  Defects are crucial in determining a variety of material properties
especially in low dimensions. In this work, we study point defects in monolayer
alpha-phase Ruthenium (III) chloride (alpha-RuCl3), a promising candidate to
realize quantum spin liquid with nearly degenerate magnetic states. Our
first-principles simulations reveal that Cl vacancies, Ru vacancies, and oxygen
substitutional defects are the most energetically stable point defects.
Besides, these point defects break the magnetic degeneracy: Cl vacancies and
oxygen substitutional defects energetically favor the zigzag-antiferromagnetic
configuration while Ru vacancies favor the ferromagnetic configuration,
shedding light on understanding the observed magnetic structures and further
defect engineering of magnetism in monolayer {\alpha}-RuCl3. We further
calculated their electronic structures and optical absorption spectra. The
polarization symmetry of optical responses provides a convenient signature to
identify the point defect types and long-range magnetic orders.
