Inhomogeneous electronic structure probed by spin-echo experiments in the electron doped high-Tc superconductor Pr_{1.85}Ce_{0.15}CuO_{4-y}

63Cu nuclear magnetic resonance (NMR) spin-echo decay rate (T_2^{-1}) measurements are reported for the normal and superconducting states of a single crystal of Pr_{1.85}Ce_{0.15}CuO_{4-y} (PCCO) in a magnetic field B_0=9T over the temperature range 2K<T<200K. The spin-echo decay rate is temperature-dependent for T<55K, and has a substantial dependence on the radio frequency (rf) pulse parameters below T~25K. This dependence indicates that T_2^{-1} is strongly effected by a local magnetic field distribution that can be modified by the rf pulses, including ones that are not at the nuclear Larmor frequency. The low-temperature results are consistent with the formation of a static inhomogeneous electronic structure that couples to the rf fields of the pulses.Comment: 4 pages, 4 figure

Highly reflective distributed Bragg reflectors using a deeply etched semiconductor/air grating for InGaN/GaN laser diodes

2002-2003 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

The Mre11-Rad50-Nbs1 complex mediates activation of TopBP1 by ATM

The activation of ATR-ATRIP in response to double-stranded DNA breaks (DSBs) depends upon ATM in human cells and Xenopus egg extracts. One important aspect of this dependency involves regulation of TopBP1 by ATM. In Xenopus egg extracts, ATM associates with TopBP1 and thereupon phosphorylates it on S1131. This phosphorylation enhances the capacity of TopBP1 to activate the ATR-ATRIP complex. We show that TopBP1 also interacts with the Mre11-Rad50-Nbs1 (MRN) complex in egg extracts in a checkpoint-regulated manner. This interaction involves the Nbs1 subunit of the complex. ATM can no longer interact with TopBP1 in Nbs1-depleted egg extracts, which suggests that the MRN complex helps to bridge ATM and TopBP1 together. The association between TopBP1 and Nbs1 involves the first pair of BRCT repeats in TopBP1. In addition, the two tandem BRCT repeats of Nbs1 are required for this binding. Functional studies with mutated forms of TopBP1 and Nbs1 suggested that the BRCT-dependent association of these proteins is critical for a normal checkpoint response to DSBs. These findings suggest that the MRN complex is a crucial mediator in the process whereby ATM promotes the TopBP1-dependent activation of ATR-ATRIP in response to DSBs

Radiation chemistry provides nanoscopic insights into the role of intermediate phases in CeO₂ mesocrystal formation

The role of intermediate phases in CeO2 mesocrystal formation from aqueous Ce(III) solutions subjected to gamma-radiation was studied. Radiolytically formed hydroxyl radicals convert soluble Ce(III) into less soluble Ce(IV). Transmission electron microscopy (TEM) and X-ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO2 mesocrystal evolution following the oxidation to Ce(IV): (1) formation of hydrated Ce(IV)-hydroxides, serving as intermediates in the liquid-to-solid phase transformation; (2) CeO2 primary particle growth inside the intermediate phase; (3) alignment of the primary particles into "pre-mesocrystals" and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of "mineral bridges". Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures

Elastic properties of the Non-Fermi liquid metal CeRu4Sb12Ce Ru_4 Sb_{12} and the Dense Kondo semiconductor CeOs4Sb12Ce Os_4 Sb_{12}

We have investigated the elastic properties of the Ce-based filled skutterudite antimonides CeRu$_{4}$Sb$_{12}$ and CeOs$_{4}$Sb$_{12}$ by means of ultrasonic measurements. CeRu$_{4}$Sb$_{12}$ shows a slight increase around 130 K in the temperature dependence of the elastic constants $C$$_{11}$, ($C$$_{11}$-$C$$_{12}$)/2 and $C$$_{44}$. No apparent softening toward low temperature due to a quadrupolar response of the 4$f$-electronic ground state of the Ce ion was observed at low temperatures. In contrast CeOs$_{4}$Sb$_{12}$ shows a pronounced elastic softening toward low temperature in the longitudinal $C$$_{11}$ as a function of temperature ($T$) below about 15 K, while a slight elastic softening was observed in the transverse $C$$_{44}$ below about 1.5 K. Furthermore, CeOs$_{4}$Sb$_{12}$ shows a steep decrease around a phase transition temperature of 0.9 K in both $C$$_{11}$ and$C$$_{44}$. The elastic softening observed in $C$$_{11}$ below about 15 K cannot be explained reasonably only by the crystalline electric field effect. It is most likely to be responsible for the coupling between the elastic strain and the quasiparticle band with a small energy gap in the vicinity of Fermi level. The elastic properties and the 4$f$ ground state of Ce ions in CeRu$_{4}$Sb$_{12}$ and CeOs$_{4}$Sb$_{12}$ are discussed from the viewpoint of the crystalline electric field effect and the band structure in the vicinity of Fermi level.Comment: 9 pages, 11 figures, regular pape