A candidate quantum detergent for spintronics

The understanding of the interplay between magnetism and topological properties in a quantum material is one of the most challenging research directions in materials science, holding potentialities for future spintronics applications, where the spin carried by an electron could be manipulated and used as information carrier in a device and/or as a single quantum bit of information. 
Examples of systems providing an important platform to innovative design of topological spintronics devices are Magnetic Topological Insulators, where exotic new quantum phases induced by magnetic order have been observed and postulated, such as the quantum anomalous Hall effect, and the topological magneto-electric effect. In particular in this latter case, the mutual control of magnetic/electric polarization led to the theoretical prediction of a new quantum phenotype – the axion insulator – which is the condensed matter analogue of the axion elementary particle postulated in high energy physics by quantum chromo-dynamics, with exotic properties related to the magneto-electric effect, and with the role of a ‘quantum detergent’ (the name ‘axion’ was given by F. Wilczek inspired by a very popular detergent in the US (see title figure)).
In this work, by combining chemical and magnetic-state sensitive electron and X-ray spectroscopies at APE LE and HE beamlines at Elettra as well as I09 beamline at Diamond, and the first-principle calculations we reveal that EuSn₂P₂ holds the characteristics of an axion insulator as it displays topological electronic properties driven by its layer-dependent ferromagnetic character. 
Our XMCD data in Figure 1A demonstrate the presence of in-plane long-range ferromagnetic order. Spin-ARPES results shown in Fig. 1 B and C provide clear spectroscopic evidence of the topological character of the observed electronic states. Spin-ARPES energy distribution curves at two opposite momentum values display clear spin inversion, signature of the spin-momentum locking of the related electronic bands. The same spin inversion is observed both above and below T_Néel. These results indicate that the observed spin polarization has a nonmagnetic origin and that the time reversal symmetry is preserved as expected for a topological surface. The topological character is strong and not influenced by in-plane layered ferromagnetism, as opposed to what has been observed in MnBi₂Te₄ for the out-of-plane direction of the magnetization (see e.g., J. Li et al., Intrinsic magnetic topological insulators in van der Waals layered MnBi₂Te₄-family materials. Sci. Adv. 5, eaaw5685, 2019).
Our data therefore provide a clear evidence that layer-dependent spin-momentum locking coexists with ferromagnetism in this material, a cohabitation that promotes EuSn₂P₂ as a prime candidate for axion insulator for topological antiferromagnetic spintronics applications.


 

Figure 1.  (A) top panel: the total electron yield absorption spectrum of EuSn₂P₂ at the M_4,5 edges measured at T = 15 K, i.e. below Néel temperature, summing Circular Right (CR) and Circular Left (CL) polarized spectra; bottom panel: normalized XMCD demonstrating layer ferromagnetism; (B) Isoenergetic ARPES color maps at Fermi level and at -0.4 eV; the two vertical dashed lines identify the two points +K and -K along the k_x axis where the spin resolved spectra in (C) were taken; (C) Spin-resolved spectra (top panel) measured at 15 K (i.e., below the magnetic transition) in the two opposite momenta -K and +K. The quantization axis of the spin is parallel to k_y direction.  In the lower panels, the resulting spin polarization showing the inversion between +K and -K, typical for topological states.
 

This research was conducted by the following research team:

Gian Marco Pierantozzi¹,  Alessandro De Vita^1,2, Chiara Bigi¹, Xin Gui³, Hung-Ju Tien⁴, Debashis Mondal^1,5, Federico Mazzola¹, Jun Fujii¹, Ivana Vobornik¹, Giovanni Vinai¹, Alessandro Sala¹, Cristina Africh¹, Tien-Lin Lee⁶,  Giorgio Rossi^1,2,  Tay-Rong Chang^4,7,8,  Weiwei Xie⁹, Robert J. Cava³, and Giancarlo Panaccione¹

¹ Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, Trieste, Italy
² Dipartimento di Fisica, University of Milano, Italy
³ Department of Chemistry, Princeton University, US
⁴ Department of Physics, National Cheng Kung University, Tainan, Taiwan
⁵ International Centre for Theoretical Physics, Trieste, Italy
⁶ Diamond Light Source, Harwell Science and Innovation Campus, Didcot, UK
⁷ Center for Quantum Frontiers of Research and Technology, Tainan, Taiwan;
⁸ Physics Division, National Center for Theoretical Sciences, National Taiwan University, Taipei Taiwan;
⁹ Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, US

Contact persons:

Ivana Vobornik, email: