配对对称性与带间作用

研究d p模型的超导性质.可出现d p配对占主导的情形，配对对称性取决于CuO2面内空穴作用的各向异性，可以是纯d波配对，也可以是纯s波配对.CuO2面内各向排斥作用不能导致空穴配对.欠掺杂区域可以出现“预配对”.当库伯对是局域的则不能根据配对函数求超导临界温度.空穴的退局域以及配对参量对称性的演化也能得到理解. 关键词： d波对称性 d p配对 超导电性     

由超辐射引起的迁移率边和重返局域化

研究了束缚在梯子形光晶格中的中性原子与高精度腔耦合的系统,发现由超辐射引起的准周期调制可以导致迁移率边和重返局域化现象的出现.在平均场近似下,超辐射现象可以引起两种不同频率的准周期调制,它们可以由腔场和泵浦场有效调节.在本文的观测范围内,当高频调制强度小于某一临界值时,系统随着低频调制强度的增加经历一次局域转变.通过数值求解分形维度、密度分布、平均参与率、平均逆参与率以及标度分析,证明了局域转变历经的临界相区存在迁移率边;当高频调制强度大于临界值时,随着低频调制强度的增强,系统依次经历完全扩展相-临界相-完全局域相-临界相-完全局域相,这是一个典型的重返局域化现象.最后给出了局域化相图.该研究结果为超辐射相变和重返局域化现象的研究建立了联系,也为重返局域化的研究搭建了新的平台.     

含有Dzyaloshinskii-Moriya相互作用的自旋1键交替海森伯模型的量子相变和拓扑序标度

利用张量网络表示的无限矩阵乘积态算法研究了含有Dzyaloshinskii-Moriya (DM)相互作用的键交替海森伯模型的量子相变和临界标度行为.基于矩阵乘积态的基态波函数计算了系统的量子纠缠熵及非局域拓扑序.数据表明,随着键交替强度变化,系统从拓扑有序的Haldane相转变为局域有序的二聚化相.同时DM相互作用抑制了系统的二聚化,并最终打破系统的完全二聚化.另外,通过对相变点附近二聚化序的一阶导数和长程弦序的数值拟合,分别得到了此模型相变的特征临界指数a和b的值.结果表明,随着DM相互作用强度的增强, a逐渐减小,同时b逐渐增大. DM相互作用强度影响着此模型的临界行为.针对此模型的临界性质的研究,揭示了量子自旋相互作用的彼此竞争机制,对今后研究含有DM相互作用的自旋多体系统中拓扑量子相变临界行为提供一定的借鉴与参考.     

一维超冷原子动量光晶格中的手征对称性破缺拓扑相

对称性在理解物质的拓扑态方面具有关键作用.过去人们认为手征对称性保证了一维晶格的量子化Zak相位及其对应的非平庸拓扑相.本文展现了在一维手征对称性破缺的情况下,晶格系统仍具有量子化Zak相位和非平庸拓扑相.具体而言,在超冷原子动量晶格系统中有效地模拟了一个链长为26、手征对称性破缺的Zigzag模型,其中相等的次近邻耦合强度能够在保留空间反演对称性的同时破坏手征对称性.通过测量原子的时间平均波包位移来获得系统的拓扑不变量,并得到了其对应的量子化的Zak相位.此外,还观测到系统随着最近邻耦合强度比例的变化会从非平庸拓扑相转变为平庸拓扑相.本文不仅为对称性及拓扑相的相关研究提供了一个完全可控的平台,还可以通过控制格点间耦合强度和原子间相互作用,探索例如Tasaki, Aharonov-Bohm caging模型中的平带拓扑以及引入相互作用研究的非线性拓扑现象.     

重费米子超导体CeCoIn_5中的局域电子结构研究

重费米子材料CeCoIn_5具有相似于高温铜氧化物和铁基超导体的奇异物性,大量实验揭示其超导电性可能由反铁磁自旋涨落引起,表现出d波非常规超导配对态.在本文中,我们从CeCoIn_5的双能带模型出发,通过考虑杂质散射效应,应用T矩阵近似和格林函数方法,在弱散射和强散射情况下,分别计算费米能级附近的局域电子密度态性质.我们发现杂质诱导的共振态局域在杂质周围,其空间调制结构强烈依赖于d波配对序参量的节线方向,这些新奇物性有助于确认其d_(x~2-y~2)波配对对称性.     

拓扑材料中的超导

近来,人们在凝聚态体系中发现了由拓扑不变量定义的物相,其中最重要的有拓扑绝缘体、拓扑半金属和拓扑超导体等.这些物相的拓扑性质由非平凡的拓扑数描述,相应的材料被称为拓扑材料,具有诸多新奇的物理特性.其中拓扑超导体由于边界上有满足非阿贝尔统计的Majorana零能模,成为实现拓扑量子计算的主要候选材料.除了探索本征的拓扑超导体外,由于拓扑性质上的相似性,在不超导的拓扑材料中调制出超导自然成为了实现拓扑超导的重要手段.目前,人们发展了栅极调制、掺杂、高压、近邻效应调制和硬针尖点接触等多种技术,已经成功地在许多拓扑绝缘体和半金属中诱导出了超导,并对超导的拓扑性和Majorana零能模进行了研究.本文回顾了本征拓扑超导候选材料,以及拓扑绝缘体和半金属中诱导出超导的代表性工作,评述了不同实验手段的优势和缺陷、分析了其超导拓扑性的证据,并提出展望.     

应变调控下Tl2Ta2O7中的拓扑相变

拓扑电子材料因为具有非平庸的拓扑态,所以会展现出许多奇异的物理性质.本文通过第一性原理计算对应变调控下的烧绿石三元氧化物Tl₂Ta₂O₇中的拓扑相变进行了研究.首先分析了原子轨道投影能带,发现体系费米能级附近O原子的(px+py)与pz轨道发生了能带反转,再构造了紧束缚模型计算得到体系的Z₂拓扑不变量确定了其拓扑非平庸性,最后研究了表面态等拓扑性质.研究发现未施加应变的Tl₂Ta₂O₇是一个在费米能级处具有二次能带交叉点的半金属,而平面内应变会破缺晶体对称性进而使体系发生拓扑相变.当对体系施加–1%的压缩应变时,它会转变为狄拉克半金属;当对体系施加1%的拉伸应变时,体系相变为拓扑绝缘体.本研究对于在三维材料中调控拓扑相变有着较重要的指导意义,并且为低能耗电子器件的设计提供了良好的材料平台.     

手性ABM态p波超导体上临界磁场的角依赖关系

利用格林函数方法,通过Klemm-Clem变换,计算具有正交晶格结构椭球形费米面的手性ABM态p波超导体上临界磁场的角依赖关系.超导序参量选取具有手性ABM对称性的等自旋配对单分量形式.当椭球形费米面满足一定条件时,上临界磁场随角度呈现非单调变化,表明除配对电子有效质量的各向异性外,手性ABM态p波超导体也具有超导序参量的各向异性.计算结果可用于判断重费米子超导体Sr2RuO4的配对电子空间结构.     

单晶Ce冲击相变的分子动力学模拟

金属Ce在室温条件下当压力达到约0.7 GPa时会发生一阶相变,体积突变减小14%—17%,相变前后两相分别为γ-Ce和α-Ce,均为面心立方结构.实验中发现冲击波在Ce中传播,其波形存在明显的多波结构依次为γ-Ce弹性前驱波、γ-Ce塑性波、γ-Ce→α-Ce相变波.基于新发展的金属Ce的嵌入原子势,对单晶Ce的冲击相变行为进行了分子动力学模拟.模拟结果表明,在一定强度下,单晶Ce中的冲击波阵面分裂为多波结构,波形结构与加载晶向明显相关:在[001]和[011]晶向加载下表现为双波结构,依次为前驱波和相变波;在[111]晶向加载下波阵面分裂为弹性前驱波、γ-Ce塑性波、γ→α相变波,与已有的实验观察相一致.冲击波速的Hugoniot关系在低强度加载下与实验符合得较好.同时在此冲击相变过程中,应力偏量对相变起促进作用,相较于静水压加载,冲击加载的相变压力条件更低一些.     

AuCu3 型超导体SrPb3 中德哈斯-范阿尔芬振荡及非平庸拓扑能带

AuCu3 型超导体兼具非平庸的拓扑能带, 是近期受到关注的拓扑超导体候选材料之一. 理论预言 SrPb3具有非平庸的拓扑能带, 然而, 实验上尚未得到证实. 本工作观察到德哈斯-范阿尔芬振荡, 验证了 SrPb3 中电子能带的非平庸拓扑性质. 通过分析提取出两个振荡频率Fα =3.54 T 和Fβ =7.43 T. 进一步利用 Lifshitz-Kosevich 公式以及朗道量子化理论, 得到两个对应费米面的载流子有效质量分别为 mα* =0.006m0 和 mβ* =0.008m0 , 同时它们的贝里相位分别为?αB = (1 .32±0.25)π 和?βB = (1 .10±0.25)π. 这些结果表明 SrPb3 中存在有效质量接近于零的相对论费米子和非平庸拓扑能带. SrPb3 中超导电性与非平庸拓扑能带的共存, 为研究超导与拓扑的关联以及探索拓扑超导体提供新的研究体系.     

Topological phase transition in a ladder of the dimerized Kitaev superconductor chains

We investigate the topological properties of a ladder model of the dimerized Kitaev superconductor chains.The topological class of the system is determined by the relative phase θ between the inter-and intra-chain superconducting pairing.One topological class is the class BDI characterized by the Z index,and the other is the class D characterized by the Z_2 index.For the two different topological classes,the topological phase diagrams of the system are presented by calculating two different topological numbers,i.e.,the Z index winding number W and the Z_2 index Majorana number M,respectively.In the case of θ=0,the topological class belongs to the class BDI,multiple topological phase transitions accompanying the variation of the number of Majorana zero modes are observed.In the case of θ = π/2 it belongs to the class D.Our results show that for the given value of dimerization,the topologically nontrivial and trivial phases alternate with the variation of chemical potential.     

Thermal stability of uni-size Pt cluster disk constructed on silicon substrate

We propose a scheme to implement controlled not gate for topological qubits in a quantum-dot and Majorana fermion hybrid system. Quantum information is encoded on pairs of Majorana fermions, which live on the the interface between topologically trivial and nontrivial sections of a quantum nanowire deposited on an s-wave superconductor. A measurement based two-qubit controlled not gate is produced with the help of parity measurements assisted by the quantum-dot and followed by prescribed single-qubit gates. The parity measurement, on the quantum-dot and a topological qubit, is achieved by the Aharonov-Casher effect.     

Evolution of edge states in topological superfluids during the quantum phase transition

The quantum phase transition between topological and nontopological insulators or between fully gapped superfluids/superconductors can occur without closing the gap. We consider the evolution of the Majorana edge states on the surface of topological superconductor during transition to the topologically trivial superconductor on example of non-interacting Hamiltonian describing spin-triplet superfluid ³He-B. In conventional situation when the gap is nullified at the transition, the spectrum of Majorana fermions shrinks and vanishes after the transition to the trivial state. If the topological transition occurs without the gap closing, the Majorana fermion spectrum disappears by escaping to ultraviolet, where the Green’s function approaches zero. This demonstrates the close connection between the topological transition without closing the gap and zeroes in the Green’s function. Similar connection takes place in interacting systems where zeroes may occur due to interaction.     

Superconducting proximity effect and majorana fermions at the surface of a topological insulator

We study the proximity effect between an s-wave superconductor and the surface states of a strong topological insulator. The resulting two-dimensional state resembles a spinless px+ipy superconductor, but does not break time reversal symmetry. This state supports Majorana bound states at vortices. We show that linear junctions between superconductors mediated by the topological insulator form a nonchiral one-dimensional wire for Majorana fermions, and that circuits formed from these junctions provide a method for creating, manipulating, and fusing Majorana bound states.     

Topological Superconductivity in Cu(x)Bi(2)Se(3)

A topological superconductor (TSC) is characterized by the topologically protected gapless surface state that is essentially an Andreev bound state consisting of Majorana fermions. While a TSC has not yet been discovered, the doped topological insulator Cu(x)Bi(2)Se(3), which superconducts below ～3 K, has been predicted to possess a topological superconducting state. We report that the point-contact spectra on the cleaved surface of superconducting Cu(x)Bi(2)Se(3) present a zero-bias conductance peak (ZBCP) which signifies unconventional superconductivity. Theoretical considerations of all possible superconducting states help us conclude that this ZBCP is due to Majorana fermions and gives evidence for a topological superconductivity in Cu(x)Bi(2)Se(3). In addition, we found an unusual pseudogap that develops below ～20 K and coexists with the topological superconducting state.     

Majorana fermions in equilibrium and in driven cold-atom quantum wires

We introduce a new approach to create and detect Majorana fermions using optically trapped 1D fermionic atoms. In our proposed setup, two internal states of the atoms couple via an optical Raman transition-simultaneously inducing an effective spin-orbit interaction and magnetic field-while a background molecular BEC cloud generates s-wave pairing for the atoms. The resulting cold-atom quantum wire supports Majorana fermions at phase boundaries between topologically trivial and nontrivial regions, as well as "Floquet Majorana fermions" when the system is periodically driven. We analyze experimental parameters, detection schemes, and various imperfections.     

Topological phase diagrams and Majorana zero modes of the Kitaev ladder and tube

In this paper,we study two quasi-one-dimensional(1 D) Kitaev models with ladder-like and tube-like spatial structures,respectively.Our results provide the phase diagrams and explicit expressions of the Majorana zero modes.The topological phase diagrams are obtained by decomposing the topological invariants and the topological conditions for topologically nontrivial phases are given precisely.For systems which belongs to topological class BDI,we obtain the regions in the phase diagrams where the topological numbers show even-odd effect.For the Kitaev tube model a phase factor induced by the magnetic flux in the axial direction of the tube is introduced to alter the classification of the tube Hamiltonian from class BDI to D.The Kitaev tube of class D is characterized by the Z_2 index when the number of chains is odd while 0,1,2 when the number of chains is even.The phase diagrams show periodic behaviors with respect to the magnetic flux.The bulk-boundary correspondence is demonstrated by the observations that the topological conditions for the bulk topological invariant to take nontrivial values are precisely those for the existence of the Majorana zero modes.     

Majorana modes in time-reversal invariant s-wave topological superconductors

We present a time-reversal invariant s-wave superconductor supporting Majorana edge modes. The multiband character of the model together with spin-orbit coupling are key to realizing such a topological superconductor. We characterize the topological phase diagram by using a partial Chern number sum, and show that the latter is physically related to the parity of the fermion number of the time-reversal invariant modes. By taking the self-consistency constraint on the s-wave pairing gap into account, we also establish the possibility of a direct topological superconductor-to-topological insulator quantum phase transition.     

Majorana zero mode in the vortex of an artificial topological superconductor

Majorana fermion (MF), an exotic particle that is identical to its own antiparticle, was recently found in solid matter as a quasiparticle excitation, the Majorana zero mode (MZM), in the vortex of an artificial topological superconductor (TSC). This artificial TSC, first proposed by Fu and Kane in 2008, is a heterostructure made of a topological insulator Bi₂Te₃ and an s-wave superconductor NbSe₂. This paper will briefly review the experimental progresses based on the Bi₂Te₃/NbSe₂ heterostructure. All evidences are self-consistent and reveal that the MZM exists in the center of vortex. Those experimental results are also supported by theory. This finding is a milestone in the research of Majorana fermions in solid state physics and a starting point of MZM’s application in topological quantum computation.     

Non-Hermitian Kitaev chain with complex periodic and quasiperiodic potentials

We study the topological properties of the one-dimensional non-Hermitian Kitaev model with complex either periodic or quasiperiodic potentials. We obtain the energy spectrum and the phase diagrams of the system by using the transfer matrix method as well as the topological invariant. The phase transition points are given analytically. The Majorana zero modes in the topological nontrivial regimes are obtained. Focusing on the quasiperiodic potential, we obtain the phase transition from the topological superconducting phase to the Anderson localization, which is accompanied with the Anderson localization–delocalization transition in this non-Hermitian system. We also find that the topological regime can be reduced by increasing the non-Hermiticity.