arXiv subjects

M. Georgiev

Publications and source records attributed to M. Georgiev.

At least 19 recordsLinked to original sources

arXiv2022

Magnetostructural Dependencies in $3d^2$ Systems: The Trigonal Bipyramidal V$^{3+}$ Complex

We introduce a multi-configurational approach to study the magneto-structural correlations in $3d^2$ systems. The theoretical framework represents a restricted active space self-consistent field method, with active space optimized to the number of all non-bonding orbitals. To demonstrate the validity and effectiveness of the method, we explore the physical properties of the trigonal bipyramidal spin-one single-ion magnet (C$_6$F$_5$)$_3$trenVCN$^t$Bu. The obtained theoretical results show a good agreement with the experimental data available in the literature. This includes measurements for the magnetization, low-field susceptibility, cw-EPR and photoluminescence spectroscopy. The proposed method may be reliably applied to a variety of $3d^2$ magnetic systems. To this end, and for the sake completeness, we provide detailed analytical and numerical representations for the generic Hamiltonian's effective matrix elements related to the crystal field, exchange, spin-orbit and Zeeman interactions.

cond-mat.str-el↗

arXiv2022

Molecular magnetism in the multi-configurational self-consistent field method

We develop a structured theoretical framework used in our recent articles [Eur. Phys. J. B 92, 93 (2019) and Phys. Rev. B 101, 094427 (2020)] to characterize the unusual behavior of the magnetic spectrum, magnetization and magnetic susceptibility of the molecular magnet Ni$_4$Mo$_{12}$. The theoretical background is based on the molecular orbital theory in conjunction with the multi-configurational self-consistent field method and results in a post-Hartree-Fock scheme for constructing the corresponding energy spectrum. Furthermore, we construct a bilinear spin-like Hamiltonian involving discrete coupling parameters accounting for the relevant spectroscopic magnetic excitations, magnetization and magnetic susceptibility. The explicit expressions of the eigenenergies of the ensuing Hamiltonian are determined and the physical origin of broadening and splitting of experimentally observed peaks in the magnetic spectra is discussed. To demonstrate the efficiency of our method we compute the spectral properties of a spin-one magnetic dimer. The present approach may be applied to a variety of magnetic units based on transition metals and rare earth elements.

cond-mat.str-el↗

arXiv2020

Magnetization steps in the molecular magnet Ni$_4$Mo$_{12}$ revealed by complex exchange bridges

We study the behavior of the magnetization and the magnetic susceptibility of molecular magnets with complex bridging structure. Our computations are based on a post-Hartree-Fock method accounting for the intricate network of interatomic bonds and an effective spin-like Hamiltonian that captures the essential magnetic features of magnetic molecules. The devised method and the constructed Hamiltonian are further employed to characterize the magnetic properties of the molecular magnet Ni$_4$Mo$_{12}$. The obtained results reproduce both quantitatively and qualitatively the main features of the magnetic spectrum. Furthermore, the computations for the magnetization and the low-field susceptibility are in very good agreement with their experimental counterparts. In this respect, they improve upon the results obtained with conventional Heisenberg models.

cond-mat.str-el↗

arXiv2019

Magnetic excitations in molecular magnets with complex bridges: The tetrahedral molecule Ni$_4$Mo$_{12}$

We investigate the spectroscopic magnetic excitations in molecular magnets with complex intermediate structure among the magnetic ions. Our approach consists in introducing a modified spin Hamiltonian that allows for discrete coupling parameters accounting for all energetically favorable spatial distributions of the valence electrons along the exchange bridges connecting the constituent magnetic ions. We discuss the physical relevance of the constructed Hamiltonian and derive its eigenvalues. The model is applied to explore the magnetic excitations of the tetrameric molecular magnet Ni$_4$Mo$_{12}$. Our results are in a very good agreement with the available experimental data. We show that the experimental magnetic excitations in the named tetramer can be traced back to the specific geometry and complex chemical structure of the exchange bridges leading to the splitting and broadness of the peaks centered about 0.5 meV and 1.7 meV.

cond-mat.str-el↗

arXiv2019

Magnetic excitations in the trimeric compounds A$_3$Cu$_3$(PO$_4$)$_4$ (A = Ca, Sr, Pb)

We study the magnetic excitations of the trimeric magnetic compounds A$_3$Cu$_3$(PO$_4$)$_4$ (A = Ca, Sr, Pb). The spectra are analyzed in terms of the Heisenberg model and a generic spin Hamiltonian that accounts for the changes in valence electrons distribution along the bonds among magnetic ions. The analytical results obtained in the framework of both Hamiltonians are compared to each other and to the available experimental measurements. The results based on our model show better agreement with the experimental data than those obtained with the aid of the Heisenberg model. For all trimers, our analysis reveals the existence of one thin energy band referring to the flatness of observed excitation peaks.

physics.atm-clus↗

arXiv2018

Magnetic Exchange in Spin Clusters

We investigate the role of exchange bridges in molecular magnets. We explore their effects on the distribution of the valence electrons and their contribution to the exchange processes. The present study is focused on a spin-half dimer with nonequivalent exchange bridges. Here, we derive an effective Hamiltonian that allows for an accurate estimation of the observables associated to the magnetic properties of the magnet. Our results are compared to those obtained by means of the conventional Heisenberg model that usually fails.

physics.atm-clus↗

arXiv2018

A systematic approach to determine the spectral characteristics of molecular magnets

We devise a formalism to investigate in a systematic way the spectroscopic magnetic excitations in molecular magnets. This consists in introducing a bilinear spin Hamiltonian that allows for discrete coupling parameters accounting for distinct spin coupling mechanisms among the constituent magnetic ions, as well as the influence of the nonmagnetic ions in the system. The model is applied to explore the magnetic excitations of the trimeric magnetic compounds $\mathrm{A_3Cu_3(PO_4)_4}$ $\mathrm{(A\ =\ Ca,\ Sr,\ Pb)}$ and the tetrameric molecular magnet $\mathrm{Ni_4Mo_{12}}$. Our results are in a very good agreement with the available experimental data: For all trimers $\mathrm{A_3Cu_3(PO_4)_4}$, calculations reveal the existence of one thin energy band referring to the flatness of observed excitation peaks. Moreover for the tetramer $\mathrm{Ni_4Mo_{12}}$, we concluded that the magnetic excitations may be traced back to the specific geometry and complex chemical structure of the exchange bridges leading to the splitting and broadness of the peaks centered about 0.5 meV and 1.7 meV.

cond-mat.str-el↗

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. IV. Reorientational rate

This last Part IV is aimed at deriving relaxation rates (times) of an off-center Li+ impurity. We follow Christov's reaction rate method to define general rate equations in terms of the exact Mathieu eigenvalues, as well as of harmonic-oscillator eigenvalues approximating for the energy spectrum near the bottom of the reorientational wells. To calculate the rate in each particular case, we derive configurational tunneling probabilities by either Mathieu eigenfunctions or by harmonic oscillator eigenfunctions. The electron-transfer probability is calculated by generalizing Landau-Zener's method. Typical examples are considered and compared with experimental relaxation times in KCl:Li+.

physics.chem-ph↗

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. III. Numerical calculations

We carried out numerical calculations by an extended-Hueckel program in order to check the analytical results reported in the preceding Part I and Part II. We typically consider alkali halide clusters composed of some tens of constituent atoms to calculate electronic energies under static conditions or versus the displacements of particular atoms. Among other things, the off-center displacement of substitutional Li+ impurity in most alkali halides is evidenced. The trigonometric profile of the rotational barriers is also confirmed for KCl.

physics.chem-ph↗

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. II. In-plane rotation and polarizability

Because of its inherent 2-D character, the eigenvalue equation for the hindered rotation around a normal cation site of an off-center impurity nearest-neighboring an F center is the well-known Mathieu equation. We present an overview of literature data on Mathieu's periodic functions providing exact solutions to the Li+ reorientational problem. We compare them with bottom-well approximating solutions by harmonic-oscillator functions at an effective vibrational frequency renormalized by both first- and third- order electron-mode coupling. We finally discuss the in-plane inversion polarizability of an off-center impurity assumed to form a dipole-dipole coupling with a nearby F center.

physics.chem-ph↗

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. I. Basic equations

Extending earlier work on a vibronic theory of the FA centers in alkali halides, the reorientation is now considered of an off-center Li+ impurity, either isolated or near an F center. We derive analytically periodic potential energy barriers between metastable reorientational off-center sites, the barriers hindering the impurity rotation around the normal lattice site. Applying to a specific model, electron-vibrational mode coupling constants are calculated up to 3rd order of the expansion of the coupling energy in the T1u mode coordinates. The 3rd order coupling brings about additional renormalization of the reorientation controlling vibrational frequency.

cond-mat.mtrl-sci↗

arXiv2007

Photoinduced electrification of solids. III. Temperature dependences

Two preceding parts of a paper (cond-mat/0508457, cond-mat/0508460) considered the heuristic values of recent experiments pointing to the nearly universal occurrence of photovoltages across solid surfaces under short-circuit conditions. These voltages arise by virtue of a variety of spectrally-dependent mechanisms activated by incident photons. For the visible range, the photovoltages are obliged to the photodetachment of ions which leave the surface charged. In an attempt to learn more, we now study short-circuit photovoltages in well-defined materials including high-Tc superconductors within a broad temperature range down to liquid nitrogen. We believe our data provide a new insight into the process.

cond-mat.mtrl-sci↗

arXiv2007

Reaction rate approach to dipolar relaxation in alkali halides: Adiabaticity versus classical, activated-tunneling, and quantal dipoles

This paper is aimed at presenting a simple vibronic model for describing the dipolar reorientation in crystals by means of reaction rate theory. The Hamiltonian of an isolated dipole is simplified so as to render the problem solvable. Depending on the crossover splitting the dipoles may reorientate adiabatically with a high electron-transfer expectancy or exhibit low reorientation rates due to low expectancy. An important quantity to distinguish between adiabatic dipoles behaving classically and ones reorientating by means of quantum-mechanical tunneling is Christov's characteristic temperature which is found to relate to the barrier height and crossover splitting. ITC data on impurity-vacancy dipoles in Eu-doped alkali halides are reanalyzed.

cond-mat.mtrl-sci↗

arXiv2007

Hindered rotators by off-site ions in solids: Optical conductivity by sombrero potentials

We consider the optical conductivity of carriers scattered by local sombrero potentials associated with hindered planar rotators in solids. Examples for hindered rotators are provided by off-center substitutional Li ions in colored alkali halides and off-site apical oxygens O(A) in layered perovskites. We obtain off-center displacements by solving for the vibronic mixing of electronic states at the displacing ion site. The eigenvalue equation of a hindered rotator is solved in 2D by Mathieu's periodic functions, as the rotation is quantized in rotational bands. We discuss the optical conductivities pertaining to each of the exemplified cases.

cond-mat.mtrl-sci↗

arXiv2006

Variational band theory of vibronic polarons in crystals. III. Numerical calculations

In the preceding Part II, we derived variational equations for the phonon Fourier amplitudes and for the Fourier amplitudes of the fractional contribution of the electronic bands to the trial variational state. These equations are now solved by means of iterations for each value of the total momentum in order to obtain the energy vs. momentum relation for the ground state. Another result is mapping out the phonon and band Fourier amplitudes in the parameter space of the mixing constant and the electron hopping energy.

cond-mat.supr-con↗

arXiv2006

Variational band theory of vibronic polarons in crystals. II. Extending Merrifield's Ansatz

Merrifield's Variational Ansatz is extended so as to cover the case of two electronic bands mixed by an Einstein phonon. The Hamiltonian is composed of the local and kinetic (hopping) energies in the absence of vibrations, the vibrational energy, and a mixing band-off-diagonal part linear in the electron-phonon coupling, all expressed in second quantization terms. The variational eigenstate is a linear combination of Merrifield states for either electronic band. We derive equations for the phonon Fourier amplitudes and for the fractional contribution of either electronic band.

cond-mat.supr-con↗

arXiv2006

Variational band theory of vibronic polarons in crystals. I. Preamble

We review the basic theoretical background for working out a variational band solution for vibronic polarons in crystals. It is based on the Lee-Low-Pines proposal as extended by Thomas et al. for describing Jahn-Teller polarons along a linear chain of atoms. The variational properties of antiadiabatic itinerant polarons are also discussed.

cond-mat.supr-con↗

arXiv2006

Adiabatic theory of off-center vibronic polarons: Local dynamics on a planar lattice

We review basic theoretical concepts and developments regarding the local and itinerant properties of off-center vibronic polarons in crystals. These include the electron self-trapping and the local rotation of the species on a square planar lattice. Phase transitions within the gas of vibronic small polarons are also discussed.

cond-mat.supr-con↗