underdoping

Underdoping is a term used in condensed matter physics, particularly in the study of high-temperature superconductors, to describe a level of chemical doping that is below the value that yields the maximum superconducting transition temperature (Tc).

In many cuprate and some iron-based superconductors, carriers are introduced by substituting ions or by adjusting oxygen content. The resulting phase diagram typically shows an antiferromagnetic insulating state at very low doping, superconductivity emerging with increased doping, and a maximum Tc at an optimal doping level. Doping levels below this optimum constitute the underdoped regime, where Tc is reduced and superconductivity can coexist with other electronic orders.

The underdoped region often exhibits distinctive physical phenomena. A pseudogap phase—an anomalous suppression of the electronic density of states above Tc—is commonly observed and is associated with strong electronic correlations and fluctuations. Magnetic correlations, short-range antiferromagnetism, and various competing orders can persist in the underdoped regime, influencing transport properties and the nature of the superconducting state. Materials in this regime may show insulating or non-Fermi-liquid-like behavior at certain temperatures, despite becoming superconducting at lower temperatures.

Doping levels are controlled through chemical substitution (for example, substituting strontium for lanthanum in La2-xSrxCuO4) or by adjusting oxygen stoichiometry (as in YBa2Cu3O6+x). Tc and other properties are characterized using resistivity, magnetic susceptibility, and spectroscopic probes.

Understanding underdoping is important for exploring the mechanisms of high-temperature superconductivity and the relationship between superconductivity and competing electronic orders, as well as for mapping the full phase diagram of related materials.