Sharif Digital Repository

For more than a century, physicists have recognized the contrast between the asymmetry of thermodynamic phenomena and T-symmetry of the fundamental laws, and they have referred to it as the arrow of time. In this thesis we tried to give an overview of the past and present status of this problem, using various theories. It is shown that the problem becomes a question about the boundary conditions of thermodynamic systems, and then a question about cosmology, and finally it becomes the problem of initial conditions of universe. An analysis of initial condition theories is presented for knowing how each of them tries to solve the problem of arrow of time. Different views about the relationship... 

With the extensive use and the growing popularity of file sharing in peer-to-peer (P2P) networks, the distribution of copyrighted files and the propagation of malwares have also increased. The increasing growth of these crimes multiplies the necessity of forensic investigation in order to detect the offenders. The yet proposed methods have sought to gather some proofs for forensic investigation by the application of some changes in clients’ programs and a constant monitoring of the network. These methods, however, have some limitations which have rendered useless the results of their application in real environments. Moreover, the important issues like the detection of the file source have... 

Valve leak detection is very important due to bad direct effects on parts life and engine power. Acoustic emission (AE) has a good known ability on leak detection between fault detection methods. The aim of this thesis is presenting the ability of AE method for valve leak detection in internal combustion engines. The research method in a general framework based on the idea that firstly some evaluations were done in static tests for leakage due to three faults including clearance, quasi-crack and notch. Then, the evaluations were continued with dynamic analysis of rotational speeds and different loads on the engine. Finally, the neural network approach was used to automatically detect all... 

Network forensic analysts employ Payload Attribution Systems (PAS) as an investigative tool. With such tool large amount of network traffic, including full packet payload, is summarized and stored. Hence an investigator could query the system for a specific string and see whether any packets transmitted previously in the network contained that specific string. Wildcards are an important type of query that allows the investigator to locate strings in the payload when only part of the string is known. In this thesis we present a new payload attribution technique, named character based bloom filter, which in addition to improving on previously proposed techniques is able to support wildcard... 

The zero-energy excitations of topological superconductor promising the design of quantum computers have received a great deal of attentions in recent years. But the materialization has remained to be elusive. Our focus in this thesis is on the one- and two-dimensional systems where the necessary ingredients may conspire to realize topological superconductors. In particular, we will focus on the role of reduction of spatial symmetries near the interfaces. We will start with a one-dimensional system at the boundary of a two-dimensional system with particular orientation so that the full translational symmetry is replaced with a quasi-periodic symmetry. We show that the Fibonacci sequence... 

Nowadays, chemotherapy is one of the common ways to treat cancer. Although, chemotherapy is used to kill cancer cells , chemotherapy drugs also target healthy cells and tissues of the body. In practice, it is difficult to strike a balance between injectable doses of drugs and toxicity in the body, and physicians are always looking for an effective treatment plan. One of the most common drugs that physicians use in chemotherapy is cell-cycle-specific drugs. These types of chemotherapy drugs affect only a certain phase of cell cycle and to a certain extent. Since the lifetime of the cancer cells is uncertain, the effective use of these drugs depends on the correct injection time of the drugs... 

In this thesis, inspired by foundational discussions about the concepts of energy and entropy in the gravitational theory, we study the definitions of these concepts and the roles they play in that theory. The definitions considered in this thesis are derived from canonical formalism. To this end, first of all, the variational principle and the canonical structure in the gravitational theory, especially in situations where the spacetime is bounded, is carefully studied and the boundary and corner actions as well as the canonical structure on the boundaries in three cases of non-null, null and general boundaries are derived. Based on this study, two canonical approaches, namely the... 

Primary visual cortex plays a paramount role in processing visual information. Visual information after the primitive processing by LGN and Thalamus, will be sent to the primary visual cortex which is called V1 area. According to the fact that V1 neurons have the definitive sensitivity to the edge and contrast of visual stimuli so their behavior are very selective to the certain features, specially contrast, of visual stimuli. These neurons have different response behavior to contrast. In fact most of them have some nonlinear behavior to this feature. Normally we expect that response of them or spiking rate of them grow linearly by the contrast increases but in most many cases they have some... 

Examining and improving driving behavior leads to a reduction in road accidents. Driver behavior is generally divided into two categories: aggressive and safe. Driving culture can be enhanced by identifying aggressive behavior and informing drivers directly. This information is used to assign safe drivers to the missions of transport fleet management companies and organizations. Also, this information is used by insurance companies or traffic police to apply discounts or fines. In any case, the detection and notification of aggressive behaviors would reduce accidents and improve the lives and mental safety of passengers and drivers. The detection of driving events is an introduction to the... 

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... 

Terahertz waves which includes frequencies between 100 GHz to 10 THz of electromagnetic spectrum have found many applications in science, medical treatments and industry. One of the recent methods to produce these waves is using nonlinear properties of superconductors based on Josephson junctions which inherently appear in some superconductors called BSCCO. Stack of intrinsic junctions of these superconducting materials can radiate terahertz waves by only applying a DC voltage across them. This Thesis investigates some of these structures based on Josephson junctions to produce coherent radiation of terahertz waves. For the optimization of the radiation power, this study also investigates... 

Internet is the largest multi-purpose, self-reliant, complex and distributed computer network across the globe. The nodes of this network are placed in every place like homes, offices, military camps, schools and all other locations by utilizing many different communication protocols, media and capacities. Furthermore, by rapid technology development, there are many newly born applications utilizing internet in new ways such as Internet of Things systems. The explosive growth of technology causing accelerated addition of new nodes to the internet, and any new single node may have several new unpatched vulnerabilities. This is a serious issue for managing this vast chaotic configuration and... 

There are different approaches in modifications of General Relativity as the theory of gravity. The modified theories of gravity are important cases in this context, which mostly start by modifying the Einstein-Hilbert action, while keeping the general covariance. Within the modified theories of gravity, we consider two cases. The first is the f(R) theories of gravity, and the second is curvature squared theories of gravity with an action like S = ∫d4x pg( 116GR + (R)2 + R2 +(R)2). Curvature squared theories of gravity have interesting feautures, most important of which is renormalizability, in contrast with General Relativity. But this advantage, as noted by K.S. Stelle in 1977 comes at...