Regular Expression (sed editor)

Hello !

I have following file, called for example "a.txt"

f_a
m_b

Now I want to replace each first character at a line followed by "_", by the same character followed by "@",
in other words I wish to receive following file.

f@a
m@b

It's important for me to execute the upper task using sed editor, and I do it in the following way:

>> sed s/"^(.)_"/"\0@"/ a.txt > b.txt

But for any reason this command doesn't work.

Can you please explain what is a problem.

Txs,
Alex