The alkali-doped fullerides A$_3$C$_{60}$ are half-filled three-orbital Hubbard systems which exhibit an unconventional superconducting phase next to a Mott insulator. While the pairing is understood to arise from an effectively negative Hund coupling, the highly unusual Jahn-Teller metal near the Mott transition, featuring both localized and itinerant electrons, has not been understood. This property is consistently explained by a previously unrecognized phenomenon: the spontaneous transition of multiorbital systems with negative Hund coupling into an orbital-selective Mott state. This symmetry-broken state, which has no ordinary orbital moment, is characterized by an orbital-dependent two-body operator (the double occupancy) or an orbital-dependent kinetic energy, and may be regarded as a diagonal-order version of odd-frequency superconductivity. We propose that the recently discovered Jahn-Teller metal phase of Rb$_x$Cs$_{3-x}$C$_{60}$ is an experimental realization of this novel state of matter.
Polarized XAFS measurements on powder of LaSrCuO with aligned c-axes find that apical oxygens neighboring only the Sr dopants have a double-site distribution. This result requires that the doped holes reside in impurity states peaked on the CuO6 octahedron neighboring the dopant (denoted as Sr-octahedrons). A model of the double site is presented of two co-existing spin differentiated (singlet and triplet) Jahn-Teller (JT) distortions of the Sr-octahedrons induced by an extrinsic doped hole pairing with the intrinsic hole. It is speculated that Bose-condensation of the singlet pairs, bound by …
We argue that the Jahn-Teller theorem, if applied to 3d-ion compounds, has to be considered in the orbital+spin space. It is despite of the weakness of the intraatomic spin-orbit coupling.
We propose a mechanism for A-type antiferromagnetism in orthorombic LaMnO_3, compatible with the large Jahn-Teller splitting inferred from structural data. Orbital ordering resulting from Jahn-Teller distortions effectively leads to A-type ordering (antiferromagnetic in the c axis and ferromagnetic in the ab plane) provided the in-plane distorsion Q_2 is large enough, a condition generally fulfilled in existing data.
We investigate how Kondo phenomenon occurs in the Anderson model dynamically coupled with local Jahn-Teller phonons. It is found that the total angular moment composed of electron pseudo-spin and phonon angular moments is screened by conduction electrons. Namely, phonon degrees of freedom essentially contribute to the formation of singlet ground state. A characteristic temperature of the Kondo effect due to dynamical Jahn-Teller phonons is explained by an effective $s$-$d$ Hamiltonian with anisotropic exchange interaction obtained from the Jahn-Teller-Anderson model in a non-adiabatic region.
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 dis…
The formation of a long-range Jahn-Teller distortion in undoped manganites is studied within the dynamical mean-field theory taking both electron-phonon and electron-electron interactions into account. We find that the observed insulating behavior at temperatures above the Jahn-Teller transition and the bond-length changes below it can most naturally by explained by assuming a strong electronic repulsion and a rather weak electron-phonon coupling.
We observe two consecutive transitions in La1-xCaxMnO3, x=0.19: ferromagnetic ordering at Tc = 177 K and Jahn-Teller orbital ordering at T_JT = 150 K. The presence of a ferromagnetic insulating state below T_JT shows that the metallic phase is bound by ferromagnetic and Jahn- Teller ordering, and not induced by a critical doping concentration. The A-site shift is found to be a good indicator of Jahn-Teller ordering. Furthermore, the A-site shift does not disapppear above T_JT, indicating a dynamical behaviour of the distorted MnO6 octahedra.
Temperature dependences of a magnetic and an orbital Bragg reflection at the Mn L2 and L3 edges of La0.5Sr1.5MnO4 have been collected. The temperature evolution of the orbital reflection depend strongly on the x-ray energy and reflects the different order parameteres for orbitals and Jahn-Teller distortions. The gradual increase of the orbital order parameter indicates that the orbital order and not the Jahn-Teller distortion drives the metal-insulator transition. Correspondingly, the orbital-orbital (super) exchange interaction dominates over the Jahn-Teller strain-field interaction.
In contrary to customarily consideration of the Jahn-Teller theorem for 3d-ion compounds in the orbital space only we point out that it has to be considered in the spin-orbital space. It is despite the weakness of the intraatomic spin-orbit coupling. A direct motivation for this paper is an erroneous claim of a recent Phys. Rev. Lett. 96 (2006) 027201 paper that the high-spin t2g4eg2 state of the 3d6 configuration is not Jahn-Teller active.
A realistic modeling of manganites should include the Coulomb repulsion between $e_g$ electrons, the Hund's rule coupling to $t_{2g}$ spins, and Jahn-Teller phonons. Solving such a model by dynamical mean field theory, we report large magnetoresistances and spectra in good agreement with experiments. The physics of the unusual, insulating-like paramagnetic phase is determined by correlated electrons which are-due to strong correlations-easily trapped as Jahn-Teller polarons.
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 lowe…
We study the influence of the lattice structure, the Jahn-Teller effect and the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb). The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60 (bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases) the value Uc of the Coulomb integral at which the metal-insulator transition occurs. There is an important partial cancellation between the Jahn-Teller effect and the Hund's rule coupling.
In a two orbital double exchange model we formulate Jahn-Teller coupled charge density wave in one electron per lattice site limit. Softening of Jahn-Teller phonons corresponding to distortion modes $Q_2$ or $Q_3$ associated with perfect nesting of Fermi surface leads to this instability at low temperature. The gap equation for charge density wave state and its dependences on electron-lattice coupling are calculated explicitly when any one of the Jahn-Teller modes is excited cooperatively. We find that the $Q_2$ distortion mode yields lowest free energy. The calculated heat capacity for each m…
We investigate the electronic properties of multi-domain configurations in models for undoped manganites by means of variational and Monte Carlo techniques. These materials display simultaneous Jahn-Teller distortions and magnetic ordering. We find that a band of electronic states appears associated with Jahn-Teller domain walls, and this band is localized in the direction perpendicular to the walls. The energy and width of this band depends on the conformational properties of the domain walls. At finite temperatures, the conductance along the domain walls, induced by the localized domain wall…
It is shown that the layered antiferromagnetic order in stoechiometric LaMnO_3 cannot be understood purely from electronic interactions. On the contrary, it mainly results from strong cooperative Jahn-Teller deformations. Those involve a compression of the Mn-O octahedron along the c-axis (mode Q_3 < 0), while alternate Jahn-Teller deformations occur in the ab-plane (mode Q_2). These deformations stabilize a certain type of orbital ordering. The resulting superexchange couplings are calculated by exact diagonalization, taking into account both e_g and t_{2g} orbitals. The main result is that a…
Based on the magnetoresistance, magnetization, ultrasound, and crystallographic data, we studied the role of the cooperative Jahn-Teller effect in the charge ordered (CO) state for La1-xCaxMnO3. We found that, with increasing the fraction of Q3 mode of Jahn-Teller distortion and decreasing that of Q2 mode in the CO state, the magnetic structure evolves from CE-type to C-type and the orbital ordering changes from 3d(x2-r2)/3d(y2-r2)-type to 3d(x2-z2)-type, with the strength of ferromagnetism and the phase separation tendency being suppressed. At the same time, the stability of the CO state and …
The Raman scattering (RS) spectra of rhombohedral $LaMnO_3$ is considered. It is shown that broad high intensity RS line at 640 $cm^{-1}$ could be explained in the framework of cooperative dynamical Jahn-Teller effect. This hypothesis is illustrated through the lattice dynamics calculations explicitly allowing for the electron-phonon term both in crystal energy and in dynamical matrix. The averaging scheme for the one mode cooperative dynamical effect is proposed
The Jahn-Teller effect in the charge-ordered (CO) state for La1-xCaxMnO3 (0.5<x<0.87) was studied by measuring the low-temperature powder x-ray diffraction, internal friction, and shear modulus. We find that the electron-lattice interaction with the static Jahn-Teller distortion is the strongest near x=0.75 in the CO state. It was particularly observed that a crossover of the Jahn-Teller vibration mode from Q2 to Q3 near x=0.75 induces crossovers of the crystal structure from tetragonally compressed to tetragonally elongated orthorhombic, and of the magnetic structure from CE-type to C-type ne…
We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appear…
It is shown that the d8 electron system occurring in the Ni2+ ion can have the non-magnetic ground state in the atomic scale formed by the Jahn-Teller effect. The tetragonal/trigonal off-cubic lattice distortion for the octahedral site causes the splitting of the triplet 3A2g into a singlet-doublet structure with the non-magnetic singlet lying lower provided the intra-atomic spin-orbit coupling is taken into account.
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 mecha…
We study the unconventional insulating state in A_4C_{60} with a variety of approaches, including density functional calculations and dynamical mean-field theory. While the former predicts a metallic state, in disagreement with experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and important excitations (such as optical and spin gap) should be essentially intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively c…
We investigate the bipolaronic crossover and the pairing transition for a two-orbital model with Jahn-Teller coupling to a two-fold degenerate phonon mode. The evolution from weak to strong coupling is reminiscent of the behavior of the single-band Holstein model: The polaron crossover in which the electrons and phonons become strongly entangled occurs for a weaker coupling than the binding of bipolarons, which gives rise to a metal-insulator transition. Interestingly, a single bipolaronic transition takes place also when the two bands have significantly different bandwidths, as opposed to the…
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 $\ge$65 cm$^{-1}$ while the net kinetic contribution to the Jahn--Teller stabilization energy constitutes ca 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 si…
We analyze the electronic structure of a mixed-valence ladder system. We find that structural anisotropy and complex electron correlations lead to on-rung charge localization and insulating character. Charge fluctuations within the rung of the ladder interact strongly to the lattice degrees of freedom, which gives rise to large pseudo Jahn--Teller effects. The phase transition in NaV$_2$O$_5$ should be driven by this kind of mechanism.
We employ a combination of density functional theory and dynamical mean-field theory to investigate the electronic structure of the recently synthesized insulator BaCrO$_3$. Our calculations show that Hund's coupling is responsible for strong correlation effects, which are however not sufficient to turn the system insulating. A finite Jahn-Teller distortion lifting the orbital degeneracy is necessary to stabilize an insulating state with orbital ordering and consequent magnetic ordering.
The properties of the polaron and bipolaron are explored in the 1D Jahn-Teller model with dynamical quantum phonons. The ground-state properties of the polaron and bipolaron are computed using a recently developed variational method. Dynamical properties of the ground state of a polaron are investigated by calculating the optical conductivity $σ(ω)$. Our numerical results suggest that the Jahn-Teller and Holstein polarons are similar. However, in the strong-coupling regime qualitative differences in $σ(ω)$ between the two models are found and discussed. The influence of the electron-phonon cou…
We have synthesized a new spinel oxide LiRh2O4 with a mixed-valent configuration of Rh3+ and Rh4+. At room temperature it is a paramagnetic metal, but on cooling, a metal-insulator transition occurs and a valence bond solid state is formed below 170 K. We argue that the formation of valence bond solid is promoted by a band Jahn-Teller transition at 230 K and the resultant confinement of t2g holes within the xy band. The band Jahn-Teller instability is also responsible for the observed enhanced thermoelectric power in the orbital disordered phase above 230 K.
The effect of Jahn-Teller phonons on the magnetic and orbital structure of LaMnO$_3$ is investigated using a combination of relaxation and Monte Carlo techniques on three-dimensional clusters of MnO$_6$ octahedra. In the physically relevant region of parameter space for LaMnO$_3$, and after including small corrections due to tilting effects, the A-type antiferromagnetic and C-type orbital structures were stabilized, in agreement with experiments.
The two-orbital Kondo model with classical Jahn-Teller phonons is studied using Monte Carlo techniques. The observed phase diagram is rich, and includes a novel regime of phase separation induced by the orbital degrees of freedom. Experimental consequences of our results are discussed. In addition, the optical conductivity $σ(ω)$ of the model is presented. It is shown to have several similarities with experimental measurements for manganites.
A synchrotron x-ray diffraction study of the crystallographic structure of PrO2 in the Jahn-Teller distorted phase is reported. The distortion of the oxygen sublattice, which was previously ambiguous, is shown to be a chiral structure in which neighbouring oxygen chains have opposite chiralities. A temperature dependent study of the magnetic excitation spectrum, probed by neutron inelastic scattering, is also reported. Changes in the energies and relative intensities of the crystal field transitions provide an insight into the interplay between the static and dynamic Jahn-Teller effects.
The Jahn-Teller effect is one of the most fundamental phenomena important not only for physics, but also for chemistry and material science. Solving the Jahn-Teller problem and taking into account strong electron correlations we show that quantum entanglement of the spin and orbital degrees of freedom via spin--orbit coupling strongly affects this effect. Depending on the number of $d$ electrons it may quench (electronic configurations $t_{2g}^2$, $t_{2g}^4$, and $t_{2g}^5$), partially suppress ($t_{2g}^1$) or in contrast induce ($t_{2g}^3$) Jahn-Teller distortions. Moreover, in certain situat…
A recursive method is developed to treat electrons coupled to phonons. It is applied to small systems with $E\otimesβ$ Jahn-Teller coupling. Two cases are considered, a model with one electron and two orbitals on a single site (related to the Rabi Hamiltonian) and a model with two electrons on two sites. The corresponding Green's functions are represented by rational functions. It is found that the spectra change substantially when one phonon couples to the electron but are relatively robust under an increasing number of phonons.
We investigate the stability of the charge exchange (CE) phase within a microscopic model which describes a single plane as in La$_{0.5}$Sr$_{1.5}$MnO$_4$. The model includes Coulomb interactions (on-site and intersite), the Jahn-Teller term and the superexchange interactions due to $e_g$ and $t_{2g}$ electrons. By investigating the phase diagram at T=0 in mean-field approximation we conclude that the superexchange interactions can stabilize the CE phase, but only if they are stronger than estimated from spectroscopy.
The infinite-$U$ Anderson model coupled to a Jahn-Teller phonon is studied using the slave boson method on the basis of the large degeneracy expansion (1/N) scheme. The model Hamiltonian acts on the orbital degrees of freedom. The main focus is on the interplay between strong local electron correlation and weak Jahn-Teller electron-phonon interaction. The Kondo temperature is found to decrease by Jahn-Teller interaction. The influence of the Jahn-Teller interaction on dynamical correlation functions is very significant in sharp contrast with the case of the Holstein-type phonon which couples t…
Resonant soft x-ray diffraction has been used to probe the temperature dependent orbital and magnetic structure of $\mathrm{LaSr_{2}Mn_{2}O_7}$. Previous crystallographic studies have shown that this material has almost no MnO$_{6}$ oxygen displacement due to Jahn-Teller distortions at low temperatures. Within the low-temperature A-type antiferromagnetic phase, we found strong intensity at the $({1/4},{1/4},0)$ orbital and $LaSr_{2}Mn_{2}O_7$ magnetic reflections. This shows that even in the near absence of Jahn-Teller distortion, this compound is strongly orbitally ordered. A fit to the Mn $L…
Cs3C60 is a correlated superconductor under pressure, but an insulator under ambient conditions. The mechanism causing this insulating behavior is the combination of Mott localization and the dynamic Jahn-Teller effect. We show evidence from infrared spectroscopy for the dynamic Jahn-Teller distortion. The continuous change with temperature of the splitting of infrared lines is typical Jahn-Teller behavior, reflecting the change in population of solid-state conformers. We conclude that the electronic and magnetic solid-state properties of the insulating state are controlled by molecular phenom…
The charge order of CE phase in half-doped manganites is studied, based on an argument that the charge-ordering is caused by the Jahn-Teller distortions of MnO6 octahedra rather than Coulomb repulsion between electrons. The uantitative calculation on the ferromagnetic zigzag chain as the basic structure unit of CE phase within the framework of two-orbital double exchange model including Jahn-Teller effect is performed, and it is shown that the charge-disproportionation of Mn cations in the charge-ordered CE phase is less than 13%. In addition, we predict the negative charge-disproportionation …
The analysis of the properties of rare-earth based materials relies on the Crystalline Electric Field theory. This theory has to be reconsidered in case of cage-type compounds, where the rare-earth ion can substantially depart from its, high symmetry, average position. It is shown that, in case of an orbital degeneracy at the cage center, a specific Jahn-Teller effect develops in the paramagnetic range: at low temperature, the distribution of the magnetic ion spreads inside the cage, the magnetic entropy is reduced, whereas the cage multipolar susceptibilities are increased. These consequences…
The Jahn-Teller effect is a fascinating and ubiquitous phenomenon in modern quantum physics and chemistry. We propose a class of oxides with melilite structure Ba$_2$TGe$_2$O$_7$ (T=V,Ni) where Jahn-Teller distortions are the main responsibles for the appearance of electric polarization. At the hearth of the proposed mechanism lies the lack of inversion symmetry displayed by tetrahedrally coordinated transition-metal ions, which allows for the condensation of polar Jahn-Teller distortions, at odds with octahedral coordination typical of conventional ferroelectric oxides with perovskite structu…
Distortions of the oxygen sublattice couple to $e_g$ orbitals of Mn$^{3+}$ and drive a cooperative Jahn-Teller (orbital ordering) transition in LaMnO$_3$. A simple model for this transition is studied. Without further adjustment, the model predicts the shape and stability of small (anti-Jahn-Teller) polarons which form when holes are doped into the material. This leads to a new description of the lightly doped insulator, the anti-ferromagnetic to ferromagnetic transition, and the metal-insulator transition.
In this work we study the complex entanglement between spin interactions, electron correlation and Janh-Teller structural instabilities in the 5d$^1$ $J_{eff}=\frac{3}{2}$ spin-orbit coupled double perovskite $\rm Ba_2NaOsO_6$ using first principles approaches. By combining non-collinear magnetic calculations with multipolar pseudospin Hamiltonian analysis and many-body techniques we elucidate the origin of the observed quadrupolar canted antifferomagnetic. We show that the non-collinear magnetic order originates from Jahn-Teller distortions due to the cooperation of Heisenberg exchange, quadr…
We use an extended two-band Kondo lattice model (KLM) to investigate the occurrence of different (anti-)ferromagnetic phases or phase separation depending on several model parameters. With regard to CMR-materials like the manganites we have added a Jahn-Teller term, direct antiferromagnetic coupling and Coulomb interaction to the KLM. The electronic properties are self-consistently calculated in an interpolating self-energy approach with no restriction to classical spins and going beyond mean-field treatments. Further on we do not have to limit the Hund's coupling to low or infinite values. Ze…
A mechanism of non-magnetic Kondo effect is proposed on the basis of a multiorbital Anderson model coupled with dynamical Jahn-Teller (JT) phonons. An electron system coupled dynamically with JT phonons has a vibronic ground state with double degeneracy due to clockwise and anti-clockwise rotational modes with entropy of $\log 2$. When a temperature is lower than a characteristic energy to turn the rotational direction, the rotational degree of freedom is eventually suppressed and the corresponding entropy $\log 2$ is released, leading to quasi-Kondo behavior. We discuss possible relevance of …
The phase diagram of half-doped manganite systems of formula A_{0.5}A'_{0.5}MnO_3 is investigated within a single-orbital model incorporating magnetic double-exchange and superexchange, together with intersite Coulomb and electron-lattice interactions. Strong Jahn-Teller and breathing mode deformations compete together and result in shear lattice deformations. The latters stabilize the charge-ordered CE-type phase, which undergo first-order transitions with temperature or magnetic field to either Ferromagnetic metallic or Paramagnetic insulating phases. An essential feature is the self-consist…
The Jahn-Teller effect explains distortions and non-degenerate energy levels in molecular and solid-state physics via a coupling of effective spins to collective bosons. Here we propose and theoretically analyze the quantum simulation of a many-body Jahn-Teller model with linear ion crystals subjected to magnetic field gradients. We show that the system undergoes a quantum magnetic structural phase transition which leads to a reordering of particle positions and the formation of a spin-phonon quasi-condensate in mesoscopic ion chains.
Through 13C NMR spin lattice relaxation (T1) measurements in cubic Na2C60, we detect a gap in its electronic excitations, similar to that observed in tetragonal A4C60. This establishes that Jahn-Teller distortions (JTD) and strong electronic correlations must be considered to understand the behaviour of even electron systems, regardless of the structure. Furthermore, in metallic Na2CsC60, a similar contribution to T1 is also detected for 13C and 133Cs NMR, implying the occurence of excitations typical of JT distorted C60^{2-} (or equivalently C60^{4-}). This supports the idea that dynamic JTD …
The role of elastic interactions between Jahn-Teller ions in formation of various orbital- and charge-ordered structures in manganites and related compounds is analyzed. It is shown that such interactions alone are often sufficient to reproduce the structures observed in different regions of the phase diagram. A special attention is focused on stripe structures at high doping levels.
Ordering and quantum fluctuations of orbital degrees of freedom are studied theoretically for LaVO$_3$ in spin-C-type antiferromagnetic state. The effective Hamiltonian for the orbital pseudospin shows strong one-dimensional anisotropy due to the negative interference among various exchange processes. This significantly enhances the instability toward lattice distortions for the realistic estimate of the Jahn-Teller coupling by first-principle LDA+$U$ calculations, instead of favoring the orbital singlet formation. This explains well the experimental results on the anisotropic optical spectra …
The presence/absence of a Berry phase depends on the topology of the manifold of dynamical Jahn-Teller potential minima. We describe in detail the relation between these topological properties and the way the lowest two adiabatic potential surfaces get locally degenerate. We illustrate our arguments through spherical generalizations of the linear T x h and H x h cases, relevant for the physics of fullerene ions. Our analysis allows us to classify all the spherical Jahn-Teller systems with respect to the Berry phase. Its absence can, but does not necessarily, lead to a nondegenerate ground stat…
We present a thorough analysis of the interplay of magnetic moment and the Jahn-Teller effect in the $Γ_8$ cubic multiplet. We find that in the presence of dynamical Jahn-Teller effect, the Zeeman interaction remains isotropic, whereas the $g$ and $G$ factors can change their signs. The static Jahn-Teller distortion also can change the sign of these $g$ factors as well as the nature of the magnetic anisotropy. Combining the theory with state-of-the-art {\it ab initio} calculations, we analyzed the magnetic properties of Np$^{4+}$ and Ir$^{4+}$ impurity ions in cubic environment. The calculated…
We report the first direct observation of the influence of high magnetic fields on the Jahn-Teller (JT) transition in TbVO4. Contrary to spectroscopic and magnetic methods, X-ray diffraction directly measures the JT distortion; the splitting between the (311)/(131) and (202)/(022) pairs of Bragg reflections is proportional to the order parameter. Our experimental results are compared to mean field calculations, taking into account all possible orientations of the grains relative to the applied field, and qualitative agreement is obtained.
To understand the orbital ordering of $LaMnO_3$ in the ground state, we study it in the A-type spin antiferromagnetic state. We calculate the two-dimensional response functions associated with the Jahn-Teller $Q_2$ and $Q_3$ distortions and find that they diverge at the wavevector $π$,$π$). Furthermore, the $Q_2$ response function diverges faster which indicates that perhaps the ordering of the orbitals at low temperatures is dominated by the $Q_2$ distortion. We calculate the ground state energy for the cases when only one of the $Q_2$ and $Q_3$ modes is excited cooperatively and find that th…
In order to clarify possible multipole states of Sm-based filled skutterudite compounds, we investigate multipole susceptibility of a multiorbital Anderson model dynamically coupled with Jahn-Teller phonons by using a numerical renormalization group method. Here we take a procedure to maximize the multipole susceptibility matrix to determine the multipole state. When the electron-phonon coupling term is simply ignored, it is found that the dominant multipole state is characterized by 2u octupole for the $Γ_{67}^-$ quartet ground state, while the low-temperature phase is governed by magnetic fl…
Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in double-exchange models are examined for Mn perovskite oxides. We study the undoped Mott insulator as well as metal-insulator transitions by hole doping, and especially strong incoherence of ferromagnetic metal. We derive models where all the spins are fully polarized in two-dimensional planes as in the experimental indications, and investigate their ground-state properties by quantum Monte Carlo method. At half filling where the number of $e_{g}$ electron is one per site on average, the Coulomb interaction opens a Mot…
We investigate the origin of the orbital ordering in the paramagnetic phase of KCrF$_3$. All previous studies described structural parameters of the paramagnetic phase using a magnetic ordering in the compound. Our simulations of real paramagnetic KCrF$_3$ were performed within an approach combining density functional theory and dynamical mean field theory (DFT+DMFT). As a result, it was found that the experimentally observed cooperative Jahn-Teller effect is successfully described in a lattice relaxation calculation for structure without any long-range magnetic ordering. It is established tha…
We demonstrate with LSDA+U calculations that x-ray scattering at the K edge of Mn is sensitive to orbital ordering in one energy range and Jahn-Teller distortion in another. Contrary to what is suggested by atomic or cluster models used to date we show that band structure effects rather than local Coulomb interactions dominate the polarization dependence of the K edge scattering and therefore it is sensitive to nearest neighbour bond length distortions and next nearest neighbour orbital occupation. Based on this we propose a new mechanism for K edge x-ray scattering in the manganates which we …
