CuprateEdit
Cuprates are a family of copper-oxide materials that exhibit high-temperature superconductivity, a phenomenon that challenged decades of conventional wisdom about how electrons pair and move through solids. Discovered in the late 1980s, these materials are built from copper-oxide planes embedded in more complex block structures. The central structural motif is the CuO2 plane, where the electronic states most closely tied to superconductivity reside. In their undoped form, many cuprates are Mott insulators with antiferromagnetic order, but the deliberate introduction of charge carriers turns them into superconductors with critical temperatures far above those of traditional metal-based superconductors. The cuprates thereby opened a new chapter in condensed-matter physics and technology, even as they remain a site of ongoing scientific debate about the mechanism behind high-temperature superconductivity.
The cuprates include several prominent families, such as La2-xSrxCuO4, YBa2Cu3O7-δ, and Bi2Sr2CaCu2O8+x, each with its own chemistry and crystal chemistry. In all cases, the common denominator is a layered structure in which one or more copper-oxide planes are separated by charge-reservoir layers that donate or remove electrons. This layered architecture is closely linked to their anisotropic electronic properties and their sensitivity to how charge is added or removed from the CuO2 planes. The parent compounds are typically antiferromagnetic Mott insulators; doping with holes or electrons suppresses the insulating state and, over a range of carrier concentration, yields superconductivity with a dome-shaped dependence of the critical temperature on doping.
Structure and composition
The defining feature of cuprates is the CuO2 plane, a square lattice of copper ions coordinated by oxygen. The copper ions are typically in a Cu2+ (3d9) state in the parent compounds, and the CuO2 planes host the essential electronic states that participate in conduction and pairing. Stacking this active layer with layers that act as charge reservoirs allows precise tuning of the carrier concentration. Examples of well-studied cuprates and their nominal compositions include La2-xSrxCuO4, which achieves hole doping by substituting lanthanum with strontium; YBa2Cu3O7-δ, in which oxygen content controls doping; and Bi2Sr2CaCu2O8+x, which likewise relies on oxygen stoichiometry to set carrier density. The layers are organized in a quasi-two-dimensional fashion, which contributes to unusual normal-state properties and to the robustness of superconductivity against certain perturbations.
Charge transfer from the reservoir blocks into the CuO2 planes is the main route by which doping occurs. This can be achieved chemically, by substituting ions in the reservoir layers (as in La2-xSrxCuO4), or chemically by tuning the oxygen content in the charge reservoir layers (as in YBa2Cu3O7-δ). The degree of doping is commonly described in terms of hole concentration per Cu site, denoted p, with superconductivity typically emerging once p exceeds a small threshold and disappearing beyond a higher critical value.
Electronic structure and superconductivity
The electronic structure of cuprates is dominated by the copper-oxide planes. The parent compounds are Mott insulators with strong electron-electron interactions, and their low-energy physics is governed by correlated electron behavior rather than by a simple band picture. When charge is added or removed, the system traverses a complex phase diagram in which antiferromagnetism gives way to superconductivity and, at other ranges of doping, to strange-metal or pseudogapped states.
A defining experimental signature is that the superconducting order parameter in most cuprates has dx2−y2 symmetry (often described as d-wave). This angular dependence of the superconducting gap reflects a pairing mechanism that is sensitive to the lattice and to the strong correlations in the CuO2 planes. The exact pairing glue—the mediator that binds electrons into Cooper pairs in these materials—remains a topic of active research and vigorous debate. The leading candidates emphasize electronic mechanisms, with spin fluctuations arising from proximity to antiferromagnetic order as a natural candidate. See for example discussions of spin fluctuations and competing theories such as RVB (Resonating Valence Bond) ideas, which are linked to the broader notion of strong correlations guiding pairing.
Several experimental observations shape the current outlook. ARPES (angle-resolved photoemission spectroscopy) and tunneling probes reveal a characteristic Fermi surface and a superconducting gap with the dx2−y2 form, while neutron scattering and other probes show strong magnetic correlations that evolve with doping. The normal state exhibits unconventional properties, including linear-in-temperature resistivity and a pseudogap that opens at temperatures above Tc in the underdoped regime. The pseudogap remains one of the central puzzles: is it a precursor to superconductivity, a competing order, or both in different regimens of doping? See pseudogap for in-depth discussion.
The interplay between competing orders—antiferromagnetic order, charge-density waves, stripe-like arrangements, and other forms of electronic organization—adds further complexity. Observations of charge-density waves in some cuprates indicate that superconductivity competes with, and sometimes coexists with, other ordered states. The precise way these orders interact with superconductivity varies among cuprate families and across the doping range, illustrating why a single, universal pairing mechanism remains elusive.
Phase diagram and notable phenomena
The canonical phase diagram of a cuprate describes Tc as a function of hole concentration p. At very low doping, the material is an antiferromagnetic Mott insulator. As carriers are introduced, the antiferromagnetic state collapses and a superconducting dome forms, with Tc peaking at intermediate doping (often around p ≈ 0.15 for many families). Beyond this optimal doping, Tc decreases and the material eventually exits superconductivity. Alongside the superconducting dome, a pseudogap region appears in the underdoped regime, and various non-Fermi-liquid and strange-metal behaviors emerge at higher temperatures.
Doping can also be tuned to study the resilience of superconductivity under perturbations such as magnetic fields, pressure, and chemical substitution. Some cuprates show relatively robust high-temperature superconductivity under modest pressure, while others reveal sensitive dependencies on lattice structure and oxygen ordering. The details vary by material family, reflecting the delicate balance of electronic, magnetic, and lattice interactions.
Key experimental tools used to study cuprates include ARPES, scanning tunneling microscopy/spectroscopy (STM/STS), neutron scattering, muon spin rotation, and transport measurements. These methods help map the Fermi surface, measure the superconducting gap, reveal magnetic correlations, and illuminate the evolution of electronic states with doping.
Controversies and debates
The cuprate problem is one of the best-known examples in condensed-matter physics of a scientific debate that endures decades after discovery. A central question concerns the mechanism that yields high Tc in these materials. The dominant view emphasizes strong electronic correlations and unconventional pairing interactions—often associated with spin fluctuations in proximity to the antiferromagnetic parent state—rather than conventional electron-phonon coupling. The dx2−y2 pairing symmetry observed across many cuprates is cited as evidence for a non-phononic glue. The role of the pseudogap is similarly debated: is it a harbinger of superconductivity, a competing phase that suppresses Tc, or a more nuanced phenomenon that participates in both roles depending on doping and material?
Competing theories include various incarnations of resonating valence bond ideas, spin-fluctuation mediated pairing, and proposals that emphasize more exotic electronic orders. In practice, different cuprate families can emphasize different aspects of the same underlying physics, and experimental interpretations can depend on sample quality, measurement technique, and environmental conditions. The presence of stripe-like or charge-density-wave order in some materials further complicates the picture by introducing competing orders that interact with superconductivity in material-specific ways.
From a broad scientific perspective, the debates highlight a pragmatic point: high-temperature superconductivity in cuprates arises from a rich, strongly correlated electronic system where multiple energy scales, orders, and lattice couplings interplay. While there is broad consensus that the superconducting mechanism is not captured by the traditional BCS framework as applied to simple metals, the precise nature of the pairing glue, the status of the pseudogap, and the relative importance of competing orders remain active areas of investigation.