Change of carrier density at the pseudogap critical point of a cuprate superconductor

Abstract: The pseudogap is a central puzzle of cuprate superconductors. Its connection to the Mott insulator at low doping $p$ remains ambiguous and its relation to the charge order that reconstructs the Fermi surface at intermediate $p$ is still unclear. Here we use measurements of the Hall coefficient in magnetic fields up to 88 T to show that Fermi-surface reconstruction by charge order in YBa$_2$Cu$_3$O$_y$ ends sharply at a critical doping $p = 0.16$, distinctly lower than the pseudogap critical point at $p^* = 0.19$. This shows that pseudogap and charge order are separate phenomena. We then find that the change of carrier density from $n = 1 + p$ in the conventional metal at high p to $n = p$ at low $p$ - a signature of the lightly doped cuprates - starts at $p^*$. This shows that pseudogap and antiferromagnetic Mott insulator are linked.

• The paper establishes that FSR from CDW order ends at p = 0.16, distinguishing it from the pseudogap critical point at p* = 0.19.
• The paper demonstrates a crucial carrier density transition from n = 1+p at high doping to n = p at the onset of the pseudogap phase.
• The paper employs high-field Hall effect measurements on clean YBCO samples to precisely chart electronic transitions, refining the cuprate phase diagram.

Change of Carrier Density at the Pseudogap Critical Point of a Cuprate Superconductor

The paper presented provides a significant advancement in understanding the pseudogap phenomena in cuprate superconductors, with a particular focus on YBa₂Cu₃O₈ (YBCO). Through extensive investigations of the Hall coefficient up to unprecedented magnetic fields of 88 T, this research distinguishes the critical doping points related to the pseudogap and charge-density-wave (CDW) phenomena.

Key Findings

The study reports two principal findings concerning the carrier density and the critical doping points. Firstly, it establishes that the Fermi-surface reconstruction (FSR) induced by CDW order concludes at a critical doping p = 0.16. This value distinctly contrasts with the pseudogap critical point identified at p* = 0.19. These results separate the CDW phenomena from the pseudogap effects, a distinction that had remained elusive due to previous research complexities involving simultaneous pseudogap and CDW onsets.

Secondly, the paper identifies a significant transition in carrier density coinciding with the onset of the pseudogap phase. The carrier density departs from the regime of n = 1 + p typical of a conventional metal at high doping, to n = p associated with low-doped cuprates, precisely beginning at p*. The increase in Hall coefficient observed across temperatures indicates this transformation, reinforcing the linkage between the pseudogap and antiferromagnetic Mott insulator phases.

Methodology

This research employed Hall effect measurements to scrutinize the normal-state properties of YBCO samples, circumventing the superconductivity challenge by leveraging extreme magnetic fields. The use of clean, homogeneous YBCO allowed for precise determination of critical points, aided by well-characterized transitions such as the Lifshitz transition and CDW modulations.

Implications and Future Directions

The findings presented have broad implications for the theoretical framework and experimental investigations of high-temperature superconductivity. By distinguishing between the CDW and pseudogap transitions, this research paves the way for more nuanced hypotheses regarding the electronic phases in cuprates. The observed drop in carrier density with the onset of the pseudogap adds credence to the idea of a connected antiferromagnetic or Mott insulating base for the pseudogap phase.

The implications extend to exploring quantum critical points and the comprehensive mapping of the cuprate phase diagram. Future work may continue to refine our understanding of collective phenomena in these materials, especially focusing on conditions that could stabilize new phases or transitions under varying external fields or doping levels. Additionally, unraveling the complete electronic structure in the pseudogap phase remains a crucial line of inquiry, potentially involving new techniques in spectroscopy and scattering experiments. These findings could eventually contribute to the development of superconducting materials with optimized properties for technological applications.