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mohammadizadeh--erfan
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Pr at Gd or Ba site in GdBa2Cu3O7: Appearance of superconductivity
, Article Physical Review B - Condensed Matter and Materials Physics ; Volume 68, Issue 10 , 2003 ; 10980121 (ISSN) ; Akhavan, M ; Sharif University of Technology
2003
Abstract
Gd(Ba2-xPrx)Cu3O7+δ single phase polycrystalline samples with 0.0 < − x < − 1.0 were investigated for their structural and electronic properties. Due to the solubility limit, the 123 structure forms in the 0.0 < − x < −0.50 range; for x > −0.6, a decomposition of the perovskite-type phases occurs. For x=0.2, there is an orthorhombic-tetragonal phase transition, concurrent with a metal-insulator transition, which is evident in the normal state resistivity. For xc=0.35, the superconductor-insulator transition occurs. An unusual hump has been observed on the resistivity vs temperature curve of the samples for particular values of Pr doping. Based on the Rietveld refinement of the x-ray...
Charge transfer in YBCO under pressure with bond valence sum approach
, Article Journal of Superconductivity and Novel Magnetism ; Volume 18, Issue 2 , 2005 , Pages 299-307 ; 15571939 (ISSN) ; Akhavan, M ; Sharif University of Technology
2005
Abstract
The bond valence sum method has been developed to high pressures considering B0(p) for calculating the valences of atoms. This approach has the advantages of finding a universal B0(P) for all the atoms -at least in YBCO, ability to evaluate the valency of each atom, and charge neutrality requirement of the unit cell. As a result, a new structural refinement has led to a more exact structural detail. The position of apical oxygen is found to be specially crucial for hole distribution between the superconducting CuO2 planes and Cu-O charge reservoir chains. Under pressure, the hole concentration in CuO2 planes increases, but in the Cu-O chains decreases, consistent with the pressure induced...
Pseudogap in Gd-based 123 HTSC
, Article Physica B: Condensed Matter ; Volume 336, Issue 3-4 , 2003 , Pages 410-419 ; 09214526 (ISSN) ; Akhavan, M ; Sharif University of Technology
2003
Abstract
The Gd(Ba2-xPrx)Cu3O7+δ single phase polycrystalline samples with 0.00 ≤ x < 1.00 were investigated for pseudogap phenomenon in comparison with the (Gd1-xPrx)Ba2Cu3O7-δ compound. The measured XRD and its Rietveld refinement show that Pr is substituted at Ba site, with its stoichiometric order, and an orthorhombic-tetragonal transition occurs at x = 0.20. For x ≥ 0.60, the solubility limit of the system exceeds. Based on the resistivity measurements, the effect of Pr substitution in 123 structure of HTSC at Gd or Ba site is to increase the pseudogap temperature Ts, although, Pr at Ba site has a stronger effect on the suppression of Ts and superconductivity. Pr-doping not only reduces the...
Normal state conduction in Gd(Ba2-xPrx)Cu 3O7+δ
, Article European Physical Journal B ; Volume 33, Issue 4 , 2003 , Pages 381-390 ; 14346028 (ISSN) ; Akhavan, M ; Sharif University of Technology
2003
Abstract
The normal state resistivity of single phase polycrystalline Gd(Ba 2-x Pr x )Cu3O7+δ samples with 0.0 ≤ x ≤ 0.50 have been investigated. There is a distinct metal-insulator transition at xMITc = 0.2 and a superconductor-insulator transition at xSITc = 0.35 with the increase of x. The two-dimensional variable range hopping is dominant in the normal state resistivity of the samples. The localization length, hopping range, and hopping energy of carriers show that Pr doping strongly localizes the carriers in normal state, and finally causes the suppression of superconductivity. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2003
Thermally activated flux creep in the Gd(Ba2-xPrx)Cu3O7+δ system
, Article Superconductor Science and Technology ; Volume 16, Issue 4 , 2003 , Pages 538-544 ; 09532048 (ISSN) ; Akhavan, M ; Sharif University of Technology
2003
Abstract
The magnetoresistance of single-phase polycrystalline Gd(Ba2-xPrx)Cu3O7+δ samples has been studied within the thermally activated flux creep model. The decrease of pinning energy with applied magnetic field can be scaled to two power-law relations for magnetic fields, smaller and also larger than about 1 kOe. The derived Hc2(T), Hc2(O), and superconducting coherence length ξ show that the Pr doping, similar to weak links, decreases the vortex flux pinning energy. It is also concluded that the substitution of Pr at the Ba site has a more destructive effect on the flux dynamics than Pr at the rare-earth site
Magnetoresistance in Gd(Ba2-xPrx)Cu3O7+δ system
, Article Physica C: Superconductivity and its Applications ; Volume 390, Issue 2 , 2003 , Pages 134-142 ; 09214534 (ISSN) ; Akhavan, M ; Sharif University of Technology
2003
Abstract
The magnetoresistance measurements of single phase polycrystalline Gd(Ba2-xPrx)Cu3O7+δ and (Gd1-xPrx)Ba2Cu3O7-δ samples have been compared and analyzed by the Ambegaokar and Halperin phase slip model. The derived power law dependences of pinning energy to applied magnetic field and temperature have been studied for different amounts of Pr doping. The power factors depend on the Pr doping. Moreover, in both systems, the magnetic field dependence of the pinning energy power factor increases for small amounts of Pr doping, while for larger amount of Pr doping, it decreases. The derived critical magnetic field at zero temperature limit is in good agreement with other model-based and...
Two dimensionality aspects of HTSC
, Article Superconductor Science and Technology ; Volume 16, Issue 10 , 2003 , Pages 1216-1223 ; 09532048 (ISSN) ; Akhavan, M ; Sharif University of Technology
2003
Abstract
Based on transport and magnetic measurements on Gd(Ba2-xPrx)Cu3O7+δ, and some other properties of high temperature superconductors (HTSC), we have extracted similarities between superconductors, two-dimensional electron gas (2D-EG) i.e. MOSFETs and ultrathin films of conventional superconductors. These are based on properties such as superconductor-insulator transition in superconductors and metal-insulator transition (MIT) in 2D-EG with doping and magnetic field, localization in transport conduction, quantum unit of resistance at MIT, larger change in resistance from critical doping to the insulating side in comparison with change from critical doping to the metallic side and strong...
Flux dynamics, superconducting, and normal state properties of Gd(Ba 2-xPrx)Cu3O7+δ
, Article European Physical Journal B ; Volume 42, Issue 3 , 2004 , Pages 321-336 ; 14346028 (ISSN) ; Akhavan, M ; Sharif University of Technology
2004
Abstract
Gd(Ba2-xPrx)Cu3O7+δ single phase polycrystalline samples with 0.0 ≤ x ≤ 1.0 were investigated for structural, electronic and flux dynamics properties. Two-dimensional variable range hopping (VRH) is the dominant conduction mechanism in the normal state of the system. Pr doping strongly localizes the carriers in normal state, and finally causes the suppression of superconductivity. The effect of Pr substitution in 123 structure of HTSC at R or Ba sites is to increase the pseudogap temperature Ts, although, Pr at Ba sites has a stronger effect on the increase of Ts and suppression of superconductivity. The magnetoresistance of the samples have been studied within thermally activated flux creep...
Appearance of a new superconducting phase in Gd(Ba2-xPr x)Cu3O7+δ
, Article Physica Status Solidi C: Conferences ; Volume 1, Issue 7 , 2004 , Pages 1879-1882 ; 16101634 (ISSN) ; Akhavan, M ; Sharif University of Technology
2004
Abstract
An unusual hump on the resistivity vs. temperature curve of Gd(Ba 2-xPrx)Cu3O7+δ samples have been observed for particular values of Pr doping. Investigation of the origin of this effect has led to an important result. Based on the Rietveld refinement of the XRD patterns, we have found that the Ba atom substitution at the rare earth site could lead to superconductivity in some parts of the grains at T p ∼ 80-90 K, which appears as a hump on the p(T) curve. Our result is in line with the previously proposed possibility of existence of superconductivity in Pr-123 due to the Ba atom substituted at Pr site. © 2004 WILEY-VCH Verlag GmbH & Co. KOaA, Weinheim
High temperature superconductors as a two-dimensional electron gas
, Article Physica Status Solidi C: Conferences ; Volume 1, Issue 7 , 2004 , Pages 1828-1831 ; 16101634 (ISSN) ; Akhavan, M ; Sharif University of Technology
2004
Abstract
Based on transport and magnetic measurements on Gd(Ba2-xPr x)Cu3O7+δ, and some other properties of high temperature superconductors (HTSC), we have extracted similarities between superconductors, two-dimensional electron gas (2D-EG) i.e., MOSFETs. These are based on properties such as superconductor-insulator transition in superconductors and metal-insulator transition (MIT) in 2D-EG with doping and magnetic field, localization in transport conduction, quantum unit of resistance at MIT, larger change in resistivity from critical doping to the insulating side in comparison with change from critical doping to the metallic side, and strongly electron-electron coupling. These similarities could...
Magnetic field effects on electrical resistivity of (Gd 1-xPrx)Ba2Cu3O7-δ and Gd(Ba2-xPrx)Cu3O7+δ
, Article Physica Status Solidi C: Conferences ; Volume 1, Issue 7 , 2004 , Pages 1887-1890 ; 16101634 (ISSN) ; Akhavan, M ; Sharif University of Technology
2004
Abstract
The magnetoresistance measurements of single phase polycrystalline Gd(Ba2-xPrx)Cu3O7+δ and Gd1-xPrx)Ba2Cu3O 7-δ samples have been compared and analyzed by the Ambegaokar and Halperin phase slip model. The derived power law dependences of pinning energy to applied magnetic field and temperature have been studied for different amounts of Pr doping. The power factors depend on the Pr doping. Moreover, in both systems, the magnetic field dependence of the pinning energy power factor increases for small amounts of Pr doping, while for larger amount of Pr doping, it decreases. The critical current power law dependence to magnetic field decreases with increasing Pr doping. Pr substitution at Ba or...
Conduction mechanism in Pr-doped GdBa2Cu3O 7
, Article Physica Status Solidi C: Conferences ; Volume 1, Issue 7 , 2004 , Pages 1851-1854 ; 16101634 (ISSN) ; Akhavan, M ; Sharif University of Technology
2004
Abstract
The normal state resistivity of single phase polycrystalline Gd(Ba 2-xPrx)Cu3O7+δ samples with 0.0 ≤ x ≤ 0.50 have been investigated. The two-dimensional variable range hopping is dominant in the normal state resistivity of the samples. The conduction shows that Pr-doping strongly localizes the carriers in normal state, and finally causes the suppression of superconductivity. Based on the resistivity measurements, the effect of Pr substitution in 123 structure of HTSC at Gd or Ba site is to increase the pseudogap temperature Ts, although, Pr at Ba site has a stronger effect on the suppression of T s and superconductivity. Pr-doping not only reduces the carrier density and induces pseudogap,...
Effects of Pr doping and magnetic field on vortex pinning in Gd-123 based HTSC
, Article Physica Status Solidi C: Conferences ; Volume 1, Issue 7 , 2004 , Pages 1883-1886 ; 16101634 (ISSN) ; Akhavan, M ; Sharif University of Technology
2004
Abstract
The magnetoresistance of single phase polycrystalline Gd(Ba 2-xPrx)Cu3O7-δ samples have been studied within thermally activated flux creep model. The decrease of pinning energy with applied magnetic field can be scaled to two power law relations for magnetic fields, smaller and also larger than about 1 kOe. The derived pinning energy shows that the Pr-doping, similar to weak links, decreases the vortex flux pinning energy. It is also concluded that the substitution of Pr at Ba site has a more destructive effect on the flux dynamics than Pr at rare earth site. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
A Process Mining Approach to Analyze Customer Journeys to Improve Customer Experience
, M.Sc. Thesis Sharif University of Technology ; Hassannayebi, Erfan (Supervisor)
Abstract
With the growth of the number of online service providers and the need to innovate in these services, in this study, the processes and the journeys taken by visitors of a website that provides employment services and employment insurance has been analyzed. In this research, process mining techniques and predictive process monitoring were implemented. With the use of a supervised and unsupervised learning algorithm, it attempted to identify the customer journeys' output and the existing patterns that lead to the complaint. In the first step, the website event log is extracted. Afterward, by using frequency-based encoding methods, the journeys traveled by users were clustered based on the...
Predictive Business Process Monitoring Using Machine Learning Algorithms
, M.Sc. Thesis Sharif University of Technology ; Hassannayebi, Erfan (Supervisor)
Abstract
In order to survive in today's business world, which is changing at a very fast pace, organizations can detect deviations even before they occur, quickly and with a high percentage of confidence, by analyzing their processes, in order to prevent disruptions in the processes. by monitoring the information systems that automatically execute business processes, it is possible to ensure the correct implementation of the existing processes. For this purpose, various techniques for monitoring business processes have been presented so that managers have a comprehensive and real view of how implement processes and be able to identify possible deviations in the future and try to fix them because the...
Data-Driven Prediction for Monitoring Business Process Pperformances Based on Classification Algorithms
, M.Sc. Thesis Sharif University of Technology ; Hassannayebi, Erfan (Supervisor)
Abstract
In recent years, several studies have been conducted in the field of data mining techniques in the field of process mining with the aim of improving the performance of organizations. Predictive process monitoring is a data-driven approach that helps business managers to improve the status and conditions of their organization. In this approach, the event log, which includes a set of completed examples of a process, is received as input, and machine learning methods are used to predict the outcome and results of the organization's processes before the process is completed. This prediction can include the prediction of the final result, the next event, the time remaining until the completion of...
Green Supplier Selection under Supply Risks with Respect to Supplier’s Financial Performance using Integrated Fuzzy MCDM Methods
, M.Sc. Thesis Sharif University of Technology ; Hassannayebi, Erfan (Supervisor)
Abstract
The supplier selection problem is considered as one of the most strategic and critical issues for any organization. This issue is mainly relevant to traditional and manufacturing businesses. However, if we examine its relationship with the emerging concept of Vendor Acquisition, which has become increasingly significant in modern businesses and startups, the importance of supplier selection becomes even greater. Today, choosing the right supplier or vendor determines the level of success organizations achieve in any new project, and the performance of vendors plays a crucial role in shaping and directing these projects. This study aims to present a comprehensive framework for supplier...
Operations Optimization in Supply Chain Systems using Simulation and Reinforcement Learning
, M.Sc. Thesis Sharif University of Technology ; Hassan Nayebi, Erfan (Supervisor)
Abstract
The inventory costs constitute a significant portion of the supply chain costs. Therefore, choosing an optimal inventory policy for orders is of great importance. The aim of this research is to find the optimal inventory policy for a distribution center in a three-tier supply chain consisting of a manufacturer, a distribution center, and a retailer. This research simulates a supply chain in agent-based framework and optimizes it using reinforcement learning. The optimization KPI in this research is the mean daily cost of the supply chain. Finally, the result obtained from reinforcement learning is compared with the optimized result of AnyLogic and the mean daily cost in the model optimized...
Predictive Process Monitoring Based on Optimized Deep Learning Methods
, M.Sc. Thesis Sharif University of Technology ; Hassannayebi, Erfan (Supervisor)
Abstract
Business processes are an essential part of every business as they provide insights on how to optimize and make them more efficient. Predictive Business Process Monitoring has garnered significant attention in recent years due to its capability to forecast process outcomes and predict the next activity within an ongoing process. In the last few years, there have been works that focused on deep learning and its applications in predicting the next activity. Some research used Long Term Short Memory, while others used Convolutional Neural Networks. However, long term short term memory models have the constraint of relatively slow training, while Convolutional Neural Networks are fast but may...
Optimal Vehicle Routing Problem with Pickup and Delivery for Same-Day Delivery Based on Machine Learning Approach
, M.Sc. Thesis Sharif University of Technology ; Hassan Nayebi, Erfan (Supervisor)
Abstract
The increasing demand for urban logistics services and the growing customer expectations regarding delivery speed and quality have made the vehicle routing problem a critical operational challenge for logistics companies. In this study, the same-day pickup and delivery vehicle routing problem was investigated using real-world data from a logistics company in Tehran. For this purpose, a mixed integer linear programming model was presented with the aim of minimizing the total travel cost, fixed vehicle operating cost, and delay costs. The problem formulation considered open vehicle routing, pickup and delivery, same-day delivery, stochastic and time-dependent travel times, and heterogeneous...