arXiv subjects

Manuel Müller

Publications and source records attributed to Manuel Müller.

17 recordsLinked to original sources

arXiv2026

Niobium Titanium Nitride as a High Tensile Stress Material for Nanomechanics

Over the past decades, high-coherence mechanical resonators have been continuously pushed to new limits, using techniques such as dissipation dilution and clamp-tapering to enhance their quality factor beyond intrinsic material limitations. Today, these mechanical resonators are often fabricated from silicon-nitride, silicon-carbide or aluminum. Recently, however, interest in novel material platforms has grown, especially those allowing for the integration within superconducting circuits. Among these, superconducting nitrides stand out as particularly promising due to their high transition temperatures compared to elementary superconductors. Here, we introduce them as nanomechanical resonators and report on the fabrication and characterization of highly stressed, doubly clamped niobium titanium nitride (NbTiN) nanostring resonators. Using optical interferometry, we determine the elastic properties and mechanical quality factor from room temperature to 13 K. We observe high tensile stress up to 0.81 GPa along with a Young's modulus of 181 GPa and an intrinsic mechanical quality factor up to 850. With these favorable mechanical properties, NbTiN constitutes a promising material platform for future applications in cavity electro- and nanomechanics.

quant-ph↗

arXiv2026

Temperature dependence of the magnon-phonon coupling in yttrium iron garnet/gadolinium gallium garnet high overtone bulk acoustic resonators

Weexperimentally study the temperature dependence of the magnon-phonon coupling in a yttrium iron garnet (YIG)/gadolinium gallium garnet (GGG) heterostructure. More specifically, we use broadband ferromagnetic resonance to investigate the magneto-elastic coupling between the Kittel mode of a YIG thin film and the transverse acoustic phonon modes of the YIG/GGG high overtone bulk acoustic wave acoustic resonator for in and out-of-plane field directions in the temperature range between T = 5K and 300K. We find that for a magnetic field applied normal to the film surface, magneto-elastic coupling decreases with decreasing temperature, whereas it increases for the in-plane magnetic field configuration. The observed temperature dependence differs from earlier observations on bulk YIG samples, which might be due to the temperature dependent stress imposed by the GGG substrate.

cond-mat.mtrl-sci↗

arXiv2025

On-chip Dicke-type magnon polaritons in the ultrastrong coupling regime via spatially separated nanomagnets

Quantum electrodynamics lies at the heart of hybrid quantum systems essential for future technologies. The thermodynamic limit of the Dicke model, a fundamental model describing these systems, predicts exotic quantum phenomena, such as equilibrium superradiant phase transitions and ground-state two-mode squeezing. However, the experimental realization of genuine Dicke systems has remained elusive due to the inevitable existence of gauge-invariant self-interaction terms that hinder such phenomena. Here, we report on the on-chip realization of a Dicke-type system utilizing ultrastrong magnetic-dipole interactions between collective excitations in a spatially separated ferromagnetic array and a superconducting resonator, resulting in creation of magnon polaritons. Crucially, this spatially separated architecture allows the cooperative enhancement of the coupling strength without increasing the self-interaction energy. We experimentally confirm the Bloch-Siegert shift, originating from the counter-rotating terms, alongside the suppression of self-excitation terms required to observe critical Dicke physics. Our results establish a versatile platform, which provides the playground to explore quantum collective coupling physics and open pathways towards integrated quantum devices harnessing Dicke physics.

quant-ph↗

arXiv2025

Theory of polarization-dependent phonon pumping in ferromagnetic/non-magnetic bilayers

We develop a theoretical model for polarization-selective phonon pumping induced by magnon-phonon coupling in a ferromagnetic/non-magnetic acoustic bilayer structure, focusing on the effects arising from a misalignment between the magnetic and crystallographic symmetry axes. Our model considers the coupled equations of motion describing uniform magnetization dynamics (the Kittel mode) and elastic waves in both layers, incorporating phonon pumping and boundary conditions at the interface. We show that even small misalignments lift the degeneracy of transverse shear elastic modes, resulting in phononic birefringence characterized by distinct propagation velocities for linearly polarized modes. Furthermore, our analysis reveals that magnon-phonon hybridization gives magnetic-field-dependent properties to otherwise non-magnetic phonons. We show that the polarization transfer between linearly polarized phonons and the circularly polarized Kittel mode can be tuned with an external magnetic field. Our theoretical results quantitatively reproduce recent experimental findings [1].

cond-mat.mes-hall↗

arXiv2024

Planar scanning probe microscopy enables vector magnetic field imaging at the nanoscale

Planar scanning probe microscopy is a recently emerging alternative approach to tip-based scanning probe imaging. It can scan an extended planar sensor, such as a polished bulk diamond doped with magnetic-field-sensitive nitrogen-vacancy (NV) centers, in nanometer-scale proximity of a planar sample. So far, this technique has been limited to optical near-field microscopy, and has required nanofabrication of the sample of interest. Here we extend this technique to magnetometry using NV centers, and present a modification that removes the need for sample-side nanofabrication. We harness this new ability to perform a hitherto infeasible measurement - direct imaging of the three-dimensional vector magnetic field of magnetic vortices in a thin film magnetic heterostructure, based on repeated scanning with NV centers with different orientations within the same scanning probe. Our result opens the door to quantum sensing using multiple qubits within the same scanning probe, a prerequisite for the use of entanglement-enhanced and massively parallel schemes.

cond-mat.mes-hall↗

arXiv2023

Temperature dependence of the magnon-phonon interaction in high overtone bulk acoustic resonator-ferromagnetic thin film hybrids

Tailored magnon-phonon hybrid systems, where high overtone bulk acoustic resonators couple resonantly to the magnonic mode of a ferromagnetic thin film, are considered optimal for the creation of acoustic phonons with a defined circular polarization. This class of devices is therefore ideal for the investigation of phonon propagation properties and assessing their capacity to transport angular momentum in the classical and potentially even in the quantum regime. Here, we study the coupling between the magnons in a ferromagnetic \ch{Co25Fe75} thin film and the transverse acoustic phonons in a bulk acoustic wave resonators formed by the sapphire substrate onto which the film is deposited. Using broadband ferromagnetic resonance experiments as a function of temperature, we investigate the strength of the coherent magnon-phonon interaction and the individual damping rates of the magnons and phonons participating in the process. This demonstrates that this coupled magnon-phonon system can reach a cooperativity $C\approx 1$ at cryogenic temperatures. Our experiments also showcase the potential of strongly coupled magnon-phonon systems for strain sensing applications.

cond-mat.mes-hall↗

arXiv2023

Site-Selective Enhancement of Superconducting Nanowire Single-Photon Detectors via Local Helium Ion Irradiation

Achieving homogeneous performance metrics between nominally identical pixels is challenging for the operation of arrays of superconducting nanowire single-photon detectors (SNSPDs). Here, we utilize local helium ion irradiation to post-process and tune single-photon detection efficiency, switching current, and critical temperature of individual devices on the same chip. For 12nm thick highly absorptive SNSPDs, which are barely single-photon sensitive prior to irradiation, we observe an increase of the system detection efficiency from $< 0.05\,\%$ to $(55.3 \pm 1.1)\,\%$ following irradiation. Moreover, the internal detection efficiency saturates at a temperature of 4.5 K after irradiation with $1800\, \mathrm{ions}\, \mathrm{nm}^{-2}$. For irradiated 10 nm thick detectors we observe a doubling of the switching current (to $20\, μ\mathrm{A}$) compared to 8 nm SNSPDs of similar detection efficiency, increasing the amplitude of detection voltage pulses. Investigations of the scaling of superconducting thin film properties with irradiation up to a fluence of $2600\, \mathrm{ions}\, \mathrm{nm}^{-2}$ revealed an increase of sheet resistance and a decrease of critical temperature towards high fluences. A physical model accounting for defect generation and sputtering during helium ion irradiation is presented and shows good qualitative agreement with experiments.

quant-ph↗

arXiv2023

Chiral phonons and phononic birefringence in ferromagnetic metal - bulk acoustic resonator hybrids

Magnomechanical devices, in which magnetic excitations couple to mechanical vibrations, have been discussed as efficient and broadband microwave signal transducers in the classical and quantum limit. We experimentally investigate the magnetoelastic coupling between the ferromagnetic resonance (FMR) modes in a metallic Co$_{25}$Fe$_{75}$ thin film, featuring ultra-low magnetic damping as well as sizable magnetostriction, and standing transverse elastic phonon modes in sapphire, silicon and gadolinium gallium garnet by performing broadband FMR spectroscopy at cryogenic temperatures. For all these substrate materials, we observe an interaction between the resonant acoustic and magnetic modes, which can be tailored by the propagation direction of the acoustic mode with respect to the crystallographic axes. We identify these phonon modes as transverse shear waves propagating with slightly different velocities with relative magnitudes of $Δv/v\simeq10^{-5}$, i.e., all substrates show phononic birefringence. Upon appropriately choosing the phononic mode, the hybrid magnomechanical system enters the Purcell enhanced coupling regime.

cond-mat.mes-hall↗

arXiv2022

Reduced effective magnetization and damping by slowly-relaxing impurities in strained $γ$-$\mathrm{Fe_2O_3}$ thin films

We study the static and dynamic magnetic properties of epitaxially strained $γ$-$\mathrm{Fe_2O_3}$ (maghemite) thin films grown via pulsed-laser deposition on MgO substrates by SQUID magnetometry and cryogenic broadband ferromagnetic resonance experiments. SQUID magnetometry measurements reveal hysteretic magnetization curves for magnetic fields applied both in- and out of the sample plane. From the magnetization dynamics of our thin films, we find a small negative effective magnetization in agreement with a strain induced perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution of the ferromagnetic resonance-linewidth as function of the microwave frequency and explain this finding with a model based on slowly relaxing impurities, the so-called slow relaxor model. By investigating the magnetization dynamics in our maghemite thin films as a function of frequency and temperature, we can isolate the temperature dependent contribution of the slowly relaxing impurities to the resonance linewidth and, in particular, observe a sign change in the effective magnetization. This finding provides evidence for a transition of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane anisotropy for reduced temperatures.

cond-mat.mes-hall↗

arXiv2021

Magnetic field robust high quality factor NbTiN superconducting microwave resonators

We systematically study the performance of compact lumped element planar microwave $\mathrm{Nb_{70}Ti_{30}N}$ (NbTiN) resonators operating at 5 GHz in external in-plane magnetic fields up to 440 mT, a broad temperature regime from 2.2 K up to 13 K, as well as mK temperatures. For comparison, the resonators have been fabricated on thermally oxidized and pristine, (001) oriented silicon substrates. When operating the resonators in the multi-photon regime at $T=2.2$ K, we find internal quality factors $Q_{\mathrm{int}}\simeq$ $2\cdot10^5$ for NbTiN resonators grown on pristine Si substrates, while resonators grown on thermally oxidized substrates show a reduced value of $Q_{\mathrm{int}}\simeq$ $1\cdot10^4$, providing evidence for additional loss channels for the latter substrate. In addition, we investigate the $Q$-factors of the resonators on pristine Si substrates at millikelvin temperatures to asses their applicability for quantum applications. We find $Q_{\mathrm{int}}\simeq$ $2\cdot10^5$ in the single photon regime and $Q_{\mathrm{int}}\simeq$ $5\cdot10^5$ in the high power regime at $T=7$ mK.

physics.app-ph↗

arXiv2021

Aluminum nitride integration on silicon nitride photonic circuits: a new hybrid approach towards on-chip nonlinear optics

Aluminum nitride (AlN) is an emerging material for integrated quantum photonics due to its large $χ^{(2)}$ nonlinearity. Here we demonstrate the hybrid integration of AlN on silicon nitride (SiN) photonic chips. Composite microrings are fabricated by reactive DC sputtering of c-axis oriented AlN on top of pre-patterned SiN. This new approach does not require any patterning of AlN and depends only on reliable SiN nanofabrication. This simplifies the nanofabrication process drastically. Optical characteristics, such as the quality factor, propagation losses and group index, are obtained. Our hybrid resonators can have a one order of magnitude increase in quality factor after the AlN integration, with propagation losses down to \SI{0.7}{dB/cm}. Using finite-element simulations, phase matching in these waveguides is explored.

physics.optics↗

arXiv2021

Magnon transport in $\mathrm{\mathbf{Y_3Fe_5O_{12}}}$/Pt nanostructures with reduced effective magnetization

For applications making use of magnonic spin currents damping effects, which decrease the spin conductivity, have to be minimized. We here investigate the magnon transport in an yttrium iron garnet thin film with strongly reduced effective magnetization. We show that in a three-terminal device the effective magnon conductivity can be increased by a factor of up to six by a current applied to a modulator electrode, which generates damping compensation above a threshold current. Moreover, we find a linear dependence of this threshold current on the applied magnetic field. We can explain this behavior by the reduced effective magnetization and the associated nearly circular magnetization precession.

cond-mat.mes-hall↗

arXiv2021

Robust formation of nanoscale magnetic skyrmions in easy-plane thin film multilayers with low damping

We experimentally demonstrate the formation of room-temperature skyrmions with radii of about 25\,nm in easy-plane anisotropy multilayers with interfacial Dzyaloshinskii-Moriya interaction (DMI). We detect the formation of individual magnetic skyrmions by magnetic force microscopy and find that the skyrmions are stable in out-of-plane fields up to about 200 mT. We determine the interlayer exchange coupling as well as the strength of the interfacial DMI. Additionally, we investigate the dynamic microwave spin excitations by broadband magnetic resonance spectroscopy. From the uniform Kittel mode we determine the magnetic anisotropy and low damping $α_{\mathrm{G}} < 0.04$. We also find clear magnetic resonance signatures in the non-uniform (skyrmion) state. Our findings demonstrate that skyrmions in easy-plane multilayers are promising for spin-dynamical applications.

cond-mat.mtrl-sci↗

arXiv2021

Growth optimization of TaN for superconducting spintronics

We have optimized the growth of superconducting TaN thin films on \ch{SiO2} substrates via dc magnetron sputtering and extract a maximum superconducting transition temperature of $T_{\mathrm{c}}=5$ K as well as a maximum critical field $μ_0H_{\mathrm{c2}}=(13.8\pm0.1)$ T. To investigate the impact of spin-orbit interaction in superconductor/ferromagnet heterostructures, we then analyze the magnetization dynamics of both normal state and superconducting TaN/\ch{Ni80Fe20}(Permalloy, Py)-bilayers as a function of temperature using broadband ferromagnetic resonance (bbFMR) spectroscopy. The phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inverse current-induced torques of the bilayers. The results are compared to our previous study on NbN/Py-bilayers. In the normal state of TaN, we detect a positive damping-like current-induced torque $σ_{\mathrm{d}}$ from the inverse spin Hall effect (iSHE) and a small field-like torque $σ_{\mathrm{f}}$ attributed to the inverse Rashba-Edelstein effect (iREE) at the TaN/Py-interface. In the superconducting state of TaN, we detect a negative $σ_{\mathrm{d}}$ attributed to the quasiparticle mediated inverse spin Hall effect (QMiSHE) and the unexpected manifestation of a large positive field-like $σ_{\mathrm{f}}$ of unknown origin matching our previous results for NbN/Py-bilayers.

cond-mat.mes-hall↗

arXiv2020

Development, Processing and Applications of a UV-Curable Polymer with Surface Active Thiol Groups

We present here a novel resist formulation with active thiol groups at the surface. The material is UV curable, and can be patterned at the micro- and nanoscale by UV nanoimprint lithography. The resist formulation development, its processing, patterning and surface characterization are presented here. In addition, a possible application, including its use to modify the electrical properties of graphene devices is shown. The cured material is highly transparent, intrinsically hydrophilic and can be made more hydrophilic following a UV-ozone or an O2 plasma activation. We evaluated the hydrophilicity of the polymer for different polymer formulations and curing conditions. In addition, a protocol for patterning of the polymer in the micro and nanoscale by nanoimprinting is given and preliminary etching rates together with the polymer selectivity are measured. The main characteristic and unique advantage of the polymer is that it has thiol functional groups at the surface and in the bulk after curing. These groups allow for direct surface modifications with thiol-based chemistry e.g., thiol-ene reactions. We prove the presence of the thiol groups by Raman spectroscopy and perform a thiol-ene reaction to show the potential of the easy click chemistry. This opens the way for very straightforward surface chemistry on nanoimprinted polymer samples. Furthermore, we show how the polymer improves the electrical properties of a graphene field effect transistor, allowing for optimal performance at ambient conditions.

cond-mat.mtrl-sci↗

arXiv2020

Temperature-dependent spin-transport and current-induced torques in superconductor/ferromagnet heterostructures

We investigate the injection of quasiparticle spin currents into a superconductor via spin pumping from an adjacent FM layer.$\;$To this end, we use NbN/\ch{Ni80Fe20}(Py)-heterostructures with a Pt spin sink layer and excite ferromagnetic resonance in the Py-layer by placing the samples onto a coplanar waveguide (CPW). A phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inductive coupling strength between sample and CPW, interpreted in terms of inverse current-induced torques, in our heterostructures as a function of temperature. Below the superconducting transition temperature $T_{\mathrm{c}}$, we observe a suppression of the damping-like torque generated in the Pt layer by the inverse spin Hall effect (iSHE), which can be understood by the changes in spin current transport in the superconducting NbN-layer. Moreover, below $T_{\mathrm{c}}$ we find a large field-like current-induced torque.

cond-mat.mes-hall↗

arXiv2020

Effect of interfacial oxidation layer in spin pumping experiments on Ni$_{80}$Fe$_{20}$/SrIrO$_3$ heterostructures

SrIrO$_3$ with its large spin-orbit coupling and low charge conductivity has emerged as a potential candidate for efficient spin-orbit torque magnetization control in spintronic devices. We here report on the influence of an interfacial oxide layer on spin pumping experiments in Ni$_{80}$Fe$_{20}$ (NiFe)/SrIrO$_3$ bilayer heterostructures. To investigate this scenario we have carried out broadband ferromagnetic resonance (BBFMR) measurements, which indicate the presence of an interfacial antiferromagnetic oxide layer. We performed in-plane BBFMR experiments at cryogenic temperatures, which allowed us to simultaneously study dynamic spin pumping properties (Gilbert damping) and static magnetic properties (such as the effective magnetization and magnetic anisotropy). The results for NiFe/SrIrO$_3$ bilayer thin films were analyzed and compared to those from a NiFe/NbN/SrIrO$_3$ trilayer reference sample, where a spin-transparent, ultra-thin NbN layer was inserted to prevent oxidation of NiFe. At low temperatures, we observe substantial differences in the magnetization dynamics parameters of these samples, which can be explained by an antiferromagnetic interfacial layer in the NiFe/SrIrO$_3$ bilayers.

cond-mat.mes-hall↗