arXiv subjects

Carsten Müller

Publications and source records attributed to Carsten Müller.

At least 19 recordsLinked to original sources

arXiv2025

Condensation of slow $\gamma$-quanta in strong magnetic fields

The implications of the root singularity of the vacuum polarization tensor near the first pair creation threshold on blackbody radiation are investigated for magnetic fields above the characteristic scale of quantum electrodynamics. We show that the vacuum birefringence in such a strong background leads to an anisotropic behavior of the Planck radiation law. The thermal spectrum is characterized by a resonance that competes with the Wien maximum, causing a crossover in the low $\gamma$-spectrum of the heat radiation. A light state resembling a many-body condensate with slow motion is linked to the high-temperature phase. This novel state of radiation may coexist with nuclear or quark matter in a neutron star's core, increasing its compactness and influencing its stability.

hep-ph↗

arXiv2025

Optimizing the incident electron momentum for resonant few-photon Kapitza-Dirac scattering in bichromatic laser fields

Nonrelativistic Kapitza-Dirac scattering of electrons from counterpropagating bichromatic laser waves is studied in the resonant Bragg regime, taking the electron spin into account. We show that the intrinsic field-induced detuning, which arises in the Rabi oscillation dynamics between initial and scattering state of the electron, can be compensated by a suitable adjustment of its incident momentum. Analytical formulas of the optimized electron momentum for spin-dependent three-photon and spin-independent four-photon Kapitza-Dirac scattering are obtained from simplified model systems in reduced dimensionality, which preserve the characteristic properties of the process.

physics.atom-ph↗

arXiv2024

Magnetic-field-tunable anisotropic blackbody radiation and condensation of slow thermal light in dynamical axion insulators

Thermal radiation features of dynamical axion insulators, which are characterized by an antiferromagnetic order with simultaneously broken time-reversal and space-inversion symmetries, are investigated. Planck's radiation law is shown to exhibit remarkable anisotropic behavior as a result of the strong dispersion caused by the light-matter interaction. A crossover scenario at low temperature is identified and an associated phase highly populated by slow thermal photons is revealed. We show that the asymmetry degree of the heat radiation and its angular distribution can be controlled via a magnetic field, paving the way toward a directional-tunable mechanism for thermal quantum manipulation and storage. Analogies are drawn with the expected behavior of blackbody radiation in the core of neutron stars.

cond-mat.mes-hall↗

arXiv2023

Light-amplified Landau-Zener conductivity in gapped graphene monolayers: a simulacrum of photo-catalyzed vacuum instability

Interband transitions of electrons in a gapped graphene monolayer are highly stimulated near the Fermi surface when a high-frequency electric wave of weak intensity and a strong constant electric field are superposed in the plane of the flake. We consider this phenomenon equivalent to the Franz-Keldysh effect, paying particular attention to the regime where the photon energy linked to the fast-oscillating field is just below the graphene gap, so that the quantum transitions still occur through tunneling effects while being facilitated by the one-photon absorption channel. In the considered parameter regime the photo-catalyzed current linked to the described setup is shown to exceed the one driven by the strong field solely by several orders of magnitude. Conditions to relieve the impact of the field's finite extension are discussed, and a formula for the residual current density is derived. The robustness of our assessment supports the viability of detecting this phenomenon in graphene, thus providing a simulation of the dynamically-assisted Schwinger mechanism in QED.

cond-mat.mes-hall↗

arXiv2022

Axion Electrodynamics in Strong Magnetic Backgrounds

The overcritical regime of axion-electrodynamics (AED) is investigated. For magnetic fields larger than the characteristic scale linked to AED, quantum vacuum fluctuations due to axion-like fields can dominate over those associated with the electron-positron fields. This hypothetical regime of the dominance of AED over QED is predicted to induce strong birefringence and screening effects. We show that, if the magnetic field lines are curved, extraordinary photons could be canalized along the field direction. It is also shown that the running QED coupling depends on the magnetic field strength, and for certain energy regimes, it could be screened almost to zero, making the QED building blocks very weakly interacting between each other. The impact of this phenomenon on the radiation mechanism of pulsars is discussed.

hep-ph↗

arXiv2022

Non-linear Breit-Wheeler pair production in collisions of bremsstrahlung $γ-$quanta and a tightly focussed laser pulse

Experimental efforts toward the detection of the nonperturbative strong-field regime of the Breit-Wheeler pair creation process plan to combine incoherent sources of GeV $γ$ quanta and the coherent fields of tightly focussed optical laser pulses. This endeavour calls for a theoretical understanding of how the pair yields depend on the applied laser field profile. We provide estimates for the number of produced pairs in a setup where the high-energy radiation is generated via bremsstrahlung. Attention is paid to the role of the transversal and longitudinal focussing of the laser field, along with the incorporation of a Gaussian pulse envelope. We compare our corresponding results with predictions from plane-wave models and determine the parameters of focused laser pulses which maximize the pair yield at fixed pulse energy. Besides, the impact of various super-Gaussian profiles for the laser pulse envelope and its transverse shape is discussed.

hep-ph↗

arXiv2021

Strong-field Breit-Wheeler pair production in $\mathrm{QED}_{2+1}$

The Breit-Wheeler pair production process in $2+1$ dimensional spacetime is investigated. In the perturbative regime, non-vanishing rates at the energy threshold are found when odd numbers of photons take part on the reaction. This behaviour is understood as a direct consequence of the reduced dimensionality and resembles a corresponding prediction made in gapped graphene monolayers. In the non-perturbative strong field regime the effect of the dimensionality manifests itself in a different rate dependence on the quantum non-linearity parameter. The consequence of this deviation is discussed briefly in line with the applicability of perturbation theory. We argue that, in addition to large values of the quantum non-linearity parameter, the superrenormalisable character of quantum electrodynamics in $2+1$ dimensional spacetime might give rise to a breakdown of perturbation theory within certain energy scales.

hep-ph↗

arXiv2020

Linear Breit-Wheeler pair production by high-energy bremsstrahlung photons colliding with an intense X-ray laser pulse

A possible setup for the experimental verification of linear Breit-Wheeler pair creation of electrons and positrons in photon-photon collisions is studied theoretically. It combines highly energetic bremsstrahlung photons, which are assumed to be generated by an incident beam of GeV electrons penetrating through a high-$Z$ target, with keV photons from an X-ray laser field, which is described as a focused Gaussian pulse. We discuss the dependencies of the pair yields on the incident electron energy, target thickness, laser parameters, and collision geometry. It is shown that, for suitable conditions which are nowadays in reach at X-ray laser facilities, the resulting number of created particles seems to be well accessible for enabling the first experimental observation of the linear Breit-Wheeler process $γγ\to e^+e^-$.

hep-ph↗

arXiv2019

Signatures of the Schwinger mechanism assisted by a fast-oscillating electric field

The spontaneous production of electron-positron pairs from the vacuum--in a field configuration composed of a high-frequency electric mode of weak intensity and a strong constant electric field--is investigated. Asymptotic expressions for the single-particle distribution function ruling this nonperturbative process are established by considering the low-density approximation in the Boltzmann-Vlasov equation. An analytical formula for the density rate of yielded particles is established which is shown to manifest a nonperturbative dependence on both the strong and weak electric fields and to interpolate between the tunneling and multi-photon regimes. It is shown that--under appropriate circumstances--the produced plasma of electrons and positrons might reach densities for which their recombinations into high-energy photons occurs copiously. On the basis of this feature, an experimental setup for observing the dynamically-assisted Schwinger effect is put forward.

hep-ph↗

arXiv2018

Simulating dynamically assisted production of Dirac pairs in gapped graphene monolayers

In a vicinity of the Fermi surface, graphene layers with bandgaps allow for closely simulating the vacuum of quantum electrodynamics and, thus, its yet unverified strong-field phenomenology with accessible field strengths. This striking feature is exploited to investigate a plausible materialization of dynamically assisted pair production through the analog production of light but massive pairs of Dirac quasiparticles. The process is considered in a field configuration combining a weak high-frequency electric mode and a strong low-frequency electric field oscillating in time. Its theoretical study is carried out from a quantum kinetic approach, similar to the one governing the spontaneous production of pairs in QED. We show that the presence of the weak assisting mode can strongly increase the number of produced massive Dirac pairs as compared with a setup driven by the strong field only. The efficiency of the process is contrasted, moreover, with the case of gapless graphene to highlight the role played by the quasiparticle mass.

cond-mat.mes-hall↗

arXiv2018

Axion-modified photon propagator, Coulomb potential and Lamb-shift

A consistent renormalization of a quantum theory of axion-electrodynamics requires terms beyond the minimal coupling of two photons to a neutral pseudoscalar field. This procedure is used to determine the self-energy operators of the electromagnetic and the axion fields with an accuracy of second-order in the axion-diphoton coupling. The resulting polarization tensor is utilized for establishing the axion-modified Coulomb potential of a static pointlike charge. In connection, the plausible distortion of the Lamb-shift in hydrogenlike atoms is established and the scopes for searching axionlike particles in high-precision atomic spectroscopy and in experiments of Cavendish-type are investigated. Particularly, we show that these hypothetical degrees of freedom are ruled out as plausible candidates for explaining the proton radius anomaly in muonic hydrogen. A certain loophole remains, though, which is linked to the nonrenormalizable nature of axion-electrodynamics.

hep-ph↗

arXiv2018

Criticality and dominance of axion physics in highly magnetized vacuum

In a constant and homogeneous magnetic background, quantum vacuum fluctuations due to axion-like fields can dominate over those associated with the electron-positron fields. Considering the framework of axion-electrodynamics, the self-energy operator for the electromagnetic field is determined with an accuracy to second-order in the axion-diphoton coupling. This outcome is utilized for establishing modifications to the propagation characteristics of photons and to the Coulomb potential of a static pointlike charge. Notably, in the magnetosphere of a neutron star, the effect of photon capture by the magnetic field, known in QED as relating to gamma-quanta, is extended in axion electrodynamics to include X-ray photons with the result that a specially polarized part of the heat radiation from the surface is canalized along the magnetic field. Besides, for field strengths larger than the critical scale associated with this theory, the modified Coulomb potential is of Yukawa-type in the direction perpendicular to the magnetic field at distances much smaller than the Compton wavelength of an axion, while along the field it follows approximately the Coulomb law at any length scale. We find that at unlimitedly large magnetic fields the longstanding problem -- overcome in QED -- that the ground-state energy of a hydrogen atom is unbounded from below, is reinstated. However, in axion-electrodynamics this unboundedness is cut off because the largest magnetic field treatable within this theory is limited by the unitarity of the associated scattering matrix.

hep-ph↗

arXiv2017

Controlling electron spin dynamics in bichromatic Kapitza-Dirac scattering by the laser field polarization

Spin-dependent Kapitza-Dirac scattering of electron beams from counterpropagating bichromatic laser waves in various polarization geometries is studied. The corresponding scattering probabilities are obtained by analytical and numerical solutions of the time-dependent Dirac equation, assuming a field frequency ratio of 2. When the fundamental field mode is circular-polarized, we show that spin dynamics are generally suppressed at low intensities, but can become distinct at high intensities. Conversely, when a linearly or elliptically polarized fundamental mode is combined with a second harmonic of circular polarization, strong spin effects arise already at low field intensities. In particular, a polarization configuration is identified which acts as a spin filter for free electrons.

physics.atom-ph↗

arXiv2017

Spin-polarizing interferometric beam splitter for free electrons

A spin-polarizing electron beam splitter is described which relies on an arrangement of linearly polarized laser waves of nonrelativistic intensity. An incident electron beam is first coherently scattered off a bichromatic laser field, splitting the beam into two portions, with electron spin and momentum being entangled. Afterwards, the partial beams are coherently superposed in an interferometric setup formed by standing laser waves. As a result, the outgoing electron beam is separated into its spin components along the laser magnetic field, which is shown by both analytical and numerical solutions of Pauli's equation. The proposed laser field configuration thus exerts the same effect on free electrons like an ordinary Stern-Gerlach magnet does on atoms.

quant-ph↗

arXiv2016

Polarization-operator approach to optical signatures of axion-like particles in strong laser pulses

Hypothetical oscillations of probe photons into axion-like particles might be revealed by exploiting the strong fields of high-intensity laser pulses. Considering an arbitrary plane-wave background, we determine the polarization tensor induced by the quantum fluctuations of the axion field and use it to calculate how the polarimetric properties of an initially linear-polarized probe beam are modified. We find that various experimental setups based on contemporary facilities and instrumentation might lead to new exclusion bounds on the parameter space of these particle candidates. The impact of the pulse shape on the discovery potential is studied via a comparison between the cases in which the wave is modulated by a Gaussian envelope and a $\sin^2$ profile. This analysis shows that the upper limits resulting from the ellipticity are relatively insensitive to this change, whereas those arising from the rotation of the polarization plane turn out to be more dependent on the field shape.

hep-ph↗

arXiv2016

Low-dimensional approach to pair production in an oscillating electric field: Application to bandgap graphene layers

The production of particle-antiparticle pairs from the quantum field theoretic ground state in the presence of an external electric field is studied. Starting with the quantum kinetic Boltzmann-Vlasov equation in four-dimensional spacetime, we obtain the corresponding equations in lower dimensionalities by way of spatial compactification. Our outcomes in $2+1$-dimensions are applied to bandgap graphene layers, where the charge carriers have the particular property of behaving like light massive Dirac fermions. We calculate the single-particle distribution function for the case of an electric field oscillating in time and show that the creation of particle-hole pairs in this condensed matter system closely resembles electron-positron pair production by the Schwinger effect.

hep-ph↗

arXiv2016

Strong-Field Breit-Wheeler Pair Production in Two Consecutive Laser Pulses with Variable Time Delay

Photoproduction of electron-positron pairs by the strong-field Breit-Wheeler process in an intense laser field is studied. The laser field is assumed to consist of two consecutive short pulses, with a variable time delay in between. By numerical calculations within the framework of scalar quantum electrodynamics, we demonstrate that the time delay exerts a strong impact on the pair-creation probability. For the case when both pulses are identical, the effect is traced back to the relative quantum phase of the interfering S-matrix amplitudes and explained within a simplified analytical model. Conversely, when the two laser pulses differ from each other, the pair-creation probability depends not only on the time delay but, in general, also on the temporal order of the pulses.

quant-ph↗

arXiv2016

Minicharged particles search by strong laser pulse-induced vacuum polarization effects

Laser-based searches of the yet unobserved vacuum birefringence might be sensitive for very light hypothetical particles carrying a tiny fraction of the electron charge. We show that, with the help of contemporary techniques, polarimetric investigations driven by an optical laser pulse of moderate intensity might allow for excluding regions of the parameter space of these particle candidates which have not been discarded so far by laboratory measurement data. Particular attention is paid to the role of a Gaussian wave profile. It is argued that, at energy regimes in which the vacuum becomes dichroic due to these minicharges, the transmission probability of a probe beam through an analyzer set crossed to the initial polarization direction will depend on both the induced ellipticity as well as the rotation of the initial polarization plane. The weak and strong field regimes, relative to the attributes of these minicharged particles, and the relevance of the polarization of the strong field are investigated.

hep-ph↗