Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.     

铁基超导体的量子临界行为

铁基超导体呈现丰富的电子相图, 各种有序态相互交叠. 本文主要介绍利用核磁共振手段在空穴型和电子型掺杂的BaFe₂As₂以及LaFeAsO_1-xF_x这三种具有代表性的铁基超导体中探测到的反铁磁序与超导序的微观共存、量子临界点和量子临界行为. 实验发现, 无论在空穴型还是电子型掺杂的铁基超导体中, 反铁磁相变温度都随着掺杂被抑制, 并最终在某个掺杂量降到零温而形成量子临界点. 在反铁磁转变温度之上存在结构相变, 其转变温度也随着掺杂而降低. 核磁共振谱证实结构相变也形成一个量子临界点. 本文介绍核磁共振及输运测量揭示的这两种量子临界点附近存在的量子临界行为, 共存态下奇异的超导性质等.     

铁基超导体中的反铁磁序和自旋动力学

类似于其他非常规超导材料,铁基高温超导电性通常出现在静态长程反铁磁序被抑制之后,并且强烈的自旋涨落始终与超导电性相伴相生,因此理解磁性相互作用是建立铁基超导微观机理的重要前提.中子散射作为研究凝聚态物质中磁性相互作用的有力工具,在揭示铁基超导电性的磁性起源方面起到了关键作用.本文系统总结了近十年来铁基超导材料的中子散射研究结果,包括铁基超导材料中的静态磁结构、磁性相变、动态磁激发、电子向列相等,并探讨它们与超导电性之间的关系.     

Universal behavior of the upper critical field in iron-based superconductors

The newly discovered iron-based high temperature superconductors have demonstrated rich physical properties. Here we give a brief review on the recent studies of the upper critical field and its anisotropy in a few typical series of the iron-based superconductors (FeSCs). In spite of their characters of a layered crystal structure, all the FeSCs possess an extremely large upper critical field and a weak anisotropy of superconductivity, being unique among the layered superconductors. These particular properties indicate potential applications of the FeSCs in the future. Based on the experimental facts of the FeSCs, we will discuss the possible mechanisms of pair breaking in high magnetic fields and its restrictions on the theoretical analysis of the superconducting pairing mechanisms.     

新型铁基超导体角分辨光电子能谱研究

最近发现的新型122结构的铁基超导体掀起了铁基高温超导研究的又一轮热潮.文章利用角分辨光电子能谱实验手段,研究了这类新型铁基材料的电子结构、费米面拓扑和超导能隙.实验结果表明,其在布里渊区中心的能带结构及费米面与其他铁基超导体存在明显差异,并导致嵌套在粒子-空穴通道的费米面消失.另外,在布里渊区边缘的电子型费米面发现了较强的并且有各向同性的超导能隙.这些结果对可能的超导配对机制提出了严格的限制.     

新型铁基超导体角分辨光电子能谱研究

最近发现的新型122结构的铁基超导体掀起了铁基高温超导研究的又一轮热潮.文章利用角分辨光电子能谱实验手段,研究了这类新型铁基材料的电子结构、费米面拓扑和超导能隙.实验结果表明,其在布里渊区中心的能带结构及费米面与其他铁基超导体存在明显差异,并导致嵌套在粒子-空穴通道的费米面消失.另外,在布里渊区边缘的电子型费米面发现了较强的并且有各向同性的超导能隙.这些结果对可能的超导配对机制提出了严格的限制.     

Angle-resolved photoemission spectroscopy study on iron-based superconductors

Angle-resolved photoemission spectroscopy （ARPES） has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. In this paper, from the ARPES perspective, we briefly review the main results from our group in recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.     

Spin, charge, and orbital orderings in iron-based superconductors

In this article, we briefly review spin, charge, and orbital orderings in iron-based superconductors, as well as the multi-orbital models. The interplay of spin, charge, and orbital orderings is a key to understand the high temperature superconductivity. As an illustration, we use the two-orbital model to show the spin and charge orderings in iron-based superconductors based on the mean-field approximation in real space. The typical spin and charge orderings are shown by choosing appropriate parameters, which are in good agreement with experiments. We also show the effect of Fe vacancies, which can introduce the nematic phase and interesting magnetic ground states. The orbital ordering is also discussed in iron-based superconductors. It is found that disorder may play a role to produce the superconductivity.     

铁基超导体的扫描隧道显微镜研究进展

铁基高温超导体自2008年发现以来,对其超导电性的研究一直是一个热门的课题.扫描隧道显微镜能够在原子尺度进行表面形貌和隧道谱测量,从微观角度研究电子态密度的信息,是研究超导的重要谱学手段.近年来,在铁基超导电性方面,扫描隧道显微镜实验已经积累了一些有价值的结果,本文进行了总结介绍.铁基超导体是多带多超导能隙的超导体,不同材料的费米面结构有很大的变化.扫描隧道显微镜证明,同时有电子和空穴费米面最佳掺杂的铁基样品超导能隙结构是无节点并带有能隙符号变化的s^±波.而进一步的实验发现在没有空穴费米面的FeSe基超导体中也存在能隙符号的相反,对统一铁基超导体的配对对称性提供了重要实验证据.此外,扫描隧道显微镜在研究铁基超导体的电子向列相、浅能带特性、可能的拓扑特性方面,提供了重要的实验数据.本文对上述相关内容进行了总结,并做了相应分析和讨论.     

Hybrid crystals of cuprates and iron-based superconductors

We propose two possible new compounds, Ba_2CuO_2Fe_2As_2and K_2CuO_2Fe_2Se_2, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of square CuO_2 layers and antifluorite-type Fe_2X_2(X = As, Se) layers separated by Ba/K. The calculations of binding energies and phonon spectra indicate that they are dynamically stable, which ensures that they may be experimentally synthesized. The Fermi surfaces and electronic structures of the two compounds inherit the characteristics of both cuprates and iron-based superconductors. These compounds can be superconductors with intriguing physical properties to help to determine the pairing mechanisms of high Tc superconductivity.     

铁基高温超导体的超导能隙和自旋涨落：核磁共振研究

文章介绍了利用核磁共振法对铁基高温超导体的研究结果.笔者研究组断定铁基超导是自旋单态配对,并首先发现超导多能隙的存在.正常态的反铁磁自旋涨落在自旋空间是各向异性的,它与超导的关系,是人们关心的热点之一.     

Crystal chemistry and structural design of iron-based superconductors

The second class of high-temperature superconductors （HTSCs）, iron-based pnictides and chalcogenides, necessarily contain Fe2X2 （＂X＂ refers to a pnictogen or a chalcogen element） layers, just like the first class of HTSCs which possess the essential CuO2 sheets. So far, dozens of iron-based HTSCs, classified into nine groups, have been discovered. In this article, the crystal-chemistry aspects of the known iron-based superconductors are reviewed and summarized by employing ＂hard and soft acids and bases （HSAB）＂ concept. Based on these understandings, we propose an alternative route to exploring new iron-based superconductors via rational structural design.     

核磁共振技术在生物研究中的应用

核磁共振（nuclear magnetic resonance,NMR）是以原子核自旋的共振跃迁为探测对象的谱学方法.当自旋量子数不为零的原子核处于外磁场中时,会引起能级的Zeeman分裂.若再施加能量等于Zeeman能级差的射频场,则会诱发原子核自旋的共振跃迁,这种现象即为核磁共振.核自旋的共振频率与原子的类型有关,且受原子所处化学和物理环境的影响.此外,NMR能量较低,不会影响探测对象（常为分子）的状态.因此,NMR能够在无损条件下提供多种具有原子和分子分辨的物质组成、结构、形态、动态变化等丰富信息.     

铁基超导体的输运性质

在铁基超导体中存在着多种有序态,例如电子向列相和自旋密度波等,从而呈现出丰富的物理现象.输运性质的测量能为认识铁基超导体的低能激发提供极为有用的信息.铁砷超导体由于其电子结构的多能带特性,其电阻率和霍尔系数与温度的关系出现多样性的变化,但在正常态并没有看到有类似铜氧化物超导体的赝能隙打开等奇异行为.在空穴型掺杂的铁基超导体中观测到霍尔系数在低温下变号,对应温区的电阻率上出现一个很宽的鼓包等,可能是从非相干到相干态的转变.热电势行为也表现出与铜氧化物超导体的明显差异,比如铁基超导体的正常态热电势的绝对值反而在最佳掺杂区是最大的,这也许跟强的带间散射有关.能斯特效应表明铁基超导体在Tc以上的超导位相涨落并不明显,与铜氧化物超导体存在明显差别.在铁基超导体上所显示出来的这些反常热电性质,并没有在类似结构的镍基超导体(如LaNiAsO)上观测到,镍基超导体表现得更像一个通常的金属.这些均说明铁基超导体的奇异输运性质与其高温超导电性存在内在的关联,这些因素是建立其超导机理时需要考虑进去的.     

Optical pumping nuclear magnetic resonance system rotating in a plane parallel to the quantization axis

A model of an optical pumping nuclear magnetic resonance system rotating in a plane parallel to the quantization axis is presented. Different coordinate frames for nuclear spin polarization vector are introduced, and theoretical calculation is conducted to analyze this model. We demonstrate that when the optical pumping nuclear magnetic resonance system rotates in a plane parallel to the quantization axis, it will maintain a steady state with respect to the quantization axis which is independent of rotational speed and direction.     

扫场对核磁共振测量的影响

在连续波核磁共振实验中,通常采用低频扫场产生重复再现的共振吸收信号。当处于磁场中的射频线圈引线回路等效面积不为零时,简谐扫场必然引起该闭合回路的磁通量变化,由此而产生同频简谐感生电动势并与核磁共振信号叠加共同构成振荡器输出信号。由于扫场上升和下降过程的感生电动势方向相反,从而扫场前半周和后半周所对应的共振吸收测量信号存在差异。实验结果表明：扫场强度及方向对核磁共振测量影响来源于实验测量技术缺陷,而非核磁共振的物理本质。     

Dynamic H nuclear magnetic resonance of rotating solids

The sensitivity of one-dimensional dynamic magic-angle spinning (MAS) and off-MAS ²H nuclear magnetic resonance spectra to changes in the parameters of jump-type molecular motions is studied. The Floquet theory approach is used to simulate spectra of spins with I = 1, which are involved in exchange processes in rotating solids. The solution of the Bloch-McConnell equations for rotating samples are derived and some simulated frequency spectra are shown. The dependence of the lineshapes of the center and sidebands of the MAS and off-MAS spectra on the exchange parameters are discussed. Experimental results of ²H spectra of perdeuterated dimethyl sulfone, obtained in the temperature range 20–55 °C, are demonstrated. The methyl groups in this molecule undergo π flips at rates that can be detected by MAS and off-MAS NMR. The shapes of the experimental sidebands are compared with simulated results.     

磁共振波谱与成像技术

文章简要介绍了核磁共振的基本原理,详细阐述了液体核磁共振在蛋白质结构、功能和动力学等方面的研究进展,论述了增强固体核磁共振分辨率的方法及其应用,讲述了磁共振成像的原理并综述了不同磁共振成像方法的应用研究进展,并对核磁共振的发展作了展望。     

核磁共振量子信息处理研究的新进展

过去的二十年中,量子信息相关研究取得了显著的进展,重要的理论和实验工作不断涌现.与其他量子信息处理系统相比,基于自旋动力学的核磁共振系统,不仅具有丰富而且成熟的控制技术,还拥有相干时间长、脉冲操控精确、保真度高等优点.这也是核磁共振体量子系统能够精确操控多达12比特的量子系统的原因.因此,核磁共振量子处理器在量子信息领域一直扮演着重要角色.本文介绍核磁共振量子计算的基本原理和一些新研究进展.研究的新进展主要包括量子噪声注入技术、量子机器学习在核磁共振平台上的实验演示、高能物理和拓扑序的量子模拟以及核磁共振量子云平台等.最后讨论了液态核磁共振的发展前景和发展瓶颈,并对未来发展方向提出展望.     

A reflection on the contrast between the Cooper pairing in iron-based and conventional superconductors

In this Perspective article we review retrospectively the streamline of our work on iron-based superconductors, and reflect on the mechanism of Cooper pairing in conventional and unconventional, such as iron-based superconductors. The main theme of this review is the concept of effective interaction and renormalization group.