arXiv subjects

Joachim Burgdörfer

Publications and source records attributed to Joachim Burgdörfer.

At least 19 recordsLinked to original sources

arXiv2025

Identifying Electronic Doorway States in the Secondary Electron Emission from Layered Materials

We investigate the secondary low-energy electron emission induced by inelastic electron scattering from graphene and layered materials thereof. By applying a coincidence detection of the primary scattered and the emitted secondary electron we unravel pronounced resonance features otherwise overshadowed by the largely structureless secondary electron energy distribution. Supported by density functional theory calculations we show that these structures are the signature of prominent Feshbach resonances above the vacuum threshold which originate from interlayer states acting as a doorway state for electron emission. Remarkably, some of these doorway states open up only for samples with more than five layers.

cond-mat.mes-hall↗

arXiv2025

Reduced density matrix description of high-harmonic generation in multi-electron atoms: exploring sub-cycle correlation effects

High-harmonic generation (HHG) is one of the fundamental processes at the heart of attosecond physics. Traditionally viewed as an effective single-particle effect, recent advances have focused on contributions beyond this single-particle picture to the harmonic spectrum, as well as on probing electron correlations through HHG in atoms, molecules, and solids. In this paper, we introduce a reduced density matrix description to explore and to quantify correlation effects on a sub-cycle time scale and apply this approach to prototypical multi-electron atoms. By comparing noble gas atoms (He, Ne) with alkaline earth atoms (Be, Mg) exposed to driving fields with similar Keldysh parameters, we show that the sub-cycle variation of correlation parameters differs markedly for noble gases and alkaline earth atoms. We provide an intuitive explanation of these surprising effects based on the dynamics of natural orbitals and discuss the effect of this ultrafast correlation dynamics on the HHG spectrum.

physics.atom-ph↗

arXiv2023

Non-equilibrium correlation dynamics in the one-dimensional Fermi-Hubbard model: A testbed for the two-particle reduced density matrix theory

We explore the non-equilibrium dynamics of a one-dimensional Fermi-Hubbard system as a sensitive testbed for the capabilities of the time-dependent two-particle reduced density matrix (TD2RDM) theory to accurately describe time-dependent correlated systems. We follow the time evolution of the out-of-equilibrium finite-size Fermi-Hubbard model initialized by a quench over extended periods of time. By comparison with exact calculations for small systems and with matrix product state (MPS) calculations for larger systems but limited to short times, we demonstrate that the TD2RDM theory can accurately account for the non-equilibrium dynamics in the regime from weak to moderately strong inter-particle correlations. We find that the quality of the approximate reconstruction of the three-particle cumulant (or correlation) required for the closure of the equations of motion for the reduced density matrix is key to the accuracy of the numerical TD2RDM results. We identify the size of the dynamically induced three-particle correlations and the amplitude of cross correlations between the two- and three-particle cumulants as critical parameters that control the accuracy of the TD2RDM theory when current state-of-the art reconstruction functionals are employed.

cond-mat.str-el↗

arXiv2022

Subcycle resolved strong-field tunneling ionization: Identification of magnetic dipole and electric quadrupole effects

Interaction of a strong laser pulse with matter transfers not only energy but also linear momentum of the photons. Recent experimental advances have made it possible to detect the small amount of linear momentum delivered to the photoelectrons in strong-field ionization of atoms. Linear momentum transfer is a unique signature of the laser-atom interaction beyond its dipolar limit. Here, we present a decomposition of the subcycle time-resolved linear momentum transfer in term of its multipolar components. We show that the magnetic dipole contribution dominates the linear momentum transfer during the dynamical tunneling process while the post-ionization longitudinal momentum transfer in the field-driven motion of the electron in the continuum is primarily governed by the electric quadrupole interaction. Alternatively, exploiting the radiation gauge, we identify nondipole momentum transfer effects that scale either linearly or quadratically with the coupling to the laser field. The present results provide detailed insights into the physical mechanisms underlying the subcycle linear momentum transfer induced by nondipole effects.

physics.atom-ph↗

arXiv2022

Canonical density matrices from eigenstates of mixed systems

One key issue of the foundation of statistical mechanics is the emergence of equilibrium ensembles in isolated and closed quantum systems. Recently, it was predicted that in the thermodynamic ($N\rightarrow\infty$) limit of large quantum many-body systems canonical density matrices emerge for small subsystems from almost all pure states. This notion of canonical typicality is assumed to originate from the entanglement between subsystem and environment and the resulting intrinsic quantum complexity of the many-body state. For individual eigenstates it has been shown that local observables show thermal properties provided the eigenstate thermalization hypothesis holds, which requires the system to be quantum chaotic. In the present paper, we study the emergence of thermal states in the regime of a quantum analog of a mixed phase space. Specifically, we study the emergence of the canonical density matrix of an impurity upon reduction from isolated energy eigenstates of a large but finite quantum system the impurity is embedded in. Our system can be tuned by means of a single parameter from quantum integrability to quantum chaos and corresponds in between to a system with mixed quantum phase space. We show that the probability for finding a canonical density matrix when reducing the ensemble of energy eigenstates of the finite many-body system can be quantitatively controlled and tuned by the degree of quantum chaos present. For the transition from quantum integrability to quantum chaos we find a continuous and universal (i.e. size independent) relation between the fraction of canonical eigenstates and the degree of chaoticity as measured by the Brody parameter or the Shannon entropy.

quant-ph↗

arXiv2022

Semiclassical strong-field theory of phase delays in $ω-2ω$ above-threshold ionization

Phase and time delays of atomic above-threshold ionization were recently experimentally explored in an $ω-2ω$ setting [Zipp et al, Optica 1, 361 (2014)]. The phases of wavepackets ejected from argon by a strong $2ω$ pulse were probed as a function of the relative phase of a weaker $ω$ probe pulse. Numerical simulations solving the time-dependent Schrödinger equation (TDSE) displayed a sensitive dependence of the doubly differential momentum distribution on the relative phase between the $ω$ and $2ω$ fields. Moreover, a surprisingly strong variation of the extracted phase delays on the intensity of the probe pulse was found. We present a semiclassical strong-field description of the phase delays in the emission of electrons in an $ω-2ω$ setting and apply it to atomic hydrogen. Non-perturbative effects in both the $2ω$ pump and the $ω$ probe field are included. The semiclassical description allows tracing phase delays to path interferences between emission during different points in time of emission within the temporal unit cell of the two-color laser field. We find good agreement between the semiclassical saddle-point approximation, the full strong field approximation (SFA), and previous results applicable in the perturbative limit of probe fields. We show that the RABBIT-like perturbative description of phase delays breaks down for stronger fields and higher-energy electron emission. In this regime, characterization of the ionization signal requires an entire ensemble of phase delays {$δ_i(E)$} with $i=1,2,\ldots$ the difference in photon numbers of the strong $2ω$ field involved in the interfering paths. Comparison between SFA and TDSE calculations reveals the influence of the Coulomb field even in this strong-field scenario.

physics.atom-ph↗

arXiv2022

Zeptosecond Angular Streak Camera

Time-resolved electronic processes on the attosecond scale have recently become experimentally accessible through the development of laser-based pump-probe interrogation techniques such as the attosecond streak camera, the reconstruction of attosecond beating by interference of two-photon transitions, and the attoclock. In this work, we demonstrate that by combining the concepts of the attosecond streak camera and the attoclock, time resolved processes down to the time scale of tens of zeptoseconds come into reach. Key to advancing to this remarkable level of time precision by this method termed the zeptosecond angular streak camera (ZASC) is its substantial intrinsic time-information redundancy. The ZASC results in a remarkably simple streaking trace, which is largely independent of the precise temporal structure of the streaking pulse, thereby bypassing the need for detailed characterization of the streaking field. Moreover, it is capable of retrieving information on the duration of the pump pulse. It is also capable of reaching attosecond-level precision in a single-shot mode that may be useful for free-electron-laser experiments. This concept promises to open pathways towards the chronoscopy of zeptosecond-level ultrafast processes.

physics.atom-ph↗

arXiv2021

Wannier Diagram and Brown-Zak Fermions of Graphene on Hexagonal Boron-Nitride

The moiré potential of graphene on hexagonal boron nitride (hBN) generates a supercell sufficiently large as to thread a full magnetic flux quantum $Φ_0$ for experimentally accessible magnetic field strengths. Close to rational fractions of $Φ_0$, $p/q \cdotΦ_0$, magnetotranslation invariance is restored giving rise to Brown-Zak fermions featuring the same dispersion relation as in the absence of the field. Employing a highly efficient numerical approach we have performed the first realistic simulation of the magnetoconductance for a 250 nm wide graphene ribbon on hexagonal boron nitride using a full ab-initio derived parametrization including strain. The resulting Hofstadter butterfly is analyzed in terms of a novel Wannier diagram for Landau spectra of Dirac particles that includes the lifting of the spin and valley degeneracy by the magnetic field and the moiré potential. This complex diagram can account for many experimentally observed features on a single-particle level, such as spin and valley degeneracy lifting and a non-periodicidy in $Φ_0$.

cond-mat.mes-hall↗

arXiv2021

Influence of coherent vibronic excitation on the high harmonics generation of diatomic molecules

The generation of high harmonics (HHG) in atomic systems by the highly non-linear response to an intense laser field is a prominent pathway to the synthesis of ever shorter laser pulses at increasingly higher photon energies. Extensions of this process to molecules add to the complexity but also offers novel opportunities as multi-center effects and the coupling to nuclear degrees of freedom can influence HHG. In this work we theoretically explore the impact of coherent vibronic excitations of diatomic molecules on the HHG spectrum within the framework of time-dependent density functional theory. We observe the appearance of novel interference structures in the HHG spectra controlled by resonances between the driving field and the vibronic wavepacket.

physics.atom-ph↗

arXiv2021

Magnetic-field effect in high-order above-threshold ionization

We experimentally and theoretically investigate the influence of the magnetic component of an electromagnetic field on high-order above-threshold ionization of xenon atoms driven by ultrashort femtosecond laser pulses. The nondipole shift of the electron momentum distribution along the light-propagation direction for high energy electrons beyond the classical cutoff is found to be vastly different from that below the cutoff. A V-shape structure in the momentum dependence of the nondipole shift above the cutoff is identified for the first time. With the help of classical and quantum-orbit analysis, we show that large-angle rescattering of the electrons strongly alters the partitioning of the photon momentum between electron and ion. The sensitivity of the observed nondipole shift to the electronic structure of the target atom is confirmed by three-dimensional time-dependent Schrödinger equation simulations for different model potentials.

physics.atom-ph↗

arXiv2020

Theory of Subcycle Linear Momentum Transfer in Strong-Field Tunneling Ionization

Interaction of a strong laser pulse with matter transfers not only energy but also linear momentum of the photons. Recent experimental advances have made it possible to detect the small amount of linear momentum delivered to the photoelectrons in strong-field ionization of atoms. We present numerical simulations as well as an analytical description of the subcycle phase (or time) resolved momentum transfer to an atom accessible by an attoclock protocol. We show that the light-field-induced momentum transfer is remarkably sensitive to properties of the ultrashort laser pulse such as its carrier-envelope phase and ellipticity. Moreover, we show that the subcycle resolved linear momentum transfer can provide novel insights into the interplay between nonadiabatic and nondipole effects in strong-field ionization. This work paves the way towards the investigation of the so-far unexplored time-resolved nondipole nonadiabatic tunneling dynamics.

physics.atom-ph↗

arXiv2020

Angle-resolved time delays for shake-up ionization of helium

Recent angle-resolved RABBITT experiments have shown that the photoionization time delay depends on the emission angle of the photoelectron. In this work we demonstrate that for photoemission from helium accompanied by shake-up (correlation satellites), the angular variation of the time delay is dramatically enhanced by the dipolar coupling between the photoelectron and the highly polarizable bound electron in the IR field. We show that the additivity rule for the time delays due to the atomic potential, the continuum-continuum (cc) coupling by the IR field, and due to this two-electron process remains valid for angle-resolved RABBITT. Our results are expected to be also applicable to other multi-electron systems that are highly polarizable or feature a permanent dipole moment.

physics.atom-ph↗

arXiv2020

60 years of attosecond physics at ICPEAC: from collisions to ultrashort pulses

The field of attosecond physics has seen an almost explosive growth since the early 2000's and represents by now an increasing fraction of contributions to the bi-annual series of International Conferences of Photonic, Electronic, and Atomic Collisions (ICPEAC). The latter is anything but a coincidence as many of the underlying concepts of electronic and photonic dynamics are closely intertwined with atomic-scale collision processes. We illustrate this fruitful connection and its implications with the help of a few prototypical examples of current topical interest.

quant-ph↗

arXiv2019

Time delays from one-photon transitions in the continuum

Attosecond photoionisation time delays reveal information about the potential energy landscape an outgoing electron wavepacket probes upon ionisation. In this study we experimentally quantify, for the first time, the dependence of the time delay on the angular momentum of the liberated photoelectrons. For this purpose, electron quantum-path interference spectra have been resolved in energy and angle using a two-color attosecond pump-probe photoionisation experiment in helium. A fitting procedure of the angle-dependent interference pattern allows us to disentangle the relative phase of all four quantum pathways that are known to contribute to the final photoelectron signal. In particular, we resolve the dependence on the angular momentum of the delay of one-photon transitions between continuum states, which is an essential and universal contribution to the total photoionization delay observed in attosecond pump-probe measurements. For such continuum-continuum transitions, we measure a delay between outgoing s- and d-electrons as large as 12 as close to the ionisation threshold in helium. Both single-active-electron and first-principles ab initio simulations confirm this observation for helium and hydrogen, demonstrating the universality of the observed delays.

physics.atom-ph↗

arXiv2019

Circular holographic ionization-phase meter

We propose an attosecond XUV-pump IR-probe photoionization protocol that employs pairs of counter-rotating consecutive harmonics and angularly resolved photoelectron detection, thereby providing direct measurement of ionization phases. The present method, which we call circular holographic ionization-phase meter (CHIP), gives also access to the phase of photoemission amplitudes of even-parity continuum states from a single time-delay measurement, since the relative phase of one- and two-photon ionization pathways is imprinted in the photoemission anisotropy. The method is illustrated with \emph{ab initio} simulations of photoionization via autoionizing resonances in helium. The rapid phase excursion in the transition amplitude to both the dipole-allowed $(2s2p)^1$P$^{\rm o}$ and the dipole-forbidden $(2p^2)^1$D$^{\rm e}$ states are faithfully reproduced.

physics.atom-ph↗

arXiv2018

Polarization tagging of two-photon double ionization by elliptically polarized XUVs

We explore the influence of elliptical polarization on the (non)sequential two-photon double ionization of atomic helium with ultrashort extreme ultraviolet (XUV) light fields using time-dependent full ab-initio simulations. The energy and angular distributions of photoelectrons are found to be strongly dependent on the ellipticity. The correlation minimum in the joint angular distribution becomes more prominently visible with increasing ellipticity. In a pump-probe sequence of two subsequent XUV pulses with varying ellipticities, polarization tagging allows to discriminate between sequential and nonsequential photoionization. This clear separation demonstrates the potential of elliptically polarized XUV fields for improved control of electronic emission processes.

physics.atom-ph↗

arXiv2018

Topologically non-trivial valley states in bilayer graphene quantum point contacts

We present measurements of quantized conductance in electrostatically induced quantum point contacts in bilayer graphene. The application of a perpendicular magnetic field leads to an intricate pattern of lifted and restored degeneracies with increasing field: at zero magnetic field the degeneracy of quantized one-dimensional subbands is four, because of a twofold spin and a twofold valley degeneracy. By switching on the magnetic field, the valley degeneracy is lifted. Due to the Berry curvature states from different valleys split linearly in magnetic field. In the quantum Hall regime fourfold degenerate conductance plateaus reemerge. During the adiabatic transition to the quantum Hall regime, levels from one valley shift by two in quantum number with respect to the other valley, forming an interweaving pattern that can be reproduced by numerical calculations.

cond-mat.mes-hall↗

arXiv2018

Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

Coherent manipulation of binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid state systems, while exploitation of the valley has only recently been started, yet without control on the single electron level. Here, we show that van-der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunneling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.

cond-mat.mes-hall↗