arXiv subjects

Ádám Ganyecz

Publications and source records attributed to Ádám Ganyecz.

9 recordsLinked to original sources

arXiv2026

Accurate description of the electronic states of the V$_{\text{B}}^-$ center in hBN by wavefunction theory

Ensembles of negatively charged boron vacancy (V$_{\text{B}}^-$) centers in hexagonal boron nitride (hBN) have emerged as a two-dimensional spin qubit system interfaced with optics to advance nanoscale quantum sensing. However, a comprehensive description of its optically detected magnetic resonance (ODMR) signal remains challenging due to the strongly correlated nature of the excited electronic states involved in its optical cycle. In this work, we model the energetics, structural relaxation, and transition rates of the V$_{\text{B}}^-$ center using a high-level wave-function-based electron correlation method (CASSCF-NEVPT2). We provide a thorough analysis of the excited state fine structure and pseudo Jahn-Teller effects, singlet-triplet quasi-degeneracies, photoluminescence parameters, intersystem crossing pathways, and stress-dependence of the fine structure and decay parameters. Our findings not only clarify the fundamental behavior of the V$_{\text{B}}^-$ center in hBN but also establish the theoretical foundation for advancing the V$_{\text{B}}^-$ center's readout for integrated 2D quantum sensors.

cond-mat.mtrl-sci↗

arXiv2025

Assessing the Reliability of Truncated Coupled Cluster Wavefunction: Estimating the Distance from the Exact Solution

A new approach is proposed to assess the reliability of the truncated wavefunction methods by estimating the deviation from the full configuration interaction (FCI) wavefunction. While typical multireference diagnostics compare some derived property of the solution with the ideal picture of a single determinant, we try to answer a more practical question, how far is the solution from the exact one. Using the density matrix renormalization group (DMRG) method to provide an approximate FCI solution for the self-consistently determined relevant active space, we compare the low-level CI expansions and one-body reduced density matrixes to determine the distance of the two solutions ($\tilde{d}_Φ$, $\tilde{d}_γ$). We demonstrate the applicability of the approach for the CCSD method by benchmarking on the W4-17 dataset, as well as on transition metal-containing species. We also show that the presented moderate-cost, purely wavefunction-based metric is truly unique in the sense that it does not correlate with any popular multireference measures. We also explored the usage of CCSD natural orbitals ($\tilde{d}_{γ,\mathrm{NO}}$) and its effect on the active space size and the metric. The proposed diagnostic can also be applied to other wavefunction approximations, and it has the potential to provide a quality measure for post-Hartree-Fock procedures in general.

physics.chem-ph↗

arXiv2025

Orbital optimization of large active spaces via AI-accelerators

We present an efficient orbital optimization procedure that combines the highly GPU accelerated, spin-adapted density matrix renormalization group (DMRG) method with the complete active space self-consistent field (CAS-SCF) approach for quantum chemistry implemented in the ORCA program package. Leveraging the computational power of the latest generation of Nvidia GPU hardware, we perform CAS-SCF based orbital optimizations for unprecedented CAS sizes of up to 82 electrons in 82 orbitals [CAS(82,82)] in molecular systems comprising of active spaces sizes of hundreds of electrons in thousands of orbitals. For both the NVIDIA DGX-A100 and DGX-H100 hardware, we provide a detailed scaling and error analysis of our DMRG-SCF approach for benchmark systems consisting of polycyclic aromatic hydrocarbons and iron-sulfur complexes of varying sizes. Our efforts demonstrate for the first time that highly accurate DMRG calculations at large bond dimensions are critical for obtaining reliably converged CAS-SCF energies. For the more challenging iron-sulfur benchmark systems, we furthermore find the optimized orbitals of a converged CAS-SCF calculation to depend more sensitively on the DMRG parameters than those for the polycyclic aromatic hydrocarbons. The ability to obtain converged CAS-SCF energies and orbitals for active spaces of such large sizes within days reduces the challenges of including the appropriate orbitals into the CAS or selecting the correct minimal CAS, and may open up entirely new avenues for tackling strongly correlated molecular systems.

physics.chem-ph↗

arXiv2025

Identifying high-energy electronic states of NV$^-$ centers in diamond

The negatively charged nitrogen-vacancy center in diamond is a prototype photoluminescent point defect spin qubit with promising quantum technology applications, enabled by its efficient optical spin polarization and readout. Its low-lying electronic states and optical spin polarization cycle have been well characterized over decades, establishing it as a benchmark system for state-of-the-art computational methods in point defect research. While the optical cycle is well understood, a comprehensive energetic analysis of higher-lying states has received less attention until recently. In this joint experimental theoretical study, we identify and characterize five high-energy states beyond those involved in the optical cycle. Using transient absorption spectroscopy, we determine their transition energies and relative oscillator strengths. Additionally, we perform two independent numerical studies employing two state-of-the-art post-DFT methods to support the experimental findings and assign energy levels. These results enhance our understanding of the NV center's energy spectrum and providing a broader reference for benchmarking high-level first-principles methods.

cond-mat.mtrl-sci↗

arXiv2024

Accurate and convergent energetics of color centers by wavefunction theory

Ab initio description of point defects in semiconductors, characterized by in-gap states of significant multideterminant character, presents a longstanding theoretical challenge for density functional theory (DFT) methods. In this study, we devise a wavefunction theory (WFT) based ab initio methodology as a competing alternative approach. Specifically, we apply perturbation theory (NEVPT2 level) on top of a defect-localized many-body wavefunction (CASSCF level), which provides a balanced description of dynamic and static correlation effects, respectively. This quantum chemical methodology, exemplified for the NV$^-$ center in diamond in this study, is not only used for the calculation of energies and properties, but also for geometry optimization, performed for each electronic state individually. By relaxing cluster models of increasing size and investigating convergence behavior, we quantitatively reproduce (i) the full energy spectrum of NV$^-$ including the recently characterized high-energy states, (ii) the effect of Jahn-Teller distortion on measurable properties, (iii) the fine structure of ground and excited states, (iv) the pressure dependence of zero-phonon lines. Our findings showcase that applying conventional wave-function-based quantum chemistry on carefully crafted clusters can be a robust routine tool for discussing defect-state energetics.

cond-mat.mtrl-sci↗

arXiv2024

Carbon-contaminated topological defects in hexagonal boron nitride for quantum photonics

Topological defects, such as Stone-Wales defects and grain boundaries, are common in 2D materials. In this study, we investigate the intricate interplay of topological defects and carbon contamination in hexagonal boron nitride revealing an intriguing class of color centers. We demonstrate that both carbon contamination and strain can stabilize Stone-Wales configurations and give rise to emitters with desirable optical properties in the visible spectral range. Inspired by these results, we further demonstrate that carbon atoms at grain boundaries can resolve energetic B-B and N-N bonds leading to highly favorable atomic structures that may facilitate the accumulation of carbon contamination at the boundaries. Similarly to contaminated Stone-Wales defects, carbon-doped grain boundaries can also give rise to color centers emitting in the visible spectral range with short radiative lifetime and high Debye-Waller factors. Our discoveries shed light on an exciting class of defects and pave the way toward the identification of color centers and single photon emitters in hBN.

cond-mat.mtrl-sci↗

arXiv2024

Near-coherent quantum emitters in hexagonal boron nitride with discrete polarization axes

Hexagonal boron nitride (hBN) has recently gained attention as a solid state host of quantum emitters. However, hBN emitters reported to date lack the properties needed for their deployment in scalable quantum technologies. Here we employ spectral hole burning spectroscopy and resonant polarization measurements to observe nearly-coherent hBN quantum emitters, both as singles and in ensembles, with three discrete polarization axes indicative of a C2v symmetry defect. Our results constitute an important milestone towards the implementation of hBN quantum emitters in integrated quantum photonics.

quant-ph↗

arXiv2023

First principles theory of the nitrogen interstitial in hBN: a plausible model for the blue emitter

Color centers in hexagonal boron nitride (hBN) have attracted considerable attention due to their remarkable optical properties enabling robust room temperature photonics and quantum optics applications in the visible spectral range. On the other hand, identification of the microscopic origin of color centers in hBN has turned out to be a great challenge that hinders in-depth theoretical characterization, on-demand fabrication, and development of integrated photonic devices. This is also true for the blue emitter, which is an irradiation damage in hBN emitting at 436 nm wavelengths with desirable properties. Here, we propose the negatively charged nitrogen split interstitial defect in hBN as a plausible microscopic model for the blue emitter. To this end, we carry out a comprehensive first principles theoretical study of the nitrogen interstitial. We carefully analyze the accuracy of first principles methods and show that the commonly used HSE hybrid exchange-correlation functional fails to describe the electronic structure of this defect. Using the generalized Koopman's theorem, we fine tune the functional and obtain a zero-phonon photoluminescence (ZPL) energy in the blue spectral range. We show that the defect exhibits high emission rate in the ZPL line and features a characteristic phonon side band that resembles the blue emitter's spectrum. Furthermore, we study the electric field dependence of the ZPL and numerically show that the defect exhibits a quadratic Stark shift for perpendicular to plane electric fields, making the emitter insensitive to electric field fluctuations in first order. Our work emphasize the need for assessing the accuracy of common first principles methods in hBN and exemplifies a workaround methodology. Furthermore, our work is a step towards understanding the structure of the blue emitter and utilizing it in photonics applications.

cond-mat.mtrl-sci↗

arXiv2023

Symmetric carbon tetramers forming chemically stable spin qubits in hBN

Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy related defects, such as the NV center in diamond and the VB$^-$ in hexagonal boron nitride (hBN), are at the forefront of high spatial resolution and low dimensional sensing. On the other hand, vacancies' reactive nature and instability at the surface limit further developments. Here, we study the symmetric carbon tetramers in hBN and propose them as a chemically stable spin qubit for sensing in low dimensions. We utilize periodic-DFT and quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect. We show that the nitrogen centered symmetric carbon tetramer gives rise to spin state dependent optical signals with strain sensitive intersystem crossing rates. Furthermore, the weak hyperfine coupling of the defect to their spin environments results in a reduced electron spin resonance linewidth that may enhance sensitivity.

cond-mat.mtrl-sci↗