Response to a twist in systems with Z_p symmetry: The two-dimensional p-state clock model

We study response to a twist in the two-dimensional p-state clock model, which has the discrete Z_p symmetry. The response is measured in terms of helicity modulus, which is usually defined with respect to an infinitesimal twist. However, we demonstrate that such a definition is inappropriate for the clock model. The helicity modulus must be defined with respect to a finite, quantized twist which matches the discrete Z_p symmetry of the model. Numerical study of the appropriately defined helicity modulus resolves controversy over the clock model, showing the existence of two Berezinskii-Kosterlitz-Thouless transitions for p>4.