Recently, possible evidence from the low-temperature specific heat ͑LTSH͒ for the lines of nodes in the superconducting order parameter of cuprate superconductors has attracted much attention and is still debated. To clarify this issue and to cover the studies in different carrier doping regimes, we have measured LTSH of La 2Ϫx Sr x CuO 4 ͑xϭ0.10, 0.16, and 0.22͒ both in zero and applied magnetic fields. In all doping regimes, it is found that the increase in the linear-T coefficient ␥ is proportional to H 1/2 , consistent with d-wave superconductivity. The data shows clear evidence for an ␣T 2 term at zero magnetic field in LTSH of La 1.78 Sr 0.22 CuO 4 . Furthermore, our results are compared with the recently proposed scaling theory.
We have studied the impurity effects on the superconducting transition temperature T c and the upper critical field H c2 in electron irradiated YBa 2 Cu 3 O y with in-plane oxygen defects and YBa 2 (Cu 1Ϫx Zn x ) 3 O y . It is found that the effects of the same type of defects or impurities on T c are the same regardless of the oxygen contents of the samples. Furthermore, T c decreases slower in irradiated YBa 2 Cu 3 O y than in YBa 2 (Cu 1Ϫx Zn x ) 3 O y . This may be well explained by the model that the scattering due to in-plane oxygen defects is more anisotropic than that due to Zn impurities. The different behavior of the reduced slopes (dH c2 /dT) T c /(dH c2 /dT) T c0 in these two types of samples can also be understood in this context. ͓S0163-1829͑99͒08309-5͔
Depth profiling measurements of photoluminescence on GaN epitaxial films grown on c-plane sapphire with metalorganic chemical vapor deposition have been performed. Dry etching technique of reactive ion etching is used with reactive gas of CCl2F2/H2/Ar under an operation power of 200 W. Before and after each etching, reflectivity and photoluminescence spectra are measured. Film thickness is determined from both the scanning electron microscopy and the interference oscillations of the reflectivity spectra. An excellent steady etching rate of 19.2 nm/min is established. The photoluminescence measurements show that both the near-band-edge and the yellow luminescence remain fairly constant until the film thickness of about 700 nm, and a large drop is obtained in the ratio of near-band-edge to yellow emission intensity under about 300 nm. Analysis shows that the yellow luminescence emitters are mostly confined within the near interface region, and supports the origin of yellow luminescence to be due to native defects instead of impurities.
The carrier concentration of a two-dimensional electron gas in a Zn 0.2 Cd 0.8 Se quantum well was persistently reduced by red-light illumination at low temperature. The deep-level donors were ''frozen out'' at 50 K and the thermal activation energy was about 42.6 meV. We believe that these deep-level donors are unlikely DX centers, and the observed negative persistent photoconductivity probably arises from the trapping of electron by the empty localized state of random-local-potential fluctuations in the barrier.
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