arXiv subjects

Gerrit E. W. Bauer

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

At least 55 records · Page 3Linked to original sources

arXiv2022

Analog quantum control of magnonic cat states on-a-chip by a superconducting qubit

We propose to directly and quantum-coherently couple a superconducting transmon qubit to magnons - the quanta of the collective spin excitations, in a nearby magnetic particle. The magnet's stray field couples to the qubit via a superconducting quantum interference device (SQUID). We predict a resonant qubit-magnon exchange and a nonlinear radiation-pressure interaction that are both stronger than dissipation rates and tunable by an external flux bias. We additionally demonstrate a quantum control scheme that generates qubit-magnon entanglement and magnonic Schrödinger cat states with high fidelity.

quant-ph↗

arXiv2022

Nonlocal Drag Thermoelectricity Generated by Ferroelectric Heterostructures

The \textquotedblleft ferron\textquotedblright\ excitations of the electric-dipolar order carry energy as well as electric dipoles. Here we predict a nonlocal ferron drag effect in a ferroelectric on top of a metallic film: An electric current in the conductor generates a heat current in the ferroelectric by long-range charge-dipole interactions. The non-local Peltier and its reciprocal Seebeck effect can be controlled by electric gates and detected thermographically. We predict large effects for van der Waals ferroelectric films on graphene.

cond-mat.mes-hall↗

arXiv2022

Broadband microwave detection using electron spins in a hybrid diamond-magnet sensor chip

Quantum sensing has developed into a main branch of quantum science and technology. It aims at measuring physical quantities with high resolution, sensitivity, and dynamic range. Electron spins in diamond are powerful magnetic field sensors, but their sensitivity in the microwave regime is limited to a narrow band around their resonance frequency. Here, we realize broadband microwave detection using spins in diamond interfaced with a thin-film magnet. A pump field locally converts target microwave signals to the sensor-spin frequency via the non-linear spin-wave dynamics of the magnet. Two complementary conversion protocols enable sensing and high-fidelity spin control over a gigahertz bandwidth, allowing characterization of the spin-wave band at multiple gigahertz above the sensor-spin frequency. The pump-tunable, hybrid diamond-magnet sensor chip opens the way for spin-based sensing in the 100-gigahertz regime at small magnetic bias fields.

cond-mat.mes-hall↗

arXiv2022

Bright and dark states of two distant macrospins strongly coupled by phonons

We study the collective dynamics of two distant magnets coherently coupled by acoustic phonons that are transmitted through an intercalated crystal. By tuning the ferromagnetic resonances of the two magnets to an acoustic resonance of the crystal, we control a coherent three levels system. We show that the parity of the phonon mode governs the nature of the indirect coupling between the magnets: the resonances with odd / even phonon modes correspond to out-of-phase / in-phase lattice displacements at the magnets, leading to bright / dark states in response to uniform microwave magnetic fields, respectively. The sample is a tri-layer garnet consisting of two thin magnetic layers epitaxially grown on both sides of a half-millimeter thick non-magnetic single crystal. In spite of the relatively weak magneto-elastic interaction, the long lifetimes of the magnon and phonon modes in the sample are the key to unveil this long range strong coupling. This demonstrates that garnets are a great platform to study multi-partite hybridization process between magnon and phonons at microwave frequencies.

cond-mat.mtrl-sci↗

arXiv2022

Excitations of the ferroelectric order

We identify the bosonic excitations in ferroelectrics that carry electric dipoles from the phenomenological Landau-Ginzburg-Devonshire theory. The "ferron" quasi-particles emerge from the concerted action of anharmonicity and broken inversion symmetry. In contrast to magnons, the transverse excitations of the magnetic order, the ferrons in displacive ferroelectrics are longitudinal with respect to the ferroelectric order. Based on the ferron spectrum, we predict temperature dependent pyroelectric and electrocaloric properties, electric-field-tunable heat and polarization transport, and ferron-photon hybridization.

cond-mat.mes-hall↗

arXiv2022

Magnetization dynamics affected by phonon pumping

"Pumping" of phonons by a dynamic magnetization promises to extend the range and functionality of magnonic devices. We explore the impact of phonon pumping on room-temperature ferromagnetic resonance (FMR) spectra of bilayers of thin yttrium iron garnet (YIG) films on thick gadolinium gallium garnet substrates over a wide frequency range. At low frequencies the Kittel mode hybridizes coherently with standing ultrasound waves of a bulk acoustic resonator to form magnon polarons that induce rapid oscillations of the magnetic susceptibility, as reported before. At higher frequencies, the phonon resonances overlap, merging into a conventional FMR line, but with an increased line width. The frequency dependence of the increased line broadening follows the predictions from phonon pumping theory in the thick substrate limit. In addition, we find substantial magnon-phonon coupling of a perpendicular standing spin wave (PSSW) mode. This evidences the importance of the mode overlap between the acoustic and magnetic modes, and provides a route towards engineering the magnetoelastic mode coupling.

cond-mat.mes-hall↗

arXiv2022

Magnon Spectrum of the Amorphous Ferromagnet Co$_4$P from Atomistic Spin Dynamics

An anomaly in the magnon dispersion of the amorphous ferromagnet Co$_{4}$P, often referred to as a `roton-like' excitation, attracted much attention half a century ago. With the current interest in heat and spin currents in amorphous magnets, we apply modern simulation methods, combining reverse Monte Carlo to build the atomic structure and the stochastic Landau-Lifshitz equation for spin dynamics, to re-investigate the magnetic excitation spectrum. We find two magnon valleys, one at the origin and another at a finite wavenumber close to the observations, but without a magnon gap. We conclude that the second dip is due to Umklapp scattering caused by residual long-range order, which may be an alternative explanation of the putative roton excitation. Our study paves the way to study magnon transport in amorphous magnets and related spintronic applications.

cond-mat.mes-hall↗

arXiv2021

Theory of the Magnon Parametron

The 'magnon parametron' is a ferromagnetic particle that is parametrically excited by microwaves in a cavity. Above a certain threshold of the microwave power, a bistable steady state emerges that forms an effective Ising spin. We calculate the dynamics of the magnon parametron as a function of microwave power, applied magnetic field and temperature for the interacting magnon system, taking into account thermal and quantum fluctuations. We predict three dynamical phases, viz. a stable Ising spin, telegraph noise of thermally activated switching, and an intermediate regime that at lower temperatures is quantum correlated with significant distillible magnon entanglement. These three regimes of operation are attractive for alternative computing schemes.

cond-mat.mes-hall↗

arXiv2021

Seebeck Effect in Nanomagnets

We present a theory of the Seebeck effect in nanomagnets with dimensions smaller than the spin diffusion length, showing that the spin accumulation generated by a temperature gradient strongly affects the thermopower. We also identify a correction arising from the transverse temperature gradient induced by the anomalous Ettingshausen effect and an induced spin-heat accumulation gradient. The relevance of these effects for nanoscale magnets is illustrated by ab initio calculations on dilute magnetic alloys.

cond-mat.mes-hall↗

arXiv2021

Magnonics vs. Ferronics

Magnons are the elementary excitations of the magnetic order that carry spin, momentum, and energy. Here we compare the magnon with the ferron, i.e. the elementary excitation of the electric dipolar order that transports polarization and heat in ferroelectrics.

cond-mat.mes-hall↗

arXiv2021

Cavity Magnonics

Cavity magnonics deals with the interaction of magnons - elementary excitations in magnetic materials - and confined electromagnetic fields. We introduce the basic physics and review the experimental and theoretical progress of this young field that is gearing up for integration in future quantum technologies. Much of its appeal is derived from the strong magnon-photon coupling and the easily-reached nonlinear regime in microwave cavities. The interaction of magnons with light as detected by Brillouin light scattering is enhanced in magnetic optical resonators, which can be employed to manipulate magnon distributions. The cavity photon-mediated coupling of a magnon mode to a superconducting qubit enables measurements in the single magnon limit.

cond-mat.mes-hall↗

arXiv2021

Imaging spin-wave damping underneath metals using electron spins in diamond

Spin waves in magnetic insulators are low-damping signal carriers that could enable a new generation of spintronic devices. The excitation, control, and detection of spin waves by metal electrodes is crucial for interfacing these devices to electrical circuits. It is therefore important to understand metal-induced damping of spin-wave transport, but characterizing this process requires access to the underlying magnetic films. Here we show that spins in diamond enable imaging of spin waves that propagate underneath metals in magnetic insulators, and then use this capability to reveal a 100-fold increase in spin-wave damping. By analyzing spin-wave-induced currents in the metal, we derive an effective damping parameter that matches these observations well. We furthermore detect buried scattering centers, highlighting the technique's power for assessing spintronic device quality. Our results open new avenues for studying metal - spin-wave interaction and provide access to interfacial processes such as spin-wave injection via the spin-Hall effect.

cond-mat.mes-hall↗

arXiv2021

Dynamic Magnetoelastic Boundary Conditions and the Pumping of Phonons

We derive boundary conditions at the interfaces of magnetoelastic heterostructures under ferromagnetic resonance for arbitrary magnetization directions and interface shapes. We apply our formalism to magnet$\vert$nonmagnet bilayers and magnetic grains embedded in a nonmagnetic thin film, revealing a nontrivial magnetization angle dependence of acoustic phonon pumping.

cond-mat.mes-hall↗

arXiv2021

The Ferroelectric Point Contact

We formulate a scattering theory of polarization and heat transport through a ballistic ferroelectric point contact. We predict a polarization current under either an electric field or a temperature difference that depends strongly on the direction of the ferroelectric order and can be detected by its magnetic stray field and associated thermovoltage and Peltier effect.

cond-mat.mes-hall↗

arXiv2021

Magnon dispersion in bilayers of two-dimensional ferromagnets

We determine magnon spectra of an atomic bilayer magnet with ferromagnetic intra- and both ferro- and anti- ferromagnetic interlayer coupling. Analytic expressions for the full magnon band of the latter case reveal that both exchange interactions govern the fundamental magnon gap. The inter and intralayer magnetic ordering are not independent: the intralayer ferromagnetism stabilizes antiferromagnetic inter-layer order. The topology of these exchange-anisotropy spin models without spin-orbit interaction turns out to be trivial.

cond-mat.mtrl-sci↗

arXiv2021

Equilibrium current vortices in rare-earth-doped simple metals

Dilute alloys of rare earths have played a vital role in understanding magnetic phenomena. Here, we model the ground state of dilute 4f rare-earth impurities in light metals. When the 4f subshells are open (but not half-filled), the spin-orbit coupling imprints a rotational charge current of conduction electrons around rare-earth atoms. The sign and amplitude of the current oscillate similar to the RKKY spin polarization. We compute the observable effect, namely the Oersted field generated by the current vortices and the Knight shift.

cond-mat.mes-hall↗

arXiv2021

Spin-Wave Doppler Shift by Magnon Drag in Magnetic Insulators

The Doppler shift of the quasiparticle dispersion by charge currents is responsible for the critical supercurrents in superconductors and instabilities of the magnetic ground state of metallic ferromagnets. Here we predict an analogous effect in thin films of magnetic insulators in which microwaves emitted by a proximity stripline generate coherent chiral spin currents that cause a Doppler shift in the magnon dispersion. The spin-wave instability is suppressed by magnon-magnon interactions that limit spin currents to values close to but below the threshold for the instability. The spin current limitations by the backaction of magnon currents on the magnetic order should be considered as design parameters in magnonic devices.

cond-mat.mes-hall↗

arXiv2021

A Hopfield neural network in magnetic films with natural learning

Macroscopic spin ensembles possess brain-like features such as non-linearity, plasticity, stochasticity, selfoscillations, and memory effects, and therefore offer opportunities for neuromorphic computing by spintronics devices. Here we propose a physical realization of artificial neural networks based on magnetic textures, which can update their weights intrinsically via built-in physical feedback utilizing the plasticity and large number of degrees of freedom of the magnetic domain patterns and without resource-demanding external computations. We demonstrate the idea by simulating the operation of a 4-node Hopfield neural network for pattern recognition.

cond-mat.dis-nn↗