Magnetic Weyl semimetals have novel transport phenomena related to pairs of Weyl nodes in the band structure. Although the existence of Weyl fermions is expected in various oxides, the evidence of Weyl fermions in oxide materials remains elusive. Here we show direct quantum transport evidence of Weyl fermions in an epitaxial 4d ferromagnetic oxide SrRuO₃. We employ machine-learning-assisted molecular beam epitaxy to synthesize SrRuO₃ films whose quality is sufficiently high to probe their intrinsic transport properties. Experimental observation of the five transport signatures of Weyl fermions—the linear positive magnetoresistance, chiral-anomaly-induced negative magnetoresistance, π phase shift in a quantum oscillation, light cyclotron mass, and high quantum mobility of about 10,000 cm²V⁻¹s⁻¹—combined with first-principles electronic structure calculations establishes SrRuO₃ as a magnetic Weyl semimetal. We also clarify the disorder dependence of the transport of the Weyl fermions, which gives a clear guideline for accessing the topologically nontrivial transport phenomena.

PbMO₃ (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO₃ remain unclear. Herein, we reveal that PbFeO₃ crystallizes into an unusual 2a_p × 6a_p × 2a_p orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb²⁺_0.5Pb⁴⁺_0.5FeO₃ lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb²⁺ and Pb⁴⁺) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe³⁺ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Negative thermal expansion (NTE) is an intriguing physical phenomenon. Layered Ca₂RuO₄ exhibits giant NTE over a wide temperature range from 200 to 400 K, which makes it attractive for fundamental research and industrial applications. However, a clear physical understanding is lacking for the appearance of NTE over such a wide temperature range and the oxygen-content-dependent switch from NTE to positive thermal expansion (PTE). Herein, we present insights into the average crystal structure, local structure, and electronic and orbital states of Ca₂RuO₄. Surprisingly, a previously overlooked monoclinic distortion is identified by electron diffraction and synchrotron X-ray diffraction (SXRD). X-ray absorption fine structure (XAFS) and synchrotron X-ray pair distribution function (PDF) analyses show large local distortions in monoclinic Ca₂RuO₄. Moreover, local stress on Ru cations is confirmed by the existence of over-bonding states, which relaxes along with NTE. Theoretical calculations indicate that d_xy orbital ordering and disordering in the monoclinic structure are the origins of NTE. Moreover, interstitial oxygen plays a critical role in stabilizing elongated RuO₆ and locally breaks the d_xy orbital ordering, facilitating the occurrence of PTE. This work elucidates the electronic and orbital states in NTE materials with defective lattices and provides a different route to designing unconventional NTE materials.

We present a dynamical mean-field theory study of the charge and orbital correlations in finite-size ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (LCMO) nanoclusters. Upon nanostructuring LCMO to clusters of 3 nm diameter, the size reduction induces an insulator-to-metal transition in the high-temperature paramagnetic phase. This is ascribed to the reduction in charge disproportionation between Mn sites with different nominal valence [H. Das et al., Phys. Rev. Lett. 107, 197202 (2011)]. Here we show that upon further reducing the system size to nanoclusters of a few atoms, quantum confinement effects come into play. These lead to the opposite effect: the nanocluster turns insulating again and the charge disproportionation between Mn sites and the orbital polarization are enhanced. Electron doping by means of external gate voltage on few-atom nanoclusters is found to trigger a site- and orbital-selective Mott transition. Our results suggest that LCMO nanoclusters could be employed for the realization of technological devices, exploiting the proximity to the Mott transition and its control by size and gate voltage.

Here, we propose an idea to realize noncollinear ferrimagnetic orders with a considerably high magnetization $\mathbf{M}$ and their coupling with an improper ferroelectric (FE) order in hexagonal ${\mathrm{LuFeO}}_3$ type systems which exhibit interesting topological orders. These magnetic and magnetoelectric phenomena are driven by the antisymmetric Dzyaloshinskii-Moriya interactions between the magnetic ions and their coupling with the ferroelectricity. The proposed two-sublattice magnetic system, generated by a specific charge-ordered state, forms multiple energetically close, noncollinear ferrimagnetic orders. This offers a platform to manipulate the microscopic magnetic interactions and to trigger spin-reorientation (SR) transitions by various efficient means. The two-sublattice structure was realized in the hexagonal phase of ${\mathrm{LuFeO}}_3$ doped with electrons. The proposed electron-doped systems are expected to exhibit switchable electric polarization ($P\sim 6–15 \mu \mathrm{C}/{\mathrm{cm}}^2$), considerably high magnetization ($M\sim 1.1–1.3{\mu }_B/\mathrm{Fe}$), and magnetic transition near room temperature ($\sim 275–290\mathrm{K}$). Based on the coupling between the magnetic interactions and the FE primary order parameter observed in this system, microscopic mechanisms to achieve electric field $\mathbf{E}$ induced SR transitions and ${180}^{\circ }$ switching of the direction of $\mathbf{M}$ are discussed.