Roton energy gap and spontaneous symmetry breaking

We study elementary excitations in superfluid helium-4 employing an approach based on the spontaneous symmetry breaking. In particular, we calculate the roton energy gap at zero temperature $Δ(0)$. The relation that we have derived is $Δ(0) / k_B T_λ= 4$. The theoretical value of $Δ(0)/k_B$ is 8.707 K, which is significantly close to the experimental value of 8.712 K. The deviation between the theoretical and experimental values is less than 0.1%.

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