다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-t...

Defects in ZnO

Contemporary theories of defects and impurities in semiconductors rely to a large extent on supercell calculations within density-functional theory using the approximate local-density approximation (LDA) or generalized gradient approximation (GGA) functionals. Such calculations are, however, affected by considerable uncertainties associated with: (i) the ``band-gap problem,'' which occurs not only in the Kohn-Sham single-particle energies but also in the quasiparticle gap (LDA or GGA) calculated from total-energy differences, and (ii) supercell finite-size effects. In the case of the oxygen vacancy in ZnO, uncertainties (i) and (ii) have led to a large spread in the theoretical predictions, with some calculations suggesting negligible vacancy concentrations, even under Zn-rich conditions, and others predicting high concentrations. Here, we critically assess (i) the different methodologies to correct the band-gap problem. We discuss approaches based on the extrapolation of perturbations which open the band gap, and the self-consistent band-gap correction employing the $\text{LDA}+U$ method for $d$ and $s$ states simultaneously. From the comparison of the results of different gap-correction, including also recent results from other literature, we conclude that to date there is no universal scheme for band gap correction in general defect systems. Therefore, we turn instead to classification of different types of defect behavior to provide guidelines on how the physically correct situation in an LDA defect calculation can be recovered. (ii) Supercell finite-size effects: We performed test calculations in large supercells of up to 1728 atoms, resolving a long-standing debate pertaining to image charge corrections for charged defects. We show that once finite-size effects not related to electrostatic interactions are eliminated, the analytic form of the image charge correction as proposed by Makov and Payne leads to size-independent defect formation energies, thus allowing the calculation of well-converged energies in fairly small supercells. We find that the delocalized contribution to the defect charge (i.e., the defect-induced change of the charge distribution) is dominated by the dielectric screening response of the host, which leads to an unexpected effective $1/L$ scaling of the image charge energy, despite the nominal $1/{L}^{3}$ scaling of the third-order term. Based on this analysis, we suggest that a simple scaling of the first order term by a constant factor (approximately 2/3) yields a simple but accurate image-charge correction for common supercell geometries. Finally, we discuss the theoretical controversy pertaining to the formation energy of the O vacancy in ZnO in light of the assessment of different methodologies in the present work, and we review the present experimental situation on the topic.

Assessment of correction methods for the band-gap problem and for finite-size effects in supercell defect calculations: Case studies for ZnO and GaAs

Monoclinic gallium oxide (β-Ga2O3) is attracting intense focus as a material for power electronics, thanks to its ultra-wide bandgap (4.5-4.8 eV) and ability to be easily doped n-type. Because the holes self-trap, the band-edge luminescence is weak; hence, β-Ga2O3 has not been regarded as a promising material for light emission. In this work, optical and structural imaging methods revealed the presence of localized surface defects that emit in the near-UV (3.27 eV, 380 nm) when excited by sub-bandgap light. The PL emission of these centers is extremely bright-50 times brighter than that of single-crystal ZnO, a direct-gap semiconductor that has been touted as an active material for UV devices.

/pdf/localized-uv-emitters-on-the-surface-of-b-ga2o3-14hblhbsip.pdf

Localized UV emitters on the surface of β-Ga2O3.

Native defects in complex oxides play a crucial role in determining their optical, electrical, and magnetic properties and it is di cult to identify and characterize them. Positron lifetime spectroscopy is a powerful technique to study vacancy defects; however its application to complex oxides has been limited. In this work we apply positron lifetime spectroscopy to study open volume defects in rare earth doped yttrium aluminum garnet (YAG) complex oxides grown in argon atmosphere. In YAG single crystals, positron lifetime measurements identi ed isolated aluminum vacancies and complexes of aluminum vacancy and neighbor oxygen vacancies. Thermoluminescence measurements were also performed to elucidate the interaction between trapping defects and luminescence centers. By combining positron lifetime and thermoluminescence, both the defect type and its e ect on the optical properties of YAG crystals were revealed.

/pdf/positron-lifetime-measurements-of-vacancy-defects-in-complex-yuunfj7k1q.pdf

Positron Lifetime Measurements of Vacancy Defects in Complex Oxides

Green luminescence from Cu-diffused LiGaO2 crystals

A positron lifetime spectroscopy (PLS) technique was developed using coincident γ rays induced by proton capture. Proton capture in some light elements induce coincident MeV γ rays, allowing positron lifetime to be measured. One γ quantum provides a start signal for the positron lifetime spectrometer, whereas the other γ quantum bombards the sample under investigation, generating a positron inside it through pair production. The stop signal is obtained from the detection of one of the two 511keV photons emitted from positron annihilation with the sample electrons. This new technique can extend PLS, which is a powerful tool to identify the size and concentration of defects, to thick materials and a broad range of applications. It also eliminates the source contribution from the measured spectra, which may lead to the identification of more defect types in a sample.

Positron lifetime measurements by proton capture

Room temperature photoconductivity measurements have been carried out on various as-grown SrTiO3 single crystals provided from different suppliers. We observed an increase in the conductivity of samples based on photon energy and photon intensity. While low energy photons decreased the conductivity, photons with energy close to the band gap of SrTiO3 enhanced the conductivity. No persistent photoconductivity was observed in these samples. Defects play the main role in inducing the photoconductivity. Positron annihilation lifetime spectroscopy and digital coincidence Doppler broadening spectroscopy were used to investigate the presence and nature of defects in the photoconductive samples. The measurements revealed high concentration of defects and the dependence of photoconductivity on defect concentration under 365 and 400 nm illumination.

https://iopscience.iop.org/article/10.1088/2053-1591/aaa094/pdf

Farida Selim

Papers

Localized UV emitters on the surface of β-Ga2O3.

Positron Lifetime Measurements of Vacancy Defects in Complex Oxides

Green luminescence from Cu-diffused LiGaO2 crystals

Positron lifetime measurements by proton capture

Photoconductivity of bulk SrTiO3 single crystals at room temperature