Preview:
library(ggplot2)
ggplot(mtcars, aes(hp, mpg)) + 
       geom_point() +
       labs(x = bquote('x axis'~(Å^2)), y = "y axis") +
       #or
       #labs(x = bquote('x axis'~(ring(A)^2)), y = "y axis") 
       theme_bw()
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