arXiv subjects

Roel Tempelaar

Publications and source records attributed to Roel Tempelaar.

At least 19 recordsLinked to original sources

arXiv2026

How Symmetry Governs the Dihedral Angle Dependence of Intermolecular Spin-Orbit Coupling

Spin-orbit, charge-transfer intersystem crossing (SOCT-ISC) allows for the efficient production of triplet excited states in donor-acceptor (DA) dyads without the involvement of heavy atoms, for use in a myriad of technologies. This process is commonly believed to proceed optimally when the dihedral angle between donor and acceptor moieties is orthogonal. Here, we challenge this idea through a theoretical study unveiling a scenario where spin-orbit couplings (SOCs) are minimized under orthogonal conditions. This scenario is rationalized based on an analysis of the structure-imposed symmetry properties of the involved singlet and triplet states. Notably, in this scenario, finite SOCs demand oblique orientation angles, which in turn requires molecular chirality, suggesting chirality to be a prerequisite for activating the involved SOC pathways.

physics.chem-ph↗

arXiv2025

QC Lab: A Python Package for Quantum-Classical Dynamics

QC Lab is an open-source Python package for QC dynamics simulations aimed to promote the development of QC algorithms, and their application to a wide variety of relevant model problems. It follows a modular design that facilitates cross-compatibility between algorithms and models. By decomposing algorithms and models into a series of tasks and ingredients that can be substituted and reused, it minimizes development efforts and code redundancy. In this Paper, we introduce the first stable release of QC Lab, and describe its design philosophy.

physics.chem-ph↗

arXiv2025

Focused Sampling for Low-Cost and Accurate Ehrenfest Modeling of Cavity Quantum Electrodynamics

An economic modeling approach for cavity quantum electrodynamics is provided by mean-field dynamics, wherein the optical field is described classically while a self-consistent interaction with quantum emitters is incorporated through the Ehrenfest theorem. However, conventional implementations of mean-field dynamics are known to suffer from a catastrophic leakage of zero-point energy, to lose accuracy in the short-cavity limit, and to require large numbers of trajectories to be sampled. Here, we address these three shortcomings within a single integrated approach. This approach builds on our recently-proposed modification of the Ehrenfest theorem, referred to as decoupled mean-field (DC-MF) dynamics, in combination with a focused sampling scheme that enforces zero-point energy at the single-trajectory level. The approach is shown to yield high accuracy in both short and long-cavity limits while reaching convergence within a minimal amount of trajectories.

physics.chem-ph↗

arXiv2025

Optically Addressing Circularly-Polarized Vibrations in Molecules

Circularly-polarized (CP) vibrations are the nuclear-motion analog of CP light, emerging in molecules with non-Abelian point-group symmetry that support orthogonal and degenerate vibrational normal modes. Here, we explore the optical addressability of CP vibrations, motivated by their potential as nanoscale angular momentum states suitable for information storage and manipulation. We investigate how symmetry-breaking chemical modifications affect a molecule's capacity to support CP vibrations. Notably, we find cases where modes retain their orthogonality and degeneracy in spite of such modifications, broadening the opportunities afforded by CP vibrations beyond non-Abelian point-group molecules. Prospects for experimental implementations using CP pump-probe techniques are briefly discussed.

physics.chem-ph↗

arXiv2025

Surface hopping simulations show valley depolarization driven by exciton-phonon resonance

Resonances between excitonic transitions and nuclear coordinates have been shown to drive a variety of excited-state dynamical phenomena in molecular systems. Here, we report mixed quantum--classical simulations showing similar resonances to primarily contribute to valley depolarization in monolayer MoS$_2$. The applied simulation framework combines reciprocal-space surface hopping with microscopic models of the quasiparticle band structure, electron--hole interactions, and carrier--phonon interactions, parametrized against ab initio calculations. This enables low-cost excited-state dynamics simulations that are microscopic, non-Markovian, and non-perturbative in the carrier--phonon interaction. The framework furthermore retains explicit information on transient phonon occupancies, through which we show a resonance between the dominant optical phonon branch and the lowest exciton band to largely drive valley depolarization, by activating a Maialle--Silva--Sham mechanism. Resulting valley polarization times are consistent with experimental measurements across temperatures.

cond-mat.mtrl-sci↗

arXiv2025

Mixed Quantum-Classical Dynamics Yields Anharmonic Rabi Oscillations

We apply a mixed quantum-classical (MQC) approach to the quantum Rabi model, involving a classical optical field coupled self-consistently to a quantum two-level system. Under the rotating wave approximation, we analytically show this approach to yield persistent yet anharmonic Rabi oscillations, governed by an undamped and unforced Duffing equation. We consider the single-quantum limit, where we find such anharmonic Rabi oscillations to closely follow full-quantum results once zero-point energy is approximately enforced when initializing the optical field coordinate. Our findings provide guidance in the application of MQC dynamics to classes of problems involving small quantum numbers and far away from decoherence.

quant-ph↗

arXiv2024

Mixed quantum-classical modeling of exciton-phonon scattering in solids: Application to optical linewidths of monolayer MoS2

We present a mixed quantum-classical framework for the microscopic and non-Markovian modeling of exciton-phonon scattering in solid-state materials, and apply it to calculate the optical linewidths of monolayer MoS2. Within this framework, we combine reciprocal-space mixed quantum-classical dynamics with models for the quasiparticle band structure as well as the electron-hole and carrier-phonon interactions, parametrized against ab initio calculations, although noting that a direct interfacing with ab initio calculations is straightforward in principle. We introduce various parameters for truncating the Brillouin zone to select regions of interest. Variations of these parameters allow us to determine linewidths in the limit of asymptotic material sizes. Obtained asymptotic linewidths are found to agree favorably with experimental measurements across a range of temperatures. As such, our framework establishes itself as a promising route towards unraveling the non-Markovian and microscopic principles governing the nonadiabatic dynamics of solids.

cond-mat.mtrl-sci↗

arXiv2024

Unitary Basis Transformations in Mixed Quantum-Classical Dynamics

A common approach to minimizing the cost of quantum computations is by transforming a quantum system into a basis that can be optimally truncated. Here, we derive classical equations of motion subjected to similar unitary transformations, and propose their integration into mixed quantum-classical dynamics, enabling this class of methods to be applied within arbitrary bases for both the quantum and classical coordinates. To this end, canonical positions and momenta are combined into a set of complex-valued classical coordinates amenable to unitary transformations. We demonstrate the potential of the resulting approach by means of surface hopping calculations of an electronic carrier scattering onto a single impurity in the presence of phonons. Appropriate basis transformations, capturing both the localization of the impurity and the delocalization of higher-energy excitations, are shown to faithfully capture the dynamics within a fraction of the classical and quantum basis sets.

quant-ph↗

arXiv2023

Ehrenfest Modeling of Cavity Vacuum Fluctuations and How to Achieve Emission from a Three-Level Atom

A much-needed solution for the efficient modeling of strong coupling between matter and optical cavity modes is offered by mean-field mixed quantum--classical dynamics, where a classical cavity field interacts self-consistently with quantum states of matter through Ehrenfest's theorem. We previously introduced a modified mean-field approach, referred to as decoupled mean-field (DC-MF) dynamics, wherein vacuum fluctuations of the cavity field are decoupled from the quantum-mechanical ground state as a means to resolve an unphysical drawing of energy from the vacuum fluctuations by a two-level atom. Here, we generalize DC-MF dynamics for an arbitrary number of (nondegenerate) atomic levels, and show that it resolves an unphysical lack of emission from a three-level atom predicted by conventional mean-field dynamics. We furthermore show DC-MF to provide an improved description of reabsorption and (resonant) two-photon emission processes.

physics.chem-ph↗

arXiv2023

Treating geometric phase effects in nonadiabatic dynamics

We present an approach for eliminating the gauge freedom for derivative couplings in nonadiabatic dynamics in the presence of geometric phase effects. This approach relies on a bottom-up construction of a parametric quantum Hamiltonian in terms of functions of a dynamical variable, which can be associated with real and imaginary-valued contributions to the Hamiltonian in a given diabatic basis. By minimizing the deviation of the imaginary functions from a constant we identify a set of diabatic bases that recover the real-valued gauge commonly used for topologically-trivial systems. This minimization, however, also confines the gauge freedom in the topologically-nontrivial case, opening a path towards finding gauge-invariant derivative couplings under geometric phase effects. Encouraging results are presented for fewest-switches surface hopping calculations of a nuclear wavepacket traversing a single avoided crossing, for which fully gauge-invariant derivative couplings are found.

physics.chem-ph↗

arXiv2023

A Mean-Field Treatment of Vacuum Fluctuations in Strong Light-Matter Coupling

Mean-field mixed quantum--classical dynamics could provide a much-needed means to inexpensively model quantum electrodynamical phenomena, by describing the optical field and its vacuum fluctuations classically. However, this approach is known to suffer from an unphysical transfer of energy out of the vacuum fluctuations when the light--matter coupling becomes strong. We highlight this issue for the case of an atom in an optical cavity, and resolve it by introducing an additional set of classical coordinates to specifically represent vacuum fluctuations whose light--matter interaction is scaled by the instantaneous ground-state population of the atom. This not only rigorously prevents the aforementioned unphysical energy transfer, but is also shown to yield a radically improved accuracy in terms of the atomic population and the optical field dynamics, generating results in excellent agreement with full quantum calculations. As such, the resulting method emerges as an attractive solution for the affordable modeling of strong light--matter coupling phenomena involving macroscopic numbers of optical modes.

physics.chem-ph↗

arXiv2022

UV/vis-to-IR Photonic Down Conversion Mediated by Excited State Vibrational Polaritons

This work proposes a new photophysical phenomenon whereby UV/vis excitation of a molecule involving a Franck-Condon (FC) active vibration yields infrared (IR) emission by strong coupling to an optical cavity. The resulting UV/vis-to-IR photonic down conversion process is mediated by vibrational polaritons in the electronic excited state potential. It is shown that the formation of such excited state vibrational polaritons (ESVP) via UV/vis excitation only occurs with molecules having vibrational modes with both a non-zero FC activity and IR activity in the excited state. Density functional theory calculations are shown to effectively identify a candidate molecule, 1-Pyreneacetic acid (PAA), with this property and the dynamics of ESVP are modeled using the truncated Wigner approximation. Overall, this work presents a new avenue of polariton chemistry where excited state dynamics, driven by photoexcitation, are influenced by the formation of vibrational polaritons. Along with this, the photonic down conversion is potentially useful in both the sensing of excited state vibrations and in quantum transduction schemes.

physics.chem-ph↗

arXiv2022

Overcoming positivity violations for density matrices in surface hopping

Fewest-switches surface hopping (FSSH) has emerged as one of the leading methods for modeling the quantum dynamics of molecular systems. While its original formulation was limited to adiabatic populations, the growing interest in the application of FSSH to coherent phenomena prompts the question how one should construct a complete density matrix based on FSSH trajectories. A straightforward solution is to define adiabatic coherences based on wavefunction coefficients. In this Paper, we demonstrate that inconsistencies introduced in the density matrix through such treatment may lead to a violation of positivity. We furthermore show that a recently proposed coherent generalization of FSSH results in density matrices that satisfy positivity, while yielding an improved accuracy throughout much (but not all) of parameter space.

physics.chem-ph↗

arXiv2022

Chiral polaritons based on achiral Fabry-Perot cavities using apparent circular dichroism

Polariton states with high levels of chiral dissymmetry offer exciting prospects for quantum information, sensing, and lasing applications. Such dissymmetry must emanate from either the involved optical resonators or the quantum emitters. Here, we theoretically demonstrate how chiral polaritons can be realized by combining (high quality factor) achiral Fabry-Perot cavities with samples exhibiting a phenomenon known as "apparent circular dichroism" (ACD), which results from an interference between linear birefringence and dichroic interactions. By introducing a quantum electrodynamical theory of ACD, we identify the design rules based on which the dissymmetry of chiral polaritons can be optimized.

physics.chem-ph↗

arXiv2022

Perturbation theory under the truncated Wigner approximation reveals how system-environment entanglement formation drives quantum decoherence

Quantum decoherence is the disappearance of simple phase relations within a discrete quantum system as a result of interactions with an environment. For many applications, the question is not necessarily how to avoid (inevitable) system-environment interactions, but rather how to design environments that optimally preserve a system's phase relations in spite of such interactions. The formation of system-environment entanglement is a major driving mechanism for decoherence, and a detailed understanding of this process could inform strategies for conserving coherence optimally. This requires scalable, flexible, and systematically improvable quantum dynamical methods that retain detailed information about the entanglement properties of the environment, yet very few current methods offer this combination of features. Here, we address this need by introducing a theoretical framework wherein we combine the truncated Wigner approximation with standard time-dependent perturbation theory allowing for computing expectation values of operators in the combined system-environment Hilbert space. We demonstrate the utility of this framework by applying it to the spin-boson model, representative of qubits and simple donor-acceptor systems. For this model, our framework provides an analytical description of perturbative contributions to expectation values. We monitor how quantum decoherence at zero temperature is accompanied by entanglement formation with individual environmental degrees of freedom. Based on this entanglement behavior, we find that the selective suppression of low-frequency environmental modes is particularly effective for mitigating quantum decoherence.

quant-ph↗

arXiv2021

A Reciprocal-Space Formulation of Surface Hopping

Surface hopping has seen great success in describing molecular phenomena where electronic excitations tend to be localized, but its application to materials with band-like electronic properties has remained limited. Here, we derive a formulation of fewest-switches surface hopping where both the quantum and classical equations of motion are solved entirely in terms of reciprocal-space coordinates. The resulting method is directly compatible with band structure calculations, and allows for the efficient description of band-like phenomena by means of a truncation of the Brillouin zone. Using the Holstein and Peierls models as examples, we demonstrate the formal equivalence between real-space and reciprocal-space surface hopping, and assess their accuracy against mean-field mixed quantum--classical dynamics and numerically-exact results.

physics.chem-ph↗

arXiv2021

A Reciprocal-Space Formulation of Mixed Quantum-Classical Dynamics

We derive a formulation of mixed quantum-classical dynamics for describing electronic carriers interacting with phonons in reciprocal space. For dispersionless phonons, we start by expressing the real-space classical coordinates in terms of complex variables. A Fourier series over these coordinates then yields the reciprocal-space coordinates. Evaluating the electron-phonon interaction term through Ehrenfest's theorem, we arrive at a reciprocal-space formalism that is equivalent to mean-field mixed quantum-classical dynamics in real space. This equivalence is numerically verified for the Holstein and Peierls models, for which we find the reciprocal-space Hellmann-Feynman forces to involve momentum derivative contributions in addition to the position derivative terms commonly seen in real space. We close by presenting a proof of concept for the inexpensive modeling of low-momentum carriers interacting with phonons by means of a truncated basis in reciprocal space, which is not possible within a real space formulation.

physics.chem-ph↗

arXiv2020

Microscopic theory of cavity-confined monolayer semiconductors: polariton-induced valley relaxation and the prospect of enhancing and controlling the valley pseudospin by chiral strong coupling

We apply a microscopic theory of exciton-polaritons in cavity-confined monolayer transition-metal dichalcogenides including both optical polarizations in the monolayer plane, allowing to describe how chiral cavity photons interact with the valley degrees of freedom of the active material. Upon polariton formation, the degenerate excitons inhabiting the two inequivalent valleys are shown to assume bonding and antibonding superpositions as a result of cavity-mediated intravalley interactions combined with intervalley Coulomb interactions. This is representative of a polariton-induced coherent mixing of the valley polarization. In combination with disorder, this mixing is prone to open a new valley relaxation channel which attains significance with increasing cavity coupling. Importantly, we show that optical cavities with an asymmetric reflectance of left- and right-handed circularly-polarized photons offer a considerably more robust platform to realize a conserved valley polarization, as the valley localization of excitons is reinstated by an asymmetric Rabi splitting which lifts their degeneracy. Moreover, we show this degeneracy lifting to allow for wavelength-selective access to the valley pseudospin by means of a polariton-induced chiral Stark effect, offering interesting opportunities for valleytronic applications.

cond-mat.mtrl-sci↗