Preprint: Activation of anomalous Hall effect and orbital magnetization in altermagnets
Preprint: Activation of anomalous Hall effect and orbital magnetization in altermagnets
Sopheak Sorn, Yuriy Mokrousov
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In altermagnets, the anomalous Hall effect depends crucially on the orientation of the Neel vector. In the so-called pure altermagnets, the easy-axis direction yields a high-symmetry condition of the ordered state, which forbids the anomalous Hall effect. We show that in this case the domain walls lift the symmetry constraint and activate the anomalous Hall effect, which can then be viewed as originating solely from the domain walls. We dub this the domain wall anomalous Hall effect. We also find that it is closer related to orbital magnetization than the spin magnetization. We discuss which symmetry group of altermagnets supports such domain wall Hall effect and which experimental setting is conducive to observe such Hall effect. Our work demonstrates the important role of the domain walls in the orbital magnetism physics and the Hall transport in altermagnets.
Profile of a Bloch domain wall of the Neel vector (double-sided arrows), which lifts the symmetry constraint and activate the anomalous Hall effect.
Preprint: Topological dipole of quantum skyrmions
Preprint: Topological dipole of quantum skyrmions
Sopheak Sorn, Joerg Schmalian, Markus Garst
Skyrmions are spatially localized swirl of spin moments, featuring a topological charge and a well-defined topological charge density in real space (see Fig.) The interplay between translational invariance and the topology leads to the conservation of the topological charge dipole moment. Three immediate consequences are resulted:
(1) We identify skyrmions as fractons--exotic quasiparticles subject to similar conservation of dipole and, in some cases, also multipole moments. As a result, similar to fractons, skyrmions exhibit restricted mobility and peculiar many-body behaviors. Our work enables the use of skyrmions to explore fractonic phenomena experimentally, which is so far inhibited by the scarcity of realistic systems hosting fractons.
(2) The topological dipole moment leads to a natural center-of-mass definition of the collective coordinate of skyrmions to specify their positions. Due to the conservation law, the dynamics of this collective coordinate is greatly simplified, i.e. its time derivative equals zero. This rules out the Newtonian term involving mass and acceleration in the force equation, amounting to a dynamical description with a zero skyrmion mass. Skyrmion mass has been controversial, argued to be non-zero and sometime appearing non-universal. Our work exploits the fundamental conservation law to arrive at the universally zero value. The main point is that one has to utilize the center-of-mass definition.Â
(3) The zero mass and the conserved topological dipole are intimately related to the remarkable feature of the topological charge density being the generator of an area-preserving diffeomorphism. We show explicitly that the topological charge density obeys the Girvin-MacDonald-Platzman algebra, which governs the intra-Landau charge-neutral excitations in quatum Hall problems. This strongly suggests the rise of exotic quantum liquids of skyrmion, akin to the fractional quantum Hall states.
Antichiral surface states and SSH physics in rutile altermagnets
Antichiral surface states and SSH physics in rutile altermagnets
Sopheak Sorn
An emerging class of unconventional collinear antiferromagnets, called altermagnets, feature many intriguing properties including by not limited to (i) momentum-dependent d-wave, g-wave and i-wave spin splitting in the band structure, (ii) large response functions such as nonrelativistic spin-Hall-like effects useful for spintronic applications, (iii) effective multipolar order parameters and the associated nonlinear response signatures in Hall effects and magnetoelectric effects. The rather unconventional altermagnetic symmetry is the main player responsible for all these. In this work, we consider electronic spectral properties when the symmetry is altered in the presence of interfaces with vacuum and domain wall textures. Using a tight-binding model on a rutile crystal within a slab geometry, we find that the altermagnets can support exotic anti-chiral surface states: surface states from opposite surfaces propagate in the same direction. This is in contrast with the more conventional chiral surface states, such as the chiral edge modes in Chern insulators and the Fermi arc states in Weyl semimetals, which propagates in an antiparallel fashion between the opposite surfaces. The anti-chiral surface states can also be switched to chiral surface states by modifying the surface termination. We explain their topological-like origin using a map from the altermagnet to a family of modified SSH chains. The latter are a type of obstructed atomic insulators which support surface states in accordance with a bulk-boundary correspondence determined by a Zak phase. The surface states in the modified SSH chains and the altermagnet are identifiable with one another. Anti-chiral surface states were first theoretically proposed in a variant of the Haldane honeycomb model for Chern insulators, where the original flux pattern is modified. So far, there are very few condensed-matter platforms that have been proposed to host the anti-chiral surface states. Those include twisted van der Waals multi-layer hetero-structures and in Dirac fermion systems where electron-phonon couplings are important. Here, we show that rutile altermagnets are another potential candidate.
Band structure for slab geometry in the surface Brillouin zone featuring in-gap surface states in red color. These are the anti-chiral surface states. For instance, the red curve between the star and the square is double degenerate, corresponding to two surface states on the two surfaces of the slab. They propagate in the same direction (negative y-axis.)
Editors' Suggestion: Signature of hidden octupolar orders from nonlinear Hall effects
Editors' Suggestion: Signature of hidden octupolar orders from nonlinear Hall effects
Sopheak Sorn, Adarsh S. Patri
Phys. Rev. B 110, 125127 (2024)
Hidden octupolar orders have been proposed in many condensed matter systems including heavy-fermion compounds and altermagnets. They are challenging to detect using conventional probes of magnets. We propose using nonlinear Hall effects, beyond the linear response, to uncover such hidden octupolar orderings. In this work, the signature of the octupolar ordering is the third-order Hall response, which is the leading Hall effect, since the linear and second-order Hall effect are forbidden by symmetry.
Illustration of Hall experiment to detect hidden octupolar ordering
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