SQL Server 2012 :: Removing Greek Characters From A String

I have a varchar field which contains some Greek characters (α, β, γ, etc...) among the regular Latin characters. I need to replace these characters with a word (alpha, beta, gamma etc...). When I try to do this, I find that it is also replacing some of the Latin characters.

DECLARE @Letters TABLE (Letter NVARCHAR(10))
INSERT INTO @Letters VALUES ('a'), ('A'), ('b'), ('B'), ('α')
SELECTLetter, REPLACE(Letter,'α','alpha')
FROM@Letters

In this case, the "α" is being replaced, but so are "a" and "A".

I have tried changing the datatype from varchar to nvarchar and also changing the collation.