I have very strong desire to use superscript as the index of the variable.

However, it looks like that the Mathematica automatically recognize the superscript as the power and I got message that my variable with superscript is 'protected'.

Could you help me to make the superscript used as the index of the variable instead of power?

UPDATE (16-June-2015):

This question is being reopened and a bounty is being awarded on this. Previous answers are very good, however the bounty is to be awarded on an answer which solve this specific problem: 


  How to change the behaviour of Ctrl+^ keybinding so that it produces Superscript instead of Power.



Question: How to update non-property value with entity_metadata_wrapper?
Answer: The payload column is a property of the commerce payment transaction type already (property is a Drupal core term for any base fields on an entity), but for whatever reason the authors of the module have chosen not to expose it to the Entity API module through commerce_payment_entity_property_info().

Fortunately you can just use hook_entity_property_info_alter() in a custom module to provide the metadata on its behalf. See hook_entity_property_info() for details of the options available.