arXiv subjects

M. Krüger

Publications and source records attributed to M. Krüger.

12 recordsLinked to original sources

arXiv2025

Elementary Steps of Energy Conversion in Strongly Correlated Systems: Beyond Single Quasiparticles and Rigid Bands

Energy conversion in materials can be considered as a sequence of elementary steps initiated by a primary excitation. While these steps are quite well understood in classical semiconductors in terms of quasiparticle (QP) excitations and interactions, their understanding in strongly correlated materials is still elusive. Here, we review the progress which has been achieved over recent years by studies of manganite perovskite oxides as a model system for materials with strong correlations. They show a subtle interplay of different types of correlations, i.e., electron-phonon, electron-electron and spin-spin, resulting in rich physical phenomena due to competition between different ground states accompanied by temperature- and field-induced phase transitions. They strongly impact various types of energy conversion and transport processes including friction at surfaces, thermal transport, time-, energy- and power-dependent optical excitations as well as photovoltaic energy conversion. The underlying microscopic processes can be broken down to the behavior of the low-energy thermal and high-energy optical excitations, their interactions, transport and conversion which are theoretically analyzed by using models of interacting and tunable QPs: Their nature and interactions can change during excitation, transport and phase transitions, thus modifying electronic structure. At sufficiently high stimulation, QP excitations can even induce or actuate phase transitions. As a result, we obtained a comprehensive understanding of energy conversion steps going far beyond single QP pictures and rigid band approximations well-known for conventional semiconductors.

cond-mat.str-el↗

arXiv2025

Squeezed states of light after high-harmonic generation in excited atomic systems

High-harmonic generation (HHG) has recently emerged as a promising method for generating non-classical states of light with frequencies spanning from the infrared up to the extreme ultraviolet regime. In this work, we theoretically investigate the generation of squeezed states of light through HHG processes in atomic systems that had been initially driven to their first excited state. Our study reveals significant single-mode squeezing in both the driving field and low-order harmonic modes. Additionally, we characterize two-mode squeezing features in the generated states, both between fundamental and harmonic modes, and among the harmonic modes themselves. Using these correlations, we demonstrate the generation of optical Schrödinger kitten states through heralding measurements, specifically via photon subtraction in one of the modes influenced by two-mode squeezing.

quant-ph↗

arXiv2024

High-Order Harmonic Generation Driven by Perfect Optical Vortex Beams: Exploring the Orbital Angular Momentum Upscaling Law

Orbital angular momentum (OAM) light beams for high-order harmonic generation (HHG) provide an additional degree of freedom to study the light-matter interaction at ultrafast timescales. A more sophisticated configuration is a perfect optical vortex (POV) beam, a light beam with a helical wavefront characterized by a phase singularity at its center and an azimuthal phase variation. POV beams are characterized by a radial profile which is independent of the OAM. Here we study the non-perturbative process of gas-phase HHG using a linearly polarized POV beam. We observe that the harmonics are emitted with similar divergence due the perfectness of the POV-driven harmonics. Furthermore, the topological charge upscaling is rigorously followed. We show that a POV beam is more advantageous than that of the Laguerre-Gaussian beam for cases where a large topological charge with a small core size is required. Our research establishes a pathway for producing bright structured extreme ultraviolet (XUV) coherent radiation sources--a pivotal tool with multifaceted applications across various technological domains.

physics.optics↗

arXiv2019

Towards temporal characterization of intense isolated attosecond pulses from relativistic surface high harmonics

Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet (XUV) and X-ray spectral range. However, its experimental realization is still a challenging task requiring identification of the optimum conditions for the generation of isolated attosecond pulses as well as their temporal characterization. Here, we demonstrate measurements in both directions. Particularly, we have made a first step towards the temporal characterization of the emitted XUV radiation by adapting the attosecond streak camera concept to identify the time domain characteristics of relativistic surface high harmonics. The results, supported by PIC simulations, set the upper limit for the averaged (over many shots) XUV duration to <6 fs, even when driven by not CEP controlled relativistic few-cycle optical pulses. Moreover, by measuring the dependence of the spectrum of the relativistic surface high harmonics on the carrier envelope phase (CEP) of the driving infrared laser field, we experimentally determined the optimum conditions for the generation of intense isolated attosecond pulses.

physics.plasm-ph↗

arXiv2017

Stresses in non-equilibrium fluids: Exact formulation and coarse grained theory

Starting from the stochastic equation for the density operator, we formulate the exact (instantaneous) stress tensor for interacting Brownian particles, whose average value agrees with expressions derived previously. We analyze the relation between the stress tensor and forces on external potentials, and observe that, out of equilibrium, particle currents give rise to extra forces. Next, we derive the stress tensor for a Landau-Ginzburg theory in non-equilibrium situations, finding an expression analogous to that of the exact microscopic stress tensor, and discuss the computation of out-of-equilibrium (classical) Casimir forces. We use these relations to study the spatio-temporal correlations of the stress tensor in a Brownian fluid, which we derive exactly to leading order in the interaction potential strength. We observe that, after integration over time, the spatial correlations generally decay as power laws in space. These are expected to be of importance for driven confined systems. We also show that divergence-free parts of the stress tensor do not contribute in the Green-Kubo relation for the viscosity.

cond-mat.soft↗

arXiv2017

Stress-strain relations in bulk metallic glasses and colloidal dispersions

A comparison is made between the nonlinear rheological response of bulk metallic glass formers and of colloidal dispersions. Stress-strain curves measured after switch-on of constant deformation rates are analyzed quantitatively using a schematic model of mode coupling theory generalized to homogeneous and incompressible flows. A mapping between metallic and dispersion rheology is possible when stresses are rescaled by an entropic scale, accumulated strains by geometrical factors, and rates by the intrinsic relaxation time. Exploiting this similarity and the possibility to directly observe individual colloidal particles, we investigate the structural distortions in the colloidal system using confocal microscopy. The distortions exhibit the (from elasticity theory) expected quadrupolar but also a strong isotropic component.

cond-mat.soft↗

arXiv2016

Attosecond physics at the nanoscale

Recently two emerging areas of research, attosecond and nanoscale physics, have started to come together. Attosecond physics deals with phenomena occurring when ultrashort laser pulses, with duration on the femto- and sub-femtosecond time scales, interact with atoms, molecules or solids. The laser-induced electron dynamics occurs natively on a timescale down to a few hundred or even tens of attoseconds, which is comparable with the optical field. On the other hand, the second branch involves the manipulation and engineering of mesoscopic systems, such as solids, metals and dielectrics, with nanometric precision. Although nano-engineering is a vast and well-established research field on its own, the merger with intense laser physics is relatively recent. In this article we present a comprehensive experimental and theoretical overview of physics that takes place when short and intense laser pulses interact with nanosystems, such as metallic and dielectric nanostructures. In particular we elucidate how the spatially inhomogeneous laser induced fields at a nanometer scale modify the laser-driven electron dynamics. Consequently, this has important impact on pivotal processes such as ATI and HHG. The deep understanding of the coupled dynamics between these spatially inhomogeneous fields and matter configures a promising way to new avenues of research and applications. Thanks to the maturity that attosecond physics has reached, together with the tremendous advance in material engineering and manipulation techniques, the age of atto-nano physics has begun, but it is in the initial stage. We present thus some of the open questions, challenges and prospects for experimental confirmation of theoretical predictions, as well as experiments aimed at characterizing the induced fields and the unique electron dynamics initiated by them with high temporal and spatial resolution.

physics.atom-ph↗

arXiv2016

Non-equilibrium Fluctuational Quantum Electrodynamics: Heat Radiation, Heat Transfer, and Force

Quantum and thermal fluctuations of electromagnetic waves are the cornerstone of quantum and statistical physics, and inherent to such phenomena as thermal radiation and van der Waals forces. While the basic principles are the material of elementary texts, recent experimental and technological advances have made it necessary to come to terms with counterintuitive consequences of electromagnetic fluctuations at short scales -- in the so called {\it near-field} regime. We focus on three manifestations of such behavior: {\bf (i)} The Stefan--Boltzmann law describes thermal radiation from macroscopic bodies, but fails to account for magnitude, polarization and coherence of radiation from small objects (say compared to the skin depth). {\bf (ii)} The heat transfer between two bodies at similar close proximity is dominated by evanescent waves, and can be several orders of magnitude larger than the classical contribution due to propagating waves. {\bf (iii)} Casimir/van der Waals interactions are a dominant force between objects at sub-micron separation; the non-equilibrium analogs of this force (for objects at different temperatures) have not been sufficiently explored (at least experimentally). To explore these phenomena we introduce the tool of fluctuational quantum electrodynamics (QED) originally introduced by Rytov in the 1950s. Combined with a scattering formalism, this enables studies of heat radiation and transfer, equilibrium and non-equilibrium forces for objects of different material properties, shapes, separations and arrangements.

cond-mat.other↗

arXiv2013

High-order harmonic generation driven by metal nanotip photoemission: theory and simulations

We present theoretical predictions of high-order harmonic generation (HHG) resulting from the interaction of short femtosecond laser pulses with metal nanotips. It has been demonstrated that high energy electrons can be generated using nanotips as sources; furthermore the recollision mechanism has been proven to be the physical mechanism behind this photoemission. If recollision exists, it should be possible to convert the laser-gained energy by the electron in the continuum in a high energy photon. Consequently the emission of harmonic radiation appears to be viable, although it has not been experimentally demonstrated hitherto. We employ a quantum mechanical time dependent approach to model the electron dipole moment including both the laser experimental conditions and the bulk matter properties. The use of metal tips shall pave a new way of generating coherent XUV light with a femtosecond laser field.

physics.optics↗

arXiv2013

Residual Stresses in Glasses

The history dependence of the glasses formed from flow-melted steady states by a sudden cessation of the shear rate $\dotγ$ is studied in colloidal suspensions, by molecular dynamics simulations, and mode-coupling theory. In an ideal glass, stresses relax only partially, leaving behind a finite persistent residual stress. For intermediate times, relaxation curves scale as a function of $\dotγt$, even though no flow is present. The macroscopic stress evolution is connected to a length scale of residual liquefaction displayed by microscopic mean-squared displacements. The theory describes this history dependence of glasses sharing the same thermodynamic state variables, but differing static properties.

cond-mat.soft↗

arXiv2010

Density profiles of a colloidal liquid at a wall under shear flow

Using a dynamical density functional theory we analyze the density profile of a colloidal liquid near a wall under shear flow. Due to the symmetries of the system considered, the naive application of dynamical density functional theory does not lead to a shear induced modification of the equilibrium density profile, which would be expected on physical grounds. By introducing a physically motivated dynamic mean field correction we incorporate the missing shear induced interparticle forces into the theory. We find that the shear flow tends to enhance the oscillations in the density profile of hard-spheres at a hard-wall and, at sufficiently high shear rates, induces a nonequilibrium transition to a steady state characterized by planes of particles parallel to the wall. Under gravity, we find that the center-of-mass of the density distribution increases with shear rate, i.e., shear increases the potential energy of the particles.

cond-mat.soft↗

arXiv1996

Two--Dimensional BF Model Quantized in the Axial Gauge

The two--dimensional topological BF model is quantized in the axial gauge. We show that this theory is trivially ultraviolet finite and that the usual infrared problem of the propagator of the scalar field in two dimensions is replaced by an easily solvable long distances problem inherent to the axial gauge. It will also be shown that contrarily to the 3--dimensional case, the action principle cannot be completely replaced by the various Ward identities expressing the symmetries of the model; some of the equation of motion are needed.

hep-th↗