Strong Jahn-Teller Coupling Research Articles

Proton Storage in Metallic H1.75MoO3 Nanobelts through the Grotthuss Mechanism.

The proton, as the cationic form of the lightest element-H, is regarded as most ideal charge carrier in "rocking chair" batteries. However, current research on proton batteries is still at its infancy, and they usually deliver low capacity and suffer from severe acidic corrosion. Herein, electrochemically activated metallic H1.75MoO3 nanobelts are developed as a stable electrode for proton storage. The electrochemically pre-intercalated protons not only bond directly with the terminal O3 site via strong O-H bonds but also interact with the oxygens within the adjacent layers through hydrogen bonding, forming a hydrogen-bonding network in H1.75MoO3 nanobelts and enabling a diffusion-free Grotthuss mechanism as a result of its ultralow activation energy of ∼0.02 eV. To the best of our knowledge, this is the first reported inorganic electrode exhibiting Grotthuss mechanism-based proton storage. Additionally, the proton intercalation into MoO3 with formation of H1.75MoO3 induces strong Jahn-Teller electron-phonon coupling, rendering a metallic state. As a consequence, the H1.75MoO3 shows an outstanding fast charging performance and maintains a capacity of 111 mAh/g at 2500 C, largely outperforming the state-of-art battery electrodes. More importantly, a symmetric proton ion full cell based on H1.75MoO3 was assembled and delivered an energy density of 14.7 Wh/kg at an ultrahigh power density of 12.7 kW/kg, which outperforms those of fast charging supercapacitors and lead-acid batteries.

Intracenter optical transitions in Mn2+ ion in CdMnTe crystals: the effect of strong Jahn–Teller coupling in excited state

Intracenter resonance optical transitions between ground and first excited states of a Mn2+ ion in bulk CdMnTe crystal are theoretically and numerically studied. An algorithm is proposed to calculate the matrix elements of the optical transitions under the influence of electromagnetic waves of different polarizations from a six-fold degenerate ground state 6A1 to a twelve-fold degenerate first excited state 4T1. The calculations were carried out with the account of a strong Jahn–Teller effect, which is due to the interaction of charge carriers in the excited state of the ion with local tetragonal distortions of the CdMnTe crystal lattice. By comparison with previously obtained experimental data, it is shown that polarization properties of the photoluminescence of manganese ions in such crystals as well as its spectral features could be explained only with the account of strong Jahn–Teller coupling in the excited state. The same calculation technique based on symmetry group analysis combined with numerical methods could be applied to intracenter Mn-transitions in other II–VI materials and it could be extended to other optically addressed atoms with highly degenerate electronic states coupled to vibrational modes.

Infrared properties of perovskite manganites La1−xCaxMnO3 with 0.25 ≤ x ≤ 0.45

The systematical investigations of infrared phonon spectra were performed on the perovskite manganites La1−xCaxMnO3 with 0.25 ≤ x ≤ 0.45. Both frequency shift of stretching mode and effective number of carriers neff∗(T) change dramatically near the Curie temperature for all compounds. Moreover, Ca concentration dependences of variations of the phonon frequencies and neff∗(T) shows their extreme values around x = 0.35 and 0.40 respectively at the measuring temperatures. With the varying Ca concentration, the competition between the strong Jahn-Teller coupling and the magnetic double exchange interaction may be responsible for those results.

A variational theory of Hall effect of Anderson lattice model: Application to colossal magnetoresistance manganites (Re1−x Ax MnO3)

An anomalous Hall constant RH has been observed in various rare earth manganites doped with alkaline earths namely Re1−xAxMnO3 (where Re = La, Pr, Nd etc., and A = Ca, Sr, Ba etc.) which exhibit colossal magnetoresistance (CMR), metal- insulator transition and many other poorly understood phenomena. We show that this phenomenon of anomalous Hall constant can be understood using two band (ℓ-b) Anderson lattice model Hamiltonian alongwith (ℓ-b) hybridization recently studied by us for manganites in the strong electron-lattice Jahn-Teller (JT) coupling regime an approach similar to the two – fluid models. We use a variational method in this work to study the temperature variation of Hall constant RH (T) in these compounds. We have already used this variational method to study the zero field electrical resistivity ρ (T) and magnetic susceptibility of doped CMR manganites. In the present study, we find that the Hall constant RH (T) reduces with increasing magnetic field parameters h&m and the metal-insulator transition temperature (Tρ) shifts towards higher temperature region. We have also observed the role of the model parameters e.g. local Coulomb repulsion U, Hund’s rule coupling JH between eg spins and t2g spins, ferromagnetic nearest neighbor exchange coupling JF between t2g core spins and hybridization Vk between ℓ-polarons and d-electrons on Hall constant RH (T) of these materials at different magnetic fields. Here we find that RH (T) for a particular value of h and m shows a rapid initial increase, followed by a sharp peak at low temperature say 50K in our case and a slow decrease at high temperatures, resembling with the key feature of many CMR compounds like La0.8Ba0.2 MnO3.The magnitude of RH (T) reduces and the anomaly (sharp peak) in RH becomes broader and shifts towards higher temperature region on increasing Vk or JH or doping x and even vanishes on further increasing these parameters. Our results of anomalous Hall constant (RH) have same qualitative behavior as the zero-field electrical resistivity. Moreover Hall Constant (RH) shows positive values indicating that the carriers in these manganites are holes.

Quantum dynamics on a three-sheeted six-dimensional ab initio potential-energy surface of the phosphine cation: Simulation of the photoelectron spectrum and the ultrafast radiationless decay dynamics.

A diabatic three-sheeted six-dimensional potential-energy surface has been constructed for the ground state and the lowest excited state of the PH3 (+) cation. Coupling terms of Jahn-Teller and pseudo-Jahn-Teller origin up to eighth order had to be included to describe the pronounced anharmonicity of the surface due to multiple conical intersections. The parameters of the diabatic Hamiltonian have been optimized by fitting the eigenvalues of the potential-energy matrix to ab initio data calculated at the CASSCF/MRCI level employing the correlation-consistent triple-ζ basis. The theoretical photoelectron spectrum of phosphine and the non-adiabatic nuclear dynamics of the phosphine cation have been computed by propagating nuclear wave packets with the multiconfiguration time-dependent Hartree method. The theoretical photoelectron bands obtained by Fourier transformation of the autocorrelation function agree well with the experimental results. It is shown that the ultrafast non-radiative decay dynamics of the first excited state of PH3 (+) is dominated by the exceptionally strong Jahn-Teller coupling of the asymmetric bending vibrational mode together with a hyperline of conical intersections with the electronic ground state induced by the umbrella mode. Time-dependent population probabilities have been computed for the three adiabatic electronic states. The non-adiabatic Jahn-Teller dynamics within the excited state takes place within ≈5 fs. Almost 80% of the excited-state population decay to the ground state within about 10 fs. The wave packets become highly complex and delocalized after 20 fs and no further significant transfer of electronic population seems to occur up to 100 fs propagation time.

Vibronic excitation dynamics in orbitally degenerate correlated electron system

Orbital-lattice coupled excitation dynamics in orbitally degenerate correlated systems are examined. We present a theoretical framework, where both local vibronic excitations and superexchange-type inter-site interactions are dealt with on an equal footing. We generalize the spin-wave approximation so as to take local vibronic states into account. Present method is valid from weak to strong Jahn-Teller coupling magnitudes. Two characteristic excitation modes coexist; a low-energy dispersive mode and high-energy multi-peak mode. These are identified as a collective vibronic mode, and Flanck-Condon excitations in a single Jahn-Teller center modified by the inter-site interactions, respectively. Present formalism covers vibronic dynamics in several orbital-lattice coupled systems.

Room temperature magnetocaloric effect of La-deficient bulk perovskite manganite La 0.7MnO 3− δ

Room temperature magnetocaloric effect in La-deficient bulk perovskite manganite La 0.7MnO 3− δ prepared by conventional solid-state reaction has been reported. The maximum value of the magnetic entropy change (about−1.32 J/kg K) and the refrigerant capacity (approximately close to 37 J/kg) had been obtained at 290 K corresponding to a magnetic field variation of 1 T for La 0.7MnO 3− δ . It is the strong Jahn–Teller coupling that changes Mn–O bond length and Mn–O–Mn bond angles and then the canted spin arrangement and induces the strong double-exchange coupling to a comparatively high magnetic transition temperature. This Curie temperature near room temperature with easy fabrication and higher chemical stability makes La 0.7MnO 3− δ a potential candidate as a working substance in magnetic refrigeration technology.

Excited Electronic States and Nonadiabatic Effects in Contemporary Chemical Dynamics

The question of how to describe the crossing of molecular electronic states is one of the most challenging issues in contemporary chemical dynamics. In recent years, the fundamental concept of conical intersections (CIs) of electronic potential energy surfaces (PESs) has emerged, which allows extremely fast and efficient switching of a molecule between its excited and ground electronic states. CIs are ubiquitous in polyatomic molecules. Because they generically allow the crossings of the Born-Oppenheimer (BO) adiabatic PESs, they have become the crucial mechanistic elements of the rapidly growing area of nonadiabatic chemistry. The most critical consequence of CIs is a complete breakdown of the adiabatic BO approximation. That means that the reorganization of fast-moving electrons and nuclear vibrations must be treated concurrently. Ideally, the theoretical description should be quantum mechanical in this situation. However, because of the complexity, the necessary approximations often make it difficult to conclusively predict dynamic behavior of large polyatomic molecules. In addition, a nonunique diabatic electronic representation (describing coupling between states in the electronic Hamiltonian) is essential to avoid the singular nature of the nuclear kinetic coupling terms of the unique adiabatic electronic representation. This Account describes both the challenges and some recent advances in quantum mechanical studies of nonadiabatic molecular processes, highlighting results from our recent work examining the static aspects of CIs and their dynamical consequences. The spectroscopic implications of the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) intersections in complex molecular systems are discussed. Our work probes the underlying details of complex vibronic spectra of systems of growing sizes in terms of both electronic and nuclear degrees of freedom. The necessity of extension of the theoretical treatment beyond a linear vibronic coupling approach is addressed. Our results establish highly overlapping band structures due to JT and PJT CIs, a bimodal distribution of spectral intensity that originates from strong JT coupling, and the role of intermode bilinear coupling in the progressions of vibronic bands. Investigations of the quantum dynamics of the prototypical naphthalene radical cation were aimed at understanding its photostability, lack of fluorescence emissions, and diffuse interstellar bands. This work established extremely fast relaxation of this radical cation through CIs. Simulations of the interplay of electronic and relativistic spin-orbit coupling in the photodetachment spectroscopy of ClH(2)(-), in conjunction with experimental data, support the existence of a shallow van der Waals well in the reactive Cl + H(2) PES. These results also reveal a quenching of electronic coupling by the relatively strong spin-orbit coupling. In addition, we studied the dynamics of the prototypical H + H(2) reaction from a new perspective by explicitly including the coupling between the two energy surfaces of its JT split degenerate ground electronic state. Although individual reaction probabilities show partial sensitivity to nonadiabatic effects, the theoretical results reveal that they are not important for the dynamical outputs such as integral reaction cross sections and thermal rate constants.

Colossal magnetoresistance manganites: A new approach

Manganites of the LA1−x Ca x MnO3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach recently proposed by us, where we argue that due to strong Jahn-Teller (JT) coupling with phonons the twofold degeneratee g states at the Mn sites dynamically reorganize themselves into localised, JT polaronsl with exponentially small inter-site hopping, and band-like, nonpolaronic statesb, leading to anew 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment of the model which explains quantitatively a wide variety of experimental results, including insulator-metal transitions and CMR, in terms of the influence of physical conditions on the relative energies and occupation of thel andb states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and spatial correlation is the key to manganite physics.

The Jahn-Teller effect in the SrLaGa3O7:Co2+system

We have presented absorption spectra related to octahedrallycoordinated Co2+ in the Sr2+ position inSrLaGa3O7. Quantitative analysis of the line shape of the4T1→4T2 transition shows the existence of thestrong T⊗τ Jahn-Teller effect in the4T1 ground state of the system. We have estimated theJahn-Teller stabilization energy to be equal to 507 cm-1. Theprocedure of estimation of the crystal-field strength parameter10Dq for the system that is affected by the strong Jahn-Tellercoupling has been discussed.

Optical absorption and electron paramagnetic resonance of in GaP

The results of optical absorption, electrical conductivity, and electron paramagnetic resonance (EPR) experiments on different p-type GaP:V:Zn samples are reported. The transition of gives rise to a broad triple-peaked absorption band and a zero-phonon line (ZPL) at . These features are explained as due to a strong Jahn-Teller coupling of the state. The reduced spin-orbit splitting is smaller than the strain-broadened line-width of the ZPL. The EPR spectrum of , which is angular dependent at T < 4 K, is analysed (g = 1.960, ). Its intensity shows dependence on microwave power characteristics for an impurity with an ground state. The photo-induced recharging is monitored by optical absorption as well as EPR and requires the recharging of anti-site defects.

Nonmagnetic molecular Jahn-Teller Mott insulators

Narrow-band conductors may turn insulating and magnetic as a consequence of strong electron-electron correlation. In molecular conductors, the concomitance of a strong Jahn-Teller coupling may give rise to the alternative possibility of a non-magnetic insulator, with or without a static cooperative Jahn-Teller distortion. In the latter case the insulator has Mott-like properties, with an interesting interplay between electron-electron repulsion and the Jahn-Teller effect, which is dynamical. We study this kind of non-magnetic insulator in a very simple $E\otimes e$ Jahn-Teller model and we discuss its general properties in a more general context, also in connection with the insulating state of K_{4}C_{60} and Rb_{4}C_{60}.

Vanadium in CdTe

Vanadium doped CdTe is studied by electron paramagnetic resonance (EPR), photo-EPR, and optical absorption. The EPR measurements identify vanadium substitutional for Cd in the oxidation states V 2+ and V 3+. Whereas V 3+ shows the well-known isotropic EPR spectrum, the V 2+ spectrum features triclinic symmetry due to a strong Jahn-Teller coupling. By photo-EPR, the V 2+ V 3+- donor level was determined at 0.67 eV below the conduction band. In absorption, additionally to internal crystal field transitions of vanadium, we also observe the recharging process V 3+ to V 2+.

Vanadium in CdTe: An electron-paramagnetic-resonance study.

The transition-metal ion vanadium in CdTe is studied using electron paramagnetic resonance (EPR) and photo-EPR. Based on the observation of central hyperfine interaction, the EPR measurements identify two vanadium related defects. They are assigned to ${\mathrm{V}}^{3+}$ and ${\mathrm{V}}^{2+}$, both substitutional for Cd. Whereas ${\mathrm{V}}^{3+}$ shows the classic isotropic EPR spectrum, the ${\mathrm{V}}^{2+}$ spectrum features triclinic symmetry caused by a strong Jahn-Teller coupling to both ${\ensuremath{\tau}}_{2}$ and $\ensuremath{\epsilon}$ vibronic modes. By means of photo-EPR the ${\mathrm{V}}^{2+}$/${\mathrm{V}}^{3+}$-donor level is determined at 0.67 eV below the conduction band.

Electronic structure of the copper(II) ion doped in cubic KZnF3

The absorption and magnetic circular dichroism spectra of 1%–3% copper(II) doped into the cubic perovskite host KZnF3 are measured over the temperature range 1.8–300 K. Sharp magnetic dipole allowed transitions 2Eg(Γ8)→2T2g(Γ7,Γ8) are observed together with accompanying vibrational fine structure. The spectra are interpreted on the basis of a tetragonally elongated CuF4−6 ground state geometry which arises from strong Jahn–Teller coupling. This results in a statistical distribution of three equivalent elongations which can undergo reorientation on the electron paramagnetic resonance time scale. The Jahn–Teller coupling within the 2T2g multiplet is partially quenched by spin–orbit coupling and the lowest Kramers doublet Γ7 has an octahedral geometry, while the higher lying Γ8 state has a small tetragonal distortion. This system presents an excellent opportunity to study the spectroscopy of the copper(II) ion in a cubic environment.

Zeeman spectroscopy of V 3+-luminescence in ZnO

ZnO crystals exhibit in the near infrared spectral region a well-known but up to now contradictorily discussed luminescence structure. We show on the basis of carefully performed fine structure measurements in dependence on polarizations, temperatures and magnetic fields that the mentioned luminescence is due to the 3T 2(F)− 3A 2(F) transition of isolated substitutional V 3+ on the cation place. The observed fine-structures show the action of a strong Jahn-Teller coupling of ε-modes of the 3T 2(F) term. The excitation spectra prove the presence of V 3+ in the unexcited crystal. In the excited 3T 1(F) term an interaction between distinct phonon modes and the phonon continua takes place, resulting in the occurence of Fano resonances in the excitation spectra.

Photoelectron spectroscopy and electronic structure of clusters of the group V elements. II. Tetramers: Strong Jahn–Teller coupling in the tetrahedral 2E ground states of P+4, As+4, and Sb+4

High resolution HeI (584 Å) photoelectron spectra have been obtained for the tetrameric clusters of the group V elements: P4, As4, and Sb4. The spectra establish that the ground 2E states of tetrahedral P+4, As+4, and Sb+4 are unstable with respect to distortion in the ν2(e) vibrational coordinate. The E⊗e Jahn–Teller problem has been treated in detail, yielding simulated spectra to compare with experimental ones. Vibronic calculations, extended to second order (quadratic coupling) for P+4, account for vibrational structure which is partially resolved in its photoelectron spectrum. A Jahn–Teller stabilization energy of 0.65 eV is derived for P+4, which can be characterized in its ground vibronic state as being highly distorted, and highly fluxional. Linear-only Jahn–Teller coupling calculations performed for As+4 and Sb+4, show good qualitative agreement with experimental spectra, yielding stabilization energies of 0.84 and 1.4 eV, respectively.

The Jahn-Teller effect in Mössbauer spectroscopy

The relaxation and new fluctuation effects shown in Mossbauer hyperfine spectra of a Jahn-Teller (JT) ion are briefly discussed. The analysis is carried out in cases of both weak and strong JT coupling.

Fe3+as near-infrared luminescence center in ZnS

A richly structured luminescence is observed in the 1.2-\ensuremath{\mu}m spectral region in ``pure'' and Fe-doped ZnS crystals for the first time. By means of emission, excitation, and magneto-optical measurements an unambiguous assignment to the $^{4}$${\mathit{T}}_{1}$(G)${\mathrm{\ensuremath{-}}}^{6}$${\mathit{A}}_{1}$(S) transition of isolated ${\mathrm{Fe}}^{3+}$ on ${\mathrm{Zn}}^{2+}$ lattice sites becomes possible. Basic arguments are the sixfold degeneracy of the ground state with an isotropic g factor of 2.026\ifmmode\pm\else\textpm\fi{}0.010, the long decay time of about 4 ms, and the influence of additional iron doping. The excitation mechanism can be described by an energy transfer via free holes from ${\mathrm{Cu}}^{2+}$ to ${\mathrm{Fe}}^{2+}$ centers and a charge-transfer process ${\mathrm{Fe}}^{3+}$long-${&gt;}^{\mathit{h}\ensuremath{\nu}}$${\mathrm{Fe}}^{2+}$+${\mathit{h}}_{\mathrm{VB}}$ (VB denotes valence band). A comparison with the well-known isoelectronic ${\mathrm{Mn}}^{2+}$ centers reveals striking similarities. The numerous zero-phonon lines at T=1.8 K are due to the polytypic structure of the investigated crystals, offering several different ${\mathrm{Zn}}^{2+}$ lattice sites. The observed fine structures of the $^{4}$${\mathit{T}}_{1}$(G) terms and their behavior in a magnetic field indicate a strong Jahn-Teller coupling to E modes as is known for the ${\mathrm{Mn}}^{2+}$ centers.

Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. II. Temperature dependence of steady state and time-resolved luminescence

The visible emission resulting from photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in the solid methanes is analyzed with spectral and time resolution. Two emission bands are observed in both solid methane (CH4) and solid perdeuteromethane (CD4) at 516 and 560 nm. The measured radiative lifetimes of the two bands are 12 and 19 ms, respectively, and independent of isotopic variation of the host. On the basis of their spectral position and radiative lifetimes, the emission features are assigned to the spin-forbidden 3P–1S intercombination band of atomic magnesium. Emission spectra recorded at various temperatures in the range 12–33 K were found to show reversible temperature dependence whereby the intensity of the higher energy 516 nm band decreased monotonically with a concomitant increase in that of the lower energy 560 nm band. Time-resolved measurements of the two bands in Mg/CH4 and Mg/CD4 show that the radiative lifetime of the 516 nm band decreases with increasing temperature while that of the 560 nm band remains constant but showing a rise time component. The magnitude of this rise time is correlated with that of the decreasing lifetime of the 516 nm band. With a combination of the temperature dependence observed in the radiative lifetimes and spectral intensities of the 516 and 560 nm emission bands in the Mg/CH4 and Mg/CD4 systems, dynamical information is extracted which allows for the creation of a simple kinetic model and an identification of the mechanism giving rise to this temperature dependent phenomenon. The experimental data is found to exhibit linear behavior on an Arrhenius plot indicating an activated process of population interconversion between the emitting levels. In solid methane the barrier in this activated process is 177 and 125 cm−1 in solid perdeuteromethane. The origin of this process is discussed in terms of (a) structural phase transitions of the solid methanes and (b) the effect of strong Jahn–Teller coupling between phonon modes of the solid methane host and the spin triplet electronic state of the atomic magnesium guest yielding symmetry-distinct minima on this surface. The lengthened values of the measured radiative lifetimes in the solid methanes, indicate population equilibration amongst the total degeneracy of the triplet state J sublevels of atomic magnesium in these solids. Because of the extremely low temperatures at which the experiment is conducted, extensive orbital reduction factors are implied to achieve such equilibration conditions. This is a manifestation of very strong Jahn–Teller coupling, a necessary condition for the stabilization of symmetry-distinct minima on the triplet spin state surface and the mechanism proposed to explain the presence of the temperature dependent emission features.