A sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to observe a large fraction of the northern sky to hunt for γ-ray sources at energies above 10 TeV. Even though the detector construction is still underway, half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the KM2A data analysis pipeline and the first observation of the Crab Nebula, a standard candle in very high energy γ-ray astronomy. We detect γ-ray signals from the Crab Nebula in both energy ranges of 10 100 TeV and 100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance, including angular resolution, pointing accuracy and cosmic-ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE = (1.13 0.05 0.08 ) 10 (E/20 TeV) cm s TeV . It is consistent with previous measurements by other experiments. This opens a new window of γ-ray astronomy above 0.1 PeV through which new ultrahigh-energy γ-ray phenomena, such as cosmic PeVatrons, might be discovered.

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A simple method was proposed to control the nanotribology behaviors of monocrystalline silicon against SiO2 microsphere by adjusting relative humidity (RH). Experimental results indicated that adhesion work, friction coefficient, and nanowear of silicon against SiO2 microsphere significantly varied between 60% and 90% RH. Under 60% RH, adhesion work was 119 mN/m, and friction coefficient was about 0.53. However, adhesion work and friction coefficient decreased to ∼70 mN/m and ∼0.3 under 90% RH, respectively. An apparent wear track ∼13 nm deep formed on the silicon surface under 60% RH, whereas no obvious wear scar was observed on the silicon surface under 90% RH. Analysis indicated that such tribological behaviors were due to different water condensations on the silicon surface under 60% and 90% RH. Under 60% RH, the water that condensed on the surfaces of the silicon sample and SiO2 tip mainly consisted of ice-like water. As a result, adhesion work was enlarged by the breaking force of the ice-like water bridge in the contact area. Given that a ≡Si–O–Si≡ bonding bridge easily formed between the silicon surface and the SiO2 tip with the help of water condensation under 60% RH instead of 90% RH, the friction coefficient was large and the nanowear of the silicon sample was severe under 60% RH. These results may help elucidate the nanotribology behaviors of silicon and facilitate the tribological design of dynamic microelectromechanical systems working under humid conditions.