arXiv subjects

Georg Schmidt

Publications and source records attributed to Georg Schmidt.

At least 19 recordsLinked to original sources

arXiv2026

Coupling of two individual magnon resonators via a superconducting microwave resonator operating in the strong coupling regime

We demonstrate coherent coupling of ferromagnetic resonance in two individual permalloy stripes via a superconducting coplanar waveguide resonator. The two stripes are placed on top of the resonator with an angle of 50$^{\circ}$ between their respective long axes. This alignment enables us to separately tune their ferromagnetic resonance frequency by exploiting their shape anisotropy by simply rotating the external bias magnetic field in the sample plane. Both individual ferromagnets exhibit strong coupling with the microwave resonator. This method avoids the use of local individual bias coils and facilitates the integration of similar experiments into planar device infrastructures.

cond-mat.mtrl-sci↗

arXiv2026

Spatially-Localized Second Harmonic Generation via Spin Wave Concentration in Patterned YIG Structures

The anisotropic dispersion and inherent nonlinearity of magnetostatic spin waves in thin films and confined structures provide unique opportunities for implementation in next-generation magnonic devices for data and signal processing. A particular challenge is to establish an effective means to locally generate higher harmonics and subsequently exploit them while avoiding extraneous nonlinear losses. Here we demonstrate that deterministically and locally tuning the dispersion relation by geometric confinement through standard patterning processes, allows the creation spatially localized, high-intensity magnons hundreds of $\mu m$ or even further from the excitation source. The local intensity obtained in passive, lithographically patterned YIG funnel structures is sufficient to achieve second harmonic generation in localized regions via conventional magnon scattering processes. We verify these effects are truly nonlinear processes by direct measurement and comparison of the 1-$\omega$ and 2-$\omega$ magnon signals as determined by highly sensitive frequency- and spatially-resolved SNS-MOKE technique. This lays the foundation for using similar devices in future magnon-based infrastructures to localize and enhance sensitivity of readout, downstream magnon-based logic operations, and for other higher harmonic generation-related phenomena and low-power magnonics applications.

cond-mat.mtrl-sci↗

arXiv2026

Strong coupling between quantized magnon modes in a YIG microstucture and microwaves in a superconducting resonator

Strong-coupling experiments based on magnons enable the exploration into on-chip demonstrations involving numerous long-lived excitations. Yttrium iron garnet (YIG) has been considered for decades as a gold standard material for magnonics due to its low-loss magnonic properties. While YIG has successfully demonstrated strong-coupling in macroscopic device geometries, the strong coupling of magnons in truly sub-10 micron YIG structures to date has not yet been realized. This obstacle is due to the difficulty producing large enough effective magnonic mode volume necessary primarily due to thickness limitations of YIG deposition and device fabrication techniques. Here, we demonstrate the use of a microplatelet of YIG, manufactured from a single crystal of YIG via focused ion beam (FIB) techniques, placed on a constricted inductive line of an optimized superconducting lumped element LC resonator to achieve strong coupling between numerous magnon modes and the LC resonator photons. These experimental findings are qualitatively backed by micromagnetic simulations and quantitatively supported by analytical calculations to identify the magnon modes corresponding to the experimentally observed anti-crossings in the microwave transmission signal. Further, we show that these anti-crossings remain even at incredibly low device input powers ($\leq 10$ fW). The fabrication techniques and device geometry enable the deterministic use of numerous confined magnon modes in micron-scale YIG structures for various magnetic field strengths and orientations at substantially reduced device powers. The results here establish a foundational path forward to achieving efficient magnon-based strong-coupling experiments in micron-scale YIG magnetic elements for effective on-chip studies.

cond-mat.mtrl-sci↗

arXiv2023

Shaping THz emission spectra by using sub-wavelength nanopatterned spintronic THz emitters

We show in theory and experiment that in periodically patterned spintronic THz emitters (STE), charge dynamics can modify the emission spectrum in a well-controlled way. Characterization of sub-wavelength patterned STE at frequencies up to 30 THz shows that the STE's emission spectrum systematically changes with emitter size. The spectral intensity exhibits significant reductions at frequencies below 4 THz, accompanied by pronounced dips at around 15 THz and 24 THz. While reducing the STE size enhances the modulation of all features, it does not alter the dip frequencies. The effect originates from the charging of the structure's edges by THz currents, causing a backflow that interferes with the primary current pulse. An analytical model quantitatively reproduces these results and agrees well with control experiments. Our findings enable a detailed investigation of the charge dynamics in STE and provide additional means for controlled shaping of STE emission spectra by nano patterning.

physics.app-ph↗

arXiv2022

Machine Learning guided high-throughput search of non-oxide garnets

Garnets, known since the early stages of human civilization, have found important applications in modern technologies including magnetorestriction, spintronics, lithium batteries, etc. The overwhelming majority of experimentally known garnets are oxides, while explorations (experimental or theoretical) for the rest of the chemical space have been limited in scope. A key issue is that the garnet structure has a large primitive unit cell, requiring an enormous amount of computational resources. To perform a comprehensive search of the complete chemical space for new garnets,we combine recent progress in graph neural networks with high-throughput calculations. We apply the machine learning model to identify the potential (meta-)stable garnet systems before systematic density-functional calculations to validate the predictions. In this way, we discover more than 600 ternary garnets with distances to the convex hull below 100~meV/atom with a variety of physical and chemical properties. This includes sulfide, nitride and halide garnets. For these, we analyze the electronic structure and discuss the connection between the value of the electronic band gap and charge balance.

cond-mat.mtrl-sci↗

arXiv2022

Charge dynamics in spintronic THz emitters

We show that to correctly describe the ultrafast currents in spintronic THz emitters it is necessary to take charge equilibration into account. The charge current which is locally induced by a fs laser pulse and the inverse spin-Hall effect (ISHE) leads to ultrafast charging phenomena at the edge of the illuminated area. Subsequent discharging leads to a current backflow with a delay and a time constant that mainly depends on the conductivity of the emitter. On the one hand, only this delayed charge equilibration allows the detection of the primary current pulse via THz emission because an instantaneous backflow would cancel any far field emission. On the other hand, especially for longer light pulses the backflow can significantly change the emitted spectrum compared to the initial spin-current pulse by suppressing low frequency components. For the analysis of spin physics based on the charge current profile it is important to understand that the timing of the spin current cannot be inferred from the charge current unless the contribution by the ISHE and the backflow can be deconvoluted.

cond-mat.mes-hall↗

arXiv2020

Enhanced THz emission from spintronic Fe/Pt emitters through crystal growth optimization

We investigate the THz emission characteristics of ferromagnetic/non-magnetic metallic heterostructures, focusing on thin Fe/Pt bilayers. In particular, we report on the impact of optimized crystal growth of the epitaxial Fe layers on the THz emission amplitude and spectral bandwidth. We demonstrate an enhancement of the emitted intensity along with an expansion of the emission bandwidth. Both are related to reduced spin scattering and higher interface transmission. Our work provides a pathway for devicing optimal spintronic THz emitters based on epitaxial Fe. It also highlights how THz emission measurements can be utilized to characterize the changes in out-of-equilibrium spin current dynamics in metallic heterostructures, driven by subtle structural refinement.

physics.app-ph↗

arXiv2020

Correlation of interface transmission in THz spintronic emitters with spin mixing conductance in spin pumping experiments

The field of THz spintronics is a novel direction in the research field of spintronics that combines magnetism with optical physics and ultrafast photonics. The experimental scheme of the field involves the use of femtosecond laser pulses to trigger ultrafast spin and charge dynamics in bilayers composed of ferromagnetic (FM) and non-magnetic (NM) thin films where the NM layer features a strong spin-orbit coupling. The key technological and scientific challenges of THz spintronic emitters is to increase their intensity and frequency bandwidth. To achieve this the control of the source of the radiation, namely the transport of the ultrafast spin current is required. However, the transfer of a spin current from a FM to a NM layer is a highly interface-sensitive effect. In this work we study the properties of the spin current transport through the interface measuring the strength of the THz emission and compare it to the effective spin mixing conductance, one of the key concepts in the spin current transport through interfaces. The results show an enhancement of the spin mixing conductance for interfaces with higher degree of epitaxy similarly to the improvement of the THz emission. The proportionality between spin mixing conductance and THz emission can define new directions in engineering the emission of spintronic THz emitters.

physics.app-ph↗

arXiv2020

Periodic metal resonator chains for Surface Enhanced Raman Scattering (SERS)

A periodic arrangement of chains of gold discs shows pronounced plasmonic grating resonances. These have a clear impact on the Surface Enhanced Raman Scattering (SERS)-signal from 4-methylbenzenethiol molecules which form self-assembled monolayers on the gold surface: Besides a clear polarisation dependence the SERS-spectra also exhibit a maximum when the excitation laser wavelength matches the plamonic grating resonance. These features are explained by a combined near and far field coupling of the individual plasmonic dipoles allowing the design of optimized nanostructures for effective SERS-substrates in the future.

physics.optics↗

arXiv2020

Luminescence Enhancement in One-Dimensional Mie-Resonant Arrays

In this paper, we demonstrate the infrared photoluminescence emission from Ge(Si) quantum dots enhanced with collective Mie modes of silicon nanopillars. We show that the excitation of band edge dipolar modes of a linear nanopillar array results in strong reshaping of the photoluminescence spectra. Among other collective modes, the magnetic dipolar mode with the polarization along the array axis contributes the most to the emission spectrum exhibiting an experimentally measured Q-factor of around 500 for an array of 11 pillars. The results belong to the first experimental evidence of light emission enhancement of quantum emitters applying collective Mie resonances and therefore represent an important contribution to the new field of active all-dielectric meta-optics.

physics.optics↗

arXiv2019

Enhancement of spin mixing conductance in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/LaNiO$_{3}$/SrRuO$_{3}$ heterostructures

We investigate spin pumping and the effective spin mixing conductance in heterostructures based on magnetic oxide trilayers composed of La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO), LaNiO$_3$ (LNO), and SrRuO$_3$ (SRO). The heterostructures serve as a model system for an estimation of the effective spin mixing conductance at the different interfaces. Our results show that by introducing a LNO interlayer between LSMO and SRO, the total effective spin mixing conductance increases due to the much more favourable interface of LSMO/LNO with respect to the LSMO/SRO interface. Neverheless, the spin current into the SRO does not decrease because of the spin diffusion length of $\lambda_\text{LNO}\approx$3.3 nm in the LNO. This value is two times higher than that of SRO. Our results show the potential of using oxide interfaces to tune the effective spin mixing conductance in heterostructures and to bring novel functionalities into spintronics by implementing complex oxides.

cond-mat.mes-hall↗

arXiv2019

Fetus: the radar of maternal stress, a cohort study

Objective: We hypothesized that prenatal stress (PS) exerts lasting impact on fetal heart rate (fHR). We sought to validate the presence of such PS signature in fHR by measuring coupling between maternal HR (mHR) and fHR. Study design: Prospective observational cohort study in stressed group (SG) mothers with controls matched for gestational age during screening at third trimester using Cohen Perceived Stress Scale (PSS) questionnaire with PSS-10 equal or above 19 classified as SG. Women with PSS-10 less than 19 served as control group (CG). Setting: Klinikum rechts der Isar of the Technical University of Munich. Population: Singleton 3rd trimester pregnant women. Methods: Transabdominal fetal electrocardiograms (fECG) were recorded. We deployed a signal processing algorithm termed bivariate phase-rectified signal averaging (BPRSA) to quantify coupling between mHR and fHR resulting in a fetal stress index (FSI). Maternal hair cortisol was measured at birth. Differences were assumed to be significant for p value less than 0.05. Main Outcome Measures: Differences for FSI between both groups. Results: We screened 1500 women enrolling 538 of which 16.5 % showed a PSS-10 score equal or above 19 at 34+0 weeks. Fifty five women eventually comprised the SG and n=55 served as CG. Median PSS was 22.0 (IQR 21.0-24.0) in the SG and 9.0 (6.0-12.0) in the CG, respectively. Maternal hair cortisol was higher in SG than CG at 86.6 (48.0-169.2) versus 53.0 (34.4-105.9) pg/mg. At 36+5 weeks, FSI was significantly higher in fetuses of stressed mothers when compared to controls [0.43 (0.18-0.85) versus 0.00 (-0.49-0.18)]. Conclusion: Our findings show a persistent effect of PS affecting fetuses in the last trimester.

q-bio.QM↗

arXiv2019

Spin-wave Confinement and Coupling in Organic-Based Magnetic Nanostructures

Vanadium tetracyanoethylene (V[TCNE]$_\text{x}$) is an organic-based ferrimagnet that exhibits robust magnetic ordering (T$_\text{C}$ of over 600 K), high quality-factor (high-Q) microwave resonance (Q up to 3,500), and compatibility with a wide variety of substrates and encapsulation technologies. Here, we substantially expand the potential scope and impact of this emerging material by demonstrating the ability to produce engineered nanostructures with tailored magnetic anisotropy that serve as a platform for the exploration of cavity magnonics, revealing strongly coupled quantum confined standing wave modes that can be tuned into and out of resonance with an applied magnetic field. Specifically, time-domain micromagnetic simulations of these nanostructures faithfully reproduce the experimentally measured spectra, including the quasi-uniform mode and higher-order spin-wave (magnon) modes. Finally, when the two dominant magnon modes present in the spectra are brought into resonance by varying the orientation of the in-plane magnetic field, we observe anti-crossing behavior indicating strong coherent coupling between these two magnon modes at room temperature. These results position V[TCNE]$_\text{x}$ as a leading candidate for the development of coherent magnonics, with potential applications ranging from microwave electronics to quantum information.

cond-mat.mtrl-sci↗

arXiv2016

Resistive Switching and Voltage Induced Modulation of Tunneling Magnetoresistance in Nanosized Perpendicular Organic Spin Valves

Nanoscale multifunctional perpendicular organic spin valves have been fabricated. The devices based on an La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/Co trilayer show resistive switching of up to 4-5 orders of magnitude and magnetoresistance as high as -70% the latter even changing sign when voltage pulses are applied. This combination of phenomena is typically observed in multiferroic tunnel junctions where it is attributed to magnetoelectric coupling between a ferromagnet and a ferroelectric material. Modeling indicates that here the switching originates from a modification of the La$_{0.7}$Sr$_{0.3}$MnO$_3$ surface. This modification influences the tunneling of charge carriers and thus both the electrical resistance and the tunneling magnetoresistance which occurs at pinholes in the organic layer.

cond-mat.mes-hall↗

arXiv2011

Optimal control of vortex core polarity by resonant microwave pulses

In a vortex-state magnetic nano-disk, the static magnetization is curling in the plane, except in the core region where it is pointing out-of-plane, either up or down leading to two possible stable states of opposite core polarity p. Dynamical reversal of p by large amplitude motion of the vortex core has recently been demonstrated experimentally,raising fundamental interest for potential application in magnetic storage devices. Here we demonstrate coherent control of p by single and double microwave pulse sequences, taking advantage of the resonant vortex dynamics in a perpendicular bias magnetic field. Optimization of the microwave pulse duration required to switch p allows to experimentally infer the characteristic decay time of the vortex core in the large oscillation regime. It is found to be more than twice shorter than in the small oscillation regime, raising the fundamental question of the non-linear behaviour of magnetic dissipation.

cond-mat.mtrl-sci↗

arXiv2008

Bivariate phase-rectified signal averaging

Phase-Rectified Signal Averaging (PRSA) was shown to be a powerful tool for the study of quasi-periodic oscillations and nonlinear effects in non-stationary signals. Here we present a bivariate PRSA technique for the study of the inter-relationship between two simultaneous data recordings. Its performance is compared with traditional cross-correlation analysis, which, however, does not work well for non-stationary data and cannot distinguish the coupling directions in complex nonlinear situations. We show that bivariate PRSA allows the analysis of events in one signal at times where the other signal is in a certain phase or state; it is stable in the presence of noise and impassible to non-stationarities.

physics.data-an↗

arXiv2007

Spin-polarized transport in II-VI magnetic resonant tunneling devices

We investigate electronic transport through II-VI semiconductor resonant tunneling structures containing diluted magnetic impurities. Due to the exchange interaction between the conduction electrons and the impurities, there arises a giant Zeeman splitting in the presence of a moderately low magnetic field. As a consequence, when the quantum well is magnetically doped the current-voltage characteristics shows two peaks corresponding to transport for each spin channel. This behavior is experimentally observed and can be reproduced with a simple tunneling model. The model thus allows to analyze other configurations. First, we further increase the magnetic field, which leads to a spin polarization of the electronic current injected from the leads, thus giving rise to a relative change in the current amplitude. We demonstrate that the spin polarization in the emitter can be determined from such a change. Furthermore, in the case of a magnetically doped injector our model shows a large increase in peak amplitude and a shift of the resonance to higher voltages as the external field increases. We find that this effect arises from a combination of giant Zeeman splitting, 3-D incident distribution and broad resonance linewidth.

cond-mat.mes-hall↗

arXiv2007

Syndrome Decoding of Reed-Solomon Codes Beyond Half the Minimum Distance based on Shift-Register Synthesis

In this paper, a new approach for decoding low-rate Reed-Solomon codes beyond half the minimum distance is considered and analyzed. Unlike the Sudan algorithm published in 1997, this new approach is based on multi-sequence shift-register synthesis, which makes it easy to understand and simple to implement. The computational complexity of this shift-register based algorithm is of the same order as the complexity of the well-known Berlekamp-Massey algorithm. Moreover, the error correcting radius coincides with the error correcting radius of the original Sudan algorithm, and the practical decoding performance observed on a q-ary symmetric channel (QSC) is virtually identical to the decoding performance of the Sudan algorithm. Bounds for the failure and error probability as well as for the QSC decoding performance of the new algorithm are derived, and the performance is illustrated by means of examples.

cs.IT↗