Comment on "Spin-orbit coupling induced ultrahigh-harmonic generation from magnetic dynamics" with prescriptions on how to validate scientific software for computational quantum transport

TL;DR

This paper critiques a recent study on high-harmonic generation in spintronics, demonstrating that its results violate fundamental theorems of quantum transport and are likely artifacts of computational errors, emphasizing the need for proper validation of scientific software.

Contribution

It provides a critical analysis of previous computational results, illustrating the importance of validating quantum transport codes and algorithms to ensure accurate scientific conclusions.

Findings

High harmonics in pumped current are real but not ultrahigh as claimed.

The original results violate fundamental theorems of quantum transport.

Proper validation strategies are essential for reliable computational physics.

Abstract

In a recent paper [Phys. Rev. B {\bf 105}, L180415 (2022)], Ly and Manchon used open source code {\tt TKWANT} for time-dependent computational quantum transport to predict surprising features in the mature field of current pumping by magnetization dynamics in spintronics -- in the presence of spin-orbit (SO) coupling, the pumped charge current oscillates at both the frequency $\omega_0$ of magnetization precession and high harmonics $N=\omega/\omega_0$, reaching {\em astonishingly high} cutoff \mbox{$N_\mathrm{max} \simeq 1000$} by increasing the SO coupling. However, results in the paper violate two basic theorems of time-dependent quantum transport: ({\em i}) current response to time-periodic external field {\em must be perfectly periodic} itself in the long time limit for a two-terminal device because its active region is attached to two semi-infinite leads bringing continuous energy spectrum; and ({\em ii}) no DC component of charge current is allowed in the left-right symmetric two-terminal devices, or in asymmetric devices its value cannot be changed by simply increasing the SO coupling. We illustrate these two theorems by using completely different calculations applied to one-dimensional two-terminal devices with either ferromagnetic (for which the device is left-right symmetric) and antiferromagnetic (for the device is left-right asymmetric) active region hosting the Rashba SO coupling. We conclude that harmonics in pumped current in the presence of SO coupling do exist, but their ``ultrahigh'' cutoff is an artifact of either ``bugs'' or inadequate algorithms selected within {\tt TKWANT}. Finally, we suggest strategies for {\em validating} time-dependent quantum transport codes, or selection of algorithms by a user within putatively validated (by developers) code, prior to deploying them to produce research papers.