Al x Ga 1-x N/GaN high-electron-mobility transistor (HEMT) structures with Al composition ranging from x = 0.13 to 0.36 are grown on sapphire substrates by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The effects of Al content on crystal quality, surface morphology, optical and electrical characteristics of the AlGaN/GaN heterostructures have been analyzed. Although high Al-content (36%) heterostructure exhibits a distinguished photoluminescence peak related to recombination between the two-dimensional electron gas and photoexcited holes (2DEG-h), its crystal quality and rough surface morphology are poor. 2DEG mobility increases with the Al content up to 26% and then it apparently decreases for high Al-content (36%) AlGaN/GaN heterostructure. The increase of sheet carrier density with the increase of Al content has been observed. A high mobility at room temperature of 2105 cm 2 /V s with a sheet carrier density of n s = 1.10 × 10 13 cm -2 , for a 26% Al-content AlGaN/GaN heterostructure has been obtained, which is approaching state-of-the-art for HEMT grown on SiC. Sheet resistance as low as 274 Ω/□ has also been achieved.
DING GuoJian , GUO LiWei, XING ZhiGang, CHEN Yao, XU PeiQiang, JIA HaiQiang, ZHOU JunMing & CHEN Hong Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
This paper studies a spin ladder model which possesses frustrating interactions.By using both the bosonization and the density matrix renormalization group techniques,it shows that the intermediate columnar dimerized phase,which exists in a narrow parameter region of the so-called J 1 J 2 model,vanishes if the interchain frustration is weak and anisotropic.Therefore,it concludes that the frustrating interaction indeed plays an important role in producing such a phase.As a complementary to our previous investigation,it reaches a more complete picture of the quantum phase transition in the frustrated spin ladder systems.
We study the properties of the two-dimensional quantum compass model in a staggered field.Using the Perron-Frbenius theorem and the reflection positivity method,we rigorously determine the low energy spectrum of this model and its global ground state Ψ 0.Furthermore,we show that Ψ 0 has a directional long-range order.
The alloys of non-centrosymmetric superconductor, Re 3 W, which were reported to have an α-Mn structure [P. Greenfield and P. A. Beck, J. Metals, N. Y. 8, 265 (1959)] with T c = 9 K, are prepared by arc melting. The values of ac susceptibility and the low-temperature specific heat of these alloys are measured. It is found that there are two superconducting phases coexisting in the samples with T c1 ≈ 9 K and T c2 ≈ 7 K, which are both non-centrosymmetric in structure as reported previously. By analysing the specific heat data measured in various magnetic fields down to a temperature of 1.8 K, we find that the absence of the inversion symmetry does not lead to an obvious deviation from an s-wave pairing symmetry in Re 3 W.
