arXiv subjects

R. Matthias Geilhufe

Publications and source records attributed to R. Matthias Geilhufe.

At least 19 recordsLinked to original sources

arXiv2026

Unifying microscopic theories for the phono-magnetic effect

Phonon angular momentum induces an effective magnetic field, a phenomenon called phono-magnetic effect, which has been measured to vary significantly in size. Here, we compare three approaches for deriving the effective magnetic field of a phonon, using electron-phonon coupling and orbital magnetism. The adiabatic approach assumes a slow ionic motion, keeping electrons in the ground state. The perturbative approach treats the electron-phonon interaction as a perturbation to the electronic ground state, and the Floquet approach is based on the time-periodicity of the circular ionic motion. We show that all three approaches lead to the same effective Hamiltonian in the low-frequency limit, which moves us closer towards a unified theory of the phono-magnetic effect. Furthermore, we identify two phononic contributions to the sample magnetization, spontaneous and induced. Thus, we clarify the role of the effective magnetic field in the phonon-induced magnetization. We conclude by providing a numerical estimate for the effective magnetic field in SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2026

Multiband Superconductivity in the Exactly Solvable Hatsugai-Kohmoto Model

Multiband superconductivity gives rise to a rich landscape of possible pairing states. Here we study superconductivity in the multiband extension of the Hatsugai-Kohmoto model, an exactly solvable model of correlated electrons with momentum-local interactions, which provides a minimal framework to explore the interplay of strong correlations, orbital structure and pairing symmetry. Focusing on a two-orbital system with point-group symmetry $\rm D_{4h}$, we classify the symmetry-allowed superconducting gap structures, taking into account spin, orbital and momentum degrees of freedom. We further compute the critical temperature and the superconducting order parameter for selected pairing channels as functions of interaction and pairing strength within a mean-field treatment. Our results provide a systematic framework for analyzing superconductivity in the orbital Hatsugai-Kohmoto model and extend symmetry-based approaches to correlated multiband settings.

cond-mat.supr-con↗

arXiv2026

Collective Buckling in Metal-Organic Framework Materials

We develop a framework to describe collective buckling in metal-organic frameworks (MOFs). Starting from the microscopic structure of a single organic linker, we define a buckling coordinate governed by an effective double-well potential. Coupling between linkers is introduced within a dipole-dipole approximation, resulting in an effective lattice Hamiltonian. We analyze the transition between ordered and disordered phases within a mean-field approximation and estimate the critical temperature. As an illustrative example for our theory, we discuss the collective buckling instability for the prototypical cubic framework MOF-5 under different values of uniaxial strain. Our approach provides a quantitative description of collective buckling in framework materials.

cond-mat.stat-mech↗

arXiv2026

Non-Markovian heat production in ultrafast phonon dynamics

High-intensity THz laser pulses enable the light-mediated control of lattice vibrations by resonantly driving selected phonon modes. On ultrafast timescales, memory effects influence the phonon dynamics and must be accounted for to describe the heat production associated with energy dissipation. Here, we establish a microscopic framework for non-Markovian phonon dynamics by deriving the noise and dissipation kernels governing a driven phonon mode. Using large-scale molecular dynamics simulations, we reconstruct these kernels directly from the many-body lattice dynamics and determine the corresponding heat production rate. Our results provide a quantitative picture of the crossover between Markovian and non-Markovian dynamics on picosecond timescales and show how the finite bandwidth of the driving field limits the dynamically relevant bath spectrum. Furthermore, we demonstrate that thermodynamic quantities such as heat production can be inferred directly from the dynamics of an individual phonon mode, enabling their experimental measurement using time-resolved spectroscopy.

cond-mat.mtrl-sci↗

arXiv2026

Fluctuation-induced quadrupole order in magneto-electric materials

Phases that go beyond dipolar ordering and into multipolar ordering have recently been observed in magneto-electric materials. The resulting phase diagram is commonly explained using the concept of competing orders and exact microscopic interactions. In contrast, we propose an approach based on composite order emerging from a parent phase to explain quadrupoling above magnetic or electric dipolar orders. We include thermal fluctuations and symmetry and show their influence on the emergence of quadrupolar order. We find an analytical expression for the quadrupolar transition temperature, the critical anisotropy and explain the coupling of the quadrupolar order to mechanical strain, in agreement with experiments. The shift in perspective on quadrupolar ordering from competing to composite order is universal and can be extended to other types of multipolar ordering. This offers the possibility of understanding tunability and material-specific predictions of the related phase transitions without explicit knowledge of the microscopic mechanisms.

cond-mat.stat-mech↗

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

Axial phono-magnetic effects

Axial or circularly polarized phonons are collective lattice vibrations with angular momentum. Over the past decade they have emerged as a promising mechanism for the manipulation of magnetism, in parallel to well established optical protocols. In particular, coherent axial phonons were shown to induce magnetization in materials without spin-ordering, making them a viable tool for ultrafast magnetic switching. The experimental evidence suggests that the size of this magnetization is significant, opening a new research area on the phono-magnetic effect. Remarkably, the coupling of axial phonons to magnetism has been observed a broad class of materials, pointing to a universal nature of the underlying mechanisms. In this review article, we present the recent progress in the field. We give an introduction to the phenomenological perspective and an overview of the experimental evidence for the magnetization emerging from axial phonons, which includes discussing the observations of phonon Zeeman effect, the magneto-optical Kerr effect and the proximity-induced magnetization switching. We present recently proposed microscopic theories for the phono-magnetic effects, based on perturbation theory, adiabatic motion and Floquet theory as well as the emergence of the phonon magnetic moment due to artificial gauge fields or inertial effects. This summary allows us to see correspondences between the seemingly different theoretical approaches, facilitating a more complete perspective of the effect.

cond-mat.mtrl-sci↗

arXiv2025

Inertial Repulsion from Quantum Geometry

We derive a repulsive, charge-dipole-like interaction for a Dirac particle in a rotating frame, arising from a geometric $U(1)$ gauge symmetry associated with the Berry phase. The Lagrangian of this system includes a non-inertial correction due to centrifugal field coupling. By imposing gauge symmetry and treating it as a full gauge theory, the Lagrangian is extended to include Berry connection and curvature terms. Upon integrating out the geometric gauge field, the effective action is obtained. This leads to the emergence of a repulsive, long-range effective interaction in the Lagrangian. Explicitly, in the non-inertial frame of the observer, the geometric gauge invariance effectively leads to a repulsive Coulomb-interaction in momentum space. In real space, the inertial repulsion manifests in a $1/\vert r\vert^{2}$ potential, which is symmetric about the origin of rotation and mirrors charge-dipole interaction.

cond-mat.other↗

arXiv2025

Prediction of Molecular Single-Photon Emitters: A Materials-Modelling Approach

Interfacing light with quantum systems is an integral part of quantum technology, with the most essential building block being single-photon emitters. Although various platforms exist, each with its individual strengths, molecular emitters boast a unique advantage -- namely the flexibility to tailor their design to fit the requirements of a specific task. However, the characteristics of the vast space of possible molecular configurations are challenging to understand and explore. Here, we present a theoretical and computational framework to initiate exploration of this vast potential by integrating database analysis with microscopic predictions. Using a model system of dibenzoterrylene in an anthracene host as benchmark, our approach identifies promising new candidates, among them a chiral molecular emitter. Future extensions of our approach integrated with machine learning routines hold the promise of ultimately unlocking the full potential of molecular quantum light-matter interfaces.

quant-ph↗

arXiv2025

Correlated Electrons and Magnetism in Double Perovskites

This paper is an overview of some recent work done on the double perovskites. We discuss the physics of selected double perovskite compounds emphasizing the relevant interactions and resulting observable phenomena such as the magnetic order using different theoretical approaches. Spin-Orbit interaction, which is comparable to other relevant interaction strengths, plays a central role in determining the physics of such 4d-5d perovskites.

cond-mat.str-el↗

arXiv2024

Ultrafast Entropy Production in Non-Equilibrium Magnets

We present an ultrafast thermodynamics framework to model heat generation and entropy production in laser-driven ferromagnetic systems. By establishing a connection between the magnetic field strength of the laser pulse and magnetization dynamics we model time-dependent entropy production rates and deduce the associated heat dissipation in epitaxial and polycrystalline FeNi and CoFeB thin films. Our theoretical predictions are validated by comparison to experimental magnetization dynamics data, shedding light on thermodynamic processes on picosecond timescales. Crucially, we incorporate recently observed inertial spin dynamics, to describe their impact on heat generation in pump-probe experiments. As such, this formalism provides novel insights into controlling heat production in magnetic systems, and contributes to advancing the understanding of non-equilibrium thermodynamics in magnetic systems, with implications for future experimental protocols in spintronics and nanotechnology.

cond-mat.stat-mech↗

arXiv2024

Composite quadrupole order in ferroic and multiferroic materials

The formalism of composite and intertwined orders has been remarkably successful in discussing the complex phase diagrams of strongly correlated materials and high-$T_c$ superconductors. Here, we propose that composite orders are also realized in ferroelectric and ferromagnetic materials when lattice anisotropy is taken into account. This composite order emerges above the ferroic phase transition, and its existence is determined by the easy axis of magnetization or polarization, respectively. In multiferroic materials, where polarization and magnetization are coupled, composites of both orders are possible. This formalism of composite orders naturally accounts for magnetoelectric monopole, toroidal, and quadrupole orders. More broadly, composite orders may explain precursor phenomena in incipient (multi)ferroic materials, arising at temperatures above the ferroic phase transition and potentially contributing to the characterization of currently hidden orders.

cond-mat.str-el↗

arXiv2024

Phonon Inverse Faraday effect from electron-phonon coupling

The phonon inverse Faraday effect describes the emergence of a DC magnetization due to circularly polarized phonons. In this work we present a microscopic formalism for the phonon inverse Faraday effect. The formalism is based on time-dependent second order perturbation theory and electron phonon coupling. While our final equation is general and material independent, we provide estimates for the effective magnetic field expected for the ferroelectric soft mode in the oxide perovskite SrTiO$_3$. Our estimates are consistent with recent experiments showing a huge magnetization after a coherent excitation of circularly polarized phonons with THz laser light. Hence, the theoretical approach presented here is promising for shedding light into the microscopic mechanism of angular momentum transfer between ionic and electronic angular momentum, which is expected to play a central role in the phononic manipulation of magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast entropy production in pump-probe experiments

The ultrafast control of materials has opened the possibility to investigate non-equilibrium states of matter with striking properties, such as transient superconductivity and ferroelectricity, ultrafast magnetization and demagnetization, as well as Floquet engineering. The characterization of the ultrafast thermodynamic properties within the material is key for their control and design. Here, we develop the ultrafast stochastic thermodynamics for laser-excited phonons. We calculate the entropy production and heat absorbed from experimental data for single phonon modes of driven materials from time-resolved X-ray scattering experiments where the crystal is excited by a laser pulse. The spectral entropy production is calculated for SrTiO$_3$ and KTaO$_3$ for different temperatures and reveals a striking relation with the power spectrum of the displacement-displacement correlation function by inducing a broad peak beside the eigenmode-resonance.

cond-mat.mtrl-sci↗

arXiv2023

Correlation Driven Magnetic Frustration and Insulating Behavior of TiF$_3$

We investigate the halide perovskite TiF$_3$, renowned for its intricate interplay between structure, electronic correlations, magnetism, and thermal expansion. Despite its simple structure, understanding its low-temperature magnetic behavior has been a challenge. Previous theories proposed antiferromagnetic ordering. In contrast, experimental signatures for an ordered magnetic state are absent down to 10~K. Our current study has successfully reevaluated the theoretical modeling of TiF$_3$, unveiling the significance of strong electronic correlations as the key driver for its insulating behavior and magnetic frustration. In addition, our frequency-dependent optical reflectivity measurements exhibit clear signs of an insulating state. Analysis of the calculated magnetic data gives an antiferromagnetic exchange coupling with a net Weiss temperature of order 25~K as well as a magnetic response consistent with a $S$=1/2 local moment per Ti$^{3+}$. Yet, the system shows no susceptibility peak at this temperature scale and appears free of long-range antiferromagnetic order down to 1~K. Extending ab initio modeling of the material to larger unit cells shows a tendency for relaxing into a non-collinear magnetic ordering, with a shallow energy landscape between several magnetic ground states, promoting the status of this simple, nearly cubic perovskite structured material as a candidate spin liquid.

cond-mat.str-el↗

arXiv2022

Electron magnetic moment of transient chiral phonons in KTaO$_3$

High intensity THz lasers allow for the coherent excitation of individual phonon modes. The ultrafast control of emergent magnetism by means of phonons opens up new tuning mechanisms for functional materials. While theoretically predicted phonon magnetic moments are tiny, recent experiments hint towards a significant magnetization in various materials. To explain these phenomena, we derive a coupling mechanism between the phonon angular momentum and the electron spin. This coupling introduces the transient level-splitting of spin-up and spin-down channels and a resulting magnetization. We estimate this magnetization on the example of the lowest infrared active mode in the perovskite KTaO$_3$. Our results show an electronic magnetic moment of $\approx 10^{-1}$ $μ_B$ per unit cell, depending on the doping level and electron temperature.

cond-mat.mtrl-sci↗

arXiv2022

Coordination chemistry in molecular symmetry adapted spin space (mSASS)

Many areas of chemistry are devoted to the challenge of understanding, predicting, and controlling the behavior of strongly localized electrons. Examples include molecular magnetism and luminescence, color centers in crystals, photochemistry and quantum sensing to name but a few. Over the years, an amalgam of powerful quantum chemistry methods, simple intuitive models, and phenomenological parameterizations have been developed, providing increasingly complex and specialized methodologies. Even with increasing specialization, a pervasive challenge remains that is surprisingly universal - the simultaneous description of continuous symmetries (e.g. spin and orbital angular momenta) and discrete symmetries (e.g. crystal field). Modeling behavior in these complex systems is increasingly important for metal ions of unusual or technologically relevant behavior. Additionally, development of broad-scope models with physically-meaningful parameters carries the potential to facilitate interdisciplinary collaboration and large-scale meta analysis. We propose a generalized algorithmic approach, the molecular symmetry adapted spin space (mSASS), to localized electronic structure. We derive the Hamiltonian in symmetry-constrained matrix form with an exact account of free parameters and examples. Although preliminary in its implementation, a fundamental benefit of this approach is the treatment of spatial and spin-orbit symmetries without the need for perturbative approximations. In general, the mSASS Hamiltonian is large but finite and can be diagonalized numerically with high efficiency, providing a basis for conceptual models of electronic structure that naturally incorporates spin while leveraging the intuition and efficiency benefits of crystallographic symmetry. For the generation of the mSASS Hamiltonian, we provide an implementation into the Mathematica Software Package, GTPack.

physics.chem-ph↗

arXiv2021

Quantum buckling in metal-organic framework materials

Metal organic frameworks are porous materials composed of metal ions or clusters coordinated by organic molecules. As a response to applied uniaxial pressure, molecules of straight shape in the framework start to buckle. Under sufficiently low temperatures, this buckling is of quantum nature, described by a superposition of degenerate buckling states. Buckling states of adjacent molecules couple in a transverse Ising type behavior. On the example of the metal organic framework topology MOF-5 we derive the phase diagram under applied strain, showing a normal, a parabuckling, and a ferrobuckling phase. At zero temperature, quantum phase transitions between the three phases can be induced by strain. This novel type of order opens a new path towards strain induced quantum phases.

cond-mat.mes-hall↗