Y. W. Xie

Entropy changes due to the first-order phase transition in the Gd5SixGe4−x system

Entropy changes due to magnetostructrual phase transition in Gd5SixGe4−x intermetallics have been studied based on a systematic analysis of experiment data and mean-field theory calculations. It is found that the magnetic and lattice entropy changes have the same sign. Further analysis indicates that the main entropy change (∼60%–∼80%) comes from the field-induced change of the magnetic order, while the rest arise from the entropy difference of the two crystallographic modifications joined by the structural transition, probably due to the variation of the lattice vibration mode. The present work reveals the importance of lattice entropy for a system experiencing a first-order transition.

Reversible electroresistance at the Ag∕La0.67Sr0.33MnO3 interface

We report the observation of reversible electroresistance in a metallic film La0.67Sr0.33MnO3 (LSMO). The transport behavior of the Ag/LSMO system is found to be sensitive to external electric field, and a switching of the resistance between two definite resistive states can be induced by electric pulses of different polarity. The current-voltage relation, measured by tuning bias voltage, is significantly nonlinear, asymmetric against field polarity, and hysteretic in the field increase/decrease processes. This relation can be well described by a simple equation I=σV+kVn with n being a number between 1.8 and 2.4. It is found that the rough surface of the LSMO film favors the electroresistance effects.

Electronic transport of the manganite-based heterojunction with high carrier concentrations

The transport property of the manganite heterojunction La0.9Ca0.1MnO3+δ∕SrTiO3 (doped by 1wt%Nb) has been experimentally studied. The most important results of the present work are the discovery of the charge tunneling-dominated transport process, characterized by the appearance of the rectifying behaviors fairly described by the Newman equation I∝exp(αT)exp(βV) in a considerable temperature range (α and β are constants, and I and V are current and voltage, respectively). Significant modification of magnetic field to charge tunneling is also observed. It is believed that magnetic field depresses junction resistance by reducing depletion width of the junction.

Rectifying properties of magnetite-based Schottky diode and the effects of magnetic field

Rectifying properties, with and without magnetic field, of a high quality Fe3O4∕SrTiO3:Nb Schottky diode have been experimentally studied. The junction exhibits an excellent rectifying behavior both below and above the Verwey temperature (TV) of Fe3O4. Magnetic field has a weak but visible effect on the transport process of the junction, producing a negative magnetoresistance for T TV. Based on an analysis of the current-voltage characteristics, the spin polarization of Fe3O4 has been deduced. It is a strong function of temperature, varying between −78% and 18%.

Semiconducting ZnSnN2 thin films for Si/ZnSnN2 p-n junctions

ZnSnN2 is regarded as a promising photovoltaic absorber candidate due to earth-abundance, non-toxicity, and high absorption coefficient. However, it is still a great challenge to synthesize ZnSnN2 films with a low electron concentration, in order to promote the applications of ZnSnN2 as the core active layer in optoelectronic devices. In this work, polycrystalline and high resistance ZnSnN2 films were fabricated by magnetron sputtering technique, then semiconducting films were achieved after post-annealing, and finally Si/ZnSnN2 p-n junctions were constructed. The electron concentration and Hall mobility were enhanced from 2.77 × 1017 to 6.78 × 1017 cm−3 and from 0.37 to 2.07 cm2 V−1 s−1, corresponding to the annealing temperature from 200 to 350 °C. After annealing at 300 °C, the p-n junction exhibited the optimum rectifying characteristics, with a forward-to-reverse ratio over 103. The achievement of this ZnSnN2-based p-n junction makes an opening step forward to realize the practical application of the...

Magnetic field effects on the manganite junction with different electronic processes

A manganite junction with two distinguishable electronic processes has been fabricated and its rectifying properties are experimentally studied. The current-voltage characteristics of the junctions are found to be dominated by leakage current and thermal current under low and high bias voltages, respectively. The responses of these two processes to magnetic field are found to be different, and the magnetoresistance (MR) of the junction arises mainly from the modification of magnetic field to leakage current. Although the MR shows a monotonic decrease with bias voltage (V), the MR-V dependence is different for the two processes. An approximately linear, yet slow, decrease of MR with V is observed for the leakage process, while an exponential reduction for the thermal one. These results show that the electronic processes undergoing in the junction can be identified based on the analysis of the MR-V relations.

Interfacial potential in La1−xCaxMnO3∕SrTiO3:Nb junctions with different Ca contents

Manganite-based heterojunctions La1−xCaxMnO3∕SrTiO3:Nb (0.05wt%) with x=0.1, 0.2, 0.33, 0.65, 0.75, and 1 have been fabricated, and the effects of Ca content on the interfacial potential are experimentally studied. Rectifying behavior well described by the Shockley equation is observed, and the interfacial potential (VD) is obtained for all of the junctions based on an analysis of the current-voltage characteristics. The most remarkable result of the present work is the strong dependence of the interfacial potential on the carrier content of La1−xCaxMnO3 films: VD increases monotonously from ∼0.6to∼1.1V as x sweeps from 0.1 to 1. Influence on VD of the Fermi energy and Jahn–Teller effect in La1−xCaxMnO3 films are discussed.

Anisotropic conduction induced by current processing in the La0.8Ca0.2MnO3 film

Effects of current processing have been experimentally studied for the La0.8Ca0.2MnO3 film. An anisotropic conduction is observed when the film is processed by a current of the density of ∼2.4×105A∕cm2. Difference between the resistances in the direction of the processing current and the reverse direction can be as high as 10 000%. Different from the original film, which exhibits a metal-to-insulator transition at ∼214K, the sample becomes insulating below 300K under low voltage bias and undergoes a metallic transition at very different temperatures in the two directions when the voltage bias is high enough. These features remain after reordering the electrodes, which reveals the intrinsic origin of the anisotropy, instead of interfacial barrier due to current processing.

Effects of lattice strains on the interfacial potential in La0.67Ca0.33MnO3/SrTiO3:Nb heterojunctions

Oxide p-n heterojunctions composed of La0.67Ca0.33MnO3 (LCMO) films with different thickness and SrTiO3:Nb 0.1 wt % are fabricated and the effects of thickness on the interfacial potential are experimentally studied. Excellent rectifying behavior of the junctions well described by the Shockley equation is observed and the interfacial potential eVD is obtained for all of the junctions based on an analysis of the current-voltage characteristics. The remarkable result of the present work is the strong dependence of the interfacial potential on the thickness of LCMO films: eVD increases from 0.5 to 0.72 eV as the thickness increase from 3.6 to 33 nm. The strain in the LCMO film, which affects the carrier density through modulating the Jahn–Teller effect, is believed to be responsible for the observation.

Rectifying behaviours of heterojunctions composed of manganites with different resistive properties

Oxide heterojunctions composed of manganite films of different tolerance factors and SrTiO3 : Nb are fabricated and their rectifying properties are experimentally studied. The current-voltage characteristics of the junctions are found to be dominated by diffusion current at high temperatures and by tunnelling current at low temperatures. Further analyses on the rectifying behaviours of the junctions indicate the occurrence of a built-in voltage independent of manganites, despite the great resistivity change of the manganites and the metal-to-insulator transition above 350 K ( for La0.67Sr0.33MnO3) and similar to 87K ( for La0.29Pr0.38Ca0.33MnO3). In contrast, the increase in the Nb content causes a reduction of the built-in voltage by similar to 0.2 eV. The presence of the depletion layer insensitive to the phase transition of the manganites is believed to be responsible for the present observations.