thickness and the DL dielectric constant are respectively estimated to be 2.8 nm and 42.6.

Leakage current of Pt∕(Ba0.7Sr0.3)TiO3 interface with dead layer

Leakage current of Pt∕(Ba0.7Sr0.3)TiO3 (BST)∕YBa2Cu3O7−δ capacitors on a (001) SrTiO3 substrate was studied. By modeling a low-dielectric constant layer, a so-called dead layer, between the Pt∕BST interface as a parasitic capacitor in series with the bulk layer capacitor, the leakage current of Pt∕BST interface was well analyzed based on the modified Schottky emission equation. Furthermore, a two-step schematic energy band diagram is proposed to explain the carrier transport through the Pt∕BST interface.

Giant enhancement of second harmonic generation in multiple photonic quantum well structures made of nonlinear material

We present a general solution of second harmonic generation (SHG) in one-dimensional (1D) inhomogeneous systems. The structure consists of 1D multiple photonic quantum wells (PQWs) made of optical nonlinear material. The optimal arrangement of the polarization directions of the ferroelectric domains in multiple PQWs is determined by a simulated annealing method. We find that the conversion efficiency of SHG can be significantly enhanced when the fundamental wave frequency is aimed at one of the defect states. By tuning the polarization directions of the domains, the conversion efficiencies of “forward” and “backward” SHGs can be flexibly changed.

Electron-doped superconducting (La,Ce)2CuO4 thin films grown by dc magnetron sputtering and their transport properties

Electron-doped La2?xCexCuO4 (LCCO) thin films were successfully prepared on (100) SrTiO3 substrates by the dc magnetron sputtering method. The optimal-doped films show a highly c-axis oriented single -type structure, and the zero resistance temperature TC0 is 25?K. In the normal state, the nearly quadratic temperature dependence of the resistivity was found and attributed to the Landau?Fermi liquid behaviour due to electron?electron scattering. At the same time, the negative sign of the Hall coefficient for such films well above TC obviously confirms the electron-type nature. The optimal conditions for the growth of single-phase -type LCCO thin films are also discussed in detail.

Evaluation of flow units and free volumes in metallic glasses

We investigate the changes of the fractions of flow units and free volumes in two typical metallic glasses by variation of their preparation cooling rates. It is found that the fractions of the flow units and the free volumes show a similar dependence on the cooling rate, which is suggestive of the existence of a strong connection between them. The microstructural correlation between flow units and free volumes of the metallic glasses is discussed, which could be helpful for understanding the formation and structural features of the flow units as well as their effects on mechanical properties and relaxation behaviors in metallic glasses.

Revealing localized plastic flow in apparent elastic region before yielding in metallic glasses

The microscopically localized flow before yielding in metallic glass (MG) is hard to be detected using conventional mechanical tests due to its extremely slow process with a long time scale and lack of structural information. We present a stress relaxation approach to circumvent the experimental problem and to reveal the evolution of the flow before yielding in MG by detecting the evolution of structural heterogeneity and activation energy barrier spectra of flow units. We report on explicit correlations among dynamical heterogeneities, nanoscale flow units, and yielding, revealing a transition from the microscopically flow to macroscopic yielding in MG.

Quantitative determination of Eliashberg function and evidence of strong electron coupling with multiple phonon modes in heavily overdoped (Bi,Pb)(2)Sr2CuO6+delta

It is known that the parity of a reflection amplitude can either be even or odd under a mirror operation. Up to now to our knowledge, all the parities of a reflection amplitude in the one-mode energy region have been even under a mirror operation. In this paper, we give an example of odd parity for Andreev reflection (AR) in a three-terminal graphene-supercondutor hybrid system. We found that the parity is even for the Andreev retroreflection and odd for specular Andreev reflection (SAR). We attribute this remarkable phenomenon to the distinct topology of the band structure of graphene and the specular Andreev reflection involving two energy bands with different parity symmetry. As a result of the odd parity of SAR, the SAR probability of a four-terminal system with two superconducting leads (two reflection interfaces) can be 0 even when the system is asymmetric due to the quantum interference of two ARs.

Transport properties of electron-doped La 2 − x Ce x CuO 4 cuprate thin films

Electron-doped La2-xCexCuO4 (x=0.06-0.20) thin films with two orientations (100) and (103) were successfully grown and their transport properties were investigated. It is found that the temperature dependence of the in-plane resistivity rho(ab) evolves from semiconducting behavior for underdoped films to two-dimensional Fermi liquid behavior for optimally and overdoped ones, while the c-axis resistivity rho(c) shows nonmetallic behavior except for heavily overdoped films. The doping-dependent anisotropy factor rho(c)/rho(ab) is found to vary from 60 to 1500 at 50 K, and the largest anisotropy value related to the highest T-C is obtained at the optimal doping (x=0.10).

Evolution of structural and dynamic heterogeneities during elastic to plastic transition in metallic glass

We investigate the evolution of microscopically localized flow under a constant applied strain in apparent elastic region of a prototypical metallic glass (MG). The distribution and evolution of energy barriers and relaxation time spectra of the activated flow units in MG with time are obtained via activation-relaxation method. The results show that the unstable nano-scale liquid-like regions acting as flow units in the glass can be activated by external stress, and their evolution with time shows a crossover from localized activation to cascade as the proportion of the flow units reaches a critical percolation value. The flow unit evolution leads to a mechanical elastic-to-plastic transition or macroscopic plastic flow. A plausible diagram involved in time, stress, and temperature is established to understand the deformations and the flow mechanisms of MGs and could provide insights on the intriguing dilemmas of glassy nature, the flow units, and their correlations with the deformation behaviors in MGs.

Microstructural heterogeneity perspective on the yield strength of metallic glasses

We report an intrinsic universal correlation between yielding and dynamic activation of flow units in various metallic glasses, analogous to yielding in crystalline materials accompanied by activation of structural defects of dislocation. A transition point of yield strength when test temperature reaches a critical value is observed, and the evolution of the yield strength corresponds well to the evolution of activation energy and fraction of flow units with temperature, indicating the correlation between yield strength and intrinsic structural heterogeneity in glasses. We propose a model based on the flow units to understand the structural origin of yielding phenomenon and the nature of the yield strength.