Xuebang Wu

Growth of thicker zinc-blende CrSb layers by using (In,Ga)As buffer layers

Zinc-blende CrSb (zb-CrSb) layers with room-temperature ferromagnetism have been grown on (In,Ga)As buffer layers epitaxially prepared on (001) GaAs substrates by molecular-beam epitaxy. Compared with the typical thickness [2-3 ML (ML denotes monolayers)] of zb-CrSb grown directly on GaAs, the thickness of zb-CrSb grown on (In,Ga)As has been increased largely; the maximum can be up to similar to 9 ML. High-resolution cross sectional transmission electron microscopy images show that the zb-CrSb layer is combined with (In,Ga)As buffer layer without any dislocations at the interface

Slow dynamics of the α and α' relaxation processes in poly(methyl methacrylate) through the glass transition studied by mechanical spectroscopy

In this paper, low-frequency mechanical spectroscopy has been used to study poly(methyl methacrylate) (PMMA) melt around the glass transition temperature Tg for shedding light on its unique relaxation behaviors. The mechanical spectra show an asymmetrical broad structure with a maximum peak on the high-temperature side and a shoulder peak on the low-temperature side. The shoulder peak corresponds to the α relaxation due to the local segmental motion; while the maximum peak is assigned to the α′ relaxation due to the slow motion of longer chain segments or chains. The α relaxation mode has a stronger temperature dependence of relaxation time than the α′ mode, causing the α′ mode to merge with the α mode with decreasing temperature toward Tg. Time-temperature superposition (TTS) breaks down in the entire temperature range due to the different friction coefficients of the α and α′ relaxations. For only the α′ process, TTS is found to hold for PMMA melt, where the high-frequency decay of the mechanical loss d...

Longer-scale segmental dynamics of amorphous poly(ethylene oxide)/poly(vinyl acetate) blends in the softening dispersion

To study the influence of poly(ethylene oxide) (PEO) on the relaxation process of poly(vinyl acetate) (PVAc) in the glass to rubber softening dispersion, the mechanical loss behaviour of PEO/PVAc blends (with PEO contents up to 20 wt%) were measured as a function of temperature and frequency. It is shown that the PEO component has a plasticizing effect on the α and α′ relaxation modes of PVAc, where the α mode is ascribed to the local segmental mode and the α′ mode is related to the motion of longer chain segments in the softening dispersion, composed of the sub-Rouse modes and the Rouse modes. Both the α and α′ modes shifted to lower temperatures with increase of PEO concentration. Time–temperature superposition breaks down for PEO/PVAc blends in the entire temperature range due to the different friction coefficients of the α and α′ modes. A single master curve of the α′ mode for pure PVAc and the PEO/PVAc blends is obtained, suggesting that the size of chain segments containing on the order of 10 to 50 or more backbone bonds does not change upon blending. Furthermore, the dynamics of the α′ mode of the PVAc component in the blends is found to show a characteristic crossover through the temperature dependence of relaxation time and relaxation strength. The crossover temperature TB decreases with increasing PEO content, while the crossover relaxation time for the blends is constant with a value about 0.08 s, much longer than 10−7 ± 1 s for the α mode. According to the coupling model, the crossover is suggested to be due to the variation of intermolecular coupling at TB.

Mechanical relaxation studies of α and slow β processes in Nd65Fe15Co10Al10 bulk metallic glass

The relaxation dynamics of the primary α and secondary β processes in Nd65Fe15Co10Al10 bulk metallic glass has been investigated by using low-frequency mechanical spectroscopy, differential scanning calorimetry (DSC) and x-ray diffraction. From the shift of internal friction peaks by the frequency change, the activation energy of β relaxation (Eβ) is found to be about 1.01 eV, and a correlation between Eβ and glass transition temperature (Tg) is found Eβ ≈ 24 RTg, indicating that the β relaxation is intrinsic in metallic glasses. According to the coupling model, the uncoupled activation energy of α relaxation (Eα*) is found to be about 1.38 eV, still larger than the value of Eβ corresponding to a simple Debye relaxation process. The possible mechanism and the connection between α and β relaxations are discussed.

Revisit to phase diagram of poly(N-isopropylacrylamide) microgel suspensions by mechanical spectroscopy

Microgels are soft particles that can be deformed and compressed, which would induce intriguing phase behaviors at high packing fractions. Poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33 °C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature. In this work, the linear viscoelastic properties of PNIPAM microgel suspensions have been investigated using mechanical spectroscopy. A particular attention is focused on the phase behaviors at high concentrations. With increasing concentration the system undergoes a repulsive glass-to-gel transition below the LCST, while, as temperature is raised across the LCST, the system undergoes a gel-to-attractive glass transition. A mechanism of these transitions for the microgels is proposed based on the directional interaction between the particles. In moderate concentration or de-swelling microgels the interaction is isotropic leading to the glass phase, while in concentrated and deformed microgels the interaction is directional leading to the gel phase. Our results enrich the current understanding of the phase transition in microgel systems and shed new light on the phase diagram of colloidal suspensions in general.

Room-temperature ferromagnetism in zinc-blende and deformed CrAs thin films

We try to clarify the controversy about the origin of room-temperature ferromagnetism in a CrAs compound. Two kinds of CrAs thin films were grown on GaAs by molecular-beam epitaxy. Structural analyses confirm that the as-grown CrAs film is a pure zinc-blende phase. Magnetic measurements suggest that room-temperature ferromagnetism exists in zinc-blende CrAs. In contrast, the CrAs film turns into a mixture of zinc-blende and deformed CrAs after annealing. A ferromagnetic signal measured at room temperature demonstrates that zinc-blende CrAs remains room-temperature ferromagnetism even when it is partly deformed into a non-zinc-blende phase.

Low-frequency mechanical spectroscopy study of conformational transition of polymer chains in concentrated solutions

A low-frequency mechanical spectroscopy approach for liquids was proposed for studying conformational transition of polymer chains in concentrated solutions. The technique is applied to aqueous solutions of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer in order to determine if chain conformation is altered in response to temperature. Two transitions are revealed by mechanical spectra and verified by differential thermal analysis with increasing temperature, which may be related to the unimer-to-micelle transition and the phase separation, respectively. The transitions are also found to be much dependent on the concentration of the solution and the addition of NaCl. Moreover, it reveals that the PEO blocks play a more important role in the micellar crystallization process. This study may be helpful in understanding the dynamics of polymer chains in concentrated solutions.

Effects of polyethylene oxide on the dynamics of the α and α′ relaxations observed in polystyrene by low-frequency anelastic spectroscopy

The relaxation dynamics in amorphous polystyrene and blends with polyethylene oxide (PEO) has been investigated by low-frequency anelastic spectroscopy. The dissipation spectra exhibit two relaxation peaks, corresponding to the α relaxation and the α′ (liquid-liquid) transition, respectively. The origin of the liquid-liquid transition is the inhomogeneity in the melt due to the chain clustering. With increasing PEO content, the peak temperature Tα′ and the relaxation strength Δα′ decrease, while Tα decreases and Δα increases. Moreover, the fractional free volume sharply increases above Tα′ for all the samples, due to the bigger value of thermal expansion coefficient αf.

Interplay between s-d exchange interaction and Rashba effect: Spin-polarized transport

The authors investigate the spin-polarized transport properties of a two-dimensional electron gas in a n-type diluted magnetic narrow gap semiconductor quantum well subjected to perpendicular magnetic and electric fields. Interesting beating patterns in the magnetoresistance are found which can be tuned significantly by varying the electric field. A resonant enhancement of spin-polarized current is found which is induced by the competition between the s-d exchange interaction and the Rashba effect [Y. A. Bychkov and E. I. Rashba, J. Phys. C 17, 6039 (1984)]. (c) 2006 American Institute of Physics.

A convenient way of determining the ferromagnetic transition temperature of metallic (Ga,Mn)As

We have studied magnetic and transport properties of insulating and metallic (Ga,Mn)As layers before and after annealing. A dramatic increase of the ferromagnetic transition temperature T-C by postgrowth annealing has been realized in both insulating and metallic (Ga,Mn)As. The as-grown insulating (Ga,Mn)As can be turned into metallic by the low-temperature annealing. For all the metallic (Ga,Mn)As, a characteristic feature in the temperature dependence of sheet resistance appears around T-C. This phenomenon may provide a simple and more convenient method to determine the T-C of metallic (Ga,Mn)As compared with superconducting quantum interference device (SQUID) measurement. Moreover, the T-C of the metallic (Ga,Mn)As obtained by this way is in good agreement with that measured by a SQUID magnetometer