arXiv subjects

C. Müller

Publications and source records attributed to C. Müller.

At least 19 recordsLinked to original sources

arXiv2026

Coherently Enhanced Cherenkov Radiation by Highly Relativistic and Ultra-Compact Electron Beams

Cherenkov radiation is emitted when charged particles penetrate dielectric media with velocity higher than the speed of light in the medium. Recent developments in plasma-based accelerator research enable creation of highly relativistic electron beams with femtosecond length and tens of micrometer scale radii, at energies of up to several GeV. Here, we consider Cherenkov radiation emitted by such extreme beams penetrating a gas target with refractive index very close to one. We demonstrate coherent enhancement in the emitted Cherenkov radiation up to the optical or even near-ultraviolet regime, which is due to the high beam energy and density. We also explore the dependence of the coherently enhanced Cherenkov radiation spectrum on the beam shape.

physics.atom-ph↗

arXiv2026

Comparative study of nonperturbative electron-positron pair production by intense laser fields colliding with either bremsstrahlung $\gamma$-rays or relativistic ions

It is well known that electron-positron pairs can be created in the strong-field environments formed by (i) a high-intensity laser field and a high-energy $\gamma$-photon (nonlinear Breit-Wheeler process) or (ii) a high-intensity laser field and a nuclear Coulomb field (nonlinear Bethe-Heitler process). Both of these processes are particularly interesting in the interaction regime where the laser field enters nonperturbatively. Various experimental collaborations are currently aiming at detecting for the first time the nonperturbative Breit-Wheeler process, by exploiting high-intensity laser pulses and $\gamma$-ray sources based on bremsstrahlung. In contrast, an experimental observation of the nonlinear, nonperturbative Bethe-Heitler process still lies further ahead in the future because its technical realization appears at present more challenging. Our comparative study shows, however, that the physical properties of the total rates for the processes (i) and (ii) can become remarkably similar when the parameters for the bremsstrahlung-driven nonperturbative Breit-Wheeler process are properly chosen. In this sense, the upcoming experiments on the nonlinear Breit-Wheeler process could also be used to closely ``simulate'' the currently hard to observe nonlinear Bethe-Heitler process.

physics.atom-ph↗

arXiv2026

Muon pair production in electron-positron collisions close to threshold in the presence of a strong laser field

Within the framework of strong-field quantum electrodynamics, we study the creation of muon-antimuon pairs in laser-assisted collisions of electrons with positrons, whose center-of-mass energy is close to the threshold of the process. Our focus lies on incident collision energies slightly below the threshold, where the associated energy gap has to be overcome by multiphoton absorption from the applied high-intensity laser field while the collision occurs. We calculate the cross section of the process and discuss its nonperturbative dependencies on the energy gap and the laser parameters. Three qualitatively different interaction regimes are identified, where the influence of the laser field either has a classical or fully quantum nature. In the latter case, an exponential dependence of the cross section on the collision parameters is found, which resembles the Schwinger effect.

hep-ph↗

arXiv2026

Superconductivity in epitaxial PtSb(0001) thin films

We report superconductivity in epitaxial PtSb(0001) thin films grown on SrF2(111). Electrical transport measurements reveal a superconducting transition at $T_{\mathrm c}=1.72$ K. The field-induced broadening of the transition and the presence of finite upper critical fields are consistent with type-II superconductivity. We determine the resistively defined upper critical fields for magnetic fields applied perpendicular and parallel to the film plane and parameterize their temperature dependence using an anisotropic Ginzburg-Landau approach. For the thickest film ($d=50$ nm), this yields coherence lengths of $\xi_{ab}\approx 55$ nm and $\xi_c\approx 14$ nm. Current-voltage characteristics show sizeable critical currents, with a critical current density reaching $J_{\mathrm c}\approx 6e4$ A/cm$^2$ at 0.5 K. These results establish epitaxial PtSb as a superconducting thin-film platform compatible with lattice-matched heterostructures in the NiAs-type materials family.

cond-mat.supr-con↗

arXiv2025

Bound-free electron-positron pair production in combined Coulomb and constant crossed electromagnetic fields: a Schwinger-like process with intrinsic assistance

The bound-free channel of electron-positron pair production by a highly charged bare ion in the presence of a strong constant crossed electromagnetic field is studied. To calculate the pair production rate, two different methods are applied and compared with each other: (i) a quasiclassical tunneling theory and (ii) a strong-field approximation, both equipped with appropriate Coulomb correction factors. The resulting rate, which depends nonperturbatively on both the Coulomb field of the ion and the constant crossed field, is calculated in a broad range of applied field strengths and nuclear charge numbers. Its functional form resembles the rate for a dynamically assisted Schwinger-like process, with the assistance being provided by the atomic binding energy of the created electron.

physics.atom-ph↗

arXiv2025

Ionization of atoms by dense and compact beams of extreme relativistic electrons

Ionization is one of the basic physical processes, occurring when charged particles penetrate atomic matter. When atoms are bombarded by very dense and compact beams of extreme relativistic electrons, two qualitatively new -- and very efficient -- ionization mechanisms arise: the tunnel or over-barrier ionization and the coherent impact ionization, which are driven by the low- and high-frequency parts, respectively, of the beam field. In these mechanisms significant fractions of the beam electrons act coherently, strongly enhancing the ionization process. They are also very sensitive to the spatiotemporal structure of the beam that can be used for analysing the beam properties.

physics.atom-ph↗

arXiv2025

Relative-phase dependence of dynamically assisted electron-positron pair creation in the superposition of strong oscillating electric-field pulses

Production of electron-positron pairs in the superposition of oscillating electric-field pulses with largely different frequencies is studied, focussing on the impact of relative phases between the pulses. Various field configurations are considered: superpositions of either two or three pulses of equal duration as well as combinations of a long low-frequency and a short high-frequency pulse. We show that the relative phase of superimposed high-frequency modes can exert a sizeable effect on the total numbers of produced pairs, enhancing them by about 10-30% for the considered field parameters.

physics.atom-ph↗

arXiv2025

Single-crystalline CrSb(0001) thin films grown by dc magnetron co-sputtering

The recent discovery of altermagnetism has sparked renewed interest in the growth of epitaxial films of the NiAs-phase polymorph of CrSb. This paper describes the magnetron sputtering-based fabrication and characterization of high-quality single crystalline CrSb(0001) thin films supported by an isostructural nonmagnetic PtSb buffer. X-ray diffraction and scanning transmission electron microscopy show that the films are phase pure and possess a very high crystalline quality (mosaicity ~0.05deg), while also being free of extended crystallographic defects. Both scanning electron microscopy and atomic force microscopy confirm their smooth and homogeneous topography. Additionally, the elemental composition of our films was found to be close to stoichiometric via electron probe microanalysis and x-ray fluorescence. Thus, the developed samples represent an ideal platform for further investigation of the material properties of CrSb.

cond-mat.mtrl-sci↗

arXiv2025

Phenomenological rate formulas for over-barrier ionization of hydrogen and helium atoms in strong constant electric fields

Nonrelativistic over-barrier ionization (OBI) of atoms in strong electric fields is studied, focussing on hydrogen and helium as concrete examples. Our goal is, on the one hand, to develop an intuitive physical picture behind established empirical formulas for the ionization rate. We show that the ionization rate in a near OBI regime can be modelled quantitatively by extending corresponding tunneling rates by the combined action of the Stark effect and a widened electron emission angle. On the other hand, we present analytical rate formulas in a far OBI regime which closely agree with available numerical data. In result, compact rate expressions describing OBI of hydrogen-like and helium atoms in a broad range of applied field strengths are obtained. They can be useful, for example, in numerical laser-plasma simulation codes to describe elementary ionization events.

physics.atom-ph↗

arXiv2025

Entering the overcritical regime of nonlinear Breit-Wheeler pair production in collisions of bremsstrahlung $\gamma$-rays and superintense, tightly focused laser pulses

Near-future high-intensity lasers offer prospects for the observation of nonlinear Breit-Wheeler pair production in an overcritical field regime, where the quantum nonlinearity parameter substantially exceeds unity. This experimentally yet unexplored scenario is envisaged here to be reached via the collision of a tightly focused laser pulse with high-energy bremsstrahlung photons. We calculate the achievable number of pairs in a range of laser intensities around 10$^{23}$ W/cm$^2$ and GeV-energies of the incident bremsstrahlung-generating electron beam. We investigate under which conditions the attenuation of the $\gamma$-beam due to the production process must be taken into account and how much the second generation of created pairs contributes to the total yield. In the considered interaction regime, where the local production rate grows rather moderately with higher field intensities, it is shown that the range of mostly contributing bremsstrahlung frequencies is generally very broad. For sufficiently large values of the quantum nonlinearity parameter, an optimum domain of frequencies emerges which is located far below the spectral endpoint. Furthermore, we show that it is beneficial for achieving the optimum pair yield to increase the interaction volume by a wider laser focus at the expense of decreased field intensity.

hep-ph↗

arXiv2024

Double interatomic Coulombic electron capture induced by a single incident electron in two- and three-center atomic systems

In the process of interatomic Coulombic electron capture, an incident free electron is captured at an atomic center $A$ and the transition energy is transferred radiationlessly over a rather large distance to a neighboring atom $B$ of different species, upon which the latter is ionized. We consider two-step cascade processes where the electron emitted from atom $B$ is subsequently captured as well, either at center $A$ or yet another atomic center $C$, leading to emission of a second electron from one of the centers. We derive formulas for the cross section of this double interatomic Coulombic electron capture and discuss the relevance of this process, which leads to a substantial rearrangement of the electronic configuration, in various two- and three-center atomic systems.

physics.atom-ph↗

arXiv2024

Impact of laser focussing and radiation reaction on particle spectra from nonlinear Breit-Wheeler pair production in the nonperturbative regime

Near-future experiments intend to detect strong-field Breit-Wheeler pair creation from the collision between bremsstrahlung bursts containing GeV-$γ$ quanta and high-intensity laser pulses. In this theoretical study, we investigate the influence of laser focusing, radiation reaction and a broad bremsstrahlung $γ$ spectrum on the energy and angular distributions of created pairs. Understanding the role of these inherent reaction attributes provides relevant insights for experimental detection strategies and data interpretation. We show that the inclusion of radiation reaction leads to a narrow energy spectrum of the yielded particles, whose maximum is shifted to substantially lower energies as compared to the case in which radiative energy losses are ignored. The broad bremsstrahlung spectrum also has distinct influence on the particle distributions, whereas the impact of laser focusing turns out to be rather moderate in the considered parameter regime.

hep-ph↗

arXiv2024

Synergistic Effect of Multi-Walled Carbon Nanotubes and Ladder-Type Conjugated Polymers on the Performance of N-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have the potential to revolutionize the field of organic bioelectronics. To date, most of the reported OECTs include p-type (semi-)conducting polymers as the channel material, while n-type OECTs are yet at an early stage of development, with the best performing electron-transporting materials still suffering from low transconductance, low electron mobility, and slow response time. Here, the high electrical conductivity of multi-walled carbon nanotubes (MWCNTs) and the large volumetric capacitance of the ladder-type π-conjugated redox polymer poly(benzimidazobenzophenanthroline) (BBL) are leveraged to develop n-type OECTs with record-high performance. It is demonstrated that the use of MWCNTs enhances the electron mobility by more than one order of magnitude, yielding fast transistor transient response (down to 15 ms) and high uC* (electron mobility x volumetric capacitance) of about 1 F/cmVs. This enables the development of complementary inverters with a voltage gain of > 16 and a large worst-case noise margin at a supply voltage of < 0.6 V, while consuming less than 1 uW of power.

physics.app-ph↗

arXiv2023

Breit interaction overtaking Coulomb force at low energies: an unexpectedly efficient mechanism for ionization in slow collisions

It is generally assumed that ionization in slow collisions of light atomic particles, whose constituents (electrons and nuclei) move with velocities orders of magnitude smaller than the speed of light, is driven solely by the Coulomb force. Here we show, however, that the Breit interaction -- a relativistic correction to the Coulomb interaction between electrons -- can become the main actor when the colliding system couples resonantly to the quantum radiation field. Our results demonstrate that this ionization mechanism can be very efficient in various not too dense physical environments, including stellar plasmas and atomic beams propagating in gases.

physics.atom-ph↗

arXiv2023

Two-center resonant photoionization-excitation driven by combined intra- and interatomic electron correlations

Ionization-excitation of an atom induced by the absorption of a single photon in the presence of a neighbouring atom is studied. The latter is, first, resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its ionization with simultaneous excitation. The process relies on the combined effects of interatomic and intraatomic electron correlations. Under suitable conditions, it can dominate by several orders of magnitude over direct photoionization-excitation and even over direct photoionization. In addition, we briefly discuss another kind of two-center resonant photoionization with excitation where the ionization and residual excitation in the final state are located at different atomic sites.

physics.atom-ph↗

arXiv2023

Dynamically-assisted nonlinear Breit-Wheeler pair production in bichromatic laser fields of circular polarization

Production of electron-positron pairs by a high-energy $γ$ photon and a bichromatic laser wave is considered where the latter is composed of a strong low-frequency and a weak high-frequency component, both with circular polarization. An integral expression for the production rate is derived that accounts for the strong laser mode to all orders and for the weak laser mode to first order. The structure of this formula resembles the well-known expression for the nonlinear Breit-Wheeler process in a strong laser field, but includes the dynamical assistance from the weak laser mode. We analyze the dependence of the dynamical rate enhancement on the applied field parameters and show, in particular, that it is substantially higher when the two laser modes have opposite helicity.

hep-ph↗

arXiv2023

Electron-positron pair creation in the superposition of two oscillating electric field pulses with largely different frequency, duration and relative positioning

Production of electron-positron pairs in two oscillating strong electric field pulses with largely different frequencies and durations is considered. In a first scenario, the influence of a low-frequency background field on pair production by a short main pulse of high frequency is analyzed. The background field is shown to cause characteristic modifications of the momentum spectra of created particles which, in turn, may be used for imaging of the background pulse. In a second scenario, an ultrashort, relatively weak assisting pulse is superimposed onto a strong main pulse. By studying the dependence of the pair production on the field parameters it is shown that duration and relative position of the ultrashort pulse modify the momentum spectra of produced particles in a distinctive way. Both scenarios enable, moreover, to extract partial information about the time periods when pairs with certain momenta are produced predominantly.

physics.atom-ph↗

arXiv2022

Resonantly enhanced interatomic Coulombic electron capture in a system of three atoms

In interatomic Coulombic electron capture, the capture of a free electron at an atomic center is accompanied by the radiationless transfer of the excess energy to a neighboring atom of different species, leading to ionization of the latter. We show that this interatomic process can be strongly enhanced by the presence of an additional third atom, provided the energy of the free-bound capture transition in the first atom is resonant to a dipole-allowed excitation energy in this assisting atom. The relation of the resonantly enhanced three-center electron capture with other processes is discussed, and its dependencies on the incident electron energy and the spatial geometry of the triatomic system are illustrated.

physics.atom-ph↗