Showing papers in "Physica C-superconductivity and Its Applications in 2015"

Iron-based superconductors: Current status of materials and pairing mechanism

Abstract: Since the discovery of high Tc iron-based superconductors in early 2008, more than 15,000 papers have been published as a result of intensive research. This paper describes the current status of iron-based superconductors (IBSC) covering most up-to-date research progress along with the some background research, focusing on materials (bulk and thin film) and pairing mechanism.

Hole-doped cuprate high temperature superconductors

Abstract: Hole-doped cuprate high temperature superconductors have ushered in the modern era of high temperature superconductivity (HTS) and have continued to be at center stage in the field. Extensive studies have been made, many compounds discovered, voluminous data compiled, numerous models proposed, many review articles written, and various prototype devices made and tested with better performance than their nonsuperconducting counterparts. The field is indeed vast. We have therefore decided to focus on the major cuprate materials systems that have laid the foundation of HTS science and technology and present several simple scaling laws that show the systematic and universal simplicity amid the complexity of these material systems, while referring readers interested in the HTS physics and devices to the review articles. Developments in the field are mostly presented in chronological order, sometimes with anecdotes, in an attempt to share some of the moments of excitement and despair in the history of HTS with readers, especially the younger ones.

Unconventional superconductivity in heavy-fermion compounds

Abstract: Over the past 35 years, research on unconventional superconductivity in heavy-fermion systems has evolved from the surprising observations of unprecedented superconducting properties in compounds that convention dictated should not superconduct at all to performing explorations of rich phase spaces in which the delicate interplay between competing ground states appears to support emergent superconducting states. In this article, we review the current understanding of superconductivity in heavy-fermion compounds and identify a set of characteristics that is common to their unconventional superconducting states. These core properties are compared with those of other classes of unconventional superconductors such as the cuprates and iron-based superconductors. We conclude by speculating on the prospects for future research in this field and how new advances might contribute towards resolving the long-standing mystery of how unconventional superconductivity works.

Pristine and intercalated transition metal dichalcogenide superconductors

Abstract: Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds that exhibit strongly competing effects of charge-density wave (CDW) formation and superconductivity (SC). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows many polytypes to form. In the single layer 1 T polytype materials, one or more CDW states can form, but the pristine TMDs are not superconducting. The 2 H polytypes have two or more Fermi surfaces and saddle bands, allowing for dual orderings, which can be coexisting CDW and SC orderings, two SC gaps as in MgB2, two CDW gaps, and possibly even pseudogaps above the onset T CDW s of CDW orderings. Higher order polytypes allow for multiple CDW gaps and at least one superconducting gap. The CDW transitions T CDW s usually greatly exceed the superconducting transitions at their low T c values, their orbital order parameters (OPs) are generally highly anisotropic and can even contain nodes, and the SC OPs can be greatly affected by their simultaneous presence. The properties of the CDWs ubiquitously seen in TMDs are remarkably similar to those of the pseudogaps seen in the high- T c cuprates. In 2H-NbSe2, for example, the CDW renders its general s-wave SC OP orbital symmetry to be highly anisotropic and strongly reduces its Josephson coupling strength ( I c R n ) with the conventional SC, Pb. Hence, the pristine TMDs are highly “unconventional” in comparison with Pb, but are much more “conventional” than are the ferromagnetic superconductors such as URhGe. Applied pressure and intercalation generally suppress the TMD CDWs, allowing for enhanced SC formation, even in the 1 T polytype materials. The misfit intercalation compound (LaSe)1.14(NbSe2) and many 2 H -TMDs intercalated with organic Lewis base molecules, such as TaS2(pyridine)1/2, have completely incoherent c-axis transport, dimensional-crossover effects, and behave as stacks of intrinsic Josephson junctions. Except for the anomalously large apparent violation of the Pauli limit of the upper critical field of (LaSe)1.14(NbSe2), these normal state and superconducting properties of these intercalation compounds are very similar to those seen in the high- T c superconductor, Bi2Sr2CaCu2O8+δ and in the organic layered superconductor, κ-(ET)2Cu[N(CN)2]Br, where ET is bis(ethylenedithio) tetrathiafulvalene. Electrolytic intercalation of TMDs with water and metallic ions leads to compounds with very similar properties to cobaltates such as Nax CoO 2 · y H2O.

Bismuthates: BaBiO3 and related superconducting phases

Abstract: BaBiO3 has the perovskite structure, but tilting of the BiO6 octahedra destroy the ideal cubic symmetry except at temperatures above 820 K. BaBiO3 is a diamagnetic semiconductor due to a charge density wave (CDW), which is equivalent to a Ba2Bi3+Bi5+O6 representation. Recent calculations and experimental results confirm that there is no significant deviation from the oxidation states of 3+ and 5+. Superconductivity with a Tc as high as 13 K occurs for BaPb1−xBixO3 phases where the 6s band is about 25% filled, and superconductivity with a Tc as high as 34 K occurs for Ba1−xKxBiO3 phases where the 6s band is about 35% filled. Structures in these two solid solutions can have cubic, tetragonal, orthorhombic, or monoclinic symmetry. However, superconductivity has only been observed when the symmetry is tetragonal.

Superconductivity in the metallic elements at high pressures

Abstract: Although the highest superconducting critical temperature, T c , found in an elemental solid at ambient pressure is 9.2 K (niobium), under the application of ultra-high pressures, several elements exhibit T c values near or above 20 K. This review includes a survey of the occurrence and understanding of pressure-induced superconductivity in the subset of elements that are metallic at ambient pressure. A particular focus is directed towards those elements that display the highest superconducting critical temperatures or exhibit substantial increases in T c with pressure. A separate article in this issue by Shimizu will cover pressure-induced superconductivity in elements that are insulating at ambient pressure.

Superconductivity in the elements, alloys and simple compounds

Abstract: We give a brief review of superconductivity at ambient pressure in elements, alloys, and simple three-dimensional compounds. Historically these were the first superconducting materials studied, and based on the experimental knowledge gained from them the BCS theory of superconductivity was developed in 1957. Extended to include the effect of phonon retardation, the theory is believed to describe the subset of superconducting materials known as ‘conventional superconductors’, where superconductivity is caused by the electron–phonon interaction. These include the elements, alloys and simple compounds discussed in this article and several other classes of materials discussed in other articles in this Special Issue.

Superconductivity in the A15 structure

Abstract: The cubic A15 structure metals, with over 60 distinct member compounds, held the crown of highest Tc superconductor starting in 1954 with the discovery of Tc = 18 K in Nb3Sn. Tc increased over the next 20 years until the discovery in 1973 of Tc = 22.3 K (optimized to ≈23 K a year later) in sputtered films of Nb3Ge. Attempts were made to produce – via explosive compression – higher (theorized to be 31–35 K) transition temperatures in not-stable-at-ambient-conditions A15 Nb3Si. However, the effort to continue the march to higher Tc’s in A15 Nb3Si only resulted in a defect-suppressed Tc of 19 K by 1981. Focus in superconductivity research partially shifted with the advent of heavy Fermion superconductors (CeCu2Si2, UBe13, and UPt3 discovered in 1979, 1983 and 1984 respectively) and further shifted away from A15’s with the discovery of the perovskite structure cuprate superconductors in 1986 with Tc = 35 K. However, the A15 superconductors – and specifically doped Nb3Sn – are still the material of choice today for most applications where high critical currents (e.g. magnets with dc persistent fields up to 21 T) are required. Thus, this article discusses superconductivity, and the important physical properties and theories for the understanding thereof, in the A15’s which held the record Tc for the longest time (32 years) of any known class of superconductor since the discovery of Tc = 4.2 K in Hg in 1911. The discovery in 2008 of Tc = 38 K at 7 kbar in A15 Cs3C60 (properly a member of the fullerene superconductor class), which is an insulator at 1 atm pressure and otherwise also atypical of the A15 class of superconductors, will be briefly discussed.

Superconducting materials classes: Introduction and overview

Abstract: An introduction to and overview of the contents of this Special Issue are given. 32 classes of superconducting materials are discussed, grouped under the three categories “conventional”, “possibly unconventional” and “unconventional” according to the mechanism believed to give rise to superconductivity.

Superconductivity of very thin films: The superconductor–insulator transition

Abstract: The study of thin superconducting films has been an important component of the science of superconductivity for more than six decades. It played a major role in the development of currently accepted views of the macroscopic and microscopic nature of the superconducting state. In recent years the focus of research in the field has shifted to the study of ultrathin films and surface and interface layers. This has permitted the exploration of one of the important topics of condensed matter physics, the superconductor–insulator transition. This review will discuss this phenomenon as realized in the study of metallic films, cuprates, and metallic interfaces. These are in effect model systems for behaviors that may be found in more complex systems of contemporary interest.

Unconventional superconductivity in Sr2RuO4

Abstract: Sr2RuO4, featuring a layered perovskite crystalline and quasi-two-dimensional electronic structure, was first synthesized in 1959. Unconventional, p-wave pairing was predicted for Sr2RuO4 by Rice and Sigrist and Baskaran shortly after superconductivity in this material was discovered in 1994. Experimental evidence for unconventional superconductivity in Sr2RuO4 has been accumulating in the past two decades and reviewed previously. In this article, we will first discuss constraints on the pairing symmetry of superconductivity in Sr2RuO4 and summarize experimental evidence supporting the unconventional pairing symmetry in this material. We will then present several aspects of the experimental determination of the unconventional superconductivity in Sr2RuO4 in some detail. In particular, we will discuss the phase-sensitive measurements that have played an important role in the determination of the pairing symmetry in Sr2RuO4. The responses of superconductivity to the mechanical perturbations and their implications on the mechanism of superconductivity will be discussed. A brief survey of various non-bulk Sr2RuO4 will also be included to illustrate the many unusual features resulted from the unconventional nature of superconductivity in this material system. Finally, we will discuss some outstanding unresolved issues on Sr2RuO4 and provide an outlook of the future work on Sr2RuO4.

Non-BCS thermodynamic properties of H2S superconductor

Abstract: The present paper determines the thermodynamic properties of the superconducting state in the H 2 S compound. The values of the pressure from 130 GPa to 180 GPa were taken into consideration. The calculations were performed in the framework of the Eliashberg formalism. In the first step, the experimental course of the dependence of the critical temperature on the pressure was reproduced: T C ∈ 〈 31 , 88 〉 K, whereas the Coulomb pseudopotential equal to 0.15 was adopted. Next, the following quantities were calculated: the order parameter at the temperature of zero Kelvin ( Δ ( 0 ) ), the specific heat jump at the critical temperature ( Δ C ( T C ) ≡ C S ( T C ) - C N ( T C ) ), and the thermodynamic critical field ( H C ( 0 ) ). It was found that the values of the dimensionless ratios: R Δ ≡ 2 Δ ( 0 ) / k B T C , R C ≡ Δ C T C / C N ( T C ) , and R H ≡ T C C N ( T C ) / H C 2 ( 0 ) deviate from the predictions of the BCS theory: R Δ ∈ 〈 3.64 , 4.16 〉 , R C ∈ 〈 1.59 , 2.24 〉 , and R H ∈ 〈 0.144 , 0.163 〉 . Generalizing the results on the whole family of the H n S -type compounds, it was shown that the maximum value of the critical temperature can be equal to ∼290 K, while R Δ , R C and R H adopt the following values: 6.53, 3.99, and 0.093, respectively.

Chevrel phases: Past, present and future

Abstract: The ternary molybdenum chalcogenides MxMo6X8 (X = chalcogen), known as Chevrel phases, constitute an outstanding family of materials presenting numerous and spectacular properties. More than 100 examples of these compds. have been synthesized thanks to their versatile crystal structure. Numerous variants are found, from the binary material formed just by the molybdenum clusters Mo6X8 leaving a three-dimensional lattice where the third element M can be inserted, up to a condensation of clusters giving rise to a monodimensional material. The great interest in these compds., discovered more than 40 years ago, came from their superconducting crit. temp. and upper crit. fields (15 K for the former, 50 T at 4.2 K for the latter), both being reasonably high values at the time of their discovery thus opening enormous hopes for their use in the fabrication of magnets. Other fundamental features are found, such as the coexistence of magnetic order with the superconducting state. These features are still of interest for the scientific community, but other potential applications are now foreseen, such as their use in batteries, catalysis and thermopower technol.We recall herein some basic characteristic of Chevrel-phases, mentioning several important families, their crystal structure and mode of elaboration. This contribution being focused on the superconducting properties, we put an accent on some fundamental aspects, such as the structural and electronic transitions, the vortex lattice, their granular behavior, crit. current densities, upper field and anisotropy, to finally discuss the so-called unconventional supercond., classifying these materials among the "exotic superconductors" and making a parallel with other superconductors which, in spite of their quite different electronic and crystal structures, present similar features.Chevrel phases have a long and incredible past as outstanding materials for basic and applied research but, in addn. to that, they have a bright future ahead because of a large range of potentialities.

Development of high-performance iron-based superconducting wires and tapes

Abstract: Conventional powder-in-tube (PIT) method has been the most effective technique for fabricating iron-based superconducting wires and tapes. Tremendous advances have been made during the last few years, especially for 122 family pnictide tapes. Here we review some of the most recent and significant developments in making high-performance iron-based tapes by the ex-situ PIT process, paying particular attention to several fabrication techniques to realize high-field J c performance in terms of increase of core density and improvement of texture. At 4.2 K, the practical level transport J c up to 0.12 MA/cm 2 in 10 T and 0.1 MA/cm 2 in 14 T have been achieved in the K-doped 122/Ag tapes. As for multifilamentary 122 iron-based wires and tapes, the highest J c values reached so far are 61 kA/cm 2 and 35 kA/cm 2 at 4.2 K and 10 T, respectively for 7- and 19-core Sr-122 tapes. Recently, high J c Cu-cladded and stainless steel/Ag double-sheathed 122 tapes have also been produced in order to improve either mechanical properties or thermal stability. More importantly, the scalable rolling process has been used for the first time to demonstrate high J c values in 122 conductor tapes of 11 m in length.

T′ and infinite-layer electron-doped cuprates

Abstract: In this short review, we survey several physical properties of electron-doped cuprates. We focus on a comparison of the T′ family of general formula Ln 2− x Ce x CuO 4 (Ln = La, Nd, Pr, Sm, Eu and Gd) and the infinite-layer cuprates of general formula Sr 1− x Ln x CuO 2 (Ln = La, Nd, Pr, Sm). We identify the common features of these high-temperature superconductors and underline the likely origin of their surprising properties while emphasizing the difficulties related to their growth.

Superconductivity in layered BiS2-based compounds

Abstract: A novel family of superconductors based on BiS2-based superconducting layers were discovered in 2012. In short order, other BiS2-based superconductors with the same or related crystal structures were discovered with superconducting critical temperatures T c of up to 10 K. Many experimental and theoretical studies have been carried out with the goal of establishing the basic properties of these new materials and understanding the underlying mechanism for superconductivity. In this selective review of the literature, we distill the central discoveries from this extensive body of work, and discuss the results from different types of experiments on these materials within the context of theoretical concepts and models.

Hole superconductivity in H 2 S and other sulfides under high pressure

Abstract: Superconductivity at temperatures up to 190 K at high pressures has recently been observed in H 2 S and interpreted as conventional BCS-electron–phonon-driven superconductivity (Drozdov et al., 2014). Instead we propose that it is another example of the mechanism of hole superconductivity at work. Within this mechanism high temperature superconductivity arises when holes conduct through negatively charged anions in close proximity. We propose that electron transfer from H to S leads to conduction by holes in a nearly full band arising from direct overlap of S = p orbitals in a planar structure. The superconductivity is non-phononic and is driven by pairing of heavily dressed hole carriers to lower their kinetic energy. Possible explanations for the observed lower critical temperature of D 2 S are discussed. We predict that high temperature superconductivity will also be found in other sulfides under high pressure such as Li 2 S, Na 2 S and K 2 S .

Superconductivity in doped semiconductors

Abstract: A historical survey of the main normal and superconducting state properties of several semiconductors doped into superconductivity is proposed. This class of materials includes selenides, tellurides, oxides and column-IV semiconductors. Most of the experimental data point to a weak coupling pairing mechanism, probably phonon-mediated in the case of diamond, but probably not in the case of strontium titanate, these being the most intensively studied materials over the last decade. Despite promising theoretical predictions based on a conventional mechanism, the occurrence of critical temperatures significantly higher than 10 K has not been yet verified. However, the class provides an enticing playground for testing theories and devices alike.

Superconductivity in non-centrosymmetric materials

Abstract: Superconductivity in absence of inversion symmetry of the crystal structure is basically controlled by a Rashba-like antisymmetric spin orbit coupling which splits the Fermi surface and removes the spin degeneracy of electrons. The Fermi surface splitting can originate a mixing of spin-singlet and spin-triplet states in the superconducting condensate. The presence of spin-triplet states is expected to be responsible for various uncommon features of the superconducting ground state. Experimentally, distinct deviations from the expectations of the BCS theory are found, in general, only in those systems where beside the missing of inversion symmetry strong correlations among electrons are present. Materials of this group are primarily based on Ce, Yb or U. For the much larger group of materials without substantial electronic correlations, BCS-like superconductivity was observed in the overwhelming number of known examples. Hence, unconventional superconductivity requires, in general, the mutual presence of electronic correlations and non-centrosymmetric crystal structures.

Superconducting doped topological materials

Abstract: Recently, the search for Majorana fermions (MFs) has become one of the most important and exciting issues in condensed matter physics since such an exotic quasiparticle is expected to potentially give rise to unprecedented quantum phenomena whose functional properties will be used to develop future quantum technology. Theoretically, the MFs may reside in various types of topological superconductor materials that is characterized by the topologically protected gapless surface state which are essentially an Andreev bound state. Superconducting doped topological insulators and topological crystalline insulators are promising candidates to harbor the MFs. In this review, we discuss recent progress and understanding on the research of MFs based on time-reversal-invariant superconducting topological materials to deepen our understanding and have a better outlook on both the search for and realization of MFs in these systems. We also discuss some advantages of these bulk systems to realize MFs including remarkable superconducting robustness against nonmagnetic impurities.

Superconductivity in graphite intercalation compounds

Abstract: Anisotropic superconductivity has been observed in a variety of layered materials including deliberately structured materials,1 transition metal dichalcogenides intercalated with large organic molecules,2 and graphite intercalation compounds (GICs). Because of the long range of the superconducting coherence distance ξ, the observation of 2D superconductivity focuses on samples with superlattice repeat distances I c large compared with the superconducting coherence distance to achieve the condition I c > ξ. Molecular beam epitaxy and magnetron sputtering offer the greatest promise for quantitative studies of 2D superconductivity and the 2D–3D crossover because of the flexibility of these synthesis techniques for the generation and control of large superlattice repeat distances in deliberately structured superconductors.1 Historically, early work on this subject was carried out with transition metal dichalcogenides intercalated with large organic molecules, where repeat distances of I c ≈ 60 A were achieved for example by intercalation of n-octadecylamine into TaS2.3 Though intercalate repeat distances greater than the superconducting coherence length have not yet been achieved in GICs, the superconducting GICs have nevertheless provided an interesting system for the study of anisotropic superconductivity phenomena. In this paper, we briefly review superconductivity in GICs and provide some comparison with anisotropic superconducting behavior in deliberately structured materials and intercalated transition metal dichalcogenides.

Conventional magnetic superconductors

Abstract: We discuss several classes of conventional magnetic superconductors including the ternary rhodium borides and molybdenum chalcogenides (or Chevrel phases), and the quaternary nickel-borocarbides. These materials exhibit some exotic phenomena related to the interplay between superconductivity and long-range magnetic order including: the coexistence of superconductivity and antiferromagnetic order; reentrant and double reentrant superconductivity, magnetic field induced superconductivity, and the formation of a sinusoidally-modulated magnetic state that coexists with superconductivity. We introduce the article with a discussion of the binary and pseudobinary superconducting materials containing magnetic impurities which at best exhibit short-range “glassy” magnetic order. Early experiments on these materials led to the idea of a magnetic exchange interaction between the localized spins of magnetic impurity ions and the spins of the conduction electrons which plays an important role in understanding conventional magnetic superconductors. These advances provide a natural foundation for investigating unconventional superconductivity in heavy-fermion compounds, cuprates, and other classes of materials in which superconductivity coexists with, or is in proximity to, a magnetically-ordered phase.

Superconductivity in compressed hydrogen-rich materials: Pressing on hydrogen

Abstract: Periodic table of elements starts with hydrogen, a simplest element of all. The simplicity is lost when the element is compressed to high densities or participates in a chemical bonding in compounds, being subjected to “chemical pressure” of surrounding atoms or molecules. The chemical nature of hydrogen is dictated by its simplest electronic shell, which has only one electron. Hydrogen can donate this electron and behave like alkali metal, or accept an extra electron and form a hydride ion with closed shell resembling a group VII element. The complexity of hydrogen goes beyond these simplest configurations, when hydrogen is involved in a multicenter bonding or in hydrogen bonds. This complex behavior is tightly related to the ability of hydrogen to participate in the process of electronic transport in solids and potentially be able to contribute to the superconductivity in a material. Hydrogen by itself when compressed to immense pressures of 400–500 GPa may form a simple atomic phase with very high critical superconducting temperatures (Tc) well above room temperature. While this theoretical insight awaits confirmation at pressures at the limit of current experimental capabilities, a variety of other hydrogen-rich materials have been suggested recently to have record high Tc values. The very existence of many of these materials still lacks experimental confirmation. In this review article, we will present an extensive list of such predicted materials. We will also review superconductivity in classical hydrides (mostly metal hydrides) and current theoretical understanding of relatively low Tc‘s in metal hydrides of transition and noble metals.

Unconventional superconductivity in electron-doped layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I)

Abstract: In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron–phonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electron-doped layered nitride superconductors with other superconductors.

Organic superconductors: The Bechgaard salts and relatives

Abstract: Organic conductors were originally considered a route to achieving high temperature superconductivity. While that goal could not be met, what came to be was a class of materials in which the interplay between correlations and dimensionality, and sometimes geometric frustration, lead to a spectacular diversity of phases and phenomena that are tuned by magnetic field, pressure, and temperature. Highlighted here are the physical properties of the superconducting and normal states of the first family of organic superconductors, the quasi-one dimensional Bechgaard salts (TMTSF)2X, as well as the quasi-two dimensional compounds κ -(BEDT-TTF)2X. In both cases, the preponderance of experiments indicate that the superconductivity is nodal. As well, the importance of correlations is evident in the temperature/pressure phase diagrams, and the influence of low-energy magnetic fluctuations over the normal state properties above the superconducting transition temperature is substantial.

Electronic structure of cuprate superconductors in a full charge-spin recombination scheme

Abstract: A long-standing unsolved problem is how a microscopic theory of superconductivity in cuprate superconductors based on the charge-spin separation can produce a large electron Fermi surface. Within the framework of the kinetic-energy driven superconducting mechanism, a full charge-spin recombination scheme is developed to fully recombine a charge carrier and a localized spin into a electron, and then is employed to study the electronic structure of cuprate superconductors in the superconducting-state. In particular, it is shown that the underlying electron Fermi surface fulfills Luttinger’s theorem, while the superconducting coherence of the low-energy quasiparticle excitations is qualitatively described by the standard d-wave Bardeen–Cooper–Schrieffer formalism. The theory also shows that the observed peak-dip-hump structure in the electron spectrum and Fermi arc behavior in the underdoped regime are mainly caused by the strong energy and momentum dependence of the electron self-energy.

Superconductivity of magnesium diboride

Abstract: Over the past 14 years MgB 2 has gone from a startling discovery to a promising, applied superconductor. In this article we present a brief overview of the synthesis and the basic superconducting properties of this remarkable compound. In particular, the effect of pressure, substitutions and neutron irradiation on superconducting properties are discussed.

What Tc tells

Abstract: Superconductivity has continued to be a fascinating phenomenon ever since its discovery in 1911. The magnitude of the transition temperature, T c , provides valuable insight into the underlying physics. Here we provide select examples of the extensive research that has been done towards understanding T c , and some cases where further investigation is called for. We believe that searching for new and enhanced T c ’s remains a fertile frontier.

Superconductivity in aromatic hydrocarbons

Abstract: ‘Aromatic hydrocarbon’ implies an organic molecule that satisfies the (4 n + 2) π-electron rule and consists of benzene rings. Doping solid aromatic hydrocarbons with metals provides the superconductivity. The first discovery of such superconductivity was made for K-doped picene (K x picene, five benzene rings). Its superconducting transition temperatures ( T c ’s) were 7 and 18 K. Recently, we found a new superconducting K x picene phase with a T c as high as 14 K, so we now know that K x picene possesses multiple superconducting phases. Besides K x picene, we discovered new superconductors such as Rb x picene and Ca x picene. A most serious problem is that the shielding fraction is ⩽15% for K x picene and Rb x picene, and it is often ∼1% for other superconductors. Such low shielding fractions have made it difficult to determine the crystal structures of superconducting phases. Nevertheless, many research groups have expended a great deal of effort to make high quality hydrocarbon superconductors in the five years since the discovery of hydrocarbon superconductivity. At the present stage, superconductivity is observed in certain metal-doped aromatic hydrocarbons (picene, phenanthrene and dibenzopentacene), but the shielding fraction remains stubbornly low. The highest priority research area is to prepare aromatic superconductors with a high superconducting volume-fraction. Despite these difficulties, aromatic superconductivity is still a core research target and presents interesting and potentially breakthrough challenges, such as the positive pressure dependence of T c that is clearly observed in some phases of aromatic hydrocarbon superconductors, suggesting behavior not explained by the standard BCS picture of superconductivity. In this article, we describe the present status of this research field, and discuss its future prospects.

High-temperature superconducting nanowires for photon detection

Abstract: The possible use of high-temperature superconductors (HTS) for realizing superconducting nanowire single-photon detectors is a challenging, but also promising, aim because of their ultrafast electron relaxation times and high operating temperatures. The state-of-the-art HTS nanowires with a 50-nm thickness and widths down to 130 nm have been fabricated and tested under a 1550-nm wavelength laser irradiation. Experimental results presenting both the amplitude and rise times of the photoresponse signals as a function of the normalized detector bias current, measured in a wide temperature range, are discussed. The presence of two distinct regimes in the photoresponse temperature dependence is clearly evidenced, indicating that there are two different response mechanisms responsible for the HTS photoresponse mechanisms.