[Paper Review] Macroscopic time reversal symmetry breaking by staggered spin-momentum interaction

Helena Reichlová, Rafael Lopes Seeger+18 more|arXiv (Cornell University)|Dec 31, 2020

Laser-Matter Interactions and Applications21 citations

TL;DR

This paper demonstrates macroscopic time-reversal (T) symmetry breaking in a collinear antiferromagnet, Mn₅Si₃, driven by a staggered spin-momentum interaction that induces two oppositely spin-split valleys in momentum space. First-principles calculations and experimental measurements of the spontaneous Hall effect confirm the emergence of a T-symmetry-broken state without net magnetization, enabling topological quantum phases in non-relativistic, low-Z materials.

ABSTRACT

Time-reversal (T) symmetry breaking is a fundamental physics concept underpinning a broad science and technology area, including topological magnets, axion physics, dissipationless Hall currents, or spintronic memories. A best known conventional model of macroscopic T-symmetry breaking is a ferromagnetic order of itinerant Bloch electrons with an isotropic spin interaction in momentum space. Anisotropic electron interactions, on the other hand, have been a domain of correlated quantum phases, such as the T-invariant nematics or unconventional superconductors. Here we report discovery of a broken-T phase of itinerant Bloch electrons with an unconventional anisotropic spin-momentum interaction, whose staggered nature leads to the formation of two ferromagnetic-like valleys in the momentum space with opposite spin splittings. We describe qualitatively the effect by deriving a non-relativistic single-particle Hamiltonian model. Next, we identify the unconventional staggered spin-momentum interaction by first-principles electronic structure calculations in a four-sublattice antiferromagnet Mn5Si3 with a collinear checkerboard magnetic order. We show that the staggered spin-momentum interaction is set by nonrelativistic spin-symmetries which were previously omitted in relativistic physics classifications of spin interactions and topological quasiparticles. Our measurements of a spontaneous Hall effect in epilayers of antiferromagnetic Mn5Si3 with vanishing magnetization are consistent with our theory predictions. Bloch electrons with the unconventional staggered spin interaction, compatible with abundant low atomic-number materials, strong spin-coherence, and collinear antiferromagnetic order open unparalleled possibilities for realizing T-symmetry broken spin and topological quantum phases.

Motivation & Objective

• To identify a novel mechanism for macroscopic time-reversal symmetry breaking in itinerant electron systems beyond conventional ferromagnetism.
• To explore the role of anisotropic, staggered spin-momentum interactions in generating T-symmetry-broken electronic states in non-relativistic materials.
• To demonstrate that such symmetry breaking can occur in collinear antiferromagnets with zero net magnetization, enabling new pathways for topological quantum materials.
• To establish a link between unconventional spin-momentum coupling and measurable macroscopic Hall effects in epitaxial Mn₅Si₃ thin films.
• To show that this mechanism is compatible with abundant low-atomic-number materials and strong spin coherence, offering a scalable route to topological spintronics.

Proposed method

• First-principles electronic structure calculations using density functional theory (DFT) to identify the staggered spin-momentum interaction in Mn₅Si₃ with a four-sublattice collinear checkerboard antiferromagnetic order.
• Derivation of a non-relativistic single-particle Hamiltonian model to qualitatively describe the unconventional spin-momentum coupling and its valley-locked spin splitting.
• Epitaxial growth of Mn₅Si₃ thin films on Si substrates via molecular beam epitaxy, followed by structural characterization using XRD at room and low temperatures.
• Magnetotransport measurements via Hall bar devices fabricated by optical lithography and argon plasma etching, with simultaneous acquisition of longitudinal and transverse resistivity.
• Separation of symmetric (ordinary Hall effect) and antisymmetric (anomalous and topological Hall effect) components of the transverse resistivity using cosh fitting to extract the anomalous Hall contribution.
• Low-temperature XRD and SQUID magnetometry to confirm structural and magnetic properties, including diamagnetic substrate contributions and field-dependent magnetization.

Experimental results

Research questions

• RQ1 Can time-reversal symmetry be broken macroscopically in a non-magnetic, collinear antiferromagnet via a staggered spin-momentum interaction?
• RQ2 What is the role of nonrelativistic spin-symmetries in enabling unconventional spin-momentum coupling not captured by standard relativistic classifications?
• RQ3 Can a spontaneous Hall effect emerge in a material with zero net magnetization due to such a staggered interaction?
• RQ4 How does the momentum-space valley structure with opposite spin splittings influence the macroscopic transport response?
• RQ5 To what extent can this mechanism be realized in abundant, low-atomic-number materials with strong spin coherence?

Key findings

• A staggered spin-momentum interaction in Mn₅Si₃ leads to two ferromagnetic-like valleys in momentum space with opposite spin splittings, breaking time-reversal symmetry macroscopically.
• First-principles DFT calculations confirm the presence of this unconventional interaction, which arises from nonrelativistic spin-symmetries previously omitted in topological classification schemes.
• The spontaneous Hall effect measured in Mn₅Si₃ epilayers with vanishing net magnetization reaches 5–20 S/cm, correlating with crystal quality and confirming theoretical predictions.
• The anomalous Hall resistivity amplitude extracted via cosh fitting shows a robust signal, with additional bump-like features indicating a topological Hall contribution.
• Low-temperature XRD confirms the dominant growth of the Mn₅Si₃ hexagonal phase along the c-axis, supporting structural integrity of the epitaxial films.
• SQUID magnetometry confirms the absence of net magnetization, with diamagnetic substrate contributions being negligible in small fields, validating the antiferromagnetic ground state.