Although the method of spectral moments offers unique insight into electron-phonon interactions in activated crystals, it has not been sufficiently exploited to date. Here, we use the spectral distribution moment method as a tool for the analysis of multiphonon luminescence bands in thiogallate phosphors MGa2S4 (M = Ca, Sr, and Ba) doped with Eu2+ ions. The exact quantum-mechanical expressions for the first three spectral moments are presented using the traditional (basic) model of the impurity ion, which deals with the two well separated electronic levels, takes into account only linear (with respect to normal vibrational coordinates) electron-phonon interaction, and implies harmonic approximation for the lattice vibrations. By analyzing the experimental data on the profiles of the emission bands corresponding to the 4f(6)5d(1) to 4f(7)(S-8(7/2)) transitions combined with the Raman spectra, we identify vibrational mode producing dominant contribution to the observed optical bands and evaluate the first three spectral moments and the Pekar-Huang-Rhys (heat release) parameters quantifying the vibronic interaction strength. When discussing the relationship between the Stokes shift and the heat release parameters in the vibronically assisted optical emission of MGa2S4: Eu2+ crystals, special attention is paid to the ambiguity of the diverse existing expressions for the Stokes shift. Critical discussion of the basic vibronic model of an impurity ion is given by comparing the experimental data with theoretical findings. Finally, based on the analysis of the spectral moments in a more general context, we discuss the challenges emerging in the problem of comparison of the optical emission bands of thiogallate phosphors in the framework of the basic model, such as role of the quadratic vibronic interaction and phonon dispersion.

Based on the theoretical framework recently developed by some of us, we predict and justify the possibility of a nonequilibrium magnetic cooling effect in the Mn12 family of clusters by considering a monocrystalline sample of the prototypical single-molecule magnet Mn12Ac as a representative example. In contrast to the quasi-static processes underlying the conventional magnetocaloric effect (MCE), we address a dynamic regime involving sudden magnetic field quenching. The proposed cooling mechanism is determined by the relaxation kinetics arising after a sudden change in the spin Hamiltonian that generates a nonequilibrium population distribution within the spin subsystem and therefore does not rely on the standard equilibrium entropy cycles. During the subsequent restoration of thermal equilibrium, heat is redistributed between the phonon bath and the spin degrees of freedom. Under appropriate conditions, this relaxation-driven process results in cooling of the lattice. The central result is that the strong easy-axis magnetic anisotropy associated with a significant magnetization reversal barrier, features typically considered detrimental to conventional magnetocaloric cooling, becomes advantageous in the nonequilibrium regime. These properties enhance both the magnitude of the cooling effect and the practical feasibility of the sudden-quench approach. This study therefore broadens the potential cryogenic applicability of single-molecule magnets by identifying a cooling mechanism that operates precisely in the parameter range where classical magnetocaloric approaches are least efficient.

In this article, we extend the recently proposed theoretical framework for nonequilibrium magnetothermal effects induced by a sudden magnetic field quenching to anisotropic 3d-metal complexes with arbitrary spins. The formalism is applicable not only to the case of complete magnetic field switching off, but also to the case of partial field quenching. A simple and universal semiquantitative rule is formulated, which allows for the prediction of the sign of a thermal effect (that means heat absorption or heat release) from the magnetic field dependencies of the spin energy levels. In many specific cases, this rule can be used to predict the sign of the magnetothermal effect prior to calculations, based on an analysis of the field dependencies of the spin levels of the complexes under study. According to this rule, each excited state contributes to cooling or heating depending on whether it becomes destabilized or stabilized as the field decreases. The performed numerical analysis of the specific heat release, as a function of temperature and initial and final magnetic fields for complexes with spins S = 1, 3/2, 2, and 5/2, demonstrates that systems with easy-axis magnetic anisotropy (D < 0) exhibit heat absorption in cases of complete and incomplete field quenching, with the effect being strongly enhanced in the latter case. In contrast, in complexes with easy-plane-type anisotropy (D > 0), the sign of the thermal effect is shown to be dependent on the temperature, the initial and final values of the magnetic field, and also on whether the spin of the complex is integer or half-integer. These results provide clear and practical guidelines for the design of low-temperature molecular magnetic refrigerants operating in fast field-quenching regimes.

The main focus of this paper is to reveal the influence of the pseudo-Jahn-Teller effect (PJTE) on the exchange interaction in a pair of magnetic ions having a degenerate or pseudo-degenerate ground state. We consider the combined effect of the Anderson's type kinetic exchange (that is called superexchange) between the two one-electron centers each having two nearly-degenerate low-lying magnetic orbitals mixed by the vibration of a suitable symmetry. The superexchange is assumed to be comparable with the energy gap between the two orbitals. Under such condition the electronic energy spectrum of the dimer is shown to consist of three superimposed pairs of S = 1 and S = 0 levels, with the order of the levels inside each group obeying GoodenoughKanamori rules. The overall effect of superexchange is antiferromagnetic if the energy gap between the two orbitals is large as compared with the superexchange and it is ferromagnetic otherwise. The PJT effect proves to be spin-dependent that drastically modifies the energy pattern arising from the superexchange. The most spectacular effect of the vibronic coupling is the predicted possibility of changing spin of the dimer. In all cases strong PJT coupling stabilizes an antiferromagnetic ground state. We demonstrate that depending on the electronic parameters involved, one of the two types of minima proves to be the global one. One of these types describes ferrodistortive antiferromagnetic state, while for other one the system proves to be ferromagnetic and antiferrodistortive. In a broader context, these results show that a dimeric PJT cluster can serve as a molecular model for interconnected cooperative phenomena in PJT crystals such as magnetic, structural and orbital types of ordering.

Molecular quantum cellular automata (QCA) devices are typically based on the square planar two-electron mixed valence (MV) molecules playing the role of QCA cells. The functional properties of such cells are determined by the Coulomb repulsion between the two excess electrons, which stabilize antipodal charge configurations that encoding binary information. The inner sphere electron transfer of the excess electrons transforming different charge configurations into each other, as well as by the vibronic coupling, which tends to localize the mobile charges. Previously, the most topical and theoretically complex case of arbitrary Coulomb repulsion has been considered by implying essential restricting assumptions on the network of the electron transfer pathways. Therefore, the electron transfer occurring along the sides of the molecular square has been taken into account. Meanwhile, the effects of diagonal transfer have been discussed only for the limiting case of strong intracell Coulomb repulsion, which is peculiar to predominantly ionic compounds that are unlikely to be relevant to the MV cells. Here, we go beyond these simplifying assumptions and consider the general situation when all electron transfer pathways are involved, providing arbitrary interrelations between the key electronic and vibronic parameters. By solving the adiabatic and quantum-mechanical three-mode vibronic problems, we reveal the influence of electron transfer network topology on key properties of QCA, such as the stabilization of different spin-states in the free and interacting cells, the extent of localization of the pair of excess electrons, the shape of the cell-cell response functions, and the heat release necessarily occurring in the course of the non-adiabatic switching cycle.

In this paper we address the long standing problem of the relationship between the Stokes shift EStokes and PekarHuang-Rhys (heat release) parameter S, in vibronically assisted optical spectra of activated crystals. We focus on resolving the ambiguity of the alternative definitions EStokes = 2S & hbar;w and EStokes = (2S-1)& hbar;w widely accepted in textbooks and numerous articles. We propose a new framework for the analysis of the Stokes shift in the broad multiphonon optical bands based on the analysis of the spectral distributions within the linear electron- vibrational coupling model. Using the exact quantum-mechanical expressions for the moments of the Pekarian type spectral distribution we demonstrate that the Stokes shift can be described by the universal expression EStokes = (2S- S(T))& hbar;w where the temperature dependent term S(T) varies in the range between 1 (low temperature) and 0 (high temperature). This result is valid for a discrete Pekarian, when the vibronic sidebands are spectrally resolvable (and maximum of the band cannot be strictly defined) and for a more common case of structureless bands arising under the condition of prevailing interactions with phonons having continuous spectrum. It is emphasized that the parameter S(T) can be determined experimentally by studying the measured spectral moments of the optical band and does not require numerical modeling, which is necessarily based on a specific (often not entirely justified) assumption regarding the shape and broadening of the individual lines of a discrete Pekarian. In a more general context we emphasize that the use of the single-mode Pekarian approach in the cases of a significant contribution of crystal vibrations is associated with a significant risk of misinterpretation of experimental data.

The article is devoted to the theoretical development of the mechanisms of molecular refrigeration, the area combining molecular magnetism and material science with promise for low-temperature physics and quantum computing, where conventional principles of refrigeration become inefficient. Given this general trend, we propose the concept of the magnetothermal effect in magnetically anisotropic complexes of 3d metal ions, caused by fast magnetic field quenching. Within this concept, the most topical case of an axially magnetically anisotropic system isolated from the environment by adiabatic envelope is analyzed. We evaluate the temperature change as a function of the initial temperature and magnetic field and also its dependence on the sign and the magnitude of the axial zero-field splitting parameter and the Debye temperature. Correlations are revealed between the sign of the magnetic anisotropy (dictated by the sign of the axial zero field splitting parameter) and the sign of the thermal effect (heating versus cooling) caused by field quenching. The temperature change is shown to be negative (cooling) in the case of complexes exhibiting easy-axis-type magnetic anisotropy, while for the case of easy-plane-type anisotropy, it proves to be positive (heating). The thermal effects are shown to have an efficient control by varying the initially applied field. These findings allow us to propose complexes exhibiting easy-axis-type magnetic anisotropy as candidates for achieving a low-temperature refrigeration effect caused by fast field quenching and also to employ the established magnetothermal correlations to the analysis of magnetic anisotropy.

In this article, we analyze the thermal processes induced in the Heisenberg-type exchange coupled binuclear clusters by suddenly switching off the magnetic field initially applied to the system. The key result of the present study is the established correlation between the sign of the exchange parameter and the sign of the thermal effect. We have demonstrated that in the case of a ferromagnetic exchange, heat absorption (cooling) occurs, while if the exchange is antiferromagnetic, heat release (heating) takes place. This conclusion suggests a possibility to regard Heisenberg type dimers exhibiting ferromagnetic exchange as a class of low-temperature magnetic refrigerants that could be promising for practical applications.

In this study, we analyze the thermal processes in Ising-type exchange-coupled binuclear clusters induced by the sudden switching off of an initially applied magnetic field. The main result is the established correlation between the sign of the exchange parameter and the sign of the thermal effect caused by the fast field switching off. We demonstrate that in the case of ferromagnetic Ising-type superexchange associated with easy-axis magnetic anisotropy, the spin system absorbs heat, resulting in the cooling of the surrounding. Conversely, in the case of antiferromagnetic exchange corresponding to easy-plane anisotropy, heat is released by the spin system, leading to heating. These results reveal a possibility to regard Ising-type dimers exhibiting ferromagnetic exchange as a class of low- and ultralow-temperature magnetic refrigerants with potential for practical applications. We also discuss the conditions under which such cooling could be most efficient.

The purpose of this short review article is to discuss at a simple qualitative level some key requirements the mixed-valence (MV) molecules should meet to be potentially applicable as cells of quantum cellular automata (QCA), and also how different interactions affect their fulfillment. We focus on two requirements, which are closely related to encoding and propagating of binary information within the electronic circuits and power dissipation caused by the logical operations. The physical features behind these requirements are the following: the ability of MV molecules to be efficiently switched between two logical binary states which assumes high polarizability manifesting itself in a strong non-linear cell-cell response and a low heat release caused by molecular rearrangements accompanying logical operations. We discuss the role of such electronic interactions as intramolecular electron transfer, intramolecular interelectronic Coulomb repulsion and the interaction of the excess electrons of a molecular cell with the electric field produced by the neighboring polarized cell. The pivotal role of the interaction of the excess electrons with the molecular vibrations (pseudo Jahn-Teller vibronic coupling) is discussed as well. Finally, the optimal conditions expressed as a parametric regime ensuring simultaneous fulfillment of the aforenamed requirements are discussed.

Quantum Dot Cellular Automata (QCA) is an emerging trend in the field of nanoelectronics, and computing can be regarded as an alternative to the traditional complementary metal–oxide–semiconductor technology. The paper is devoted to the study of the key functional properties of the cells for molecular QCA based on mixed valence molecules. The theoretical results for the heat dissipation under the conditions of the fast nonadiabatic switching event and cell–cell response function are obtained in the framework of the quantum-mechanical vibronic approach. These results are parameterized using the previous reliable ab initio calculations performed for oxidized norbornadiene and its polycyclic derivatives with variable lengths of the bridge. The comparative analysis of the dimeric and bidimeric molecular cells composed of these compounds is given. It is underlined that the conditions of a strong non-linear response and a low heat release are contradictory. However, despite this problem, a parametric regime is proposed, which provides a low heat release in combination with a strong nonlinear response of the working cell to the electric field induced by the polarized driver cell.

This article is largely oriented towards the theoretical foundations of the rational design of molecular cells for quantum cellular automata (QCA) devices with optimized properties. We apply the vibronic approach to the analysis of the two key properties of such molecular cells, namely the cell–cell response and energy dissipation in the course of the non-adiabatic switching of the electric field acting on the cell. We consider two kinds of square planar cells, namely cells represented by a two-electron tetrameric mixed valence (MV) cluster and bidimeric cells composed of two one-electron MV dimeric half-cells. The model includes vibronic coupling of the excess electrons with the breathing modes of the redox sites, electron transfer, intracell interelectronic Coulomb repulsion, and also the interaction of the cell with the electric field of polarized neighboring cells. For both kinds of cells, the heat release is shown to be minimal in the case of strong delocalization of excess electrons (weak vibronic coupling and/or strong electron transfer) exposed to a weak electric field. On the other hand, such a parametric regime proves to be incompatible with a strong nonlinear cell–cell response. To reach a compromise between low energy dissipation and a strong cell–cell response, we suggest using weakly interacting MV molecules with weak electron delocalization as cells. From this point of view, bidimeric cells are advantageous over tetrameric ones due to their smaller number of electron transfer pathways, resulting in a lower extent of electron delocalization. The distinct features of bidimeric cells, such as their two possible mutual arrangements (“side-by-side” and “head-to-tail”), are discussed as well. Finally, we briefly discuss some relevant results from a recent ab initio study on electron transfer and vibronic coupling from the perspective of the possibility of controlling the key parameters of molecular QCA cells.

In this article, we develop a vibronic theory of clocking in molecular quantum cellular automata (QCA). The clocking mechanism is considered for a trigonal trimeric mixed-valence (MV) system with one mobile electron, which is shown to act as the dimeric unit encoding binary information (Boolean states 0 or 1) coupled to a third redox center (Null state). The model includes the electron transfer between the three centers; vibronic coupling of the mobile charge with the "breathing" modes, forming a double degenerate Jahn-Teller vibration of the molecular triangle; and two electric fields, one collinear to the dimeric unit, which controls the binary states, and the other perpendicular to this unit, performing clocking. In the framework of the adiabatic approximation, the potential surface of the trimeric system has been studied and the condition determining switching and clocking has been analyzed in terms of the two controlling fields and the vibronic and transfer parameters. A thorough understanding of the site populations is achieved through the quantum-mechanical solution of the vibronic problem, maintaining the adiabatic condition for the controlling fields. It is shown that a MV trimer can act as a molecular clocked QCA cell, with favorable conditions being a positive electron transfer parameter and sufficiently strong vibronic coupling.

In this article we analyze the thermal processes in magnetically anisotropic metal complexes under the action of non-adiabatic switching of magnetic field. Using the non-stationary perturbation theory for the case of sudden perturbation, we show that this field can cause not only heat release, but also heat absorption, interconnected with the axial zero field splitting (parameter D) in a paramagnetic metal complex. As an illustrative example we consider the simplest S = 1-complexes having "easy axis" and "easy plane" types of anisotropy influenced by the magnetic field that is suddenly turned off. We demonstrate that the character of the thermal processes (heat dissipation or absorption) depends on the sign of D and direction of applied field and so the analysis of these processes can be in principle used as a complementary tool (in addition to SQIUD magnetometry, EPR spectroscopy and INS) for studying magnetic anisotropy. The conditions under which the non-adiabatic switching of the magnetic field gives rise to the heat absorption are revealed. This unusual phenomenon, which can be called "nonadiabatic field switching cooling", may have practical applications. We analyze the thermal processes in magnetically anisotropic metal complexes under the action of non-adiabatic switching of magnetic field. The results can be used for studying the magnetic anisotropy and may have practical implementation as a method of cooling.

The theory of the magnetic coupling between the localized spins, mediated by the mobile excess electron, is generalized to the case of a trigonal, six-center, four-electron molecule with partial valence delocalization. The combination of the electron transfer occurring within the valence-delocalized subsystem and the interatomic exchange producing coupling of the spin of the mobile electron of valence-delocalized fragment with the three localized spins forming the valence-localized subsystem leads to the appearance of a special kind of double exchange (DE), termed the "external core double exchange" (ECDE), in order to distinguish such DE from the conventional "internal core double exchange" for which the mobile electron is coupled with the spin-cores on the same center via the intra-atomic exchange. The effect of the ECDE on the ground spin state of the considered trigonal molecule is compared with earlier reported effect produced by DE in the four-electron, mixed-valence (MV) trimer. A high diversity of the ground spin states is revealed, depending on the relative magnitudes and signs of the electron transfer and interatomic exchange parameters, with part of these states not appearing to be the ground states in a trigonal trimer exhibiting DE. We briefly discuss some examples of trigonal MV systems from the point of view of the possibility to have different combinations of signs of the transfer and exchange parameters and, accordingly, different ground spin states. The tentative role of the considered systems in molecular electronics and spintronics is also noticed.

The rare-earth ions in crystals such as terbium (YTaO4:Tb3+) and europium (YTaO4:Eu3+)-activated yttrium tantalate phosphors have a number of attractive features that predetermine their crucial role in practical application in contemporary optoelectronic devices. In this article, we employ the group-theoretical arguments aimed to reveal the group-theoretical classification of the crystal field levels and selection rules for the allowed optical transition between the crystal field components of Tb3+ and Eu3+ of the low symmetry crystal field in the activated yttrium tantalate phosphors. We also establish possible polarization rules for the lines corresponding to the allowed transitions. We deduce the symmetry-assisted results for the selection rules in the optical transitions accompanied by the absorption/emission of the vibrational quanta. The selection rules for the vibronic satellites of the zero-phonon lines are expected to be useful for the identification of the lines in the spectra of rare-earth ions with a weak vibronic coupling. The results of the low-temperature measurements of photoluminescence under the 325 nm excitation are in compliance with the group-theoretical analysis. The aim of the paper is to establish symmetry-assisted results that are the background of the quantitative crystal field theory based on the quantum-mechanical consideration.

In this article, we analyze power dissipation in the nonadiabatic switching event in mixed-valence (MV) molecular cells of quantum cellular automata (QCA) in combination with a key functional property of cells such as polarizability in the applied electric field. We demonstrate that although the requirements for a strong nonlinear response of the cell to the applied electric field and low heat release are competing from the point of view of molecular parameters, this by no means can be regarded as an insurmountable obstacle for achieving functional advantages and possibility of practical application of QCA. The general theoretical consideration is applied to the series of MV compounds exemplifying electric field-switchable MV molecules, which include oxidized norbornadiene [C7H8]+ (I) and its polycyclic derivatives [C12H12]+ (II), [C17H16]+, (III), [C27H24]+ (IV), and [C32H28]+ (V). Based on the results of high-level ab initio calculations performed for the series of compounds with variable length of the bridge connecting redox groups, we show that strongly localized cation radicals with long bridges can be easily polarized even by a fairly weak electric field. This ensures quite low power dissipation, which is shown to coexist with a rather strong nonlinear cell-cell response. We thus conclude that consideration of the series of MV dimers with controllable electron transfer provides a reasonable way to design molecule-based QCA cells with the required properties.

The effects of electronic and vibronic interactions on the specific heat release occurring in the course of nonadiabatic switching of the electric field polarizing a one-electron mixed-valence dimer is analyzed within the framework of the Piepho-Krausz-Schatz vibronic model. The search for an optimal parametric regime from the point of view of minimizing heat release is carried out taking into account the requirement to maintain a strong nonlinear response of the dimer to the applied electric field. Calculations of the specific heat release and the response performed in the framework of the quantum mechanical vibronic approach show that although the heat release is minimal under a weak electric field acting on the dimer in combination with weak vibronic coupling and/or strong electron transfer, such a combination of the parameters is incompatible with the requirement of a strong nonlinear response. Unlike this, for molecules exhibiting strong vibronic interactions and/or weak transfer, a rather strong nonlinear response can be obtained even with a very weak electric field, which, in turn, ensures low heat release. Thus, we can conclude that an efficient strategy to improve characteristics of molecular quantum cellular automata devices or other molecular switchable devices based on mixed-valence dimers consists in usage of molecules subjected to the action of a weak polarizing field, which are characterized by strong vibronic coupling and/or weak transfer.