Positron annihilation spectroscopy (PAS) is a sensitive probe for studying the electronic structure of defects in solids. We show that the high-momentum part of the Doppler-broadened annihilation spectra can be used to distinguish different elements. This is achieved by using a new two-detector coincidence system to examine the line shape variations originating from high-momentum core electrons. Because the core electrons retain their atomic character even when atoms form a solid, these results can be directly compared to simple theoretical predictions. The new approach adds increased elemental specificity to the PAS technique, and is useful in studying the elemental variations around a defect site.[ S0031-9007(96) Positron annihilation spectroscopy (PAS) is a sensitive probe for studying defects in solids [1,2]. The method relies on the propensity of positrons to become localized at open-volume regions of a solid and the emission of annihilation gamma rays that escape the test system without any final state interaction. These gamma rays hold information about the electronic environment around the annihilation site. PAS measurements for defect characterization generally utilize two observables: positron lifetime and the conventional Doppler broadening of the annihilation gamma rays using a single detector. Both of these techniques are not very sensitive to elemental variations around an annihilation site, such as the one occurring when a material is lightly doped with another or when a vacancy is tied with an impurity atom. A third observable, angular correlation of annihilation radiation, can overcome this deficiency. However, this observable is not used routinely in defect spectroscopy owing to the difficulties associated with the low counting rates at many of the existing facilities. Here we present the results from a new two-detector setup that measures the elemental variations around the annihilation site. The new setup improves the peak to background ratio in the annihilation spectrum to ϳ10 5 , and as a result the variations of the Doppler-broadened spectra resulting from annihilations with different core electrons can be mapped. Because the core electrons retain their atomic character even when atoms form a solid, the new results can be easily verified with straightforward theoretical calculations. In the past, Lynn et al. have shown the advantage of using a two-detector setup in a study of thermal generation of vacancies in aluminum [3,4].Upon entering the solid, positrons lose most of their kinetic energy and reach thermal equilibrium with the host material (within about 10 psec). In a crystal, the thermalized positrons experience a periodic repulsive potential that is centered on the ionic cores, and their wave function is confined to the interstitial region. Their subsequent motion is dominated by phonon scattering, and in the absence of an overall electric field in the medium, this motion is nearly an isotropic random walk. Open-volume defects and negative charge centers provide isolated minima in t...
ABSTRACT:The interface between graphene and the ferroelectric superlattice PbTiO 3 /SrTiO 3 (PTO/STO) is studied. Tuning the transition temperature through the PTO/STO volume fraction minimizes the adsorbates at the graphene-ferroelectric interface, allowing robust ferroelectric hysteresis to be demonstrated. "Intrinsic" charge traps from the ferroelectric surface defects can adversely affect the graphene channel hysteresis, and can be controlled by careful sample processing, enabling systematic study of the charge trapping mechanism.
The interplay of the massless Dirac fermions in graphene and the Cooper pair states in a superconductor has the potential to give rise to exotic physical phenomena and useful device applications. But to date, the junctions formed between graphene and superconductors on conventional substrates have been highly disordered. Charge scattering and potential fluctuations caused by such disorder are believed to have prevented the emergence or observation of new physics. Here we propose to address this problem by forming suspended graphene-superconductor junctions. We demonstrate the fabrication of high-quality suspended monolayer graphene-NbN Josephson junctions with device mobility in excess of 150,000 cm 2 per Vs, minimum carrier density below 10 10 cm À 2 , and the flow of a supercurrent at critical temperatures greater than 2 K. The characteristics of our Josephson junctions are consistent with ballistic transport, with a linear dependence on the Fermi energy that reflects of linear dispersion of massless Dirac fermions.
We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to 1.43×10 20 protons on target, we observe 31 fully-contained single µ-like ring events in Super-Kamiokande, compared with an expectation of 104 ± 14 (syst) events without neutrino oscillations. The best-fit point for two-flavor νµ → ντ oscillations is sin 2 (2θ23) = 0.98 and |∆m 2 32 | = 2.65 × 10 −3 eV 2 . The boundary of the 90% confidence region includes the points (sin 2 (2θ23), |∆m 2 32 |) = (1.0, 3.1×10 −3 eV 2 ), (0.84, 2.65×10 −3 eV 2 ) and (1.0, 2.2×10 −3 eV 2 ).
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