Indices of Kummer extensions of prime degree

Let p be a prime and let ζp be a primitive p-th root of unity. For a finite extension k of Q containing ζp, we consider a Kummer extension L/k of degree p. In this paper, we show that if k=Q(ζp) and the class number of k is one, the index of L/k is one. We also show that if L/k is tamely ramified with a normal integral basis, the index is at most a power of p. In the last section, we show that there exist infinitely many cubic Kummer extensions of Q(ζ3) for both wildly and tamely ramified cases, whose integer rings do not have a power integral basis over that of Q(ζ3).