Nonthermal magnetization pathways in photoexcited semiconductors

Dear Editor,

Thank you for your careful assessment of my manuscript and for the opportunity to clarify its scope with respect to the expectations of SciPost Physics.

The previously-identified and long-standing research stumbling block addressed by this work concerns the existence of a dynamical pathway leading to the emergence of transient magnetic order in nominally non-magnetic semiconductors following ultrafast laser irradiation. To this aim, I employed a simple yet physically transparent model to show that such a pathway does exist and to identify the fundamental microscopic mechanisms that underlie it. Additional context is provided by the discussion presented in the present work as well as in Refs.[1-3].

The present findings are relevant to the progress and interpretation of first-principles calculations in the context of ultrafast magnetization dynamics: over the past decade, first-principles real-time TDDFT studies have consistently demonstrated that spin–orbit coupling enables ultrafast spin dynamics following laser excitation, even in non-magnetic materials. However, a fundamental unresolved issue has remained: why and under which conditions such laser-triggered spin dynamics can (or cannot) evolve into a genuine broken-symmetry magnetic state, as opposed to remaining a coherent, reversible excitation. In particular, the role of energy relaxation and dissipation, unavoidable in real materials but typically absent in TDDFT, has remained conceptually unclear.

The breakthrough presented in this work is the identification and explicit demonstration of a minimal and general dynamical mechanism showing that:

1) Non-dissipative (TDDFT-like) dynamics are intrinsically unable to reach transient magnetic broken-symmetry states, even when strong spin dynamics is initiated by laser excitation.

2) The inclusion of dissipation is not a quantitative correction but a qualitative requirement to enable relaxation toward a constrained low-energy magnetic state.

3) A minimal spin–orbital model suffices to demonstrate this principle in a controlled and transparent way, thereby isolating a universal mechanism that is not material-specific.

4) This microscopic insight naturally connects to a time-dependent GL description of the order parameter, providing a bridge between ultrafast quantum dynamics and mesoscopic symmetry breaking.

To my knowledge, this is the first work that clearly disentangles the initiation of laser-induced spin dynamics from the subsequent formation of transient magnetic order, and that explains on fundamental grounds why real-time TDDFT simulations generically fail to access photoinduced magnetic phase transitions in the absence of explicit relaxation channels. As such, the conclusions presented here provide valuable guidance for the future development of first-principles methodologies aimed at describing light-induced broken-symmetry states.

In the light of this, I believe the manuscript meets the flagship criteria for SciPost Physics because it:

1) Addresses a foundational and timely problem involving ultrafast magnetism and light-induced phase transitions. 2) Provides a conceptual advance rather than a case-specific result. 3) Clarifies the strengths and limitations of a widely used first-principles methodology (real-time TDDFT) in the context of ultrafast magnetism, with implications for materials physics and ultrafast science.

I hope this clarification addresses the Editor’s concerns and helps convey why the manuscript is intended as a contribution to SciPost Physics.

With kind regards,

Dr. Giovanni Marini

[1]: Marini, G. and Calandra, M. Phys. Rev. B 105, L220406 (2022) [2]: Neufeld O et al. npj Computational Materials volume 9, Article number: 39 (2023) [3]: Neufeld, O Phys. Rev. Lett. 135, 206902 (2025)

Following comments by Referee 1 I made the following changes:

1) I modified both the abstract and introduction to more clearly emphasize: (i) the conceptual novelty of initiating magnetization solely through an impulsive kick of the electronic orbital angular momentum and (ii) the importance of relaxation in governing the long-time decay of the induced magnetic state. These points are now stated explicitly and concisely. I expanded the relevant abstract sentence, that now reads: “In this work, I introduce a minimal real-time spin-orbital model and simulate the dynamics after an orbital angular momentum kick, identifying the fundamental microscopic mechanisms that enable the emergence of a transient magnetic order and studying the role of dissipation.” I further added the following phrase in the Introduction: “The effect of a laser pulse on the angular momentum dynamics is schematically included through an angular momentum kick, mimicking the direct coupling to the orbital degrees of freedom of electrons in real materials.” 2) I moved the equations of motion from the Appendix directly into the main text also for the Ginzburg-Landau part, while keeping more technical details in the Appendix to improve readability. 3) I have expanded the discussion of how the kick amplitude, direction, and polarization affect the resulting magnetization dynamics at the end of Appendix B. I also added a new figure, Fig.7, where I explicitly study the role of pulse intensity. 4) I added a new subsection to the manscript (now 2.3), named: ”Significance and limitations of the present model”, where I discuss both the limitation of this model when compared to a more realistic study which includes microscopic interactions. 5) I modified the discussion in the Ginzburg-Landau model Sec. 2.4 (old Sec 2.3) in order to clarify the illustrative the scope of Fig 3. 6) Added a short discussion commenting on the long-time oscillations to Appendix A, when commenting on Figures 4 and 5. 7) I expanded the conclusion to include suggestions to identify desirable platforms for photoinduced magnetism, which could stimulate further work on the topic.

Following Referee 2's comments I made the following changes:

1) I removed the following wrong unfinished sentence from the manuscript: "In this situation sever" 2) I corrected an error in Eq. 4, which is discussed in the response to Referee 2.

List of additional changes:

1) corrected various typos, added the definition of E_int that was erroneously missing from the previous version. 2) Updated one arxiv reference to an arxiv article, now published on prl (https://journals.aps.org/prl/abstract/10.1103/75tm-3t9b)