Juhua Xu

Preparation of a novel stable nitroxide based on [60]fulleropyrrolidine and its magnetic properties

A novel stable nitroxide based on C60 integrated into special structures was synthesized through the oxidization of [60]fulleropyrrolidine with m-chloroperbenzoic acid. The magnetic properties of the nitroxide were measured by ESR and a SQUID-magnetometer.

The First Crown Ether-Bearing [60]Fullero-Pyrrolidine: Synthesis and Regulation of Absorption Spectrum by ions Binding

Abstract A monoadduct of [60]Fulleropyrrolidine having a 15-crown-5 structure has been prepared and characterised. It is found that the absorption spectrum of this crowned fullerene can be regulated hypsochromically by metal ions complexation.

Effect of growth interruption on the optical properties of InAs/GaAs quantum dots

The effect of growth interruption (GI) on the optical properties of InAs/GaAs quantum dots was investigated by cw and time-resolved photoluminescence (PL). It is found that this effect depends very much on the growth conditions, in particular, the growth rate. In the case of low growth rate, we have found that the GI may introduce either red-shift or blue-shift in PL with increase of the interruption time, depending on the InAs thickness. The observed red shift in our 1.7 monolayer (ML) sample is attributed to the evolution of the InAs islands during the growth interruption. While the blue-shift in the 3 ML sample is suggested to be mainly caused by the strain effect. In addition, nearly zero shift was observed for the sample with thickness around 2.5 ML.

An unexpected reaction: [2 +1] cycloaddition of [60]fullerene with 4, 4, 5, 5-tetramethylimidazolidine-2-thione and DL-valine

C60 reacted with DL-valine and 4, 4, 5, 5-tetramethylimidazolidine-2-thione in a new reaction to give a new C60 spiro-compound 3 which was characterized by FT-IR, NMR, and FD-mass spectra. The possible reaction mechanism was put forward.

Synthesis and characterization of stable nitroxides based on fullerenes (C60, C70) and their magnetic study

Abstract C 60 or C 70 reacts with 2-aminoisobutyric acid and cyclohexanone in chlorobenzene at refluxing through 1,3-dipolar cycloaddition to give [60]fulleropyrrolidine 1 or [70]fulleropyrrolidine 3 which can be oxidized with MCPBA at room temperature to yield stable nitroxides 2 or 4 . The compounds 1 and 3 were characterized by mass spectrum, FT-i.r., n.m.r. spectra. The compounds 2 and 4 were characterized by mass spectrum, FT-i.r., e.s.r. spectra. The magnetic properties of compounds 2 and 4 were measured by SQUID-magnetometer (MPMC Quantum Design), we found that they were paramagnetic.

Optical gain in GaAs/GaAlAs graded-index separate-confinement single-quantum-well heterostructures

Optical model gain in both the TE and TM polarizations of graded-index separate-confinement single-quantum-well heterostructure lasers measured at various levels of injection current on samples with different quantum-well widths is discussed. Lasers with wide quantum wells (>or=120 AA) have emission and gain spectra which exhibit two peaks, caused by the n=1 and n=2 subband transitions. With ordinary cavity parameters, the saturation gain of the n=1 subband transitions is lower than the cavity loss of the laser, and the lasers always lase at the n=2 transitions. Reducing the quantum-well width increases the saturation gain of the n=1 transitions enough to allow lasing from them, even in cases of higher cavity loss. Further, for a fixed cavity loss, reduction of the quantum-well width decreases the threshold current density for n=1 lasing transitions, while that for n=2 lasing increases. The superlinear increase of the material gain with the decrease of the well width reduces the minimum cavity length for n=1 subband lasing. Narrower quantum wells with higher mirror reflectivity allow shorter cavity lengths while retaining n=1 lasing, resulting in low threshold current. >

Covalently linked [60,70] fullerenes–nitroxide unit for synthesis and characterization

Oxidization of those [60] and [70] fulleropyrolidine obtained from the reaction of C60 and C70 with diketone and 2-aminoisobutyric acid by m-chloroperoxybenzoic acid to give two new nitroxides integrated into C60 and C70. The magnetic property of the nitroxide based on C60 was measured by SQUID.

A novel reaction: [60,70]fullerene reacting with 4,4,5,5-tetramethylimidazoline-2-thione and α-aminoacids as carbene reaction

The results of characterization had shown that it was [1+2] cycloaddition reaction when [60,70]fullerene reacted with 4,4,5,5-tetramethylidazoline-2-thione (1) and dl-valine by FT-IR, FD-mass, 1H NMR, 13C NMR, UV–Vis spectra. [60]fullerene reacting with 4,4,5,5-tetramethylidazoline-2-thione (1) and other α-aminoacids such as l-leucine and α-aminoisobutyric acid had been also done in order to understand the reaction better, and the possible mechanism was put forward based on the results of characterization. In this reaction, reaction temperature was very important due to the reactivity of different α-aminoacids and the result of the experiment had also shown that the reactivity of dl-valine was more reactive than that of other two α-aminoacids.

A comparative study of ferromagnetic transition temperatures for some perovskite manganates

Three series of bulk samples, La-2/3(Ca1-zSrz)(1/3)MnO3, (La1-ySmy)(1/3)Nd-1/3 Sr1/3MnO3 and La1/3Nd1/3(Ca1-xSrx)(1/3)MnO3, were carefully prepared. Paramagnetic insulator to ferromagnetic metal transitions were observed for all of the samples. Their T-c-(r(A)) relation fell onto three distinct plots in the phase diagram, the sequence of which, from high to low values of T-c, is consistent with the increasing occupancy of A site magnetic rare earth ions. Close investigation into the data revealed that the large variance of T-c with common (r(A)) could not be explained by the present understanding. In view of the fact that T-c is essentially related to A site magnetic rare earth contents, and the recent finding that the Mn-Re coupling might contribute to the overall magnetic structure in the perovskite manganates, we propose a site-percolation model in which the Mn-Re interaction could influence the ferromagnetic coupling between Mn ions. Before the content of A site magnetic Re3+ reaches the critical value of 0.311 for a simple cubic lattice, the effect could be negligible; otherwise, the effect could be significant, manifesting itself by the lowering of T-c, which would be enhanced by more A site magnetic Re3+ occupancy. Such description agrees well with our current study and some other previously reported ones.

Magnetic properties and structure of the intermetallic compounds La-2(Co1-xFex)(16)Ti

The effect of substitution of Fe for Co in the compounds La-2(Co1-xFex)(16)Ti (x = 0.0-0.5) on the crystal structure and magnetic properties has been investigated. The lattice parameters and the unit-cell volume of La-2(Co1-xFex)(16)Ti increase with x in the range 0 less than or equal to x less than or equal to 0.44. The magnetic moment per formula unit in La-2(Co1-xFex)(16)Ti increases linearly as the Fe concentration increases. The Curie temperature is found to increase for x less than or equal to 0.1 and then decrease for x > 0.1. The easy-magnetization direction changes from the easy-axis direction (0 less than or equal to x less than or equal to 0.3) to the easy-plane direction (x = 0.4), and the anisotropy field H-A decreases with the Fe substitution. Evidence for spin reorientation in the compounds La-2(Co1-xFex)(16)Ti for 0.3 less than or equal to x less than or equal to 0.44 was observed in the M-T and chi(ac)-T curves.