Authorship disagreement
You should openly discuss your concern with the co-authors. As a matter of fact, all co-authors have to agree on the author order before submission. Nature Physics requires a statement of authors's contributions and this the ground you can use to initiate the conversation. According to what you wrote, you deserve to be the first author and the postdoc should be the corresponding author, which is often regarded as equally important as the first author. Your PI should be the one who makes the final decision. Also, he does not neccessarily have to be the last author, if he did not make substantial contributions to the project. For your situation, the reasonable author order that would make everyone happy is: you, everyone else, the postdoc. There might be political game involved. In that case, be prepared to fight for your rights.