[Paper Review] Induced Monolayer Altermagnetism in MnP(S,Se)$_3$ and FeSe

I. I. Mazin, Rafael González‐Hernández+1 more|arXiv (Cornell University)|Sep 5, 2023

Iron-based superconductors research18 citations

TL;DR

Demonstrates how monolayer antiferromagnets MnP(S,Se)3 and FeSe can be functionalized into strong altermagnets by breaking PT symmetry with out-of-plane electric fields, Janusization, or substrates, and shows accompanying magneto-optical and topological features.

ABSTRACT

Altermagnets (AM) are a recently discovered third class of collinear magnets, distinctly different from conventional ferromagnets (FM) and antiferromagnets (AF). AM have been actively researched in the last few years, but two aspects so far remain unaddressed: (1) Are there realistic 2D single-layer altermagnets? And (2) is it possible to functionalize a conventional AF into AM by external stimuli? In this paper we address both issues by demonstrating how a well-known 2D AF, MnP(S,Se)$_3$ can be functionalized into strong AM by applying out-of-plane electric field. Of particular interest is that the induced altermagnetism is of a higher even-parity wave symmetry than expected in 3D AM with similar crystal symmetries. We confirm our finding by first-principles calculations of the electronic structure and magnetooptical response. We also propose that recent observations of the time-reversal symmetry breaking in the famous Fe-based superconducting chalchogenides, either in monolayer form or in the surface layer, may be related not to an FM, as previously assumed, but to the induced 2D AM order. Finally, we show that monolayer FeSe can simultaneously exhibit unconventional altermagnetic time-reversal symmetry breaking and quantized spin Hall conductivity indicating possibility to research an intriquing interplay of 2D altermagnetism with topological and superconducting states within a common crystal-potential environment.

Motivation & Objective

• Explore whether realistic two-dimensional altermagnets can be created from 2D antiferromagnets by external symmetry breaking.
• Demonstrate functionalization routes (electric field, Janusization, substrate effects) that induce altermagnetic order in MnP(S,Se)3 and FeSe monolayers.
• Characterize the resulting spin-splitting symmetry and its magneto-optical responses.
• Assess potential connections to TRS breaking observations in Fe-based superconductors and topological aspects in FeSe.
• Provide first-principles support for the induced altermagnetic states and their observable consequences.

Proposed method

• First-principles density functional theory (DFT) calculations with VASP and comparison with WIEN2k.
• Use GGA-PBE for exchange-correlation with DFT+U to treat Mn (U_eff ≈ 4 eV) and modest U for FeSe to emulate experimental Fermiology.
• Spin-orbit coupling included to assess band splitting and magneto-optical responses.
• Wannier90 to construct a tight-binding representation for evaluating spin Hall conductivity.
• Symmetry analysis to identify altermagnetic spin channels and possible d- or i-wave waveforms in 2D.
• Compute magneto-optical Kerr effect (MOKE) and spin Hall conductivity to connect altermagnetic order with observable signals.

Experimental results

Research questions

• RQ1 Can realistic two-dimensional altermagnets be realized from monolayer antiferromagnets by breaking PT symmetry while preserving altermagnetic mirrors?
• RQ2 What practical routes (electric field gating, chemical Janusization, substrate interfacing) induce altermagnetic order in MnP(S,Se)3 and FeSe monolayers?
• RQ3 What are the symmetry character and angular dependence of altermagnetic spin splitting in these 2D systems?
• RQ4 Do induced altermagnets exhibit measurable magneto-optical responses and topological transport such as quantized spin Hall conductivity?
• RQ5 Can observed TRS-breaking signals in FeSe-based surfaces be explained by induced altermagnetism rather than ferromagnetism?

Key findings

• Electric-field gating can induce strong altermagnetic spin splitting in MnPSe3, up to ~25 meV, by breaking PT symmetry while preserving mirror symmetries.
• Janusization (top/bottom ligand inequivalence) yields altermagnetic band structures with sizable Kerr rotation, with maximum Kerr angles reaching about 0.4 degrees.
• Monolayer FeSe can host altermagnetic spin splitting of dx2−y2 type, and SOC opens a topologically nontrivial gap at the M point, yielding a quantized spin Hall conductivity plateau.
• FeSe monolayers on SrTiO3 substrates, or surface effects, can explain time-reversal symmetry breaking signals observed in EELS and MOKE experiments without invoking ferromagnetism.
• 2D altermagnetism in FeSe is compatible with a quantum spin Hall scenario, suggesting interplay between altermagnetism, topology, and superconductivity in a common crystal-potential environment.
• The work connects induced altermagnetism to observed TRS-breaking phenomena in Fe-based superconductors and proposes practical platforms for exploring altermagnetic topology in 2D.