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Valentin Ahrens

Publications and source records attributed to Valentin Ahrens.

16 recordsLinked to original sources
arXiv2026

Establishing the Magnetoelastic Origin of Spin-Wave Routing through Focused Ion Beam Patterning

Spin waves are promising information carriers for analog and wave-based computing, where functionality relies on compact and precisely engineered scattering landscapes. Focused ion beam (FIB) irradiation enables such control by locally tailoring the spin-wave dispersion in yttrium iron garnet (YIG). However, a non-monotonic dependence of the spin-wave wavelength on increasing ion dose hinders predictive landscape design. Here, we present an experimentally validated framework that explains this non-monotonic spin-wave steering by linking phenomenological strain-induced anisotropy to its magnetoelastic origin. Irradiation-induced lattice dislocations drive elastic and plastic deformation, which evolve into partial amorphization, each stage contributing distinctly to the dispersion behavior. We combine post-irradiation wet-chemical etching and atomic force microscopy (AFM) to quantify thickness changes, and track the dispersion in etched regions using time-resolved magneto-optical Kerr effect (trMOKE) microscopy. Fitting the data to the Kalinikos--Slavin formalism with an added effective magnetoelastic field isolates contributions from elastic and plastic deformation. Validation is achieved by mapping the deformation evolution onto a three-phase scenario based on SRIM simulations, reproducing the extracted field trends, and by consistent strain tensor and micromagnetic analyses. These results establish a physical basis for FIB-engineered graded-index (GRIN) spin-wave landscapes and magnetoelastically programmable magnonic devices.

physics.app-ph↗
arXiv2023

Secondary Excitation of Spin-Waves: How Electromagnetic Cross-Talk Impacts on Magnonic Devices

This work examines the impact of electromagnetic cross-talk in magnonic devices when using inductive spin-wave (SW) transducers. We present detailed electrical SW spectroscopy measurements showing the signal contributions to be considered in magnonic device design. We further provide a rule of thumb estimation for the cross-talk that is responsible for the secondary SW excitation at the output transducer. Simulations and calibrated electrical characterizations underpin this method. Additionally, we visualize the secondary SW excitation via time-resolved MOKE imaging in the forward-volume configuration in a 100nm Yttrium-Iron-Garnet (YIG) system. Our work is a step towards fast yet robust joint electromagentic-micromagnetic magnonic device design.

physics.app-ph↗
arXiv2022

Experimental Demonstration of a Spin-Wave Lens Designed with Machine Learning

We present the design and experimental realization of a device that acts like a spin-wave lens i.e., it focuses spin waves to a specified location. The structure of the lens does not resemble any conventional lens design, it is a nonintuitive pattern produced by a machine learning algorithm. As a spin-wave design tool, we used our custom micromagnetic solver "SpinTorch" that has built-in automatic gradient calculation and can perform backpropagation through time for spin-wave propagation. The training itself is performed with the saturation magnetization of a YIG film as a variable parameter, with the goal to guide spin waves to a predefined location. We verified the operation of the device in the widely used mumax3 micromagnetic solver, and by experimental realization. For the experimental implementation, we developed a technique to create effective saturation-magnetization landscapes in YIG by direct focused-ion-beam irradiation. This allows us to rapidly transfer the nanoscale design patterns to the YIG medium, without patterning the material by etching. We measured the effective saturation magnetization corresponding to the FIB dose levels in advance and used this mapping to translate the designed scatterer to the required dose levels. Our demonstration serves as a proof of concept for a workflow that can be used to realize more sophisticated spin-wave devices with complex functionality, e.g., spin-wave signal processors, or neuromorphic devices.

physics.app-ph↗
arXiv2022

Spin-Wave Optics in YIG by Ion-Beam Irradiation

We demonstrate direct focused ion beam (FIB) writing as an enabling technology for realizing spin-wave-optics devices. It is shown that ion-beam irradiation changes the characteristics of YIG films on a submicron scale in a highly controlled way, allowing to engineer the magnonic index of refraction adapted to desired applications. This technique does not physically remove material, and allows rapid fabrication of high-quality architectures of modified magnetization in magnonic media with minimal edge damage (compared to more common techniques such as etching or milling). By experimentally showing magnonic versions of a number of optical devices (lenses, gratings, Fourier-domain processors) we envision this technology as the gateway to building magnonic computing devices that rival their optical counterparts in their complexity and computational power.

physics.app-ph↗
arXiv2021

Experimental Demonstration of a Rowland Spectrometer for Spin Waves

We experimentally demonstrate the operation of a spin-wave Rowland spectrometer. In the proposed device geometry, spin waves are coherently excited on a diffraction grating and form an interference pattern that spatially separates spectral components of the incoming signal. The diffraction grating was created by focused-ion-beam irradiation, which was found to locally eliminate the ferrimagnetic properties of YIG, without removing the material. We found that in our experiments spin waves were created by an indirect mechanism, by exploiting nonlinear resonance between the grating and the coplanar waveguide. Our work paves the way for complex spin-wave optic devices -- chips that replicate the functionality of integrated optical devices on a chip-scale.

physics.app-ph↗
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

Controlling Domain-Wall Nucleation in Ta/CoFeB/MgO Nanomagnets via Local Ga+ Ion Irradiation

Comprehensive control of the domain wall nucleation process is crucial for spin-based emerging technologies ranging from random-access and storage-class memories over domain-wall logic concepts to nanomagnetic logic. In this work, focused Ga+ ion-irradiation is investigated as an effective means to control domain-wall nucleation in Ta/CoFeB/MgO nanostructures. We show that analogously to He+ irradiation, it is not only possible to reduce the perpendicular magnetic anisotropy but also to increase it significantly, enabling new, bidirectional manipulation schemes. First, the irradiation effects are assessed on film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Subsequent time-domain nucleation characteristics of irradiated nanostructures reveal substantial increases in the anisotropy fields but surprisingly small effects on the measured energy barriers, indicating shrinking nucleation volumes. Spatial control of the domain wall nucleation point is achieved by employing focused irradiation of pre-irradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from a Stoner-Wohlfarth particle's coherent rotation to depinning from an anisotropy gradient. Dynamic micromagnetic simulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal.

cond-mat.mes-hall↗
arXiv2014

Renormalization-Group Improved Prediction for Higgs Production at Hadron Colliders

We use renormalization-group methods in effective field theory to improve the theoretical prediction for the cross section for Higgs-boson production at hadron colliders. In addition to soft-gluon resummation at NNNLL, we also resum enhanced contributions of the form (C_Aπα_s)^n, which arise in the analytic continuation of the gluon form factor to time-like momentum transfer. This resummation is achieved by evaluating the matching corrections arising at the Higgs-boson mass scale at a time-like renormalization point μ^2<0, followed by renormalization-group evolution to μ^2>0. We match our resummed result to NNLO fixed-order perturbation theory and give numerical predictions for the total production cross section as a function of the Higgs-boson mass. Resummation effects are significant even at NNLO, where our improved predictions for the cross sections at the Tevatron and the LHC exceed the fixed-order predictions by about 13% and 8%, respectively, for m_H=120 GeV. We also discuss the application of our technique to other time-like processes such as Drell-Yan production, e^+ e^- --> hadrons, and hadronic decays of the Higgs boson.

hep-ph↗
arXiv2012

Structure of Infrared Singularities of Gauge-Theory Amplitudes at Three and Four Loops

The infrared divergences of massless n-parton scattering amplitudes can be derived from the anomalous dimension of n-jet operators in soft-collinear effective theory. Up to three-loop order, the latter has been shown to have a very simple structure: it contains pairwise color-dipole interactions among the external partons, governed by the cusp anomalous dimension and a logarithm of the kinematic invariants s_{ij}, plus a possible three-loop correlation involving four particles, which is described by a yet unknown function of conformal cross ratios of kinematic invariants. This function is constrained by two-particle collinear limits and by the known behavior of amplitudes in the high-energy limit. We construct a class of relatively simple functions satisfying these constraints. We also extend the analysis to four-loop order, finding that three additional four-particle correlations and a single five-particle correlation appear, which again are governed by functions of conformal cross ratios. Our results suggest that the dipole conjecture, which states that only two-particle color-dipole correlations appear in the anomalous dimension, may need to be generalized. We present a weaker form of the conjecture, stating that to all orders in perturbation theory corrections to the dipole formula are governed by functions of conformal cross ratios, and are O(1/N_c^2) suppressed relative to the dipole term. If true, this conjecture implies that the cusp anomalous dimension obeys Casimir scaling to all orders in perturbation theory.

hep-ph↗
arXiv2011

The top-pair forward-backward asymmetry beyond NLO

We make use of recent results in effective theory and higher-order perturbative calculations to improve the theoretical predictions of the QCD contribution to the top-quark pair production forward-backward asymmetry at the Tevatron. In particular, we supplement the fixed-order NLO calculation with higher-order corrections from soft gluon resummation at NNLL accuracy performed in two different kinematic schemes, which allows us to make improved predictions for the asymmetry in the $p\bar p$ and $t\bar t$ rest frames as a function of the rapidity and invariant mass of the $t\bar t$ pair. Furthermore, we provide binned results which can be compared with the recent measurements of the forward-backward asymmetry in events with a large pair invariant mass or rapidity difference. Finally, we calculate at NLO+NNLL order the top-quark charge asymmetry at the LHC as a function of a lower rapidity cut-off for the top and antitop quarks.

hep-ph↗
arXiv2011

Precision predictions for the t+t(bar) production cross section at hadron colliders

We make use of recent results in effective theory and higher-order perturbative calculations to improve the theoretical predictions of the top-quark pair production cross section at hadron colliders. In particular, we supplement the fixed-order NLO calculation with higher-order corrections from soft gluon resummation at NNLL accuracy. Uncertainties due to power corrections to the soft limit are estimated by combining results from single-particle inclusive and pair invariant-mass kinematics. We present our predictions as functions of the top-quark mass in both the pole scheme and the MS(bar) scheme. We also discuss the merits of using threshold masses as an alternative, and calculate the cross section with the top-quark mass defined in the 1S scheme as an illustrative example.

hep-ph↗
arXiv2010

Renormalization-Group Improved Predictions for Top-Quark Pair Production at Hadron Colliders

Precision predictions for phenomenologically interesting observables such as the t-tbar invariant mass distribution and forward-backward asymmetry in top-quark pair production at hadron colliders require control over the differential cross section in perturbative QCD. In this paper we improve existing calculations of the doubly differential cross section in the invariant mass and scattering angle by using techniques from soft-collinear effective theory to perform an NNLL resummation of threshold logarithms, which become large when the invariant mass M of the top-quark pair approaches the partonic center-of-mass energy. We also derive an approximate formula for the differential cross section at NNLO in fixed-order perturbation theory, which completely determines the coefficients multiplying the singular plus distributions. We then match our results in the threshold region with the exact results at NLO in fixed-order perturbation theory, and perform a numerical analysis of the invariant mass distribution, the total cross section, and the forward-backward asymmetry. We argue that these are the most accurate predictions available for these observables at present. Using MSTW2008NNLO parton distribution functions (PDFs) along with α_s(M_Z)=0.117 and m_t=173.1 GeV, we obtain for the inclusive production cross sections at the Tevatron and LHC the values σ_{Tevatron} = (6.30 +-0.19 +0.31-0.23) pb and σ_{LHC} = (149 +- 7 +- 8) pb, where the first error results from scale variations while the second reflects PDF uncertainties.

hep-ph↗
arXiv2010

Updated Predictions for Higgs Production at the Tevatron and the LHC

We present updated predictions for the total cross section for Higgs boson production through gluon fusion at hadron colliders. In addition to renormalization-group improvement at next-to-next-to-next-to-leading logarithmic accuracy, we incorporate the two-loop electroweak corrections, which leads to the most precise predictions at present. Numerical results are given for Higgs masses between 115 GeV and 200 GeV at the Tevatron with \sqrt{s}=1.96 TeV and the LHC with \sqrt{s}=7-14 TeV.

hep-ph↗
arXiv2010

Threshold expansion at order alpha_s^4 for the t-tbar invariant mass distribution at hadron colliders

We calculate the leading O(alpha_s^4) contributions to the invariant mass distribution of top-quark pairs produced at the Tevatron and LHC, in the limit where the invariant mass of the t-tbar pair approaches the partonic center-of-mass energy. Our results determine at NNLO in alpha_s the coefficients of all singular plus distributions and scale-dependent logarithms in the differential partonic cross sections for q-qbar, gg -> t-tbar + X. A numerical analysis showing the effects of the NNLO corrections on the central values and scale dependence of the invariant mass distribution is performed. The NNLO corrections are found to significantly enhance the cross section and reduce the perturbative uncertainties compared to the NLO calculation.

hep-ph↗
arXiv2008

Origin of the Large Perturbative Corrections to Higgs Production at Hadron Colliders

The very large K-factor for Higgs-boson production at hadron colliders is shown to result from enhanced perturbative corrections of the form (C_Aπα_s)^n, which arise in the analytic continuation of the gluon form factor to time-like momentum transfer. These terms are resummed to all orders in perturbation theory using the renormalization group. After the resummation, the K-factor for the production of a light Higgs boson at the LHC is reduced to a value close to 1.3.

hep-ph↗