Physical Review uses the delimiter $ for any TEX in-line math, e.g.,
the quantity azis obtained from the input
the quantity $a^z$
Another example of in-line math is
... difference between $\langle J_z \rangle
(t)$ and $\langle J_z\rangle_{\rm HF}(t) ...$
... difference between 〈Jz〉(t) and 〈Jz〉HF(t)...
Standard mathematical style conventions are followed for in-line math such as smaller point size for superscripts/subscripts, appropriate use of roman, italic, and Greek fonts, and the use of special math symbols. (See the Style and Notation Guide for help.) TEX will take care of setting the point size appropriately for variables and numbers in the superscript/subscript position. To get the appropriately sized superscript/subscript in the roman font, use the \rm command:
input:
$... J_{\rm HF}(t) ...$
...JHF(t)...TEX will assume that you want the superscript or subscript to consist of the first token (generally a single character or command) following the ˆ or _, unless you use curly brackets to delimit the subscript/superscript. It is safest to use the curly brackets if unsure. The curly brackets here also serve to limit the scope of the command. Again, don't use any vertical motion commands in math; horizontal motion commands are unnecessary as well.