Thickness-dependent quantum transport of Weyl fermions in ultra-high-quality SrRuO3 films

Abstract

The recent observation of Weyl fermions in the itinerant 4d ferromagnetic perovskite SrRuO3 points to this material being a good platform for exploring novel physics related to a pair of Weyl nodes in epitaxial heterostructures. In this letter, we report the thickness-dependent magnetotransport properties of ultra-high-quality epitaxial SrRuO3 films grown under optimized conditions on SrTiO3 substrates. Signatures of Weyl fermion transport, i.e., unsaturated linear positive magnetoresistance accompanied by a quantum oscillation having a π Berry phase, were observed in films with thicknesses as small as 10 nm. Residual resistivity increased with decreasing film thickness, indicating disorder near the interface between SrRuO3 and the SrTiO3 substrate. Since this disorder affects the magnetic and electrical properties of the films, the Curie temperature decreases and the coercive field increases with decreasing thickness. Thickness-dependent magnetotransport measurements revealed that the threshold residual resistivity ratio (RRR) to observe Weyl fermion transport is 21. These results provide guidelines for realizing quantum transport of Weyl fermions in SrRuO3 near heterointerfaces.

Bayesian optimization with experimental failure for high-throughput materials growth

“…The itinerant 4d ferromagnetic perovskite SrRuO 3 is one of the most promising materials for oxide electronics owing to its high metallic conduction, chemical stability, compatibility with other perovskite oxides, and ferromagnetism with strong uniaxial magnetic anisotropy [30][31][32][33][34][35][36][37][38][39] . In addition, the recent observation of the Weyl fermions in SrRuO 3 points to this material as an appropriate platform to integrate two emerging fields: topology in condensed matter and oxide electronics [40][41][42][43][44] . The RRR value, defined as the ratio of resistivity at 300 K [ρ(300 K)] to that at 4 K [ρ(4 K)], is a good measure of the purity of metallic systems, and accordingly, the quality of single-crystalline SrRuO 3 thin films 35,39,[45][46][47] .…”

“…For practical applications of SrRuO 3 , such as electrodes in dynamic random access memory application, high-quality SrRuO 3 films are necessary 48,49 . In terms of physical interest, high-quality SrRuO 3 films are also indispensable because only SrRuO 3 films with high RRR values above 20 have enabled the observation of quantum transport of Weyl fermions 41,42 . Thus, we adopted RRR as the evaluation value.…”

Science-based, data-driven developments in plasma processing for material synthesis and device-integration technologies

“…Wakabayashi et al demonstrated Bayesian optimization of the molecular beam epitaxy of ternary compounds, such as SrRuO 3 . [328][329][330][331][332][333][334][335][336][337][338][339][340] HTS techniques can reduce the number of trials and reach a goal using predictive conditions obtained from the data of previous runs. 341) Predictive RbR control has enabled the efficient development of a processing method for new materials.…”

Strain- and Oxygen-Modulated Anomalous Hall Effect in the SrRuO3/Sr3Al2O6 Heterostructure

“…SrRuO 3 (SRO), a 4d itinerant ferromagnet, is an extensively explored oxide with the nontrivial band structure, which is highly sensitive to the intrinsic Berry curvature, especially with the Fermi level near the band crossing position. − This nontrivial band structure triggers various exotic physical properties in SRO, including anomalous Hall effect (AHE), topological Hall effect (THE), and so forth. , Recently, the emergence of hump-like AHE in thin SRO films (few of nanometers) and heterostructures has garnered considerable attention as a promising signature of nontrivial spin texture as well as various potential applications in spintronics. − However, the origination of these hump-like features in AHE remains the subject of ongoing debate. − Some studies propose that these features arise from the topological Hall effect resulting from magnetic skyrmions (Berry curvature in band structure), − while the two-channel AHE (inhomogeneous magnetic domains) is suggested as the origination of hump-like AHE, , which is strongly related to thickness variation, , structural defects, and stoichiometric deviation (Ru contents). − In the two-channel AHE model, the two anomalous Hall resistances have opposite polarities, which are temperature dependent . Although hump-like features are observed in SRO films of varying thicknesses in the literature, such characteristics are typically observed in thinner films.…”

“…Although hump-like features are observed in SRO films of varying thicknesses in the literature, such characteristics are typically observed in thinner films. Such features are typically observed in thinner films (≤6 nm). − ,− The hump-like characteristics are absent in thick films unless they possess a high density of structural defects . Thus, a comprehensive investigation of AHE in SRO films is necessary to understand the origin of hump-like AHE.…”