[Paper Review] Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO$_2$

O. Fedchenko, J. Minář+21 more|arXiv (Cornell University)|Jun 3, 2023

Magnetic properties of thin films14 citations

TL;DR

The study directly visualizes time-reversal symmetry breaking in the band structure of altermagnetic RuO₂ using magnetic circular dichroism in angle-resolved photoemission, supported by ab initio calculations.

ABSTRACT

Altermagnets are an emerging third elementary class of magnets. Unlike ferromagnets, their distinct crystal symmetries inhibit magnetization while, unlike antiferromagnets, they promote strong spin polarization in the band structure. The corresponding unconventional mechanism of timereversal symmetry breaking without magnetization in the electronic spectra has been regarded as a primary signature of altermagnetism, but has not been experimentally visualized to date. We directly observe strong time-reversal symmetry breaking in the band structure of altermagnetic RuO$_2$ by detecting magnetic circular dichroism in angle-resolved photoemission spectra. Our experimental results, supported by ab initio calculations, establish the microscopic electronic-structure basis for a family of novel phenomena and functionalities in fields ranging from topological matter to spintronics, that are based on the unconventional time-reversal symmetry breaking in altermagnets.

Motivation & Objective

• Motivate the search for altermagnets as a third elementary class with strong band-structure spin polarization but no net magnetization.
• Demonstrate time-reversal symmetry breaking in the electronic structure of RuO₂ through magnetic circular dichroism in ARPES.
• Corroborate experimental results with ab initio (DFT+U) calculations within a one-step photoemission framework.
• Establish the microscopic electronic-structure basis for altermagnetic phenomena with potential spintronic and topological implications.

Proposed method

• Use magnetic circular dichroism in angle-resolved photoemission spectroscopy (MCD-ARPES) to visualize TRS breaking in momentum space.
• Employ both soft X-ray and ultraviolet photon excitation to map four-dimensional spectral density in energy-momentum space and to isolate MCD from CDAD.
• Decompose the measured dichroic signal into MCD and CDAD components to attribute observed asymmetries to magnetism.
• Compare experimental MCD signals with first-principles calculations based on DFT+U and one-step photoemission theory (KKR) to validate the altermagnetic band structure.
• Rotate the sample to confirm the magnetic origin of A_MCD by observing sign changes consistent with spin-axis orientation along the c-axis.

Experimental results

Research questions

• RQ1 Can time-reversal symmetry breaking in the altermagnetic band structure be directly visualized in RuO₂?
• RQ2 Does MCD-ARPES reveal strong TRS breaking consistent with altermagnetic theory and exchange-dominated spin splitting?
• RQ3 Do experimental MCD results agree with ab initio predictions for altermagnetic RuO₂ across soft X-ray and UV excitations?
• RQ4 Is the observed dichroism dominated by MCD rather than CDAD, indicating a magnetic origin?

Key findings

• Strong time-reversal symmetry breaking is observed in the RuO₂ band structure via MCD-ARPES, consistent with altermagnetic predictions.
• The measured MCD spectra agree with ab initio calculations based on DFT+U and one-step photoemission theory, indicating exchange-dominated TRS breaking.
• MCD dominates over CDAD in the measured dichroism, supporting a magnetic origin rather than geometric effects.
• Ultraviolet excitation results corroborate the soft X-ray findings, with consistent MCD signals and spin-axis orientation along the c-axis.
• Tomographic mapping of the three-dimensional Brillouin zone shows Fermi-surface features in line with altermagnetic phase predictions, matching experimental data across photon-energy scans (560–660 eV).
• Rotating the sample by 180 degrees reverses the MCD signal as expected for a spin-axis-aligned TRS-breaking state, reinforcing the magnetic origin of A_MCD.