EuAgAs: Tunable Magnetic Topological Semimetal
- EuAgAs is a hexagonal rare-earth pnictide whose magnetic order crucially shapes its topological phases, including Dirac, Weyl, and triply-degenerate states.
- It integrates a complex interplay of crystal symmetry, layered magnetic interactions, and varied experimental signatures from ARPES, transport, and ultrafast optics.
- Multiple control pathways via magnetic field, pressure, and laser excitation make EuAgAs a promising platform for advanced spintronic and topological applications.
EuAgAs is a hexagonal rare-earth pnictide in space group (No. 194) whose defining feature is the strong dependence of its band topology on magnetic order. Across first-principles, neutron, transport, ARPES, resonant x-ray, and ultrafast optical studies, it has been described as a Dirac semimetal in the paramagnetic or symmetry-protected antiferromagnetic regime, and as a system that can host magnetic triply-degenerate points, mirror-topological semimetal phases, linear and double Weyl points, higher-order topology, and pressure-accessible altermagnetism (Jin et al., 2021, Gazzah et al., 15 May 2026, Laha et al., 2021, Mudgal et al., 22 May 2026, Liu et al., 2024, Soh et al., 19 May 2026). Its status as a model magnetic topological semimetal rests not on a single fixed phase, but on a closely competing manifold of magnetic states whose electronic structures are topologically distinct.
1. Crystal chemistry and symmetry framework
EuAgAs crystallizes in the hexagonal structure with two formula units per cell. The literature assigns related prototype labels including a modified NiIn–type hexagonal structure, a NiIn–type hexagonal structure, and a BeZrSi–type structure, while retaining the same space-group description (Jin et al., 2021, Liu et al., 2024, Gazzah et al., 15 May 2026). Reported lattice parameters include Å and Å from experiment, Å and Å at $250$ K, and room-temperature values of Å, 0 Å; other studies quote approximate values 1 Å, 2 Å or note that this family typically has lattice parameters on the order of 3 Å and 4 Å (Jin et al., 2021, Gazzah et al., 15 May 2026, Soh et al., 19 May 2026, Liu et al., 2024, Laha et al., 2021).
All cited studies place Eu on the 5 Wyckoff site. For Ag and As, several works use Ag at 6 and As at 7, whereas one neutron-based refinement reports Ag at 8 and As at 9 (Jin et al., 2021, Laha et al., 2021, Liu et al., 2024, Soh et al., 19 May 2026, Gazzah et al., 15 May 2026). Symmetry generators explicitly invoked in topological analyses include 0, 1, inversion 2, screw 3, mirror 4, glide 5, and mirror 6 (Jin et al., 2021). The structure is nonsymmorphic, with 7 screw axes and glide planes, and these operations are central to the protection or reshaping of magnetic topological states, including altermagnetic symmetry and higher-order Weyl nodes (Gazzah et al., 15 May 2026, Soh et al., 19 May 2026).
Structurally, EuAgAs may be viewed as stacked Eu layers alternating with 8 honeycomb nets along 9 (Soh et al., 19 May 2026). This layered arrangement underlies the prominence of interlayer magnetic competition and the repeated appearance of propagation vectors with a substantial 0-axis component.
2. Magnetic order: established phases, competing models, and exchange hierarchy
Eu carries a local moment 1 in DFT-based treatments and an ordered moment 2 per Eu in neutron diffraction, consistent with Eu3 and 4 (Jin et al., 2021, Gazzah et al., 15 May 2026). Magnetic ordering is consistently reported near 5 K: 6 K, 7 K, 8 K, or a two-step sequence with 9 K and 0 K (Jin et al., 2021, Laha et al., 2021, Gazzah et al., 15 May 2026, Soh et al., 19 May 2026).
Early descriptions emphasized A-type antiferromagnetism: ferromagnetic alignment within each 1 plane and antiferromagnetic coupling between adjacent planes along 2, corresponding to a propagation vector 3 (Laha et al., 2021). A weak ferromagnetic canting was inferred from the small bifurcation of the ZFC–FC susceptibility below 4 K and a metamagnetic anomaly in 5 at 6 T (Laha et al., 2021). In that phenomenology, the minimal spin Hamiltonian was written as
7
with 8, easy-plane anisotropy 9, and Dzyaloshinskii–Moriya vectors 0 allowed by the 1 symmetry (Laha et al., 2021).
Subsequent neutron diffraction refined the bulk ground state as a 2 AFM structure with in-plane moments along the crystallographic 3-axis and a ferromagnetic-layer stacking sequence 4 along 5, doubling the primitive cell (Gazzah et al., 15 May 2026). The same work extracted a critical exponent from 6 with 7, consistent with a 8D second-order transition (Gazzah et al., 15 May 2026).
A further neutron and resonant elastic x-ray study resolved an incommensurate transverse helical state. Below 9 it found
0
with 1 decreasing from 2 at 3 K to 4 at 5 K, and below 6 an additional
7
with 8 nearly temperature-independent down to 9 K (Soh et al., 19 May 2026). Spherical neutron polarimetry identified both as transverse helices with moments in the 0 plane rotating about the 1 axis. The turn angle per Eu–Eu layer was reported as
2
implying a helix period of 3–4 layers, or 5 (Soh et al., 19 May 2026).
The magnetic competition in EuAgAs is unusually strong. One first-principles study of collinear states considered PM, AFM-6, AFM-7, AFM-8, and FM-9 configurations, finding AFM-$250$0 as the lowest total energy, AFM-$250$1 only $250$2 meV/f.u. higher, AFM-$250$3 $250$4–$250$5 meV/f.u. higher, and FM-$250$6 $250$7–$250$8 meV/f.u. higher (Jin et al., 2021). A later DFT study centered on the experimentally refined $250$9 AFM ground state found an even tighter hierarchy: FM with moments parallel to the 0-axis at 1 meV/f.u., AM with 2 at 3 meV/f.u., and FM with moments parallel to the 4-axis at 5 meV/f.u. (Gazzah et al., 15 May 2026).
To reproduce the commensurate 6 stacking, that later work introduced the effective one-dimensional model
7
with
8
Pure bilinear exchange yields 9 and generally favors an incommensurate spin spiral, whereas the negative biquadratic term penalizes non-collinear alignments and stabilizes the commensurate 00 state when sufficiently large (Gazzah et al., 15 May 2026).
A recurrent point of interpretation is therefore the magnetic ground state itself. Earlier topological analyses were formulated for A-type AFM and closely related collinear configurations, whereas later diffraction studies reported either a commensurate 01 02 state or an incommensurate transverse helix with two nearby propagation vectors (Jin et al., 2021, Gazzah et al., 15 May 2026, Soh et al., 19 May 2026). This suggests an exceptionally shallow magnetic energy landscape.
3. Order-dependent bulk topology
The central theoretical result on EuAgAs is that distinct magnetic orders realize different quasiparticle spectra and bulk invariants (Jin et al., 2021). In the paramagnetic state, with time-reversal and inversion both present, EuAgAs is a Dirac semimetal with two accidental fourfold Dirac points 03 and 04 on 05–06 at 07, where
08
The little co-group is 09, and the two crossing bands transform as the 10D irreps 11 and 12 (Jin et al., 2021). The low-energy 13 Hamiltonian around 14 is
15
with 16 eV·Å (Jin et al., 2021). The Dirac points are accidental in the sense that they disappear if the band ordering at 17 is reversed, and the band inversion at 18 yields 19D 20 in the 21 plane (Jin et al., 2021).
Under magnetic ordering, the same parent band structure reorganizes into several distinct topological phases.
| Magnetic state | Symmetry/topology outcome | Representative feature |
|---|---|---|
| PM | Dirac semimetal | Two accidental Dirac points on 22–23 |
| AFM-24 | Magnetic triply-degenerate semimetal | Each Dirac splits into two TDPs |
| AFM-25, AFM-26 | Topological mirror semimetals | Direct gap with band inversion at 27 |
| FM-28 | Weyl semimetal | Linear and double Weyl points on 29–30 |
| Helical phase | Higher-order Weyl state | 31 Weyl nodes near 32 |
| AM phase | Altermagnetic Dirac-semimetal regime | Twofold spin-split crossings along 33–34 |
In AFM-35, breaking 36 but preserving 37 along 38–39 splits each Dirac point into two magnetic triply-degenerate points. The first, 40, is the crossing of 41 and 42 at 43; the second, 44, is the crossing of 45 and 46 at 47 (Jin et al., 2021).
In AFM-48 and AFM-49, the systems are fully gapped throughout the Brillouin zone in the direct-gap sense but retain band inversion at 50; their topology is captured by mirror Chern numbers
51
with DFT yielding nonzero 52, 53, and 54 for AFM-55, and nonzero 56 for AFM-57 (Jin et al., 2021). These phases were described as adiabatically connected to topological crystalline insulators. All AFM states in that framework also carry a nontrivial parity-based 58 index
59
indicating second-order topology and implying hinge modes (Jin et al., 2021).
In FM-60, exchange splitting breaks all degeneracies and produces multiple twofold Weyl points on 61–62. Four nodes were explicitly identified:
63
64
65
66
The double Weyl point 67 is stabilized by 68 and disperses linearly along 69 and quadratically in 70 (Jin et al., 2021).
The more recent altermagnetic analysis extends this landscape. In the AM phase, bulk Dirac-semimetal crossings along 71–72 at 73 remain, but are twofold split by non-relativistic spin polarization with “g-wave” altermagnetism; splittings up to 74 meV were reported in Eu-75 bands (Gazzah et al., 15 May 2026). Hydrostatic pressure drives the AFM and AM energies through a crossing at 76 GPa, with the sign change of 77 moving the system across the 78 phase diagram from the AFM sector into the AM sector (Gazzah et al., 15 May 2026).
In the helical phase, the exchange field
79
folds the band structure in a supercell doubled along 80, and the combination of broken 81 and 82 with preserved 83 and 84 produces protected higher-order Weyl nodes with Chern number 85 along 86–87 (Soh et al., 19 May 2026). DFT places these double-Weyl points at or very near the 88 point in the unshifted helical band structure (Soh et al., 19 May 2026).
4. Electronic structure in experiment: transport, ARPES, and Fermi-level alignment
Magnetotransport established EuAgAs as an antiferromagnetic Dirac semimetal with additional real-space-topological response (Laha et al., 2021). In the AFM ground state, each band remains twofold degenerate by the combination of inversion 89 and the effective time-reversal symmetry 90, and a pair of fourfold-degenerate Dirac points occurs along the 91 axis close to the Fermi energy (Laha et al., 2021). The low-energy Hamiltonian about one node was written as
92
The Hall resistivity was decomposed as
93
with 94 and 95 (Laha et al., 2021). The topological Hall component appears only below 96 and in the field window 97–98 T where the weak metamagnetic transition occurs. At 99 K, the maximum amplitude reaches 00 and the corresponding Hall angle 01 reaches 02 (Laha et al., 2021). This signal was attributed to real-space Berry phases from a noncollinear spin texture with finite scalar spin chirality
03
Longitudinal magnetoresistance measurements for 04 found a negative LMR that reaches 05 at 06 K and persists up to 07 K in the paramagnetic regime (Laha et al., 2021). Fits using
08
with 09 and 10, yielded a positive 11 and a chirality-changing scattering time 12 s from the temperature dependence of 13 versus 14 (Laha et al., 2021). In the field-polarized regime, the breaking of 15 splits each Dirac point into two Weyl nodes with calculated separations 16 Å17 and 18 Å19; the intrinsic anomalous Hall conductivity was estimated as 20 (Laha et al., 2021).
ARPES and DFT later refined the orbital content and near-21 dispersion. Polarization-dependent ARPES at 22 eV found a small 23-centered Fermi-surface ring formed by nearly linear Dirac-like bands observed along 24–25 and 26–27, with the contour expanding systematically at binding energies 28, 29 meV, 30 meV, and 31 meV (Mudgal et al., 22 May 2026). The minimal in-plane Hamiltonian was written as
32
with 33 and 34 on the order of 35–36 m/s extracted from ARPES slopes (Mudgal et al., 22 May 2026). Orbital-projected DFT showed that these low-energy bands derive predominantly from As 37 and Ag 38 orbitals, with the Eu 39 manifold sitting 40 eV below 41 (Mudgal et al., 22 May 2026). In polarization-dependent measurements, 42 polarization enhanced even-parity orbitals and 43 polarization enhanced odd-parity orbitals, producing complementary intensity modulation on the Fermi-surface ring (Mudgal et al., 22 May 2026).
The same ARPES study reported a higher-order van Hove point at 44 eV in DFT, arising from opposite curvature along 45 and 46, with an enhanced bulk DOS at the same binding energy (Mudgal et al., 22 May 2026). Across the AFM transition, Fermi surfaces and dispersions at 47 K and 48 K were essentially identical within experimental resolution, and DFT comparisons between nonmagnetic and AFM phases likewise showed negligible splitting or reconstruction near 49 (Mudgal et al., 22 May 2026). This observation directly counters the simple expectation of a large low-energy exchange reconstruction.
A separate ARPES study performed in the helical-order context found that all measured bands lie 50 eV lower in energy than in DFT; aligning theory with experiment required a rigid upward shift of 51 eV to the calculated bands (Soh et al., 19 May 2026). In that alignment, the higher-order Weyl nodes predicted for the helical phase lie just above the ARPES chemical potential and were not directly observed (Soh et al., 19 May 2026).
5. Surface and boundary phenomena
Surface electronic structure calculations reflect the magnetic-order dependence of the bulk topology (Jin et al., 2021). In the paramagnetic Dirac-semimetal phase, the side surface normal to 52 exhibits surface-projected Dirac points connected by two Fermi arcs, one for each Dirac-point pair (Jin et al., 2021). This is a characteristic surface manifestation of the bulk accidental Dirac nodes on 53–54.
For AFM-55, surface features are obscured by bulk bands, although the nontrivial 56 index implies higher-order topology and expected hinge modes (Jin et al., 2021). In the mirror-topological semimetal regimes, the side surface normal to 57 hosts a Dirac-type surface cone at 58 in AFM-59, protected by 60 and 61, while AFM-62 supports a single Dirac cone shifted away from 63, protected by 64 only (Jin et al., 2021). These states represent the surface counterpart of the nonzero mirror Chern numbers.
In FM-65, the side surface normal to 66 displays the expected Weyl connectivity: each projected double Weyl point 67 emits two Fermi arcs, whereas the linear Weyl points emit single arcs (Jin et al., 2021). In this sense, EuAgAs offers within a single material a direct comparison among Dirac-point projections, crystalline-topological surface cones, and Weyl-arc networks.
Surface calculations in the 68 geometry for the AFM phase also showed gapped Dirac crossings and topological surface resonances, and this was used to argue that access to the AM phase could realize symmetry-protected spin-split topological states (Gazzah et al., 15 May 2026). By contrast, in the helical phase the experimentally measured Fermi surface changed only minimally because the folded bands near 69 carried negligible spectral weight (Soh et al., 19 May 2026). The boundary phenomenology is therefore highly state selective: some phases yield clear arc or cone signatures, while others mainly advertise their topology through hinge or resonance features.
6. Tunability, nonequilibrium control, and broader significance
The most distinctive materials-level attribute of EuAgAs is the multiplicity of control parameters that can move it among proximate magnetic and topological sectors. Magnetic-order control was proposed to switch reversibly among a Dirac semimetal, a triply-degenerate semimetal, a mirror TCI-like semimetal, and a Weyl semimetal, with concomitant changes from surface Fermi arcs to surface Dirac cones (Jin et al., 2021). The FM phase can be accessed in calculation or by applied field; one later study explicitly described the forced FM phase as reachable for example by 70 T, where each Dirac point splits into a pair of Weyl points protected by the 71 screw symmetry (Gazzah et al., 15 May 2026).
Hydrostatic pressure provides a second route. DFT predicts an AFM-to-AM transition at 72 GPa, driven by the change in sign of 73 and accompanied by momentum-dependent non-relativistic spin splitting characteristic of the AM phase (Gazzah et al., 15 May 2026). This places EuAgAs among the few systems proposed as controllable platforms for topological altermagnetism rather than merely static altermagnetic candidates.
Ultrafast optical excitation provides a third route. Time-resolved differential reflectivity at 74 nm using 75 fs pulses found at low fluence a fast rise followed by a 76 ps decay, described by
77
with 78 fs and an instrument-response width 79 fs (Liu et al., 2024). Above 80, the relaxation followed a two-temperature model with 81 W m82 K83 (Liu et al., 2024). At 84 K, the fluence dependence showed a sign reversal of 85 at a critical fluence 86J/cm87, with 88 increasing by 89 through 90; this was interpreted as a laser-driven AFM91FM switch that transiently transforms an AFM Dirac semimetal into an FM Weyl semimetal on a subpicosecond timescale (Liu et al., 2024). Above 92, at 93 K, the same sign reversal was absent (Liu et al., 2024).
These control pathways are the basis of the proposed applications. The literature associates EuAgAs with spin-polarized surface transport, magneto-optical devices, high-sensitivity magnetic sensors, low-dissipation interconnects, 1D dissipationless channels based on higher-order hinge modes, spin-orbit-torque devices, topological magnetic memory elements, chiral-anomaly-based sensors, and THz-speed magnetic memory (Jin et al., 2021, Laha et al., 2021, Liu et al., 2024). A cautious reading is nevertheless warranted. The ground-state magnetic structure is not presented identically across all studies, and some topological features remain prediction-led or lie above the experimentally accessed Fermi level (Gazzah et al., 15 May 2026, Soh et al., 19 May 2026). Even so, the accumulated evidence converges on a narrower conclusion: EuAgAs is a strongly tunable magnetic topological semimetal in which subtle changes of magnetic order, field, pressure, or optical excitation reorganize both the quasiparticle content and the accessible boundary responses.