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Register a new userFerromagnetic Weyl Fermions in Two-Dimensional Layered Electride Gd$_2$C
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Recently, two-dimensional layered electrides have emerged as a new class of materials which possess anionic electron layers in the interstitial spaces between cationic layers. Here, based on first-principles calculations, we discover a time-reversal-symmetry-breaking Weyl semimetal phase in a unique two-dimensional layered ferromagnetic (FM) electride Gd$_2$C. It is revealed that the crystal field mixes the interstitial electron states and Gd 5$d$ orbitals near the Fermi energy to form band
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Recent experimental observations of Weyl fermions in materials opens a new frontier of condensed matter physics. Based on first-principles calculations, we here discover Weyl fermions in a two-dimensional layered electride material Y$_2$C. We find that the Y 4$d$ orbitals and the anionic $s$-like orbital confined in the interstitial spaces between [Y$_2$C]$^{2+}$ cationic layers are hybridized to give rise to van Have singularities near the Fermi energy $E_{rm F}$, which induce a ferromagnetic (FM) order via the Stoner-type instability. This FM phase with broken time-reversal symmetry hosts the rotation-symmetry protected Weyl nodal lines near $E_{rm F}$, which are converted into the multiple pairs of Weyl nodes by including spin-orbit coupling (SOC). However, we reveal that, due to its small SOC effects, Y$_2$C has a topologically nontrivial drumhead-like surface state near $E_{rm F}$ as well as a very small magnetic anisotropy energy with several ${mu}$eV per unit cell, consistent with the observed surface state and paramagnetism at low temperatures below ${sim}$2 K. Our findings propose that the Brillouin zone coordinates of Weyl fermions hidden in paramagnetic electride materials would fluctuate in momentum space with random orientations of the magnetization direction.
Two-dimensional (2D) electrides are a new concept material in which anionic electrons are confined in the interlayer space between positively charged layers. We have performed angle-resolved photoemission spectroscopy measurements on Y$_2$C, which is a possible 2D electride, in order to verify the formation of 2D electride states in Y$_2$C. We clearly observe the existence of semimetallic electride bands near the Fermi level, as predicted by ${ab}$ ${initio}$ calculations, conclusively demonstrating that Y$_2$C is a quasi-2D electride with electride bands derived from interlayer anionic electrons.
Magnetic properties of the electride compound Y$_2$C were investigated by muon spin rotation and magnetic susceptibility on two samples with different form (poly- and single-crystalline), to examine the theoretically-predicted Stoner ferromagnetism for the electride bands. There was no evidence of static magnetic order in both samples even at temperatures down to 0.024 K. For the poly-crystalline sample, the presence of a paramagnetic moment at Y sites was inferred from the Curie-Weiss behavior of the muon Knight shift and susceptibility, whereas no such tendency was observed in the single-crystalline sample. These observations suggest that the electronic ground state of Y$_2$C is at the limit between weak-to-strong electronic correlation, where onsite Coulomb repulsion is sensitive to a local modulation of the electronic state or a shift in the Fermi level due to the presence of defects/impurities.
High-mobility two-dimensional carriers originating from pairs of Weyl nodes in magnetic Weyl semimetals is highly desired for accessing exotic quantum transport phenomena and for topological electronics applications. Here, we report thickness- and angle-dependent magnetotransport, including quantum oscillations, in magnetic Weyl semimetal SrRuO3 epitaxial films grown by machine-learning-assisted molecular beam epitaxy. The exceptionally high quality of our SrRuO3 films enables observation of the quantum transport of Weyl fermions even when the film thickness is as thin as 10 nm. The quantum oscillations for the 10-nm film show a high quantum mobility of 3500 cm2/Vs, a light cyclotron mass of 0.25m0 (m0: the free electron mass in a vacuum), and two-dimensional angular dependence. When the film thickness is 63 nm, which is too large to observe the quantum confinement effect, we still observe the two-dimensional angular dependence of the quantum oscillations, suggesting that the high-mobility two-dimensional carriers originate from surface Fermi arcs. By measuring the magnetoresistance up to 52 T, we also observed the saturation of the negative magnetoresistance (MR) in the quantum limit, confirming the negative MR is induced by the chiral anomaly of Weyl nodes in SrRuO3. These findings make SrRuO3 an intriguing platform for topological oxide electronics and pave the way for exploring exotic quantum transport phenomena in magnetic Weyl semimetals, which can be controlled by both magnetic and electric fields.
Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) we study the electronic structure of layered BaZnBi$_2$. Our experimental results show no evidence of Dirac states in BaZnBi$_2$ originated either from the bulk or the surface. The calculated band structure without spin-orbit interaction shows several linear dispersive band crossing points throughout the Brillouin zone. However, as soon as the spin-orbit interaction is turned on, the band crossing points are significantly gapped out. The experimental observations are in good agreement with our DFT calculations. These observations suggest that the Dirac fermions in BaZnBi$_2$ are trivial and massive. We also observe experimentally that the electronic structure of BaZnBi$_2$ comprises of several linear dispersive bands in the vicinity of Fermi level dispersing to a wider range of binding energy.
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