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Article Citation - WoS: 2Citation - Scopus: 1Comparison of the Performance of Structural Break Tests in Stationary and Nonstationary Series: a New Bootstrap Algorithm(Springer, 2024) Camalan, Ozge; Hasdemir, Esra; Omay, Tolga; Kucuker, Mustafa CanStructural breaks are considered as permanent changes in the series mainly because of shocks, policy changes, and global crises. Hence, making estimations by ignoring the presence of structural breaks may cause the biased parameter value. In this context, it is vital to identify the presence of the structural breaks and the break dates in the series to prevent misleading results. Accordingly, the first aim of this study is to compare the performance of unit root with structural break tests allowing a single break and multiple structural breaks. For this purpose, firstly, a Monte Carlo simulation study has been conducted through using a generated homoscedastic and stationary series in different sample sizes to evaluate the performances of these tests. As a result of the simulation study, Zivot and Andrews (J Bus Econ Stat 20(1):25-44, 1992) are the best-performing tests in capturing a single break. The most powerful tests for the multiple break setting are those developed by Kapetanios (J Time Ser Anal 26(1):123-133, 2005) and Perron (Palgrave Handb Econom 1:278-352, 2006). A new Bootstrap algorithm has been proposed along with the study's primary aim. This newly proposed Bootstrap algorithm calculates the optimal number of statistically significant structural breaks under more general assumptions. Therefore, it guarantees finding an accurate number of optimal breaks in real-world data. In the empirical part, structural breaks in the real interest rate data of the US and Australia resulting from policy changes have been examined. The results concluded that the bootstrap sequential break test is the best-performing approach due to the general assumption made to cover real-world data.Article Citation - WoS: 18Citation - Scopus: 24Fusion of Smartphone Sensor Data for Classification of Daily User Activities(Springer, 2021) Sengul, Gokhan; Ozcelik, Erol; Misra, Sanjay; Damasevicius, Robertas; Maskeliunas, RytisNew mobile applications need to estimate user activities by using sensor data provided by smart wearable devices and deliver context-aware solutions to users living in smart environments. We propose a novel hybrid data fusion method to estimate three types of daily user activities (being in a meeting, walking, and driving with a motorized vehicle) using the accelerometer and gyroscope data acquired from a smart watch using a mobile phone. The approach is based on the matrix time series method for feature fusion, and the modified Better-than-the-Best Fusion (BB-Fus) method with a stochastic gradient descent algorithm for construction of optimal decision trees for classification. For the estimation of user activities, we adopted a statistical pattern recognition approach and used the k-Nearest Neighbor (kNN) and Support Vector Machine (SVM) classifiers. We acquired and used our own dataset of 354 min of data from 20 subjects for this study. We report a classification performance of 98.32 % for SVM and 97.42 % for kNN.Article Citation - WoS: 4Citation - Scopus: 4Optical Characterization of (tlins2)0.5< Crystal by Ellipsometry: Linear and Optical Constants for Optoelectronic Devices(Springer, 2023) Guler, I.; Isik, M.; Gasanly, N.TlInSSe [(TlInS2)(0.5)(TlInSe2)(0.5)] crystals have garnered significant attention as promising candidates for optoelectronic applications due to their exceptional optoelectrical characteristics. This study focused on investigating the linear and nonlinear optical properties of TlInSSe layered single crystals through ellipsometry measurements. The X-ray diffraction analysis revealed the presence of four distinct peaks corresponding to a monoclinic crystalline structure. In-depth analysis was conducted to examine the variations of refractive index, extinction coefficient, and complex dielectric function within the energy range of 1.25-6.15 eV. By employing derivative analysis of the absorption coefficient and utilizing the Tauc relation, the indirect and direct bandgap energies of TlInSSe crystals were determined to be 2.09 and 2.26 eV, respectively. Furthermore, this research paper presents findings on oscillator energy, dispersion energy, Urbach energy, zero and high frequency dielectric constants, plasma frequency, carrier density to effective mass ratio, nonlinear refractive index, and first-order and third-order nonlinear susceptibilities of TlInSSe crystals.Article Citation - WoS: 2Citation - Scopus: 2Effects of Laser Excitation and Temperature on Ag/Gase0.5< Microwave Filters(Springer, 2014) Qasrawi, A. F.; Khanfar, H. K.The effects of temperature, illumination, and microwave signals on Ag/GaS0.5S0.5/C Schottky-type microwave filters have been investigated. The devices, which were produced from thin layers of GaSe0.5S0.5 single crystal, had room temperature barrier height and ideality factor of 0.65 eV and 3.28, respectively. Barrier height increased uniformly with increasing temperature, at 2.12 x 10(-2) eV/K, and the ideality factor approached ideality. The devices can even function at 95A degrees C. A current switching phenomenon from low to high injection ("On/Off") was also observed; this current switching appears at a particular voltage, V (s), that shifts toward lower values as the temperature is increased. When the devices were reverse-biased and illuminated with a laser beam of wavelength 406 nm, a readily distinguishable V (s) was observed that shifted with increasing laser power. When the devices were run in passive mode and excited with an ac signal of power 0.0-20.0 dBm and frequency 0.05-3.0 GHz they behaved as band filters that reject signals at 1.69 GHz. Device resistance was more sensitive to signal amplitude at low frequencies (50 MHz) than at high frequencies. The features of these Ag/GaS0.5S0.5/C Schottky devices imply that they may be used as optical switches, as self standing, low band-pass, band reject filters, and as high band-pass microwave filters.Article Citation - WoS: 8Citation - Scopus: 8Spectroscopic ellipsometry study of Bi12TiO20 single crystals(Springer, 2021) Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.Bi12XO20 (X: Si, Ge, Ti, etc.) ternary compounds have attracted attention especially due to their fascinating photorefractive characteristics. The present paper introduces the structural and optical characteristics of Bi12TiO20 single crystals grown by Czochralski method. X-ray diffraction pattern of the compound exhibited sharp and intensive peaks corresponding to parallel planes of cubic crystalline structure. The lattice constant of the cubic structure was determined as a = 1.0118 nm using a diffraction pattern indexing program. The optical characterization of the Bi12TiO20 single crystals was carried through spectroscopic ellipsometry experiments performed in the 1.2-5.0 eV spectral range. The spectral dependencies of refractive index, extinction coefficient, and complex dielectric function were revealed analyzing experimental ellipsometric data under the light of sample-air optical model. The band gap energy of the compound was determined as 3.34 eV from the analyses of absorption coefficient. Three critical points at 3.51, 4.10, and 4.71 eV were obtained from the analyses of components of dielectric function using their second-energy derivative spectra.Article Citation - WoS: 16Citation - Scopus: 18Left Behind: Intergenerational Transmission of Human Capital in the Midst of Hiv/Aids(Springer, 2013) Akbulut-Yuksel, Mevlude; Turan, BelgiThis paper provides evidence on how adverse health conditions affect the transfer of human capital from one generation to the next. We explore the differential exposure to HIV/AIDS epidemic in sub-Saharan Africa as a substantial health shock to both household and community environment. We utilize the recent rounds of the Demographic and Health Surveys for 11 countries in sub-Saharan Africa. First, we find that an additional year of maternal education leads to a 0.37-year increase in children's years of schooling in the developing economies in sub-Saharan Africa. Second, our results show that mother's HIV status has substantial detrimental effects on inheritability of human capital. We find that the association between infected mothers' and their children's human capital is 30 % less than the general population. Finally, focusing only on noninfected mothers and their children, we show that HIV prevalence in the community also impairs the intergenerational human capital transfers even if mother is HIV negative. The findings of this paper are particularly distressing for these already poor, HIV-torn countries as in the future they will have even lower overall level of human capital due to the epidemic.Article Citation - WoS: 1Citation - Scopus: 1Modeling of Kappa Factor Using Multivariate Adaptive Regression Splines: Application To the Western Türkiye Ground Motion Dataset(Springer, 2024) Kurtulmus, Tevfik Ozgur; Yerlikaya-Ozkurt, Fatma; Askan, AysegulThe recent seismic activity on Turkiye's west coast, especially in the Aegean Sea region, shows that this region requires further attention. The region has significant seismic hazards because of its location in an active tectonic regime of North-South extension with multiple basin structures on soft soil deposits. Recently, despite being 70 km from the earthquake source, the Samos event (with a moment magnitude of 7.0 on October 30, 2020) caused significant localized damage and collapse in the Izmir city center due to a combination of basin effects and structural susceptibility. Despite this activity, research on site characterization and site response modeling, such as local velocity models and kappa estimates, remains sparse in this region. Kappa values display regional characteristics, necessitating the use of local kappa estimations from previous earthquake data in region-specific applications. Kappa estimates are multivariate and incorporate several characteristics such as magnitude and distance. In this study, we assess and predict the trend in mean kappa values using three-component strong-ground motion data from accelerometer sites with known VS30 values throughout western Turkiye. Multiple linear regression (MLR) and multivariate adaptive regression splines (MARS) were used to build the prediction models. The effects of epicentral distance Repi, magnitude Mw, and site class (VS30) were investigated, and the contributions of each parameter were examined using a large dataset containing recent seismic activity. The models were evaluated using well-known statistical accuracy criteria for kappa assessment. In all performance measures, the MARS model outperforms the MLR model across the selected sites.Article Citation - WoS: 24Citation - Scopus: 28Palladium(0) Nanoparticles Supported on Hydroxyapatite Nanospheres: Active, Long-Lived, and Reusable Nanocatalyst for Hydrogen Generation From the Dehydrogenation of Aqueous Ammonia-Borane Solution(Springer, 2014) Karatas, Yasar; Yurderi, Mehmet; Gulcan, Mehmet; Zahmakiran, Mehmet; Kaya, MuratAmong the solidmaterials considered in the chemical hydrogen storage, ammonia-borane (NH3-BH3) appears to be one of the promising candidates as it can release hydrogen throughout hydrolysis in the presence of suitable catalyst under mild conditions. Herein we report, for the first time, the preparation and characterization of palladium(0) nanoparticles supported on nanohydroxyapatite and their catalytic use in the hydrolysis of ammonia-borane under air at room temperature. These new palladium(0) nanoparticles were generated in situ during the catalytic hydrolysis of ammonia-borane starting with palladium(II) immobilized nanohydroxyapatite. The preliminary characterization of the palladium(0) nanoparticles supported on nanohydroxyapatite was done by the combination of complimentary techniques, which reveals that the formation of well-dispersed Pd(0)NPs nanoparticles (1.41 +/- 0.52 nm) on the surface of hydroxyapatite nanospheres (60-150 nm). The resulting palladium nanocatalyst achieves hydrogen generation from the hydrolysis of ammonia-borane with an initial turnover frequency value (TOF) of 11 mol H-2 mol(-1) Pd x min at room temperature under air. In addition to their high activity, the catalytic lifetime experiment showed that they can also act as a long-lived heterogeneous catalyst for this reaction (TTON = 14,200 mol H-2 mol(-1) Pd) at room temperature under air. More importantly, nanohydroxyapatite- supported palladium(0) nanoparticles were found to be highly stable against to leaching and sintering throughout the catalytic runs that make them isolable, bottleable, and reusable heterogeneous catalyst for the hydrolysis of ammonia-borane.Article Time Complexity Comparison of Stopping at First Failure and Completely Running the Test(Springer, 2020) Yucesan, Ongun; Ozkil, AltanFor a given test set, we compare two test application techniques, "stopping at first failure" and "complete run", respectively. The former stops the test whenever a failure is encountered. The latter however, carries on till the end. Our comparison concludes that although the first technique is generally faster, there is no strong evidence to support a complete rejection of the second technique for it provides statistics useful in test planning. Results support an interchangeable application of both techniques with more frequent use of stopping at first failure. Even though there are numerous studies to decide how to prioritize and select test cases (TC), or to examine dependencies of TCs and codes, the presented comparison of the two test application techniques for a given "test set" has the potential to reduce the total run time of the test.Article Clinicopathological Predictors of Central Lymph Node Metastasis in Clinically Node-Negative Papillary Thyroid Carcinoma: A Retrospective Cohort Analysis(Springer, 2025) Sakiz, Davut; Calapkulu, Murat; Sencar, Muhammed Erkam; Unsal, Ilknur Ozturk; Kartal, Mehmet Zabit; Ucan, Bekir; Cakal, ErmanPurposeThis retrospective cohort study aimed to identify clinicopathological predictors of central lymph node metastasis (CLNM) in patients with papillary thyroid carcinoma (PTC).MethodsA total of 503 patients with PTC who underwent thyroidectomy with central lymph node dissection were included. Preoperative clinical data, ultrasonographic features, and postoperative histopathological characteristics were analyzed.ResultsCLNM was detected in 209 (41.55%) patients. Male sex (odds ratio [OR] = 3.12, 95% CI: 1.82-5.35), irregular nodule borders (OR = 2.02, 95% CI: 1.25-3.26) and higher risk categories in the American Thyroid Association (ATA) ultrasonographic pattern stratification system (OR = 13.312, 95% CI: 8.623-20.551) were identified as independent preoperative predictors of CLNM. A simplified model incorporating only the ATA ultrasonographic pattern stratification system and male sex achieved 83.5% accuracy in predicting CLNM. Histopathological evaluation revealed lymphovascular invasion (OR = 4.61, 95% CI: 2.81-7.55), extrathyroidal extension (OR = 2.41, 95% CI: 1.52-3.83), and multifocality (OR = 1.58, 95% CI: 1.04-2.40) as independent predictors of CLNM, while lymphocytic thyroiditis (OR = 0.59, 95% CI: 0.39-0.89) was a protective factor.ConclusionThis study establishes that ATA ultrasonographic suspicion patterns combined with the male sex provide an accurate and simplified model for preoperative CLNM prediction in PTC, outperforming complex nomograms. Key independent predictors included male sex, irregular nodule margins, ATA high-risk patterns, lymphovascular invasion, extrathyroidal extension, and multifocality, while lymphocytic thyroiditis was protective. According to the current study, risk assessment and personalized management should favor a holistic approach, rather than focusing solely on individual risk factors.
