Layered Superconductors

1. Introduction

Since the discovery of cuprates (Cu-oxide superconductors) in 1986 [1,2,3,4], layered superconductors have attracted much attention, due to the emergence of high-transition-temperature (high-T_c) and unconventional superconductivity. The highest record of the cuprates, T_c >130 K, is still the highest among superconductors at ambient pressure. Furthermore, several kinds of new superconductors with exotic mechanisms and properties have been discovered. In 2001, MgB₂ with T_c = 39 K was discovered [5]. In 2008, FeAs-based (Fe-based) superconductor LaFeAsO_1−xF_x was discovered [6] and T_c exceeding 50 K has been recorded [7]. Thus, the discovery of new layered superconductors and increased understanding of superconductivity mechanisms are very important for exploring new high-T_c superconductors. Furthermore, application of layered superconductors is an important and developing field of superconductivity. Therefore, this special issue is aimed to publish review and original papers on various layered superconductors and its application.

2. The Present Issue

This special issue consists of 11 papers, which covers the topics on the physics, chemistry, and application of layered superconductors. The target systems are cuprates, Fe-based superconductors, low-dimensional systems, and newly discovered layered superconductors.

The cuprates and the Fe-based superconductors have been a hot issue in the fields of science and application. In this issue, Adachi et al. give an overview on the novel electronic state and superconductivity in the electron-doped high-T_c T′-superconductors [8]. The superconductivity in the T′ structure is one of the hot topics in the field of high-T_c superconductivity. In this review paper, recent advances on this matter and future prospects for elucidating the mechanisms of the superconductivity in this system have been shown. Miller et al. introduces time-correlated vortex tunneling in layered superconductors [9]. Their model can well explain the experimental observation for measured critical current vs. film thickness of HTS-coated conductors, and the concept proposed in this paper can be applicable to other correlated electron systems. Geahel et al. discuss edge contamination, bulk disorder, flux front roughening, and multiscaling in type II superconducting thin films [10]; they particularly investigate the roughening of magnetic flux fronts penetrating into strongly pinning YBCO (YBa₂Cu₃O_7−d) superconducting films with different degrees of edge and bulk disorder. They also point the similarity to other systems like Fe-based superconductors. Imai, Nabeshima and Maeda give a comparative review on thin film growth of Fe-based superconductors [11]. Since the discovery of the Fe-based superconductors, study on thin films has been actively performed to clarify the mechanisms and to achieve a high performance for superconductivity application. They particularly show the growth of Fe chalcogenide films and their superconducting properties.

According to the experiences of material developments of the cuprates and Fe-based superconductors, several kinds of layered superconductors have been recently discovered. The first example is 122-type pnictides, which have a crystal structure similar to that of FeAs-based Ba_1−xK_xFe₂As₂ [12]. Zhang and Zhai review superconductivity in 122-type pnictides without iron [13]. They show the similarity and differences in the structural and physical properties between Fe-based and Fe-free systems. Yajima reviews the properties of titanium pnictide oxide superconductors [14]. The Ti-based system contains Ti₂O₂Pn (Pn:pnictogen) superconducting layers and various kinds of blocking (spacer) layers; the crystal structure is interesting due to the similarity to high-T_c systems. Similarly, BiS₂-based superconductors [15,16], discovered in 2012, also consist of superconducting BiS₂ layers and various kinds of blocking layers. Since new superconductors can be designed by replacing the structure of the blocking layers, such a layered superconductor system is a good playground for novel superconductivity with low dimensionality. In the paper by Mizuguchi, review of the discovery of the BiS₂-based superconductor and material design concept is introduced [17]. In addition, original work on the single crystal growth and superconducting properties of antimony substituted NdO_0.7F_0.3BiS₂ is reported by Demura et al. [18]

Layered materials have been revisited due to the discovery of topological insulators and related superconductors. In this issue, magnetoresistance, gating and proximity effects in ultrathin NbN-Bi₂Se₃ bilayers are reported [19]. Furthermore, in a layered conductor, Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase can emerge. Croitoru and Buzdin give a review on the signatures and the underlying theoretical framework for FFLO states in quasi-low dimensional superconductors [20].

To study physics, chemistry, and application of layered superconductors, one of the most important contributions is growth of high quality single crystals. Therefore, we invited Nagao to review crystal growth technique for layered superconductors [21]. For various layered superconductors, the suitable method to grow high quality crystal is introduced.

3. Conclusions

As introduced here, we have published 11 papers related to Layered Superconductors. Although about 30 years have passed since the discovery of the cuprate system, the field of layered superconductors is still developing. One of the ultimate goals of this field may be the discovery of a room-temperature superconductor and its application. Recently, superconductivity with T_c = 203 K under extremely high pressure has been achieved in H₃S [22,23]. This fact encourages our field because of the realization of very high T_c exceeding the cuprate record. If we could reproduce the superconducting states of H₃S in a layered structure, we will be able to approach room-temperature superconductivity. I hope that this special issue is useful for further development of this field toward the ultimate goal.