Gaplessness indicator by topologically trivial twisting operators

We propose several general necessary conditions for quantum many-body system in one dimension respecting U(1) symmetry to be gapped.

We show that the ground-state expectation value of topologically trivial twisting operators must approach unity in the thermodynamic limit with a certain finite-size scaling.

Equivalently, its violation can indicate gaplessness of U(1)-symmetric Hamiltonians.

The topological triviality of such a twisting operator enables us to derive infinitely many other gaplessness indicators by static structure factor to any order in real experiments, which are impossible to obtain by earlier topologically nontrivial twisting operators.

We also apply analytic and numerical calculations to test the efficiency and consistency of our results.