费米冷原子气体中的奇异超流相研究

发布时间：2018-06-03 02:17

本文选题：冷原子气体 + 光晶格　；参考：《中国科学技术大学》2016年博士论文

【摘要】：冷原子物理是利用磁光阱囚禁极低温的碱金属原子,并对其进行测量、操控的新研究领域。其利用Feshbach共振技术,可以人为调控冷原子系统的多体碰撞相互作用。因此冷原子气体系统的实现和研究,可以用于量子模拟凝聚态系统,实现可人为调控的强关联体系,并对多体系统基本原理的研究,寻找强关联体系中的新奇量子态,提供了理想的实验平台。1911年Onnes发现的超导现象,是凝聚态物理研究的热点。1957年, Bardeen、 Cooper和Schrieffer提出BCS平均场理论,开启了理论解释超导／超流现象的先河。随着重费米子系统超导体(CeColn5)、有机物超导体((TMTSF)2PF2)、拓扑超导体(Kitaev链)等新现象的发现,BCS平均场理论已经不足以解释这些新奇的超导态。一方面,Fulde-Ferrell等奇异超流相的提出,为理论解释这些新超导态提供了新思路。另一方面,Fulde-Ferrell等奇异超流相在真实的物理系统中的实现,因为技术上的困难而仍缺乏实验上的观测。近年来拓扑材料及拓扑相变的研究得到广泛关注。所谓拓扑性,是指系统依靠时间反演对称性、粒子·空穴对称性、和手性对称性这三种对称性的有无,使得系统的拓扑序发生变化,其显著特征是系统中存在空间分布局域在材料边界的拓扑边缘态。对拓扑相变的研究能够解释诸如量子霍尔效应、拓扑绝缘体、拓扑超导体等特殊量子态出现的物理机制。其中,拓扑超导体因其支持以手性边缘态形式存在的Majorana费米子态.而成为拓扑材料领域的一个焦点。对拓扑超导体的研究,能够为拓扑量子计算和量子存储提供理想的实验平台,在容错拓扑量子计算上有着重要的应用前景。基于以上背景,对奇异超流相出现的物理机制和拓扑性质的研究,及其在冷原子系统中的制备、实现,是这篇论文关注的焦点。我们展开的研究工作具体如下：1、利用自旋轨道耦合作用制备Fulde-Ferrell超流相。在近年来的冷原子实验工作中,人们利用Raman激光,将被囚禁的冷原子的两个超精细能级耦合起来。由于在冷原子物理研究中,超精细能级通常被标记为自旋量子数,而Raman激光耦合的过程中原子能级的跃迁存在动量传递。因此这个实验过程实际上模拟了凝聚态物理系统中的自旋轨道耦合作用。同时,两个超精细能级间的调谐模拟了作用在赝自旋空间的Zeeman场。我们的工作是利用自旋轨道耦合作用和Zeeman场,研究这两个外场的同时存在对冷原子的超流态影响。我们展开的研究如下：(1)寻找奇异超流相Fulde-Ferrell超流相存在的可能性,并研究其出现的物理机制。(2)讨论Fulde-Ferrell超流相在拓扑相变中表现的拓扑性质,寻找奇异超流相中Majorana费米子态存在的踪迹,并讨论拓扑相变出现的物理机制。(3)研究两个外场作用对多体系统热力学性质影响。2、利用震荡光晶格技术制备Fulde-Ferrell超流相。冷原子物理中的震荡光晶格技术是近年来发展的实验方法,它的核心是利用激光,对原有的光晶格系统在某个维度上进行光晶格震荡。通过调节震荡的频率,使得系统不同轨道能带间发生原子跃迁耦合,进而改变系统的单粒子性质。我们的工作就是利用震荡光晶格所引入的不同轨道能带耦合,诱导Fulde-FeI]rell超流相,并研究其出现的物理机制和拓扑性质。3、利用驱动光晶格技术制备Fulde-Ferrell超流相。受震荡光晶格技术的启发,我们利用激光对原有的光晶格体系里,在某个维度上再加一套运动的光晶格势场。通过调节驱动的光晶格的运动速度,可以耦合不同轨道的能带,从而改变系统单粒子性质。由于单粒子能带的空间对称性被破坏,Fulde-Ferrell超流相应运而生。这个的方案构造简单,能够规避自旋轨道耦合带来的热效应等带来的技术阻碍。另外,由于系统的自旋简并没有被破坏,这种方案诱导的Fulde-Ferrell超流相没有自旋极化率,这一特性明显区别于先前依靠破坏自旋简并制备Fulde-Ferrell超流相的工作。因此、我们的工作对Fulde-Ferrell超流相出现的物理机制提供了新的思路。4、利用自旋依赖的光品格系统制备p波超流相。在冷原子物理中,自旋依赖的光晶格技术是通过不同的激光将两种超精细态的原子囚禁在两套光晶格里。我们的工作是将这两套光晶格在空间上错位,使得一套光晶格囚禁的原子可以同时与另一套光晶格囚禁的原子发生关联作用,进而诱导出p波超流相。这里,p波超流相是一种具有拓扑非平庸性质的超导态,在p波超流相中可以寻找到在基本粒子物理、暗物质等领域有重要作用的Majorana费米子态。我们的工作提供了在冷原子系统中实现p波超流相的方案,相比先前研究文献的结果.我们的方案构造简单,能够规避先前实验上p波Feshbach共振的技术困难,具有更好的实验可行性。
[Abstract]:Cold atom physics is a new field of research, which uses a magnetic optical trap to imprism extremely low temperature alkali metal atoms, and is a new field of control. By using Feshbach resonance technology, it can artificially regulate the interaction of multibody collisions between cold atomic systems. Therefore, the realization and research of cold atomic gas system can be used in quantum simulation condensed state system, and it can be realized. The strong association system which can be controlled, and the study of the basic principle of the multibody system, search for the novel quantum state in the strong association system, provide the superconducting phenomenon found in the ideal experimental platform.1911 Onnes, which is a hot.1957 year in condensed matter physics. Bardeen, Cooper and Schrieffer put forward the theory of BCS mean field, which opens the theoretical explanation. Superconductor / supercurrent phenomenon. With the discovery of new phenomena such as heavy fermion system superconductor (CeColn5), organic superconductor ((TMTSF) 2PF2), topological superconductor (Kitaev chain) and other new phenomena, BCS mean field theory is not enough to explain these new superconducting states. On the one hand, the new supercurrent of Fulde-Ferrell and other singular superfluid phase is put forward to explain these new superconductors in theory. State of state provides new ideas. On the other hand, the realization of Fulde-Ferrell and other singular superfluid phases in real physical systems, because of technical difficulties, still lacks experimental observation. In recent years, the research of topological materials and topological phase transition has been widely concerned. The so-called topology, which refers to the inversion of symmetry by time, and particle cavity symmetry. Whether there are three kinds of symmetry, such as sex, and chiral symmetry, make the topological order of the system change. Its remarkable feature is that there is a topological edge state of the spatial distribution in the material boundary in the system. The study of the topological phase transition can explain the physics of special quantum states such as the quantum Holzer effect, the topological insulator, the topological superconductor and so on. The topological superconductor is a focal point in the field of topological materials because of its support for the Majorana fermion states in the form of chiral edge states. The study of topological superconductors can provide an ideal experimental platform for topological quantum computation and quantum storage. It has an important application prospect in fault-tolerant quantum computing. In the above background, the research on the physical mechanism and topological properties of the singular superfluid phase and its preparation and implementation in the cold atomic system are the focus of this paper. 1, 1, using spin orbit coupling to prepare the superfluid phase. In the cold atom experiment in recent years, The Raman laser is used to combine the two ultra fine energy levels of the trapped cold atoms. Because in the cold atom physics, the hyperfine level is usually labeled as the spin quantum number, while the transition of the atomic energy level in the Raman laser coupling exists in the momentum transfer. So this experimental process actually simulates the condensed matter physics. The spin orbit coupling in the system. At the same time, the tuning between two hyperfine energy levels simulates the Zeeman field acting in the pseudo spin space. Our work is to study the effect of the two external fields on the superflow state of the cold atoms by using the spin orbit coupling action and the Zeeman field. The possibility of the existence of the flow phase Fulde-Ferrell supercurrent and its physical mechanism. (2) the topological properties of the Fulde-Ferrell superfluid phase in the topological phase transition are discussed, the trace of the Majorana fermion state in the singular superfluid phase is found, and the physical mechanism of the appearance of the topological phase transition is discussed. (3) the study of two external fields to the multibody system The thermodynamic properties affect.2, using the concussion optical lattice technology to prepare the Fulde-Ferrell superfluid phase. The shock optical lattice technology in cold atom physics is an experimental method developed in recent years. Its core is to use laser to shake the original optical lattice system in a certain dimension. By adjusting the frequency of the shock, the system is different. The coupling of atomic transition between the orbital energy bands will change the single particle properties of the system. Our work is to use the different orbital energy bands introduced by the oscillating light lattice to induce the Fulde-FeI]rell superfluid phase, and to study the physical and topological properties of the.3, and to prepare the Fulde-Ferrell superfluid phase by the drive optical lattice technology. Inspired by the optical lattice technology, we use the laser to add a set of light lattice potential fields to a certain dimension in the original optical lattice system. By adjusting the velocity of the driven optical lattice, we can coupling the energy bands of different orbits, thus changing the single particle properties of the system. Because the spatial symmetry of the single particle energy band is destroyed, Fuld This scheme is simple and can avoid the technical impediments caused by the thermal effect of spin orbit coupling. In addition, because the spin degeneracy of the system is not destroyed, the Fulde-Ferrell superflow phase induced by this scheme has no spin polarization, which is obviously different from the previous reliance on spins. Degenerate and prepare the Fulde-Ferrell superfluid phase. Therefore, our work provides a new idea for the physical mechanism of the Fulde-Ferrell superfluid phase,.4, using the spin dependent optical character system to prepare the P wave superfluid phase. In cold atom physics, the spin dependent optical lattice technology is the two hyperfine atoms by different lasers. It is imprisoned in two sets of optical lattices. Our work is to mislocate the two sets of optical lattices in space, so that a set of atoms trapped by a set of optical lattices can be associated with another set of atoms trapped in a set of optical lattices, and then the P wave superfluid phase is induced. Here, the P wave superfluid phase is a superconducting state with a topological non mediocre property, in the P wave superstructure. We can find the Majorana fermion states that have important roles in the fields of elementary particle physics, dark matter and other fields. Our work provides a scheme for the realization of the P wave superfluid phase in the cold atomic system. Compared with the previous research literature, our scheme is simple and can avoid the technical difficulties of the P wave Feshbach resonance in previous experiments. It has better experimental feasibility.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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