arXiv subjects

Sergey A. Varganov

Publications and source records attributed to Sergey A. Varganov.

7 recordsLinked to original sources

arXiv2025

The role of spin-orbit coupling and state-crossing topography in the non-radiative decay of Ir(III) complexes

A pillar of our current understanding of the photoluminescence of Ir(III) complexes is the assumption that the population of triplet metal-centered states determines an efficient non-radiative decay to the ground state minimum. Based on that assumption, the energy separation between the emitting state and the minimum-energy crossing point of the triplet metal-centered and the ground states has been employed as a key variable for evaluating the ability of Ir(III) complexes to decay non-radiatively. We demonstrate that the strong spin-orbit coupling between the triplet metal-centered and the ground state of Ir(III) complexes, together with the sloped topography of their crossing, lead to a significant energy separation between the two states, resulting in a reduced rate of non-radiative ground state recovery. Therefore, we propose that the role of metal-centered states is defined by the tendency of the excited state population to remain trapped in the metal-centered minima.

physics.chem-ph↗

arXiv2025

Effects of Coupling Between Chiral Vibrations and Spins in Molecular Magnets

In single molecular magnets, chiral vibrations carrying vibrational angular momentum ($\hat{L}^{\text{vib}}$) emerge due to the splitting of a doubly degenerate vibrational mode. Here, we identify a new type of effective spin-vibrational coupling responsible for lifting this degeneracy, which can facilitate optically selective excitations. In the presence of an external Zeeman field, this coupling breaks both inversion (in-plane parity) $\mathcal{P}$ and time-reversal $\mathcal{T}$ symmetries, imparting distinct geometric phases to the resulting dressed spin-vibronic states. The wave function of the spin-vibronic state is characterized by a $π$-Berry phase, which results in magneto-optical circular dichroism. This framework is validated using density functional theory and multi-reference \emph{ab initio} calculations on the Ce(trenovan) molecular magnet.

cond-mat.mes-hall↗

arXiv2024

Excited-State Dynamics and Optically Detected Magnetic Resonance of Solid-State Spin Defects from First Principles

Optically detected magnetic resonance (ODMR) is an efficient and reliable method that enables initialization and readout of spin states through spin-photon interface. In general, high quantum efficiency and large spin-dependent photoluminescence contrast are desirable for reliable quantum information readout. However, reliable prediction of the ODMR contrast from first-principles requires accurate description of complex spin polarization mechanisms of spin defects. These mechanisms often include multiple radiative and nonradiative processes in particular intersystem crossing (ISC)among multiple excited electronic states. In this work we present our implementation of the first-principles ODMR contrast, by solving kinetic master equation with calculated rates from \textit{ab initio} electronic structure methods then benchmark the implementation on the case of the negatively-charged nitrogen vacancy center in diamond. We show the importance of correct description of multi-reference electronic states and pseudo Jahn-Teller effect for quantitatively, even qualitatively correct prediction of spin-orbit coupling (SOC) and the rate of ISC. We present the complete calculation of SOC for different ISC processes that align with both group theory and experimental observations. Moreover, we provide a comprehensive picture of excitation and relaxation dynamics, including previously unexplored internal conversion processes. We show good agreement between our first-principles calculations and the experimental ODMR contrast under magnetic field. We then demonstrate reliable predictions of magnetic field direction, pump power, and microwave frequency dependency, as important parameters for ODMR experiments. Our work provides a predictive computational platform for spin polarization and optical readout of solid-state quantum defects from first principles.

cond-mat.mtrl-sci↗

arXiv2023

Anthropic constraint on transient variations of fundamental constants

The anthropic principle implies that life can emerge and be sustained only in a narrow range of values of fundamental constants. Here we show that anthropic arguments can set powerful constraints on {\em transient} variations of the fine-structure constant $α$ over the past 4 billion years since the appearance of lifeforms on Earth. We argue that the passage through Earth of a macroscopic dark matter clump with a value of $α$ inside differing substantially from its nominal value would make Earth uninhabitable. We demonstrate that in the regime of extreme variation of $α$, the periodic table of elements is truncated, water fails to serve as a universal solvent, and protons become unstable. Thereby, the anthropic principle constrains the likelihood of such encounters on a 4-billion-year timescale. This enables us to improve existing astrophysical bounds on certain dark matter model couplings by several orders of magnitude.

physics.atom-ph↗

arXiv2019

Spin coherence and optical properties of alkali-metal atoms in solid parahydrogen

We present a joint experimental and theoretical study of spin coherence properties of 39K, 85Rb, 87Rb, and 133Cs atoms trapped in a solid parahydrogen matrix. We use optical pumping to prepare the spin states of the implanted atoms and circular dichroism to measure their spin states. Optical pumping signals show order-of-magnitude differences depending on both matrix growth conditions and atomic species. We measure the ensemble transverse relaxation times (T2*) of the spin states of the alkali-metal atoms. Different alkali species exhibit dramatically different T2* times, ranging from sub-microsecond coherence times for high mF states of 87Rb, to ~100 microseconds for 39K. These are the longest ensemble T2* times reported for an electron spin system at high densities (n > 10^16 cm^-3). To interpret these observations, we develop a theory of inhomogenous broadening of hyperfine transitions of ^2S atoms in weakly-interacting solid matrices. Our calculated ensemble transverse relaxation times agree well with experiment, and suggest ways to longer coherence times in future work.

physics.atom-ph↗

arXiv2018

Enhanced spin coherence of rubidium atoms in solid parahydrogen

We measure the transverse relaxation of the spin state of an ensemble of ground-state rubidium atoms trapped in solid parahydrogen at cryogenic temperatures. We find the spin dephasing time of the ensemble (T$_2^*$) is limited by inhomogeneous broadening. We determine that this broadening is dominated by electrostatic interactions with the host matrix, and can be reduced by preparing nonclassical spin superposition states. Driving these superposition states gives significantly narrower electron paramagnetic resonance lines and the longest reported electron spin $T_2^*$ in any solid-phase system other than solid helium.

physics.atom-ph↗

arXiv2014

Accurate potential energy, dipole moment curves, and lifetimes of vibrational states of heteronuclear alkali dimers

We calculate the potential energy curves, the permanent dipole moment curves, and the lifetimes of the ground and excited vibrational states of the heteronuclear alkali dimers XY (X, Y = Li, Na, K, Rb, Cs) in the X1Σ+ electronic state using the coupled cluster with singles doubles and triples (CCSDT) method. All-electron quadruple-ζ basis sets with additional core functions are used for Li and Na, and small-core relativistic effective core potentials with quadruple-ζ quality basis sets are used for K, Rb and Cs. The inclusion of the coupled cluster non-perturbative triple excitations is shown to be crucial for obtaining the accurate potential energy curves. Large one-electron basis set with additional core functions is needed for the accurate prediction of permanent dipole moments. The dissociation energies are overestimated by only 14 cm-1 for LiNa and by no more than 114 cm-1 for the other molecules. The discrepancies between the experimental and calculated harmonic vibrational frequencies are less than 1.7 cm-1, and the discrepancies for the anharmonic correction are less than 0.1 cm-1. We show that correlation between atomic electronegativity differences and permanent dipole moment of heteronuclear alkali dimers is not perfect. To obtain the vibrational energies and wave functions the vibrational Schrödinger equation is solved with the B-spline basis set method. The transition dipole moments between all vibrational states, the Einstein coefficients and the lifetimes of the vibrational states are calculated. We analyze the decay rates of the vibrational states in terms of spontaneous emission, and stimulated emission and absorption induced by black body radiation. In all studied heteronuclear alkali dimers the ground vibrational states have much longer lifetimes than any excited states.

physics.chem-ph↗