I was wondering how to get my superscript/subscript variables to look more clean, as I have seen in publications. For example, I would like letters within a superscript closer together, to limit visual clutter on certain variables. A visual example of this is below:
Here is what my variables look like:

Versus what I have seen in a publication:

In the publication, the PH in the superscript is closer together. Any idea how to clean up these kind of variables that have a superscript and subscript index?
Here is my code (I use R Markdown to create my pdfs):
\frac{\rho{_N^{CN}}}{n{_N^{CN}}}

