Comment on "Time reversal polarization and a Z2 adiabatic spin pump"

TL;DR

This paper critically examines a proposed Z2 spin pump, demonstrating through calculations and examples that the system does not return to its original state after two cycles, thus challenging the original claim of its Z2 topological nature.

Contribution

The authors refute the claim that the spin pump is a Z2 pump by showing the system's behavior contradicts the original proposal and providing detailed analysis and examples.

Findings

The system's first excited state degeneracy is not split as claimed.

Level crossing occurs at the end of the cycle, contradicting previous assumptions.

No system behaves as the original figure suggested, invalidating the Z2 pump claim.

Abstract

In Ref 1[Phy. Rev. B 74, 195312(2006)] Fu and Kane propose a spin pump for onedimensional (1D) insulating Hamiltonians. They claim that this spin pump is a Z2 pump because For an isolated system, a single closed cycle of the pump changes the expectation value of the spin at each end even when spin-orbit interactions violate the conservation of spin. A second cycle, however, returns the system to its original state. A Z2 topological invariant is proposed to characterize the Z2 pump. In this comment we show their discussion on the spin pump is inaccurate. Their reason why the isolated system return to its original state after second cycle is unjustified and several claims contradict to this return of the system are made in Ref 1. Detailed calculations and concrete examples show the degeneracy of the first excited state at t = 0, T; is not split by the electron-electron interaction in the way described in Ref 1 and there is level crossing at t = T. In fact, despite of a detailed search, not a single system behave as described in Fig. 1(d) in Ref 1 has been found. Thus we conclude the isolated system won't return to its original state after two cycles and the spin pump is not a Z2 pump in general.