Sharif Digital Repository

Superconducting SQUID sensors are one of the most sensitive magnetic sensors that have been ever produced. Due to their high sensitivity, these sensors have many applications in different fields. Magnetic Imaging and far away targets detections are of these applications. In this project, a magnetic imaging system based on Superconducting SQUID sensors have been designed and implemented. In SQUID applications one of the most important problems is the cancelation of the unwanted fields including background low frequency and high frequency fields. In residential and urban areas the interference of Electricity network is considered as one of the major noise sources. While the high frequency... 

AdS/CFT Correspondence since it origination has found numerous applications in various aspects of Physics such as ion collision in Accelerators, Condensed matter Physics and etc. this orrespondence is a consequence of String Theory. In this project we have tried to analyze some of its applications in Condensed matter Physics and in the controversial phenomenon of superconductivity without resorting to the complexities of String Theory. Having applied this approach we have found a dual for it in Gravity and having calculated some common physical quantities and equations in these two sets such as Energy gap, Conductivity, London Equations and etc, we have tried to show the correspondence,... 

There is an ever increasing trend to apply AdS/CFT to explain physical phenomena and this endeavor has recently begun concerning Superconductivity. Using black holes with differing structures and gravitational content has proved to be uitful in this explanation. In this thesis we try to explain superconductivity for a charged ro-tating black hole with Kerr-Newman-Ads metric in four dimensions. Having calcu-lated condensation for a rotating black hole we obtained diagrams of phase transition in superconductivity with respect to temperature. In addition a brief discussion on calculating the conductivity has been presented  

Nonuniform superconducting microstrip transmission lines in microwave regime have been investigated. Nonuniform transmission lines provide the capability of controlling line parameters. Photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous yet spatially dispersive structures. This idea is here applied to nonuniform superconducting transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. This modelling is quite useful in geometrical and optical control of quasi-TEM wave propagation and dispersion engineering along microwave superconducting lines. This approach has applications... 

In this thesis, we have investigated nonuniform normal and superconducting microstrip transmission lines in microwave regime. Photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous structures. This idea is here applied to nonuniform transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. We present an analytical model based on telegraphers’ equations in order to obtain line propagation characteristics; characteristic impedance, propagation constant, and group velocity. The accuracy of analysis in normal and superconducting lines is evaluated by measurement and electromagnetic... 

Josephson junctions are the basic elements of superconducting technology. They play the same role of p-n junctions in semiconductor technology. Here, we investigated on the optical responses of the fabricated Josephson junctions in order to gain high sensitive optical sensors. The emission has both thermal and non-thermal effect on the current-voltage characteristics of the junctions. While the bolometric effect is the result of slight changes in the bias temperature of device, the non-thermal effect is due to depairing of Cooper pairs and generation of quasi-particles, which is dominant in temperatures far below TC especially in the case the superconducting layer is optically thin. In this... 

Fabrication of bulk superconductors with high critical current density widely has been studied because they have many applications, for example, Fault Current Limiter (SFCL), Superconducting Magnetic Energy Storage (SMES), and magnetic levitation. Providing pinning center by addings impurity into superconductor is usual and simple solution for increasing the superconductor critical current density. In this project we have fabricated superconductor YBCO bulk and optimized its critical current density. There are two major stream in this thesis, first design of Vibrating System Magnetization (VSM) for critical current measurements and then fabrication of YBCO bulk and optimization of its... 

Superconducting Quantum Interference Devices (SQUIDs) are one of the most sensitive magnetic flux detectors. According to their high sensitivity, these sensors have many applications such as magnetic imaging, magnetocardiography and magnetoencephalography. In this project we have designed and implemented a fluxgate based active shield that would be used in magnetocardiography. One of the main concerns in RF-SQUID applications is suppression of the low and high frequency noises. These noise sources includes earth magnetic force, humming noise and RF noise caused by cell phones. These sources could unlock RF-SQUID sensors and hence needs to be shielded. Low frequency noise suppression could be... 

The continuous rise in electricity demand and the ever growing power generation in different power networks over the world have caused the short circuit level in the power systems to exceed the operational rating of the installed power devices (including cables, circuit breakers and buses). Replacing these devices with new higher rating devices is an expensive solution. From this point of view, utilizing fault current limiters is an economical solution. Superconducting fault current limiters form an efficient category of current limiters. This type is expected to be studied and used widely in future due to their advantages. In this thesis, the first chapter reviews the superconductivity... 

Motion of magnetic flux, as a result of exerting electrical current, causes dissipation and voltage difference in the type II superconducting slab. So fixing these vortices by pinning centers will increase the critical current . Enhancing the critical current by different kinds of pinning centers is interesting for a lot of theoretical and experimental research groups. At first in this project dynamics of vortices in a triangular lattice is investigated by changing the pinning force. For the case of fp=2f0 we have seen two phase transition in velocity-force plot. And also a practical pinning array of parallel lines is presented. The effect of pinning force, pinning potential width and the... 

Synthesis and Investigation of Effect of Pr Doping on the Compound Y_(3-x) 〖Pr〗_x 〖Ba〗_5 〖Cu〗_8 O_(19-δ)-FInvestigation of doping of the parent compounds with different elements is important to understand the origin of high-Tc superconductivity. Several research groups have attempted to improve the properties of superconductivity and better understand the superconductivity mechanism of cuprate through doping different elements. In this research, at first, optimization of Y_3 〖Ba〗_5 〖Cu〗_8 O_(19-δ) F_z samples with z=0.2 ,0.4 ,0.6 ,0.8 ,1 ,2 ,3 ,4 have been done. By doping fluorine instead of oxygen, we expected an increase in the transition temperature, based on some the reports. But after... 

In this article some systems wich have a resolution beyond the diffraction limit and much more than the conventional lenses (like convex lenses, optical microscopes that have a resolution in order of wavelength) have been reviewed. These structures can be used in many different areas like Electron-Beam lithography (Photoleptography and Nano lithography), printing integrated optical circuits and chips, accurate cell imagings (entering and exiting of proteins in cells) and etc. This kind of imaging is called ”subwavelength imaging”. Generally some remarkable properties of these structures are propagation of all the spatial frequencies or amplifying the evanescent waves. One of the most known... 

In this thesis, first we study superconductivity and its mechanisms then classify different types of that based on symmetry of gap function. After that, we shall discuss metallic systems that have impurities in them and propose two successful model describing this phenomenon, Anderson’s model and Kondo’s model. In the next step, by introducing Kondo lattice model which is an ordered array of impurities in system, try to solve it by mean-field approximation and will be seen a phase transition in a characteristic temperature known as Kondo’s temperature. Then generalize these models to situation when background system is a superconductor and study it’s behavior and quantum phases. Although...