arXiv subjects

Samir Lounis

Publications and source records attributed to Samir Lounis.

At least 19 recordsLinked to original sources

arXiv2026

Quantum dynamics of ultrasmall magnetic skyrmions in atomic-defect landscapes

Magnetic skyrmions are prime candidates for future information technology, and their fate in devices is settled by the defects they encounter. In ultrathin films, skyrmions can be as small as a couple of nanometers, carrying only a few tens of magnetic moments, while atomic defects reshape their energy landscape on the atomic scale. Here, we demonstrate that such ultrasmall skyrmions can behave quantum mechanically and that their quantum dynamics is unusual. Since the gyrotropic force dominates, the two coordinates of a ferromagnetic skyrmion are canonically conjugate and the skyrmion behaves as a particle in the lowest Landau level of an effective magnetic field, with a magnetic length of 0.6~Å. Utilizing skyrmion-defect interaction profiles of Cr and Fe adatoms and dimers in PdFe/Ir(111), obtained from atomistic spin simulations parametrized from first principles, we unveil quantized gyrotropic orbits with level spacings of 20--85~$μ$eV, bound not only by pinning defects but also, counter-intuitively, by repulsive ones. Saddle points of the energy landscape act as quantum beam splitters for skyrmions, and quantum depinning reaches rates of 1--100~Hz below the critical current. Embedded impurities, in contrast, bind the skyrmion so strongly that it ceases to behave as a rigid particle, defining a strong-coupling regime. For the Gilbert damping typical of ultrathin films, the quantized levels remain well resolved. The thermal-to-quantum crossover lies between 40 and 160~mK, which is within reach of millikelvin scanning tunneling microscopy, where tip-induced escape statistics and electron spin resonance STM can reveal the quantized gyrotropic motion of skyrmions pinned at defects crafted atom by atom.

cond-mat.mtrl-sci↗

arXiv2026

Zeeman-field-induced three-spin interactions and relativistic coupling to the scalar spin chirality

Magnetic fields are commonly incorporated in atomistic spin models through the Zeeman energy of individual magnetic moments. Starting from a microscopic electronic Hamiltonian, we show that the Zeeman coupling of conduction electrons also generates field-linear three-spin interactions, the lowest-order corrections allowed by time-reversal symmetry. Using a Green-function expansion of the grand potential, we identify two classes of interactions. Isotropic terms of the form $(\mathbf S_i\cdot\mathbf S_j)(\mathbf S_k\cdot\mathbf B)$ exist without spin-orbit coupling, and their total strength obeys a sum rule involving the three-site electronic loop at the Fermi energy. Relativistic terms require spin-orbit coupling and include $\mathbf K_{ijk}\cdot\mathbf B,χ_{ijk}$, where $χ_{ijk}=\mathbf S_i\cdot(\mathbf S_j\times\mathbf S_k)$ is the scalar spin chirality. We show that this coupling is forbidden in the spin channel without spin-orbit coupling, whereas the orbital channel recovers the orbital-flux mechanism of Mott insulators. Exact diagonalization of the Hubbard trimer confirms the independence and additivity of both channels. For magnetic trimers in Rashba electron gases, we derive analytical expressions showing that $\mathbf K_{ijk}$ lies in-plane at first order in spin-orbit coupling but vanishes for $C_{3v}$-symmetric trimers, where an out-of-plane component emerges at second order. These interactions can shift magnetization-step fields and modify field-induced noncoplanarity. In the continuum limit, the chirality coupling splits skyrmions from antiskyrmions, while the relativistic terms generate a field-dependent Dzyaloshinskii-Moriya interaction.

cond-mat.mtrl-sci↗

arXiv2026

Giant orbital Zeeman effects in a magnetic topological van der Waals interphase

Van der Waals (vdW) heterostructures allow the engineering of magnetic and electronic properties via interfacial orbital hybridisation and charge transfer. The resulting electric fields can give rise to Rashba spin-orbit coupling and Dzyaloshinskii-Moriya interactions, which may stabilise novel two-dimensional magnetic phases. While such emergent magnetic 'interphases' may offer a promising platform for spin-based electronics, direct spectroscopic evidence for them is still lacking. Here, we report electric-field tunable Zeeman effects with Landé $g$-factors up to $\approx230$ at the interface of graphene and the vdW ferromagnet Fe$_3$GeTe$_2$. They arise from an interplay of local-moment canting and the itinerant orbital moments of Fe$_3$GeTe$_2$, producing a giant asymmetric level splitting. Exploiting the inelastic phonon gap of graphene, scanning tunnelling spectroscopy allows us to access the buried interface properties, and quantify Faraday-like screening. Our findings provide new insights and opportunities for electrically controlled interface magnetism, e.g. for spintronics and orbitronics devices.

cond-mat.mes-hall↗

arXiv2026

Orbital Hall effect and orbital altermagnetism in even- and odd-parity-wave magnetic Lieb lattices

Altermagnets combine compensated antiferromagnetic order with ferromagnet-like signatures such as spin-polarized bands and, under appropriate conditions, an anomalous Hall response. Their characteristic momentum-dependent spin splitting originates from the interplay of magnetic order and crystal structure and therefore does not require spin-orbit coupling (SOC), whereas the anomalous Hall effect relies on SOC. This raises the question whether the ferromagnet-like transport character of altermagnets can manifest already in the nonrelativistic limit. Here, we investigate the transport of orbital angular momentum in magnetic Lieb lattices. We show that a collinear antiferromagnetic texture realizes a d-wave altermagnetic state and simultaneously generates an orbital Hall effect, both in the complete absence of SOC. In this nonrelativistic limit, the orbital Hall response closely resembles that of the corresponding ferromagnet. When SOC is included, the orbital Hall effect is accompanied by a spin Hall response, while the altermagnetic spin texture acquires a corresponding orbital texture, realizing orbital altermagnetism. In contrast, an anomalous, or crystal, Hall effect requires SOC and is additionally subject to crystal-symmetry constraints. We extend the analysis from d-wave altermagnetism to odd-parity p-wave magnetism, where the orbital Hall effect becomes anisotropic and the orbital conductivity tensor develops a symmetric transverse component analogous to the planar Hall response in charge transport. Our results establish the orbital Hall effect as a nonrelativistic transport manifestation of the close relation between altermagnets and ferromagnets and extend this connection to unconventional magnetic orders beyond altermagnetism.

cond-mat.str-el↗

arXiv2026

Chiral-Angle-Controlled Spin Splitting and Spin Transport in Nanotubes Rolled from d-wave Altermagnets

Altermagnets combine compensated collinear magnetic order with momentum-dependent spin splitting in the electronic band structure. Here, we show that rolling a two-dimensional (2D) $d$-wave altermagnet into a nanotube converts this momentum dependence into chiral-angle-controlled one-dimensional (1D) spin splitting through dimensional projection. A minimal tight-binding model reveals a characteristic nodal--antinodal dependence on the chiral angle $θ$, with the central circumferential subband exhibiting a $\cos(2θ)$ scaling and the projected spin splitting vanishing for the nodal orientation and reversing sign between orthogonal antinodal orientations. First-principles calculations for V$_2$O and representative symmetric and Janus systems demonstrate that this nodal--antinodal selection rule persists despite curvature-induced structural asymmetry and magnetic moment imbalance. We further show that the projected electronic structure produces chiral-angle-controlled spin-polarized transport: antinodal nanotubes exhibit spin-polarized transmission, whereas the nodal nanotube remains conducting with identical spin-channel transmission. These results demonstrate how dimensional projection can translate the momentum-space spin splitting of a 2D altermagnet into geometrically controlled electronic and transport properties in nanotubes.

cond-mat.mtrl-sci↗

arXiv2026

Exact theory of chirality-dependent p-wave magnetism and Edelstein effect in spin spirals

Spin-momentum locking is widely regarded as a hallmark of relativistic spin-orbit coupling. Here we demonstrate analytically that it can instead emerge solely from magnetic chirality. Solving the minimal tight-binding model of electrons coupled to a spin spiral using a generalized Bloch theorem, we show that the spiral generates chirality-dependent p-wave magnetism characterized by the antisymmetric spin texture $\boldsymbol{s}(\boldsymbol{k})=-\boldsymbol{s}(-\boldsymbol{k})$. The exact solution further yields closed-form expressions for the electrical conductivity and the spin Edelstein susceptibility, revealing a microscopic mechanism by which magnetic chirality alone can generate spin polarization without spin-orbit coupling, with direct implications also for chirality-induced spin selectivity. In the strong exchange-coupling regime, the spin-dependent physics of the spin spiral becomes directly analogous to the orbital-dependent physics of electrons propagating through a non-magnetic helix. Our work establishes a minimal exactly solvable model of chirality-induced spin-momentum locking and spin-charge conversion beyond the conventional spin-orbit coupled paradigm.

cond-mat.mes-hall↗

arXiv2026

Coupled Spin-Orbital $p$-Wave Magnetism via Structural and Magnetic Chirality

Helical spin textures represent the minimal realization of $p$-wave magnetism which is characterized by momentum-odd spin polarization. Independently, structurally chiral crystals exhibit momentum-odd orbital polarization arising from broken inversion symmetry. Here, we demonstrate that spin-orbit coupling couples these two independent microscopic chirality degrees of freedom, allowing the orbital polarization of a chiral crystal to generate an additional contribution to the $p$-wave spin splitting. The resulting spin-orbital state is naturally classified by the relative chirality $η=χ_{\mathrm c}χ_{\mathrm m}$, giving rise to two symmetry-distinct $p$-wave phases corresponding to homochiral and heterochiral configurations which can be directly probed by the longitudinal conductivity. These phases exhibit distinct transport signatures, establishing a unified framework linking orbitronics and unconventional magnetism through coupled spin-orbital $p$-wave order.

cond-mat.mes-hall↗

arXiv2026

$d$-wave Surface Altermagnetism in Centrosymmetric Collinear Antiferromagnets

Broken inversion symmetry at the surfaces of centrosymmetric collinear antiferromagnets lifts combined inversion and time-reversal symmetry ($PT$) and can, in principle, enable nonrelativistic d-wave spin splitting, termed surface altermagnetism. Combining symmetry analysis with first-principles calculations, we show that surface inversion breaking, while necessary, is not sufficient for this effect. Surface altermagnetism emerges only when no antiunitary symmetry survives at the surface that exchanges the two antiferromagnetically coupled surface sublattices and enforces spin degeneracy. We demonstrate this mechanism explicitly for the centrosymmetric G-type antiferromagnets V$_3$Al and BaMn$_2$Sb$_2$, and contrast it with MnPt, where a sublattice-exchanging symmetry survives at the surface in the form of translation-time-reversal symmetry ($tT$), thereby preserving spin degeneracy despite broken inversion symmetry. The mechanism is shown to apply across multiple classes of centrosymmetric antiferromagnets and remains robust against spin-orbit coupling, although relativistic spin mixing in heavier-element compounds may reduce the observable spin polarization. These results establish a symmetry-based route toward realizing robust nonrelativistic momentum-dependent spin polarization at antiferromagnetic surfaces and interfaces.

cond-mat.mtrl-sci↗

arXiv2026

Chiral spin-textures in van der Waals heterostructures

Chiral spin textures such as skyrmions have attracted considerable attention due to their nontrivial topology, chirality, stability at the nanoscale, and potential for low-power spintronic devices. The recent discovery of intrinsic magnetism in van der Waals (vdW) materials and the ability to engineer their heterostructures has opened a new platform to study and manipulate such textures. In these layered systems, atomically sharp interfaces, strong spin-orbit coupling, and tunable symmetry breaking provide unique opportunities to stabilize and control chiral magnetic states. This review summarizes the fundamental mechanisms underlying the formation of chiral spin textures in vdW heterostructures, including the roles of exchange interactions, magnetic anisotropy, Dzyaloshinskii-Moriya interaction, and dipolar effects. We highlight key experimental advances in the observation and manipulation of chiral textures, discuss their dynamical properties and transport signatures, while overviewing selected theoretical investigations. Finally, we outline current challenges and future directions toward realizing robust, room-temperature chiral spin textures for practical spintronic technologies.

cond-mat.mes-hall↗

arXiv2025

Three-dimensional topological orbital Hall effect caused by magnetic hopfions

Magnetic hopfions are non-collinear spin textures that are characterized by an integer topological invariant, called Hopf index. The three-dimensional magnetic solitons can be thought of as a tube with a twisted magnetization that has been closed at both ends to form a torus. The tube consists of a magnetic whirl called in-plane skyrmion or bimeron. Although hopfions have been observed by microscopy techniques, their detection remains challenging as they lack an electronic hallmark so far. Here we predict a three-dimensional orbital Hall effect caused by hopfion textures: When an electric field is applied, the hopfion generates a transverse current of orbital angular momentum. The effect arises due to the local emergent field that gives rise to in-plane and out-of-plane orbital Hall conductivities. This orbital Hall response can be seen as a hallmark of hopfions and allows us to distinguish them from other textures, like skyrmioniums, that look similar in real-space microscopy experiments. While the two-dimensional topological invariant of a skyrmion determines its topological Hall transport, the unique three-dimensional topological orbital Hall effect can be identified with the three-dimensional topological invariant that is the Hopf index. Our results make hopfions attractive for spin-orbitronic applications because their orbital signatures allow for their detection in devices and give rise to large orbital torques.

cond-mat.mes-hall↗

arXiv2025

Chirality-induced selectivity of angular momentum by orbital Edelstein effect in carbon nanotubes

Carbon nanotubes (CNTs) are promising materials exhibiting exceptional strength, electrical conductivity, and thermal properties, making them promising for various technologies. Besides achiral configurations with a zigzag or armchair edge, there exist chiral CNTs with a broken inversion symmetry. Here, we demonstrate that chiral CNTs exhibit chirality-induced orbital selectivity (CIOS), which is caused by the orbital Edelstein effect and could be detected as chirality-induced spin selectivity (CISS). We find that the orbital Edelstein susceptibility is an odd function of the chirality angle of the nanotube and is proportional to its radius. For metallic CNTs close to the Fermi level, the orbital Edelstein susceptibility increases quadratically with energy. This makes the CISS and CIOS of metallic chiral nanotubes conveniently tunable by doping or applying a gate voltage, which allows for the generation of spin- and orbital-polarized currents. The possibility of generating large torques makes chiral CNTs interesting candidates for technological applications in spin-orbitronics and quantum computing.

cond-mat.mes-hall↗

arXiv2025

Ab-initio exploration of Gd monolayer interfaced with WSe$_2$: from electronic and magnetic properties to the anomalous Hall effect

Heterostructures involving transition metal dichalcogenides (TMDs) have attracted significant research interest due to the richness and versatility of the underlying physical phenomena. In this work, we investigate a heterostructure consisting of a rare-earth material, specifically a Gd monolayer, interfaced with WSe$_2$. We explore its electronic structure, magnetic properties, and transport behavior, with particular emphasis on the emergence of the anomalous Hall effect (AHE). Both Gd and W are heavy elements, providing strong spin-orbit coupling (SOC), which plays a crucial role in triggering the AHE. The combination of strong SOC and inversion symmetry breaking leads to pronounced asymmetries between the $Γ-K$ and $Γ-K^\prime$ directions in the Brillouin zone. Our calculations reveal a substantial anomalous Hall conductivity (AHC) at the ferromagnetic interface, primarily originating from numerous avoided crossings involving the d-states of both Gd and W near the Fermi level. Moreover, we demonstrate that the AHC is highly tunable, either by adjusting the in-plane lattice constant or by reducing the separation between Gd and WSe$_2$.

cond-mat.mtrl-sci↗

arXiv2025

Oxygen-driven altermagnetic symmetry inducing d-wave superconductivity in the cuprates and nickelates

Since the discovery of cuprate high-Tc superconductivity, numerous theoretical frameworks have been proposed to explain its mechanism; Anderson's RVB picture [Science 235, 1196-1198, 1987] and U(1) gauge theory [Phys. Rev. Lett. 76, 503-506, 1996] motivate a minimal one-band view that largely integrates out oxygen. By contrast, altermagnetism [Phys. Rev. X 12, 040501, 2022] yields a d-wave-like k-space magnetic texture from alternatingly rotated nonmagnetic cages; La2CuO4 (the parent of a high-Tc cuprate) is a prototypical example. As a proof of principle, we show in La2CuO4 that an alternating local pairing potential on the two Cu sublattices (plus/minus s(r)) produces a nodal, d-wave-like Delta(k). As orthorhombic tilts are, however, not the driver (and even suppress superconductivity in nickelates; [Nature 621, 493, (2023)], we then show that the in-plane oxygen sublattice of CuO2/NiO2 layers, ubiquitous in cuprates and nickelates, intrinsically realizes the same symmetry. Imposing an oxygen-centered, staggered s pairing yields a d-wave gap with perfect C4 symmetry, demonstrated self-consistently in NdNiO2 from first principles. While the underlying mechanism that drives this order is unclear, we outline possible origins. Further, this description of superconductivity enables mapping a real-space superconducting order parameter onto a lattice picture, allowing superconductivity and Hubbard physics to be treated on the same footing.

cond-mat.supr-con↗

arXiv2025

Electronic properties of Kagome metal YbV$_3$Sb$_4$: A First-Principles Study

We have investigated the vanadium-based Kagome metal YbV$_3$Sb$_4$ using density functional theory (DFT) combined with the Wannier function analysis. We explore the electronic properties, de Haas-van Alphen (dHvA) effect and Fermi surface. The inclusion of spin-orbit coupling SOC induces the splitting of Yb-4f states, while its impact on the V-3d states is moderate. Furthermore, we have incorporated SOC+U, where U being the Hubbard parameter, which drastically changes the Yb-4f states creating additional splitting leading to three distinct peaks in the density of states (DOS). The V-3d atoms with the Kagome lattice contribute maximum to the transport properties, exhibits flat bands near the EF while being protected under SOC and U+SOC. Herein, we report the vulnerability of the Yb-4f states under SOC and U+SOC. Furthurmore, The Fermi surface is found to comprise of quasi-2D cylindrical sheets centered at the Gamma-point, along with smaller pockets near the Brillouin zone boundaries, which under combined U+SOC, a small spherical pocket emerges and the cylindrical sheet exhibits slight deformations. The dHvA frequencies reach as high as 70 kilotesla, which increase with tilt angle, exhibiting a nearly parabolic trend as expected for cylindrical orbits, while a low-frequency branch remains below 1 kT. Only the U+SOC case shows noticeable modification in both the Fermi surface and the dHvA oscillation. Crucially, the $Z_2$ invariant calculation identifies YbV$_3$Sb$_4$ as a strong topological metal ($r_0 = 1$). These findings not only advance our understanding of the underlying quantum phenomena in rare-earth Kagome systems, but also establish YbV$_3$Sb$_4$ as a compelling and promising platform for exploring intertwined topology and electron correlations in kagome lattices, thereby offering valuable insights for engineering quantum phases in layered materials.

cond-mat.str-el↗

arXiv2025

Effect of applied pressure on the non-relativistic spin-splitting (NRSS) of FeSb2 altermagnet: A first-principles study

We have investigated the pressure-dependent electronic structure, phonon stability, and anomalous Hall response of the recently discovered altermagnet FeSb2 from density functional theory (DFT) and Wannier function analysis. From density functional perturbation theory (DFPT) calculations, we have found that FeSb2 remains dynamically stable up to 10 GPa, evidenced by positive phonon frequencies. Our spin-polarised band structure shows that the node of band crossing between spin-up and spin-down bands around the Fermi energy exactly lies at the Gamma and A-symmetry points. The Fermi crossing is mostly exhibited by band-24, band-25 and band-26. The non-relativistic spin-splitting (NRSS) along M'-Gamma-M and A-Z-A' symmetry is attributed to the broken time-reversal (PT ) symmetry. There are significant changes in the band profile under applied pressure, as one can see the shifting of the node of band-24 and band-26 towards the lower energy side. The NRSS exhibited by band-24 along M'-Gamma-M symmetry is notably small. Although the strength of NRSS of band-26 along A-Z-A' symmetry is significant but reduces under applied pressure. The anomalous Hall conductivity (AHC) values are prominent in -1 to 1 eV range. A sharp peaked and positive AHC values at ambient pressure, becomes spectrally broadened and negative at 10 GPa due to pressure-induced band crossings and redistribution of Berry curvature near the Fermi level. We have observed that the values of spin hall conductivity (SHC) are around 2-2.5 times lower as compared to AHC and prominent in between -1.0 eV to 1.0 eV. Our results establish FeSb2 as a tunable altermagnetic candidate where pressure can modulate both topological transport and dynamic stability, offering opportunities for strain-engineered Hall responses in compensated magnetic systems.

cond-mat.str-el↗

arXiv2025

Influence of Interlayer Stacking on Optical Behavior in WSe$_{2}$/MoS$_{2}$ van der Waals Heterostructures

We investigate the impact of crystal alignment on excitonic behavior in WSe$_{2}$/MoS$_{2}$ van der Waals heterostructures by comparing eclipsed (AA) and staggered (AB) stacking configurations. Our first-principles and symmetry-based analysis reveal that interlayer stacking symmetry plays a central role in determining the nature of electron-hole pairs. We uncover a rich variety of excitonic states, including spatially confined two-dimensional (2D) excitons, delocalized three-dimensional (3D) excitons, and charge-transfer (CT) excitons with interlayer character. The dimensionality and optical activity of these excitons are governed by the interplay among orbital character, interlayer hybridization, and symmetry-imposed selection rules. Our findings establish general principles for engineering excitonic properties in van der Waals heterostructures through controlled layer orientation and stacking order.

cond-mat.str-el↗

arXiv2025

Unlocking Hidden Potential in Electron Holography of Non-Collinear Spin Textures

Due to their particle-like properties, three-dimensional (3D) spin textures have garnered significant interest, particularly for their potential applications in next-generation information storage devices. However, efficiently identifying these textures remains a major challenge. Here, we approach this problem from a new perspective. Rather than relying solely on the magnetic stray field, which vanishes in antiferromagnets, we use multiple-scattering theory to demonstrate that spin textures carry nontrivial charges due to the noncollinearity of magnetic moments. This induced charge encodes magnetic information driven by spin-mixing and spin-orbit interactions. We propose leveraging electron holography to extract this information by reconstructing phase images obtained from transmission electron microscopy (TEM). To quantify this effect, we systematically calculate and compare the contributions of both conventional and newly identified mechanisms to the phase images, considering different electronic structure parameters. Our findings mark a significant milestone in advancing the exploration and possible application of 3D spin textures in next-generation spintronic devices.

cond-mat.mes-hall↗

arXiv2025

Triple-Q state in magnetic breathing kagome lattice

Magnetic frustration in two-dimensional spin lattices with triangular motifs underpins a series of exotic states, ranging from multi-Q configurations to disordered spin-glasses. The antiferromagnetic kagome lattice, characterized by its network of corner-sharing triangles, represents a paradigmatic frustrated system exhibiting macroscopic degeneracy. Expanding upon the kagomerization mechanism, we focus on the magnetic breathing kagome lattice formed by a Mn monolayer deposited on a heavy metal substrate and capped with h-BN. The Mn kagome arrangement induces pronounced magnetic frustration, as evidenced by the nearly flat bands derived from spin spiral energy calculations. Including further-neighbor interactions reveals a spin spiral energy minimum along the $Γ$-K line and an intriguing triple-Q state with nonzero topological charge, potentially leading to highly nonlinear Hall effects. Furthermore, the flat band properties can further give rise to an even more complex spin configuration, marked by several Q-pockets in the spin structure factor. These results present a fertile ground for advancing the study of multi-Q states and exploring emergent topological phenomena.

cond-mat.mtrl-sci↗