Recent Publications

A publication with Jairo Sinova, Libor Šmejkal, Atasi Chakraborty and Rafael González Hernández about strain-induced phase transitions from antiferromagnets to altermagnets has been published in Physical Review B.

A key challenge for future applications and functionalization of altermagnets is to demonstrate controlled transitioning to the altermagnetic phase from other conventional phases in a single material. Here they prove a viable path toward overcoming this challenge through a strain-induced transition from an antiferromagnetic to an altermagnetic phase in ReO2. Combining spin group symmetry analysis and ab initio calculations, they demonstrate that under compressive strain ReO2 undergoes such transition, lifting the Kramers degeneracy of the band structure of the antiferromagnetic phase in the nonrelativistic regime. In addition, they show that this magnetic transition is accompanied by a metal-insulator transition, and calculate the distinct spin-polarized spectral functions of the two phases, which can be detected in angle-resolved photoemission spectroscopy experiments.

A publication with Libor Šmejkal about x-ray magnetic circular dichroism in altermagnetic -MnTe has been published in Physical Review Letters.

In this work they use symmetry, ab initio theory, and experiments to explore x-ray magnetic circular dichroism (XMCD) in the altermagnetic class. As a representative material for our XMCD study they choose -MnTe with compensated antiparallel magnetic order in which an anomalous Hall effect has been already demonstrated. They predict and experimentally confirm a characteristic XMCD line shape for compensated moments lying in a plane perpendicular to the light propagation vector. Their results highlight the distinct phenomenology in altermagnets of this time-reversal symmetry breaking response, and its potential utility for element-specific spectroscopy and microscopy.

A publication with Olena Gomonay about current-controlled chirality dynamics in a mesoscopic magnetic domain wall has been published in Physical Review B.

Chirality as internal degree of freedom of a mesoscopic domain wall inside a quasi-one-dimensional fixture can be controlled by spin-polarized current for ferro- as well as antiferromagnetic domain walls. They show that the current density required for the chirality manipulation can be significantly reduced in the low-temperature regime where the chirality dynamics exhibits quantum effects. In this quantum regime, weak currents can excite Bloch oscillations of the domain wall angular rotation velocity, with the oscillation frequency proportional to the current, modulated by a much higher magnon-range frequency. In addition to that, the Wannier-Stark localization effects enable controlled switching between different chiral states, suppressing inertial effects characteristic for the classical regime. They also show that for recently discovered novel class of magnetic materials — altermagnets — chirality switching can be driven by the usual charge current (not spin polarized).

A publication with Jairo Sinova, Libor Šmejkal, Nayra Alvarez and Venkata K. Bharadwaj about Strain control of band topology and surface states in antiferromagnetic EuCd2As2 has been published in Physical Review B.

They investigate via ab initio density functional theory calculations, the effects of shear strain on the bulk and surface states in two antiferromagnetic EuCd2⁢As2 phases with out-of-plane and in-plane spin configurations. When magnetic moments are along the axis, a 3% longitudinal or diagonal shear strain can tune the Dirac semimetal phase to an axion insulator phase, characterized by the parity-based invariant 4⁢=2. For an in-plane magnetic order, the axion insulator phase remains robust under all shear strains. They further find that for both magnetic orders, the bulk gap increases, and a surface gap opens on the (001) surface up to 16 meV. Because of a nonzero 4⁢ index and gapped states on the (001) surface, hinge modes are expected to happen on the side surface states between those gapped surface states. This result can provide valuable insight into the realization of the long-sought axion states.

A publication with Jairo Sinova, Libor Šmejkal, Anna B. Hellenes, Rodrigo Jaeschke Ubiergo and Venkata K. Bharadwaj about supercell altermagnets has been published in Physical Review B.

They substantially broaden the family of altermagnetic candidates by predicting supercell altermagnets. Their magnetic unit cell is constructed by enlarging the nonmagnetic primitive unit cell, resulting in a nonzero propagation vector for the magnetic structure. This connection of the magnetic configuration to the ordering of sublattices gives an extra degree of freedom to supercell altermagnets, which can allow for the control over the order parameter spatial orientation. They identify realistic candidates ${\mathrm{MnSe}}_2$ with a $d$-wave order, and ${\mathrm{RbCoBr}}_3,{\mathrm{CsCoCr}}_3$, and ${\mathrm{BaMnO}}_3$ with $g$-wave order. They demonstrate the reorientation of the order parameter in ${\mathrm{MnSe}}_2$, which has two different magnetic configurations, whose energy difference is only 5 meV, opening the possibility of controlling the orientation of the altermagnetic order parameter by external perturbations.

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.

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.

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.

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.

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.