arXiv subjects

Mariya Romanova

Publications and source records attributed to Mariya Romanova.

7 recordsLinked to original sources

arXiv2025

Surface coupling of NV centers over nanoscale lengths

Shallow nitrogen-vacancy (NV-) centers in diamond are among the most promising quantum sensors, offering high sensitivity and nanoscale spatial resolution. These systems are, however, prone to decoherence due to coupling with surface states. Here, we study sub-surface NV- centers embedded into large diamond slabs (8 nm) using various surface orientations (100 and 111) and terminations (hydrogen and nitrogen terminators) and compute the quasiparticle states of the defect. Our results show how dynamical charge fluctuations near the surface influence defect stability. We find that the (100) N-terminated surface introduces strong surface-state instabilities, while the (111) N-terminated surface provides a more favorable configuration. However, many-body calculations (within the GW approximation) reveal that defects placed shallower than ~ 4 nm are prone to surface-induced ionization. These findings establish an accurate theoretical limit on the minimum depth required for stable NV- centers, guiding the design of NV- based quantum sensors.

cond-mat.mtrl-sci↗

arXiv2022

Dynamical downfolding for localized quantum states

We introduce an approach to treat localized correlated electronic states in the otherwise weakly correlated host medium. Here, the environment is dynamically downfolded on the correlated subspace. It is captured via renormalization of one and two quasiparticle interaction terms which are evaluated using many-body perturbation theory. We outline the strategy on how to take the dynamical effects into account by going beyond the static limit approximation. Further, we introduce an efficient stochastic implementation that enables treating the host environment with a large number of electrons at a minimal computational cost. For small explicitly correlated subspace, the dynamical effects are critical. We demonstrate the methodology by reproducing optical excitations in the negatively charged NV center defect in diamond, that are in excellent agreement with experimental results.

cond-mat.str-el↗

arXiv2022

Reduced scaling of optimal regional orbital localization via sequential exhaustion of the single-particle space

Wannier functions have become a powerful tool in the electronic structure calculations of extended systems. The generalized Pipek-Mezey Wannier functions exhibit appealing characteristics (e.g., reaching an optimal localization and the separation of the $σ$-$π$ orbitals) when compared with other schemes. However, when applied to giant nanoscale systems, the orbital localization suffers from a large computational cost overhead when one is interested in localized states in a small fragment of the system. Herein we present a swift, efficient, and robust approach for obtaining regionally localized orbitals of a subsystem within the generalized Pipek-Mezey scheme. The proposed algorithm introduces a reduced workspace and sequentially exhausts the entire orbital space until the convergence of the localization functional. It tackles systems with $\sim$10000 electrons within 0.5 hours with no loss in localization quality compared to the traditional approach. Regionally localized orbitals with a higher extent of localization are obtained via judiciously extending the subsystem's size. Exemplifying on large bulk and a 4-nm wide slab of diamond with NV$^-$ center, we demonstrate the methodology and discuss how the choice of the localization region affects the excitation energy of the defect. Furthermore, we show how the sequential algorithm is easily extended to stochastic methodologies that do not provide individual single-particle eigenstates. It is thus a promising tool to obtain regionally localized states for solving the electronic structure problems of a subsystem embedded in giant condensed systems.

cond-mat.mtrl-sci↗

arXiv2021

Stochastic many-body calculations of moiré states in twisted bilayer graphene at high pressures

Atomistic first principles many-body computational studies were so far limited by the system size. In this work, we apply and expand the stochastic GW method allowing calculations of quasiparticle energies of giant systems. We introduce three new developments to the stochastic many-body formalism: efficient evaluation of off-diagonal self-energy terms, construction of Dyson orbitals, and introduce stochastic cRPA, enabling efficient downfolding onto model Hamiltonians. We exemplify the new methodology by exploring twisted bilayer graphene (tBLG); its moire periodicity is associated with giant unit-cells hosting correlated electrons in flat bands. The resulting behavior of tBLG is governed by the coupling between the weakly and strongly correlated electrons in individual monolayers, controlled by twist or applied pressure. Here we study the latter scenario. The calculations document the formation of charge carrier localization at the Fermi level while most weakly correlated states are merely shifted in energy. This contrasts with the mean-field results. We show how to efficiently downfold the correlated subspace on a model Hamiltonian, namely the Hubbard model with a screened frequency dependent on-site interaction, computed with newly developed stochastic cRPA. For the $6^{\circ}$ tBLG system, the onsite interactions range between 200 and 300 meV under compression. The Dyson orbitals exhibit spatial distribution similar to the mean-field single particle states. Under pressure, the electron-electron interactions in the localized states increase; the dynamical screening does not fully compensate the dominant bare Coulomb interaction. The correlated moiré states thus appear only moderately screened at high pressures suggesting the presence of insulating states in compressed tBLG.

physics.comp-ph↗

arXiv2020

Are multi-quasiparticle interactions important in molecular ionization?

Photo-emission spectroscopy directly probes individual electronic states, ranging from single excitations to high-energy satellites, which simultaneously represent multiple quasiparticles (QPs) and encode information about electronic correlation. First-principles description of the spectra requires an efficient and accurate treatment of all many-body effects. This is especially challenging for inner valence excitations where the single QP picture breaks down. Here, we provide the full valence spectra of small closed-shell molecules, exploring the independent and interacting quasiparticle regimes, computed with the fully-correlated adaptive sampling configuration interaction (ASCI) method. We critically compare these results to calculations with the many-body perturbation theory, based on the $GW$ and vertex corrected $GWΓ$ approaches. The latter explicitly accounts for two-QP quantum interactions, which have been often neglected. We demonstrate that for molecular systems, the vertex correction universally improves the theoretical spectra, and it is crucial for accurate prediction of QPs as well as capturing the rich satellite structures of high-energy excitations. $GWΓ$ offers a unified description across all relevant energy scales. Our results suggest that the multi-QP regime corresponds to dynamical correlations, which can be described via perturbation theory.

physics.chem-ph↗

arXiv2020

Decomposition and embedding in the stochastic $GW$ self-energy

We present two new developments for computing excited state energies within the $GW$ approximation. First, calculations of the Green's function and the screened Coulomb interaction are decomposed into two parts: one is deterministic while the other relies on stochastic sampling. Second, this separation allows constructing a subspace self-energy, which contains dynamic correlation from only a particular (spatial or energetic) region of interest. The methodology is exemplified on large-scale simulations of nitrogen-vacancy states in a periodic hBN monolayer and hBN-graphene heterostructure. We demonstrate that the deterministic embedding of strongly localized states significantly reduces statistical errors, and the computational cost decreases by more than an order of magnitude. The computed subspace self-energy unveils how interfacial couplings affect electronic correlations and identifies contributions to excited-state lifetimes. While the embedding is necessary for the proper treatment of impurity states, the decomposition yields new physical insight into quantum phenomena in heterogeneous systems.

physics.comp-ph↗

arXiv2019

Symmetry aspects of spin-filtering in molecular junctions: hybridization and quantum interference effects

Control and manipulation of electric current and, especially, its degree of spin polarization (spin filtering) across single molecules are currently of great interest in the field of molecular spintronics. We explore one possible strategy based on the modification of nanojunction symmetry which can be realized, for example, by a mechanical strain. Such modification can activate new molecular orbitals which were inactive before due to their orbital mismatch with the electrode's conduction states. This can result in several important consequences such as (i) quantum interference effects appearing as Fano-like features in electron transmission and (ii) the change in molecular level hybridization with the electrode's states. We argue that the symmetry change can affect very differently two majority- and minority-spin conductances and thus alter significantly the resulting spin-filtering ratio as the junction symmetry is modified. We illustrate the idea for two basic molecular junctions: Ni/benzene/Ni (perpendicular vs tilted orientations) and Ni/Si chain/Ni (zigzag vs linear chains). In both cases, one highest occupied molecular orbital (HOMO) and one lowest unoccupied molecular orbital (LUMO) (out of HOMO and LUMO doublets) are important. In particular, their destructive interference with other orbitals leads to dramatic suppression of majority-spin conductance in low-symmetry configurations. For a minority-spin channel, on the contrary, the conductance is strongly enhanced when the symmetry is lowered due to an increase in hybridization strength. We believe that our results may offer a potential route for creating molecular devices with a large on-off ratio of spin polarization via quantum interference effects.

cond-mat.mes-hall↗