arXiv subjects

Kalman Varga

Publications and source records attributed to Kalman Varga.

At least 19 recordsLinked to original sources

arXiv2026

Exact Electron Dynamics in Linear and Chiral Cavities: Complete Field versus Single-Helicity Reduction

Can a chiral cavity turn a free electron into a Landau-like orbiting particle? We answer with exact solutions of three quadratic electron--cavity models in the dipole limit, valid at arbitrary coupling. A single linearly polarized mode dresses the electron mass, which delays its flight, slows the spreading of a wavepacket and entangles the packet with the cavity. The complete transverse field, with both circular helicities, keeps two commuting, gapless translations at any coupling, mode number and helicity splitting, so the electron stays ballistic and no synthetic magnetic field appears. Keeping a single helicity gives two inequivalent models that are not related by a gauge transformation: reducing before the Power--Zienau--Woolley transformation gives ballistic motion, whereas reducing after it gives a Landau-like crossed-quadrature Hamiltonian. They differ by a real uniform field $B=sλ^2/(2ω_c)$ ($ω_c$ the mode frequency, $λ$ the coupling, $s=\pm1$ the helicity). In the complete theory the frequency $ω_{-s}$ of the partner helicity, measured against $g^2=λ^2/(2ω_c)$, decides between them: the electron is ballistic for a stiff partner, and Landau-like only at the soft-partner point, through an exact canonical reorganization that holds for the active sector but not for observables such as the electron--photon entanglement. Projecting onto one helicity after the PZW transformation is therefore not by itself a mechanism for synthetic magnetism; the mode content of the cavity is.

cond-mat.str-el↗

arXiv2026

Plane-Wave Photon-Fock Cavity QED-DFT: Chiral-Cavity-Induced Topology in Graphene

We develop an explicit plane-wave (PW) $\times$ photon-Fock approach to the quantized, velocity-gauge Pauli--Fierz Hamiltonian for periodic, first-principles calculations: because the quantized vector potential is spatially uniform, the light--matter coupling reduces to an operator-valued shift of the crystal momentum, $K\to K+\hat{\mathbf A}/c$, and the existing plane-wave machinery of ordinary solid-state DFT is reused essentially unchanged. This PW$\times$Fock construction puts first-principles cavity QED of periodic solids on the same footing as the Fock-space coupled-cluster and configuration-interaction methods already developed for molecules, opening hitherto inaccessible systems---materials in a linear or chiral cavity, in particular---to first-principles plane-wave calculations. We illustrate the method with monolayer graphene in linear and chiral cavities, obtaining a polarization-selective Haldane gap together with the corresponding density-of-states, real-space, and circular-dichroism optical signatures. A full Brillouin-zone Berry-curvature calculation confirms the topological character of the chiral-cavity gap directly: the occupied manifold carries a quantized Chern number that steps through a non-monotonic sequence, $C=1\to3\to{-1}\to1\to2\to{-1}\to1$, with two narrow, sign-reversed windows---a property of extended, vacuum-dressed matter with no counterpart in a finite molecular system.

cond-mat.str-el↗

arXiv2026

Size Effects in the Strong-Field Ionization and Dissociation Dynamics of (H$_2$O)$_n$ (n=1-4)

The size-dependent strong-field ionization and dissociation dynamics of (H$_2$O)$_n$ (n=1-4) are investigated using real-time time-dependent density functional theory (RT-TDDFT) coupled to Ehrenfest molecular dynamics under a common few-cycle near-infrared laser pulse. It is found that the net ionization per monomer varies only weakly on cluster size, whereas the protonic and oxygen response is changed much more strongly once the cluster size grows beyond the dimer. In particular, H-ejection activity is observed to rise sharply from the dimer to the trimer/tetramer regime, while stable H-transfer is essentially absent in the dimer under the present criterion but becomes substantial in the trimer and is further amplified in the tetramer. Through timing analyses, it is shown that the dimer exhibits a weak and temporally broad response, whereas the larger clusters display a much stronger early-time protonic response concentrated within and immediately after the laser pulse window. By endpoint oxygen statistics, a systematic increase in dissociation propensity with cluster size is likewise shown. For a clean subset of direct two-body dimer breakup trajectories, the asymptotic kinetic energy release is estimated to be 4.47 $\pm$ 1.03 eV, in reasonably good agreement with the experimental value for the unprotonated two-body Coulomb-explosion channel. Overall, it is shown by the results that increasing water-cluster size primarily reshapes the strong-field response through proton-mediated and topology-level nuclear dynamics rather than through a large change in net ionization alone.

physics.chem-ph↗

arXiv2026

Explicitly Correlated Gaussian Basis Approach to Periodic Systems

Closed-form expressions for all matrix elements required for variational calculation of the electronic structure of periodic solids have been derived using a basis of explicitly correlated Gaussians (ECGs). Periodic basis functions are constructed by summing shifted correlated Gaussians over all composite lattice translations, where a generalized unfolding theorem reduces the resulting double lattice sum to a single sum through a unified computational framework for overlap, kinetic energy, and Coulomb potential operators. The formalism has been validated through application to an infinite one-dimensional hydrogen chain, where the ground-state energy per atom computed in the thermodynamic limit is shown to agree with finite-chain results extrapolated by other many-body methods.

quant-ph↗

arXiv2026

A New Algorithm for Applying Sequences of Affine Transformations in Quantum Circuits

This paper introduces a robust and scalable framework for implementing nested affine transformations in quantum circuits. Utilizing Hadamard-supported conditional initialization and block encoding, the proposed method systematically applies sequential affine transformations while preserving state normalization. This approach provides an effective method for generating combinatorial amplitude patterns within quantum states with demonstrated applications in combinatorics and signal processing. The utility of the framework is exemplified through two key applications: financial risk assessment, where it efficiently computes portfolio returns using combinatorial sum of amplitudes, and discrete signal processing, where it enables precise manipulation of Fourier coefficients for enhanced signal reconstruction.

quant-ph↗

arXiv2026

Quantum-Electrodynamical Time-Dependent Density Functional Theory Description of Molecules Interacting with Light

We study light-mediated interactions between spatially separated molecules using real-time quantum electrodynamical time-dependent density functional theory based on the Pauli-Fierz Hamiltonian. An ultrashort delta-kick excitation selectively perturbs a single molecule, while a second, distant molecule remains initially unexcited. In free space, the excitation stays localized and no response is observed in the second molecule. In contrast, when both molecules are coupled to the same cavity mode, the initial excitation induces coherent dynamics in the distant molecule through the shared quantized electromagnetic field.

physics.chem-ph↗

arXiv2026

Time-dependent density functional theory study of strong-field laser-induced coulomb explosion of the HCl dimer

We present a channel-resolved interpretation of laser-driven Coulomb explosion of the HCl dimer from an ensemble of trajectories. Three dominant outcomes are identified: a minor three-body channel and two four-body channels (sequential and near-simultaneous dissociation of both molecules). The key result is that pathway selection is strongly correlated with the degree of ionization during the laser interaction, which is in turn strongly modulated by laser-molecule orientation. Higher early-time ionization predisposes the system toward near-simultaneous four-body breakup, whereas lower ionization favors sequential and three-body fragmentation; for low-ionization cases, a fragment-resolved charge metric further differentiates three-body and sequential behavior. These charge-dependent trends consistently map onto experimentally accessible observables: the simultaneous mechanism dominates the high-energy tail of the kinetic energy release (KER) spectrum and populate distinct regions of the emission-angle distributions, while sequential events concentrate at lower KER. Overall, early-time charge evolution provides a unifying explanation for channel branching and for the channel-resolved fragmentation signatures.

physics.chem-ph↗

arXiv2025

Quantum-electrodynamical time-dependent density functional theory description of molecules in optical cavities

A quantum electrodynamical time-dependent density functional theory framework is applied to describe strongly coupled light--matter interactions in cavity environments. The formalism utilizes a tensor product approach, coupling real-space electronic wave functions with Fock space photonic states. Various molecular systems serve as test cases to examine how coupling parameters and cavity frequencies affect molecular geometry, polaritonic spectra, and intermolecular binding.

physics.chem-ph↗

arXiv2025

Ultrafast Relaxation Dynamics of Inner-Shell Vacancies in Hydrated Pyrrole

We employ real-space, real-time time-dependent density functional theory (TDDFT) combined with Ehrenfest dynamics to investigate ultrafast intermolecular relaxation following inner-valence ionization in hydrated pyrrole. This time-dependent approach treats electronic and nuclear motions simultaneously, allowing the description of electronic excitation, charge transfer, ionization, and nuclear motion.When the initial vacancy in the O 2s 1 state is created on the water molecule, the system predominantly undergoes intermolecular Coulombic decay (ICD) and electron-transfer mediated decay (ETMD), accompanied by pronounced charge transfer between pyrrole and water. In contrast, ionization of the pyrrole site for N 2s electron leads to both ICD and Auger decay channels. These results demonstrate that the decay dynamics are strongly governed by the initial vacancy location, offering microscopic insight into intermolecular energy-transfer mechanisms in hydrated molecular systems.

physics.chem-ph↗

arXiv2025

Low-energy proton impact dynamics on hydrocarbons: Dependence on kinetic energy and incident site

The dynamics of low-energy proton collisions with hydrocarbon with hydrocarbon molecules are investigated using real-time time-dependent density functional theory. Through systematic variation of proton kinetic energy and impact site on the molecular surface, the resulting scattering, proton capture, and bond dissociation pathways are analyzed. The simulations reveal a strong dependence of reaction outcomes on both incident energy and collision geometry, with the interplay between electronic and nuclear degrees of freedom highlighted as governing molecular fragmentation and reaction mechanisms.

physics.chem-ph↗

arXiv2025

Time-dependent density functional theory investigation of the formation of H$^{3+}$ from alkanes

The formation of H$^{3+}$ from ethane, propane, and butane dications was investigated with time-dependent density-functional theory (TDDFT) simulations. This approach offers the benefit of simultaneously addressing nuclear and electronic dynamics, enabling the investigation of electronic excitation, charge transfer, ionization, and nuclear motion. For each dication we determined the ground-state HOMO, the branching ratios of all dissociation channels, and the mechanism leading to H$^{3+}$. The simulated branching ratios for ethane and propane are similar, while butane is markedly lower. Ethane follows the minimum-energy pathway (MEP) proposed previously; propane forms H$^{3+}$ mainly via H$_2$ roaming. In butane, H$^{3+}$ appears only through the MEP within the present trajectory set; roaming H$_2$ was not observed under the same conditions.

physics.chem-ph↗

arXiv2025

Time-dependent density-functional study of intermolecular Coulombic decay for 2a$_1$ ionized water dimer

A real-space, real-time time-dependent density functional theory (RT-TDDFT) with Ehrenfest dynamics is used to simulate intermolecular Coulombic decay (ICD) processes following the ionization of an inner-valence electron. The approach has the advantage of treating both nuclear and electronic motion simultaneously, allowing for the study of electronic excitation, charge transfer, ionization, and nuclear motion. Using this approach, we investigate the decay process for the 2a$_1$ ionized state of the water dimer. For the 2a$_1$ vacancy in the proton donor water molecule, ICD is observed in our simulations. In addition, we have identified a novel dynamical process: at the initial stage, the proton generally undergoes a back-and-forth motion. Subsequently, the system may evolve along two distinct pathways: in one, no proton transfer occurs; in the other, the proton departs again from its original position and ultimately completes the transfer process. In contrast, when the vacancy resides in the proton acceptor water molecule, no proton transfer occurs, and ICD remains the sole decay channel.

physics.chem-ph↗

arXiv2025

The effect of finite mass in cavity QED calculations

The effect of finite nuclear mass is investigated in coupled light matter systems in cavity quantum electrodynamics (cavity QED) using the Pauli-Fierz Hamiltonian. Three different systems, the He atom, the H$^-$ ion and the H$_2^+$ ion is investigated. There are small, but significant differences in the behavior of the binding energies as the function of the coupling strength. The probability of coupling to light is found to be very small but even this small coupling has a very strong effect on the energies of the systems.

physics.chem-ph↗

arXiv2024

Time-dependent density-functional study of hydrogen adsorption and scattering on graphene surfaces

Time-dependent density-functional theory simulations are performed to examine the effects of varying incident points and kinetic energies of hydrogen atom projectiles on a graphene-like structure. The simulations reveal that the incident point significantly influences the hydrogen atom's kinetic energy post-interaction, the vibrational dynamics of the graphene lattice, and the scattering angles. Incident points that do not directly collide with carbon atoms result in prolonged interaction times and reduced energy transfer, increasing the likelihood of overcoming the graphene's potential energy barrier and hydrogen atom adsorption. The study also explores the role of initial kinetic energy in determining adsorption, scattering, or transmission outcomes. These results emphasize the critical influence of initial parameters on the hydrogenation process and provide a foundation for future experimental validation and further exploration of hydrogen-graphene interactions.

cond-mat.mtrl-sci↗

arXiv2024

CH(A) Radical Formation in Coulomb Explosion from Butane Seeded Plasma Generated with Chirp-Controlled Ultrashort Laser Pulses

We experimentally studied the formation of CH(A) radicals in butane seeded plasma generated with chirp-controlled ultrashort laser pulses ($\sim 760 \, μ\text{J}/\text{pulse}$, 890 nm, 1 kHz, 8 fs). The focused beam with high peak intensity ($\sim 10^{14} - 10^{16} \, \text{W/cm}^2$) caused Coulomb explosion (CE). The time dependent emission spectra were observed with the Fourier-transform Visible spectroscopy (FTVis) step-scan method. The average signal intensity decreased with the chirp in the Ar$^+$ > C$_2$ > H-$α$ $\sim$ CH(A) order with a plateau for CH(A) in the $-200$ to $-100 \, \text{fs}^2$ range. The short rise time of the CH(A) emission signal, the monoexponential emission decay, and the nearly constant rotational and vibrational temperatures of the CH(A) radicals ($\sim 3000 \, \text{K}$ and $\sim 3800 \, \text{K}$) all support their formation as a primary product. Our TDDFT calculations predict that CH and many other fragments can be formed beyond CE at $\sim 7 \times 10^{14} \, \text{W/cm}^2$ intensity. The average charge of CH (+0.6) and its relative abundance (0.5\%) support the formation of detectable CH(A) within 120 fs.

physics.chem-ph↗

arXiv2021

Stochastic Variational Approach to Small Atoms and Molecules Coupled to Quantum Field Modes

In this work, we present a stochastic variational calculation (SVM) of energies and wave functions of few particle systems coupled to quantum fields in cavity QED. The light-matter coupled system is described by the Pauli-Fierz Hamiltonian. The spatial wave function and the photon spaces are optimized by a random selection process. Examples for a two-dimensional trion and confined electrons as well as for the He atom and the Hydrogen molecule are presented showing that the light-matter coupling drastically changes the electronic states.

cond-mat.mes-hall↗

arXiv2021

Deformed Explicitly Correlated Gaussians

Deformed correlated Gaussian basis functions are introduced and their matrix elements are calculated. These basis functions can be used to solve problems with nonspherical potentials. One example of such potential is the dipole self-interaction term in the Pauli-Fierz Hamiltonian. Examples are presented showing the accuracy and necessity of deformed Gaussian basis functions to accurately solve light-matter coupled systems in cavity QED.

physics.chem-ph↗

arXiv2021

Energy spectrum and structure of one-dimensional few-electron Wigner crystals with and without coupling to light in cavity

Explicitly Correlated Gaussian basis is used to calculate the energies and wave functions of one dimensional few-electron systems in confinement potentials created by external potentials or coupling to light in cavity. The appearance and properties of Wigner crystal-like structures are discussed. It is shown that one dimensional Wigner crystals can be formed by coupling electrons to light due to the dipole self-interaction term in the light-matter Hamiltonian, provided an additional extremely weak confining potential is present.

quant-ph↗