Recently,intrinsic antiferromagnetic topological insulator MnBi_(2)Te_(4) has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous H...Recently,intrinsic antiferromagnetic topological insulator MnBi_(2)Te_(4) has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect,axion insulator state,and other quantum phases.An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi_(2)Te_(4).However,the experimental observations of the surface gap remain controversial.Here,we report the observation of the surface gap via the point contact tunneling spectroscopy.In agreement with theoretical calculations,the gap size is around 50 me V,which vanishes as the sample becomes paramagnetic with increasing temperature.The magnetoresistance hysteresis is detected through the point contact junction on the sample surface with an out-of-plane magnetic field,substantiating the surface ferromagnetism.Furthermore,the non-zero transport spin polarization coming from the ferromagnetism is determined by the point contact Andreev reflection spectroscopy.Combining these results,the magnetism-induced gap in topological surface states of MnBi_(2)Te_(4) is revealed.展开更多
Nature reserves in the Yangtze River,China,are crucial for protecting the Yangtze finless porpoise(Neophocaena asiaeorientalis asiaeorientalis)and their habitats.Despite a 10-year fishing ban along the middle and lowe...Nature reserves in the Yangtze River,China,are crucial for protecting the Yangtze finless porpoise(Neophocaena asiaeorientalis asiaeorientalis)and their habitats.Despite a 10-year fishing ban along the middle and lower reaches of the Yangtze River instated on January 2020 to alleviate pressures on the porpoise's prey,other stressors,particularly underwater noise,continue to impact these rare cetaceans,which rely heavily on vocalization for prey detection,communication and navigation.Between September 2020 and September 2021,underwater noise monitoring was carried out in the Xinluo Baiji National Nature Reserve,located in Hubei province,China,with the aim of assessing the potential impact of underwater noise pollution on the Yangtze finless porpoise.The reserve was divided into 30 sub-sections to systematically monitor underwater noise across the reserve.Passive acoustic monitoring was conducted at each site within these sub-sections during four sampling periods,and these data were subjected to comparative analysis with data collected in 2012 and 2017.The results showed that the root mean square sound pressure levels at each site ranged from 117 to 162 dB.Power spectral levels(11.2–152 kHz)exceeded the average audiogram levels for the porpoise at most sites.94.2%and 93.3%of the sites exceeded underwater acoustic thresholds known to induce a hearing threshold shift for the porpoise in terms of both the averaged cumulative unweighted sound exposure level,and cumulative weighted sound exposure level respectively.Noise levels in this section during 2020–2021 were approximately 25 and 30 dB higher compared to the levels recorded in 2012 and 2017,and likely due to increased shipping activity and the proximity of sampling sites to the shipping channel.The rise in noise levels might contribute to the observed declines in the porpoise population within this area.To mitigate these impacts,implementing noise control measures is essential,such as regulating navigation speed and limiting the number of vessels in the reserve.展开更多
基金Supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303302,2018YFA0305604,2018YFA0307100)the National Natural Science Foundation of China (Grant Nos. 11888101,11774008,11704279,11874035,51788104)+2 种基金the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB28000000)the Beijing Natural Science Foundation (Grant Nos. Z180010 and 1202005)the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics,Tsinghua University (Grant No. KF202001)
文摘Recently,intrinsic antiferromagnetic topological insulator MnBi_(2)Te_(4) has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect,axion insulator state,and other quantum phases.An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi_(2)Te_(4).However,the experimental observations of the surface gap remain controversial.Here,we report the observation of the surface gap via the point contact tunneling spectroscopy.In agreement with theoretical calculations,the gap size is around 50 me V,which vanishes as the sample becomes paramagnetic with increasing temperature.The magnetoresistance hysteresis is detected through the point contact junction on the sample surface with an out-of-plane magnetic field,substantiating the surface ferromagnetism.Furthermore,the non-zero transport spin polarization coming from the ferromagnetism is determined by the point contact Andreev reflection spectroscopy.Combining these results,the magnetism-induced gap in topological surface states of MnBi_(2)Te_(4) is revealed.
文摘Nature reserves in the Yangtze River,China,are crucial for protecting the Yangtze finless porpoise(Neophocaena asiaeorientalis asiaeorientalis)and their habitats.Despite a 10-year fishing ban along the middle and lower reaches of the Yangtze River instated on January 2020 to alleviate pressures on the porpoise's prey,other stressors,particularly underwater noise,continue to impact these rare cetaceans,which rely heavily on vocalization for prey detection,communication and navigation.Between September 2020 and September 2021,underwater noise monitoring was carried out in the Xinluo Baiji National Nature Reserve,located in Hubei province,China,with the aim of assessing the potential impact of underwater noise pollution on the Yangtze finless porpoise.The reserve was divided into 30 sub-sections to systematically monitor underwater noise across the reserve.Passive acoustic monitoring was conducted at each site within these sub-sections during four sampling periods,and these data were subjected to comparative analysis with data collected in 2012 and 2017.The results showed that the root mean square sound pressure levels at each site ranged from 117 to 162 dB.Power spectral levels(11.2–152 kHz)exceeded the average audiogram levels for the porpoise at most sites.94.2%and 93.3%of the sites exceeded underwater acoustic thresholds known to induce a hearing threshold shift for the porpoise in terms of both the averaged cumulative unweighted sound exposure level,and cumulative weighted sound exposure level respectively.Noise levels in this section during 2020–2021 were approximately 25 and 30 dB higher compared to the levels recorded in 2012 and 2017,and likely due to increased shipping activity and the proximity of sampling sites to the shipping channel.The rise in noise levels might contribute to the observed declines in the porpoise population within this area.To mitigate these impacts,implementing noise control measures is essential,such as regulating navigation speed and limiting the number of vessels in the reserve.
