Research

08.03.2024 New Joint Publication in Altermagnetic Transport

A joint publication with Jairo Sinova, Libor Šmejkal, Anna B. Hellenes, Rodrigo Jaeschke Ubiergo, Warlley H. Campos, Venkata K. Bharadwaj and Atasi Chakraborty about the direct observation of altermagnetic band splitting in CrSb thin films has been published in Nature Communications.

They investigate directly this unconventional band splitting near the Fermi energy through spin-integrated soft X-ray angular resolved photoemission spectroscopy. The experimentally obtained angle-dependent photoemission intensity, acquired from epitaxial thin films of the predicted altermagnet CrSb, demonstrates robust agreement with the corresponding band structure calculations. In particular, they observe the distinctive splitting of an electronic band on a low-symmetry path in the Brilliouin zone that connects two points featuring symmetry-induced degeneracy. The measured large magnitude of the spin splitting of approximately 0.6 eV and the position of the band just below the Fermi energy underscores the significance of altermagnets for spintronics based on robust broken time reversal symmetry responses arising from exchange energy scales, akin to ferromagnets, while remaining insensitive to external magnetic fields and possessing THz dynamics, akin to antiferromagnets.

You can find the publication under Nat Commun 15, 2116 (2024).

26.02.2024 New Joint Publication in Antiferromagnetic Spintronics

A joint publication with Jairo Sinova, Ricardo Zarzuela and V.K. Bharadwaj about homochiral antiferromagnetic merons, antimerons and bimerons in synthetic antiferromagnets has been published in Nature Communications.

They realize chiral in-plane topological antiferromagnetic spin textures namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. They demonstrate multimodal vector imaging of the three-dimensional Néel order parameter, revealing the topology of those spin textures and a globally well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Their analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role to significantly reduce the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as antiferromagnetic merons, in synthetic antiferromagnets, making them a promising platform for next-generation spintronics applications.

You can find the publication under Nat Commun 15, 1641 (2024).

14.02.2024 New Joint Publication in Altermagnetism

A joint publication with Jairo Sinova and Libor Šmejkal about the altermagnetic lifting of the Kramers spin degeneracy has been published in Nature.

They write that the lifted Kramers spin degeneracy (LKSD) underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology to topological quantum matter. They continue, that LKSD has been considered to originate from two possible internal symmetry-breaking mechanisms. The first refers to time-reversal symmetry breaking by magnetization of ferromagnets and tends to be strong because of the non-relativistic exchange origin. The second applies to crystals with broken inversion symmetry and tends to be comparatively weaker, as it originates from the relativistic spin–orbit coupling (SOC). A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic, that allows for LKSD without net magnetization and inversion-symmetry breaking. Here they provide the confirmation using photoemission spectroscopy and ab initio calculations. They identify two distinct unconventional mechanisms of LKSD generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization. Their observation of the altermagnetic LKSD can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors, that have been either identified recently or perceived for many decades as conventional antiferromagnets.

You can find the publication under Nature 626, 517–522 (2024).

31.01.2024 New Joint Publication in Altermagnetism

A joint publication with Jairo Sinova, Libor Šmejkal and Anna Hellenes about the observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO2 has been published in Science Advances.

They directly observe strong time-reversal symmetry breaking in the band structure of altermagnetic RuO2 by detecting magnetic circular dichroism in angle-resolved photoemission spectra. Their experimental results, supported by ab initio calculations, establish the microscopic electronic structure basis for a family of interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets.

You can find the publication under Sci. Adv. 10, eadj4883 (2024).

29.01.2024 New Joint Publication in Altermagnetic Transport

A joint publication with Jairo Sinova and Libor Šmejkal about the crystal thermal transport in altermagnetic
RuO2 has been published in Physical Review Lettters.

They demonstrate the emergence of a pronounced thermal transport in the recently discovered class of magnetic materials—altermagnets. From symmetry arguments and first-principles calculations performed for the showcase altermagnet, RuO2, they uncover that crystal Nernst and crystal thermal Hall effects in this material are very large and strongly anisotropic with respect to the Néel vector. They find the large crystal thermal transport to originate from three sources of Berry’s curvature in momentum space: the Weyl fermions due to crossings between well-separated bands, the strong spin-flip pseudonodal surfaces, and the weak spin-flip ladder transitions, defined by transitions among very weakly spin-split states of similar dispersion crossing the Fermi surface. Moreover, they reveal that the anomalous thermal and electrical transport coefficients in RuO2 are linked by an extended Wiedemann-Franz law in a temperature range much wider than expected for conventional magnets. Their results suggest that altermagnets may assume a leading role in realizing concepts in spin caloritronics not achievable with ferromagnets or antiferromagnets.

You can find the publication under Phys. Rev. Lett. 132, 056701 (2024).

27.01.2024 New Joint Publication in Antiferromagnetic Spintronics

A joint publication with Jairo Sinova, Libor Šmejkal and Helen Gomonay about phase transitions associated with magnetic-field induced topological orbital momenta in a non-collinear antiferromagnet has been published in Nature communications.

They show that in polycrystalline Mn3Zn0.5Ge0.5N with non-collinear antiferromagnetic order, changes in the diagonal and, rather unexpected, off-diagonal components of the resistivity tensor occur at low temperatures indicating subtle transitions between magnetic phases of different symmetry. This is supported by neutron scattering and explained within a phenomenological model which suggests that the phase transitions in magnetic field are associated with field induced topological orbital momenta. The fact that they observe transitions between spin phases in a polycrystal, where effects of crystalline anisotropy are cancelled suggests that they are only controlled by exchange interactions. The observation of an off-diagonal resistivity extends the possibilities for realising antiferromagnetic spintronics with polycrystalline materials.

You can find the publication under Nat. Comm. 15, 822 (2024).

18.01.2024 New Publication in Altermagnetism

A joint publication with Libor Šmejkal about the broken Kramers degeneracy in altermagnetic MnTe has been published in Physical Review Letters.

By combining high-quality film growth and in situ angle-resolved photoemission spectroscopy, they report the electronic structure of an altermagnetic candidate, αMnTe. Temperature-dependent study reveals the lifting of Kramers degeneracy accompanied by a magnetic phase transition at TN=267Kwith spin splitting of up to 370 meV, providing direct spectroscopic evidence for altermagnetism in MnTe.

You can find the publication under Phys. Rev. Lett. 132, 036702 (2024).

20.12.2023 New Joint Publication in Altermagnetic Magnonics

A joint publication with Jairo Sinova and Libor Šmejkal about crystal magnons in altermagnets has been published in Physical Review Letters.

They theoretically demonstrate a new class of magnons on a prototypical d-wave altermagnet
RuO2 with the compensated antiparallel magnetic order in the ground state. Based on density-functional-theory calculations they observe that the THz-range magnon bands in RuO2 have an alternating chirality splitting, similar to the alternating spin splitting of the electronic bands, and a linear magnon dispersion near the zero wave vector. They also show that, overall, the Landau damping of this metallic altermagnet is suppressed due to the spin-split electronic structure, as compared to an artificial antiferromagnetic phase of the same RuO2 crystal with spin-degenerate electronic bands and chirality-degenerate magnon bands.

You can find the publication under Phys. Rev. Lett. 131, 256703 (2023).

08.12.2023 New Publication in Antiferromagnetic Transport Theory

A joint publication with Libor Šmejkal and Rafeal González-Hernández about the Anomalous Hall Effect in in doped AgCrSe2 has been published in Advanced Science.

They report the observation of a spontaneous AHE in doped AgCrSe2, a layered polar semiconductor with an antiferromagnetic coupling between Cr spins in adjacent layers. The anomalous Hall resistivity is comparable to the largest observed in compensated magnetic systems to date, and is rapidly switched off when the angle of an applied magnetic field is rotated to ≈80° from the crystalline c-axis. The ionic gating experiments show that the anomalous Hall conductivity magnitude can be enhanced by modulating the p-type carrier density. They also present theoretical results that suggest the AHE is driven by Berry curvature due to noncollinear antiferromagnetic correlations among Cr spins, which are consistent with the previously suggested magnetic ordering in AgCrSe2. Their results open the possibility to study the interplay of magnetic and ferroelectric-like responses in this fascinating class of materials.

You can find the publication under Adv. Sci. 2023, 2307306 (2023).

04.12.2023 New Joint Publication in Topological Altermagnetism

A joint publication with Jairo Sinova and Libor Šmejkal about the influence of magnetism, strain, and pressure on the band topology of EuCd2As2 has been published in Physical Review B.

They analyze the role of the delicate interplay of Eu magnetism, strain, and pressure on the realization of nontrivial topological phases. For that they invoke a combination of a group theoretical analysis with ab initio density functional theory calculations and uncover a rich phase diagram with various nontrivial topological phases beyond a Weyl semimetallic state, such as axion and topological crystalline insulating phases, and discuss their realization.

You can find the publication under Phys. Rev. B 108, 235113 (2023).