arXiv subjects

Gerrit E. W. Bauer

Publications and source records attributed to Gerrit E. W. Bauer.

At least 19 recordsLinked to original sources

arXiv2026

Observation of nonlocal ferron-drag thermoelectricity

The Peltier effect induces a heat current when a charge current passes through a conductor. Since a charge current is conserved at the junction between two different conductors, the difference between the heat flowing in both conductors for the same charge current leads to heating or cooling of the interface, providing an operating mechanism of solid-state heat pumps. Here, we report observation of heat absorption and release signals even in a junction-free, homogeneous metal when placed in proximity to a ferroelectric insulator. Our experiments using active thermographic imaging techniques confirm the prediction of the ferron-drag effect, i.e., the nonlocal excitation of ferrons, the collective excitation of the ferroelectric order, by conduction electrons in the adjacent metal. We reveal the electric-polarization-direction dependence of the temperature change signals and their unexpected increase with the metal thickness beyond the charge screening length, uncovering additional electron-phonon-ferron interactions in the metal/ferroelectric hybrid structure. The discovery of crosstalk between metals and ferroelectrics via remote ferrons could become both a nuisance and an opportunity for highly integrated circuits with ferroelectric barrier materials and revolutionize the design architecture of thermoelectric devices.

cond-mat.mes-hall↗

arXiv2026

Thermoelectric response of a ferroelectric insulator

Thermoelectric effects enable the conversion between heat and electricity without moving parts. While conventionally associated with mobile charges, we report thermoelectricity caused by bound charges in the form of temperature changes measured by multi-harmonic lock-in thermography of a ferroelectric under an ac electric field. The observed temperature gradient depends on the field-induced displacement current, a Peltier effect in a dielectric material. Its coefficient exceeds 100 V around the ferroelectric-paraelectric phase transition, which is several orders of magnitude greater than reported values in conductors. Our findings uncover previously hidden functionalities of ferroelectric materials for thermal management by directional heat transport in ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Fractional Magnonic Frequency Combs

Magnonic frequency combs (MFCs) are spectacular phenomena in microwave-driven high-quality magnets. Like the equally spaced prongs in a comb, conventional \textit{integer} MFCs are sharp resonances with an equal and constant frequency difference. Here we report \textit{fractional} MFCs in a high-quality magnetic sphere that emerges when adding a low-power, precisely detuned microwave to the main drive that compresses the frequency spacings to a rational fraction of the original comb, generating high-density spectral grids with hundreds of lines. The theoretical analysis finds that parametric three-magnon scattering is the dominant non-linear process that reproduces the observation well. This mechanism is unique to magnets: it does not exist in an optomechanical system, where the Kerr and optical nonlinearities govern comb formation at a much higher power input. Since our platform operates as a frequency ``vernier caliper" with much higher sensitivity than integer MFCs, it has application potential in precision metrology.

cond-mat.mtrl-sci↗

arXiv2026

Dynamical control of random telegraph noise in magnetic tunnel junctions

Faster random telegraph noise (RTN) in magnetic tunnel junctions (MTJs) would be beneficial for probabilistic computing applications. However, the interactions between the macrospin and spin waves with finite wave numbers reduce the RTN attempt frequency. We theoretically show that mode-selective heating and cooling by parametric excitation of Kittel mode or propagating spin waves can substantially increase or decrease the RTN frequency, respectively, and propose a nonlinear cooling mechanism that accelerates the switching dynamics. We outline experimental pathways to characterize the nonlinear processes that maximize the operation speed of MTJ-based probabilistic (p-) bits.

cond-mat.mes-hall↗

arXiv2026

Universal Right-Hand Chirality of Evanescent Vector Fields

A locking between propagation direction $\hat{\bf q}$, spin $\hat{\bf S}$, and surface or interface normal $\hat{\bf n}$ -- a phenomenon broadly termed \textit{chirality} -- pervades evanescent or surface waves in optics, magnetism, plasmonics, and acoustics. Yet, it is not known whether the phenomenon is universal or bound to conditions. Here, we unveil that any \textit{source-free} vector field that propagates along $\hat{\bf q}$ and is evanescent along $\hat{\bf n}$, the existence of a spin automatically enforces a rigid, right-handed locking among $\hat{\bf q}$, $\hat{\bf S}$, and $\hat{\bf n}$, that is characterized by a chirality index $C_{\bf q}\equiv \hat{\bf n}\cdot(\hat{\bf S}\times \hat{\bf q})>0$. For an arbitrary propagation direction defined by $\hat{\bf q}\cdot \hat{\bf S}\equiv η\in [-1,1]$, i.e., not necessarily transverse spin, we derive a fundamental upper bound $C_{\bf q}\le \sqrt{1-η^2}$. The universal absence of left-handed evanescent (source-less) vector fields underscores the fundamental physical non-equivalence of mirror-image configurations -- a principle echoed by parity violation, the natural excess of enantiomers in chiral molecules, and the fixed chirality of DNA.

cond-mat.mes-hall↗

arXiv2026

Spin-caloritronic signatures of soft magnons in bilayer CrSBr

Spin transport in magnetic insulators is often treated by assuming that magnons carry a fixed spin angular momentum of $\hbar$, which does not hold in general, however. Here we calculate the magnon spin angular momentum of a layered antiferromagnet as a function of applied magnetic field and wave vector. We show that the triaxial anisotropy and intralayer dipolar interactions in bilayer CrSBr renormalize the magnon spin angular momentum, which diverges upon field-induced magnon softening. This divergence gives rise to a pronounced peak in the thermal spin Seebeck response and provides a clear spin-caloritronic signature of soft magnons.

cond-mat.mes-hall↗

arXiv2026

Polarization transfer force on ferroelectric domain walls

We investigate the dynamics of ferroelectric textures driven by polarization currents. We show that, ferrons, the quanta of collective polarization excitations, provide an exotic driving mechanism for domain wall (DW) dynamics, compared with their magnonic counterparts. By mapping the linear polarization dynamics of a DW onto a Schrödinger-like problem with a Pöschl-Teller potential, we show that polarization waves are fully transmitted and therefore do not exert a net force on the DW in the linear regime. However, intrinsic nonlinearities give rise to a negative radiation pressure that pulls the DW toward the source. This mechanism allows efficient DW control by optical excitation and temperature gradients with application potential in ferroelectric memory and logic devices.

cond-mat.mtrl-sci↗

arXiv2026

Magnetoelastic conversion in integrated YIG nanostructures

Motivated by the recent proposal of two-step transduction from microwave to optical domain using magnetic and elastic intermediate stages arXiv:2205.05088, we consider the coupling between resonant magnetic and elastic modes within a simple axially-symmetric nanodevice designed to host high-quality-factor acoustic modes: A suspended YIG ring structure supported by a central stem, fabricated from a continuous single-crystal film. We study the modes of the system with our custom finite element solvers. We identify the lowest order ``breathing'' mode of a magnetic vortex and the lowest order elastic breathing mode as having the largest mode overlap. For this pair of modes, the external out-of-plane magnetic bias field is critical for bringing them into resonance; however, we show that at the same time it also affects the strength of the coupling. To counteract this, we optimize the radius of the ring at fixed thickness. For the 100 nm-thick film the resonant coupling is maximized at $g/2π= 8\text{MHz}$ at $R\approx1.7μ\text{m}$, indicating that the overlap integral approaches the idealized limit assumed in previous order-of-magnitude estimates. Our results pave the way for the design of tunable frequency-conversion devices based on magnetoelastics.

cond-mat.mes-hall↗

arXiv2026

Dynamical stability by spin transfer in nearly isotropic magnets

Spin transfer torques (STTs) control magnetisation by electric currents, enabling a range of nano-scale spintronic applications. They can destabilise the equilibrium magnetisation state by counteracting magnetic relaxation. Here, we maximise the STT effect through a dedicated growth-annealing protocol for CoFeB thin films, such that magnetic anisotropies originating from the interface and shape almost cancel each other. The nearly isotropic magnets enable low-current dynamical stabilisation of the magnetisation in the direction opposite to an applied magnetic field, thereby realising a spintronic analogue of the Kapitza pendulum. In an intermediate current regime, the STT drives large magnetisation vector fluctuations that cover the entire Bloch sphere. The continuous variable associated with the stochastic magnetisation direction may serve as a resource for probabilistic computing and neuromorphic hardware. Our results establish isotropic magnets as a platform to study as-yet-uncharted, far-from-equilibrium spin dynamics including anti-magnonics, with promising implications for unconventional computing paradigms.

cond-mat.mes-hall↗

arXiv2025

Polarization Controlled Supercurrent in Ferroelectric Josephson Junction

Josephson junctions are essential devices in superconducting electronics and quantum computing hardware. Here we predict electrical control of the supercurrent in composite superconductor-insulator-ferroelectric-insulator-superconductor (S-I-FE-I-S) Josephson junctions. Inversion symmetry broken by unequal dielectric barrier thicknesses and/or potentials converts ferroelectric polarization reversal into a substantial change of the critical current. With a WKB tunneling model we obtain non-volatile switching of the critical current with on-off efficiency up to 0.9 for physically realistic parameters. This can be achieved by optimizing the thicknesses and potential barriers of the insulating layers, as well as the thickness and dielectric constant of the ferroelectric layer. We also derive a compact linear expression for the critical current valid for small polarizations. Our results identify ferroelectric Josephson junctions as electrically programmable superconducting current switches for cryogenic memory and logic applications.

cond-mat.supr-con↗

arXiv2025

Multiferroic-like Quasiparticles in Ferroelectrics

Multiferroics are materials with coexisting electric and magnetic orders that are of central importance for fundamental research and technological applications. Unfortunately, intrinsic multiferroics that operate at room temperature remain rare due to an apparent incompatibility between magnetism and ferroelectricity. Here we predict that a pure ferroelectric support multiferroic-like quasiparticles, termed ``multiferrons", that simultaneously carry \emph{static} magnetic and electric dipoles. The electric dipole moment emerges from the parity-odd anharmonicity of the ferroelectric dynamics, while the magnetic moment has both paramagnetic and diamagnetic origins generated by circularly polarized transverse fluctuations of the ferroelectric polarization. In contrast to the established ``dynamical multiferroicity" of circularly polarized phonons, which involve only \emph{oscillating} electric dipoles, multiferrons cause, apart from Zeeman and Einstein-de Haas effects, a linear dc Stark response, giant electric-field-tunable second-harmonic generation in the THz-frequency regime, and a finite magnetoelectric cross coupling. Multiferrons open a new route toward nonlinear THz optical applications and offer multiferroic functionalities with simple ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2025

Observing Differential Spin Currents by Resonant Inelastic X-ray Scattering

Controlling spin currents, i.e., the flow of spin angular momentum, in small magnetic devices is the principal objective of spin electronics, a main contender for future energy efficient information technologies. Surprisingly, a pure spin current has never been measured directly since the associated electric stray fields and/or shifts in the non-equilibrium spin-dependent distribution functions are too small for conventional experimental detection methods optimized for charge transport. Here we report that resonant inelastic x-ray scattering (RIXS) can bridge this gap by measuring the spin current carried by magnons -- the quanta of the spin wave excitations of the magnetic order -- in the presence of temperature gradients across a magnetic insulator. This is possible due to the sensitivity of the momentum- and energy-resolved RIXS intensity to minute changes in the magnon distribution under non-equilibrium conditions. We use the Boltzmann equation in the relaxation time approximation to extract transport parameters, such as the magnon lifetime at finite momentum, essential for the realization of magnon spintronics.

cond-mat.mes-hall↗

arXiv2025

Directional entanglement of spin-orbit locked nitrogen-vacancy centers by magnons

We address that the stray magnetic field emitted by the excited quantum states of the nitrogen-vacancy (NV) centers is spin-momentum locked, such that the spin transfer to nearby ferromagnetic nanostructures is unidirectional. This may allow the controlled excitation of propagating magnons by NV centers in diamond. A pair of NV spin qubits exchange virtual magnons in a magnetic nanowire in a chiral manner that leads to directional quantum entanglement. A magnon-based ``quantum-entanglement isolator" should be a useful device in future quantum information technology.

cond-mat.mes-hall↗

arXiv2025

Dipolar-exchange spin waves in thin bilayers

We investigate the dipolar-exchange spin wave spectrum in thin ferromagnetic bilayers with inplane magnetization, incorporating interlayer exchange coupling and intra- and interlayer dipolar interactions. In the continuum approximation we analyze the nonreciprocity of propagating magnetic stray fields emitted by spin waves as a function of the relative orientation of the layer magnetizations that are observable by magnetometry of synthetic antiferromagnets or weakly coupled type-A van der Waals antiferromagnetic bilayers as a function of an applied magnetic field.

cond-mat.mes-hall↗

arXiv2025

Electromagnetic Proximity Effect: Superconducting Magnonics and Beyond

The exchange interaction at interfaces between superconductors (SCs) and ferromagnets (FMs) has been a central topic in condensed matter physics for many decades, starting with the prediction of exotic phases such as the Fulde-Ferrell-Larkin-Ovchinnikov states and leading to the discovery of triplet superconductivity. This review focuses on new phenomena in SC$|$FM heterostructures caused by the \textit{non-contact dipolar interaction} between magnons, i.e., the quanta of spin wave excitations in the ferromagnet, and the superconducting order. A universal non-relativistic spin-orbit coupling locks the polarization and momentum of their evanescent stray magnetic fields and leads to chiral screening by proximate superconductors. The interaction-induced hybrid quasiparticles are magnon-Meissner collective modes, magnon-cooparon, Josephson plasmonic modes, and nodal magnon-photon polaritons. Superconducting and normal metallic gates modulate and control the magnetodipolar interaction and thereby magnetization and energy transport at interfaces and in thin films.

cond-mat.supr-con↗

arXiv2025

Observation of ferron transport in ferroelectrics

Ferroelectrics feature spontaneous electric dipolar order reconfigurable via electric fields. Recent theoretical studies of the collective excitations of this electric dipolar order give rise to the hope that "ferron" quasiparticles may complement the magnons of magnetic materials in information and heat management technologies. Yet direct experimental evidence of ferron transport remains elusive. Here we demonstrate efficient ferron injection and detection enabled by ferromagnetic metal contacts, achieving nonlocal signal transmission over micrometer distances in a prototypical ferroelectric PMN-PT. The transmission efficiency can be switched by external magnetic fields that couple to the contacts and gate electric fields that control the ferron excitations. Ferron-based devices open new power saving strategies that employ ferroelectric materials in a future sustainable information society.

physics.app-ph↗

arXiv2025

Exchange Surface Spin Waves in Type-A van der Waals Antiferromagnets

Surface spin waves in the short-wavelength regime enable ultrafast, nanoscale magnon-based devices. Here we report the emergence of surface spin-wave excitations within the bulk magnon band gap of type-A van der Waals antiferromagnets composed of antiferromagnetically coupled ferromagnetic monolayers. In contrast to the magnetostatic Damon-Eshbach modes in magnetic slabs, these surface waves are pure exchange modes owing to the reduced interlayer exchange coupling at surface layers, and thus persist in ultrathin multilayer stacks and at large wave numbers. We show that they can be efficiently excited by electromagnetic waves, with absorption power comparable to or even exceeding that of bulk modes. Moreover, their magnetic stray fields exhibit pronounced even-odd oscillations with the number of monolayers that should be observable by NV-center magnetometry.

cond-mat.mes-hall↗

arXiv2025

Spin Wave Dispersion of the van der Waals Antiferromagnet NiPS$_3$

We calculate the magnon dispersion spectra of the two-dimensional zigzag van der Waals antiferromagnet NiPS$_3$ for monolayer, bilayer, and bulk systems as a function of an external magnetic field. We calculate the exchange and anisotropy constants in our spin model by first principles. We can accurately explain the transition from a collinear to a canted ground state for a magnetic field applied normal to the (in-plane) easy-axis, and a spin-flop transition when the field is parallel to it. A topologically protected Dirac nodal line is present and robust with respect to both external and anisotropy fields.

cond-mat.mes-hall↗