arXiv subjects

Yuqi Qin

Publications and source records attributed to Yuqi Qin.

5 recordsLinked to original sources

arXiv2026

Intrinsic anomalous Hall response in the bilayer kagome ferromagnet Co$_3$Sn

Transition-metal kagome magnets provide a rich platform for investigating the interplay between layer stacking, magnetic order, and band topology. Here, we report the molecular beam epitaxy and experimental investigation of high-quality thin films of the kagome metal Co$_3$Sn, which has not been synthesized in bulk form yet. Structural and chemical analyses confirm a hexagonal lattice structure ($P6_3/mmc$) composed of direct A-B stacked Co$_3$Sn kagome bilayers. Magnetometry reveals robust easy-plane ferromagnetism with a Curie temperature exceeding $300\,\text{K}$. Magneto-transport measurements demonstrate metallic behavior (carrier density $n\approx5.01\times10^{22}\,\text{cm}^{-3}$) alongside a temperature-independent anomalous Hall conductivity of $\sigma_{\rm AHE}\approx90\,\Omega^{-1}\cdot{\rm cm}^{-1}$, extending from $2\,\text{K}$ up to room temperature. Results from first-principles density functional theory calculations attribute this anomalous Hall response to intrinsic Berry curvature hotspots near the Fermi level in the spin-split band structure. Our results establish Co$_3$Sn as a room-temperature kagome ferromagnet and highlight the impact of the layer stacking sequence on the material properties of kagome metals from the CoSn family.

cond-mat.mtrl-sci↗

arXiv2026

Active learning molecular beam epitaxy of complex quantum materials

The integration of machine learning (ML) into materials science offers a transformative pathway toward fully autonomous synthesis workflows. For precise thin-film deposition techniques like molecular beam epitaxy (MBE), this automation is critical to overcome the time-consuming, manual navigation of high-dimensional thermodynamic phase spaces. Existing approaches for ML-assisted thin film growth predominantly rely on continuous Bayesian optimization (BO) models that assume smooth parameter landscapes. Consequently, they struggle to capture the abrupt crystallographic phase boundaries and narrow growth windows inherent to binary quantum materials. Here, we demonstrate an active learning protocol based on Sequential Model-Based Optimization (SMBO) designed specifically for the closed-loop MBE of such compounds. To overcome the limitations of continuous models while retaining the efficient exploration-exploitation logic of traditional BO, we combine a random forest surrogate model capable of capturing highly non-linear phase transitions and thermodynamics constraints of the growth process with an expected improvement function to predict optimum growth parameters. We apply this combined SMBO framework to the MBE of the topological Weyl ferromagnet Fe$_3$Sn, which exists as a metastable line compound. Using a small initial training set of fewer than twenty growth iterations, our active learning loop rapidly navigates a complex optimization landscape to identify an optimum growth window bounded by sharp transitions. Within only four active learning iterations, the absolute predictive error is halved to $\approx10\%$. This data-efficient framework paves the way for the autonomous discovery and thin-film synthesis of functional quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Room temperature observation of the anomalous in-plane Hall effect in epitaxial thin films of a Weyl ferromagnet

Topologically nontrivial electronic states can give rise to novel anomalous Hall effects. The potential appearance of these effects at room temperature holds promise for their application in magnetic sensing, spintronics, and energy harvesting technology. The anomalous in-plane Hall effect (IPHE) is predicted to arise in topological magnetic materials when an external magnetic field is applied within the sample plane. Because of stringent symmetry requirements, the conclusive detection of the anomalous IPHE induced by topological electronic states remains challenging, and the study of anomalous Hall effects is often confined to cryogenic conditions. Combining molecular beam epitaxy of the kagome metal Fe$_3$Sn with measurements of the electric Hall effect and theoretical calculations, we propose and experimentally demonstrate that the interplay of the kagome lattice motif with spin-orbit coupling and canted ferromagnetism with large exchange interactions gives rise to the anomalous IPHE at room temperature that is induced by topological Weyl points in the electronic band structure. Synthesizing a topological heterostructure including layers of Fe$_3$Sn and ferromagnetic CoFeB, we further show the enhancement of the anomalous IPHE through the magnetic stray field of the CoFeB layer. Our work establishes a design paradigm for topological magnets and heterostructures to discover and control novel anomalous Hall effects toward their use in technological applications.

cond-mat.mes-hall↗

arXiv2023

Magnetic-field tuned anisotropic quantum phase transition in the distorted kagome antiferromagnet Nd3BWO9

Rare-earth (RE) kagome-lattice magnets offer an excellent platform to discover the novel magnetic phase as well as quantum phase transition tuned by non-thermal control parameters, while the experimental realizations remain largely unexplored. Here, we report the discovery of magnetic-field (B)-induced anisotropic quantum phase transition in a distorted kagome antiferromagnet Nd3BWO9 with TN~0.32 K. The isothermal magnetizations at 0.05 K exhibit the spin-flop like metamagnetic crossover behaviors with different fractional magnetization anomalies for B perpendicular (B // c-axis) and parallel (B // a*-axis) to the kagome plane, respectively. In combination with the thermodynamic measurements, the field-temperature (B-T) phase diagrams for both field directions are constructed and that reveal the existence of several field-induced magnetic states. Along the c-axis, a proximate quantum bicritical point is observed near the metamagnetic crossover, which separates the low-field antiferromagnetic (AFM) phase and the intermediate AFM phase. While, for B // a*, another intermediate magnetic phase (IAFM2) appears between the low-field AFM phase and intermediate AFM (IAFM1) phase, giving rise to a tetracritical point. These results support the anisotropic field-induced metamagnetic quantum criticalities in Nd3BWO9, making it as a rare kagome antiferromagnet to investigate the quantum multi-criticality driven by spin frustration.

cond-mat.str-el↗

arXiv2023

Ba9RE2(SiO4)6 (RE=Ho-Yb): A New Family of Rare-earth based Honeycomb Lattice Magnets

Rare-earth (RE) based honeycomb-lattice materials with strong spin-orbit coupled Jeff=1/2 moments have attracted great interest as a platform to realize Kitaev quantum spin liquid (QSL) state. Herein, we report the discovery of a new family of RE based honeycomb-lattice magnets Ba9RE2(SiO4)6(RE=Ho-Yb), which crystallize into the rhombohedral structure with space group R-3. In these serial compounds, magnetic RE3+ ions are arranged on a perfect honeycomb lattice within the ab-plane and stacked in the ABCABC-type fashion along the c-axis. All Ba9RE2(SiO4)6(RE=Ho-Yb) polycrystals exhibit the dominant antiferromagnetic interactions and absence of magnetic order down to 2 K. In combination with the magnetization and electron spin resonance (ESR) results, distinct anisotropic magnetic behaviors are proposed for compounds with different RE ions. Moreover, the synthesized Ba9Yb2Si6O24 single crystals show large magnetic frustration and no long-range magnetic ordering down to 0.15 K, being a possible QSL candidate state. These serial compounds are attractive for exploring the exotic magnetic phases of Kitaev materials with 4f electrons.

cond-mat.str-el↗