arXiv subjects

Arne Brataas

Publications and source records attributed to Arne Brataas.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear Topological Orbital Responses of Antiferromagnetic Skyrmions

Antiferromagnetic skyrmions evade the skyrmion Hall effect, but compensation suppresses their conventional topological charge Hall signal. We predict a semiclassical nonlinear topological orbital response of an isolated skyrmion in a $\mathcal{PT}$-symmetric hexagonal antiferromagnet without spin-orbit coupling. In the diffusive, weak-emergent-field regime, the spin-dependent emergent Lorentz force reshapes the carrier distribution, generating a local orbital Hall-current correction and a local orbital accumulation, both quadratic in the applied electric field. The current correction requires spin-asymmetric longitudinal scattering, whereas the accumulation survives spin-symmetric scattering. When the spin-diffusion length greatly exceeds the strip width, the current correction persists while the accumulation approaches zero. Both signals are even under electric-field reversal, odd under reversal of the skyrmion topological charge $Q$, and helicity independent within this model. These symmetries enable rectified detection and distinguish this mechanism from the $Q$-even quantum-regime topological orbital Hall effect.

cond-mat.mes-hall↗

arXiv2026

Inherent electro-optic Kerr rotation

We uncover a previously overlooked contribution to the electro-optic Kerr rotation of reflected light, arising from the interplay of matter, the static electric field, and the magnetic component of light. This contribution remains nonzero even in isotropic nonmagnetic homogeneous systems. We derive analytical expressions for the Kerr rotation in both two-dimensional layers and semi-infinite systems. Within the relaxation-time approximation, we predict experimentally accessible signal magnitudes in metals. This inherent mechanism thereby opens opportunities for probing electronic properties in materials through Kerr spectroscopy.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic moment of electrons in systems with spin-orbit coupling

Magnetic effects originating from spin-orbit coupling (SOC) have been attracting major attention. However, SOC contributions to the electron magnetic moment operator are conventionally disregarded. In this work, we analyze relativistic contributions to the latter operator, including those of the SOC-type: in vacuum, for the semiconductor 8 band Kane model, and for an arbitrary system with two spectral branches. In this endeavor, we introduce a notion of relativistic corrections to the operation $\partial/\partial\boldsymbol B$, where $\boldsymbol B$ is an external magnetic field. We highlight the difference between the magnetic moment and $-\partial H/\partial\boldsymbol B$, where $H$ is the system Hamiltonian. We suggest to call this difference the abnormal magnetic moment. We demonstrate that the conventional decomposition of the total magnetic moment into the spin and orbital parts becomes ambiguous when relativistic corrections are taken into account. The latter also jeopardize the "modern theory of orbital magnetization" in its standard formulation. We derive a linear response Kubo formula for the kinetic magnetoelectric effect projected to individual branches of a two branch system. This allows us, in particular, to identify a source of this effect that stems from noncommutation of the position and $\partial/\partial\boldsymbol B$ operators' components. This is an analog of the contribution to the Hall conductivity from noncommuting components of the position operator. We comment on the relation between such contributions and the Berry curvature theory. We also report several additional observations related to the electron magnetic moment operator in systems with SOC and other relativistic corrections.

cond-mat.mes-hall↗

arXiv2026

Unidirectional magnetoresistance driven by nonequilibrium antiferromagnetic magnons

Magnetoresistive effects are typically symmetric under magnetization reversal. However, nonlinear spin transport can give rise to unidirectional magnetoresistance in systems with strong spin-orbit interaction and broken inversion symmetry. Here, we demonstrate that the nonequilibrium magnon accumulation characterized by a finite magnon chemical potential can lead to a large and robust magnonic unidirectional spin Hall magnetoresistance (USMR) in the weakly coupled van der Waals antiferromagnet CrPS4 in contact with Pt. Unlike conventional magnonic USMR driven by magnetization fluctuations, this effect persists under strong magnetic fields and low temperatures, with a pronounced peak near the spin-flip transition. The magnitude of magnonic USMR in CrPS4/Pt exceeds that of YIG/Pt by more than two orders of magnitude and surpasses the electrical USMR in metallic Ta/Co bilayers by a factor of two. The observed field and temperature dependence indicates that spin transport is dominated by magnon chemical potential gradients rather than thermal- or fluctuation-driven magnon generation. These findings establish a new mechanism for nonlinear magnetoresistance in antiferromagnetic van der Waals heterostructures and open a route to magnon-based antiferromagnetic spintronic functionalities in two-terminal device geometries.

cond-mat.mtrl-sci↗

arXiv2026

Thickness-dependent magnon spin transport in antiferromagnetic insulators: Crossover from quasi-three-dimensional to quasi-two-dimensional regimes

Motivated by the recent observation of giant room-temperature magnon spin conductivity in an ultrathin ferromagnetic insulator [X.-Y. Wei et al., Nat. Mater. 21, 1352 (2022)], we investigate thickness-dependent magnon spin transport in thin antiferromagnetic insulators (AFIs). We study the prototypical AFI hematite, known for its exceptionally low magnetic damping and two distinct magnetic phases: a low-temperature uniaxial easy-axis phase and a high-temperature biaxial easy-plane phase. Using stochastic micromagnetic simulations, we investigate thickness-dependent magnon spin transport across both magnetic phases. Our results uncover a crossover from quasi-three-dimensional to quasi-two-dimensional magnon spin transport at a critical thickness, determined by the frequency or energy of the excited magnons. Below this critical thickness, we observe a pronounced enhancement in the magnon diffusion length in both magnetic phases. This rise is attributed to a change in the effective magnon density of states, reflecting the reduced phase space available for scattering in the thinner, quasi-two-dimensional regime. Understanding and controlling long-distance magnon spin transport in AFIs is crucial for developing next-generation spintronic nanodevices, especially as materials approach the two-dimensional limit.

cond-mat.mes-hall↗

arXiv2025

Phonon- and magnon-mediated decoherence of a magnonic qubit

We investigate the decoherence of magnonic qubits in small ferromagnetic insulators and compute the relaxation and dephasing rates due to magnon-phonon and magnon-magnon interactions. We combine a Bloch--Redfield description with Keldysh non-equilibrium field theory to find explicit expressions for the rates. For a quadratic dispersion and assuming a uniform mode defines the qubit, we find that decay into two phonons is the only allowed relaxation process at zero temperature. The low resonance frequency and heavy unit cell strongly suppress this process in yttrium-iron-garnet. We also find that the dephasing rate scales with the inverse of size and damping of the magnet, and could become large for small and clean magnets. Our calculation thus provides additional insight into the viability of magnon-based quantum devices.

cond-mat.mes-hall↗

arXiv2025

Magnon and photon blockade in a hybrid antiferromagnet-cavity quantum system

We investigate both magnon and photon blockade for an antiferromagnetic insulator coupled to a linearly polarized cavity mode. We focus on the cross-Kerr nonlinearity between the two magnon modes, which can be large in antiferromagnets with a weak easy-axis magnetic anisotropy. By numerically solving the Lindblad master equations, we demonstrate that the resulting bright and dark modes, i.e., system eigenmodes that couple strongly and weakly to photons, respectively, exhibit distinct behaviors. The bright mode exhibits both magnon and photon blockade due to a weak effective nonlinearity, while the dark mode only exhibits magnon blockade for a detuned cavity photon. The blockade efficiency can further be optimized by appropriately tuning the competing interactions in the system. In addition, we show that applying a DC magnetic field, which lifts the degeneracy of antiferromagnetic chiral magnon eigenmodes, destroys the dark mode and leads to an unconventional photon blockade. These findings provide a pathway for generating single magnon and photon states useful for quantum information technology based on the underlying large squeezing of antiferromagnetic magnons.

cond-mat.mes-hall↗

arXiv2025

Stability of chiral magnon condensates in collinear antiferromagnetic insulators

Quasiequilibrium magnon Bose-Einstein condensates in ferromagnetic insulators have been a field of great interest, while condensation in antiferromagnetic systems has not yet been explored in detail. We analyze the stability of condensed chiral magnons in two antiferromagnetic insulators: a uniaxial easy-axis system and a biaxial system. We show that two-component magnon condensation and inter-magnon interactions are essential to create metastable magnon condensation. The uniaxial system with a Rashba-type Dzyaloshinskii-Moriya interaction supports two degenerate condensate populations at finite wave vectors. We find that the condensation state in this model is (meta)stable only when the distribution of condensed magnons between the two populations is symmetric. In addition, we demonstrate the emergence of a zero-sound-like Goldstone mode in antiferromagnetic systems that support two-component magnon condensation. On the other hand, in the biaxial system without Dzyaloshinskii-Moriya interaction, we predict that the magnon condensate cannot stabilize due to the breaking of the magnon degeneracy. Our results suggest that this instability is a general characteristic of single-component quasiequilibrium quasiparticle condensates.

cond-mat.mes-hall↗

arXiv2025

Shear Strain-Induced Multiferroic Response in the Altermagnetic Semiconductor CuFeS$_2$

CuFeS$_2$ is an altermagnetic semiconductor that is lattice-matched with silicon and has a high Néel temperature. It is nonpolar and magnetically compensated in its structural ground state. However, the crystal belongs to a magnetic symmetry class allowing simultaneous piezoelectricity and -magnetism, indicating that distortion by shear strain may enable functional properties not observed in its tetragonal ground state. This first-principles study explores how biaxial and shear strain affect the crystal structure and functional properties. Biaxial strain lowers crystal symmetry when applied to two of the three crystallographic {001} planes considered, enhancing the altermagnetic lifting of the Kramers degeneracy. Shear strain has a compressive effect on the crystal, enhancing the effects on the electronic structure seen under biaxial compressive strain. Applying it to any one of the three {001} planes induces a polar phase with an out-of-plane electric polarization, perpendicular to the strained plane. Moreover, applying shear strain to two out of the three {001} planes induces a net magnetization simultaneously with electric polarization, producing a multiferroic response.

cond-mat.mtrl-sci↗

arXiv2025

Second-order correlation and squeezing of photons in cavities with ultrastrong magnon-photon interactions

We investigate the second-order photon correlation function in cavity-magnon systems, focusing on ferromagnetic and antiferromagnetic cavities within the ultrastrong coupling regime, and extending beyond the rotating-wave approximation. By deriving exact integral solutions for the second-order correlation function, we demonstrate that counter-rotating magnon-photon interactions induce quadrature squeezing in the cavity mode. Furthermore, we show that tuning the anisotropic magnon-cavity couplings enhances the squeezing effect by changing the level repulsion of the magnon-cavity photon hybrid mode without increasing the cavity photon occupation number. Our study reveals distinct quantum correlation behaviors in ferromagnetic and antiferromagnetic cavities: For ferromagnetic cavities, we show that squeezing increases with coupling strength asymmetry, whereas in the antiferromagnetic case, magnon modes with opposite chirality suppress quantum effects and impose a lower bound on correlation functions. These findings provide a pathway to optimize photon blockade for quantum information technology in magnon-cavity systems in the ultrastrong coupling limit.

cond-mat.mes-hall↗

arXiv2024

Orbital Edelstein effect from the gradient of a scalar potential

We study the orbital Edelstein effect (OEE) that originates from a particular inversion symmetry breaking mechanism: an asymmetric scalar potential. We compute OEE of this kind with the help of the Kubo formula in the diffusive regime for a parabolic band Hamiltonian. We also present a qualitative derivation of the effect. Both approaches give the same result. This result does not rely on spin-orbit coupling (SOC) and scales as a cube of the momentum relaxation time. In sufficiently clean large systems with weak SOC, OEE of this nature should exceed the spin Edelstein effect by orders of magnitude. It may also provide an alternative interpretation for some experiments concerning the spin Hall effect.

cond-mat.mes-hall↗

arXiv2024

Minimal Models and Transport Properties of Unconventional $p$-Wave Magnets

New unconventional compensated magnets with a $p$-wave spin polarization protected by a composite time-reversal translation symmetry have been proposed in the wake of altermagnets. To facilitate the experimental discovery and applications of these unconventional magnets, we construct an effective analytical model. The effective model is based on a minimal tight-binding model for unconventional $p$-wave magnets that clarifies the relation to other magnets with $p$-wave spin-polarized bands. One of the most prominent advantages of our analytical model is the possibility to employ various analytical approaches while capturing essential features of $p$-wave magnets. We illustrate the effective model by evaluating the tunneling conductance in junctions with $p$-wave magnets, revealing a large magnetoresistance, spin filtering, and anisotropic bulk spin conductivity beyond linear response despite the absence of a net magnetization. These results show that unconventional $p$-wave magnets offer several useful functionalities, broadening the material selection for spintronics devices.

cond-mat.mes-hall↗

arXiv2024

Spin Seebeck in the weak exchange coupled van der Waals antiferromagnet

Spin Seebeck effect (SSE) refers to the creation of spin currents due to a temperature gradient in the magnetic materials or across magnet-normal metal interfaces, which can be electrically detected through the inverse spin Hall effect (ISHE) when in contact with heavy metals. It offers fundamental insights into the magnetic properties of materials, including the magnetic phase transition, static magnetic order, and magnon excitations. However, the SSE in van der Waals antiferromagnet is still elusive, especially across the spin-flip transition. Here, we demonstrate the SSE in the weak exchange coupled van der Waals antiferromagnet CrPS$_4$. The SSE increases as the magnetic field increases before the spin-flip transition due to the enhancement of the thermal spin current as a function of the applied field. A peak of SSE is observed at the spin-flip field, which is related to the magnon mode edges across the spin-flip field. Our results extend SSE research to van der Waals antiferromagnets and demonstrate an enhancement of SSE at the spin-flip transition.

physics.app-ph↗

arXiv2024

Position operators and interband matrix elements of scalar and vector potentials in the 8-band Kane model

We diagonalize the 8-band Kane Hamiltonian with a proper inclusion of the interband matrix elements of the scalar and vector potentials. This leads, among other results, to a modification of the conventional expression for the spin-orbit coupling (SOC) strength in narrow-gap semiconductors with the zinc blende symmetry. We find that in GaAs, at low temperatures, the correct expression for the SOC strength is actually twice as large as usually considered. In InSb it is $1.76$ times larger. We also provide a proper treatment of the interband matrix elements of the position operator. We show that the velocity operator in a crystal should be defined as a time-derivative of a fictitious position operator rather than the physical one. We compute the expressions for both these position operators projected to the conduction band of the 8-band Kane model. We also derive an expression for the projected velocity operator and demonstrate that the SOC strength in it differs from the SOC strength in the Hamiltonian. The ratio between them is not equal to $1$, as it is often assumed for the Rashba model. It does not equal $2$ either. The correct result for this ratio is given by a rational function of the parameters of the model. This function takes values between $4(23+3\sqrt{2})/73\approx 1.49$ and $2$. Our findings modify a vast number of research results obtained using the Rashba model and provide a path for a consistent treatment of the latter in future applications.

cond-mat.mes-hall↗

arXiv2024

RKKY interaction in Rashba altermagnets

The interaction between two impurity spins provides vital information about the host system and has been suggested to form a building block in quantum computation and spintronic devices. We here determine this Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in recently discovered altermagnetic materials, including the presence of a Zeeman field, in a two- and three-dimensional altermagnet. In two dimensions, we also study the effect of Rashba spin-orbit coupling. Our results reveal that the momentum-resolved spin-polarization of the itinerant carriers in the altermagnet changes the RKKY interaction qualitatively from the isotropic spin-splitting in a ferromagnet. The Ising-contribution directed parallel with the altermagnetism is found to exhibit a beating pattern reflecting the shape of the Fermi surface of the system and may thus be further influenced by an out-of-plane Zeeman field. However, the exchange interaction for in-plane impurity spins deviates only slightly from that of a normal metal. On shorter length scales, we find that the Ising, the Dzyaloshinskii-Moriya terms, and other non-collinear interaction terms in the RKKY interaction acquire a rapidly oscillating behavior as a function of the relative angle between the impurity spins, which is not present in the ferromagnetic case. We determine how this new length scale depends on the system parameters analytically. Our results show that the RKKY interaction in altermagnets is qualitatively different from that of ferromagnets despite both breaking time-reversal symmetry.

cond-mat.mes-hall↗

arXiv2024

Quasiclassical theory of superconducting spin-splitter effects and spin-filtering via altermagnets

Conducting altermagnets have recently emerged as intriguing materials supporting strongly spin-polarized currents without magnetic stray fields. We demonstrate that altermagnets enable three key functionalities, merging superconductivity and spintronics. The first prediction is a controllable supercurrent-induced edge magnetization, which acts like a dissipationless spin-splitter effect. The second and third predictions are a Cooper pair spin-splitter and a filtering effect, respectively. These effects allow for spatial separation of triplet pairs with opposite spin-polarizations and spin-selective tunneling of Cooper pairs. We derive a quasiclassical theory with associated boundary conditions that describe these phenomena and explain how they can be experimentally verified. Our results open a new path for spatial control of spin signals via triplet Cooper pairs using hybrid superconductor-altermagnet devices.

cond-mat.supr-con↗

arXiv2024

Quantum Theory of Spin-Transfer and Spin-Pumping in Collinear Antiferromagnets and Ferrimagnets

Antiferromagnets are promising candidates as active components in spintronic applications. They share features with ferrimagnets in that opposing spin orientations exist in two or more sublattices. Spin transfer torque and spin pumping are essential ingredients in antiferromagnetic and ferrimagnet spintronics. This paper develops an out-of-equilibrium quantum theory of the spin dynamics of collinear magnets containing many spins coupled to normal metal reservoirs. At equilibrium, the spins are parallel or antiparallel to the easy axis. The theory, therefore, covers collinear antiferromagnets and ferrimagnets. We focus on the resulting semi-classical spin dynamics. The dissipation in the spin dynamics is enhanced due to spin-pumping. Spin accumulations in the normal metals induce deterministic spin-transfer torques on the magnet. Additionally, each electron's discrete spin angular momentum causes stochastic fluctuating torques on the antiferromagnet or ferrimagnet. We derive these fluctuating torques. The fluctuation-dissipation theorem holds at high temperatures, including the effects of spin-pumping. At low temperatures, we derive shot noise contributions to the fluctuations.

cond-mat.mes-hall↗

arXiv2024

Impact of spin torques and spin pumping phenomena on magnon-plasmon polaritons in antiferromagnetic insulator-semiconductor heterostructures

We investigate the impact of spin torque and spin pumping on the surface magnon polariton dispersion in a antiferromagnetic insulator-semiconductor heterostructure. In the bilayer system, the surface magnon polaritons conventionally couple to the plasma-oscillations in the semiconductor via electromagnetic fields. Additionally, magnons in the antiferromagnetic insulator layer may interact with the semiconductor layer via spin torques and their reciprocal phenomena of spin pumping. Due to the spin-to-charge conversion from the spin Hall and inverse spin Hall effects in the semiconductor layer with a strong spin-orbit coupling, this can couple the magnons to the plasmons in the semiconductor layer. Our research reveals that modifications in the mode frequency and the hybridization gap induced by these phenomena depend on the thickness of the antiferromagnetic layer. In thick layers, the spin-pumping contribution to the frequency shift and damping is inversely proportional to the wavelength, while in thin layers it is inversely proportional to the thickness. Furthermore, hybridization of the surface magnon polariton and dispersive magnons in the antiferromagnet is shown to depend on both the thickness and wavelength of the modes.

cond-mat.mes-hall↗