Dynamic Jahn-Teller Research Articles

X-ray Photoelectron spectroscopic and Raman spectroscopic study of bayldonite from Wheal Carpenter, Cornwall, UK

Bayldonite [Cu3Pb(AsO4)2(OH)2], Wheal Carpenter (Cornwall, UK) was studied by X-ray Photoelectron Spectroscopy and Raman Microscopy. X-ray Photoelectron Spectroscopy revealed single copper, lead and arsenic positions in the crystal structure. Two oxygen bands with a 1:4 ratio were associated oxygen positions in arsenate- and hydroxyl-groups, excluding the presence of acidic arsenate groups. The relatively large difference in binding energy for the two oxygen bands was interpreted as being due to the dynamic Jahn-Teller distortion of the copper octahedral in the bayldonite crystal structure. Raman microscopy showed bands at 804 and 837 cm−1 assigned to arsenate antisymmetric stretching mode and the symmetric stretching mode. Supported by the X-ray Photoelectron Spectroscopic results the bands at 726, 761, 822 and 886 cm−1 were assigned to copper-hydroxyl modes. Bands around 497 cm−1 were assigned to the arsenate antisymmetric bending modes and around 427 cm−1 to the symmetric bending modes. The 539 cm−1 band was associated with a copper-hydroxyl stretching mode or another υ4. The region 250–400 cm−1 showed sharp bands at 313 and 328 cm−1 with weaker bands at 298 and 342 cm−1 assigned to copper-oxygen and/or lead-oxygen stretching modes.

Possible low spin state of Fe ions in HoBaFe4O7

Effective magnetic moment of YBaFe4O7-δ (YBFO) and HoBaZn3GaO7 is obtained as 4.88 μB (/magnetic atom) and 10 μB (/magnetic atom), respectively from magnetic susceptibility. Nevertheless that of HoBaFe4O7 (HBFO) is almost half magnitude (2.77 μB/magnetic atom) than simply summed up the former two effective moments, although magnetic interaction between Ho and Fe ions is absent. The small effective moment of HBFO suggests different spin state in comparison with YBFO. Taking consideration of our recent neutron inelastic scattering result of HBFO, the small effective moment of HBFO is considered by low spin state of Fe ions. Low spin state is hardly expected in cubic system of Fe ions usually, but low spin state is considered to be realized in HBFO due to spin multiple splitting by dynamical Jahn-Teller effect.

First-principles study of the dynamic Jahn-Teller distortion of the neutral vacancy in diamond

First-principles density functional theory methods are used to investigate the structure, energetics, and vibrational motions of the neutral vacancy defect in diamond. The measured optical absorption spectrum demonstrates that the tetrahedral $T_d$ point group symmetry of pristine diamond is maintained when a vacancy defect is present. This is shown to arise from the presence of a dynamic Jahn-Teller distortion that is stabilised by large vibrational anharmonicity. Our calculations further demonstrate that the dynamic Jahn-Teller-distorted structure of $T_d$ symmetry is lower in energy than the static Jahn-Teller distorted tetragonal $D_{2d}$ vacancy defect, in agreement with experimental observations. The tetrahedral vacancy structure becomes more stable with respect to the tetragonal structure by increasing temperature. The large anharmonicity arises mainly from quartic vibrations, and is associated with a saddle point of the Born-Oppenheimer surface and a minimum in the free energy. This study demonstrates that the behaviour of Jahn-Teller distortions of point defects can be calculated accurately using anharmonic vibrational methods. Our work will open the way for first-principles treatments of dynamic Jahn-Teller systems in condensed matter.

Two-Channel Kondo Physics due to As Vacancies in the Layered Compound ZrAs_{1.58}Se_{0.39}.

We address the origin of the magnetic-field-independent -|A|T^{1/2} term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs_{1.58}Se_{0.39}, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the nonmagnetic version of the two-channel Kondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a C_{4} symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the low superconducting transition temperature T_{c}≈0.14 K of ZrAs_{1.58}Se_{0.39} compared to the free-of-vacancies homologue ZrP_{1.54}S_{0.46} (T_{c}≈3.7 K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.

Direct and real time probe of photoinduced structure transition in colossal magnetoresistive material

We report a direct and real time measurement of photoinduced structure phase transition in single crystal La0.84Sr0.16MnO3 using femtosecond electron diffraction. The melting of orthorhombic lattice ordering under femtosecond optical excitation is found involving two distinct processes with different time scales, an initial fast melting of orthorhombic phase in about 4 ps and a subsequent slower transformation in 90 ps and longer timescales. The fast process is designated as the initial melting of orthorhombic phase induced by the Mn-O bond change that is most likely driven by the quenching of the dynamic Jahn-Teller distortion following the photo-excitation. The slow process is attributed to the growing of newly formed structure domain from the photo-excited sites to the neighboring non-excited orthorhombic sites.

Calculation of the g Factors and Local Angular Distortions for ZnO:Cu2+ Nanocrystals With Various Copper Concentrations

Based on the perturbation treatments for a tetragonally distorted tetrahedral 3d 9 cluster, the g factors and local angular distortions are calculated for ZnO:Cu2+ nanocrystals with various Cu2+ concentrations in different systems I and II under dissimilar experimental conditions. Because of the dynamic Jahn–Teller effect, the bond angles θ between the four equivalent Cu2+–O2– bonds and the C4 axis are about 1.5o larger than that (θ0 ≈ 54.736o) of an ideal tetrahedron. Consequently, the original slightly trigonally distorted oxygen tetrahedron of the host Zn2+ site is transformed into a tetragonally compressed one. The isotropy of g factors may be attributed to the appropriate angular distortions Δθ = θ – θ0 due to the dynamic Jahn-Teller effect. The slightly increasing (or decreasing) g factors with concentration x can be illustrated as the delicate increases (or decreases) of the angular distortions (Δθ) and the covalency factors (N) for system I (or II), respectively, under almost equivalent crystal-fi eld strengths (Dq).

Monitoring Structural Changes in LixNi0.8Co0.15Al0.05O2 Using Ex Situ 7li/ 27al NMR

Although layered transition metal oxides are the most commercially available battery materials, they suffer from capacity fading over time due to transition metal (TM) and Li layer mixing that can lead to decreased Li diffusion and phase segregation due to oxygen loss. We have implemented 7Li and 27Al NMR to study the structural changes in LixNi0.8Co0.15Al0.05O2 (NCA) during electrochemical cycling. Using an optimized DFT calculated structure, the predicted NMR shifts are calculated using the spin-flipping method of Middlemiss et al.1 In the pristine materials, the 7Li and 27Al spectra indicate a dynamic Jahn-Teller distortion, as expected for LiNiO2, and the absence of Li in the TM layer. 27Al NMR also indicates that Al has a preference to be surrounded by six Ni3+ compared to a random solution model structure. 7Li NMR has been widely used to monitor local changes in the Li environment arising from Li/ TM layer mixing and the formation of defects and/or other phases. Here we show that the 27Al NMR shift of the Al environment is highly sensitive to its proximity to Ni3+, allowing for the indirect monitoring of Ni migration out of the TM layer after multiple cycles. Both 7Li and 27Al NMR can be utilized to indicate the mechanisms leading to capacity fade in NCA and other Al-substituted layered transition metal oxides over multiple cycles. Reference 1. D. S. Middlemiss, A. J. Ilott, R. J. Clément, F. C. Strobridge and C. P. Grey, Chem Mater, 2013, 25, 1723-1734.

Molecular geometric phase from the exact electron-nuclear factorization

The Born-Oppenheimer electronic wavefunction $\Phi_R^{BO}(r)$ picks up a topological phase factor $\pm 1$, a special case of Berry phase, when it is transported around a conical intersection of two adiabatic potential energy surfaces in $R$-space. We show that this topological quantity reverts to a geometric quantity $e^{i\gamma}$ if the geometric phase $\gamma = \oint \mathrm{Im} \langle \Phi_R |\nabla_{\mu} \Phi_R\rangle \cdot d\mathbf{R}_{\mu}$ is evaluated with the conditional electronic wavefunction $\Phi_R(r)$ from the exact electron-nuclear factorization $\Phi_R(r)\chi(R)$ instead of the adiabatic function $\Phi_R^{BO}(r)$. A model of a pseudorotating molecule, also applicable to dynamical Jahn-Teller ions in bulk crystals, provides the first examples of induced vector potentials and molecular geometric phase from the exact factorization. The induced vector potential gives a contribution to the circulating nuclear current which cannot be removed by a gauge transformation. The exact potential energy surface is calculated and found to contain a term depending on the Fubini-Study metric for the conditional electronic wavefunction.

Multifrequency cw-EPR and DFT Studies of an Apparent Compressed Octahedral Cu(II) Complex.

The syntheses and single-crystal X-ray structures of the mononuclear complexes [Cu(bmet)](ClO4)2·H2O, [Cu(bmet)]Br2·2MeCN, and [Zn(bmet)](ClO4)2·H2O (bmet = N,N'-bis(2,2'-bipyridin-6-ylmethyl)ethane-1,2-diamine) are described. All three complexes feature a central metal ion bound to all six N atoms of the bmet ligand, which displays a meridional-facial-facial-meridional (mffm) configuration. The three complexes show one N-M-N axis to be significantly shorter than the others in agreement with an apparent compressed octahedral geometry. The X-ray structures of a single crystal of [Cu(bmet)](ClO4)2·0.375H2O resolved from data recorded at different temperatures display no remarkable structural modifications. However, they all display both as a powder and, in solution, an axial g1 > g2 ≳ g3 > g(e) electron paramagnetic resonance (EPR) pattern at low temperature, which is indicative of tetragonally elongated octahedra, while at room temperature the Q-band EPR spectra display a more rhombic g1 ≳ g2 > g3 > g(e) pattern. The fully density functional theory optimized structure of the Cu(II) complexes displays significant structural modifications only along one N(imine)-M-N(amine) axis resulting in an elongated octahedral structure. Furthermore, the EPR parameters predicted from this structure are comparable to those determined experimentally from the axial EPR signal recorded at low temperature, consistent with the unpaired electron residing mainly in the {3d(x(2)-y(2))} orbital. The structural and electronic properties of [Cu(bmet)](2+) are different from those in other previously described dynamic Jahn-Teller systems. We propose that these data can be rationalized by a dynamic Jahn-Teller effect perturbed by the strain of the hexadentate bmet ligand.

Giant permittivity and electrostriction induced by dynamic Jahn-Teller and pseudo Jahn-Teller effects

We revealed specific properties of molecular systems and solids with dynamic Jahn-Teller effect (JTE) or pseudo JTE (PJTE) that enhance permittivity and electrostriction by several orders of magnitude. These effects occur due to the spontaneous symmetry breaking at the JTE or PJTE centers resulting in several equivalent minima of the adiabatic potential energy surface in which the system is dipolar-distorted in different directions. In the absence of external perturbations the distortions resonate between the equivalent orientations via tunneling between the minima, but an external electric field, similar to external strain considered earlier [I. B. Bersuker, Appl. Phys. Lett. 106, 022903 (2015)], makes the minima nonequivalent, trapping all the centers polar-distorted along one direction. This results in an orientational contribution to the polarization and electrostriction of the system which is much stronger than the displacive one. Under this effect, the estimated permittivity and electrostriction of paraelectric BaTiO3 and similar crystals is enhanced by approximately three orders of magnitude, in agreement with experimental data. A variety of many other systems are expected to possess similar properties.

Models for Copper Dynamic Behavior in Doped Cadmium dl-Histidine Crystals: Electron Paramagnetic Resonance and Crystallographic Analysis.

Electron paramagnetic resonance and crystallographic studies of copper-doped cadmium dl-histidine, abbreviated as CdDLHis, were undertaken to gain further understanding on the relationship between site structure and dynamic behavior in biological model complexes. X-ray diffraction measurements determined the crystal structure of CdDLHis at 100 and 298 K. CdDLHis crystallizes in the monoclinic space group P21/c with two cadmium complexes per asymmetric unit. In each complex, the Cd is hexacoordinated to two histidine molecules. Both histidines are l in one complex and d in the other. Additionally, each complex contains multiple waters of varying disorder. Single crystal EPR spectroscopic splitting (g) and copper hyperfine (A(Cu)) tensors at room temperature (principal values: g = 2.249, 2.089, 2.050; A(Cu) = -453, -30.5, -0.08 MHz) were determined from rotational experiments. Alignments of the tensor directions with the host structure were used to position the copper unpaired dx(2)-y(2) orbital in an approximate plane made by four proposed ligand atoms: the N-imidazole and N-amino of one histidine, and the N-amino and O-carboxyl of the other. Each complex has two such planes related by noncrystallographic symmetry, which make an angle of 65° and have a 1.56 Å distance between their midpoints. These findings are consistent with three interpretations that can adequately explain previous temperature-dependent EPR powder spectra of this system: (1) a local structural distortion (static strain) at the copper site has a temperature dependence significant enough to affect the EPR pattern, (2) the copper can hop between the two sites in each complex at high temperature, and (3) there exists a dynamic Jahn-Teller effect involving the copper ligands.

The benzene radical anion: A computationally demanding prototype for aromatic anions

The benzene radical anion is studied with ab initio coupled-cluster theory in large basis sets. Unlike the usual assumption, we find that, at the level of theory investigated, the minimum energy geometry is non-planar with tetrahedral distortion at two opposite carbon atoms. The anion is well known for its instability to auto-ionization which poses computational challenges to determine its properties. Despite the importance of the benzene radical anion, the considerable attention it has received in the literature so far has failed to address the details of its structure and shape-resonance character at a high level of theory. Here, we examine the dynamic Jahn-Teller effect and its impact on the anion potential energy surface. We find that a minimum energy geometry of C2 symmetry is located below one D2h stationary point on a C2h pseudo-rotation surface. The applicability of standard wave function methods to an unbound anion is assessed with the stabilization method. The isotropic hyperfine splitting constants (Aiso) are computed and compared to data obtained from experimental electron spin resonance experiments. Satisfactory agreement with experiment is obtained with coupled-cluster theory and large basis sets such as cc-pCVQZ.

Charge ordering transition in (La, Pr)1/3Sr2/3FeO3−δ with different oxygen concentration investigated by mechanical spectroscopy

We present the results of the kilohertz mechanical spectrum of oxygen-varied La1/3Sr2/3FeO3−δ and Pr1/3Sr2/3FeO3−δ. On cooling, a substantial modulus softening above the charge ordering transition temperature TCO, and an abrupt stiffness and step-like decrease of the internal friction below TCO were observed in oxygen stoichiometric La1/3Sr2/3FeO3−δ. No step-like change of the mechanical spectrum was observed in charge-ordered Pr1/3Sr2/3FeO3−δ, which shows the absence of dynamic Jahn-Teller distortions in Pr1/3Sr2/3FeO3−δ. Another modulus softening below was observed in charge-ordered (La, Pr)1/3Sr2/3FeO3−δ, which was used to discriminate the charge ordering transition from the charge disproportionation transition in perovskite iron oxides.

Dynamic Jahn–Teller effect on superconducting excitations in Raman spectra of cuprates

We report a model study of the phonon response due to the interplay of the superconducting (SC) gap, both static and dynamic Jahn–Teller (DJT) distortions and predict the appearance of the Jahn–Teller (JT) distorted SC excitation peaks in the Raman spectra of high-Tc superconductors. The static JT and s-wave type SC interactions in the conduction band are introduced in this model. The phonons are coupled to the conduction electron density as well as to the JT split conduction bands giving rise to DJT interaction, which is the main objective of this communication. The phonons are considered in a harmonic approximation. The phonon Green׳s function and hence the phonon self-energy arising due to normal electron–phonon (EP) interaction and DJT interaction are calculated using Zubarev׳s technique. The phonon self-energy is calculated from the electron response function in the coexistence phase of the two order parameters. Two excitation peaks due to the SC gap and the JT distortion are predicted by phonon density. The evolution of these excitation peaks is investigated by varying different model parameters of the system.

Resonating valence-bond state in an orbitally degenerate quantum magnet with dynamical Jahn-Teller effect

Short-range resonating-valence bond states in an orbitally degenerate magnet on a honeycomb lattice is studied. A quantum-dimer model is derived from the Hamiltonian which represents the superexchange interaction and the dynamical Jahn-Teller (JT) effect. We introduce two local units termed "spin-orbital singlet dimer", where two spins in a nearest-neighbor bond form a singlet state associated with an orbital polarization along the bond, and "local JT singlet", where an orbital polarization is quenched due to the dynamical JT effect. A derived quantum-dimer model consists of the hopping of the spin-orbital singlet dimers and the JT singlets, and the chemical potential of the JT singlets. We analyze the model by the mean-field approximation, and find that a characteristic phase, termed "JT liquid phase", where both the spin-orbital singlet dimers and the JT singlets move quantum mechanically, is realized. Possible scenarios for the recently observed non magnetic-ordered state in Ba$_3$CuSb$_2$O$_9$ are discussed.

Dynamical Jahn-Teller instability in metallic fullerides

Dynamical Jahn-Teller effect has escaped so far direct observation in metallic systems. It is particularly believed to be quenched also in correlated conductors with orbitally degenerate sites such as cubic fullerides. Here the Gutzwiller approach is extended to treat electron correlation over metals with Jahn-Teller active sites and applied to the investigation of the ground state of K$_3$C$_{60}$. It is shown that dynamical Jahn-Teller instability fully develops in this material when the interelectron repulsion $U$ on C$_{60}$ sites exceeds some critical value. The latter is found to be lower than the current estimates of $U$, meaning that dynamical Jahn-Teller effect takes place in all cubic fullerides. This leads to strong splitting of LUMO orbitals on C$_{60}$ sites and calls for reconsideration of the role of orbital degeneracy in the Mott-Hubbard transition in fullerides.

Time-dependent Jahn-Teller problem: phonon-induced relaxation through conical intersection.

A theoretical study of time-dependent dynamical Jahn-Teller effect in an impurity center in a solid is presented. We are considering the relaxation of excited states in the E⊗e-problem through the conical intersection of the potential energy. A strict quantum-mechanical treatment of vibronic interactions with both the main Jahn-Teller active vibration and the nontotally symmetric phonons causing the energy loss is given. The applied method enables us to calculate the time-dependence of the distribution function of the basic configurational coordinate. We have performed a series of numerical calculations allowing us, among other relaxation features, to visualise the details of the relaxation through the conical intersection. In particular, we elucidate how the Slonczewski quantization of the states in the conical intersection affects the relaxation.

Pair correlation and dynamic Jahn-Teller effect: High-Tc in nanoclusters

Electronic states in metallic nanoclusters form energy shells and degree of their filling depends on the number of delocalized electrons. In the region close to half-filling the cluster's geometry oscillates between the prolate and oblate configurations (dynamic Jahn-Teller effect). For large clusters (N > 102; N is the number of delocalized electrons) this effect competes with pair correlation and, as a result, it is perfectly realistic to observe the transition to the superconducting state. For some clusters (e.g., for Zn76, Al70) the value of the critical temperature is rather high .

Vibrational coherences in charge-transfer dyes: a non-adiabatic picture.

Essential-state models efficiently describe linear and nonlinear spectral properties of different families of charge-transfer chromophores. Here, the essential-state machinery is applied to the calculation of the early-stage dynamics after ultrafast (coherent) excitation of polar and quadrupolar chromophores. The fully non-adiabatic treatment of coupled electronic and vibrational motion allows for a reliable description of the dynamics of these intriguing systems. In particular, the proposed approach is reliable even when the adiabatic and harmonic approximations do not apply, such as for quadrupolar dyes that show a multistable, broken-symmetry excited state. Our approach quite naturally leads to a clear picture for a dynamical Jahn-Teller effect in these systems. The recovery of symmetry due to dynamical effects is however disrupted in polar solvents where a static symmetry lowering is observed. More generally, thermal disorder in polar solvents is responsible for dephasing phenomena, damping the coherent oscillations with particularly important effects in the case of polar dyes.

Magnetic neutron spectroscopy of a spin-transitionMn3+molecular complex

We have investigated by inelastic neutron scattering (INS), neutron diffraction, andmagnetometry the magnetic properties of the mononuclear complex [Mn3+(pyrol)3(tren)] in both high-spin (5E, HS, S = 2) and low-spin (3T1, LS, S = 1) states. The system presents a spin transition (ST) around 47 K with a small hysteresis width (TST,↑ = 47.5 K and TST,↓ = 46 K) characteristic of an efficient collective transition process. In the HS state, the INS spectrum at 56 K and zero magnetic field is accounted for by a zero-field splitting with D = −5.73(3) cm−1 and |E| = +0.47(2) cm−1 which may be the result of a dynamic Jahn-Teller effect reported in the literature.In the LS state, a single magnetic peak at 4.87 meV is observed, still at zero field. Despite the existence of an unquenched orbital moment (L = 1) in the ground 3T1 state, we argue that it may be described by a genuine S = 1 spin Hamiltonian owing to the existence of a strong trigonal distortion of the Mn3+ coordination octahedron. The observed peak corresponds to a transition M = +1 within the S = 1 ground state split by a large single-ion anisotropy term D = +39.3 cm−1. A full spin-Hamiltonian model is proposed based on these first INS results obtained in a thermal ST molecular magnetic system.