Jingwei Wang

Nonvolatile resistive switching in graphene oxide thin films

Reliable and reproducible resistive switching behaviors were observed in graphene oxide (GO) thin films prepared by the vacuum filtration method. The Cu/GO/Pt structure showed an on/off ratio of about 20, a retention time of more than 104u2002s, and switching threshold voltages of less than 1 V. The switching effect could be understood by considering the desorption/absorption of oxygen-related groups on the GO sheets as well as the diffusion of the top electrodes. Our experiments indicate that GO is potentially useful for future nonvolatile memory applications.

Magnetic field-induced entropy change in phase-separated manganites

Magnetic field-induced entropy change has been experimentally studied for phase-separated manganites Eu0.55Sr0.45MnO3 and La0.27Nd0.40Ca0.33 MnO3. The entropy derived from the measured heat capacity exhibits a significant decrease under applied field in a broad temperature range below Curie temperature, and the maximum change is ∼8J∕kgK for Eu0.55Sr0.45MnO3 and is ∼4.2J∕kgK for La0.27Nd0.40Ca0.33MnO3 for a field change of 0–5T. In comparison with the calorimetric technique, Maxwell relation fails to give a proper description for the entropy change. It underestimates the entropy reduction in the low temperature range and even anticipates an entropy increase for Eu0.55Sr0.45MnO3. Failures of the Maxwell relation could be ascribed to the coexistence of two phases in the compound and to the variation of the proportion of each phase with applied field.

Field-induced entropy change in the manganite with significant short-range magnetic order

Effects of short-range magnetic order on magnetic entropy change have been studied for the manganese oxide Eu0.55Sr0.45MnO3. Superparamagnetic clusters composed of seven to ten Mn ions, depending on applied field, have been derived in the paramagnetic state of the compound based on the analysis of dc susceptibility. The presence of short-range magnetic order greatly depresses the magnetic entropy of the paramagnetic phase; thus the entropy changes at the field-induced paramagnetic to ferromagnetic phase transition. The maximum entropy change detected is only ∼7J∕kgK for a field change of 0–5T, about one-fifth of the theoretical expectation. The experimental results can be well elucidated within the mean field theory. It is suggested that a way of destroying the short-range order would enhance the magnetic entropy change greatly.

Operational region and sawteeth oscillation in the EAST tokamak

The first plasma discharges were successfully achieved on the experimental advanced superconducting tokamak (EAST) in 2006. The sawteeth behaviours were observed by means of soft x-ray diagnostics and ECE signals in the EAST. The displacement and radius of the q = 1 surface was studied and compared with the result of equilibrium calculation. The density sawtooth oscillation was also observed by the HCN laser interferometer diagnostics. The structure of the EAST operational region was studied in detail. Plasma performance was obviously improved by the boronization and wall conditioning. It was observed that lower qa and a wider stable operating region is extended by the GDC boronization.

A comparison study of the entropy changes in materials with and without short-range magnetic order

The effects of short-range magnetic order (SRMO) on magnetic entropy changes have been studied for the compounds Sm0.55Sr0.45MnO3, Eu0.55Sr0.45MnO3 and the rare-earth metal Gd. Ferromagnetic clusters composed of ~8 Mn ions are found to be formed in the paramagnetic phase of the manganites based on the analysis of susceptibility. As a result, the entropy change for a field variation of 0?5?T is ~6?J?kg?1?K?1 for Sm0.55Sr0.45MnO3 and ~7?J?kg?1?K?1 for Eu0.55Sr0.45MnO3, well below the values expected without considering the effect of SRMO. (The maximum entropy change will be ~56?J?kg?1?K?1 for both compounds.) Assuming the presence of sizable magnetic clusters, the field-induced entropy change, not only its peak value but also its temperature dependence, can be well reproduced by the mean field theory. The average cluster size is found to be m?8, in agreement with the result obtained from a Curie?Weiss analysis of the susceptibility. The presence of SRMO reduces the entropy change by ~80%. In contrast, the mean field theory gives a good description of the entropy change in Gd, for which no significant SRMO is observed. The present work reveals the important role of SRMO in modifying the magnetocaloric properties of the magnetic materials.

Magnetic, thermal and transport properties of phase-separated La0.27Nd0.4Ca0.33MnO3

The magnetic, resistive and thermal properties of the phase-separated compound La0.27Nd0.4Ca0.33MnO3 have been experimentally studied. The sample is found to experience a charge/orbital ordering transition at ∼175 K and an antiferromagnetic (AFM) transition at ∼156 K without a magnetic field. A magnetic field stabilizes the ferromagnetic (FM) order and the field-induced FM phase coexists with the AFM phase under a field below 2 T in the temperature range below TC ≈ 110 K. However, the magnetic entropy change accompanying the AFM–FM transition is negligibly small when TC TN. A general relation between resistivity and magnetization, ρ = A0T exp[(e − 800m 2 )/T ] (m = normalized magnetization), is established for the paramagnetic phase, which is also applicable to other compounds with different characters, such as the La0.67Ca0.33MnO3 film and the La0.474Bi0.193Ca0.33Mn0.994Cr0.006O3 and Eu0.55Sr0.45MnO3 ceramics. In the FM state, the resistivity is quite sensitive to the change of spin alignment and exhibits an exponential decrease with magnetization ρ = ρ0 exp(−22.7m). Field-induced phase separation is believed to be responsible for the distinct properties of La0.27Nd0.4Ca0.33MnO3. (Some figures in this article are in colour only in the electronic version)

Electronic transport properties of charge-ordered Bi0.4Ca0.6MnO3 film

The electronic transport properties of charge-ordered Bi0.4Ca0.6MnO3 films grown on a (110) SrTiO3 substrate are experimentally studied. Special attention has been paid to the Hall effect around the charge-ordering (CO) transition. The charge carriers are found to be electron-like, and the carrier density n exhibits a significant change upon the CO transition: it is nearly constant above the transition temperature Tco, ~0.36xa0electrons/Mn, and reduces with decrease of the temperature below Tco following the formula , with an activation energy EH of ~0.13xa0eV. In contrast, no obvious signatures of thermal activation for Hall mobility were observed. Meanwhile, it is revealed that magnetic field affects the resistivity by enhancing the carrier mobility of the film in the course of the CO transition.

Femtosecond laser filament induced condensation and precipitation in a cloud chamber

A unified picture of femtosecond laser induced precipitation in a cloud chamber is proposed. Among the three principal consequences of filamentation from the point of view of thermodynamics, namely, generation of chemicals, shock waves and thermal air flow motion (due to convection), the last one turns out to be the principal cause. Much of the filament induced chemicals would stick onto the existing background CCN’s (Cloud Condensation Nuclei) through collision making the latter more active. Strong mixing of air having a large temperature gradient would result in supersaturation in which the background CCN’s would grow efficiently into water/ice/snow. This conclusion was supported by two independent experiments using pure heating or a fan to imitate the laser-induced thermal effect or the strong air flow motion, respectively. Without the assistance of any shock wave and chemical CCN’s arising from laser filament, condensation and precipitation occurred. Meanwhile we believe that latent heat release during condensation /precipitation would enhance the air flow for mixing.

A universal relation between resistivity and magnetization in paramagnetic state of manganites

A systematic study on the resistivity-magnetization relation above the Curie temperature (TC) has been performed for the manganites with different properties, such as the epitaxial La0.67Ca0.33MnO3 film on a SrTiO3 substrate, which has the weakest magnetic correlation above TC, the La0.27Nd0.4Ca0.33MnO3 polycrystalline, which exhibits a charge ordering in the paramagnetic state, and the ceramics Eu0.55Sr0.45MnO3 and La0.474Bi0.193Ca0.33Mn0.994Cr0.006O3, which show strong short-range magnetic orders above TC. A quantitative, yet universal, relation of the form ρ=ATexp[(e-αm2)∕T] is observed in all of the manganites, with an essentially constant coefficient of α≈800±60K, where m is the normalized magnetization and e is the activation energy for polarons. The underlying physics for this relation has been analyzed based on the double exchange model.

Trapping of electromagnetic radiation in self-generated and preformed cavities

Laser light trapping in cavities in near-critical density plasmas is studied by two-dimensional particle-in-cell simulation. The laser ponderomotive force can create in the plasma a vacuum cavity bounded by a thin overcritical-density wall. The laser light is self-consistently trapped as a half-cycle electromagnetic wave in the form of an oscillon-caviton structure until it is slowly depleted through interaction with the cavity wall. When the near-critical density plasma contains a preformed cavity, laser light can become a standing wave in the latter. The trapped light is characterized as multi-peak structure. The overdense plasma wall around the self-generated and preformed cavities induced by the laser ponderomotive force is found to be crucial for pulse trapping. Once this wall forms, the trapped pulse can hardly penetrate.