arXiv subjects

Carsten Henkel

Publications and source records attributed to Carsten Henkel.

At least 19 recordsLinked to original sources

arXiv2026

Quantum thermodynamics near the border of a one-dimensional Bose gas

We consider an ultracold Bose gas in a half-open potential well, otherwise confined to a quasi-one-dimensional geometry. The Bogoliubov equations for its elementary excitations are solved in the continuous spectrum to explore the particle and energy content near the edge of the gas, beyond the often applied local density approximation. In particular, gradients in the condensate density enhance density-dominated excitations in the border region. We discuss excess (missing) particles and their energy by comparing to suitable reference solutions for quasi-homogeneous systems. The density profile near the edge shows no Friedel oscillations, but a dipolar feature from the spill-out of thermally excited particles. The calculations are performed in the grand-canonical ensemble and in the thermodynamic limit.

cond-mat.quant-gas↗

arXiv2026

Electrons Hopping across a Molecular Network: Spectra and Symmetries

We investigate interacting spinless electrons on finite molecular ring networks described by a tight-binding Hubbard Hamiltonian. The interplay between lattice geometry, Coulomb interactions and discrete symmetries is analysed for rings with $L=3,4,5,6$ nodes, filled with one, two or three electrons. Special attention is devoted to the role of the network symmetries in determining the structure of the many-body spectrum and the Mulliken classification of the eigenstates. Using group-theoretical methods, we examine the evolution of the spectra in the presence of an external magnetic flux. The Zeeman effect lifts degeneracies and results in combination with the Coulomb interaction to avoided crossings in symmetry sectors. We identify a qualitatively distinction between systems with an even and odd number of particles. At half-filling, particle-hole symmetry (duality) protects selected symmetry sectors against Zeeman splitting.

quant-ph↗

arXiv2025

Quantum Listenings -- Amateur Sonification of Vacuum and other Noises

The sensory perceptions of vision and sound may be considered as complementary doorways towards interpreting and understanding physical phenomena. We provide a few selected samples where scientific data of systems usually not directly accessible to humans may be listened to. The examples are chosen close to the regime where quantum mechanics is applicable. Visual and auditory renderings are compared with some connections to music, illustrating in particular a kind of fractal complexity along the time axis.

physics.pop-ph↗

arXiv2025

Reflection phase shifts of bouncing Bogoliubov waves

The Bogoliubov-de Gennes equations are solved for an inhomogeneous condensate in the vicinity of a turning point, addressing the full continuous spectrum. A basis change in the space of the two Bogoliubov "particle" and "hole" amplitudes is introduced that decouples them approximately. We find a spatially extended mode that governs mainly excitations in the condensate phase, while another mode is localised to regions with density gradients. An analytical and numerical discussion of the phase shift is provided that incident matter waves suffer upon reflection at the turning point, forming standing waves. As an application, we compute eigenfrequencies in a gravitational trap, without recourse to the local density approximation. The non-condensate density at finite temperature and its quantum depletion are discussed in a companion paper.

cond-mat.quant-gas↗

arXiv2025

Quantum Borderlines -- Fluctuation Energies in Ultracold Bose Gases

Ultracold Bose gases are many-body systems with well-defined particle interactions that may serve as models for interacting quantum fields. The impact of virtual excitations is studied in the spatial transition zone created by a soft confinement potential that separates a degenerate ideal gas from a dense quasi-condensate. We compute within Bogoliubov theory the contribution of quasi-particles to the surface energy at zero temperature.

cond-mat.quant-gas↗

arXiv2024

Electromagnetic Response of the Electron Gas and the Thermal Casimir Pressure Anomaly

A review of the nonlocal electromagnetic response functions for the degenerate electron gas, computed within standard perturbation theory, is given. These expressions due to Lindhard, Klimontovich and Silin are used to re-analyze the Casimir interaction between two thick conducting plates in the leading order at high temperatures (zero'th term of Matsubara series). Up to small corrections that we discuss, the results of the conventional Drude model are confirmed. The difference between longitudinal and transverse permittivities (or polarization tensors) yields the Landau (orbital) diamagnetism of the electron gas.

quant-ph↗

arXiv2023

Rectified Lorentz Force from Thermal Current Fluctuations

In a conducting medium held at finite temperature, free carriers are performing Brownian motion and generate fluctuating electromagnetic fields. We compute the averaged Lorentz force density that turns out nonzero in a thin sub-surface layer, pointing towards the surface, while vanishing in the bulk. This is an elementary example of rectified fluctuations, similar to the Casimir force or radiative heat transport. Our results also provide an experimental way to distinguish between the Drude and so-called plasma models.

quant-ph↗

arXiv2023

Long-time equilibration can determine transient thermality

When two initially thermal many-body systems start interacting strongly, their transient states quickly become non-Gibbsian, even if the systems eventually equilibrate. To see beyond this apparent lack of structure during the transient regime, we use a refined notion of thermality, which we call g-local. A system is g-locally thermal if the states of all its small subsystems are marginals of global thermal states. We numerically demonstrate for two harmonic lattices that whenever the total system equilibrates in the long run, each lattice remains g-locally thermal at all times, including the transient regime. This is true even when the lattices have long-range interactions within them. In all cases, we find that the equilibrium is described by the generalized Gibbs ensemble, with three-dimensional lattices requiring special treatment due to their extended set of conserved charges. We compare our findings with the well-known two-temperature model. While its standard form is not valid beyond weak coupling, we show that at strong coupling it can be partially salvaged by adopting the concept of a g-local temperature.

quant-ph↗

arXiv2023

Limit on spatial quantum superpositions with massive objects due to phonons

It has been a long-standing goal to bring massive objects into a superposition of different locations in real space, not only to confirm quantum theory in new regimes, but also to explore the interface with gravity. The main challenge is usually thought to arise from forces or scattering due to environmental fields and particles that decohere the large object's wave function into a statistical mixture. We unveil a decoherence channel which cannot be eliminated by improved isolation from the environment. It originates from sound waves within the object, which are excited as part of any splitting process and carry partial "Welcher Weg" information. This puts stringent constraints on future spatial superpositions of large objects.

quant-ph↗

arXiv2022

Coupling function from bath density of states

Modelling of an open quantum system requires knowledge of parameters that specify how it couples to its environment. However, beyond relaxation rates, realistic parameters for specific environments and materials are rarely known. Here we present a method of inferring the coupling between a generic system and its bosonic (e.g., phononic) environment from the experimentally measurable density of states (DOS). With it we confirm that the DOS of the well-known Debye model for three-dimensional solids is physically equivalent to choosing an Ohmic bath. We further match a real phonon DOS to a series of Lorentzian coupling functions, allowing us to determine coupling parameters for gold, yttrium iron garnet (YIG) and iron as examples. The results illustrate how to obtain material-specific dynamical properties, such as memory kernels. The proposed method opens the door to more accurate modelling of relaxation dynamics, for example for phonon-dominated spin damping in magnetic materials.

quant-ph↗

arXiv2022

Internal decoherence in nano-object interferometry due to phonons

We discuss the coherent splitting and recombining of a nanoparticle in a mesoscopic "closed-loop" Stern-Gerlach interferometer in which the observable is the spin of a single impurity embedded in the particle. This spin, when interacting with a pulsed magnetic gradient, generates the force on the particle. We calculate the internal decoherence which arises as the displaced impurity excites internal degrees of freedom (phonons) that may provide Welcher Weg information and preclude interference. We estimate the constraints this decoherence channel puts on future interference experiments with massive objects. We find that for a wide range of masses, forces and temperatures, phonons do not inhibit Stern-Gerlach interferometry with micro-scale objects. However, phonons do constitute a fundamental limit on the splitting of larger macroscopic objects if the applied force induces phonons.

quant-ph↗

arXiv2022

Electron spin spectroscopy of single TEMPO dimers coupled via interfering tunneling currents

We report the detection of electron spin resonance (ESR) in individual dimers of the stable free radical 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO). ESR is measured by the current fluctuations in a scanning tunnelling microscope (ESR-STM method). The multi-peak power spectra, distinct from macroscopic data, are assigned to dimers having exchange and Dzyaloshinskii-Moriya interactions in presence of spin-orbit coupling. These interactions are generated in our model by interfering electronic tunneling pathways from tip to sample via the dimer's two molecules. This is the first demonstration that tunneling via two spins is a valid mechanism of the ESR-STM method.

cond-mat.mes-hall↗

arXiv2021

No-slip boundary conditions for electron hydrodynamics and the thermal Casimir pressure

We derive modified reflection coefficients for electromagnetic waves in the THz and far infrared range. The idea is based on hydrodynamic boundary conditions for metallic conduction electrons. The temperature-dependent part of the Casimir pressure between metal plates is evaluated. The results should shed light on the "thermal anomaly" where measurements deviate from the standard fluctuation electrodynamics for conducting metals.

quant-ph↗

arXiv2021

Heat transfer and entanglement -- non-equilibrium correlation spectra of two quantum oscillators

The non-equilibrium state of two oscillators with a mutual interaction and coupled to separate heat baths is discussed. Bosonic baths are considered, and an exact spectral representation for the elements of the covariance matrix is provided analytically. A wide class of spectral densities for the relevant bath modes is allowed for. The validity of the fluctuation-dissipation theorem is established for global equilibrium (both baths at the same temperature) in the stationary state. Spectral measures of entanglement are suggested by comparing to the equilibrium spectrum of zero-point fluctuations. No rotating-wave approximation is applied, and anomalous heat transport from cold to hot bath, as reported in earlier work, is demonstrated not to occur.

quant-ph↗

arXiv2021

Spin entanglement via STM current

We consider a system of two spins under a scanning tunneling microscope bias and derive its master equation. We find that the tunneling elements to the electronic contacts (tip and substrate) generate an exchange interaction between the spins, as well as a Dzyaloshinskii-Moriya interaction in the presence of spin-orbit coupling. The tunnel current spectrum then shows additional lines compared to conventional spin resonance experiments. When the spins have degenerate Larmor frequencies and equal tunneling amplitudes (without spin-orbit), there is a dark state with vanishing decay rate. The coupling to the electronic environment generates significant spin-spin entanglement via the dark state, even if the initial state is non-entangled.

cond-mat.mes-hall↗

arXiv2020

Realization of a complete Stern-Gerlach interferometer: Towards a test of quantum gravity

The Stern-Gerlach effect, discovered a century ago, has become a paradigm of quantum mechanics. Surprisingly there has been little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Specifically, no full-loop Stern-Gerlach interferometer has been realized with the scheme as envisioned decades ago. Furthermore, several theoretical studies have explained why such an interferometer is a formidable challenge. Here we provide a detailed account of the first full-loop Stern-Gerlach interferometer realization, based on highly accurate magnetic fields, originating from an atom chip, that ensure coherent operation within strict constraints described by previous theoretical analyses. Achieving this high level of control over magnetic gradients is expected to facilitate technological as well as fundamental applications, such as probing the interface of quantum mechanics and gravity. While the experimental realization described here is for a single atom, future challenges would benefit from utilizing macroscopic objects doped with a single spin. Specifically, we show that such an experiment is in principle feasible, opening the door to a new era of fundamental probes.

quant-ph↗

arXiv2020

Vacuum-Induced Saturation in Plasmonic Nanoparticles

Vacuum fluctuations are a fundamental feature of quantized fields. It is usually assumed that observations connected to vacuum fluctuations require a system well isolated from other influences. In this work, we demonstrate that effects of the quantum vacuum can already occur in simple colloidal nano-assemblies prepared by wet chemistry. We claim that the electromagnetic field fluctuations at the zero-point level saturate the absorption of dye molecules self-assembled at the surface of plasmonic nano-resonators. For this effect to occur, reaching the strong coupling regime between the plasmons and excitons is not required. This intriguing effect of vacuum-induced saturation (VISA) is discussed within a simple quantum optics picture and demonstrated by comparing the optical spectra of hybrid gold-core dye-shell nanorods to electromagnetic simulations.

physics.optics↗

arXiv2020

Remarks about surface plasmons and their stability

We comment on the macroscopic model for surface plasmons of H.-Y. Deng [New J. Phys. 21 (2019) 043055; arXiv:1712.06101] and a claim, based on energy conversion from charges to the electric field, that surface plasmons on metallic surfaces may become unstable [J. Phys.: Cond. Matt. 29 (2017) 455002; arXiv:1606.06239, 1701.01060]. The discussion revolves around the formulation of charge conservation in the bulk and the surface of a metal. We elaborate in particular on the role of a finite electric current normal to the surface. Using a scheme of Cercignani and Lampis and of Zaremba, we point out that the model chosen by Deng for the non-specular scattering of electrons needs to be amended to prevent the disappearance of charges at the surface. Different models and approaches in the literature on surface plasmons are reviewed: the interfacial excess field approach of Bedeaux and Vlieger which contains Deng's macroscopic model, the assumption of specular reflection of Ritchie and Marusak, a hydrodynamic model with a composite charge density (partially localized at the surface), the local dielectric model, and a macroscopic method with (anti)symmetric fictitious stimuli (used, e.g., by Garc\'ıa-Moliner and Flores). This puts Deng's results into perspective and illustrates problems with his approach.

physics.optics↗