On November 12th, the journal “Nature Communications” published a paper titled “Tuning the BCS-BEC crossover of electron-hole pairing with pressure”. The Wuhan National High Magnetic Field Center served as the primary institution for the paper, with doctoral student Yuhao Ye as the first author and Professor Zengwei Zhu as the corresponding author. Other contributors to the work included Professor Kamran Behnia from the Paris Institute of Physical Chemistry, Researcher Benoît Fauqué from the École Normale Supérieure, Associate Researcher Xiaokang Li, Engineer Huakun Zuo, and doctoral students Pan Nie and Jinhua Wang.

This research is based on the electrical transport experimental testing platform of the pulsed high magnetic field facility at our university and represents the first domestic achievement to combine three extreme experimental conditions—pulsed high magnetic fields, high pressure, and low temperature. The Bridgman-type pressure cell used in the experiment was independently designed and developed by the team, and it was repeatedly optimized and improved for use in the pulsed high magnetic field environment. Its shell is made from the high-strength non-magnetic alloy MP35N, with a maximum outer diameter of only 11.8 mm and a sample hole diameter of 1 mm, as shown in Figures 1 (a) and (b). Compared to similar devices abroad, this design significantly reduces eddy current heating and greatly improves experimental efficiency.

Figure 1. Pressure cell and magneto resistivity results.

In 1985, Nozières and Schmitt-Rink demonstrated that the transition between the strong-coupling limit (the Bose-Einstein condensation or BEC of composite bosons, either excitons or Cooper pairs) to the weak-coupling limit (the Bardeen-Cooper- Schrieffer or BCS) is smooth. Graphite, a semimetal with an equal density of electrons and holes (n = p≈3×10¹⁸cm⁻³), Indeed, confining all carriers to their lowest Landau level opens the way to a nesting instability. This is the case of graphite in presence of a magnetic field exceeding 7.4 T.

WHMFC provides an important measurement platform for this work. It represents the first significant achievement in China that combines three extreme experimental conditions: pulsed high magnetic fields, high pressure, and low temperature. The pressure cell is Bridgman type pressure cell adapted from the design of D. Braithwaite et al. The cell body, with a diameter of 11.8 mm and a length of 36 mm, is crafted from MP35N (see Fig. 1(a)(b) for a photo of the cell). The anvils are machined from ZrO₂. Compared to similar devices abroad, it reduces eddy current heating and significantly improves experimental efficiency.

Professor Zhu et al. has enriched the phase diagram of graphite under low-temperature strong magnetic fields [Phys. Rev. X 9, 011058(2019)], and has confirmed the existence of a critical temperature and a critical magnetic field for the Bose-Einstein condensation of excitons in graphite [PNAS 117, 30215 (2019)]. Here, they study the evolution of the first dome (below 60 T) under hydrostatic pressure up to 1.7 GPa (see Fig. 1(c)-(h). With increasing pressure, the field-temperature phase boundary shifts towards higher magnetic fields, yet the maximum critical temperature remains unchanged, as shown in the Fig. 2. According to fermiology data, pressure amplifies the density and the in-plane effective cyclotron mass of hole-like and electron-like carriers. Thanks to this information, they verify the persistent relevance of the BCS relation between the critical temperature and the density of states in the weak-coupling boundary of the dome. In contrast, the strong-coupling summit of the dome does not show any detectable change with pressure. They argue that this is because the out-of plane BCS coherence length approaches the interplane distance that shows little change with pressure. Thus, the BCS-BEC crossover is tunable by magnetic field and pressure, but with a locked summit. The result shows that pairing interaction V is almost pressure-independent. It is the increase of DOS, induced by the magnetic field, that drives here the BCS-BEC crossover and not the tuning of the pairing interaction or the particle density.

Figure 2. Phase diagram of graphite under pressure.

This work was supported by the National Key Research and Development Program of China , the National Science Foundation of China and the Fundamental Research Funds for the Central Universities.

Link: https://www.nature.com/articles/s41467-024-54021-7