SPECIAL TOPIC—Superconductivity and its applications
超导及其应用专题编者按
DOI: 10.7498/aps.70.010101
超导及其应用专题编者按DOI: 10.7498/aps.70.010101 1911 年荷兰科学家Heike Kamerlingh Onnes 首次在金属汞中发现超导现象以来, 超导作为人类发现的第一个宏观量子现象已经有百余年的研究历史. 在这百余年的时间里, 人们对传统的低温超导材料的认识及应用已经取得了巨大的成就, 尤其是关于其超导机理的BCS 理论的建立极大地推动了凝聚态物理的发展. 在铜氧化物高温超导体发现后的近三十余年里, 源于对其机理的研究开辟了基础物理新的领域, 也为超导体的应用带来了新的技术. 然而, 非常规高温超导机理的研究和高临界参数的新超导体的探索仍面临许多挑战.
20 世纪80 年代, 铜氧化合物高温超导体的发现为超导研究开辟了一个全新的领域, 在全世界范围内掀起了高温超导研究的热潮. 以赵忠贤院士为代表的中国科学家在铜氧化物高温超导体研究领域做出了重要贡献, 为我国在国际超导界赢得了一席之地. 因铜氧化物高温超导体的发现, 1987 年“三月会议”(March Meeting) 盛况空前, 堪比于音乐界的“摇滚乐狂欢节” (“WoodstockFestival”). 在这次会议上, 赵忠贤先生作为会议特邀的五个科学家之一代表我国介绍了工作,确立了我国在高温超导研究领域中的重要国际地位. 当时, 以赵忠贤先生为代表的老一辈科学家在高温超导研究中取得的成绩激励了国内一大批年轻人(也包括笔者和本专题中许多文章的撰写人) 从事超导相关的研究, 这批“年轻人”现在也大多成为了我国在超导研究领域的中坚力量. 多年来, 国家对基础研究持续投入, 超导研究的基础条件有了长足的进步. 同时, 培养了一批在超导研究领域中具有国际影响力的学者, 我国的超导研究也逐渐走到了世界的前列. 2008 年, 赵忠贤先生和经历铜氧化物高温超导研究培养出来的中坚力量齐心协力, 在铁基高温超导体的研究中再次取得重大突破, 引领了高温超导研究的第二次“热潮”, 实现了我国在高温超导研究领域的全面赶超.
在经历了两次高温超导研究的“热潮”之后, 超导研究领域又面临着新的思考, 未来超导研究将如何发展, 这是摆在超导研究人员面前一个必须回答的问题. 这使我想起了早在1986 年铜氧化物高温超导体发现之前, 《物理》杂志1977 年刊登了赵忠贤先生题为“探索高临界温度超导体”的文章(见: 赵忠贤 1977 《物理》6(4): 211—215). 也许正是这种前瞻性的思考和积累, 才有我国在1986 年和2008 年两次非常规高温超导体研究中取得的成绩和地位. 我们应该有前瞻性的思维和布局, 超导作为一种宏观量子现象, 非常规高温超导的物理机理还没有完全解决, 这不但需要理论物理学家的努力, 也需要实验物理学家和材料科学家的齐心协力. 在未来的若干年里, 这一领域仍然是超导研究的重心之一, 高温超导体新材料的发现是极可能实现重大突破的方向. 在应用方面, 超导材料已经被广泛地应用于我们的生活、科研和生产等许多方面(如: 医院核磁共振成像、大科学装置和实验室的各种超导磁体等), 尤其是高温超导体已开始得到应用, 如超导电磁感应加热应用于铝锭的加工和电网限流器等. 超导材料的广泛应用将会极大改善我们人类的生活品质, 大力开展超导应用领域的研究将是未来超导研究的另一个重要方向. 此外, 超导体还具备许多特殊的“新性质”,例如拓扑超导被视为实现未来量子计算的重要方案之一. 超导体的这些“新性质”也将在未来有重要的发展, 这些“新性质”的潜在应用可提供相关的超导解决方案, 从而实现一些核心技术方面的突破,对其开展研究是未来超导研究的又一重要方向.
为进一步促进我国在超导研究领域的发展, 作为我国中文物理类学术期刊中影响面最广、影响力最大的刊物, 《物理学报》组织出版了“超导及其应用”专题. 本专题针对上述超导研究方向, 邀请了相关领域国内有代表性的学者进行专题评论, 很好地梳理了相关方向的近期进展, 内容紧贴当前的研究前沿, 为我国超导领域的研究人员提供了一个全面的参考资料, 非常及时和必要. 最后, 希望以此专题为契机, 激发国内超导领域的同行对超导未来的发展开展广泛的讨论和思考, 促进我国超导研究事业全面发展.
2021, 70 (1): 017406.
doi: 10.7498/aps.70.20201913
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Superconductivity represents a magic macroscopic quantum phenomenon. There have been two major categories of superconductors: the conventional superconductors represented by metals or alloys; and the unconventional superconductors represented by cuprates and iron-based high-temperature superconductors. While the superconductivity mechanism of the conventional superconductors is successfully addressed by the BCS theory of superconductivity, no consensus has been reached in understanding the high temperature superconductivity mechanism for more than 30 years, which has become one of the most prominent issues in condensed matter physics. Revealing the microscopic electronic structure of unconventional superconductors is the prerequisite and foundation in understanding their superconductivity. Angle resolved photoelectron spectroscopy (ARPES) plays an important role in the study of unconventional superconductors because it can directly measure the electronic structure of materials. In this paper, our recent progress in the ARPES study of electronic structure and superconductivity mechanism of high temperature cuprate superconductors and iron-based superconductors is reviewed. It mainly includes the electronic structure of the parent compound, the non-Fermi liquid behavior in the normal state, the band and gap structure of the superconducting state, and the many-body interactions both in the normal and superconducting states. These results will provide important information in understanding the superconductivity mechanism of Cu-based and Fe-based superconductors.
2021, 70 (1): 017401.
doi: 10.7498/aps.70.20201673
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As a novel quantum state in condensed matter physics, Majorana zero mode has become a popular research topic at present because of its potential value in topological quantum computing. Theory predicts that Majorana zero mode appears in the vortex core of the topological superconductor as a unique bound state. However, due to various factors such as the existence of conventional low energy bound states or impurity states, it is difficult to identify the Majorana zero mode and to put it into the specific applications. Nowadays, it is still urgent to find a suitable topological superconducting system and identify the clean Majorana zero mode in experiment. In this paper, we study the vortex states of electron-doped iron-selenium-based superconductors (Li, Fe)OHFeSe and single-layer FeSe/SrTiO3 with extremely high energy resolution STM. There exists a robust and clean Majorana zero mode in the free vortex core of (Li, Fe)OHFeSe, which has the quantized conductance. As for single-layer FeSe/SrTiO3 film, it has only conventional Caroli-de Gennes-Matricon (CdGM) bound states without zero energy mode. These experimental results provide a suitable platform for further studying the physical properties of Majorana zero mode, and also shed light on the source of topological superconductivity in iron-based superconductors.
2021, 70 (1): 017101.
doi: 10.7498/aps.70.20202122
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Based on the common properties exhibited in both cuprates and iron-based high temperature superconductors, we have recently proposed the “gene” concept for unconventional high temperature superconductors: those d-orbitals of transition metal elements with the strongest in-plane bonding to anion p-orbitals must be isolated near Fermi energy. Here we summarized recent progress in this research direction and discussed several electronic environments that meet the “gene” condition. We also discussed the challenge and the possibility in finding new unconventional high temperature superconductors.
2021, 70 (1): 017403.
doi: 10.7498/aps.70.20202102
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2021, 70 (1): 018502.
doi: 10.7498/aps.70.20202131
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Superconductivity is a macroscopic quantum phenomenon. Flux quantization and the Josephson effect are two physical phenomena which can best reflect the macroscopic quantum properties. Superconducting quantum interference device (SQUID) is one type of superconducting devices which uses these two characteristics. SQUID devices are widely used in the sensitive detection of magnetic signals. This paper briefly introduces the background and recent developments of low temperature superconductor and high temperature superconductor SQUID devices.
2021, 70 (1): 017407.
doi: 10.7498/aps.70.20202189
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In recent years, hydrogen-rich compounds under extremely high pressure have become the hot target materials for high-temperature superconductors. At present, two landmark progresses have been made in this field. Covalent H3S hydrogen-rich superconductors (Tc = 200 K) and ionic hydrogen-rich superconductors with hydrogen-cage structure, such as LaH10 (Tc = 260 K, –13 ℃), YH6 and YH9, have been successively synthesized, setting a new record of superconducting temperature. These studies have given rise to the hope of discovering room-temperature superconductors in hydrogen-rich compounds under high pressure. This paper focuses on the progress of hydrogen-rich superconductors with high critical temperature under high pressure, discusses the physical mechanism of high-temperature superconductivity in hydrogen-rich compounds, provide an outlook on the possibility of discovering room-temperature superconductors in hydrogen-rich compounds in the future, and offer the candidate system for high superconductivity in multiple hydrogen-rich compounds.
2021, 70 (1): 017402.
doi: 10.7498/aps.70.20201418
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Heavy fermion superconductors belong to a special class of strongly correlated systems and unconventional superconductors. The emergence of superconductivity in these materials is closely associated with the presence of quantum critical fluctuations. Heavy fermion superconductors of different structures often exhibit distinct competing orders and superconducting phase diagrams, implying sensitive dependence of their electronic structures and pairing mechanism on the crystal symmetry. Here we give a brief introduction on recent theoretical and experimental progress in several different material families. We develop a new phenomenological framework of superconductivity combining the Eliashberg theory, a phenomenological form of quantum critical fluctuations, and strongly correlated band structure calculations for real materials. Our theory provides a unified way for systematic understanding of various heavy fermion superconductors.
2021, 70 (1): 017408.
doi: 10.7498/aps.70.20202180
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High-Tc cuprates, iron-based superconductors, heavy-fermion superconductors and κ-type layered organic superconductors share some common features − the proximity of the superconducting state to the magnetic ordered state and the non-s-wave superconducting pairing function. It is generally believed that the Cooper pairings in these unconventional superconductors are mediated by spin fluctuations. In this paper, we present a brief overview on the spin dynamics and unconventional pairing, focusing on high-Tc cuprates and iron-based superconductors. In particular, we will overview the properties of the neutron spin resonance and its possible origin, the pairing mechanism in Hubbard model within the weak-coupling framework and its application to the aforesaid unconventional superconductors. We point out that the interplay between magnetism and superconductivity is still an area of active research.
2021, 70 (1): 017405.
doi: 10.7498/aps.70.20201881
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Superconductivity is achieved through macroscopic phase coherence; the charge carriers are Cooper pairs. In absence of an external magnetic field and applied current, the behavior of these Cooper pairs can be described by a single wave function $ \psi = {\psi _{\rm{0}}}{e^{i\varphi }}$ , and the phase is uniform over the space. When applying an external field but still below a certain threshold, a screening current will be established at the surface, which prohibits the entering of magnetic field, that is so-called Meissner effect. When the external field is larger than this threshold, the magnetic flux will penetrate into the sample, forming the interface of superconducting and normal state regions. According to the sign of this interface energy, we can categorize superconductors into type-I (positive interface energy) and type-II (negative interface energy). Most superconductors found so far are type-II in nature. Due to the negative interface energy in type-II superconductors, the penetrated magnetic flux will separate into the smallest bundle, namely the quantum flux line, with a quantized flux ${\varPhi _0} = h/2e$ (h is the Planck constant and e is the charge of an electron). There are weak repulsive interactions among these vortices, thus usually they will form a lattice, called mixed state. When applying a current, a Lorentz force will exert on the flux lines (vortices) and will make them to move, this will induce energy dissipation and the appreciable feature of zero resistance of a superconductor will be lost. By introducing some defects, impurities or dislocations into the system, it is possible to pin down these vortices and restore the state of zero resistance. The study concerning vortex pinning and dynamics is very important, which helps not only the understanding of fundamental physics, but also to the high power application of type-II superconductors. This paper gives a brief introduction to the vortex dynamics of type-II superconductors.
2021, 70 (1): 018501.
doi: 10.7498/aps.70.20202121
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It has been nearly 110 years since the discovery of superconductors, and more than 30 years since the discovery of high temperature superconductors (HTS). Great progress has been made in the application of superconducting electronics in the last two decades. HTS microwave devices have shown much higher perfomance than the traditional ones and have found their ways to the industry applications in mobile communication, radar, and special communication applications. Owing to the ultrahigh sensitivity to magnetic fields and currents, superconducting quantum interference devices (SQUIDs) have been used as the irresplacible sensors in geological surveying, magnetic resonanc imaging, biomagnetic imaging, and other areas. The sensitivity of superconducting radiation detectors such as superconducting SIS mixer, superconducting hot electron bolometer, superconducting transition edge sensor, superconducting nanowire single photon detector, and superconducting microwave kinetic inductance detector are near the quantum limitation. They are now key technology in geophysics, astrophysics, quantum information science, biomedicine, and so on. Superconducting Josephson parametric amplifier has become a key element for superconducting quantum computing. Superconducting integrated circuit has been included in the international roadmap for devices and systems, and shows that having the potential to become one of the mainstreams for post-Moore information processing technology. In metrology, superconducting Josephson effect and Josephson junction array devices have been widely used in the redefinition of quantum voltage reference and basic units of the International system of Units. Superconducting electronics plays an important role in the current quantum information technology boom, which in turn promotes the development of superconducting electronics. This review will brief introduce the research and application of superconducting electronics in China in recent years.
2021, 70 (1): 018401.
doi: 10.7498/aps.70.20202042
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This paper presents a brief review of the development trend of superconducting magnets in large scale applications towards high magnetic fields, depending on and pushing the Nb3Sn wire technics' continuous improvement. The focus is on analysis of the technology challenges of 14 T whole-body superconducting magnets. Using the Bonze Nb3Sn wires and on the base of a combination design of Nb3Sn and NbTi coils, an electromagnetic conception design of a 14 T whole-body MRI magnet is presented, and the thermal stability and quench protection are analyzed by simulations. The critical issues on stress, joints as well as shimming of 14 T whole-body superconducting magnets are also discussed. According to the results, this paper believes: 1) Nb3Sn wires are of the first important issue for 14 T whole-body superconducting magnets—the Bonze Nb3Sn wire is of the best choice but the performance specifications of the current products need to be improved further to match the requirements; 2) quench protection of 14 T whole-body superconducting magnets is one of the most complicated technics that covers design of the copper to superconductor (Cu/SC) ratio, coordination of the operating current and coil inductances, subdivisions of passive protection circuits and quench triggering control of active protection, as well as the stray field limitation during the transient process.
2021, 70 (1): 017404.
doi: 10.7498/aps.70.20201836
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There are a variety of order states in iron-based pnictides, such as electronic nematic phase, spin density wave, and so on, which leads to plenty of novel physical phenomena. The measurements of transport properties can provide extremely useful information for understanding of the low-energy excitations of iron-based superconductors. Due to the multi-band electronic structure in iron-based pnictides, the temperature dependence of resistivity and Hall coefficient varies with different systems, however, there are no evidence for the pseudo-gap opening in the normal state which is a common feature in underdoped high-$T_{\rm{c}}$ cuprates. In the hole-doped iron-based superconductors, the Hall coefficient changes its sign in low temperatures, and meanwhile the resistivity shows a broad hump in the same temperature range. Such a behavior is proposed as a crossover from incoherent to coherent transport. The Seebeck coefficients of iron-based superconductors also show remarkable differences from the cuprates. In iron-based superconductors, the absolute value of Seebeck coefficients in the normal state becomes the largest at the optimally doping point with highest $T_{\rm{c}}$ , which is probably related to the strong inter-band scattering. The Nernst effect in the normal state of iron-based superconductors indicates that superconducting phase fluctuations is not obvious above $T_{\rm{c}}$ , which is also significantly different from the cuprates. These unusual thermoelectric properties observed in iron-based superconductors have not been observed in the nickel-based pnictide superconductors with the analogous structure, i.e., LaNiAsO, and the nickel-based superconductors behave more like a usual metal. All these results above illustrate that these unusual transport properties of iron-based superconductors are inherently associated with their high temperature superconductivity, and these factors should be taken into account in the theory on its superconducting mechanism.
