Dynamic Jahn-Teller Effect Research Articles

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.

Percolation features of cooperative Jahn-Teller systems: Ising EFT framework

Elastic exchange between two nearest Jahn-Teller (JT) centers in two or three dimensional dense crystals, can give an ordered macroscopic distortion known as cooperative JT effect (CJTE). A very diluted JT crystal does not show this effect. In the dynamic JT effect (DJTE), tunneling between different equivalent distorted wells has a pronounced influence on the CJTE. We investigate this phenomenon using a progressive increase in the concentration of these centers in the JT crystals, based on a bond percolation vector spin analogy technique within the framework of effective field theory (EFT). Mean field theory (MFT) was extensively used in previous studies of CJTE; however it neither includes correlation between JT centers in the lattice due to the complexity of the distortion field in the crystal nor the effect of tunneling between wells. We resort to an alternative procedure, by describing a JT center as a pseudo-spin vector $$\vec S$$ , induced to represent the degenerate JT-distorted states, where two nearest JT centers interact via an elastic exchange described by an Ising type spin interaction. The DJTE is considered to be similar to an elastic transverse field term in the Hamiltonian portraying the effect of tunneling between equivalent wells in the adiabatic potential energy surface (APES). We will be particularly discussing S = 1, S = 3/2 and S = 5/2 spin cases, where 2S + 1 wells in the APES are present and what JT systems they actually represent, with a percolative mechanism applied to the interactions between different JT centers. The different lattices are distinguished by their coordination numbers. Strong tunneling effects can suppress the CJTE and lead to a new state of criticality. Generalizations to higher spin systems will be obtained using a scaling technique. For the relevant distortions, we determine single site correlations, the macroscopic average distortion describing a structural phase transition and the elastic isothermal susceptibility as a function of temperature. The critical bond percolation threshold and the critical tunneling parameter are also obtained.

Electronic structure of the negatively charged silicon-vacancy center in diamond

The negatively-charged silicon-vacancy (SiV$^-$) center in diamond is a promising single photon source for quantum communications and information processing. However, the center's implementation in such quantum technologies is hindered by contention surrounding its fundamental properties. Here we present optical polarization measurements of single centers in bulk diamond that resolve this state of contention and establish that the center has a $\langle111\rangle$ aligned split-vacancy structure with $D_{3d}$ symmetry. Furthermore, we identify an additional electronic level and evidence for the presence of dynamic Jahn-Teller effects in the center's 738 nm optical resonance.

Interaction of Helium Rydberg State Atoms with Superfluid Helium

The pair potentials between ground state helium and Rydberg He ∗ (2s, 2 p, 3s) atoms are calculated by the full configuration interaction electronic structure method for both the electronic singlet and the triplet manifolds. The obtained pair potentials are validated against existing experimental molecular and atomic data. Most states show remarkable energy barriers at long distances (R > 5 A), which can effectively stabilize He ∗ against the formation of He ∗ at low nuclear kinetic energies. Bosonic density functional theory calculations, based on the calculated pair potential data, indicate that the triplet ground state He ∗ reside in spherical bubbles in superfluid helium with a barycenter radius of 6.1 A at the liquid saturated vapor pressure. The pressure dependency of the relative He ∗ 2s 3 S → 2 p 3 P absorption line blue shift in the liquid was obtained through both the statistical line broadening theory as well as the dynamic adiabatic following method. The pronounced difference between the results from the static and dynamic models is attributed to the dynamic Jahn-Teller effect that takes places in the electronically excited state within the dephasing time of 150 fs. Transient non-thermalized liquid surroundings near He ∗ may contribute to an artificial reduction in the absorption line blue shift by up to 30 cm −1 .

Theoretical investigations of the spin Hamiltonian parameters and local angular variations for the trigonal V3+ centers in alum compounds

The spin Hamiltonian parameters (zero-field splittings D, g factors g // and g ⊥) for V3+ in the alum compounds are theoretically studied using the perturbation formulas for these parameters for a 3d2 ion in the trigonally distorted octahedra. The contributions from the dynamical Jahn–Teller effect, the configuration interactions and the ligand orbital and spin–orbit coupling interactions are considered from the cluster approach in a uniform way. The angles in the deformed octahedra around V3+ ions are increased by 1.1°–2.7°, as compared with those ones for the host. The theoretical spin Hamiltonian parameters based on the above angular variations and the related effects show good agreement with the experimental data, which reveal that these effects (especially the Jahn–Teller effect) can bring forward significant influences on the spin Hamiltonian parameters, and should be taken into account in electron paramagnetic resonance (EPR) analysis. The contributions from the configuration interactions and the ligand orbital and spin–orbit coupling interactions are quantitatively involved from the cluster approach in a uniform way. The results are discussed.

Magnetic structure in the spin liquid Tb2Ti2O7induced by a [111] magnetic field: Search for a magnetization plateau

We have studied the field-induced magnetic structures of Tb${}_{2}$Ti${}_{2}$O${}_{7}$ pyrochlore by single-crystal neutron diffraction under a field applied along the [111] axis, up to $H=12$ T and down to $T=40$ mK. We refined the magnetic structures with $\mathbit{k}=\mathbf{0}$ propagation vector by performing a symmetry analysis in the space group $R\overline{3}m$, reducing the number of free parameters to three only. The Tb moments gradually reorient towards the field direction, keeping close to a ``3-in, 1-out / 1-in, 3-out'' spin structure (magnetic space group $R\overline{3}{m}^{\ensuremath{'}}$) in the whole measured field range 0.05--12 T. Our results rule out the ``all-in/all-out'' structure previously proposed and do not support the existence of a magnetization plateau. We perform a quantitative comparison with mean-field calculations and we propose the presence of a low-temperature dynamic symmetry breaking of the local trigonal symmetry, akin to a dynamic Jahn-Teller effect, i.e., preserving the overall cubic symmetry. We discuss the possible origin of this off-diagonal mixing term in the crystal field Hamiltonian in terms of quadrupole-quadrupole interaction or magnetoelastic effects.

Dynamical Jahn-Teller effect in a spin-orbital coupled system

Dynamical Jahn-Teller (DJT) effect in a spin-orbital coupled system on a honeycomb lattice is examined, motivated from recently observed spin-liquid behavior in Ba${}_{3}$CuSb${}_{2}$O${}_{9}$. An effective vibronic Hamiltonian, where the superexchange interaction and the DJT effect are taken into account, is derived. We find that the DJT effect induces a spin-orbital resonant state where local spin-singlet states and parallel orbital configurations are entangled with each other. This spin-orbital resonant state is realized in between an orbital-ordered state, where spin-singlet pairs are localized, and an antiferromagnetic ordered state. Based on the theoretical results, a possible scenario for Ba${}_{3}$CuSb${}_{2}$O${}_{9}$ is proposed.

Dynamical Jahn-Teller Effect and Antiferromagnetism inCs3C60

The dynamical Jahn-Teller effect on fullerene sites in insulating Cs(3)C(60) is investigated fully ab initio. The vibronic excitations of rotational type are at ≥ 65 cm(-1), while the net kinetic contribution to the Jahn-Teller stabilization energy constitutes approximately 90 meV. This means that no localization of distortions by intermolecular interactions is possible in these fullerides; therefore, free rotations of deformations take place independently on each C(60). The latter destroy the orbital ordering and establish a conventional exchange interaction between S = 1/2 on fullerene sites. The corresponding exchange model is derived and predicts the Néel temperature for A15 Cs(3)C(60) close to experiment.

Syntheses, crystal structures and magnetic studies of new copper(II) complexes of (E)-N′-(phenyl(pyridin-2-yl)methylene)isonicotinohydrazide containing azide and SCN anions

Three new complexes of copper(II) with pseudohalide anions and (E)-N′-(phenyl(pyridin-2-yl)methylene)isonicotinohydrazide (HL), namely azide bridged dinuclear [Cu(L)(μ-N3)]2 (1), two-dimensional coordination polymer [Cu2(L)2(NCS)2]n (2a) and monomeric [Cu(L-H)(NCS)2] (2b), were synthesized. The complexes were characterized by elemental analyses and spectroscopic methods. The structures were unequivocally confirmed by X-ray diffraction, and further studied by means of EPR and magnetic susceptibility measurements. In all three complexes, the isonicotinohydrazide ligand coordinates to the copper center in the enolic form (–CN–N). In complex 1, the two bridging azide ligands connect the two metal centers in an end-on (EO) fashion, with the metal centers having an asymmetric square pyramidal geometry. Complex 2b is based on a square pyramid with the neutral isonicotinohydrazide ligand coordinating in a tridentate mode. By contrast, the Schiff base (L)− coordinates in a tetradentate ligand fashion in 2a as the oxygen atom of the amide unit in the enol form asymmetrically bridges two Cu centers and a third Cu center is coordinated through the pyridyl-N atom. The EPR spectrum of complex 2a, consisting of two poorly resolved resonance lines, points to the presence of a dynamical Jahn–Teller effect, typical for Cu2+ ions in octahedral environments.

Electron correlation and dynamical Jahn-Teller effect in an orbitally degenerate system

Motivated by recent experiments on Ba3CuSb2O9, we study the dynamical Jahn-Teller effect in a spin-orbital coupled system. We analyze a low-energy model Hamiltonian which includes the superexchange-type interaction and the Jahn-Teller interaction by using a combined method of the mean-field approximation and the quantum Monte-Carlo simulation. We find that magnetic orders do not appear in a wide parameter region and that the spin-dimer state with an orbital order is realized. The orbital order is strongly suppressed by the dynamical Jahn-Teller effect.

Theoretical investigations of the local structure and the spin Hamiltonian parameters for Ti2+ in MgCl2

The local structure and spin Hamiltonian parameters (zero-field splitting and g factors) for Ti2+ in MgCl2 are theoretically studied using the perturbation formulas of these parameters for a 3d2 ion under trigonally distorted octahedra. The contributions from the dynamical Jahn–Teller effect, the configuration interactions and the ligand orbital and spin–orbit coupling interactions are quantitatively involved based on the cluster approach in a uniform way. The chlorine polyhedron around the impurity Ti2+ is found to transform from a slightly compressed octahedron in pure MgCl2 to a slightly elongated one, characterized by the local angular decrease of 0.3° related to the host bond angle 54.78° due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above trigonal elongation show good agreement with the experimental data. The results are discussed.

Mott localization in the correlated superconductor Cs3C60 resulting from the molecular Jahn-Teller effect

Cs3C60 is a correlated superconductor under pressure, but an insulator under ambient conditions. The mechanism causing this insulating behavior is the combination of Mott localization and the dynamic Jahn-Teller effect. We show evidence from infrared spectroscopy for the dynamic Jahn-Teller distortion. The continuous change with temperature of the splitting of infrared lines is typical Jahn-Teller behavior, reflecting the change in population of solid-state conformers. We conclude that the electronic and magnetic solid-state properties of the insulating state are controlled by molecular phenomena. We estimate the time scale of the dynamic Jahn-Teller effect to be above 10−11 s and the energy difference between the conformers less than 20 cm−1.

Theoretical modelling of Jahn-Teller distorted C60 anions on a surface

Through the use of scanning tunnelling microscopy (STM), it is possible to directly observe the molecular orbitals associated with a particular molecule. For the charged ions of the C60 fullerene, the interpretation of these experimental images has an additional complication due to the inherent presence of the Jahn-Teller (JT) effect. In this work, the influence of the JT effect on STM images is examined. We also include interactions between the molecule and both the surface substrate on which it rests, and, when present within a monolayer, the nearest neighbours. Simple symmetry arguments are used to explain the effects of these external interactions on the energy of the different JT wells relating to molecular distortions of D3d, D5d, and D2h symmetry. We first investigate the C−60 monoanion, and then, through the construction of multi-electron states, move on to consider anions with higher charges. It is found that for high symmetry orientations of the molecule on the surface, the wells that are degenerate in the absence of external interactions split into equal energy subgroups, with the grouping dependent on the orientation of the molecule. Hückel molecular orbital theory is then used to investigate the effect this has on STM images. We show that when dynamic JT effects are considered, the images are always formed from some linear combination of the squares of the individual single electron molecular orbitals that make up the lowest unoccupied molecular orbital of the neutral molecule.

Effect of uniaxial stress on substitutional Ni in ZnO

The influence of uniaxial stress on the electronic T13(F)→T23(F) transitions of Ni2+ (d8) in ZnO at 4216, 4240, and 4247cm−1 is studied. It is shown that the split pattern and polarized properties of IR absorption lines are consistent with a dynamic Jahn–Teller effect in the T23(F) state of the defect.

On-line phase transitions of bulk and nanocrystalline La1−xPbxMnO3 (x = 0.3, 0.4, and 0.5) perovskite manganite materials using ultrasonic measurements

Bulk and nanostructured La1−xPbxMnO3 perovskite manganite samples for different Pb contents (x=0.30, 0.40 and 0.50) were synthesised using solid-state reaction and sonochemical methods, respectively. XRD, FTIR, SEM, EDAX, BET and TEM studies were carried out to characterise the prepared bulk and nanosamples. The XRD characterization studies confirmed that the prepared bulk and nano perovskite manganite samples were crystalline in nature. The SEM and TEM studies revealed that the particle sizes of bulk perovskite samples is in the range of 0.75–85μm, whereas it ranges from 50 to 90nm in the case of nano perovskite samples. Further, it has been confirmed that there is an increase in particle size with an increase in Pb content both in bulk and nano perovskite samples. In addition, ultrasonic velocities and attenuations of bulk and nanosamples were measured using the on-line ultrasonic through transmission method. The temperature-dependent ultrasonic parameters showed an interesting anomaly in both bulk and nano perovskite samples. The observed marked softening and hardening in sound velocities and attenuation were related to ferromagnetic to paramagnetic phase transition temperature (TC). In nano perovskites samples, the existence of diffused phase transition took place at a lower temperature compared with the corresponding bulk samples. The observed broad ferromagnetic (FM)–paramagnetic (PM) transition in nano perovskites confirms the absence of sharp transition, which is attributed to the coexistence of orbitally, ordered and disordered phases. The spin–phonon and electron–phonon couplings are known to exist because of single ion magnetostriction and dynamic Jahn–Teller effect, respectively, are evidenced by the observed anomalies in both velocities and attenuations.

Quadratic coupling treatment of the Jahn-Teller effect in the triply-degenerate electronic state of $\bf CH_4^+$CH4+: Can one account for floppiness?

The Jahn-Teller (JT) coupling effects in the triply degenerate ground electronic state of methane radical cation are investigated theoretically within a quadratic vibronic coupling approach. The underlying potential energy surfaces over the two-dimensional space of nuclear coordinates, subject to the T(2) ⊗ (e + t(2) + t(2)) Jahn-Teller effect, are established from extensive ab initio calculations using the multi-reference configuration interaction method and then employed to determine the various parameters of a diabatic Hamiltonian of this system. Our previous investigation [T. Mondal and A. J. C. Varandas, J. Chem. Phys. 135, 174304 (2011)], relying on the linear vibronic coupling approach augmented by only a diagonal second-order term of the totally symmetric mode, are extended here by including all possible quadratic coupling constants of JT active e and t(2) modes. Inclusion of these quadratic couplings is found to be important to reproduce correctly the broad vibrational structure and for a better description of dynamical JT effect in the first vibronic band of this radical cation. The impact of large amplitude motions (which are responsible for floppiness of the molecule) on the vibronic structure and dynamics of the first photoelectron band have been examined via readjustment of their linear coupling parameters up to ±10%.

Jahn–Teller Dynamics in a Series of High-Symmetry Co(II) Chelates Determine Paramagnetic Relaxation Enhancements

NMR paramagnetic relaxation enhancements (PREs) of a series of structurally characterized, trigonal bis-trispyrazolylborate (Tp) chelates of high-spin Co(II), spanning 100-850 MHz in field, are reported. Prior knowledge of the metal-nucleus distances allows numerical extraction of position-dependent electron spin relaxation rates (τ(c)(-1)) from direct measurement of the individual PREs of the four symmetry distinct protons in Co(Tp)(2), using available closed-form expressions. The data for this electronically complex system where spin-orbit coupling defines the ground state electronic structure are analyzed in terms of the Solomon-Bloembergen-Morgan (SBM) relations, as well as available zero-field splitting limit theories. A simple angular correction is shown to be sufficient to reconcile the individual τ(c)(T) data for the four classes of protons. The data identify a previously unrecognized dynamic Jahn-Teller effect in these historically important complexes, with a barrier of ~230 cm(-1), pointing to a level of dynamics in trispyrazolylborate chemistry that has not been described before, and further show that it is the Jahn-Teller that is responsible for the PREs in fluid solution. A field-dependent component is also identified for the two protons nearest g(//), which is suggested to arise due to Zeeman mixing of excited state character into the ground level.

Observation of elastic anomalies driven by coexisting dynamical spin Jahn-Teller effect and dynamical molecular-spin state in the paramagnetic phase of frustrated MgCr2O4

Ultrasound velocity measurements of magnesium chromite spinel MgCr$_2$O$_4$ reveal elastic anomalies in the paramagnetic phase that are characterized as due to geometrical frustration. The temperature dependence of the tetragonal shear modulus $(C_{11}-C_{12})/2$ exhibits huge Curie-type softening, which should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic phase. The trigonal shear modulus $C_{44}$ exhibits nonmonotonic temperature dependence with a characteristic minimum at $\sim$50 K, indicating a coupling of the lattice to dynamical molecular spin state. These results strongly suggest the coexistence of dynamical spin Jahn-Teller effect and dynamical molecular spin state in the paramagnetic phase, which is compatible with the coexistence of magnetostructural order and dynamical molecular spin state in the antiferromagnetic phase.

Ultrasonic, Transport, and Magnetic Properties of Nd0.5Sr0.5Mn1−x Cr x O3 Perovskites

The resistivity, magnetization and longitudinal ultrasonic velocity (V l) have been measured in single-phase polycrystalline Nd0.5Sr0.5Mn1−x Cr x O3 (x=0, 0.02, 0.05, 0.1) to clarify the Cr doping effect on the metal-insulator and charge ordering transitions. For x=0 sample, the V l shows two large stiffening just below the metal-insulator transition temperature T MI and the charge ordering temperature T CO, respectively. It is suggested that the former ultrasonic anomaly may originate from the spin-lattice effect and the dynamic Jahn-Teller effect, and the latter is due to the electron-lattice effect arising from the static Jahn-Teller effect of Mn3+. With the Cr doping, the T MI shifts to lower temperature, the maximum of magnetization becomes smaller and the resistivity remains metallic type. Furthermore, only one stiffening of ultrasonic velocity is observed below T MI in these doping samples. The analysis suggests that the Cr doping enhances the double exchange between Mn3+ and Mn4+, and thoroughly destroys the charge ordering state.

Theoretical investigations of the spin Hamiltonian parameters and local structures for the tetragonal V3+ and Ti2+ centers in MgO and CaO

Abstract The spin Hamiltonian parameters (zero-field splitting D , g factor and hyperfine structure constant) and the local structures for MgO:V 3+ , CaO:V 3+ and CaO:Ti 2+ are theoretically investigated from the perturbation formulas of these parameters for 3d 2 ions under tetragonally distorted octahedra. The contributions from the dynamical Jahn–Teller effect, the configuration interactions and the ligand orbital and spin–orbit coupling interactions are quantitatively taken into account based on the cluster approach in a uniform way. The ligand octahedra are found to suffer the relative elongations of about 1.5%, 4.4% and 4.6% for MgO:V 3+ , CaO:V 3+ and CaO:Ti 2+ , respectively, along the C 4 axis due to the dynamical Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above local tetragonal elongations show reasonable agreement with the experimental data for all the systems. The results are discussed.