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SPECIAL TOPIC—Superconductivity and its Applications

  

EDITOR'S SUGGESTION

2021, 70 (1): 010101. doi: 10.7498/aps.70.010101
Abstract +

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Searching for new unconventional high temperature superconductors
Hu Jiang-Ping
2021, 70 (1): 017101. doi: 10.7498/aps.70.20202122
Abstract +
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.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Spin fluctuations and uncoventional superconducting pairing
Li Jian-Xin
2021, 70 (1): 017408. doi: 10.7498/aps.70.20202180
Abstract +
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.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Angle-resolved photoemission spectroscopy studies on the electronic structure and superconductivity mechanism for high temperature superconductors
Zhao Lin, Liu Guo-Dong, Zhou Xing-Jiang
2021, 70 (1): 017406. doi: 10.7498/aps.70.20201913
Abstract +
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.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Combinatorial film and high-throughput characterization methods of phase diagram for high-Tc superconductors
Jin Kui, Wu Jie
2021, 70 (1): 017403. doi: 10.7498/aps.70.20202102
Abstract +
Cuprate and iron-based superconductors are known as the only two types of high-Tc superconductors. The mechanism of high-Tc superconductivity is the most challenging issue in the field. Building accurate high-dimensional phase diagram and exploring key parameters that determine Tc, would be essential to the comprehension of high-Tc mechanism. The electronic phase diagrams of cuprate superconductors show complexity and diversity, for the strong coupling and interplay among lattice, orbital, charge and spin degrees of freedom. It is tough to construct a high-dimensional holographic phase diagram and obtain quantitative laws by traditional research methods. Fortunately, the high-throughput synthesis and fast screening techniques enable to probe the phase diagram via line-by-line or map scanning modes, and thereby are expected to obtain high-dimensional phase diagram and key superconducting parameters in a much efficient way. In this article, electronic phase diagrams of cuprate superconductors that are obtained mainly by electrical transport measurements, are briefly summarized in the view of cation substitutions, oxygen variation in the parent compounds, electric double-layer gating (electrostatic/electrochemical manipulation) and magnetic field. We introduce the preparation methods for combinatorial film based on the developed pulsed laser deposition and oxide molecular beam epitaxy techniques, as well as corresponding scale-span high-throughput measurement techniques. These high-throughput techniques have been successfully applied in the research of interface superconductivity, quantum phase transition, and so on. The novel high-throughput superconductivity research mode will play an indispensable role in the construction of the high-dimensional holographic phase diagram, the comprehension of high-Tc mechanism, and practical applications of superconductors.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

The transport properties of iron-based superconductors
Li Miao-Cong, Tao Qian, Xu Zhu-An
2021, 70 (1): 017404. doi: 10.7498/aps.70.20201836
Abstract +
$T_{\rm{c}}$ 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.">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.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Vortex bound states and Majorana zero mode in electron-doped FeSe-based high-temperature superconductor
Chen Chen, Liu Qin, Zhang Tong, Feng Dong-Lai
2021, 70 (1): 017401. doi: 10.7498/aps.70.20201673
Abstract +
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.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Theoretical progress and material studies of heavy fermion superconductors
Li Yu, Sheng Yu-Tao, Yang Yi-Feng
2021, 70 (1): 017402. doi: 10.7498/aps.70.20201418
Abstract +
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.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Progress on hydrogen-rich superconductors under high pressure
Sun Ying, Liu Han-Yu, Ma Yan-Ming
2021, 70 (1): 017407. doi: 10.7498/aps.70.20202189
Abstract +
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.

SPECIAL TOPIC—Superconductivity and its applications

  

EDITOR'S SUGGESTION

Brief introduction to flux pinning and vortex dynamics in high temperature superconductors
Wen Hai-Hu
2021, 70 (1): 017405. doi: 10.7498/aps.70.20201881
Abstract +
$ \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.">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.
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