Yun-Hui Huang

Large enhancement in room-temperature magnetoresistance and dramatic decrease in resistivity in La0.7Ca0.3MnO3–Ag composites

A ferromagnet-metal-type composite, La0.7Ca0.3MnO3 (LCMO)–Ag, was fabricated by a two-step chemical route which can avoid the doping of Ag into the lattice of LCMO. The grain size of Ag can be reduced by increasing calcination temperature, which favors the penetration of Ag into LCMO matrices. A large enhancement in magnetoresistance (MR) near room temperature and a dramatic decrease in resistivity are observed for the samples calcined at above the melting temperature of Ag. We suggest that the shift of metal–insulator transition up to Curie temperature in melted-Ag-added LCMO and magnetic inhomogeneity are responsible for the enhanced MR.

Soft chemical synthesis and transport properties of La0.7Sr0.3MnO3 granular perovskites

Abstract The thermal decomposition of a precursor complex, using ethylenediaminetetraacetic acid (EDTA) as a complexing agent, was introduced to synthesize La 0.7 Sr 0.3 MnO 3 granular perovskites. The dependence of crystallization, grain size and surface morphology on calcination temperature was investigated. Spin-dependent interfacial tunneling magnetoresistance corresponding to microstructure was observed. Our results indicate that decreasing grain size leads to broadening of the ferromagnetic–paramagnetic transition and enhancement of the extrinsic magnetoresistance caused by grain boundaries. Further decrease in grain size will result in the disappearance of the metal–insulator transition and weakening of the magnetoresistance effect at room temperature.

Terahertz emission and detection both based on high-Tc superconductors: Towards an integrated receiver

We have combined a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa2Cu3O7 grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-Tc receiver.

Variation of structural, magnetic and transport properties in RE0.7Sr0.3MnO3 granular perovskites

Granular perovskite family RE0.7Sr0.3MnO3 (RE=rare earth except Ce and Sc) were prepared by thermal decomposition of complex precursors. The effects of A-site rare earth elements on structural symmetry, magnetic and transport properties were systematically investigated. While RE is varied from La to Lu, the perovskite structure is distorted from rhombohedral to orthorhombic and finally to hexagonal symmetry for small rare earths. Meanwhile, the magnetic measurement shows a variation corresponding to the phase transition. Carrier mobility and magnetoresistance also strongly depend on the rare earth elements. Our results reveal that the tolerance factor t plays an important role in the structural distortion and grain boundary effect for the granular compounds. In addition, the band-bending effects are suggested to explain the discrepancy in the transport properties of this family.

Synthesis of bismuth with various morphologies by electrodeposition

Abstract Metal bismuth with various morphologies, with particle size from nanometer to micrometer, has been successfully prepared by electrodeposition onto Pt, Au, Al and ITO electrodes at room temperature. The size and morphology of the deposits are strongly dependent on preparation conditions, such as deposition potential, current density, electrode and electrolyte. As observed by scanning and transmission electron microscopes, the deposited Bi particles exhibit plentiful appearances, such as prickly rod, banch, skeleton and strip-like shapes. A significant positive magnetoresistence effect is observed even at room temperature.

Enhanced magnetoresistance in granular La2/3Ca1/3MnO3/ polymer composites

Polymer embedded granular composites, La2/3Ca1/3MnO3/polyparaphenylene (LCMO)1−x(PPP)x (x is the weight fraction of PPP), were prepared and their magnetoresistance (MR) properties were investigated. A significant enhancement in MR is observed for the composites especially in the low temperature range. The MR reaches the maximum at x=0.1 when T=5 K and at x=0.2 when T=260 K. We argue that such enhancement in MR is attributed to the enhanced spin-polarized tunneling, which is manipulated by the spin disorder at the LCMO surfaces caused by PPP.

Enhanced ferromagnetic transition and magnetoresistance in granular Ag-added La0.7Ca0.3MnO3

Granular Ag-added La0.7Ca0.3MnO3 (LCMO) samples were prepared by a sol-gel chemical route. Significant enhancements in Curie temperature (T-C), metal-insulator transition (T-p) and magnetoresistance (MR) effects near room temperature arc observed in as-obtained samples. 10 wt% addition of Ag in LCMO causes T-C shift from 272 to 290 K T-p boost up for more than 100 K and resistivity decrease by more than 3 orders of magnitude. X-ray diffraction patterns, thermal analysis and energy dispersive analysis of X-rays evidently show the existence of metal silver in LCMO matrices. High-resolution electron microscopy illustrates a well crystallization for LCMO grains in existence of Ag. It is argued that improved grain boundary effect and better crystallization caused by Ag addition are responsible for the enhancements

Improvement of magnetoresistance over a wide temperature range in La2/3Sr1/3MnO3/polymer composites

Polymer-embedded La2/3Sr1/3MnO3 (LSMO) composites, (LSMO)1 −x(PPP)x (PPP is polyparaphenylene, and x is the weight fraction of PPP), were prepared by mixing pre-prepared LSMO and PPP powders. Thermogravimetric analysis, x-ray diffraction, and infrared spectra show that the composites are stable when calcined at 400oC. Significant enhancement in the magnetoresistance (MR) effect over a wide temperature range is observed for the composites compared with the parent LSMO. The MR enhancement reaches its maximum at x = 0.2. The magnetotransport is mainly attributed to intergrain spin-polarized tunnelling. We argue that the introduction of PPP gives rise to magnetic disorder and hence an enhanced tunnelling effect, which is responsible for the MR enhancement.

Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks with improved cooling: Coherent emission above 1 THz

We report on Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43–0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other “hot” devices, e.g., quantum cascade lasers.

Photocurrent characteristics of two-dimensional-electron-gas-based AlGaN/GaN metal-semiconductor-metal photodetectors

Photocurrent response characteristics of two-dimensional-electron-gas-based AlGaN/GaN metal-semiconductor-metal (MSM) photodetectors were investigated by changing the frequency of incident light signals. At low chopper frequencies, the contribution to the photocurrent is mainly from the GaN bulk layer at 5 V bias, as expected. With increasing chopper frequency, a peak response at 361 nm wavelength becomes more and more pronounced and dominates gradually the photocurrent spectrum, indicating that this peak response has a much less frequency dependence and a faster response rate than the spectral response from the GaN bulk layer. This peak response is attributed to the contribution of photoelectrons generated in the two-dimensional electron gas (2DEG) channel according to simulation results of electrical field distribution and analysis of carrier transport in the 2DEG-based AlGaN/GaN heterostructure. This characteristic makes the 2DEG-based MSM photodetectors a large potential to develop high-speed ultravio...