Jahn-Teller Coupling Research Articles

The Jahn–Teller effect in the ground electronic state of the tetrafluoromethane cation before dissociation: a promoter of the anisotropic fragmentation

ABSTRACTThe role of multimode Jahn–Teller coupling effect on the anisotropic fragmentation of in its electronic state is investigated theoretically via an ab initio quantum dynamics study. The potential energy surfaces up to two-dimensions of nuclear coordinates are established from extensive electronic structure calculations. A higher order (seventh-order) diabatic vibronic coupling model Hamiltonian is necessary to mimic the ab initio potentials and thereby constructed. The theoretical results are in good agreement with experimental observations. A detail analysis of the relevant static aspects and the vibrational progressions under the structureless and broad vibronic band of are made. Present study suggests that the Jahn–Teller effect is operative before the dissociation of upon photoionisation and responsible for the experimentally measured anisotropic fragmentation in the electronic state of the title cation.

Beyond Born-Oppenheimer treatment on spectroscopic and scattering processes

We review the formulation of beyond Born-Oppenheimer (BBO) theory based on first principle for the construction of diabatic potential energy surfaces (PESs) both for few important spectroscopic systems, viz., Na3 cluster, NO2 radical as well as scattering process like D+ + H2. The essential theoretical development leading to the BBO equations are thoroughly discussed. It has been found that the above molecular systems posses numerous nonadiabatic interactions that range from Jahn-Teller, Renner-Teller types of conical intersections along with strong pseudo Jahn-Teller couplings between various electronic states. We have calculated the adiabatic PESs and nonadiabatic coupling terms for those systems and subsequently performed adiabatic-to-diabatic transformation to construct smooth, symmetric and continuous diabatic potential energy matrix. Nuclear dynamics has been performed on the diabatic PESs of Na3 and NO2 to simulate the photoelectron spectra that match quite well with the experimentally measured ones. Moreover, we have carried out reactive scattering dynamics on the adiabatic and diabatic surfaces of system to reproduce experimental cross sections for reactive charge and non-charge transfer processes.

The consequences of growth modes on the magnetotransport properties of La0.4Pr0.3Ca0.3MnO3/LAO films

The impact of pulse repetition rate and film thickness on the magnetic, magnetoresistance (MR) and anisotropic magnetoresistance (AMR) has been investigated for La0.4Pr0.3Ca0.3MnO3 thin films grown on LaAlO3 (LAO) substrates by pulsed laser deposition technique. Change in repetition rate led to a difference in growth modes and altered the magnetic and electrical properties (MR and AMR) of thin films. The XRD results show that in the films with the thickness of 100 nm, by increasing the pulse repetition rate from 2 to 7 Hz the strain level doesn’t change, while strain significantly increased at the repetition rate of 10Hz. Analysis of transport and magnetization measurements show that at 2 and 5 Hz deposited films, the ferromagnetic metallic behavior is dominant, while in the film deposited at 10 Hz the antiferromagnetic (AFM) insulating phase increased. Moreover, with increasing strain in the thinner films (40 nm), deposited at 2 Hz, AFM phase is strengthened. Exchange bias confirms the existence of a dead layer in 100 nm film deposited at 10 Hz and 40 nm film deposited at 2 Hz. In addition, high strain in thinner films led to an enhanced Jahn-Teller coupling, so the angular momentum and spin-orbital/lattice interactions are significantly increased, which led to a lower magnetoresistance in thinner films. These results suggest that the pulse repetition rate provides an additional degree of freedom to tune magnetic and electrical properties of manganite thin films prepared by pulsed laser deposition technique.

Surface reconstructions and modified surface states in La1−xCaxMnO3

The unique functionalities present at surfaces and interfaces of complex oxides have attracted intense research in the past decade. Yet, the fundamental mechanisms underpinning functionality are often elusive, especially in doped manganites, limiting their implementation in functional electronic devices such as memristors and spin valves. Here, we present a local probe-based study on mixed-terminated $\mathrm{L}{\mathrm{a}}_{5/8}\mathrm{C}{\mathrm{a}}_{3/8}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) films, and reveal surface metallicity in a thin film grown by pulsed-laser deposition. Using first-principles density-functional theory calculations with Hubbard correction that are more accurate to capture effects of correlation in these systems, we show that for Ca-segregated (001) LCMO surfaces the (La,Ca)O-site terminated surfaces are half metallic due to delocalized Mn-$d$ states populating the Fermi level, whereas the ${\mathrm{MnO}}_{2}$-site terminated surfaces exhibit a half-metallic or insulating character depending on the type of surface reconstruction. Computations not only explain the current measurements, but also explain other recent surface measurements on LCMO thin films, leading to a coherent picture of how the crucial link between surface segregation and Jahn-Teller couplings in the manganese oxides tune the surface electronic/magnetic structure, thereby pointing to the fine control of transport and magnetism at the conductive oxide surface independent of the bulk.

Topological Effects in Vibronically Coupled Degenerate Electronic States: A Case Study on Nitrate and Benzene Radical Cation.

We carry out detailed investigation for topological effects of two molecular systems, NO3 radical and C6H6+ (Bz+) radical cation, where the dressed adiabatic, dressed diabatic, and adiabatic-via-dressed diabatic potential energy curves (PECs) are generated employing ab initio calculated adiabatic and diabatic potential energy surfaces (PESs). We have implemented beyond Born–Oppenheimer (BBO) theory for constructing smooth, single-valued, and continuous diabatic PESs for five coupled electronic states [J. Phys. Chem. A2017,121, 6314–6326]. In the case of NO3 radical, the nonadiabatic coupling terms (NACTs) among the low-lying five electronic states, namely, X̃2A2′ (12B2), A~2E″ (12A2 and 12B1), and B~2E′ (12A1 and 22B2), bear the signature of Jahn–Teller (JT) interactions, pseudo JT (PJT) interactions, and accidental conical intersections (CIs). Similarly, Bz+ radical cation also exhibits JT, PJT, and accidental CIs in the interested domain of nuclear configuration space. In order to generate dressed PECs, two components of degenerate in-plane asymmetric stretching modes are selectively chosen for both the molecular species (Q3x–Q3y pair for NO3 radical and Q16x–Q16y pair for Bz+ radical cation). The JT coupling between the electronic states is essentially originated through the asymmetric stretching normal mode pair, where the coupling elements exhibit symmetric and nonlinear functional behavior along Q3x and Q16x normal modes.

Exploring the methane combustion reaction: A theoretical contribution**Project supported by the National Natural Science Foundation of China (Grant Nos. 51574016 and 51604018).

This paper represents an attempt to extend the mechanisms of reactions and kinetics of a methane combustion reaction. Three saddle points (SPs) are identified in the reaction using the complete active space self-consistent field and the multireference configuration interaction methods with a proper active space. Our calculations give a fairly accurate description of the regions around the twin first-order SPs (3A′ and 3A″) along the direction of O(3P) attacking a near-collinear H–CH3. One second-order SP2nd (3E) between the above twin SPs is the result of the C3v symmetry Jahn–Teller coupling within the branching space. Further kinetic calculations are performed with the canonical unified statistical theory method with the temperature ranging from 298 K to 1000 K. The rate constants are also reported. The present work reveals the reaction mechanism of hydrogen-abstraction by the O(3P) from methane, and develops a better understanding for the role of SPs. In addition, a comparison of the reactions of O(3P) with methane through different channels allows a molecule-level discussion of the reactivity and mechanism of the title reaction.

Lattice distortion modified anisotropic magnetoresistance in epitaxial La0.67Sr0.33MnO3 thin films

Effects of lattice distortion on the anisotropic magnetoresistance (AMR) of magnetron sputtered epitaxial La0.67Sr0.33MnO3 (LSMO) thin films grown on (001)-oriented SrTiO3 and MgO substrates were investigated. Lattice mismatch at the film/substrate interface gives rise to weak and strong lattice distortion of the LSMO/SrTiO3 and LSMO/MgO films, respectively. The Jahn-Teller electron-phonon coupling in LSMO leads to significant localization of the eg electrons, causing smaller magnetization, lower transition temperatures (TC, TMI) and larger resistivity. Moreover, AMR of the LSMO films show strong correlations with the substrates. Only two-fold symmetric AMR can be observed in the LSMO/SrTiO3 films. However, the AMR curves of the LSMO/MgO system show clear transitions between two- and four-fold symmetry. This behavior is ascribed to the lattice distortion in the LSMO films which enhances the magnetocrystalline anisotropy.

Nodeless vibrational amplitudes and quantum nonadiabatic dynamics in the nested funnel for a pseudo Jahn-Teller molecule or homodimer.

The nonadiabatic states and dynamics are investigated for a linear vibronic coupling Hamiltonian with a static electronic splitting and weak off-diagonal Jahn-Teller coupling through a single vibration with a vibrational-electronic resonance. With a transformation of the electronic basis, this Hamiltonian is also applicable to the anti-correlated vibration in a symmetric homodimer with marginally strong constant off-diagonal coupling, where the non-adiabatic states and dynamics model electronic excitation energy transfer or self-exchange electron transfer. For parameters modeling a free-base naphthalocyanine, the nonadiabatic couplings are deeply quantum mechanical and depend on wavepacket width; scalar couplings are as important as the derivative couplings that are usually interpreted to depend on vibrational velocity in semiclassical curve crossing or surface hopping theories. A colored visualization scheme that fully characterizes the non-adiabatic states using the exact factorization is developed. The nonadiabatic states in this nested funnel have nodeless vibrational factors with strongly avoided zeroes in their vibrational probability densities. Vibronic dynamics are visualized through the vibrational coordinate dependent density of the time-dependent dipole moment in free induction decay. Vibrational motion is amplified by the nonadiabatic couplings, with asymmetric and anisotropic motions that depend upon the excitation polarization in the molecular frame and can be reversed by a change in polarization. This generates a vibrational quantum beat anisotropy in excess of 2/5. The amplitude of vibrational motion can be larger than that on the uncoupled potentials, and the electronic population transfer is maximized within one vibrational period. Most of these dynamics are missed by the adiabatic approximation, and some electronic and vibrational motions are completely suppressed by the Condon approximation of a coordinate-independent transition dipole between adiabatic states. For all initial conditions investigated, the initial nonadiabatic electronic motion is driven towards the lower adiabatic state, and criteria for this directed motion are discussed.

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.

Jahn-Teller effect and large-amplitude motion in [formula omitted] studied by high-resolution photoelectron spectroscopy of CH4

This article presents the current status of the knowledge of the rovibronic energy-level structure of CH4+ obtained by high-resolution photoelectron spectroscopy. The results of previous investigations are summarized and extended by new results obtained by double-resonance experiments involving vacuum-ultraviolet and mid-infrared laser radiation. These experiments have led to assignments of the nuclear-spin symmetry of 303 rovibrational levels of CH4+ with up to 3575cm−1 of internal excitation. A two-dimensional model of the pseudorotational motion of CH4+ is also presented, with which the positions and vibronic symmetry of the low-lying vibronic levels of CH4+ can be predicted. The model maps the F⊗(f⊕e) Jahn-Teller problem for the e and f2 modes corresponding to the CH bending motion onto a sphere, including the effects of both linear and quadratic Jahn-Teller coupling terms. The pseudorotation eigenstates are obtained by solving the two-dimensional Schrödinger equation using a basis of spherical harmonics.

Assigning the low lying vibronic states of CH3O and CD3O.

The assignment of lines in vibrational spectra in strongly mixing systems is considered. Several low lying vibrational states of the ground electronic X∼2E state of the CH3O and CD3O radicals are assigned. Jahn-Teller, spin-orbit, and Fermi couplings mix the normal mode states. The mixing complicates the assignment of the infrared spectra using a zero-order normal mode representation. Alternative zero-order representations, which include specific Jahn-Teller couplings, are explored. These representations allow for definitive assignments. In many instances it is possible to plot the wavefunctions on which the assignments are based. The plots, which are shown in the adiabatic representation, allow one to visualize the effects of various higher order couplings. The plots also enable one to visualize the conical seam and its effect on the wavefunctions. The first and the second order Jahn-Teller couplings in the rocking motion dominate the spectral features in CH3O, while first order and modulated first order couplings dominate the spectral features in CD3O. The methods described here are general and can be applied to other Jahn-Teller systems.

Vibronic eigenstates and the geometric phase effect in the 2E″ state of NO3.

The 2E″ state of NO3, a prototype for the Jahn-Teller effect, has been an enigma and a challenge for a long time for both experiment and theory. We present a detailed theoretical study of the vibronic quantum dynamics in this electronic state, uncovering the effects of tunnelling, geometric phase, and symmetry. To this end, 45 vibronic levels of NO3 in the 2E″ state are determined accurately and analyzed thoroughly. The computation is based on a high quality diabatic potential representation of the two-sheeted surface of the 2E″ state developed by us [W. Eisfeld et al., J. Chem. Phys. 140, 224109 (2014)] and on the multi-configuration time dependent Hartree approach. The vibrational eigenstates of the NO3- anion are determined and analyzed as well to gain a deeper understanding of the symmetry properties of such D3h symmetric systems. To this end, 61 eigenstates of the NO3- anion ground state are computed using the single sheeted potential surface of the 1A1 state published in the same reference quoted above. The assignments of both the vibrational and vibronic levels are discussed. A simple model is proposed to rationalize the computed NO3 spectrum strongly influenced by the Jahn-Teller couplings, the associated geometric phase effect, and the tunnelling. Comparison with the available spectroscopic data is also presented.

Spin-orbital order in the undoped manganite LaMnO3 at finite temperature

We investigate the evolution of spin and orbital order in undoped LaMnO$_3$ under increasing temperature with a model including both superexchange and Jahn-Teller interactions. We used several cluster mean field calculation schemes and find coexisting $A$-type antiferromagnetic ($A$-AF) and $C$-type alternating orbital order at low temperature. The value of the Jahn-Teller coupling between strongly correlated $e_g$ orbitals is estimated from the orbital transition temperature at $T_{\rm OO}\simeq 780$ K. By a careful analysis of on-site and on-bond correlations we demonstrate that spin-orbital entanglement is rather weak. We have verified that the magnetic transition temperature is influenced by entangled spin-orbital operators as well as by entangled orbital operators on the bonds but the errors introduced by decoupling such operators partly compensate each other. Altogether, these results justify why the commonly used disentangled spin-orbital model is so successful in describing the magnetic properties and the temperature dependence of the optical spectral weights for LaMnO$_3$.

Cation States of Ethane: HEAT Calculations and Vibronic Simulations of the Photoelectron Spectrum of Ethane.

High-accuracy ab initio calculations have been carried out on ethane and its radical cation. With the HEAT-345(Q) scheme, adiabatic ionization potentials of 11.52 and 11.57 eV are determined for the X̃ (2)Eg and à (2)A1g states, respectively, with an uncertainty of ±0.015 eV. Also considered in this report are linear and quadratic vibronic coupling involving both states. With this simple vibronic model, the photoelectron spectrum of ethane was simulated in the 11-15 eV region using linear and full quadratic Jahn-Teller coupling Hamiltonians, and with up to 70 billion direct product basis functions in a high-performance computing environment. Although the linear vibronic coupling model adequately reproduces the spectral envelope, the quadratic vibronic treatment results in much better agreement with the observed spectrum.

Relativistic theory of the Jahn-Teller effect: p-orbitals in tetrahedral and trigonal systems.

A relativistic generalization of Jahn-Teller theory is presented which includes spin-orbit coupling effects beyond low-order Taylor expansions in vibrational coordinates. For the example of a p-electron in tetrahedral and trigonal environments, the matrix elements of the Breit-Pauli spin-orbit-coupling operator are expressed in terms of the matrix elements of the electrostatic electronic potential. Employing expansions of the latter in invariant polynomials in symmetry-adapted nuclear coordinates, the spin-orbit induced Jahn-Teller coupling terms are derived for the T2 × (t2 + e) and (E + A) × (e + a) Jahn-Teller problems up to arbitrarily high orders. The linear G3/2 × (t2 + e) Jahn-Teller Hamiltonian of Moffitt and Thorson [Phys. Rev. 108, 1251 (1957)] for tetrahedral systems is generalized to higher orders in vibrational displacements. The Jahn-Teller Hamiltonians derived in the present work are useful for the interpolation and extrapolation of Jahn-Teller distorted potential-energy surfaces of molecules and complexes with heavy elements as well as for the calculation of vibronic spectra of such systems.

Jahn-Teller Effect in Systems with Strong On-Site Spin-Orbit Coupling.

When strong spin-orbit coupling removes orbital degeneracy, it would at the same time appear to render the Jahn-Teller mechanism ineffective. We discuss such a situation, the t_{2g} manifold of iridates, and show that, while the Jahn-Teller effect does indeed not affect the j_{eff}=1/2 antiferromagnetically ordered ground state, it leads to distinctive signatures in the j_{eff}=3/2 spin-orbit exciton. It allows for a hopping of the spin-orbit exciton between the nearest-neighbor sites without producing defects in the j_{eff}=1/2 antiferromagnet. This arises because the lattice-driven Jahn-Teller mechanism only couples to the orbital degree of freedom but is not sensitive to the phase of the wave function that defines isospin j_{z}. This contrasts sharply with purely electronic propagation, which conserves isospin, and the presence of Jahn-Teller coupling can explain some of the peculiar features of measured resonant inelastic x-ray scattering spectra of Sr_{2}IrO_{4}.

Combined first-principles and model Hamiltonian study of the perovskite seriesRMnO3(R=La,Pr,Nd,Sm,Eu, andGd)

We merge advanced ab initio schemes (standard density functional theory, hybrid functionals and the GW approximation) with model Hamiltonian approaches (tight-binding and Heisenberg Hamiltonian) to study the evolution of the electronic, magnetic and dielectric properties of the manganite family RMnO3 (R = La, Pr, Nd, Sm, Eu and Gd). The link between first principles and tight-binding is established by downfolding the physically relevant subset of 3d bands with e_g character by means of maximally localized Wannier functions (MLWFs) using the VASP2WANNIER90 interface. The MLWFs are then used to construct a tight-binding Hamiltonian. The dispersion of the TB e_g bands at all levels are found to match closely the MLWFs. We provide a complete set of TB parameters which can serve as guidance for the interpretation of future studies based on many-body Hamiltonian approaches. In particular, we find that the Hund's rule coupling strength, the Jahn-Teller coupling strength, and the Hubbard interaction parameter U remain nearly constant for all the members of the RMnO3 series, whereas the nearest neighbor hopping amplitudes show a monotonic attenuation as expected from the trend of the tolerance factor. Magnetic exchange interactions, computed by mapping a large set of hybrid functional total energies onto an Heisenberg Hamiltonian, clarify the origin of the A-type magnetic ordering observed in the early rare-earth manganite series as arising from a net negative out-of-plane interaction energy. The obtained exchange parameters are used to estimate the Neel temperature by means of Monte Carlo simulations. The resulting data capture well the monotonic decrease of the ordering temperature down the R series, in agreement with experiments.

Ab initio constructed diabatic surfaces of NO2 and the photodetachment spectra of its anion.

A thorough investigation has been performed for electronic structure, topological effect, and nuclear dynamics of NO2 molecule, where the adiabatic potential energy surfaces (PESs), conical intersections between the ground (X(2)A1) and the first excited state (A(2)B2), and the corresponding non-adiabatic coupling terms between those states are recalculated [Chem. Phys. 416, 11 (2013)] to achieve enough accuracy in dynamics. We employ beyond Born-Oppenheimer theory for these two state sub-Hilbert space to carry out adiabatic to diabatic transformation (ADT) to obtain the ADT angles and thereby, to construct single-valued, smooth, and continuous diabatic PESs. The analytic expressions for the adiabatic PESs and ADT angles are provided to represent a two-state three-mode diabatic Hamiltonian of NO2 for performing nuclear dynamics to calculate the photo-electron spectra of its anion. It appears that not only Jahn-Teller type coupling but also Renner-Teller interaction contributes significantly on the overall spectrum. The coupling between the electronic states (X(2)A1 and A(2)B2) of NO2 is essentially through the asymmetric stretching mode, where the functional form of such interaction is distinctly symmetric and non-linear.

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.