arXiv subjects

Ulf Saalmann

Publications and source records attributed to Ulf Saalmann.

At least 37 records · Page 2Linked to original sources

arXiv2016

Recovery of dynamic interference

We develop general quantitative criteria for dynamic interference, a manifestation of double-slit interference in time which should be realizable with brilliant state-of-the-art high-frequency laser sources. Our analysis reveals that the observation of dynamic interference hinges upon maximizing the difference between the dynamic polarization of the initial bound and the final continuum state of the electron during the light pulse, while keeping depletion of the initial state small. Confirmed by numerical results, we predict that this is impossible for the hydrogen ground-state but feasible with excited states explicitly exemplified with the hydrogen 2p-state.

physics.atom-ph↗

arXiv2015

Full Counting Statistics of a Non-adiabatic Electron Pump

Non-adiabatic charge pumping through a single-level quantum dot with periodically modulated parameters is studied theoretically. By means of a quantum-master-equation approach the full counting statistics of the system is obtained. We find a trinomial-probability distribution of the charge transfer, which adequately describes the reversal of the pumping current by sweeping the driving frequency. Further, we derive equations of motion for current and noise, and solve those numerically for two different driving schemes. Both show interesting features which can be fully analyzed due to the simple and generic model studied.

cond-mat.mes-hall↗

arXiv2015

Dynamical characteristics of Rydberg electrons released by a weak electric field

The dynamics of ultra-slow electrons in the combined potential of an ionic core and a static electric field is discussed. With state-of-the-art detection it is possible to create such electrons through strong intense-field photo-absorption and to detect them via high-resolution time-of-flight spectroscopy despite their very low kinetic energy. The characteristic feature of their momentum spectrum, which emerges at the same position for different laser orientations, is derived and could be revealed experimentally with an energy resolution of the order of 1meV.

physics.atom-ph↗

arXiv2014

The envelope Hamiltonian for electron interaction with ultrashort pulses

For ultrashort VUV pulses with a pulse length comparable to the orbital time of the bound electrons they couple to we propose a simplified envelope Hamiltonian. It is based on the Kramers-Henneberger representation in connection with a Floquet expansion of the strong-field dynamics but keeps the time dependence of the pulse envelope explicit. Thereby, the envelope Hamiltonian captures the essence of the physics, -- light-induced shifts of bound states, single-photon absorption, and non-adiabatic electronic transitions. It delivers quantitatively accurate ionization dynamics and allows for physical insight into the processes occurring. Its minimal requirements for construction in terms of laser parameters make it ideally suited for a large class of atomic and molecular problems.

physics.atom-ph↗

arXiv2014

Prevailing features of X-ray induced molecular electron spectra revealed with fullerenes

Intense X-ray photo-absorption from short and intense pulses by a molecule triggers complicated electron and subsequently ion dynamics leading to photo-electron spectra which are difficult to interpret. Illuminating fullerenes offers a way to separate out the electron dynamics. Moreover, the fullerene cage confines spatially the origin of photo and Auger electrons. Together with the sequential nature of the photo processes at intensities available at X-ray free electron lasers, this allows for a remarkably detailed interpretation of the photo-electron spectra as we will demonstrate. The general features derived can serve as a paradigm for less well-defined situations in other large molecules or clusters.

physics.atm-clus↗

arXiv2013

Proton ejection from molecular hydride clusters exposed to strong X-ray pulses

Clusters consisting of small molecules containing hydrogen do eject fast protons when illuminated by short X-ray pulses. A suitable overall charging of the cluster controlled by the X-ray intensity induces electron migration from the surface to the bulk leading to efficient segregation of the protons and to a globally hindered explosion of the heavy atoms even outside the screened volume. We investigate this peculiar effect systematically along the iso-electronic sequence of methane over ammonia and water to the atomic limit of neon as a reference. In contrast to core-shell systems where the outer shell is sacrificed to reduce radiation damage, the intricate proton dynamics of hydride clusters allows one to keep the entire backbone of heavy atoms intact.

physics.atm-clus↗

arXiv2013

Virtual single-photon transition interrupted: time-gated optical gain and loss

The response of matter to an optical excitation consists essentially of absorption and emission. Traditional spectroscopy accesses the frequency-resolved and time-integrated response, while the temporal evolution stays concealed. However, we will demonstrate here that the temporal evolution of a virtual single-photon transition can be mapped out by a second pulsed electromagnetic field. The resulting optical signal shows previously unexpected optical gain and loss, which can be gated and controlled via the relative delay of the electromagnetic fields. The model presented here can be applied to any system that assumes a two-level character through near-resonant optical dipole excitation, whether they are of atomic, molecular or even solid-state nature. These theoretical observations are in excellent qualitative agreement with our transient absorption spectroscopy study in helium. The presented results can act as starting point for a new scheme for creating optical gain, which is a prerequisite for the operation of lasers. It may be possible to open the doors to spectral regions, which were difficult to access until now, e.g. in the extreme ultraviolet.

physics.atom-ph↗

arXiv2012

Spatial correlations in finite samples revealed by Coulomb explosion

We demonstrate that fast removal of many electrons uncovers initial correlations of atoms in a finite sample through a pronounced peak in the kinetic-energy spectrum of the exploding ions. This maximum is the result of an intricate interplay between the composition of the system from discrete particles and its boundary. The formation of the peak can be described analytically, allowing for correlations beyond a mean-field reference model. It can be experimentally detected with short and intense light pulses from 4th-generation light sources.

physics.atm-clus↗

arXiv2012

Non-adiabatic Rectification and Current Reversal in Electron Pumps

Pumping of electrons through nano-scale devices is one of the fascinating achievements in the field of nano-science with a wide range of applications. To optimize the performance of pumps, operating them at high frequencies is mandatory. We consider the influence of fast periodic driving on the average charge transferred through a quantum dot. We show that it is possible to reverse the average current by sweeping the driving frequency only. In connection with this, we observe a rectification of the average current for high frequencies. Since both effects are very robust, as corroborated by analytical results for harmonic driving, they offer a new way of controlling electron pumps.

cond-mat.mes-hall↗

arXiv2011

Many-electron dynamics triggered by massively parallel ionization

Massively parallel ionization of many atoms in a cluster or bio-molecule is identified as new phenomenon of light-matter interaction which becomes feasible through short and intense FEL pulses. Almost simultaneously emitted from the illuminated target the photo-electrons can have such a high density that they interact substantially even after photo-ionization. This interaction results in a characteristic electron spectrum which can be interpreted as convolution of a mean-field electron dynamics and binary electron-electron collisions. We demonstrate that this universal spectrum can be obtained analytically by summing synthetic two-body Coulomb collision events. Moreover, we propose an experiment with hydrogen clusters to observe massively parallel ionization.

physics.atm-clus↗

arXiv2011

Non-adiabatic Electron Pumping through Interacting Quantum Dots

We study non-adiabatic charge pumping through single-level quantum dots taking into account Coulomb interactions. We show how a truncated set of equations of motion can be propagated in time by means of an auxiliary-mode expansion. This formalism is capable of treating the time-dependent electronic transport for arbitrary driving parameters. We verify that the proposed method describes very precisely the well-known limit of adiabatic pumping through quantum dots without Coulomb interactions. As an example we discuss pumping driven by short voltage pulses for various interaction strengths. Such finite pulses are particular suited to investigate transient non-adiabatic effects, which may be also important for periodic drivings, where they are much more difficult to reveal.

cond-mat.mes-hall↗

arXiv2011

Energy bunching in soft recollisions revealed with long-wavelength few-cycle pulses

Soft recollisions are laser-driven distant collisions of an electron with its parent ion. Such collisions may cause an energy bunching, since electrons with different initial drift momenta can acquire impacts, which exactly counterbalance these differences. The bunching generates a series of peaks in the photo-electron spectrum. We will show that this series could be uncovered peak-by-peak experimentally by means of phase-stabilized few-cycle pulses with increasing duration.

physics.atom-ph↗

arXiv2011

Electron-energy bunching in laser-driven soft recollisions

We introduce soft recollisions in laser-matter interaction. They are characterized by the electron missing the ion upon recollision in contrast to the well-known head-on collisions responsible for high-harmonic generation or above-threshold ionization. We demonstrate analytically that soft recollisions can cause a bunching of photo-electron energies through which a series of low-energy peaks emerges in the electron yield along the laser polarization axis. This peak sequence is universal, it does not depend on the binding potential, and is found below an excess energy of one fifth of the ponderomotive energy.

physics.atom-ph↗

arXiv2011

Propagation of Time-Nonlocal Quantum Master Equations for Time-Dependent Electron Transport

Time-resolved electron transport in nano-devices is described by means of a time-nonlocal quantum master equation for the reduced density operator. Our formulation allows for arbitrary time dependences of any device or contact parameter. The quantum master equation and the related expression for the electron current through the device are derived in fourth order of the coupling to the contacts. It is shown that a consistent sum up to infinite orders induces level broadening in the device. To facilitate a numerical propagation of the equations we propose to use auxiliary density operators. An expansion of the Fermi function in terms of a sum of simple poles leads to a set of equations of motion, which can be solved by standard methods. We demonstrate the viability of the proposed propagation scheme and consider electron transport through a double quantum dot.

cond-mat.mes-hall↗

arXiv2010

Coherent manipulation of charge qubits in double quantum dots

The coherent time evolution of electrons in double quantum dots induced by fast bias-voltage switches is studied theoretically. As it was shown experimentally, such driven double quantum dots are potential devices for controlled manipulation of charge qubits. By numerically solving a quantum master equation we obtain the energy- and time-resolved electron transfer through the device which resembles the measured data. The observed oscillations are found to depend on the level offset of the two dots during the manipulation and, most surprisingly, also the on initialization stage. By means of an analytical expression, obtained from a large-bias model, we can understand the prominent features of these oscillations seen in both the experimental data and the numerical results. These findings strengthen the common interpretation in terms of a coherent transfer of electrons between the dots.

cond-mat.mes-hall↗

arXiv2010

Dynamics of photo-activated Coulomb complexes

Intense light with frequencies above typical atomic or molecular ionization potentials as provided by free-electron lasers couples many photons into extended targets such as clusters and biomolecules. This implies, in contrast to traditional multi-photon ionization, multiple single-photon absorption. Thereby, many electrons are removed from their bound states and either released or trapped if the target charge has become sufficiently large. We develop a simple model for this photo activation to study electron migration and interaction. It satisfies scaling relations which help to relate quite different scenarios. To understand this type of multi-electron dynamics on very short time scales is vital for assessing the radiation damage inflicted by that type of radiation and to pave the way for coherent diffraction imaging of single molecules.

physics.atm-clus↗

arXiv2009

Propagation Scheme for Non-Equilibrium Dynamics of Electron Transport in Nanoscale Devices

A closed set of coupled equations of motion for the description of time-dependent electron transport is derived. It provides the time evolution of energy-resolved quantities constructed from non-equilibrium Green functions. By means of an auxiliary-mode expansion a viable propagation scheme for finite temperatures is obtained, which allows to study arbitrary time dependences and structured reservoirs. Two illustrative examples are presented.

cond-mat.mes-hall↗

arXiv2009

A partial fraction decomposition of the Fermi function

A partial fraction decomposition of the Fermi function resulting in a finite sum over simple poles is proposed. This allows for efficient calculations involving the Fermi function in various contexts of electronic structure or electron transport theories. The proposed decomposition converges in a well-defined region faster than exponential and is thus superior to the standard Matsubara expansion.

cond-mat.other↗