arXiv subjects

Tom G. Saunderson

Publications and source records attributed to Tom G. Saunderson.

17 recordsLinked to original sources

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

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

Higher-order Hall response arises from octupole order and scalar spin chirality in a noncollinear antiferromagnet

Noncollinear antiferromagnets can generate a transverse electrical response known as the anomalous Hall effect, even though they possess almost no net magnetization. The microscopic origin of this behaviour, however, has remained unclear because conventional measurement geometries mix different contributions to the measured response. Here, we show that applying magnetic fields in selected in-plane directions allows us to disentangle the mechanisms underlying the Hall effect in a representative noncollinear antiferromagnet. By suppressing any dipole-related signal, we isolate a purely octupole-driven Hall response that exhibits a characteristic three-fold angular symmetry. At low magnetic fields, we further observe an additional Hall-like contribution that arises from the scalar spin chirality associated with noncoplanar spin textures. Combining symmetry analysis, first-principles calculations, and transport measurements, we reveal that octupole order, dipole moments, and chirality coexist and contribute in distinct field regimes. These findings establish a framework for identifying and controlling complex magnetic order parameters for spintronic applications.

cond-mat.mes-hall↗

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

Triplet superconductivity by the orbital Rashba effect at surfaces of elemental superconductors

It is often assumed that in a superconductor without spin-triplet pairing, the formation of unconventional spin-triplet densities requires the spin-orbit interaction in combination with either broken inversion symmetry or broken time-reversal symmetry. Here, we show from first principles the existence of supercurrent-driven spin triplet densities on the surface of a variety of simple superconducting materials without spin-orbit coupling. We are able to attribute this phenomenon to the superconducting non-relativistic orbital Rashba Edelstein effect. Furthermore, we find that the spin-orbit induced spin moment is one order of magnitude smaller than the orbital moment, and has a vanishing effect on the total magnitude of the induced triplet density. Our findings imply the existence of a route to generate spin-currents without the use of heavy metals. Additionally, as an orbital moment can couple directly to a magnetic field, it shows that orbital physics is the dominant term that drives the superconducting diode effect.

cond-mat.supr-con↗

arXiv2024

Inductive detection of inverse spin-orbit torques in magnetic heterostructures

The manipulation of magnetization via Magnetic torques is one of the most important phenomena in spintronics. In thin films, conventionally, a charge current flowing in a heavy metal is used to generate transverse spin currents and to exert torques on the magnetization of an adjacent ferromagnetic thin film layer. Here, in contrast to the typically employed heavy metals, we study spin-to-charge conversion in ferromagnetic heterostructures with large spin-orbit interaction that function as the torque-generating layers. In particular, we chose perpendicular magnetic anisotropy (PMA) multilayers [Co/Ni] and [Co/Pt] as the torque-generating layers and drive magnetization dynamics in metallic ferromagnetic thin film $\mathrm{Co_{20}Fe_{60}B_{20}}$ (CoFeB) layers with in-plane magnetic anisotropy (IMA). We investigate the spin dynamics driven by spin-orbit torque (SOT) and the concomitant charge current generation by the inverse SOT process using an inductive technique based on a vector network analyzer. In our experimental findings, we find that the SOTs generated by our multilayers are of a magnitude comparable to those produced by Pt, consistent with first-principles calculations. Furthermore, we noted a significant correlation between the SOT and the thickness of the CoFeB layer.

cond-mat.mtrl-sci↗

arXiv2024

Fluctuation-mediated spin-orbit torque enhancement in the noncollinear antiferromagnet Mn3Ni0.35Cu0.65N

The role of spin fluctuations near magnetic phase transitions is crucial for generating various exotic phenomena, including anomalies in the extraordinary Hall effect, excess spin-current generation through the spin-Hall effect (SHE), and enhanced spin-pumping, amongst others. In this study, we experimentally investigate the temperature dependence of spin-orbit torques (SOTs) generated by Mn3Ni0.35Cu0.65N (MNCN), a member of the noncollinear antiferromagnetic family that exhibits unconventional magnetotransport properties. Our work uncovers a strong and nontrivial temperature dependence of SOTs, peaking near the Néel temperature of MNCN, which cannot be explained by conventional intrinsic and extrinsic scattering mechanisms of the SHE. Notably, we measure a maximum SOT efficiency of 30%, which is substantially larger than that of commonly studied nonmagnetic materials such as Pt. Theoretical calculations confirm a negligible SHE and a strong orbital Hall effect that can explain the observed SOTs. We propose a previously unidentified mechanism wherein fluctuating antiferromagnetic moments trigger the generation of substantial orbital currents near the Néel temperature due to the emergence of scalar spin chirality. Our findings present an approach for enhancing SOTs, which holds promise for magnetic memory applications by leveraging antiferromagnetic spin fluctuations to amplify both orbital and spin currents.

cond-mat.mes-hall↗

arXiv2023

Promoting $p$-based Hall effects by $p$-$d$-$f$ hybridization in Gd-based dichalcogenides

We conduct a first-principles study of Hall effects in rare-earth dichalcogenides, focusing on monolayers of the H-phase EuX$_2$ and GdX$_2$, where X = S, Se, and Te. Our predictions reveal that all EuX$_2$ and GdX$_2$ systems exhibit high magnetic moments and wide bandgaps. We observe that while in case of EuX$_2$ the $p$ and $f$ states hybridize directly below the Fermi energy, the absence of $f$ and $d$ states of Gd at the Fermi energy results in $p$-like spin-polarized electronic structure of GdX$_2$, which mediates $p$-based magnetotransport. Notably, these systems display significant anomalous, spin, and orbital Hall conductivities. We find that in GdX$_2$ the strength of correlations controls the relative position of $p$, $d$ and $f$-states and their hybridization which has a crucial impact on $p$-state polarization and the anomalous Hall effect, but not the spin and orbital Hall effect. Moreover, we find that the application of strain can significantly modify the electronic structure of the monolayers, resulting in quantized charge, spin and orbital transport in GdTe$_2$ via a strain-mediated orbital inversion mechanism taking place at the Fermi energy. Our findings suggest that rare-earth dichalcogenides hold promise as a platform for topological spintronics and orbitronics.

cond-mat.mes-hall↗

arXiv2023

Magnetic impurities on superconducting Pb surfaces

It has been predicted theoretically and found experimentally that magnetic impurities induce localized bound states within the superconducting energy gap, called Yu-Shiba-Rusinov (YSR) states. Combining symmetry analysis with experimental findings provides a convincing argument for the energy splitting and distribution of the YSR peaks, but the full details of the electronic structure remain elusive and simple models with point scatterers lack the full orbital complexity required to meet this challenge. In this work we combine a Greens function based first-principles method, which incorporates a phenomenological parameterization of the superconducting state, with orbitally complex impurity potentials to make material-specific predictions of realistic systems. We study the effect of 3d transition elements on the superconducting energy gap of a Pb (001) surface. Not only do we find a good agreement with experiment, we also show that the energetic position, strength and orbital composition of the YSR states depend strongly on the chemical makeup of the impurity and its position with respect to the surface. Such quantitative results cannot be derived from simplified models but require full material specific calculations.

cond-mat.supr-con↗

arXiv2022

Hidden interplay of current-induced spin and orbital torques in bulk Fe$_3$GeTe$_2$

Low crystal symmetry of magnetic van der Waals materials naturally promotes spin-orbital complexity unachievable in common magnetic materials used for spin-orbit torque switching. Here, using first-principles methods, we demonstrate that an interplay of spin and orbital degrees of freedom has a profound impact on spin-orbit torques in a prototype van der Waals ferromagnet: Fe$_3$GeTe$_2$ (FGT). While we show that bulk FGT hosts strong "hidden" current-induced torques harvested by each of its layers, we uncover that their origin alternates between the conventional spin flux torque and the so-called orbital torque as the magnetization direction is varied. A drastic difference in the behavior of the two types of torques results in a non-trivial evolution of switching properties with doping. Our findings promote the design of non-equilibrium orbital properties as the guiding mechanism for crafting the properties of spin-orbit torques in layered van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2022

Skyrmionic Spin Structures in Layered Fe5GeTe2 Up To Room Temperature

The role of the crystal lattice, temperature and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 is a key open question. Using Lorentz transmission electron microscopy, we experimentally observe topological spin structures up to room temperature in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temperature and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of a centrosymmetric crystal structure. In the pre-cooling phase, these bubbles are observable even without the application of an external field, while in the post-cooling phase, a transformation from bubble domains to stripe domains is seen. To understand the magnetic order in Fe5GeTe2 we compare macroscopic magnetometry characterization results with microscopic density functional theory calculation. Our results show that even up to room temperature, topological spin structures can be stabilized in centrosymmetric van der Waals magnets.

cond-mat.mes-hall↗

arXiv2022

Spin and orbital transport in rare earth dichalcogenides: The case of EuS$_2$

We perform first-principles calculations to determine the electronic, magnetic and transport properties of rare-earth dichalcogenides taking a monolayer of the H-phase EuS$_2$ as a representative. We predict that the H-phase of the EuS$_2$ monolayer exhibits a half-metallic behavior upon doping with a very high magnetic moment. We find that the electronic structure of EuS$_2$ is very sensitive to the value of Coulomb repulsion $U$, which effectively controls the degree of hybridization between Eu-$f$ and S-$p$ states. We further predict that the non-trivial electronic structure of EuS$_2$ directly results in a pronounced anomalous Hall effect with non-trivial band topology. Moreover, while we find that the spin Hall effect closely follows the anomalous Hall effect in the system, the orbital complexity of the system results in a very large orbital Hall effect, whose properties depend very sensitively on the strength of correlations. Our findings thus promote rare-earth based dichalcogenides as a promising platform for topological spintronics and orbitronics.

cond-mat.mtrl-sci↗

arXiv2021

Strong bulk spin-orbit torques quantified in the van der Waals ferromagnet Fe3GeTe2

The recent emergence of magnetic van der Waals materials allows for the investigation of current induced magnetization manipulation in two dimensional materials. Uniquely, Fe3GeTe2 has a crystalline structure that allows for the presence of bulk spin-orbit torques (SOTs), that we quantify in a Fe3GeTe2 flake. From the symmetry of the measured torques, we identify the current induced effective fields using harmonic analysis and find dominant bulk SOTs, which arise from the symmetry in the crystal structure. Our results show that Fe3GeTe2 uniquely can exhibit bulk SOTs in addition to the conventional interfacial SOTs enabling magnetization manipulation even in thick single layers without the need for complex multilayer engineering.

cond-mat.mtrl-sci↗

arXiv2021

Full orbital decomposition of Yu-Shiba-Rusinov states based on first principles

We have implemented the Bogoliubov-de Gennes (BdG) equation in a screened Korringa-Kohn- Rostoker (KKR) method for solving, self-consistently, the superconducting state for 3d crystals including substitutional impurities. In this report we extend this theoretical framework to allow for collinear magnetism and apply it to fcc Pb with 3d magnetic impurities. In the presence of magnetic impurities, there is a pair-breaking effect that results in sup-gap Yu-Shiba-Rusinov (YSR) states which we decompose into contributions from the individual orbital character. We determine the spatial extent of these impurity states, showing how the different orbital character affects the details of the YSR states within the superconducting gap. Our work highlights the importance of the first principles based description which captures the quantitative details making direct comparisons possible with experimental findings.

cond-mat.supr-con↗

arXiv2020

Real-space multiple scattering theory for superconductors with impurities

We implement the Bogoliubov-de Gennes (BdG) equation in real-space using the screened Korringa-Kohn-Rostoker (KKR) method. This allows us to solve, self-consistently, the superconducting state for 3d crystals including substitutional impurities with a full normal-state DFT band structure. We apply the theoretical framework to bulk Nb with impurities. Without impurities, Nb has an anisotropic gap structure with two distinct peaks around the Fermi level. In the presence of non-magnetic impurities those peaks are broadened due to the scattering between the two bulk superconducting gaps, however the peaks remain separated. As a second example of self-consistent real-space solutions of the BdG equations we examine superconducting clusters embedded within a non-superconducting bulk metallic host. This allows us to estimate the coherence length of the superconductor and we show that, within our framework, the coherence length of the superconductor is related to the inverse of the gap size, just as in bulk BCS theory.

cond-mat.supr-con↗

arXiv2019

Gap Anisotropy in Multiband Superconductors Based on Multiple Scattering Theory

We implement the Bogoliubov-de Gennes (BdG) equation in a screened Korringa-Kohn-Rostoker (KKR) method for solving, self-consistently, the superconducting state for 3d crystals. This method combines the full complexity of the underlying electronic structure and Fermi surface geometry with a simple phenomenological parametrisation for the superconductivity. We apply this theoretical framework to the known s-wave superconductors Nb, Pb, and MgB$_2$. In these materials multiple distinct peaks at the gap in the density of states were observed, showing significant gap anisotropy which is in good agreement with experiment. Qualitatively, the results can be explained in terms of the k-dependent Fermi velocities on the Fermi surface sheets exploiting concepts from BCS theory.

cond-mat.supr-con↗