arXiv subjects

Pieter M. Gunnink

Publications and source records attributed to Pieter M. Gunnink.

17 recordsLinked to original sources

arXiv2026

Bilinear magnon--exciton coupling in biased ferromagnetic electron--hole bilayers

The hybridization of magnons and excitons would combine magnetic and optical degrees of freedom in a single composite quasiparticle. Such a hybridization is however difficult to achieve, because of their inherent energy mismatch. We propose that in biased bipolar ferromagnetic electron--hole bilayers the excitons and magnons can be brought into resonance, with the exciton energy lowered through the voltage bias to match the magnon energies. We demonstrate the linear hybridization of magnons and spin-flip excitons in this regime, starting from the microscopic exchange interactions between electrons and localized magnetic moments. We show that further increasing the gate voltage softens the hybrid magnon--exciton mode and realizes a magnon--exciton condensate, which manifests in both the magnon and exciton sectors and is associated with spin-superfluid transport. Ferromagnetic electron--hole bilayers therefore provide a new platform for the study of composite magnon--exciton quasiparticles and the realization of spinful condensates and associated spin superfluidity.

cond-mat.str-el↗

arXiv2026

Magnetic noise of a dark exciton Bose-Einstein condensate

Excitons provide a promising platform for the realization of solid-state Bose-Einstein condensation (BEC), offering quantum coherence, strongly correlated electron-hole physics, and superfluidity. Yet, its unambiguous experimental identification remains challenging. In particular, $S_z= \pm 1$ triplet excitons are excellent candidates to realize an exciton BEC, because of their intrinsically limited recombination rate and thus long lifetimes. However, since their optical detection is inherently forbidden, experimental signatures remain elusive. In this work, we demonstrate that the magnetic nature of a $S_z= \pm 1$ triplet exciton BEC gives rise to stray magnetic field noise, that can be measured using nitrogen-vacancy (NV) center magnetometry. Using an external magnetic field to tune the system from an antiferromagnetic to a ferromagnetic ordering, the longitudinal spin sound mode of the BEC softens, bringing the mode into the characteristic gigahertz frequency range of the NV spin relaxation rate and thus allowing direct detection. Furthermore, we demonstrate an unconventional UV scaling at small distances $d$ from the sample, owing to the cubic corrections to the sound mode dispersion and damping rate in the form of Beliaev damping. Through this approach, we establish that the spin component of exciton BECs offers new approaches to detect exciton BECs.

cond-mat.mes-hall↗

arXiv2026

Floquet-induced anisotropic magnetoresistance and anomalous Hall effect in 2D $d$-wave altermagnets with Rashba spin-orbit coupling

Altermagnets (AMs) combine momentum-dependent spin splitting with zero net magnetization, making them promising for spintronics. Periodic driving enables dynamic symmetry engineering beyond static, material-specific control. We show that Floquet engineering in 2D $d$-wave AMs with out-of-plane Néel order and Rashba spin-orbit coupling unlocks equilibrium-forbidden transport responses. Monochromatic driving produces purely out-of-plane magnetization, yielding longitudinal anisotropic magnetoresistance (AMR) and an anomalous Hall effect, whereas bichromatic driving generates both in-plane and out-of-plane magnetizations and additionally activates transverse AMR via the second harmonic of the secondary beam. Comparable static magnetic fields would require hundreds of tesla, avoided in Floquet driving. These effects persist across linear, circular, and mixed light polarizations and are tunable via light parameters. Our results establish multi-color Floquet engineering for controlling magnetization and symmetry-protected transport in AMs.

cond-mat.mtrl-sci↗

arXiv2026

Topological Magnon-Plasmon Hybrids

We study magnon-plasmon coupling in effectively two-dimensional stacks of van der Waals layers in the context of the band structure topology. Invoking the quasiparticle approximation, we show that the magnetic dipole coupling between the plasmons in a metallic layer and the magnons in a neighboring magnetic layer gives rise to a Berry curvature. As a result, the hybrid quasiparticles acquire an anomalous velocity, leading to intrinsic anomalous thermal Hall and spin-Nernst effects in ferromagnets and antiferromagnets. We propose magnetic layers supporting skyrmion crystals as a platform to realize chiral magnon-plasmon edge states, inviting the notion of topological magnon-plasmonics.

cond-mat.mes-hall↗

arXiv2026

Surface acoustic wave driven acoustic spin splitter in d-wave altermagnetic thin films

The generation of spin currents is a key challenge in the field of spintronics. We propose using surface acoustic waves (SAWs) to generate spin currents in altermagnetic thin films, thereby realizing an acoustic spin splitter. Altermagnets, characterized by spin-polarized electrons and magnons, provide a versatile platform where SAWs can drive spin currents carried by both charge carriers and magnons. This acoustic spin splitter can be implemented in both metallic and insulating altermagnetic thin films, offering broad material applicability and a novel way to detect the spin splitter effect in insulating altermagnetic thin film. We examine a realistic experimental setup where a heavy metal layer, such as platinum, is used to convert the spin current into a measurable charge current via the inverse spin Hall effect. For representative material parameters, we calculate the expected spin current and the corresponding inverse spin Hall voltage. Furthermore, we demonstrate that tuning the SAW frequency allows for precise control over the spin current, highlighting the versatility and potential of the acoustic spin splitter for future spintronics applications.

cond-mat.mes-hall↗

arXiv2026

Coupling of plasmons to the two-magnon continuum in antiferromagnets

The coupling of magnons and plasmons offers a promising avenue for hybrid quantum systems, facilitating coherent energy and information transfer between magnetic and charge excitations. However, existing mechanisms often depend on spin-orbit coupling or temperature-activated processes, limiting their robustness for low-temperature quantum technologies. Here, we propose a coupling mechanism between plasmons and the two-magnon continuum in antiferromagnetic insulators, which operates at zero temperature and does not require spin-orbit coupling. Using a model system consisting of a two-dimensional electron gas on an insulating antiferromagnetic substrate, we show that the electric field of the plasmons interacts with the magnetically mediated electric polarization in the antiferromagnet, arising from bonds with broken inversion symmetry. This interaction enables a strong coupling to the spin-conserving two-magnon continuum, allowing for efficient hybridization and reaching the ultrastrong coupling regime.

cond-mat.mes-hall↗

arXiv2025

Detection of Geometric Phases in Spin Waves using Nitrogen-Vacancy Centers

Due to their robustness, the implementation of geometric phases provides a reliable and controllable way to manipulate the phase of a spin wave, thereby paving the way towards functional magnonics-based data processing devices. Moreover, geometric phases in spin waves are interesting from a fundamental perspective as they contain information about spin wave band structures and play an important role in magnon Hall effects. In this paper we propose to directly measure geometric phases in spin wave systems using the magnetic field sensing capabilities of nitrogen-vacancy (NV) centers. We demonstrate the general principles of this method on two systems in which spin waves acquire a geometric phase, namely a wire with a magnetic domain wall and a system with position-dependent anisotropy axes, and explicitly show how this phase can be deduced from the NV center signal.

cond-mat.mes-hall↗

arXiv2025

Spin demons in d-wave altermagnets

Demons are a type of plasmons, which consist of out-of-phase oscillations of electrons in different bands. Here, we show that $d$-wave altermagnets, a recently discovered class of collinear magnetism, naturally realize a spin demon, which consists of out-of-phase movement of the two spin species. The spin demon lives outside of the particle-hole continuum of one of the spin species, and is therefore significantly underdamped, reaching quality factors of $>10$. We show that the spin demon carries a magnetic moment, which inherits the $d$-wave symmetry. Finally, we consider both three and two dimensional $d$-wave altermagnets, and show that spin demons exists in both.

cond-mat.str-el↗

arXiv2025

Nonequilibrium effects in spin-torque oscillators

One of the cornerstones of spintronics is the application of a spin-transfer torque to a nanomagnet, driving the magnetization of the nanomagnet into a steady-state precession and realizing a spin-torque oscillator. Such a steady state, sustained by a balance between driving and dissipation, could be a textbook example for a nonequilibrium situation. Nevertheless, most theoretical descriptions of spin-torque oscillators simply assume local equilibrium. Here, based on a simple model, we investigate the relevance of nonequilibrium effects in spin-torque oscillators. We use a nonequilibrium Keldysh description, which allows us to treat the effects of spin relaxation, and find that, in the absence of spin relaxation, persistent precessions of the magnetization are not allowed, if magnetic anisotropies are absent. However, introducing spin relaxation enables persistent precessions, where the strength of the spin relaxation has quantitative and qualitative effects on the magnetization dynamics. In the presence of magnetic anisotropy, we find that persistent precessions are allowed even if spin relaxation is absent, but spin relaxation causes a nonlinear relation between the oscillator power and the applied voltage bias. Finally, we consider an alternative spin relaxation mechanism and study the resulting magnetization precessions, highlighting the importance of understanding the exact nature of the relaxation in nanomagnets.

cond-mat.mes-hall↗

arXiv2024

Electrical Non-Hermitian Control of Topological Magnon Spin Transport

Magnonic topological phases realize chiral edge spin waves that are protected against backscattering, potentially enabling highly efficient spin transport. Here we show that the spin transport through these magnonic chiral edge states can be electrically manipulated by non-Hermitian control. We consider the paradigmatic magnon Haldane model and show that it is transformed into an effective non-Hermitian magnon Chern insulator by including a sublattice-dependent spin-orbit torque. In linear spin-wave theory, this electrically induced torque reduces the damping of the chiral edge magnons along certain edge directions, leading to an enhancement of the spin-wave amplitude. This prediction is confirmed by numerical simulations based on the Landau-Lifshitz-Gilbert equation. For a spin-wave transport setup, in which magnons are excited by a microwave field and detected with a normal metal conductor, we find that the magnon amplification is remarkably robust against disorder, establishing non-Hermitian control as a promising avenue for topological magnonics.

cond-mat.mes-hall↗

arXiv2024

Magnon spin capacitor

In this work we show that a magnon spin capacitor can be realized at a junction between two exchange coupled ferromagnets. In this junction, the buildup of magnon spin over the junction is coupled to the difference in magnon chemical potential, realizing the magnon spin analogue of an electrical capacitor. The relation between magnon spin and magnon chemical potential difference directly follows from considering the magnon density-density interaction between the two ferromagnets. We analyse the junction in detail by considering spin injection and detection from normal metal leads, the tunneling current across the junction and magnon decay within the ferromagnet, showing that such a structure realizes a magnon spin capacitor in series with a spin resistor. Choosing yttrium iron garnet as the ferromagnet, we numerically calculate the capacitance, which ranges from picofarad to microfarad, depending on the area of the junction. We therefore conclude that the magnon spin capacitor could directly be of use in applications.

cond-mat.mes-hall↗

arXiv2023

Charge conservation in spin torque oscillators leads to a self-induced torque

Spin torque oscillators are conventionally described by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, at the onset of oscillations, the predictions of the conventional LLGS equation differ qualitatively from experimental results and thus appear to be incomplete. In this work we show that taking charge conservation into account leads to a previously-overlooked self-induced torque, which modifies the LLGS equation. We show that the self-induced torque originates from the pumping current that a precessing magnetization drives through a magnetic tunnel junction. To illustrate the importance of the self-induced torque, we consider an in-plane magnetized nanopillar, where it gives clear qualitative corrections to the conventional LLGS description.

cond-mat.mes-hall↗

arXiv2023

Zero-frequency chiral magnonic edge states protected by non-equilibrium topology

Topological bosonic excitations must, in contrast to their fermionic counterparts, appear at finite energies. This is a key challenge for magnons, as it prevents straightforward excitation and detection of topologically-protected magnonic edge states and their use in magnonic devices. In this work, we show that in a non-equilibrium state, in which the magnetization is pointing against the external magnetic field, the topologically-protected chiral edge states in a magnon Chern insulator can be lowered to zero frequency, making them directly accessible by existing experimental techniques. We discuss the spin-orbit torque required to stabilize this non-equilibrium state, and show explicitly using numerical Landau-Lifshitz-Gilbert simulations that the edge states can be excited with a microwave field. Finally, we consider a propagating spin wave spectroscopy experiment, and demonstrate that the edge states can be directly detected.

cond-mat.mes-hall↗

arXiv2022

Non-linear dynamics of the non-Hermitian Su-Schrieffer-Heeger model

We numerically determine the robustness of the lasing edge modes in a spin-torque oscillator array that realizes the non-Hermitian Su-Schrieffer-Heeger model. Previous studies found that the linearized dynamics can enter a topological regime in which the edge mode is driven into auto-oscillation, while the bulk dynamics are suppressed. Here we investigate the full non-linear and finite-temperature dynamics, whose understanding is essential for spin-torque oscillators-based applications. Our analysis shows that the lasing edge mode dynamics persist in the non-linear domain for a broad range of parameters and temperatures. We investigate the effects of perturbations relevant to experimental implementations and discuss which ones might be detrimental to the stability of the lasing edge mode. Finally, we map our model onto a photonic model. Our analysis has the potential to shed light onto the dynamics of a plethora of non-Hermitian systems with non-linearities.

cond-mat.mes-hall↗

arXiv2021

Theory for electrical detection of the magnon Hall effect induced by dipolar interactions

We derive the anomalous Hall contributions arising from dipolar interactions to diffusive spin transport in magnetic insulators. Magnons, the carriers of angular momentum in these systems, are shown to have a non-zero Berry curvature, resulting in a measurable Hall effect. For yttrium iron garnet (YIG) thin films we calculate both the anomalous and magnon spin conductivities. We show that for a magnetic field perpendicular to the film the anomalous Hall conductivity is finite. This results in a non-zero Hall signal, which can be measured experimentally using Permalloy strips arranged like a Hall bar on top of the YIG thin film. We show that electrical detection and injection of spin is possible, by solving the resulting diffusion-relaxation equation for a Hall bar. We predict the experimentally measurable Hall coefficient for a range of temperatures and magnetic field strengths. Most strikingly, we show that there is a sign change of the Hall coefficient associated with increasing the thickness of the film.

cond-mat.mes-hall↗

arXiv2020

Artificial oxide heterostructures with non-trivial topology

In the quest for topological insulators with large band gaps, heterostructures with Rashba spin-orbit interactions come into play. Transition metal oxides with heavy ions are especially interesting in this respect. We discuss the design principles for stacking oxide Rashba layers. Assuming a single layer with a two-dimensional electron gas (2DEG) on both interfaces as a building block, a two-dimensional topological insulating phase is present when negative coupling between the 2DEGs exists. When stacking multiple building blocks, a two-dimensional or three-dimensional topological insulator is artificially created, depending on the intra- and interlayer coupling strengths and the number of building blocks. We show that the three-dimensional topological insulator is protected by reflection symmetry, and can therefore be classified as a topological crystalline insulator. In order to isolate the topological states from bulk states, the intralayer coupling term needs to be quadratic in momentum. It is described how such a quadratic coupling could potentially be realized by taking buckling within the layers into account. The buckling, thereby, brings the idea of stacked Rashba system very close to the alternative approach of realizing the buckled honeycomb lattice in [111]-oriented perovskite oxides.

cond-mat.mes-hall↗

arXiv2020

Electrical detection of unconventional transverse spin-currents in obliquely magnetized thin films

In a typical experiment in magnonics, thin films are magnetized in-plane and spin waves only carry angular momentum along their spatial propagation direction. Motivated by the experiments of Bozhko et al. [Phys. Rev. Research 2, 023324 (2020)], we show theoretically that for obliquely magnetized thin films, exchange-dipolar spin waves are accompanied by a transverse spin-current. We propose an experiment to electrically detect this transverse spin-current with Pt strips on top of a YIG film, by comparing the induced spin-current for spin waves with opposite momenta. We predict the relative difference to be of the order $10^{-4}$, for magnetic fields tilted at least $30^{\circ}$ out of plane. This transverse spin-current is the result of the long range dipole-dipole interaction and the inversion symmetry breaking of the interface.

cond-mat.mes-hall↗