512 research outputs found

    The origin of magnetism in anatase Co-doped TiO2 magnetic semiconductors

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    Dilute magnetic semiconductors (DMS) can be tailored by doping a small amount of\ud elements containing a magnetic moment into host semiconductors, which leads to a new class of semiconductors with the functionality of tunable magnetic properties. Recently, oxide semiconductors have attained interests for the possibility of developing room temperature dilute magnetic semiconductors. However, a large number of conflicting results have pointed towards both an intrinsic and extrinsic origin for ferromagnetism. In earlier studies of oxides in relation to DMS, the controversy originated mainly from imprudent conclusions relying on measurements of macroscopic magnetic properties without careful structural studies of secondary phases in the materials under investigation. This thesis describes the results of experiments with a broad set of complementary tools to investigate the origin of ferromagnetism in anatase Co:TiO2, which shows room temperature magnetism and an anomalous Hall effect (AHE). In previous work carried out in the NanoElectronics group, ferromagnetism and the AHE at room temperature were observed in anatase Co:TiO2 thin films grown under oxygen poor conditions, while the remanence and coercivity disappear for films grown under oxygen rich conditions. We employed synchrotron based techniques, namely x-ray absorption spectroscopy\ud (XAS) and x-ray magnetic circular dichroism (XMCD), which rely on the excitation of\ud core-level electrons by soft x-rays. One of appealing features of these techniques for\ud DMS studies is that they can provide element specific information on the electronic and\ud magnetic properties of materials. Here, we focused on Co-derived electronic states.\ud We observed a suppressed XAS multiplet structure due to Co2+ ions at Co L-edges in\ud oxygen poor samples, as compared to oxygen rich samples. This suppressed fine structure\ud can be explained by a larger featureless Co metal contribution superimposed on the\ud spectral features related to Co2+ ions. A Co metal contribution is confirmed by XMCD\ud measurements, which show a featureless dichroism spectrum characteristic of metallic Co.\ud Both oxygen poor and oxygen rich Co:TiO2 showed a similar featureless XMCD\ud spectrum, with the circular dichroism effect being significantly weaker for the oxygen\ud rich case. These findings show that measurements of XAS L-edges alone may confirm\ud the presence of ionic species, but cannot rule out secondary phases that may not yield a\ud discernable fingerprint.\ud It can thus be concluded from the XAS/XMCD measurements that the Co distribution is\ud heterogeneous for all Co:TiO2 films, grown under oxygen rich as well as oxygen poor\ud conditions. Co-heterogeniety, in the form of metallic clusters, is also confirmed by\ud energy filtered transmission electron microscopy (EF-TEM). This means that metallic Co\ud clusters are present in all the investigated films, and that they contribute to the\ud ferromagnetism (as shown by XMCD) and AHE (as discussed below). We conclude that\ud XMCD and EF-TEM are especially useful tools to check for heterogeneity in DMS\ud research.\ud Using XAS, we also observed clear charge transfer (CT) satellite peaks at Co L edges for\ud the thickest (160 nm) oxygen poor films. The CT peaks can be explained by the\ud interaction of Co (3d) electrons with donor defect states in the band gap. It is known that\ud Co segregates to defects sites, where it may form clusters. The observation that the CT\ud peaks become stronger for thicker films, which exhibit a higher defect density resulting from the relief of strain, may be related to defect states that are more energetically close\ud to and significantly hybridized with the Co(3d) states. These findings also confirm that\ud the electronic structure of Co ions is heterogeneous, related to defect states in Co:TiO2\ud thin films.\ud The introduction of a TiO2 buffer layer at the substrate/film interface, intended to reduce\ud strain in Co:TiO2, leads to a dramatic change of magnetic and structural properties in\ud Co:TiO2. First, a reduction of the density of Co clusters and a more homogeneous Co\ud distribution in the thin films are confirmed by XRD, AFM and EF-TEM measurements.\ud Second, the AHE is suppressed. These results can be explained by a cluster-induced AHE,\ud also taking into account the results from EF-TEM analysis which showed that the\ud location at which Co clusters form is markedly different for films with/without buffer\ud layer. For Co:TiO2 films without buffer layer, Co clusters formed preferentially at the\ud substrate/film interface give rise to an anomalous transverse Hall resistivity by polarizing\ud nearby electrons. On the other hand, for Co:TiO2 films without buffer layer, Co clusters\ud where mainly observed at the surface, such that they are placed outside the current path\ud and do not contribute to the associated transverse scattering. The very small value of the\ud ratio of the anomalous Hall resistivity to the longitudinal resistivity (10-4) also confirms\ud that the AHE originates from Co clusters, since Co clusters polarize only nearby electrons.\ud Therefore, the presence of AHE cannot be considered as a definite test for carrier\ud mediated magnetism in dilute magnetic semiconductors.\ud We also observed that metallic impurity band conduction and ferromagnetism occur\ud simultaneously in both types of Co:TiO2 films, with and without a buffer layer. Since the\ud metallic impurity band conduction is found to co-occur with ferromagnetism due to Co\ud clusters, we can conclude that the phenomena of impurity band conduction and\ud ferromagnetism are not related.\ud Last, the spin polarization of the charge carriers in ultrathin anatase Co:TiO2 layers is\ud investigated by studying spin-polarized tunneling in a magnetic tunnel junction (MTJ)\ud configuration. It is found that the tunnel magnetoresistance (TMR) and junction\ud resistance of epitaxial LSMO/STO/Co magnetic tunnel junctions changes significantly\ud upon the insertion of ultrathin layers of Co:TiO2 at the STO/Co interface. The magnitude\ud of the TMR decreases but remains negative, while the junction resistance increases\ud strongly. This is consistent with an effectively insulating and paramagnetic Co:TiO2\ud adding to the tunnel barrier, with the tunneling electrons originating mostly from the\ud Co:TiO2/Co interface, and experiencing spin-flip scattering by paramagnetic Co in the\ud Co:TiO2

    Observation of tW production in the single-lepton channel in pp collisions at root s=13 TeV

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    A measurement of the cross section of the associated production of a single top quark and a W boson in final states with a muon or electron and jets in proton-proton collisions at root s = 13 TeV is presented. The data correspond to an integrated luminosity of 36 fb(-1) collected with the CMS detector at the CERN LHC in 2016. A boosted decision tree is used to separate the tW signal from the dominant t (t) over bar background, whilst the subleading W+jets and multijet backgrounds are constrained using data-based estimates. This result is the first observation of the tW process in final states containing a muon or electron and jets, with a significance exceeding 5 standard deviations. The cross section is determined to be 89 +/- 4 (stat) +/- 12 (syst) pb, consistent with the standard model.Peer reviewe

    SmartSAGE: Training Large-scale Graph Neural Networks using In-Storage Processing Architectures

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    Graph neural networks (GNNs) can extract features by learning both the representation of each objects (i.e., graph nodes) and the relationship across different objects (i.e., the edges that connect nodes), achieving state-of-the-art performance in various graph-based tasks. Despite its strengths, utilizing these algorithms in a production environment faces several challenges as the number of graph nodes and edges amount to several billions to hundreds of billions scale, requiring substantial storage space for training. Unfortunately, state-of-the-art ML frameworks employ an in-memory processing model which significantly hampers the productivity of ML practitioners as it mandates the overall working set to fit within DRAM capacity. In this work, we first conduct a detailed characterization on a state-of-the-art, large-scale GNN training algorithm, GraphSAGE. Based on the characterization, we then explore the feasibility of utilizing capacity-optimized NVM SSDs for storing memory-hungry GNN data, which enables large-scale GNN training beyond the limits of main memory size. Given the large performance gap between DRAM and SSD, however, blindly utilizing SSDs as a direct substitute for DRAM leads to significant performance loss. We therefore develop SmartSAGE, our software/hardware co-design based on an in-storage processing (ISP) architecture. Our work demonstrates that an ISP based large-scale GNN training system can achieve both high capacity storage and high performance, opening up opportunities for ML practitioners to train large GNN datasets without being hampered by the physical limitations of main memory size.Comment: Accepted for publication at the 49th IEEE/ACM International Symposium on Computer Architecture (ISCA-49), 202

    Acquirer’s Absorptive Capacity and Firm Performance: The Perspectives of Strategic Behavior and Knowledge Assets

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    In this study, we underline the importance of the relationship between absorptive capacity and an acquiring firm’s post-merger performance following the acquisition of a target firm’s knowledge through cross-border mergers and acquisitions (CBMAs). We analyzed CBMAs between developed countries to highlight how realized absorptive capacity plays a crucial part in a firm’s achievement of CBMA sustainability. Using United States CBMA transactions with other developed countries during 2000–2014, our findings suggest that an acquiring firm’s greater absorptive capacity leads to better post-merger performance. More interestingly, compared to for domestic M&As, the direct effect between absorptive capacity and post-merger performance was found to be more positively related in CBMA transactions, even when we applied propensity-score matching (PSM) and Heckman’s selection model to the same estimation. In addition, we introduce four moderating variables that could either intensify or lessen a firm’s effort to seek external knowledge for organizational growth. In terms of an acquiring firm’s strategic behavior, we find that paying in cash and past CBMA experiences positively influence a firm’s post-merger performance. For a target firm’s knowledge assets, we show that when a target firm possesses more strategic assets, they reinforce the acquiring firm’s post-merger performance, and when the target firm is in a high-tech industry, the acquiring firm’s post-merger performance is weakened. Our study contributes to the CBMA literature by incorporating the concept of a knowledge-based view and by empirically testing the different effects of absorptive capacity between domestic M&A and CBMA and how both strategic behavior and a target firm’s knowledge assets affect a firm’s post-merger performance related to CBMA sustainability

    Determinants of Market Share of For-Profit Hospitals: An Empirical Examination

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    This study estimates the effects of a prospective payment system on the growth of for-profit hospitals. The empirical results show that the proportion of patient care paid for by Medicare managed care has a positive, statistically significant relationship with the market share of for-profit hospitals. Medicare managed care reimburses health care providers prospectively, and a larger portion of prospective reimbursements is received by for-profit hospitals, whose market share consequently increases. In addition, the proportion of patients with Medi-Cal and third party managed care has a positive, statistically significant relationship with the market share of for-profit hospitals

    Memory efficient fork-based checkpointing mechanism for in-memory database systems

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    © 2020 ACM.Consistent checkpointing is an essential technique for in-memory databases (IMDBs) to achieve some persistence to data. Towards a fast consistent checkpointing with low overhead and low memory footprint, many consistent checkpointing algorithms have been proposed. However, recent work revealed that the simple fork-based checkpointing scheme used for industrial IMDBs could indeed outperform the state-of-the-arts in terms of average latency, latency spike, and implementation effort. On the other hand, the existing fork-based checkpointing scheme has a memory footprint issue, which remains unsolved; memory usage increases incrementally (up to 2x) during checkpointing for update-intensive workloads. This paper introduces a memory dump based checkpointing scheme, called MDC. With minor operating system supports, our scheme can suppress the increase in memory footprint during checkpointing. By logging the virtual addresses of the objects and by novelly exploiting memory dump, MDC allows pages to be returned to the OS sooner, before the checkpointing process is completely done. This gives the OS the opportunity to reduce the copy-on-write fault overhead because write protection can be ignored when the faulting page is private, thus no page duplication incurs. We implement and apply our scheme into Redis. Extensive evaluations show that our scheme yields a much lower maximum memory footprint and marginally higher throughput in an update-intensive workload scenario.N

    Vulvar Reconstruction Using Keystone Flaps Based on the Perforators of Three Arteries

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    Various flaps are used to reconstruct skin and soft tissue defects of the vulva following resection of malignancies. Whenever possible, reconstruction using local flaps is the standard treatment. Here, we describe vulvar defect reconstruction using keystone flaps. Standard keystone flaps are based on randomly located vascular perforators. However, we designed a keystone flap that includes perforators of three named arteries (the anterior labial artery of the external pudendal artery, cutaneous branches of the obturator artery, and posterior labial artery of the internal pudendal artery) and the pudendal nerve, which accompanies the internal pudendal artery. Four patients with squamous cell carcinoma and extramammary Paget's disease of the vulva underwent radical vulvectomy and keystone flaps including perforators of three arteries. Depending on the morphology of the defects, keystone flaps were used with different designs. For elliptical and unilateral vulvar defects, a standard keystone flap was designed, and for defects on both sides of the vulva, a double opposing keystone flap was used. For oval defects, the omega variant keystone flap was designed, and when the morphology of the defect needed rotation of the flap, a rotational keystone flap was designed. All the patients showed good function and sensation, with an acceptable cosmetic appearance
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