arXiv subjects

M. Schäfer

Publications and source records attributed to M. Schäfer.

17 recordsLinked to original sources

arXiv2026

Photoluminescence of Femtosecond Laser-irradiated Silicon Carbide

Silicon carbide (SiC) is the leading wide-bandgap semiconductor material, providing mature doping and device fabrication. Additionally, SiC hosts a multitude of optically active point defects (color centers) and is relevant for many applications in quantum technologies. A crucial step towards harnessing the full potential of the SiC platform includes technologies to create color centers with defined localization and density, e.g. to facilitate their coupling to nano-photonic structures and to observe cooperative effects. Here, silicon vacancy centers and divacancies stand out as no impurity atom is needed and high-thermal budget annealing steps can be avoided. We characterize the effect of localized, femtosecond laser irradiation of SiC, investigating surface modifications and photoluminescence including Raman spectroscopy and optical lifetime measurements. We employ commercial high-purity, semi-insulating substrates and an industrial grade laser system to explore broader applicability of the method. As a novel approach, we apply femtosecond laser irradiation to SiC substrates with an epitaxial graphene layer and find that the threshold for photoluminescence due to laser treatment is lowered.

cond-mat.mtrl-sci↗

arXiv2025

A renormalizable theory for not-so-light nuclei

We present an improved action for Pionless Effective Field Theory (EFT). Previous formulations of renormalizable nuclear EFTs have encountered instabilities in systems with more than four nucleons. We resolve this issue by introducing a finite interaction range at leading order, which is compensated for in perturbation theory at next-to-leading order. Calculated ground-state energies of $^4$He, $^6$Li, $^{12}$C, and $^{16}$O converge and agree with experiment within theoretical uncertainties. This first successful implementation of systematic renormalizability beyond the lightest nuclei enables not only applications to larger nuclei but also extensions to other EFTs in the strong-coupling regime.

nucl-th↗

arXiv2024

Improved action for contact effective field theory

We present an improved action for renormalizable effective field theories (EFTs) of systems near the two-body unitarity limit. The ordering of EFT interactions is constrained, but not entirely fixed, by the renormalization group. The remaining freedom can be used to improve the theory's convergence, to simplify its applications, and to connect it to phenomenological models. We exemplify the method on a contact theory applied to systems of up to five $^4$He atoms. We solve the EFT at leading order including a subleading interaction that accounts for part of the two-body effective range. We show that the effects of such fake range can be compensated in perturbation theory at next-to-leading order, as long as the fake range is smaller or comparable to the experimental effective range. These results open the possibility of using similar improved actions for other many-body systems.

physics.atm-clus↗

arXiv2023

In-medium $Λ$ isospin impurity from charge symmetry breaking in the ${_Λ^4}{\rm H}-{_Λ^4}{\rm He}$ mirror hypernuclei

The $Λ$ separation energies in the mirror hypernuclei ${_Λ^4}{\rm H}-{_Λ^4}{\rm He}$ exhibit large charge symmetry breaking (CSB). Analyzing this CSB within pionless effective field theory while using partially conserved baryon-baryon SU(3) flavor symmetry, we deduce a $Λ-Σ^0$ induced in-medium admixture amplitude ${\cal A}_{I=1}\approx 1.5\%$ in the dominantly isospin $I=0$ $Λ$ hyperon. Our results confirm the free-space value ${\cal A}^{(0)}_{I=1}$ inferred directly within the SU(3) baryon octet by Dalitz and von-Hippel in 1964 and reaffirmed in a recent QCD+QED lattice calculation. Furthermore, exploring the consequences of SU(3) flavor symmetry on the $Λ$-nucleon interaction, we find that CSB is expected to impact the $S=1$ and $S=0$ spin channels in opposite directions, with the latter dominating by an order of magnitude. These observations explain a recent deduction of $Λ$-nucleon CSB strengths.

nucl-th↗

arXiv2022

Consequences of increased hypertriton binding for $s$-shell $Λ$-hypernuclear systems

Consequences of increasing the binding energy of the hypertriton ground state ${_Λ^3}{\rm H}(J^P={\frac{1}{2}}^+)$ from the emulsion value $B^{\rm EMUL}_Λ({_Λ^3}{\rm H}_{\rm g.s.})$=0.13$\pm$0.05 MeV to the STAR value $B^{\rm STAR}_Λ(^{3}_Λ{\rm H}) = (0.41\pm 0.12 \pm 0.11)$ MeV are studied for $s$-shell hypernuclei within a pionless EFT approach at leading order, constrained by the binding energies of the $0^+$ and $1^+$ ${_Λ^4} {\rm H}$ states. The stochastic variational method is used in bound-state calculations, whereas the inverse analytic continuation in the coupling constant method is used to locate $S$-matrix poles of continuum states. It is found that the $Λnn({\frac{1}{2}}^+)$ resonance becomes broader and less likely to be observed experimentally, whereas the ${_Λ^3}{\rm H}({\frac{3}{2}}^+)$ spin-flip virtual state moves closer to the $Λd$ threshold to become a shallow bound state for specific $ΛN$ interaction strengths. The effect of such a near-threshold ${_Λ^3}{\rm H}({\frac{3}{2}}^+)$ state on femtoscopic studies of $Λ$-deuteron correlations, and its lifetime if bound, are discussed. Increasing $B_Λ({_Λ^3} {\rm H}_{\rm g.s.})$ moderately, up to $\sim$0.5 MeV, hardly affects calculated values of $B_Λ({_Λ^5}{\rm He})$.

nucl-th↗

arXiv2021

Influence of an external electric field on the energy dissipation at the initial stage of laser ablation

A density-dependent two-temperature model is applied to describe laser excitation and the following relaxation processes of silicon in an external electric field. Two approaches on how to describe the effects of the external electric field are presented. The first approach avoids the buildup of internal electric fields due to charge separation by assuming ambipolar diffusion and adds an additional carrier-pair current. In the second approach, electrons and holes are treated separately to account for charge separation and the resulting shielding of the external electric field inside the material. The two approaches are compared to experimental results. Both the first approach and the experimental results show similar tendencies for optimization of laser ablation in the external electric field.

physics.optics↗

arXiv2021

Magnetic-field assisted laser ablation of silicon

Understanding and manipulation of the laser processing quality during the ablation of solids have crucial importance from fundamental and industrial perspectives. Here we have studied the effect of external magnetic field on the micro-material processing of silicon by ultrashort laser pulses. It was found experimentally that such a field directed along the laser beam improves the quality and efficiency of the material removal. Additionally, we observe that the formation of laser-induced periodic surface structures (LIPSS) in a multi-pulse regime is affected by the external magnetic field. Our results open a route towards efficient and controllable ultrafast laser micromachining.

physics.optics↗

arXiv2021

Nature of the $Λnn$ $(J^π=1/2^+, I=1)$ and ${\rm ^3_ΛH^*} (J^π=3/2^+, I=0)$ states

The nature of the $Λnn$ and ${\rm ^3_ΛH^*} (J^π=3/2^+,~I=0)$ states is investigated within a pionless effective field theory at leading order, constrained by the low energy $ΛN$ scattering data and hypernuclear 3- and 4-body data. Bound state solutions are obtained using the stochastic variational method, the continuum region is studied by employing two independent methods - the inverse analytic continuation in the coupling constant method and the complex scaling method. Our calculations yield both the $Λnn$ and ${\rm ^3_ΛH^*}$ states unbound. We conclude that the excited state ${\rm ^3_ΛH^*}$ is a virtual state and the $Λnn$ pole located close to the three-body threshold in a complex energy plane could convert to a true resonance with Re$(E)>0$ for some considered $ΛN$ interactions. Finally, the stability of resonance solutions is discussed and limits of the accuracy of performed calculations are assessed.

nucl-th↗

arXiv2020

$Λ^*$ matter and its stability

We performed calculations of nuclear systems composed solely of $Λ^*$ hyperons, aiming at exploring the possibility of existence of absolutely stable $Λ^*$ matter. We considered $Λ^*$ interaction strengths compatible with the $Λ^*Λ^*$ binding energy $B_{Λ^*Λ^*}$ given by the $\bar{K}N$ interaction model by Yamazaki and Akaishi [1]. We found that the binding energy per $Λ^*$ saturates at values well below 100 MeV for mass number $A\geq120$. The $Λ^*$ matter is thus highly unstable against strong interaction decay.

nucl-th↗

arXiv2020

The Continuum Spectrum of Hypernuclear Trios

The spectrum of hypernuclear trios composed of a $Λ$ baryon and two nucleons is the subject of an ongoing experimental campaign, aiming to study the interaction of the $Λ$ particle with a neutron, and the 3-body $Λ$-nucleon-nucleon force. In this manuscript we utilize baryonic effective field theory at leading order, constrained to reproduce the available low energy light hypernuclear data, to study the continuum spectrum of such hypernuclear trios. Using the complex scaling method and the inverse analytic continuation in the coupling constant method we find the existence of a virtual state in the $Λn p$ $J^π=\frac{3}{2}^{+}$ channel, leading to cross-section enhancement near threshold. For the $Λn n$ $J^π=\frac{1}{2}^{+}$ channel we predict a resonance state. Depending, however, on the value of the $ΛN$ scattering length, the resonance pole moves from the physical to the unphysical complex energy sheet within the experimental bounds.

nucl-th↗

arXiv2019

Hyperbolic wavelet analysis of classical isotropic and anisotropic Besov-Sobolev spaces

In this paper we introduce new function spaces which we call anisotropic hyperbolic Besov and Triebel-Lizorkin spaces. Their definition is based on a hyperbolic Littlewood-Paley analysis involving an anisotropy vector only occurring in the smoothness weights. Such spaces provide a general and natural setting in order to understand what kind of anisotropic smoothness can be described using hyperbolic wavelets (in the literature also sometimes called tensor-product wavelets), a wavelet class which hitherto has been mainly used to characterize spaces of dominating mixed smoothness. A centerpiece of our present work are characterizations of these new spaces based on the hyperbolic wavelet transform. Hereby we treat both, the standard approach using wavelet systems equipped with sufficient smoothness, decay, and vanishing moments, but also the very simple and basic hyperbolic Haar system. The second major question we pursue is the relationship between the novel hyperbolic spaces and the classical anisotropic Besov-Lizorkin-Triebel scales. As our results show, in general, both approaches to resolve an anisotropy do not coincide. However, in the Sobolev range this is the case, providing a link to apply the newly obtained hyperbolic wavelet characterizations to the classical setting. In particular, this allows for detecting classical anisotropies via the coefficients of a universal hyperbolic wavelet basis, without the need of adaption of the basis or a-priori knowledge on the anisotropy.

math.FA↗

arXiv2019

The onset of $ΛΛ$ hypernuclear binding

Binding energies of light, $A\leq 6$, $ΛΛ$ hypernuclei are calculated using the stochastic variational method in a pionless effective field theory (EFT) approach at leading order with the purpose of assessing critically the onset of binding in the strangeness S=-2 hadronic sector. The EFT input in this sector consists of (i) a $ΛΛ$ contact term constrained by the $ΛΛ$ scattering length $a_{ΛΛ}$, using a range of values compatible with $ΛΛ$ correlations observed in relativistic heavy ion collisions, and (ii) a $ΛΛN$ contact term constrained by the only available $A\leq 6$ $ΛΛ$ hypernucler binding energy datum of $^{6}_{ΛΛ}$He. The recently debated neutral three-body and four-body systems $^{3}_{ΛΛ}$n and $^{4}_{ΛΛ}$n are found unbound by a wide margin. A relatively large value of $|a_{ΛΛ}| \gtrsim 1.5$ fm is needed to bind $^{4}_{ΛΛ}$H, thereby questioning its particle stability. In contrast, the particle stability of the $A=5$ $ΛΛ$ hypernuclear isodoublet $^{5}_{ΛΛ} $H--$^{5}_{ΛΛ}$He is robust, with $Λ$ separation energy of order 1 MeV.

nucl-th↗

arXiv2019

On the Stability of $Λ(1405)$ Matter

A hypothesis of absolutely stable strange hadronic matter composed of $Λ(1405)$ baryons, here denoted $Λ^*$, is tested within many-body calculations performed using the Relativistic Mean-Field approach. In our calculations, we employed the $Λ^*Λ^*$ interaction compatible with the $Λ^*Λ^*$ binding energy $B_{Λ^*Λ^*}=40$~MeV given by the phenomenological energy-independent $\bar{K}N$ interaction model by Yamazaki and Akaishi (YA). We found that the binding energy per $Λ^*$, as well as the central density in $Λ^*$ many-body systems saturates for mass number $A\geq120$, leaving $Λ^*$ aggregates highly unstable against strong interaction decay. Moreover, we confronted the YA interaction model with kaonic atom data and found that it fails to reproduce the $K^-$ single-nucleon absorption fractions at rest from bubble chamber experiments.

nucl-th↗

arXiv2014

Ray-tracing in pseudo-complex General Relativity

Motivated by possible observations of the black hole candidate in the center of our galaxy and the galaxy M87, ray-tracing methods are applied to both standard General Relativity (GR) and a recently proposed extension, the pseudo-complex General Relativity (pc-GR). The correction terms due to the investigated pc-GR model lead to slower orbital motions close to massive objects. Also the concept of an innermost stable circular orbit (ISCO) is modified for the pc-GR model, allowing particles to get closer to the central object for most values of the spin parameter $a$ than in GR. Thus, the accretion disk, surrounding a massive object, is brighter in pc-GR than in GR. Iron K$α$ emission line profiles are also calculated as those are good observables for regions of strong gravity. Differences between the two theories are pointed out.

astro-ph.GA↗

arXiv2010

Photonic SUSY Two-Loop Corrections to the Muon Magnetic Moment

Photonic SUSY two-loop corrections to the muon magnetic moment are contributions from diagrams where an additional photon loop is attached to a SUSY one-loop diagram. These photonic corrections are evaluated exactly, extending a leading-log calculation by Degrassi and Giudice. Compact analytical expressions are provided and the numerical behaviour is discussed. The photonic corrections reduce the SUSY one-loop result by 7...9%. The new terms are typically around ten times smaller than the leading logarithms, but they can be larger and have either sign in cases with large SUSY mass splittings. We also provide details on renormalization and regularization and on how to incorporate the photonic corrections into a full SUSY two-loop calculation.

hep-ph↗

arXiv1994

Dilepton Production in Nucleon-Nucleon Interactions

Starting from a realistic one--boson--exchange--model fitted to the amplitudes of elastic nucleon--nucleon scattering and the process $NN\rightarrow NΔ$ we perform a fully relativistic and gauge invariant calculation for the dilepton production in nucleon--nucleon collisions, including the important effect of propagating the $Δ$--resonance. We compare the results of our calculations with the latest experimental data on dilepton production. We also show how to implement various electromagnetic formfactors for the hadrons in our calculations without loosing gauge--invariance and discuss their influence on dilepton spectra.

nucl-th↗

arXiv1993

Pion-nucleus reactions in a microscopic transport model

We analyse pion-nucleus reactions in a microscopic transport model of the BUU type, which propagates nucleons, pions, deltas and N(1440)-resonances explicitly in space and time. In particular we examine pion absorption and inelastic scattering cross sections for pion kinetic energies T(pi) =85-315MeV and various target masses. In general, the mass-dependence of the experimental data is well described for energies up to the delta-resonance (\approx 160 MeV) while the absorption cross sections are somewhat overestimated for the higher energies. In addition we study the possible dynamical effects of delta- and pion-potentials in the medium on various observables as well as alternative models for the in-medium delta-width.

nucl-th↗