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Register a new userQuantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide
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Magnetic Weyl fermions, which occur in magnets, have novel transport phenomena related to pairs of Weyl nodes, and they are, of both, scientific and technological interest, with the potential for use in high-performance electronics, spintronics and quantum computing. Although magnetic Weyl fermions have been predicted to exist in various oxides, evidence for their existence in oxide materials remains elusive. SrRuO3, a 4d ferromagnetic metal often used as an epitaxial conducting layer in oxide heterostructures, provides a promising opportunity to seek for the existence of magnetic Weyl fermions. Advanced oxide thin film preparation techniques, driven by machine learning technologies, may allow access to such topological matter. Here we show direct quantum transport evidence of magnetic Weyl fermions in an epitaxial ferromagnetic oxide SrRuO3: unsaturated linear positive magnetoresistance (MR), chiral-anomaly-induced negative MR, Pi Berry phase accumulated along cyclotron orbits, light cyclotron masses and high quantum mobility of about 10000 cm2/Vs. We employed machine-learning-assisted molecular beam epitaxy (MBE) to synthesize SrRuO3 films whose quality is sufficiently high to probe their intrinsic quantum transport properties. We also clarified the disorder dependence of the transport of the magnetic Weyl fermions, and provided a brand-new diagram for the Weyl transport, which gives a clear guideline for accessing the topologically nontrivial transport phenomena. Our results establish SrRuO3 as a magnetic Weyl semimetal and topological oxide electronics as a new research field.
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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.
We investigate the ultrafast relaxation dynamics of hot Dirac fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission spectroscopy. We observe DT spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, the electron cooling is determined by electron-acoustic phonon scattering, found to occur on the time scale of 1 ps for highly doped layers, and 4-11 ps in undoped layers. The spectra also provide strong evidence for the multilayer stucture and doping profile of thermally grown epitaxial graphene on SiC.
The recent discoveries of strikingly large zero-field Hall and Nernst effects in antiferromagnets Mn$_3$$X$, ($X$ = Sn, Ge) have brought the study of magnetic topological states to the forefront of condensed matter research and technological innovation. These effects are considered fingerprints of Weyl nodes residing near the Fermi energy, promoting Mn$_3$$X$, ($X$ = Sn, Ge) as a fascinating platform to explore the elusive magnetic Weyl fermions. In this review, we provide recent updates on the insights drawn from experimental and theoretical studies of Mn$_3$$X$, ($X$ = Sn, Ge) by combining previous reports with our new, comprehensive set of transport measurements of high-quality Mn$_3$Sn and Mn$_3$Ge single crystals. In particular, we report magnetotransport signatures specific to chiral anomalies in Mn$_3$Ge and planar Hall effect in Mn$_3$Sn, which have not yet been found in earlier studies. The results summarized here indicate the essential role of magnetic Weyl fermions in producing the large transverse responses in the absence of magnetization.
The kagome lattice is a two-dimensional network of corner-sharing triangles known as a platform for exotic quantum magnetic states. Theoretical work has predicted that the kagome lattice may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these have evaded experimental detection to date. Here we study the d-electron kagome metal Fe$_3$Sn$_2$ designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature independent intrinsic anomalous Hall conductivity persisting above room temperature suggestive of prominent Berry curvature from the time-reversal breaking electronic bands of the kagome plane. Using angle-resolved photoemission, we discover a pair of quasi-2D Dirac cones near the Fermi level with a 30 meV mass gap that accounts for the Berry curvature-induced Hall conductivity. We show this behavior is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state with atomic spin-orbit coupling. This report provides the first evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. This offers insight into recent discoveries of exotic electronic behavior in kagome lattice antiferromagnets and may provide a stepping stone toward lattice model realizations of fractional topological quantum states.
An ideal Weyl semimetal with a single pair of Weyl points (WPs) may be generated by splitting a single Dirac point (DP) through the breaking of time-reversal symmetry by magnetic order. However, most known Dirac semimetals possess a pair of DPs along an axis that is protected by crystalline symmetry. Here, we demonstrate that a single pair of WPs may also be generated from a pair of DPs. Using first-principles band structure calculations, we show that inducing ferromagnetism in the AFM Dirac semimetal EuCd2As2 generates a single pair of WPs due to its half-metallic nature. Analysis with a low-energy effective Hamiltonian shows that this ideal Weyl semimetal is obtained in EuCd2As2 because the DPs are very close to the zone center and the ferromagnetic exchange splitting is large enough to push one pair of WPs to merge and annihilate at Gamma while the other pair survives. Furthermore, we predict that alloying with Ba at the Eu site can stabilize the ferromagnetic configuration and generate a single pair of Weyl points without application of a magnetic field.
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