arXiv subjects

Libor Šmejkal

Publications and source records attributed to Libor Šmejkal.

At least 37 records · Page 2Linked to original sources

arXiv2025

Spin polarization engineering in $d$-wave altermagnets

Altermagnets host unconventional spin-polarized bands despite zero net magnetization, but controlling their spin structure remains challenging. We propose a multi-field approach to engineer spin polarization in $d$-wave altermagnets using gating, optical driving, and in-plane electric fields, which enable tunable and switchable polarizations along multiple directions. Optical driving induces out-of-plane ($z$) polarization, while gating and in-plane fields generate $x$- and $y$-polarizations via the Edelstein effect, all of which are experimentally detectable. We further find that spin- and band-selective doping induces chiral optical activity, a feature unique to altermagnets. Our approach provides a versatile route for full control of spin polarization in altermagnets.

cond-mat.mtrl-sci↗

arXiv2025

Spin Hall magnetoresistance at the altermagnetic insulator/Pt interface

The resistance of a heavy metal can be modulated by an adjacent magnetic material through the combined effects of the spin Hall effect, inverse spin Hall effect, and dissipation of the spin accumulation at the interface. This phenomenon is known as the spin Hall magnetoresistance. The dissipation of the spin accumulation can occur via various mechanisms, with spin-transfer torque being the most extensively studied. In this work, we report the observation of spin Hall magnetoresistance at the interface between platinum and an insulating altermagnetic candidate, Ba$_2$CoGe$_2$O$_7$. Our findings reveal that this heterostructure exhibits a relatively large spin Hall magnetoresistance signal, which is anisotropic with respect to the crystal orientation of the current channel. We explore and rule out several potential explanations for this anisotropy and propose that our results may be understood in the context of the anisotropies of the spin current channels across the Pt/altermagnetic Ba$_2$CoGe$_2$O$_7$ interface.

cond-mat.mes-hall↗

arXiv2025

Non-relativistic linear Edelstein effect in helical EuIn2As2

Motivated by the ongoing interest in understanding the actual magnetic ground state of the promising axion insulator candidate EuIn2As2, we present here a spin symmetry analysis and ab-initio calculations, aiming to identify specific exchange-dominated physics that could offer insights into the current debate. We investigate two non-collinear coplanar magnetic orders reported in this compound: the helical and broken helical phases. Our symmetry analysis shows that magnetic-exchange alone results in the formation of an out-of-plane odd-wave order in momentum space with a single un-polarized nodal plane in both phases. Additionally, we identified an in-plane g-wave order that emerges exclusively in the broken helical phase, providing a distinguishing feature for this phase. Furthermore, we report a non-relativistic Edelstein effect with a distinct out-of-plane polarized spin density that dominates over spin-orbit coupling effects. Our ab-initio calculations reveal a significant contrast in the magnitude of this effect between both phases, which could serve as a means to identify the magnetic transition and distinguish them from other magnetic ground states proposed for this compound.

cond-mat.str-el↗

arXiv2025

Revealing the Altermagnetism in Hematite via XMCD Imaging and Anomalous Hall Electrical Transport

Altermagnets are a class of magnetic materials that exhibit unconventional transport properties, such as an anomalous Hall effect, despite having compensated sublattice magnetic moments. In this study, we report fundamental experimental evidence of the altermagnetic nature of hematite ($α$-\ch{Fe2O3}), combining electrical transport with advanced XPEEM imaging with linear and circular dichroism contrast. Our measurements directly visualize the Néel vector's coupling to the crystal orientation, confirming hematite's altermagnetic order and its symmetry-driven transport behavior. Transport measurements reveal an anisotropic AHE with a pronounced crystal orientation dependence, including a sign inversion for specific Néel vector alignments. Supported by first-principles theoretical calculations, we explain how the interplay between collinear spin and crystal symmetry breaking drives the observed anomalous Hall effect. These findings establish hematite as an altermagnet, paving the way for experimental identification of altermagnetic materials and their integration into spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2025

Atomic Altermagnetism

Altermagnetism has been recently experimentally verified by photoemission mapping of the spin order in momentum space in MnTe and CrSb, which feature two anisotropic sublattices with antiparallel magnetic dipole moments. In this work, we explicitly demonstrate the presence of an even-parity ferroically ordered non-dipolar spin density on the atomic sites, i.e. atomic altermagnetism, in MnTe, $La_2O_3Mn_2Se_2$ and $Ba_2CaOsO_6$. We do so through spin-symmetry analysis and partial-wave decomposition of the spin density obtained by first-principles calculations. In MnTe we show a ferroically ordered g-wave form factor in the spin density around the Mn site. In the $A_2O_3M_2Se_2$ family (A= La, Sr and M= Mn, Fe, Co), we show that there is a ferroically ordered d-wave form factor coexisting with the antiferroic magnetic dipoles in the M site, while the O site shows no dipole but a pure d-wave atomic spin density. In the Mott-insulating candidate $Ba_2CaOsO_6$, as a key result, we reveal a pure form of atomic altermagnetism - absent of any dipolar sublattice order. This highlights that the altermagnetic order can exist without a Néel vector formed by antiferroic dipole moments on an even number of crystal sublattices, underlining its distinction from collinear Néel antiferromagnetic order. Our calculations predict that $La_2O_3Mn_2Se_2$ and $Ba_2CaOsO_6$ can exhibit giant spin-splitter angles of up to 42° and 26° respectively, thus demonstrating the possibility of large altermagnetic responses without requiring the staggered Néel order of local dipole moments.

cond-mat.mtrl-sci↗

arXiv2025

Altermagnetic splitting of magnons in hematite ($α$-Fe$_2$O$_3$)

We develop a four-sublattice spin-wave theory for the $g$-wave altermagnet candidate hematite ($α$-Fe$_2$O$_3$), considering both its easy-axis phase below and its weak ferromagnetic phase above the Morin temperature. A key question is whether the defining altermagnetic feature - magnon spin splitting (also called chirality or polarization splitting) due to nonrelativistic time-reversal symmetry breaking - remains intact when relativistic corrections, which contribute to hematite's magnetic order, are included. Using a detailed symmetry analysis supported by density functional theory, we show that capturing the magnon splitting within a Heisenberg model requires exchange interactions extending at least to the 13th neighbor. We find an altermagnetic band splitting of approximately 2 meV, which contrasts with the total band width of about 100 meV. To evaluate the experimental observability of this splitting, we analyze relativistic corrections to the magnon spectrum in both magnetic phases. We show that spin-orbit coupling - manifesting as magnetocrystalline anisotropies and the Dzyaloshinskii-Moriya interaction (DMI) - does not obscure the key altermagnetic features. These findings indicate that inelastic neutron scattering can directly probe altermagnetic magnon splitting in hematite. We also discuss implications for magnon transport, particularly magnonic contributions to the thermal Hall effect (which requires spin-orbit coupling) and to spin splitter effects (which do not). Notably, we predict a third-order nonlinear magnon spin splitter effect. This result suggests that the $g$-wave magnon spin splitting in hematite enables transverse heat-to-spin conversion without requiring an external magnetic field.

cond-mat.str-el↗

arXiv2025

Even-in-magnetic field part of transverse resistivity as a probe of magnetic transitions

The component of the resistivity tensor $ρ_{ij}$ corresponding to voltage transverse to both an applied current and a magnetic field can be separated into odd and even parts with respect to the applied magnetic field. The former contains information, for example, about the ordinary or anomalous Hall effect. The latter is often ascribed to experimental artefacts and ignored. Here, we show that upon suppressing these artefacts in carefully controlled experiments, useful information remains. We first investigate the well-explored ferromagnet CoFeB, where the even part of $ρ_{yx}$ contains a contribution from the anisotropic magnetoresistance, which we confirm by Stoner--Wohlfarth modelling. We then apply our approach to magnetotransport measurements of $\rm Mn_5Si_3$ thin films, which undergo a transition from non-collinear to an altermagnetic collinear state. In this material, the even part of the transverse signal is sizable only in the low-spin-symmetry phase below $\approx 80$~K. Transverse resistivity measurements thus offer a simple and readily available probe of magnetic order transitions.

cond-mat.mes-hall↗

arXiv2025

Strain controlled g- to d-wave transition in altermagnetic CrSb

The possibility of a strain-induced transformation from $g$-wave to $d$-wave altermagnetism was recently recently proposed for MnTe using a $k\cdot p$ perturbative model. In this work, we demonstrate such a transition in CrSb for a wider array of strains, using a combination of a minimal model and first-principles calculations. Starting from a symmetry perspective, we analyze the spin elastoconductivity tensor, and determine the strain types which allow for a change in the altermagnetic symmetry. We obtain three strain directions, which allow for a $d$-wave type splitting, and one in which a net magnetic moment emerges. Using first-principles calculations in the absence of spin-orbit coupling (SOC), we confirm these symmetry predictions. Furthermore, these results do not alter qualitatively in the presence of SOC. Finally, we reveal that the resulting spin currents give rise to a spin-splitter effect of up to 5\% under realistic strains of 1\%, confirming strain as a powerful tool for tuning altermagnetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast electron-phonon scattering in antiferromagnetic Dirac semimetals

Topological antiferromagnetic systems, which exhibit anisotropic band structures combined with complex relativistic spin structures in momentum space, have shown strong magnetoresistance effects driven by Dirac fermion characteristics. While these new antiferromagnets have been studied in transport experiments, very little is known about their spin-dependent electronic dynamics on ultrafast timescales and far-from-equilibrium behavior. This paper investigates theoretically the spin-dependent electronic dynamics due to electron-phonon scattering in a model electronic band structure that corresponds to a Dirac semimetal antiferromagnet. Following a spin conserving instantaneous excitation we obtain a change of the antiferromagnetic spin polarization due to the scattering dynamics for the site-resolved spin expectation values. This allows us to identify fingerprints of the anisotropic band structure in the carrier dynamics on ultrashort timescales.

cond-mat.mes-hall↗

arXiv2025

Eightfold Degenerate Dirac Nodal Line in Collinear Antiferromagnet Mn$_5$Si$_3$

We study the electronic, magnetic, and spin transport properties of the orthorhombic Mn$_{5}$Si$_{3}$ compound in the $AF2$ phase using symmetry analysis and ab-initio calculations. Our ground state energy calculations align with experimental observations, demonstrating that the collinear antiferromagnetic (AFM) order, with Néel vector in the [010] direction, is the most stable magnetic configuration both with and without spin-orbit coupling (SOC) in a bulk lattice geometry. We identified an unconventional eight-fold degenerate Dirac nodal line (DNL) close to the Fermi level, characterized by negligible SOC. This DNL is robustly protected by a unique combination of a pure-spin symmetry and a lattice symmetry together with magnetic space group symmetries. Upon introducing SOC, this degeneracy is reduced to two four-fold DNLs, being protected by the combination of time-reversal, partial translation and nonsymmorphic symmetries within the magnetic space group. We predict also a large intrinsic spin Hall conductivity (SHC) which correlates with the presence of SOC-induced splitting of these eight-fold degenerate DNLs near the Fermi level. These intriguing characteristics position collinear antiferromagnet Mn$_{5}$Si$_{3}$ as a compelling candidate for spintronic applications, particularly in the generation and detection of spin currents, while remaining compatible with modern silicon technology.

cond-mat.mtrl-sci↗

arXiv2025

Spin and pair density waves in 2D altermagnetic metals

Altermagnetism, a recently proposed and experimentally confirmed class of magnetic order, features collinear compensated magnetism with unconventional d-, g-, or i-wave spin order. Here, we show that in a metallic 2D d-wave altermagnet with combined two-fold spin and four-fold lattice rotational symmetry $[C_2||C_4]$, secondary instabilities can arise. Using an unbiased functional renormalization group approach, we analyze the weak-coupling instabilities of a 2D Hubbard model with a preexisting altermagnetic order inspired by our ab initio electronic structure calculations of realistic material candidates from V$_2$X$_2$O (X = Te, Se) family. We identify two distinct spin density wave (SDW) states that break the underlying altermagnetic $[C_2||C_4]$ symmetry. Additionally, we find spin-fluctuation-induced instabilities leading to a singlet d-wave superconducting state and an unconventional commensurate pair density wave (PDW) state with extended s-wave and spin-triplet symmetry. We establish a general criterion for the unusual exchange statistics for these pair density waves and characterize their excitation spectrum, which exhibits Bogoliubov Fermi surfaces or nodal points depending on the gap size.

cond-mat.str-el↗

arXiv2025

Origin of Anomalous Hall effect in Cr-doped RuO$_2$

RuO$_2$ is one of the most highlighted candidates for altermagnetism. However, the most recent muon spin spectroscopy and neutron studies demonstrated the absence of magnetic order in this system. The electronic structure of RuO$_2$ hints at a possibility of realizing a magnetically ordered state upon hole doping, and such a possibility was explored experimentally in Cr-doped RuO$_2$, where it was suggested that this system exhibits the anomalous Hall effect (AHE) due to altermagnetism. In this manuscript, based on our density functional calculations, we revise the results obtained for this system and propose a different interpretation of experimental results. Our calculations suggest that extra holes are bound to Cr impurity and do not dope Ru bands, which remain nonmagnetic. Thus, the observed AHE is not due to the altermagnetism but stems entirely from magnetic Cr ions.

cond-mat.mtrl-sci↗

arXiv2024

Transport theory and spin-transfer physics in d-wave altermagnets

We develop a mesoscale transport theory for the charge and spin degrees of freedom of itinerant carriers in a $d$-wave altermagnet. Our effective Lagrangian description is built upon the slave-boson formulation of the microscopic $t-J$ model. We obtain a spin-polarized diffusive contribution to the effective Hamiltonian, with no counterpart in conventional antiferromagnetism and parametrized by the spin splitting, that is responsible for the so-called spin-splitter effect in $d$-wave altermagnets. We also elucidate the spin-transfer response of the itinerant fluid as well as the spin pumping into the altermagnet, which show previously unidentified combinations of the charge current and spatial partial derivatives (namely, {$j_{x}^{e}$,$\partial_{y}$} and {$j_{y}^{e}$,$\partial_{x}$}). The emergent spin-transfer physics in $d$-wave altermagnets opens up new possibilities for the dynamics of spin textures, such as the domain-wall motion driven by transverse charge currents. We also consider the effect of elastic distortions in the aforementioned transport properties.

cond-mat.mes-hall↗

arXiv2024

Altermagnetic multiferroics and altermagnetoelectric effect

Magnetoelectric multiferroics are highly sought after for applications in low-power electronics and for advancing fundamental research, including axion insulators and dark matter detection. However, achieving a combination of ferroic spin and electric orders, along with their controllable switching, remains a significant challenge in conventional ferromagnets and antiferromagnets. Here, we present first-principles evidence that time-reversal symmetry-breaking altermagnetic spin polarization with relatively high critical temperatures can emerge in ferroelectrics BaCuF$_4$ (T$_N$ $\sim$ 275K) and Ca$_3$Mn$_2$O$_7$ (T$_N$ $\sim$ 110K). Furthermore, we classify all possible altermagnetic polar spin groups, revealing altermagnetism in a collinear phase of BiFeO$_3$. We also propose an altermagnetoelectric effect, a nonrelativistic cross-coupling between altermagnetic spin polarization and ferroelectric polarization, mediated by a rotation of nonmagnetic polyhedra in the lattice structure. Our findings suggest an alternative pathway towards high-temperature magnetoelectric multiferroicity and the electric field control of altermagnetic order parameters.

cond-mat.mtrl-sci↗

arXiv2024

Anisotropy of the anomalous Hall effect in the altermagnet candidate Mn$_5$Si$_3$ films

Altermagnets are compensated magnets belonging to spin symmetry groups that allow alternating spin polarizations both in the coordinate space of the crystal and in the momentum space of the electronic structure. In these materials the anisotropic local crystal environment of the different sublattices lowers the symmetry of the system so that the opposite-spin sublattices are connected only by rotations, which results in an unconventional spin-polarized band structure in the momentum space. This low symmetry of the crystal structure is expected to be reflected in the anisotropy of the anomalous Hall effect. In this work, we study the anisotropy of the anomalous Hall effect in epitaxial thin films of Mn$_5$Si$_3$, an altermagnetic candidate material. We first demonstrate a change in the relative Néel vector orientation when rotating the external field orientation through systematic changes in both the anomalous Hall effect and the anisotropic longitudinal magnetoresistance. We then show that the anomalous Hall effect in this material is anisotropic with the Néel vector orientation relative to the crystal structure and that this anisotropy requires high crystal quality and unlikely correlates with the magnetocrystalline anisotropy. Our results provide further systematic support to the case for considering epitaxial thin films of Mn$_5$Si$_3$ as an altermagnetic candidate material.

cond-mat.mes-hall↗

arXiv2024

Highly Efficient Non-relativistic Edelstein effect in p-wave magnets

The origin and efficiency of charge-to-spin conversion, known as the Edelstein effect (EE), has been typically linked to spin-orbit coupling mechanisms, which require materials with heavy elements within a non-centrosymmetric environment. Here we demonstrate that the high efficiency of spin-charge conversion can be achieved even without spin-orbit coupling in the recently identified coplanar p-wave magnets. The non-relativistic Edelstein effect (NREE) in these magnets exhibits a distinct phenomenology compared to the relativistic EE, characterized by a strongly anisotropic response and an out-of-plane polarized spin density resulting from the spin symmetries. We illustrate the NREE through minimal tight-binding models, allowing a direct comparison to different systems. Through first-principles calculations, we further identify the p-wave candidate material CeNiAsO as a high-efficiency NREE material, revealing a 25 times larger response than the maximally achieved relativistic EE and other reported NREE in non-collinear magnetic systems with broken time-reversal symmetry. This highlights the potential for efficient spin-charge conversion in p-wave magnetic systems.

cond-mat.mes-hall↗

arXiv2024

Ultrafast electron dynamics in altermagnetic materials

Altermagnets constitute a new class of magnetic materials that combine properties previously thought to be exclusive to either antiferromagnets or ferromagnets, and have unique properties of their own. In particular, a combination of symmetries connecting magnetic sublattices gives rise to a band spin splitting exhibiting unconventional d, g, or i-wave character. Their unique electronic properties have already led to new spin-dependent transport effects. Here, we consider their spin and charge dynamics on ultrafast timescales. We use a minimal tight binding model that captures the main features of the altermagnetic candidate material KRu$_4$O$_8$. In the framework of this model, we compute the spin-dependent electronic scattering dynamics after ultrashort-pulse excitation and show through these microscopic calculations how electron-electron and electron-phonon scattering processes redistribute optically excited carriers in a 2D slice of the Brillouin zone. We find that the optically excited spin polarization is long lived (~1ps) compared to the electron-electron momentum scattering lifetime of roughly 10fs. This contrasts remarkably with the much shorter spin lifetimes observed in typical ultrafast electronic spin dynamics in conventional ferromagnets and antiferromagnets, making these pulse-driven spin excitation experiments a key probe of altermagnetism.

cond-mat.mtrl-sci↗

arXiv2024

Altermagnetic variants in thin films of Mn5Si3

The altermagnet candidate Mn5Si3 has attracted wide attention in the context of non-relativistic spin physics, due to its composition of light elements. However, the presumed structure of the altermagnetic phase had yet to be demonstrated. In this study, we demonstrate a hallmark of altermagnetism in Mn5Si3 thin films, namely the three options, or variants, for the checkerboard distribution of the magnetic Mn atoms. The magnetic symmetries were altered by field-rotation of the Néel vector along relevant crystal directions, resulting in anomalous Hall effect anisotropy. The experimental results in nanoscale devices were corroborated by a theoretical model involving atomic site dependent anisotropy and bulk Dzyaloshinskii-Moriya interaction for a single variant.

cond-mat.mes-hall↗