Observation of Dirac hierarchy in three-dimensional acoustic topological insulators

Dirac cones (DCs) play a pivotal role in various unique phenomena ranging from massless electrons in graphene to robust surface states in topological insulators (TIs). Recent studies have theoretically revealed a full Dirac hierarchy comprising an eightfold bulk DC, a fourfold surface DC, and a twofold hinge DC, associated with a hierarchy of topological phases including first-order to third-order three-dimensional (3D) topological insulators, using the same 3D base lattice. Here, we report the first experimental observation of the Dirac hierarchy in 3D acoustic TIs. Using acoustic measurements, we unambiguously reveal that lifting of multifold DCs in each hierarchy can induce two-dimensional (2D) topological surface states with a fourfold DC in a first-order 3D TI, one-dimensional (1D) topological hinge states with a twofold DC in a second-order 3D TI, and zero-dimensional (0D) topological corner states in a third-order 3D TI. Our work not only expands the fundamental research scope of Dirac physics, but also opens up a new route for multidimensional robust wave manipulation

Comparative validation of machine learning algorithms for surgical workflow and skill analysis with the HeiChole benchmark

Purpose: Surgical workflow and skill analysis are key technologies for the next generation of cognitive surgical assistance systems. These systems could increase the safety of the operation through context-sensitive warnings and semi-autonomous robotic assistance or improve training of surgeons via data-driven feedback. In surgical workflow analysis up to 91% average precision has been reported for phase recognition on an open data single-center video dataset. In this work we investigated the generalizability of phase recognition algorithms in a multicenter setting including more difficult recognition tasks such as surgical action and surgical skill. Methods: To achieve this goal, a dataset with 33 laparoscopic cholecystectomy videos from three surgical centers with a total operation time of 22 h was created. Labels included framewise annotation of seven surgical phases with 250 phase transitions, 5514 occurences of four surgical actions, 6980 occurences of 21 surgical instruments from seven instrument categories and 495 skill classifications in five skill dimensions. The dataset was used in the 2019 international Endoscopic Vision challenge, sub-challenge for surgical workflow and skill analysis. Here, 12 research teams trained and submitted their machine learning algorithms for recognition of phase, action, instrument and/or skill assessment. Results: F1-scores were achieved for phase recognition between 23.9% and 67.7% (n = 9 teams), for instrument presence detection between 38.5% and 63.8% (n = 8 teams), but for action recognition only between 21.8% and 23.3% (n = 5 teams). The average absolute error for skill assessment was 0.78 (n = 1 team). Conclusion: Surgical workflow and skill analysis are promising technologies to support the surgical team, but there is still room for improvement, as shown by our comparison of machine learning algorithms. This novel HeiChole benchmark can be used for comparable evaluation and validation of future work. In future studies, it is of utmost importance to create more open, high-quality datasets in order to allow the development of artificial intelligence and cognitive robotics in surgery

Observation of multiple rotons and multidirectional roton-like dispersion relations in acoustic metamaterials

Roton dispersion relations were firstly predicted by Landau and have been extensively explored in correlated quantum systems at low temperatures. Recently, the roton-like dispersion relations were theoretically extended to classical acoustics, which, however, have remained elusive in reality. Here, we report the experimental observation of roton-like dispersions in acoustic metamaterials with beyond-nearest-neighbour interactions at ambient temperatures. The resulting metamaterial supports multiple coexisting modes with different wavevectors and group velocities at the same frequency and broadband backward waves, analogous to the ‘return flow’ termed by Feynman in the context of rotons. By increasing the order of long-range interaction, we observe multiple rotons on a single dispersion band, which have never appeared in Landau’s prediction or any other condensed-matter or classical-wave studies. Moreover, we have also theoretically proposed and experimentally observed multidirectional roton-like dispersion relations in a two-dimensional nonlocal acoustic metamaterial. The realization of roton-like dispersions in metamaterials could pave the way to explore novel physics and applications on quantum-inspired phenomena in classical systems.Published versionZ.G. acknowledge support from the National Natural Science Foundation of China under grant number 12104211, 6101020101, SUSTech Start-up Grant (Y01236148, Y01236248). The work at Zhejiang University was sponsored by the Key Research and Development Program of the Ministry of Science and Technology under Grants No. 2022YFA1404902 (Y Y) and No.2022YFA1405201 (Y Y), the National Natural Science Foundation of China (NNSFC) under Grants No. 62175215 (Y Y), the Fundamental Research Funds for the Central Universities (2021FZZX001-19) (Y Y), and the Excellent Young Scientists Fund Program (Overseas) of China (Y Y). H S acknowledged the support of the National Natural Science Foundation of China (Grant Nos. 12274183 and 12174159), and the State Key Laboratory of Acoustics, Chinese Academy of Science under Grant No. SKLA202216

Spinful topological phases in acoustic crystals with projective PT symmetry

For the classification of topological phases of matter, an important consideration is whether a system is spinless or spinful, as these two classes have distinct symmetry algebra that gives rise to fundamentally different topological phases. However, only recently has it been realized theoretically that in the presence of gauge symmetry, the algebraic structure of symmetries can be projectively represented, which possibly enables the switch between spinless and spinful topological phases. Here, we report the experimental demonstration of this idea by realizing spinful topological phases in "spinless" acoustic crystals with projective space-time inversion symmetry. In particular, we realize a one-dimensional topologically gapped phase characterized by a 2Z winding number, which features double-degenerate bands in the entire Brillouin zone and two pairs of degenerate topological boundary modes. Our Letter thus overcomes a fundamental constraint on topological phases by spin classes.Published versionZ. G. acknowledges support from the National Natural Science Foundation of China under Grants No. 12104211, Shenzhen Science and Technology Innovation Commission under Grant No. 20220815111105001, and SUSTech under Grants No. Y01236148 and No. Y01236248. The work at Zhejiang University was sponsored by the Key Research and Development Program of the Ministry of Science and Technology under Grants No. 2022YFA1404704 (H. C.), No. 2022YFA1404902 (Y. Y.), and No. 2022YFA1405201 (Y. Y.), the National Natural Science Foundation of China (NNSFC) under Grants No. 11961141010 (H. C.), No. 62175215 (Y. Y.), and No. 61975176 (H. C.), the Fundamental Research Funds for the Central Universities (2021FZZX001-19) (Y. Y.), and the Excellent Young Scientists Fund Program (Overseas) of China (Y. Y.). H. S. acknowledges the support from the National Natural Science Foundation of China (Grants No. 12274183 and No. 12174159), and the National Key Research and Development Program of China (Grant No. 2020YFC1512403)