查看更多>>摘要：QED atoms are composed of unstructured and point-like lepton pairs bound together by the electromag-netic force.The smallest and heaviest QED atom is formed by a τ+τ-pair.Currently,the only known atoms of this type are the e+e-and μ+e-atoms,which were discovered 64 years ago and remain the sole examples found thus far.We demonstrate that the Jτ(τ+τ-atom with JPC=1--)atom signal can be observed with a significance larger than 5σ including both statistical and systematic uncertainties,via.the process e+e-→X+Y-(E)(X,Y=e,μ,π,K,or ρ,and(E) is the missing energy due to unobserved neutri-nos)with 1.5 ab-1 data taken around the τ pair production threshold.The τ lepton mass can be measured with a precision of 1 keV with the same data sample.This is within one year's running time of the pro-posed super tau-charm facility in China or super charm-tau factory in Russia.
查看更多>>摘要：Magnetic impurities in superconductors are of increasing interest due to emergent Yu-Shiba-Rusinov(YSR)states and Majorana zero modes for fault-tolerant quantum computation.However,a direct rela-tionship between the YSR multiple states and magnetic anisotropy splitting of quantum impurity spins remains poorly characterized.By using scanning tunneling microscopy,we systematically resolve indi-vidual transition-metal(Fe,Cr,and Ni)impurities induced YSR multiplets as well as their Zeeman effects in the K3C60 superconductor.The YSR multiplets show identical d orbital-like wave functions that are symmetry-mismatched to the threefold K3C60(1 1 1)host surface,breaking point-group symmetries of the spatial distribution of YSR bound states in real space.Remarkably,we identify an unprecedented fermion-parity-preserving quantum phase transition between ground states with opposite signs of the uniaxial magnetic anisotropy that can be manipulated by an external magnetic field.These findings can be readily understood in terms of anisotropy splitting of quantum impurity spins,and thus elucidate the intricate interplay between the magnetic anisotropy and YSR multiplets.
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