# Peer-Review Publications

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#### 2021

H. Abramowicz,
U. Acosta,
M. Altarelli,
R. Assmann,
Z. Bai,
T. Behnke,
Y. Benhammou,
T. Blackburn,
S. Boogert,
O. Borysov,
M. Borysova,
R. Brinkmann,
M. Bruschi,
F. Burkart,
K. Büßer,
N. Cavanagh,
O. Davidi,
W. Decking,
U. Dosselli,
N. Elkina,
A. Fedotov,
M. Firlej,
T. Fiutowski,
K. Fleck,
M. Gostkin,
C. Grojean,
J. Hallford,
H. Harsh,
A. Hartin,
B. Heinemann,
T. Heinzl,
L. Helary,
M. Hoffmann,
S. Huang,
X. Huang,
M. Idzik,
A. Ilderton,
R. Jacobs,
B. Kämpfer,
B. King,
H. Lahno,
A. Levanon,
A. Lévy,
I. Levy,
J. List,
W. Lohmann,
T. Ma,
A. J. Macleod,
V. Malka,
F. Meloni,
A. Mironov,
M. Morandin,
J. Moron,
E. Negodin,
G. Perez,
I. Pomerantz,
R. Pöschl,
R. Prasad,
F. Quéré,
A. Ringwald,
C. Rödel,
S. Rykovanov,
F. Salgado,
A. Santra,
G. Sarri,
A. Sävert,
A. Sbrizzi,
S. Schmitt,
U. Schramm,
S. Schuwalow,
D. Seipt,
L. Shaimerdenova,
M. Shchedrolosiev,
M. Skakunov,
Y. Soreq,
M. Streeter,
K. Swientek,
N. Hod,
S. Tang,
T. Teter,
D. Thoden,
A. I. Titov,
O. Tolbanov,
G. Torgrimsson,
A. Tyazhev,
M. Wing,
M. Zanetti,
A. Zarubin,
K. Zeil,
M. Zepf,
and A. Zhemchukov

Eur. Phys. J. Special Topics,

**Conceptual design report for the LUXE experiment**Eur. Phys. J. Special Topics,

**230**:2445 (September 2021)
F. Karbstein

J. High Energ. Phys.,

**Derivative corrections to the Heisenberg-Euler effective action**J. High Energ. Phys.,

**09**:070 (September 2021)## Abstract:

We show that the leading derivative corrections to the Heisenberg-Euler effective action can be determined efficiently from the vacuum polarization tensor evaluated in a homogeneous constant background field. After deriving the explicit parameter-integral representation for the leading derivative corrections in generic electromagnetic fields at one loop, we specialize to the cases of magnetic- and electric-like field configurations characterized by the vanishing of one of the secular invariants of the electromagnetic field. In these cases, closed-form results and the associated all-orders weak-and strong-field expansions can be worked out. One immediate application is the leading derivative correction to the renowned Schwinger-formula describing the decay of the quantum vacuum via electron-positron pair production in slowly-varying electric fields.

B. Ying,
F. Machalett,
V. Huth,
M. Kuebel,
A. Sayler,
T. Stoehlker,
G. Paulus,
and P. Wustelt

J. Phys. B,

**Experimental study of the laser-induced ionization of heavy metal and metalloid ions: Au+ and Si2+ in intense and sculpted femtosecond laser fields**J. Phys. B,

**54**:174002 (September 2021)## Abstract:

We implement a liquid metal ion source in a 3D coincidence momentum spectroscopy setup for studying the interaction of ionic targets with intense laser pulses. Laser intensities of up to 4 . 10(16) W cm(-2) allow for the observation of up to ten-fold ionization of Au+-ions and double ionization of Si2+-ions. Further, by utilizing two-color sculpted laser fields to control the ionization process on the attosecond time scale, we demonstrate the capability to resolve the recoil ion momenta of heavy metal atoms. Simulations based on a semiclassical model assuming purely sequential ionization reproduce the experimental data well. This work opens up the use of a range of metallic and metalloid ions, which have hardly been investigated in strong-field laser physics so far.

A. Mueller,
M. Martins,
J. Borovik,
T. Buhr,
A. Perry-Sassmannshausen,
S. Reinwardt,
F. Trinter,
S. Schippers,
S. Fritzsche,
and A. S. Kheifets

Phys. Rev. A,

**Role of L-shell single and double core-hole production and decay in m-fold (1 <= m <= 6) photoionization of the Ar+ ion**Phys. Rev. A,

**104**:033105 (September 2021)## Abstract:

Multiple ionization of the Ar+(3s(2)3p(5)) ion by a single photon has been investigated in the photon-energy range 250-1800 eV employing the photon-ion merged-beams technique. Absolute partial cross sections were measured for all Ar(1+m)+ product-ion channels with 1 <= m <= 6 covering a size range from several tens of Mb down to a few b. Narrow 2p-subshell excitation resonances were observed in all channels up to quadruple ionization at a photon-energy bandwidth of 52 meV. Double excitations involving a 2p and a 3s or 3p electron were also studied at high resolution and the measurements of the broad 2s excitation resonances directly showed their natural widths. Contributions of direct photo double ionization (PDI) to the production of the highest final Ar ion charge states are revealed, with PDI of the 2s subshell being mainly responsible for the production of Ar7+. The experiment made use of the PIPE setup installed at beamline P04 of the PETRA III synchrotron light source of DESY in Hamburg. The measurements were supported by theoretical calculations to identify the main contributions to the observed cross sections. Comparisons of theory and experiment show remarkable agreement but also hint to additional ionizationmechanisms that are not considered in the theoretical models such as core ionization accompanied by excitations with subsequent Auger decays leading to net m-fold ionization with m >= 4.

S. Fritzsche

Symmetry,

**Symbolic Evaluation of Expressions from Racah s Algebra**Symmetry,

**13**:1558 (September 2021)## Abstract:

Based on the rotational symmetry of isolated quantum systems, Racah s algebra plays a significant role in nuclear, atomic and molecular physics, and at several places elsewhere. For N-particle (quantum) systems, for example, this algebra helps carry out the integration over the angular coordinates analytically and, thus, to reduce them to systems with only N (radial) coordinates. However, the use of Racah s algebra quickly leads to complex expressions, which are written in terms of generalized Clebsch-Gordan coefficients, Wigner n-j symbols, (tensor) spherical harmonics and/or rotation matrices. While the evaluation of these expressions is straightforward in principle, it often becomes laborious and prone to making errors in practice. We here expand Jac, the Jena Atomic Calculator, to facilitate the sum-rule evaluation of typical expressions from Racah s algebra. A set of new and revised functions supports the simplification and subsequent use of such expressions in daily research work or as part of lengthy derivations. A few examples below show the recoupling of angular momenta and demonstrate how Jac can be readily applied to find compact expressions for further numerical studies. The present extension makes Jac a more flexible and powerful toolbox in order to deal with atomic and quantum many-particle systems.

A.-L. Viotti,
S. Ališauskas,
H. Tuennermann,
E. Escoto,
M. Seidel,
K. Dudde,
B. Manschwetus,
I. Hartl,
and C. Heyl

Opt. Lett.,

**Temporal pulse quality of a Yb:YAG burst-mode laser post-compressed in a multi-pass cell**Opt. Lett.,

**46**:4686 (September 2021)## Abstract:

Nonlinear pulse post-compression represents an efficient method for ultrashort, high-quality laser pulse production. The temporal pulse quality is, however, limited by amplitude and phase modulations intrinsic to post-compression. We here characterize in frequency and time domain with high dynamic range individual post-compressed pulses within laser bursts comprising 100-kHz-rate pulse trains. We spectrally broaden 730 fs, 3.2 mJ pulses from a Yb:YAG laser in a gas-filled multi-pass cell and post-compress them to 56 fs. The pulses exhibit a nearly constant energy content of 78% in the main peak over the burst plateau, which is close to the theoretical limit. Our results demonstrate attractive pulse characteristics, making multi-pass post-compressed lasers very applicable for pump-probe spectroscopy at, e.g., free-electron lasers or as efficient drivers for secondary frequency conversion stages.

Y. Zhang,
C. L. Zhong,
S. P. Zhu,
X. T. He,
M. Zepf,
and B. Qiao

Physical Review Applied,

**Obtaining Intense Attosecond Pulses in the Far Field from Relativistic Laser-Plasma Interactions**Physical Review Applied,

**16**:024042 (August 2021)## Abstract:

In this paper, we show that the Gouy phase shift plays a key role in the far-field waveform evolution of the reflected harmonic radiations from plasma surfaces driven by a relativistic Gaussian laser. With a proper adjustment of laser focal position away from the plasma surface, the inherent separations between the peaks of different harmonic carrier waves as well as the fundamental wave due to different wavelengths can be cleared away when they propagate from near to far field, since they experience the same Gouy phase shift of pi/2. Using this method, intense attosecond pulses can be obtained in the far field with no need of any spectral filters. Three-dimensional particle-in-cell simulations show that far-field attosecond pulses with intensity of 4 x 1015 W/cm2 (2.56 x 1017 W/sr; 65 times increase) and duration of 76 as (50% decrease) can be obtained by lasers at intensities of 1021 W/cm2. Such brilliant pulses with fully reserved spectra significantly benefit applications in attosecond science.

A. Koszorus,
L. J. Vormawah,
R. Beerwerth,
M. L. Bissell,
P. Campbell,
B. Cheal,
C. S. Devlin,
T. Eronen,
S. Fritzsche,
S. Geldhof,
H. Heylen,
J. D. Holt,
A. Jokinen,
S. Kelly,
I. D. Moore,
T. Miyagi,
S. Rinta-Antila,
A. Voss,
and C. Wraith

Phys. Rev. B,

**Proton-neutron pairing correlations in the self-conjugate nucleus Sc-42**Phys. Rev. B,

**819**:136439 (August 2021)## Abstract:

Collinear laser spectroscopy of the N = Z = 21 self-conjugate nucleus Sc-42 has been performed at the JYFL IGISOL IV facility in order to determine the change in nuclear mean-square charge radius between the I-pi = 0(+) ground state and the I-pi = 7(+) isomer via the measurement of the Sc-42g,Sc-42m isomer shift. New multi-configurational Dirac-Fock calculations for the atomic mass shift and field shift factors have enabled a recalibration of the charge radii of the Sc42-46 isotopes which were measured previously. While consistent with the treatment of proton-neutron, proton-proton and neutron-neutron pairing on an equal footing, the reduction in size for the isomer is observed to be of a significantly larger magnitude than that expected from both shell-model and ab-initio calculations. The measured nuclear magnetic dipole moment and electric quadruple moment, on the other hand, are in good agreement with simple empirical estimates and shell-model calculations.

B. Lei,
D. Seipt,
M. Shi,
B. Liu,
J. Wang,
M. Zepf,
and S. Rykovanov

Phys. Rev. A,

**Relativistic modified Bessel-Gaussian beam generated from plasma-based beam braiding**Phys. Rev. A,

**104**:021501 (August 2021)## Abstract:

We theoretically and numerically demonstrate the generation of a relativistic modified Bessel-Gaussian beam (MBGB) via plasma-based beam braiding. It is realized by injecting several intense Gaussian pulses with well-designed offsets and angles into an underdense plasma channel which acts as a laser-pulse combiner via refractive coupling. The MBGB propagates stably in the plasma channel with a well-controlled orbital angular momentum of large value, exciting a twisted plasma wave. After leaving the plasma, it becomes unguided and survives in vacuum for at least hundreds of femtoseconds. This method is insensitive to the initial laser injection conditions and thus should be robust for experimental implementation. It provides an alternative approach in generating high-quality tunable intense optical vortex beams which are desired for various applications.

M. Mueller,
C. Aleshire,
J. Buldt,
H. Stark,
C. Grebing,
A. Klenke,
and J. Limpert

Opt. Express,

**Scaling potential of beam-splitter-based coherent beam combination**Opt. Express,

**29**:27900 (August 2021)## Abstract:

The impact of nonlinear refraction and residual absorption on the achievable peakand average power in beam-splitter-based coherent beam combination is analyzed theoretically. While the peak power remains limited only by the aperture size, a fundamental average power limit is given by the thermo-optical and thermo-mechanical properties of the beam splitter material and its coatings. Based on our analysis, 100 kW average power can be obtained with state-of-the-art optics at maintained high beam quality (M-2 <= 1.1) and at only 2% loss of combining efficiency. This result indicates that the power-scaling potential of today\textquotesingle s beam-splitter-based coherent beam combination is far from being depleted. A potential scaling route to megawatt-level average power is discussed for optimized beam splitter geometry.

F. Karbstein,
C. Sundqvist,
K. S. Schulze,
I. Uschmann,
H. Gies,
and G.G. Paulus

New J. Phys.,

**Vacuum birefringence at x-ray free-electron lasers**New J. Phys.,

**23**:095001 (August 2021)## Abstract:

We study the perspectives of measuring the phenomenon of vacuum birefringence predicted by quantum electrodynamics using an x-ray free-electron laser (XFEL) alone. We devise an experimental scheme allowing two consecutive XFEL pulses to collide under a finite angle, and thus act as both pump and probe field for the effect. The signature of vacuum birefringence is encoded in polarization-flipped signal photons to be detected with high-purity x-ray polarimetry. Our findings for idealized scenarios underline that the discovery potential of solely XFEL-based setups can be comparable to those involving optical high-intensity lasers. For currently achievable scenarios, we identify several key details of the x-ray optical ingredients that exert a strong influence on the magnitude of the desired signatures.

B. Böning,
and S. Fritzsche

J. Phys. B,

**Above-threshold ionization driven by Gaussian laser beams: beyond the electric dipole approximation**J. Phys. B,

**54**:144002 (July 2021)## Abstract:

Strong-field atomic experiments have recently become sensitive to nondipole (magnetic) interactions. In particular, photoelectrons emitted in above-threshold ionization possess a nonzero momentum along the beam axis as a result of the Lorentz force. Here, we show how this longitudinal momentum can be theoretically calculated based on a nondipole strong-field approximation that accounts not only for the temporal but also the spatial dependence of the laser field in the photoelectron continuum. If the driving laser beam is approximated as a plane wave, the theoretical values differ from known experimental results by a constant offset. We demonstrate that this offset can successfully be removed if a realistic Gaussian beam profile is accounted for in the quantum description of ATI. We also discuss the influence of the size of the beam waist in the focus.

V. Schuster,
V. Hilbert,
R. Klas,
C. Liu,
M. Tschernajew,
B. Bernhardt,
J. Rothhardt,
and J. Limpert

Opt. Express,

**Agile spectral tuning of high order harmonics by interference of two driving pulses**Opt. Express,

**29**:22117 (July 2021)## Abstract:

In this work, the experimental realization of a tunable high photon flux extreme ultraviolet light source is presented. This is enabled by high harmonic generation of two temporally delayed driving pulses with a wavelength of 1030 nm, resulting in a tuning range of 0.8 eV at the 19th harmonic at 22.8 eV. The implemented approach allows for fast tuning of the spectrum, is highly flexible and is scalable towards full spectral coverage at higher photon energies.

M. Alamoudi,
M. Sattari,
M. Balubaid,
E. Eftekhari-Zadeh,
E. Nazemi,
O. Taylan,
and E. Kalmoun

Symmetry,

**Application of Gamma Attenuation Technique and Artificial Intelligence to Detect Scale Thickness in Pipelines in Which Two-Phase Flows with Different Flow Regimes and Void Fractions Exist**Symmetry,

**13**:1198 (July 2021)## Abstract:

Scale deposits can reduce equipment efficiency in the oil and petrochemical industry. The gamma attenuation technique can be used as a non-invasive effective tool for detecting scale deposits in petroleum pipelines. The goal of this study is to propose a dual-energy gamma attenuation method with radial basis function neural network (RBFNN) to determine scale thickness in petroleum pipelines in which two-phase flows with different symmetrical flow regimes and void fractions exist. The detection system consists of a dual-energy gamma source, with Ba-133 and Cs-137 radioisotopes and two 2.54-cm x 2.54-cm sodium iodide (NaI) detectors to record photons. The first detector related to transmitted photons, and the second one to scattered photons. The transmission detector recorded two signals, which were the counts under photopeak of Ba-133 and Cs-137 with the energy of 356 keV and 662 keV, respectively. The one signal recorded in the scattering detector, total counts, was applied to RBFNN as the inputs, and scale thickness was assigned as the output.

J. Gollwitzer,
L. Bocklage,
R. Röhlsberger,
and G. Meier

npj Quantum Inf,

**Connecting Fano Interference and the Jaynes-Cummings Model in Cavity Magnonics**npj Quantum Inf,

**7**:114 (July 2021)## Abstract:

We show that Fano interference can be realized in a macroscopic microwave cavity coupled to a spin ensemble at room temperature. Via a formalism developed from the linearized Jaynes-Cummings model of cavity electromagnonics, we show that generalized Fano interference emerges from the photon–magnon interaction at low cooperativity. In this regime, the reflectivity approximates the scattering cross-section derived from the Fano-Anderson model. Although asymmetric lineshapes in this system are often associated with the Fano formalism, we show that whilst Fano interference is actually present, an exact Fano form cannot be achieved from the linear Jaynes-Cummings model. In the Fano model an additional contribution arises, which is attributed to decoherence in other systems, and in this case is due to the resonant nature of the photonic mode. The formalism is experimentally verified and accounts for the asymmetric lineshapes arising from the interaction between magnon and photon channels. As the magnon–photon coupling strength is increased, these channels merge into hybridized magnon–photon modes and the generalized Fano interference picture breaks down. Our results are universally applicable to systems underlying the linearized Jaynes-Cummings Hamiltonian at low cooperativity and connect the microscopic parameters of the quantum optical model to generalized Fano lineshapes.

T. Kaaden,
V. Tympel,
M. Kober,
F. Schmidl,
M. Rettenmayr,
and S. Lippmann

Rev. Sci. Instrum.,

**Electric pulse heating device for the analysis of solid/solid phase transformations**Rev. Sci. Instrum.,

**92**:074703 (July 2021)## Abstract:

Ohmic pulse heating is applied to investigate diffusion and interface controlled solid-state phase transformations. The developed device uses extensive solid-state electronics providing a high current, low voltage approach that overcomes the limitations of existing setups, most notably the use of sample geometries that allow for the reliable measurement of local temperatures and their assignment to microstructures. Power for heating is supplied by a capacitor array with adjustable voltage, and the process is controlled by microcontrollers and a solid-state relay, which allows for controlled pulses that are adjustable in microseconds. Electric currents of up to 22 kA at 90 V can be realized by the setup. Electric data are monitored and collected during the experiments, and temperature data are captured using a high-resolution infrared camera at high frame rates (1200 fps). The capabilities of the setup are demonstrated by rapid heating (10(6) K/s) and subsequent cooling of a brass sample. Two distinct areas of the sample are analyzed in detail, showing similar heating, but different cooling curves with rates of 10(4) and 10(2) K/s. Local microstructure analysis shows that different phase transformation mechanisms were dominant, and thus, the setup fulfills its purpose.

P.-M. Hillenbrand,
K. N. Lyashchenko,
S. Hagmann,
O. Andreev,
D. Banaś,
E. P. Benis,
I. Bondarev,
C. Brandau,
E. De Filippo,
O. Forstner,
J. Glorius,
R. E. Grisenti,
A. Gumberidze,
D. L. Guo,
M. O. Herdrich,
M. Lestinsky,
Y. Litvinov,
V. Pagano,
N. Petridis,
M. S. Sanjari,
D. Schury,
U. Spillmann,
S. Trotsenko,
M. Vockert,
A. B. Voitkiv,
G. Weber,
and T. Stoehlker

Phys. Rev. A,

**Electron-loss-to-continuum cusp in collisions of U89+ with N-2 and Xe**Phys. Rev. A,

**104**:012809 (July 2021)## Abstract:

We study the electron-loss-to-continuum (ELC) cusp experimentally and theoretically by comparing the ionization of U89+ projectiles in collisions with N-2 and Xe targets, at a beam energy of 75.91 MeV/u. The coincidence measurement between the singly ionized projectile and the energy of the emitted electron is used to compare the shape of the ELC cusp at weak and strong perturbations. A significant energy shift for the centroid of the electron cusp is observed for the heavy target of Xe as compared to the light target of N-2. Our results provide a stringent test for fully relativistic calculations of double-differential cross sections performed in the first-order approximation and in the continuum-distorted-wave approach.

J. Hofbrucker,
B. Böning,
A. V. Volotka,
and S. Fritzsche

Phys. Rev. A,

**Elliptical dichroism in biharmonic ionization of atoms**Phys. Rev. A,

**104**:013102 (July 2021)## Abstract:

In multiphoton ionization of atoms, elliptical dichroism may arise in the photoelectron angular distributions due to the interference of the possible ionization pathways. We here consider the interaction of atoms with an elliptically polarized biharmonic $(ømega + 2ømega)$ field which simultaneously allows one- and two-photon ionization of the atoms. The interference between these two ionization pathways introduces contributions to the elliptical dichroism in addition to the dichroism that arises from the two-photon ionization alone. We show that these additional dichroism contributions can lead to a stronger dichroism in comparison to the one arising from two-photon ionization only. We present a relativistic analysis of the corresponding photoelectron angular distributions and discuss individual contributions to the dichroic phenomena. Detailed computations have been performed for biharmonic ionization of neutral helium atoms.

P. Wustelt,
F. Oppermann,
S. Mhatre,
M. Kuebel,
A. Sayler,
M. Lein,
S. Graefe,
and G. Paulus

Phys. Rev. Lett.,

**Laser-Driven Anharmonic Oscillator: Ground-State Dissociation of the Helium Hydride Molecular Ion by Midinfrared Pulses**Phys. Rev. Lett.,

**127**:043202 (July 2021)## Abstract:

The vibrational motion of molecules represents a fundamental example of an anharmonic oscillator. Using a prototype molecular system, HeH+, we demonstrate that appropriate laser pulses make it possible to drive the nuclear motion in the anharmonic potential of the electronic ground state, increasing its energy above the potential barrier and facilitating dissociation by purely vibrational excitation. We find excellent agreement between the frequency-dependent response of the helium hydride molecular cation to both classical and quantum mechanical simulations, thus removing any ambiguities through electronic excitation. Our results provide access to the rich dynamics of anharmonic quantum oscillator systems and pave the way to state-selective control schemes in ground-state chemistry by the adequate choice of the laser parameters.

S. Willing,
K. Schlage,
L. Bocklage,
M. M. R. Moayed,
T. Gurieva,
G. Meier,
and R. Röhlsberger

ACS Appl. Mater. Interfaces,

**Novel Tunnel Magnetoresistive Sensor Functionalities via Oblique-Incidence Deposition**ACS Appl. Mater. Interfaces,

**13**:32343 (July 2021)## Abstract:

Controlling the magnetic properties of ultrathin films remains one of the main challenges to the further development of tunnel magnetoresistive (TMR) device applications. The magnetic response in such devices is mainly governed by extending the primary TMR trilayer with the use of suitable contact materials. The transfer of magnetic anisotropy to ferromagnetic electrodes consisting of CoFeB layers results in a field-dependent TMR response, which is determined by the magnetic properties of the CoFeB as well as the contact materials. We flexibly apply oblique-incidence deposition (OID) to introduce arbitrary intrinsic in-plane anisotropy profiles into the magnetic layers. The OID-induced anisotropy shapes the magnetic response and eliminates the requirement of additional magnetic contact materials. Functional control is achieved via an adjustable shape anisotropy that is selectively tailored for the ultrathin CoFeB layers. This approach circumvents previous limitations on TMR devices and allows for the design of new sensing functionalities, which can be precisely customized to a specific application, even in the high field regime. The resulting sensors maintain the typical TMR signal strength as well as a superb thermal stability of the tunnel junction, revealing a striking advantage in functional TMR design using anisotropic interfacial roughness.