First off, thanks for turning me on to these operators -- had no idea you could even do that!
I assume you are referring to searching on a string (text) field, because dates and numeric values already use a range.
I am betting you are simply forgetting to uppercase your AND. Keywords like AND, OR, and NOT always need to be capitalized. I just did a test on a search for an index using the expression:
<= "Y" AND >= "S"
and I got all the documents where that field started with S, T, U, V, W, X, and X. (no "Y" values because any string starting with "Y" that wasn't exactly "Y" wouldn't make it in). So it looks like it works, and you can use "<=" and ">=" in addition to =, <, and >.
Looks like you can even use wildcards in the expressions to create even more powerful searches!
Hope this helps!