Electronic structures of compounds related to iron-based superconductor BaFe2As2

探討近年來研究高溫超導材料的趨勢，除了傳統的銅氧化物超導體，鐵基超導(Iron-based superconductors)材料為探討的新方向，更高臨界溫度材料也不斷被發現。而鐵基超導始終尚未有一個完美的實際理論運作機制可以解釋，而不同材料的臨界溫度Tc在特定外加壓力或參雜時均會有增加的現象，故鐵基超導的研究領域實為廣闊。 除了可利用實驗分析外，本論文利用第一原理計算軟體Vasp，亦可運用計算分析電子特性。研究分析內容分成三個部分︰ 第一部分探討基本的鐵基超導材料FeS、FeTe、FeSe的電子性質，如態密度、能帶結構及費米面。同結構的FeS、FeTe、FeSe有共通的特性，三種材料的態密度極為相似，其中Fe的d軌域貢獻在費米能量之上約在-2eV到2eV之間，而Se、Te及S的p軌域主要貢獻在費米能量之下，三種材料的費米面也極為相似。第一部分另探討√2×√2 AFMⅡ結構的FeSe，在0至120kB的外加壓力下晶格常數的趨勢變化，發現在70kB至80kB時，會有晶格常數Z軸長度小於X、Y長度的情況。 第二部分探討在BaFe_2 As_2 晶格中，以參雜不同濃度的Ru以取代Fe， ( 即Ba(Fe1-xRux)2As2 ) 的電子性質。考慮不同Ru參雜濃度X = 0.125與0.25，及不同Fe及Ru的磁性方向排列，詳盡的探討Ru削弱Fe的實際運作，考慮是否因為電荷轉移而造成Fe磁性削弱的現象，並判斷哪一種參雜結構最為為穩定。 第三部分探討在BaFe_2 As_2 晶格中，考慮同時以Ru取代Fe及以P取代As的參雜情況，( 即Ba(Fe1-xRux)2(As1-yPy)2 )，藉由不同Ru參雜濃度X = 0.125、0.25及Y = 0.125、0.25。Ru取代Fe及P取代As，以第一原理計算探討電子性質，可以發現與Fe連結的P，亦有削弱磁性的現象。In recent years, besides the traditional copper oxide superconductors, Iron-based superconductors and higher critical temperature materials have become a new trend in research of high Tc superconductors . As the traditional copper oxide superconductors are still lacking a perfect theorem to explain how they work, and the higher critical temperature materials in different outer pressures or in the situation of doping have the problem of Tc increase, the research of Iron-based superconductors is therefore considered to be more potential. Instead of experimental analysis, this thesis applies the first principle calculation software – Vasp to analyze the electron properties. The main discussions are broken down into three significant parts with conclusions as follows: Part I is the discussion on the electron properties of FeS, FeSe and FeTe, such as density of state, band structure and Fermi Surface, and the change of the lattice constant of FeSe. Part II is the discussion on the doping effect. I choose Ba(Fe1-xRux)2As2 , that is, we substitute Ru for Fe by different X. By considering different types of spin polarize and different places where Ru doped in Ba(Fe1-xRux)2As2 crystal. The doping of Ru shows a property of the suppression of the magnetic moment. Ru atoms like to remain nonmagnetic in this structure. We consider two circumstances, X = 0.125 and X = 0.25. By using the first principle calculation method, we are trying to figure out how the suppression works. Part III--- we consider Ba(Fe1-xRux)2(As1-yPy)2 . By replacing Ru and P at the same time, we find that the P substations also decrease the magnetic moment of the nearest iron atoms. We consider X = 0.125 and 0.25，Y = 0.125 and 0.25

Distributed adaptive rate system for wireless packet networks

[[abstract]]A distributed adaptive rate system for wireless packet networks is proposed. Compared to the centralized adaptive rate system that needs to know the packet retransmission probability to maximize the throughput, this system need not know the packet retransmission probability in advance and can achieve the optimal adaptive rate system that maximizes the throughput for wireless packet networks.[[note]]SC

Joint MMSE Equalization and Power Control for MIMO System under Multi-User Interference

[[abstract]]The joint MMSE equalization and power control (JMEPC) algorithm, which jointly and iteratively adjusts the equalization weights and the power level of each user, is proposed for the MIMO system under multi-user interference. Two types of MMSE equalization, namely, the parallel MMSE equalization and the optimum successive decoding (OSD), are considered. In the JMEPC algorithm, we let the mean capacity among cochannel the users be the capacity target and make the resulting capacity approach the maximum achievable capacity through the iterations that can be finished in a distributed manner. Simulation results show that the JMEPC algorithm increases the capacity of the MIMO system and the parallel MMSE equalization with JMEPC can have larger capacity than the OSD without JMEPC.[[note]]SC

Joint rate regulation and power control for cochannel interference limited wireless networks

[[abstract]]Centralized and distributed algorithms for joint rate regulation and power control are proposed for the wireless networks where the cochannel interference limits the capacity. The goal of the proposed algorithms is maximizing the data rate while minimizing the transmitting power on each hop of the wireless networks. The distributed algorithm simulates the operation of the centralized algorithm in a distributed fashion and need not measure the link gains for all transmission links and interference links. We prove that the distributed algorithm can find the maximal transmitting rate and the minimal transmitting power as the centralized algorithm does. Simulation results also show that the proposed distributed algorithm outperforms the previous distributed algorithm in the data rate achieved. Copyright (C) 2011 John Wiley & Sons, Ltd.[[note]]SC

Efficient power assignment for multi-cell CDMA networks

[[abstract]]There exists an optimum power assignment, which can be obtained by solving a set of linear equations, for multi-cell CDMA networks. According to the direct method, we have to measure all the link gains to solve these linear equations. We propose in this paper an indirect method which allows each base station to only measure the received power for each user in its cell area and the interference from each other-cell. Compared to the direct method, the number of measurements can be largely reduced.[[note]]SC

Ratioed power and rate control for CDMA wireless networks

[[abstract]]We propose in this paper the ratioed power and rate control (RPRC) algorithm to satisfy the requirements of both the data rate and the signal quality. During the execution of the RPRC algorithm, only the representative user in each cell adjusts its power level according to certain distributed power control algorithm, and the ratio of the power level for the non-representative user to that for the representative user is kept constant. When the RPRC algorithm is finished, the power levels for all users remain unchanged and the transmission rate is determined by the available rate. We show that the RPRC algorithm can be used for allocating the bandwidth of each cell and achieving the required rate of each user. Furthermore, simulation results reveal that the RPRC algorithm results in larger throughput and finds a feasible power set faster than the other algorithm. Copyright (C) 2008 John Wiley & Sons, Ltd.[[note]]SC

Integrated base station assignment, diversity and power control for cognitive radio networks

[[abstract]]An integrated base station assignment, diversity and power control algorithm is proposed to realize the base station sharing, in which primary users (PUs) and secondary users (SUs) can connect to any base station by the variable base station assignment, so that the radio resource is efficiently utilized for cognitive radio networks. We show that this algorithm reduces SU's outage rate (for failing to meet the requirement of SIN R per bit) while always meeting the requirement of SINR per bit for all PUs. (C) 2011 The Franklin Institute. Published by Elsevier Ltd. All rights reserved.[[note]]SC

Distributed rate assignment for multicarrier multi-cell networks

[[abstract]]A distributed rate assignment is proposed for the multicarrier multi-cell networks. It assigns the data rate for each subchannel in a distributed manner and needs not measure the channel gains. We show that the aggregate rate can be increased by increasing the number of iterations in this assignment.[[note]]SC

Distributed subchannel and rate assignment for multi-carrier multi-cell networks

[[abstract]]A distributed subchannel and rate assignment (DSRA) algorithm, which integrates the subchannel allocation and the rate assignment to minimize the number of subchannels needed for meeting the rate requirements, is proposed for the multi-carrier multi-cell networks. The DSRA algorithm need not measure the channel gains and executes the distributed power control to allocate the subchannel and transmission rate. One can decrease the number of subchannels used in the DSRA algorithm by increasing the number of iterations.[[note]]SC

Rate control with distributed joint diversity and power control for wireless networks

[[abstract]]Two rate control algorithms with distributed joint diversity and power control are proposed for wireless networks. We prove that with the proposed algorithms, the allowed transmitting rate can converge to the individual rate requirement with which the QoS requirement can be satisfied, and each user can share the bandwidth according to the pre-specified setting so that certain fairness can be maintained among the users in the same cell. Simulation results also show that the scheme with rate control gains larger throughput than the scheme without rate control.[[note]]SC