Pseudogap state from quantum criticality

TL;DR

This paper reveals a complex quantum state in two-dimensional materials near quantum criticality, characterized by a pseudogap and a superposition of superconductivity and quadrupole-density wave, offering insights into high-temperature superconductivity.

Contribution

It uncovers a broad region with a pseudogap state involving coexisting orders, expanding the understanding of quantum criticality beyond a single point in layered materials.

Findings

Discovery of a broad pseudogap region with coexisting orders.

Identification of a novel quantum state combining d-wave superconductivity and quadrupole-density wave.

Implication for high-temperature superconductivity in layered materials.

Abstract

Upon application of an external tuning parameter, a magnetic state can be driven to a normal metal state at zero temperature. This phenomenon is known as quantum criticality and leads to fascinating responses in thermodynamics and transport of the compound. In the standard picture, a single quantum critical point occurs at zero temperature, which results in a nontrivial critical behaviour in its vicinity. Here we show that in two dimensions the scenario is considerably more complex due to the enormous amount of quantum fluctuations. Instead of the single point separating the antiferromagnet from the normal metal, we have discovered a broad region between these two phases where the magnetic order is destroyed but certain areas of the Fermi surface are closed by a large gap. This gap reflects the formation of a novel quantum state characterised by a superposition of d-wave superconductivity and a quadrupole-density wave, id est a state in which an electron quadrupole density spatially oscillates with a period drastically different from the one of the original spin-density wave. At moderate temperatures both orders co-exist at short distances but thermal fluctuations destroy the long-range order. Below a critical temperature the fluctuations are less essential and superconductivity becomes stable. This new phenomenon may shed some light on the origin of the mysterious pseudogap state and of the high-temperature transition into the superconducting state in cuprates. Our results demonstrate that quantum phase transitions between antiferromagnets and normal metals in layered materials may be the proper playground for search of new high temperature superconductors.