Superconductivity Centennial

The following sections are included:

• Preface
• Contents

A stable and reproducible superconductivity transition between 80 and 93 K has been unambiguously observed both resistively and magnetically in a new Y-Ba-Cu-O compound system at ambient pressure. An estimated upper critical field H_c2(0) between 80 and 180 T was obtained.

The following sections are included:

• Introduction
• Experimental
• Results and Discussion

The recent discovery of superconductivity below a transition temperature (T_c) of 94 K in HgBa₂CuO_4+δ has extended the repertoire of high-T_c superconductors containing copper oxide planes embedded in suitably structured (layered) materials. Previous experience with similar compounds containing bismuth and thallium instead of mercury suggested that even higher transition temperatures might be achieved in mercury-based compounds with more than one CuO₂ layer per unit cell. Here we provide support for this conjecture, with the discovery of superconductivity above 130 K in a material containing HgBa₂Ca₂Cu₃O_1+x (with three CuO₂ layers per unit cell), HgBa₂CaCu₂O_6+x (with two CuO₂ layers) and an ordered superstructure comprising a defined sequence of the unit cells of these phases. Both magnetic and resistivity measurements confirm a maximum transition temperature of ∼133 K, distinctly higher than the previous established record value of 125–127 K observed in Tl₂Ba₂Ca₂Cu₃O₁₀.

We have used the concept of flux quantization in superconducting YBa₂Cu₃O_7–δ rings with 0, 2, and 3 grain-boundary Josephson junctions to test the pairing symmetry in high-T_c superconductors. The magnetic flux threading these rings at 4.2 K is measured by employing a scanning superconducting quantum interference device microscope. Spontaneous magnetization of a half magnetic flux quantum, ϕ₀/2 = h/4e has been observed in the 3-junction ring, but not in the 2-junction rings. These results are consistent with d-wave pairing symmetry.

⁶³Cu, ¹⁷O and ²⁰⁵Tl NMR have been performed in the high-T_c superconductor Tl₂Ba₂-Ca₂Cu₃O₁₀ whose T_c (max) is 127 K. The hole densities at Cu and oxygen sites in the CuO₂ plane have been extracted from the nuclear quadrupole frequency vQ. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T₁ and T₂G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa₂Cu₃O₇, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher T_c is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, T_c would go up to 132 K which is quite close to the record T_c realized in the Hg based compound.

It is well known that BCS mean-field theory is remarkably successful in describing conventional superconductors. A central concept of BCS theory is the energy gap in the electronic excitation spectrum below the superconducting transition temperature, T_c. The gap also serves as the order parameter: quite generally, long-range phase coherence and a non-zero gap go hand-in-hand. But in underdoped high-T_c superconductors there is considerable evidence that a pseudogap (a suppression of spectral weight) is already formed in the normal state above T_c—first, from studies of the spin excitation spectrum, which measure a ‘spin gap’, and later from a variety of other probes. Here we present a study of underdoped Bi₂Sr₂CaCu₂O_8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES), which directly measures the momentum-resolved electron excitation spectrum of the CuO₂ planes. We find that a pseudogap with d-wave symmetry opens up in the normal state below a temperature T* > T_c, and develops into the d-wave superconducting gap once phase coherence is established below T_c.

We use inelastic neutron scattering to demonstrate that the low-frequency magnetic fluctuations in YBa₂Cu₃O_6.6 (T_c = 62.7 K) change from commensurate to incommensurate on cooling with the incommensurability first appearing at temperatures above T_c. For the energies studied, the susceptibility at incommensurate positions increases on cooling below T_c, accompanied by a suppression of the spin fluctuations at the commensurate points. These results suggest that incommensurate spin fluctuations may be a common feature for all cuprate superconductors.

The parent compounds of the copper oxide high-transition-temperature (high-T_c) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are ‘doped’ away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers (‘holes’) into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-T_c superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of ∼14Å. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

The recently discovered iron-pnictide superconductors exhibit the interplay between magnetism and superconductivity. Understanding how these two phases coexist and compete with each other is of great importance to unraveling the novel properties of these superconductors. Based on a minimum two-orbital tight-binding model which fits the angle resolved photo-emission experiments, we obtain the Fermi surface and phase diagram as a function of the electron doping for the 122 family of the ironpnictides as observed by experiments. With this background, we review some theoretical aspects, material properties and experimental observations in various doping regimes of these compounds. Hopefully, this study would help us in gaining more insight into the nature of magnetism and superconductivity in the 122 iron-pnictides.

A series of layered CeO_1–xF_xFeAs compounds with x = 0 to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high T_c strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce 4f electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.

The effect of a non-magnetic Zn impurity on superconductivity may be completely different in the superconductors with different pairing symmetry. Thus the doping effect of the non-magnetic element Zn becomes an important supplementary means to explore the iron-based superconductors. We found that the non-magnetic Zn impurities severely suppresses the spin density wave (SDW) in the parent compounds. However, in the optimal doped LaFeAsO_0.9F_0.1 superconductors (T_c = 26 K), slight Zn doping has little effect on the superconducting (SC) transition temperature (T_c). Further studies showed that the doping effect of the impurity Zn is closely related with the concentration of F doping in the LaFe_1–xZn_xAsO₁–_yF_y system. In the presence of a Zn impurity, T_c increases in the under-doped regime, remains unchanged in the optimally doped regime and is severely suppressed in the over-doped regime. Comparing with the theoretical models, we propose that the superconducting pairing symmetry may change with the carrier concentration (F content) in the system. In the under-doped and optimally-doped regimes, we believe that the SC pairing may be the usual s++ pairing, where the non-magnetic impurities have little effect on T_c according to the Anderson’s theorem. In the under-doped regime, T_c increases due to the suppression of the SDW order by Zn impurity where SC order may compete with the SDW order. In the over-doped regime, the SC gap should have nodes, and the SC pairing symmetry may be d-wave or s+/− wave pairing. These observations help to understand the pairing mechanism of iron-based superconductors.

It has been more than 20 years since the high-temperature superconductors (HTSCs), including cuprate and iron-based superconductors, were discovered. The various physical properties of the normal state and the superconducting state have been studied in great details. Although the intrinsic mechanism of superconductivity has not been revealed, a variety of experiments indicate that these materials present distinct characteristics compared with conventional s-wave symmetrical superconductors, and therefore, these materials are considered as unconventional superconductors. The nature and transport behaviors of the low-energy quasiparticles (QPs) in the superconducting state are undoubtedly important to explore the unconventional superconducting electronic states and the mechanism of superconductivity. Among kinds of experimental methods, the very-low-temperature thermal conductivity (k) plays a very important role. This article mainly introduces some experimental progress on the very-low-temperature heat transport of these two types of unconventional superconductors, including the universal thermal conductivity behavior, the superconducting gap symmetry reflected by the residual thermal conductivity, the very-low-T thermal conductivity and the metal-insulator transition of the normal state in copper oxide superconductors.

Superconductors and superfluids are quantum phases characterized by the existence of off-diagonal long range order (ODLRO), and both share some similarities between physical properties. By briefly retrospecting the historic developments of superconductivity, Bose-Einstein condensation and superfluidity, we discuss the property of ODLRO and its consequences as well as the relevance to other emergent phenomena in physics. The ODLRO in several quantum systems is also reviewed, and the concept of superstate is introduced. Finally, a prospect for high temperature superconductivity is outlined. This paper is dedicated to the 100 anniversary of the discovery of superconductivity.

The strong correlation of electrons such as the magnetic ordering in transition-metaloxides and the Kondo resonance in dilute magnetic compounds rising from the local exchange interactions were well understood many-body effects in the past decades. The superconducting cuprate discovered in 1986 inspired a fascinating idea: the magnetic origin of superfluid. We speculate that the fluctuating spins on the oxygen sites in the CuO₂ layer created by the holes may pair into a resonating-valence-bond state with the quantum number S = 1, S_z = 0 via local exchange interactions, the same force responsible for magnetic ordering in the low doped cuprate. Impurity doping drives the cuprate from an antiferromagnetic insulator to a superconductor, then a Fermi liquid giving a rich phase diagram.

Since the discovery of high-transition-temperature (high-T_c) superconductivity in layered copper oxides, extensive effort has been devoted to exploring the origins of this phenomenon. A T_c higher than 40 K (about the theoretical maximum predicted from Bardeen-Cooper-Schrieffer theory), however, has been obtained only in the copper oxide superconductors. The highest reported value for non-copper-oxide bulk superconductivity is T_c = 39 K in MgB2. The layered rare-earth metal oxypnictides LnOFeAs (where Ln is La-Nd, Sm and Gd) are now attracting attention following the discovery of superconductivity at 26 K in the iron-based LaO_1–xF_xFeAs. Here we report the discovery of bulk superconductivity in the related compound SmFeAsO_1–xF_x, which has a ZrCuSiAs-type structure. Resistivity and magnetization measurements reveal a transition temperature as high as 43 K. This provides a new material base for studying the origin of high-temperature superconductivity.

We report the superconductivity in iron-based oxyarsenide Sm[O_1–xF_x]FeAs, with the onset resistivity transition temperature at 55.0 K and Meissner transition at 54.6 K. This compound has the same crystal structure as LaOFeAs with shrunk crystal lattices, and becomes the superconductor with the highest critical temperature among all materials besides copper oxides up to now…

Superconducting gap anisotropy at least an order of magnitude larger than that of the conventional superconductors has been observed in the a-b plane of Bi₂Sr₂CaCu₂O_8+δ in angle-resolved photoemission spectroscopy. For samples with T_c, of 88 K, the gap size reaches a maximum of approximately 20 meV along the Cu-O bond direction, and a minimum of much smaller or vanishing magnitude 45. away. The experimental data are discussed within the context of various theoretical models. In particular, a detailed comparison with what is expected from a superconductor with a d_x²−y² order parameter is carried out, yielding a consistent picture.

Laser-based angle-resolved photoemission measurements with superhigh resolution have been carried out on an optimally doped Bi2Sr2CaCu2O8 high temperature superconductor. New high energy features at ∼115 meV and ∼150 meV, in addition to the prominent ∼70 meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which cannot be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.

We have measured the magnetic field and temperature dependence of specific heat on Bi₂Sr_2–xLa_xCuO_6+δ single crystals in wide doping and temperature regions. The superconductivity related specific-heat coefficient γ_sc and entropy S_sc are determined. It is found that γ_sc has a humplike anomaly at T_c and behaves as a long tail which persists far into the normal state for the underdoped samples, but for the heavily overdoped samples the anomaly ends sharply just near T_c. Interestingly, we found that the entropy associated with superconductivity is roughly conserved when and only when the long tail part in the normal state is taken into account for the underdoped samples, indicating the residual superconductivity above T_c.

High-temperature superconductivity is one of the most fascinating and challenging areas of condensed matter physics. In the past two decades, a huge number of theoretical and experimental studies have been contributed to the field and many crucial progresses have been made. Raman scattering has been extensively applied to high-T_c superconductors owing to its unique abilities to probe multiple primary excitations and symmetry selection rules. In this chapter, we present a brief review on Raman scattering experimental achievements in cuprate superconductors, with emphasis on pairing mechanism related aspects such as electron-phonon coupling, pair-breaking peak and two-magnon etc. And also we will address some open questions.

The following section is included:

• Index