arXiv subjects

Felix Hartmann

Publications and source records attributed to Felix Hartmann.

5 recordsLinked to original sources

arXiv2026

Quantifying non-Markovianity in magnetization dynamics via entropy production rates

Magnetization dynamics is commonly described by the stochastic Landau-Lifshitz-Gilbert (LLG) equation. On picosecond timescales, inertial and open-system extensions of the LLG equation are necessary to interpret recent experiments. We show analytically and numerically that the standard LLG equation exhibits strictly positive entropy production rates, while inertial and open-system LLG dynamics display temporarily negative entropy production rates indicating non-Markovianity. Here we quantify the degree of non-Markovianity using established measures. Our numerical calculations show that the open-system LLG equation consistently exhibits the highest magnitude of non-Markovianity for different initial conditions and magnetic field orientations.

cond-mat.stat-mech↗

arXiv2025

Intrinsic non-Markovian magnetisation dynamics

Memory effects arise in many complex systems, from protein folding, to the spreading of epidemics and financial decisions. While so-called non-Markovian dynamics is common in larger systems with interacting components, observations in fundamental physical systems have been confined to specifically engineered cases. Here, we report the experimental observation of non-Markovian dynamics in an elemental material, crystalline cobalt. By driving this material with an intense terahertz electromagnetic field, we bring its magnetisation into a non-equilibrium state and follow its evolution. We measure the sample's low temperature magnetic response in the time domain which leads to an unexpectedly rich multi-peaked spectrum in the Fourier domain, that cannot be explained by established models. We use open quantum system theory, which predicts a non-Markovian memory kernel in the dynamical equations to capture the fundamental interaction between the spin system and the phonon bath. Simulations based on this theory produce a multi-peaked spectrum, which matches the measured one. Our non-Markovian approach is also able to reproduce the modification of the spectrum at higher temperatures. Our findings demonstrate that non-Markovian effects are observable at a much more fundamental level than previously thought, opening the door to their exploration and control in a broad range of condensed matter systems.

quant-ph↗

arXiv2025

Atomistic spin dynamics with quantum colored noise

The accurate prediction of temperature-dependent magnetization dynamics is a fundamental challenge in computational magnetism. While Atomistic Spin Dynamics (ASD) simulations have emerged as a powerful tool for studying magnetic phenomena, their classical nature leads to significant deviations from experimental observations, particularly at low temperatures. Here we present a comprehensive implementation of quantum-corrected ASD into the Vampire software package, based on the open-system Landau-Lifshitz-Gilbert equation with a quantum thermostat. Our implementation incorporates memory effects along with colored noise derived from quantum-mechanical considerations that improve the description of the equilibrium magnetization. We demonstrate excellent quantitative agreement with experimental magnetization curves for nickel and gadolinium across the full temperature range. Our results establish that incorporating quantum environmental effects and colored noise substantially enhances the predictive capabilities of ASD simulations, providing a robust framework for modeling temperature-dependent magnetic phenomena in localized moment magnetic systems.

cond-mat.mtrl-sci↗

arXiv2024

Pb transport on Si(111)-(7x7) following tip induced imbalance

Pb on Si(111)-(7x7) shows surprising nucleation and mass transport dynamics at odds with standard theories. To create local imbalances on stable Pb islands we use the tip of a scanning force microscope. We enforce a short, local contact between the island and our tip. The subsequent island height growth and the local contact potential difference are studied via scanning force microscopy and Kelvin probe force microscopy. Though the island has a large volume increase after the contact, we observe that its surrounding wetting layer shows the same Pb density decrease as the global wetting layer. This indicates a collective density thinning of the wetting layer.

cond-mat.mes-hall↗

arXiv2023

Anisotropic signatures in the spin-boson model

Thermal equilibrium properties of nanoscale systems deviate from standard macroscopic predictions due to a non-negligible coupling to the environment. For anisotropic three-dimensional materials, we derive the mean force corrections to the equilibrium state of a classical spin vector. The result is valid at arbitrary coupling strength. Specifically, we consider cubic, orthorhombic, and monoclinic symmetries, and compare their spin expectation values as a function of temperature. We underpin the correctness of the mean force state by evidencing its match with the steady state of the simulated non-Markovian spin dynamics. The results show an explicit dependence on the symmetry of the confining material. In addition, some coupling symmetries show a spin alignment transition at zero temperature. Finally, we quantify the work extraction potential of the mean force-generated inhomogeneities in the energy shells. Such inhomogeneities constitute a classical equivalent to quantum coherences.

cond-mat.stat-mech↗