The interplay between superconductivity and magnetism is a central theme in condensed matter physics. Conventional Bardeen-Cooper-Schrieffer (BCS) superconductivity arises from electron-lattice interactions, while unconventional superconductivity often involves strong magnetic correlations. Understanding this interplay is crucial for developing new superconducting materials with enhanced properties. Many unconventional superconductors exhibit magnetic order near their superconducting transition temperature, with the magnetic moments of transition metal 3d electrons playing a key role in the pairing mechanism. In some cases, the upper critical field, the magnetic field required to suppress superconductivity, is limited by the Pauli paramagnetic effect. However, some unconventional superconductors, such as UTe₂, exhibit upper critical fields that far exceed the Pauli limit, suggesting spin-triplet pairing. In both spin-singlet and spin-triplet superconductors, magnetic fluctuations are important for the formation of Cooper pairs. Even in conventional or unconventional superconductors with localized magnetic moments not directly involved in the superconducting layers, the interplay between magnetic order and superconductivity can significantly affect the material's properties. Previous studies on EuTe₂ reported colossal magnetoresistance and a small energy gap at ambient pressure. The present study examines the pressure-induced effects on EuTe₂ aiming to explore the interplay between Eu²⁺ local moments and Te 5p itinerant electrons, potentially leading to the emergence of superconductivity.

Several studies have investigated pressure-induced superconductivity in various materials, including CrSiTe₃, WTe₂, Euln₂As₂, and EuSn₂As₂. These studies reveal a complex interplay between pressure, structural transitions, and the emergence of superconductivity. A recent high-pressure study on EuTe₂ suggested an exotic superconducting pairing mechanism. The Jaccarino-Peter mechanism, which explains the enhancement of the upper critical field by the exchange field of localized magnetic moments, has been observed in other materials. This mechanism involves the compensation of the external magnetic field by the internal exchange field, leading to higher critical fields.

This research employed a multifaceted approach involving high-pressure synchrotron powder X-ray diffraction (XRD), electrical transport measurements, and magnetoresistance (MR) measurements under pressure. Single crystals of EuTe₂ were grown using the self-flux method, and their structure was confirmed by single-crystal XRD. High-pressure XRD experiments were conducted using a diamond anvil cell (DAC) with silicone oil as a pressure-transmitting medium, enabling the identification of a pressure-induced structural phase transition. Electrical transport measurements, including resistance and Hall effect measurements, were performed on single crystals in a miniature DAC using NaCl as a pressure-transmitting medium to explore the evolution of semiconducting, metallic and superconducting properties under pressure. Magnetoresistance measurements were performed to elucidate the role of magnetism in the observed superconductivity. Neutron diffraction measurements were used to characterize the magnetic structure. Density functional theory (DFT) calculations and Monte Carlo simulations were employed to investigate the magnetic exchange couplings and the Néel temperature (TN) under pressure. The CALYPSO method was used for crystal structure prediction.

High-pressure synchrotron XRD revealed a pressure-induced structural phase transition from the tetragonal 14/mcm to the monoclinic C2/m space group at ~16 GPa. Superconductivity was observed above ~5 GPa in both phases. In the low-pressure (LP) phase, magnetoresistance measurements showed strong coupling between Eu²⁺ local moments and Te 5p conduction electrons. The upper critical field (Hc2) in the LP phase was significantly higher than the Pauli limit. The Néel temperature (TN) increased with pressure in the LP phase, indicating enhanced magnetic exchange interactions. In the high-pressure (HP) phase, EuTe₂ becomes nonmagnetic, and Hc2 decreases below the Pauli limit. The superconducting transition temperature (Tc) reached a maximum of 6.1 K at 7.0 GPa, independent of the structural transition and magnetism. DFT calculations and Monte Carlo simulations revealed that the C-type antiferromagnetic order is maintained under pressure up to 11.8 GPa, and the four nearest-neighbor exchange couplings are strengthened. The high Hc2 in the spin-flop and spin-flipped states of the LP phase were well above the Pauli limit, potentially explained by the Jaccarino-Peter mechanism (exchange field compensation). The Hc2-Tc relation in the HP phase could be described by a single Ginzburg-Landau formula, suggesting a conventional BCS-like superconductivity. The localized Eu 4f electrons reside approximately 1.25 eV below the Fermi level in the low-pressure phase. The high Hc2 values in the LP phase suggest that the exchange field produced by the Eu²⁺ moments compensates for the external field, enhancing the superconducting state.

The results demonstrate a clear interplay between magnetism, pressure-induced structural transitions, and superconductivity in EuTe₂. The observation of superconductivity above 5 GPa in both the magnetic low-pressure and the non-magnetic high-pressure phases is significant. The enhanced Hc2 in the low-pressure phase exceeding the Pauli limit is attributed to the Jaccarino-Peter mechanism, whereby the exchange field from the Eu²⁺ moments compensates for the applied magnetic field. This observation highlights the importance of considering the interplay between local moments and conduction electrons in understanding superconductivity in such systems. The fact that Tc is largely unaffected by the structural transition or the change in magnetic order suggests that the superconducting pairing mechanism is robust and may not be directly mediated by magnetic fluctuations. The results provide valuable insight into the complex interplay between magnetism and superconductivity, offering potential pathways to design novel high-field superconductors.

This study reveals pressure-induced superconductivity in EuTe₂ with an unusually high upper critical field in the low-pressure antiferromagnetic phase, explained by the Jaccarino-Peter mechanism. The superconductivity persists in the high-pressure nonmagnetic phase, indicating a robust pairing mechanism possibly originating from electron-phonon interaction. Future studies could investigate the detailed microscopic pairing mechanism through techniques such as angle-resolved photoemission spectroscopy (ARPES) and other spectroscopic measurements to further elucidate the superconductivity in this material. Further explorations into the pressure-induced effects on other Eu-based compounds are warranted to expand our understanding of the interplay between magnetism and superconductivity.

The study mainly focuses on macroscopic measurements; microscopic measurements such as ARPES are needed to confirm the pairing mechanism. The pressure-transmitting media used in the experiments might not be perfectly hydrostatic at higher pressures, potentially influencing the obtained results. The study is limited to specific pressure and temperature ranges, and further investigations at broader ranges might unveil additional features.