🔬 materials science

Pressure-induced Lifshitz and quantum phase transitions in electron-doped cuprate superconductor

By combining high-pressure transport and synchrotron X-ray diffraction measurements on Pr0.87LaCe0.13CuO4, researchers discovered a pressure-induced Lifshitz transition where the Fermi surface topology reconstructs from electron- to hole-dominated at 10 GPa, followed by the complete suppression of superconductivity at 17.6 GPa, all occurring without structural phase transitions and highlighting fundamental differences in how pressure affects electron- versus hole-doped cuprates.

Original authors: Jinyu Zhao, Shu Cai, Zhaoyu Liu, Jianfeng Zhang, Shuaihang Sun, Pengyu Wang, Jing Guo, Yazhou Zhou, Shiliang Li, Fuyang Liu, Luhong Wang, Haozhe Liu, Yang Ding, Qi Wu, Tao Xiang, Richard L Greene, Lil

Published 2026-10-09

📖 5 min read🧠 Deep dive

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Original authors: Jinyu Zhao, Shu Cai, Zhaoyu Liu, Jianfeng Zhang, Shuaihang Sun, Pengyu Wang, Jing Guo, Yazhou Zhou, Shiliang Li, Fuyang Liu, Luhong Wang, Haozhe Liu, Yang Ding, Qi Wu, Tao Xiang, Richard L Greene, Liling Sun

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of materials science, some substances hold a secret that has fascinated researchers for nearly forty years: the ability to conduct electricity with absolutely no resistance when cooled to very low temperatures. These are superconductors, and among the most famous are copper-oxide compounds known as cuprates. For decades, scientists have tried to understand how these materials work, knowing that they behave very differently depending on how they are prepared. If you add extra electrons to the material, it becomes an "electron-doped" superconductor; if you remove electrons, it becomes "hole-doped." While scientists have long studied how changing the chemical recipe affects these materials, they have also known that squeezing them with immense external pressure can alter their behavior just as dramatically. The big question has been whether these two methods—changing the chemistry and applying pressure—act in the same way, or if they reveal different, hidden rules of nature. Understanding this is crucial because it could finally explain the mysterious mechanism behind high-temperature superconductivity, a discovery that could one day revolutionize how we transmit and store energy.

A team of researchers has now taken a fresh look at this problem by focusing on a specific electron-doped cuprate called Pr0.87LaCe0.13CuO4±δ, or PLCCO for short. Instead of just squeezing the material and watching it get hotter or colder, they combined extreme pressure with precise measurements of how electricity flows through it and how the atoms inside are arranged. They wanted to see if the pressure would simply make the material stop superconducting, or if it would trigger a more fundamental change in the way the electrons move. By cooling the material to very low temperatures and subjecting it to pressures up to nearly 18 gigapascals—roughly 180,000 times the pressure of the Earth's atmosphere—they discovered a surprising sequence of events that had never been seen before in this type of material.

The journey began with the material acting as a superconductor, carrying current without any loss. As the researchers increased the pressure, the temperature at which the material became superconducting stayed relatively steady at first, then began to drop. However, something far more interesting was happening inside the flow of electricity. The researchers measured the Hall coefficient, a value that acts like a signpost indicating whether the primary carriers of electricity are negative electrons or positive "holes." At the start, the sign was negative, confirming that electrons were in charge. But as the pressure reached about 10 gigapascals, the sign flipped to positive. This sudden change meant that the internal landscape of the material had been completely rearranged; the electrons had been pushed out of the way, and holes had taken over as the dominant carriers. This event is known as a Lifshitz transition, a moment where the very shape of the electron paths, or Fermi surface, reconstructs itself.

What made this discovery particularly significant was that the researchers could prove this change was purely electronic, not structural. They used powerful X-ray beams to look at the crystal structure of the material while it was under this immense pressure and at low temperatures. They found that the atoms did not shift into a new arrangement or form a different crystal shape. The material remained in the same structural phase the entire time. This ruled out the possibility that the change in electricity flow was caused by the atoms moving around. Instead, it confirmed that the pressure was forcing the electrons to reorganize themselves into a new pattern without the atoms ever moving from their spots.

After this electronic reorganization, the story took a second turn. As the pressure continued to rise beyond 10 gigapascals, the superconducting ability of the material began to fade steadily. By the time the pressure reached approximately 17.6 gigapascals, the superconductivity vanished completely. The material did not turn into an insulator, which is what happens in many other cuprates under pressure; instead, it settled into a normal metallic state where electricity flows but with resistance. This marked a second critical point, a quantum phase transition where the material moved from a superconducting state to a metallic one. The researchers found that once the initial electronic reshuffling was complete, further pressure simply weakened the force that holds the superconducting pairs together, eventually breaking them apart entirely.

The findings offer a clear contrast to what happens in hole-doped cuprates, where pressure often leads to insulating behavior rather than metallic. This study suggests that the two types of cuprates, though they look similar, follow different rules when squeezed. The electron-doped material in this experiment showed a unique path: first, a complete reconstruction of its electronic identity at 10 gigapascals, and second, a gradual loss of superconductivity leading to a metallic state at 17.6 gigapascals. These results provide a rare glimpse into how pressure can tune the delicate balance between electron interactions and superconductivity, showing that the path to understanding these complex materials requires looking at both how they conduct electricity and how their internal electronic maps change under stress.

Technical Summary: Pressure-Induced Lifshitz and Quantum Phase Transitions in Electron-Doped Cuprate Superconductor

Problem Statement
Despite nearly four decades of research into high-temperature superconductivity in copper-oxide materials, a unified mechanism remains elusive. While chemical doping (both hole and electron) is well-established as a primary control parameter for tuning superconductivity, the effects of external pressure on electron-doped cuprates have remained significantly underexplored compared to their hole-doped counterparts. Previous studies on hole-doped systems have shown that pressure can enhance the superconducting transition temperature ( $T_c$ ) or induce new phenomena, often accompanied by structural changes. However, it is unclear how pressure tunes the ground states of electron-doped systems, particularly regarding the interplay between Fermi surface topology, electronic correlations, and the suppression of superconductivity. This study addresses the gap in understanding the high-pressure behavior of the electron-doped cuprate superconductor $\text{Pr}_{0.87}\text{LaCe}_{0.13}\text{CuO}_{4\pm\delta}$ (PLCCO).

Methodology
The authors conducted a systematic investigation of PLCCO single crystals using a combination of high-pressure and low-temperature experimental techniques:

Electrical Resistance Measurements: High-pressure resistance measurements were performed on five samples (S#1–S#5) to track the evolution of the superconducting transition temperature ( $T_c$ ) and the normal-state resistivity up to pressures exceeding 18 GPa.
Hall Coefficient ( $R_H$ ) Measurements: High-pressure Hall resistance ( $R_{xy}$ ) measurements were conducted at 40 K with magnetic fields swept from 0 to 5 T perpendicular to the $ab$-plane. This was used to probe the carrier type (electron vs. hole) and Fermi surface properties.
Synchrotron X-ray Diffraction (XRD): Low-temperature (40 K) and room-temperature synchrotron XRD measurements were performed to detect any pressure-induced structural phase transitions. This was crucial to distinguish between electronic and structural origins of observed transport changes.
Theoretical Modeling: An effective two-band model based on upper and lower Hubbard bands was employed to calculate band structure evolution, incorporating pressure-dependent pairing strength ( $J$ ) and band splitting ( $R$ ).

Key Results
The study reveals two distinct critical pressures ( $P_{c1}$ and $P_{c2}$ ) driving electronic transitions in PLCCO:

Lifshitz Transition at $P_{c1}$ (~10 GPa):
- Hall Coefficient Sign Change: The Hall coefficient ( $R_H$ ) decreases continuously from a large negative value at ambient pressure, reaching zero at ~10 GPa. Beyond this point, $R_H$ changes sign from negative to positive.
- Fermi Surface Reconstruction: This sign change indicates a reconstruction of the Fermi surface from an electron-dominated topology to a hole-dominated topology.
- Absence of Structural Transition: Low-temperature XRD measurements confirm that the crystal structure remains in the tetragonal $T'$ phase (space group $I4/mmm$) across $P_{c1}$ . No structural phase transition occurs at low temperatures, confirming the purely electronic origin of the Lifshitz transition.
- Anisotropic Compression: The lattice parameter $c$ is compressed significantly (~~0.6 Å) at $P_{c1}$ , much more than parameter $a$ (~~0.1 Å), suggesting the transition is linked to pressure-induced anisotropic lattice reductions.
Quantum Phase Transition (QPT) at $P_{c2}$ (~17.6 GPa):
- Suppression of Superconductivity: While $T_c$ shows slight variation below $P_{c1}$ , it decreases monotonically above $P_{c1}$ and vanishes completely at ~17.6 GPa.
- Ground State Change: The system transitions from a superconducting state to a non-superconducting metallic state.
- Carrier Saturation: Above $P_{c1}$ , the Hall coefficient remains positive and pressure-independent, indicating that the suppression of superconductivity is not driven by further changes in carrier type or Fermi surface topology, but rather by the diminishing effective pairing interaction.
Temperature Dependence of Structural Stability:
- Room-temperature XRD reveals a partial transition from the $T'$ phase to the $T$ phase starting at ~9.3 GPa, with coexistence up to 41.4 GPa. The authors attribute the stability of the $T'$ phase at low temperatures to a lack of thermal energy to overcome the energy barrier for transformation, rendering the low-temperature $T'$ phase metastable at higher temperatures.

Significance and Claims
The paper claims several firsts and fundamental insights:

First Observation of Pressure-Induced Lifshitz Transition: This is the first report of a pressure-induced Lifshitz transition in bulk electron- or hole-doped cuprate superconductors. It demonstrates that pressure can reconstruct the Fermi surface topology without a structural phase transition in this material class.
Distinct Pressure Response vs. Hole-Doped Systems: The study highlights a fundamental difference between electron- and hole-doped cuprates under pressure. While hole-doped systems tend to become insulating-like under pressure, PLCCO transitions to a metallic state. Furthermore, the pressure-induced Lifshitz transition observed here is absent in hole-doped systems.
Decoupling of Topology and Pairing: The results suggest that the suppression of superconductivity in the high-pressure regime is driven by the reduction of the pairing interaction ( $J$ ) rather than changes in Fermi surface topology once the Lifshitz transition is complete.
Theoretical Validation: The experimental trends are well-supported by a two-band model where the Lifshitz transition corresponds to the vanishing of band splitting ( $R$ ), leading to the merging of electron and hole pockets into a single hole-dominated Fermi surface.

Conclusion
The authors conclude that pressure acts as a specific tuning parameter that depends heavily on the initial electronic structure and carrier type, rather than a generic bandwidth controller. These findings provide crucial constraints for developing a unified theoretical description of high- $T_c$ superconductivity, emphasizing the need to understand not only pairing interactions but also how superconductivity is destroyed and the nature of the resulting ground states in both electron- and hole-doped families.

Technical Summary: Pressure-Induced Lifshitz and Quantum Phase Transitions in Electron-Doped Cuprate Superconductor

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