Consecutive topological phase transitions and colossal magnetoresistance in a magnetic topological semimetal

The study explores how magnetism can control electronic band topology to yield and switch among nontrivial quantum states. In magnetic topological materials, the magnetic structure strongly influences topological band features, enabling topological phase transitions through magnetic manipulation. EuCd2As2 is a layered f-electron system that in the paramagnetic state hosts a single Dirac cone near the Fermi level. Below TN ≈ 9 K it exhibits A-type antiferromagnetic (AFM) order with moments in the ab plane; this magnetic structure gaps the Dirac point and may realize a small-gap magnetic topological insulator. With magnetic field, a fully polarized state can yield an ideal single pair of Weyl points, and a Weyl state has been suggested near and above TN. The proximity of AFM and ferromagnetic (FM) ground states (tunable by synthesis, magnetic field, or pressure) suggests EuCd2As2 as a platform to manipulate electronic topology via magnetic tuning. The present work asks whether hydrostatic pressure can drive topological phase transitions in EuCd2As2 and whether small magnetic fields can switch the system between insulating and topological semimetallic states, thereby producing large magnetoresistance.

Prior advances in magnetic topological materials reveal unusual quasiparticles (e.g., Weyl fermions) and phenomena such as anomalous Hall effects and axion insulators. High-throughput and symmetry-based classifications highlight the role of magnetic space groups in topology. EuCd2As2 specifically has been reported to host a Dirac cone at the Fermi level, A-type AFM order below ~9 K with in-plane Eu2+ moments, a magnetic-field-induced Weyl state with a single Weyl pair, and potential Weyl-like behavior near TN. Tunability between AFM and FM states by synthesis and pressure has been demonstrated. These works establish EuCd2As2 as a candidate where small changes in magnetic structure can substantially alter electronic topology.

Single crystals of EuCd2As2 were grown using the Sn-flux method. Electrical resistivity under pressure was measured on multiple samples up to 2.50 GPa and down to 0.3 K using piston–cylinder and diamond anvil setups with a pressure-transmitting medium (Diamond 7373). Pressure was determined from the shift of the superconducting transition temperature of a Pb manometer. Measurements were performed in a PPMS platform; magnetization was measured using the VSM option. At ambient pressure, resistivity, specific heat, and magnetic susceptibility were characterized to confirm the A-type AFM ground state with easy ab-plane anisotropy and saturation fields consistent with Eu2+ moments. First-principles electronic structure calculations were performed within DFT using VASP with PAW and PBE-GGA, including an on-site Coulomb interaction U = 6 eV on Eu f orbitals (LDA+U). A plane-wave cutoff of 480 eV and a 12×12×4 k-point mesh were used. Lattice constants and internal coordinates were relaxed to forces < 0.01 eV/Å. Band structures were mapped to Wannier tight-binding Hamiltonians via manually projected Wannier functions, and surface states/topology were analyzed using WannierTools. Applied pressure in calculations was modeled via reduced unit cell volumes to mimic compression.

- Electrical resistivity under pressure exhibits a metallic–insulating–metallic evolution, forming an insulating dome between pC1 ≈ 1.0 GPa and pC2 ≈ 2.0 GPa, with metallic behavior below pC1 and above pC2. At ~1.50 GPa, low-temperature ρ(T) shows activated behavior consistent with opening of an electronic gap. - The insulating state is strongly suppressed by small magnetic fields. At 1.50 GPa and T ≈ 0.3 K, resistivity is reduced by about three orders of magnitude upon applying a magnetic field, constituting a colossal negative magnetoresistance. MR saturates at fields on the order of ~0.2 T for in-plane fields and ~1.5 T for c-axis fields, tracking Eu2+ moment polarization. - DFT reveals pressure-driven topological phase transitions controlled by magnetism: within the A-type AFM state, EuCd2As2 changes from a magnetic topological insulator (bulk gap ~10 meV at V ≈ 127.60 ų) to a trivial insulator (gap ~8 meV at V ≈ 124.78 ų, increasing to ~64 meV at V ≈ 121.11 ų). For the same compressed volume (V ≈ 121.11 ų), a FM ground state yields a Weyl semimetal with bulk Weyl points and surface Fermi arcs. - The pressure evolution is consistent with two consecutive topological transitions: MTI → trivial insulator at pC1 within the AFM phase, and trivial insulator → Weyl semimetal at pC2 driven by an AFM-to-FM transition. - Field-induced polarization similarly drives a trivial-insulator-to-Weyl transition, accounting for the colossal negative MR at low fields and temperatures. - The phenomena point to weak interlayer exchange couplings and weak magnetic anisotropy as key ingredients enabling magnetic and topological tunability.

The results directly link magnetic structure to electronic topology in EuCd2As2. The insulating dome under pressure and its rapid suppression by small magnetic fields indicate that the low-temperature transport is governed by whether the system is in an AFM trivial-insulator state or a FM Weyl semimetal state. DFT captures the sequence of phases and explains the mechanism: pressure increases hopping relative to spin–orbit coupling, quenching band inversion (MTI → trivial insulator) within AFM order, while further pressure stabilizes FM order that produces Weyl points (trivial insulator → WSM). The colossal negative MR thus originates from a field-induced AFM-to-polarized transition that changes the band topology from trivial to Weyl, opening metallic conduction channels (including surface Fermi arcs) and collapsing the gap. The small fields required and the large magnitude at low temperatures highlight strong coupling between magnetism and topology and suggest practical tunability of transport via modest fields or pressures. These findings validate the hypothesis that proximate magnetic ground states in a Dirac system can be used to switch topological phases and functional properties.

This work demonstrates that EuCd2As2 undergoes two consecutive topological phase transitions under pressure: from a magnetic topological insulator to a trivial insulator near ~1.0 GPa within the AFM phase, and from a trivial insulator to a Weyl semimetal near ~2.0 GPa concomitant with an AFM-to-FM transition. A small magnetic field similarly drives the trivial insulator into a Weyl semimetal via Eu2+ moment polarization, producing colossal negative magnetoresistance that persists to the lowest measured temperatures. First-principles calculations corroborate the pressure–magnetism–topology interplay and quantify gap evolutions. These results identify weak interlayer exchange and weak magnetic anisotropy as favorable for achieving tunable magnetic topological phases with functional responses. Future directions include realizing similar colossal MR at ambient pressure via chemical pressure (e.g., partial P-for-As substitution), exploring lower temperatures to maximize MR, and investigating related compounds with comparable magnetic exchange and anisotropy to engineer switchable topological states.

- The insulating behavior and its magnitude show some sample-to-sample variation, and previous reports at similar pressures did not observe insulating behavior down to 5 K, indicating sensitivity to sample quality and magnetic state. - Transport measurements were performed down to 0.3 K; extrapolations suggest even larger MR at lower temperatures, but this remains to be demonstrated experimentally. - Pressure was limited to ~2.5 GPa; the full extent of the high-pressure phase and detailed mapping of the AFM–FM boundary could benefit from broader pressure ranges and complementary probes (e.g., neutron scattering under pressure). - Some reported values (e.g., precise MR magnitude exponents) may be affected by typographical or measurement uncertainties; comprehensive angle-resolved and temperature-dependent MR studies would refine quantitative assessments.