The interplay between static magnetism and conduction electrons has been a core topic in modern condensed matter physics spanning a broad spectrum of interesting phenomena including the Kondo physics and heavy fermions, dilute magnetic semiconductors, giant/colossal magnetoresistance, and spintronics. Recently, the addition of nontrivial band topology to this topic leads to more exotic topological quantum phenomena such as quantum anomalous Hall effect and the intrinsic large anomalous Hall effect. When the conduction electrons condense into Cooper pairs and coexist with static magnetism in the magnetic superconductors, unconventional pairing states with intriguing superconducting properties can emerge as exemplified by the U- and Eu-based magnetic superconductors. Due to the antagonistic nature between magnetism and superconductivity, however, the magnetic superconductors are rare, and the concurrence of abovementioned phenomena in a single material is even scarce. Recently, we reported on a rare case that manifests an intimated interplay among static magnetism, conduction electrons, and possible exotic superconductivity through pressure regulations on an antiferromagnetic (AF) semiconductor EuTe₂ [1].

Fig. 1. Temperature-pressure phase diagram of EuTe2. Inset shows the crystal and magnetic structures of EuTe2 at ambient pressure.

At ambient pressure (AP), EuTe₂ crystallizes in a CuAl₂-type tetragonal structure with space group I4/mcm. As seen in the inset of Fig. 1, each Eu²⁺ ion is surrounded by eight Te atoms, which form the [Te₂]²⁻ dimers stacking along the c-axis. Upon cooling down at zero field, EuTe₂ exhibits a semiconducting behavior in resistivity and the Eu²⁺ moments develop a type-A AF order below T_N = 11 K, having the c-axis-oriented ferromagnetic (FM) Eu²⁺ layers coupled antiferromagnetically. When an external magnetic field is applied along c-axis, the type-A AF order can be tuned into a canted AF state through a spin-flop transition at μ₀H₁ = 2.3 T and then a fully spin-polarized state at μ₀H₂ = 7.6 T. Interestingly, these field-induced transitions have a profound impact on the transport properties of EuTe₂; i.e., ρ(T) under μ₀H > μ₀H₁ is altered from semiconducting to metallic-like behavior below a characteristic temperature T_m >> T_N, resulting in a large negative MR with over five orders of drop in resistivity at low temperatures. According to the density-functional-theory calculations, the charge carriers near the Fermi level originate mainly from the Te-5p orbitals and the small energy gap of E_a ≈ 16 meV at zero field can be closed by lifting the band degeneracy of the Te-5p orbitals in the spin-flop state with a net FM component. This explains the field-induced metallic state and thus the large negative MR below T_m. These results thus demonstrated an intimated interplay between the static magnetism of Eu²⁺ sublattice and the charge carries from Te-5p orbitals in EuTe₂.

Considering the fact that many Te-containing materials become superconducting at ambient and/or high pressures, we are motived to pursue whether EuTe₂ can be driven into a magnetic superconductor upon compression. So, we investigate the effect of pressure on the transport and magnetic properties of EuTe₂ single crystals by using a cubic anvil cell apparatus. The main results are summarized in the temperature–pressure phase diagram of EuTe₂ in Fig. 1. We find that the application of high-pressure drives EuTe₂ from an AF semiconductor showing a large negative MR into a magnetic superconductor with a large upper critical field comparable to the Pauli paramagnetic limit. Interestingly, the emergence of superconductivity above the critical pressure of P_c ≈ 6 GPa is accompanied with a concomitant enhancement of T_N, which experiences a quicker rise with the slope increased dramatically from dT_N/dP = 0.85(14) K/GPa for P ≤ Pc to 3.7(2) K/GPa for P ≥ Pc. Moreover, the superconductivity can survive in the spin-flop state with a net FM component of the Eu²⁺ sublattice under moderate fields μ₀H ≥ 2 T, implying a possible exotic pairing state. Our findings establish the pressurized EuTe₂ as a rare magnetic superconductor, and these new results under high pressures have enriched the physics pertinent to the interplay between static magnetism and conduction electrons in this interesting compound.

References

• [1] P. T. Yang, Z. Y. Liu, K. Y. Chen, X. L. Liu, X. Zhang, Z. H. Yu, H. Zhang, J. P. Sun, Y. Uwatoko, X. L. Dong, K. Jiang, J. P. Hu, Y. F. Guo, B. S. Wang, and J.-G. Cheng, Nature Commun. 13, 2975 (2022)

Authors

• P. T. Yang^a,b, Z. Y. Liu^a, K. Y. Chen^a,b, X. L. Liu^c, X. Zhang^c, Z. H. Yu^c, H. Zhang^a,b, J. P. Sun^a,b,d, Y. Uwatoko, X. L. Dong^a,b,d, K. Jiang^a,b,d, J. P. Hu^a,b, Y. F. Guo^c B. S. Wang^a,b,d, and J.-G. Cheng^a,b
• ^aChinese Academy of Sciences
• ^bUniversity of Chinese Academy of Sciences
• ^cShanghai Tech University
• ^dSongshan Lake Materials Laboratory