Structural and magnetic characterization of the elusive Jahn-Teller active NaCrF3

Jahn-Teller Effect in Diamond-like Carbon Film

The Jahn-Teller effect used semi-theoretically to analyse UV-visible spectra of the diamond like-carbon films on Si substrates. By deconvolution of UV-visible absorption spectra of the typical films, different absorption lines found. For each sample, six lines which were related to $\V^\circ$ by Jahn-Teller effect are distinguished. Our theoretical approach based on distortion of the $\V^\circ$ as the consequence of the stress in the films, is in agreement with experimental results. The shift of the Jahn-Teller lines toward the zero phonon line of $\V^\circ$ were found in accordance to stress …

Interplay between Jahn-Teller instability, uniaxial magnetism and ferroelectricity in Ca3CoMnO6

Ca3CoMnO6 is composed of CoMnO6 chains made up of face-sharing CoO6 trigonal prisms and MnO6 octahedra. The structural, magnetic, and ferroelectric properties of this compound were investigated on the basis of density functional theory calculations. Ca3CoMnO6 is found to undergo a Jahn-Teller distortion associated with the CoO6 trigonal prisms containing high-spin Co2+ (d7) ions, which removes the C3 rotational symmetry and hence uniaxial magnetism. However, the Jahn-Teller distortion is not strong enough to fully quench the orbital moment of the high-spin Co2+ ions thereby leading to an elect…

Measurement of the local Jahn-Teller distortion in LaMnO_3.006

The atomic pair distribution function (PDF) of stoichiometric LaMnO_3 has been measured. This has been fit with a structural model to extract the local Jahn-Teller distortion for an ideal Mn(3+)O_6 octahedron. These results are compared to Rietveld refinements of the same data which give the average structure. Since the local structure is being measured in the PDF there is no assumption of long-range orbital order and the real, local, Jahn-Teller distortion is measured directly. We find good agreement both with published crystallographic results and our own Rietveld refinements suggesting that…

Jahn-Teller Distortion and Ferromagnetism in the Dilute Magnetic Semiconductors GaN:Mn

Using first-principles total-energy methods, we investigate Jahn-Teller distortions in III-V dilute magnetic semiconductors, GaAs:Mn and GaN:Mn in the cubic zinc blende structure. The results for an isolated Mn impurity on a Ga site show that there is no appreciable effect in GaAs, whereas, in GaN there is a Jahn-Teller effect in which the symmetry around the impurity changes from T$_{d}$ to D$_{2d}$ or to C$_{2v}$. The large effect in GaN occurs because of the localized d$^4$ character, which is further enhanced by the distortion. The lower symmetry should be detectable experimentally in cubi…

Structural ground states of ($A,A'$)Cr$_2$O$_4$($A$=Mg, Zn; $A^{\prime}$ = Co, Cu) spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects

We examine the effect of small amounts of magnetic substituents in the $A$ sites of the frustrated spinels MgCr$_2$O$_4$ and ZnCr$_2$O$_4$. Specifically we look for the effects of spin and lattice disorder on structural changes accompanying magnetic ordering in these compounds. Substitution of Co$^{2+}$ on the non-magnetic Zn$^{2+}$ site in Zn$_{1-x}$Co$_{x}$Cr$_2$O$_4$ where 0\,$<$\,$x$\,$\leq$\,0.2 completely suppresses the spin-Jahn-Teller distortion of ZnCr$_2$O$_4$ although these systems remain frustrated, and magnetic ordering occurs at very low temperatures of $T$\,$<$\,20\,K. On the ot…

Single-Molecule Magnets: Ligand-Induced Core Distortion and Multiple Jahn-Teller Isomerism in [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4]

The Mn12Ac single-molecule magnet has been converted for the first time into a form that contains non-carboxylate ligands. Reaction of Mn12Ac with diphenylphosphinic acid converts it into the title compound in which eight of the acetate (Ac) groups have been replaced with Ph2PO2 (P) groups. The Mn12AcP product retains the S=10 ground state and single-molecule magnetism properties of Mn12Ac, and displays hysteresis loops containing steps due to quantum tunneling of the magnetization. The crystal structures show that Mn12AcP can exist in three Jahn-Teller isomeric forms differing in the relative…

Jahn-Teller Spectral Fingerprint in Molecular Photoemission: C60

The h_u hole spectral intensity for C60 -> C60+ molecular photoemission is calculated at finite temperature by a parameter-free Lanczos diagonalization of the electron-vibration Hamiltonian, including the full 8 H_g, 6 G_g, and 2 A_g mode couplings. The computed spectrum at 800 K is in striking agreement with gas-phase data. The energy separation of the first main shoulder from the main photoemission peak, 230 meV in C60, is shown to measure directly and rather generally the strength of the final-state Jahn-Teller coupling.

Berry Phase and Ground State Symmetry in H x h Dynamical Jahn-Teller Systems

Due to the ubiquitous presence of a Berry phase, in most cases of dynamical Jahn-Teller systems the symmetry of the vibronic ground state is the same as that of the original degenerate electronic state. As a single exception, the linear H x h icosahedral model, relevant to the physics of C60 cations, is determined by an additional free parameter, which can be continuously tuned to eliminate the Berry phase from the low-energy closed paths: accordingly, the ground state changes to a totally-symmetric nondegenerate state.

Electronic properties of Jahn-Teller and photoluminescence systems under pressure

Photoluminescence (PL) properties of materials containing transition-metal ions depend on a variety of structural factors such as electronic structure, site symmetry and neighbouring atoms. These factors play a crucial role for the occurrence PL i.e. the PL quantum yield. This work investigates different mechanisms leading to non-radiative deexcitation processes and whether they can be modified by applying high pressure.

Strain induced Jahn-Teller distortions in BaFeO3 : A first-principles study

The effect of epitaxial strain on structural, magnetic and electronic properties of BaFeO 3 per- ovskite oxide are investigated from first principles calculations, using the Density Functional The- ory (DFT) plus the Hubbard approach (DFT+U) within the Generalized Gradient Approximation (GGA). Hybrid functional calculations, based on mixed exact Hartree-Fock (HF) and DFT exchange energy functionals, are also performed. For the ground state calculations,the DFT+U is found more suitable to describe the half metallic and ferromagnetic state of cubic BaFeO 3 . The possible oc- curence of oxygen oc…

Stability of Jahn Teller distortion in LaMnO3 under pressure: An x-ray absorption study

The local environment of manganese atoms in LaMnO3 under pressure up to 15.3 GPa has been studied by x-ray absorption spectroscopy. For pressures below 8 GPa, no change is detected within the MnO6 octahedra. Above this pressure a continuous reduction of the long Mn-O distance takes place, however the octahedral distortion persists over the whole pressure range. At 15.3 GPa the average Jahn-Teller splitting of the distances is reduced by about one third, indicating that a total removal of the local Jahn-Teller distortion would occur only for pressures around 30 GPa, where metallization is repor…

Electronic and magnetic properties of the Jahn-Teller active fluoride $\mathrm{NaCrF_{3}}$ from first-principles calculations

In perovskite-type compounds, the interplay of cooperative Jahn-Teller effect, electronic correlations and orbital degree of freedom leads to intriguing properties. $\mathrm{NaCrF_{3}}$ is a newly synthesized Jahn-Teller active fluoroperovskite where the $\mathrm{CrF_{6}^{4-}}$ octahedrons are considerably distorted. Based on the first-principles calculation, we analyze its electronic structure and magnetic properties. Our numerical results show that the $\mathrm{Cr^{2+}}$ ions adopt the high-spin $t_{2g\uparrow}^{3}e_{g\uparrow}^{1}$ configuration with $G$-type orbital ordering. We also estim…

Jahn-Teller like origin of the tetragonal distortion in disordered Fe-Pd magnetic shape memory alloys

The electronic structure and magnetic properties of disordered Fe$_{x}$Pd$_{100-x}$ alloys $(50 < x < 85)$ are investigated in the framework of density functional theory using the full potential local orbital method (FPLO). Disorder is treated in the coherent potential approximation (CPA). Our calculations explain the experimental magnetization data. The origin of the tetragonal distortion in the Fe-Pd magnetic shape memory alloys is found to be a Jahn-Teller like effect which allows the system to reduce its band energy in a narrow composition range. Prospects for an optimization of the alloys…

Jahn-Teller畸变驱动的相干高熵氧化物薄膜晶格参数可调性

Jahn-Teller Distortion-Driven Lattice Parameter Tunability in Coherent High-Entropy Oxide Thin Films

本研究通过制备不同Cu含量的MgCoNiCuZnO高熵氧化物薄膜，发现Cu驱动的Jahn-Teller畸变与Co价态变化共同控制晶格参数，且Cu畸变是与Co价态同等重要的结构调控参数。

Jahn-Teller, polarity and insulator-to-metal transition in BiMnO3 at high pressure

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained thin films. Here we investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four interacting mechanisms: electric polarity, octahedra tilts, magnetism, and cooperative Jahn-Teller distortion. We have probed structural transitions under high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60 GPa. We show that BiMnO3 displays under pressure a rich sequence of…

Pressure tuning the Jahn-Teller transition temperature in NaNiO$_2$

NaNiO$_2$ is a layered material consisting of alternating layers of NaO$_6$ and Jahn-Teller-active NiO$_6$ edge-sharing octahedra. At ambient pressure it undergoes a broad phase transition from a monoclinic to rhombohedral structure between $\sim$465 K and $\sim$495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO$_2$ as a function of pressure and temperature from ambient pressure to $\sim$5 GPa between 290 K and 490 K. The 290 K and 460 K isothermal compressions remained in the monoclinic p…

Comment on ``Conduction states in oxide perovskites: Three manifestations of Ti$^{3 +}$ Jahn-Teller polarons in barium titanate''

In this comment to [S. Lenjer, O. F. Schirmer, H. Hesse, and Th. W. Kool, Phys. Rev. B {\bf 66}, 165106 (2002)] we discuss the electronic structure of oxygen vacancies in perovskites. First principles computations are in favour of rather deep levels in these vacancies, and Lenjer et al suggest that the electrons' interaction energy is negative, but data on electroconductivity are against.

Phonon study of Jahn-Teller distortion and phase stability in NaMnO$_2$ for sodium-ion batteries

Cathode materials undergo various phase transitions during the charge/discharge process, and the structural transitions significantly affect the battery performance. Although phonon properties can provide a direct clue for structural stability and transitions, it has been less explored in sodium cathode materials. Here, using the first-principles calculations, we investigate phonon and electronic properties of various layered NaMnO$_2$ materials, especially focusing on the dependency of the Jahn-Teller distortion of Mn$^{3+}$. The phonon dispersion curves show that the O$'$3 and P$'$2 structur…

Interplay of the Jahn--Teller Effect and Spin-Orbit Coupling: The Case of Trigonal Vibronic Modes

We study an interplay between the orbital degeneracy and spin-orbit coupling giving rise to spin-orbital entangled states. As a specific example, we analyze the interaction of electrons occupying triply degenerate single-ion $t_{2g}$ levels with trigonal vibronic modes (the $t\otimes T$ problem). A more general problem of the electron-lattice interaction involving both tetragonal and trigonal vibrations is also considered. It is shown that the result of such interaction crucially depends on the occupation of $t_{2g}$ levels leading to either the suppression or enhancement of the Jahn-Teller ef…

Origin of the Non-Linear Pressure Effects in Perovskite Manganites: Buckling of Mn-O-Mn Bonds and Jahn-Teller Distortion of the MnO6 Octahedra Induced by Pressure

High-pressure resistivity and x-ray diffraction have been measured on La0.85MnO3-d. At low pressures the metal-insulator transition temperature (TMI) increases linearly with pressure up to a critical pressure, P* ~ 3.4 GPa, which is followed by reduction of TMI with increasing pressure. Analysis of the bond distances and bond angles reveal that a bandwidth increase drives the in-crease of TMI for pressure below P*. The reduction of TMI at higher pressures is found to result from Jahn-Teller distortions of the MnO6 octahedra. The role of anharmonic interatomic potential is discussed.

Crystal structures of spin-Jahn-Teller ordered MgCr_2O_4 and ZnCr_2O_4

Magnetic ordering in the geometrically frustrated magnetic oxide spinels MgCr_2O_4 and ZnCr_2O_4 is accompanied by a structural change that helps relieve the frustration. Analysis of high-resolution synchrotron X-ray scattering reveals that the low-temperature structures are well described by a two-phase model of tetragonal I4_1/amd and orthorhombic Fddd symmetries. The Cr_4 tetrahedra of the pyrochlore lattice are distorted at these low-temperatures, with the Fddd phase displaying larger distortions than the I4_1/amd phase. The spin-Jahn-Teller distortion is approximately one order of magnitu…

Polarons and Solitons in Jahn-Teller Systems

Using a semiclassical continuum model of an electron in a deformable molecular crystal, some properties of multicomponent generalizations of the polaron--``vector polarons''-- are elucidated. Analytical solutions for the case of two electronic bands coupled to two vibrational modes are given in detail. Within the model considered, the vector polaron can be classified by its wavefunction into several types and can have features that include: (1) a spatial variation in the electronic and vibrational character, and (2) low-energy internal degrees of freedom. For the case of electronic and vibrati…

Effects of rare earth ion size on the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites

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…

Dynamical Jahn-Teller effect of fullerene anions

The dynamical Jahn-Teller effect of C$_{60}^{n-}$ anions ($n = $ 1-5) is studied using the numerical diagonalization of the linear $p^n \otimes 8d$ Jahn-Teller Hamiltonian with the currently established coupling parameters. It is found that in all anions the Jahn-Teller effect stabilizes the low-spin states, resulting in the violation of Hund's rule. The energy gain due to the Jahn-Teller dynamics is found to be comparable to the static Jahn-Teller stabilization. The Jahn-Teller dynamics influences the thermodynamic properties via strong variation of the density of vibronic states with energy.…

$E\otimes e$ Jahn-Teller Effect in $C_{70}^{3-}$ Systems

The electron-phonon interaction in $C_{70}$ anions is studied by making use of a lattice relaxation approach. We find there exists a Jahn-Teller effect in $C_{70}^{3-}$ system, due to an extra electron being doped to the double degenerate $E_{1}^{''}$ state. As a result of this effect, the original $D_{5h}$ symmetry of the ground state becomes unstable, which causes distortion of the lattice configuration. The only symmetry maintained in the final state of the relaxation is the $x-y$ plane reflection symmetry. We further find that besides the Jahn-Teller active $A_{1}^{'}, A_{2}^{'}, E_{2}^{'}…

Structural relaxation and Jahn-Teller distortion of LaMnO3 (001) surface

We studied in detail the structural relaxation and Jahn-Teller distortion in LaMnO3 (001) surface of the orthorhombic phase by means of classical atomistic simulation. It is found that MnO2-terminated surface is more energetically favorable than LaO-terminated surface by 0.34 eV. The standard deviation of Mn-O bond lengths of MnO6 octahedra and Jahn-Teller distortion oscillate in LaMnO3 (001) surface. Our simulated atomic displacements in the surface are compared with some ab initio studies.

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 phenom…

Jahn-Teller distortion induced charge-ordering in CE phase of manganites

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 …

Jahn-Teller distortions as a novel source of multiferroicity

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…

The Anti-Jahn-Teller Polaron in LaMnO$_3$

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.

钙钛矿氟化物单层中由Jahn-Teller畸变驱动的铁磁性

Jahn-Teller distortion driven ferromagnetism in a perovskite fluoride monolayer

本研究发现钙钛矿氟化物KCrF₃单层可保留块体中的Jahn-Teller畸变，产生交错轨道有序和铁磁性，阐明了不同结构畸变对性质的影响，为二维铁磁体设计提供了新思路。

Elastic interaction and superstructures in manganites and other Jahn-Teller systems

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.

The Jahn-Teller effect and spin--orbit coupling: friends or foes?

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…

The role of the Berry Phase in Dynamical Jahn-Teller Systems

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…

Determinative Role of the Jahn-Teller Disorder in the Raman Scattering of Mixed-Valence Manganites

The mixed-valence perovskitelike manganites are characterized by unique interrelation of Jahn-Teller distortions, electric and magnetic properties. The Jahn-Teller distortion follows the Mn(3+)->Mn(4+) charge transfer with some delay. Its development depends on the lifetime of Mn in (3+) state, governed by the Mn(4+)/Mn(3+) ratio and magnetic correlation. The non-coherence of Jahn-Teller distortions in orthorhombic mixed-valence manganites and rhombohedral RMnO3 (R = rare earth) results in oxygen disorder. We demonstrate that the Raman spectra in this case are dominated by disorder-induced ban…

Jahn-Teller Solitons, Structural Phase Transitions and Phase Separation

It is demonstrated that under common conditions a molecular solid subject to Jahn-Teller interactions supports stable Q-ball-like non-topological solitons. Such solitons represent a localized lump of excess electric charge in periodic motion accompanied by a time-dependent shape distortion of a set of adjacent molecules. The motion of the distortion can correspond to a true rotation or to a pseudo-rotation about the symmetric shape configuration. These solitons are stable for Jahn-Teller coupling strengths below a critical value; however, as the Jahn-Teller coupling approaches this critical va…

Zeeman interaction and Jahn-Teller effect in $Γ_8$ multiplet

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…

Chaos in Jahn-Teller Rattling

We unveil chaotic behavior hidden in the energy spectrum of a Jahn-Teller ion vibrating in a cubic anharmonic potential as a typical model for rattling in cage-structure materials. When we evaluate the nearest-neighbor level-spacing distribution $P(s)$ of eigenenergies of the present oscillator system, we observe the transition of $P(s)$ from the Poisson to the Wigner distribution with the increase of cubic anharmonicity, showing the occurrence of chaos in the anharmonic Jahn-Teller vibration. The energy scale of the chaotic region is specified from the analysis of $P(s)$ and we discuss a poss…

Observation of the dynamic Jahn-Teller effect in the excited states of nitrogen-vacancy centers in diamond

The optical transition linewidth and emission polarization of single nitrogen-vacancy (NV) centers are measured from 5 K to room temperature. Inter-excited state population relaxation is shown to broaden the zero-phonon line and both the relaxation and linewidth are found to follow a T^5 dependence for T up to 100 K. This dependence indicates that the dynamic Jahn-Teller effect is the dominant dephasing mechanism for the NV optical transitions at low temperatures.

Static and dynamic Jahn-Teller effect in the alkali metal fulleride salts A4C60 (A = K, Rb, Cs)

We report the temperature dependent mid- and near-infrared spectra of K4C60, Rb4C60 and Cs4C60. The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C604- which brings the fulleride anion from D2h to either a D3d or a D5d distortion. In contrast to Cs4C60, low temperature neutron diffraction measurements did not reveal a structural phase transition in either K4C60 and Rb4C60. This proves that the molecular transition is driven by the molecular Jahn-Teller effect, which overrides the distorting potential field of the surrounding cations at high te…

The cooperative Jahn-Teller ordering investigation by absorption spectra in $KDy(MoO_4)_2$

Absorption spectra fine structure of $KDy(MoO_4)_2$ in the region of cooperative Jahn-Teller type ordering was studied. Temperature anomalies in the spectra occurring at phase transformation correlate with the ultrasound peculiarities observed earlier. Based on the symmetry approaching, possible activity of the irreducible representations of the rhombic $D_{2h}$ point group was discussed, which lead to the incommensurate phase at cooperative ordering. It was supposed, that coupled $A_u$-type phonon mode may lead to the incommensurate phase existence, which is possible at least in the temperatu…

Second-order nonadiabatic couplings from time-dependent density functional theory: Evaluation in the immediate vicinity of Jahn-Teller/Renner-Teller intersections

For a rigorous quantum simulation of nonadiabatic dynamics of electrons and nuclei, knowledge of not only first-order but also second-order nonadiabatic couplings (NAC), is required. Here we propose a method to efficiently calculate second-order NAC from time-dependent density functional theory (TDDFT), on the basis of the Casida ansatz adapted for the computation of first-order NAC, which has been justified in our previous work and can be shown to be valid for calculating second-order NAC between ground state and singly excited states within the Tamm-Dancoff approximation. Test calculations o…

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 th…

NaNiO$_2$ 中的位移型 Jahn--Teller 相变

Displacive Jahn--Teller transition in NaNiO$_2$

本研究利用中子全散射、NMR、EXAFS 及 AIMD 模拟作为局域探针，首次通过直接观测证明了 NaNiO$_2$ 在相变温度下发生的是位移型而非有序-无序型 Jahn-Teller 相变。

Kondo Effect of a Jahn-Teller Ion Vibrating in a Cubic Anharmonic Potential

We discuss the Kondo effect in a spinless two-orbital conduction electron system coupled with anharmonic Jahn-Teller vibration by employing a numerical renormalization group technique. When a temperature $T$ is decreased, we encounter a plateau of $\log 3$ entropy due to quasi-triple degeneracy of local low-energy states, composed of vibronic ground states and the first excited state with an excitation energy of $ΔE$. Around at $T$$\approx$$ΔE$, we observe an entropy change from $\log 3$ to $\log 2$. This $\log 2$ entropy originates from the rotational degree of freedom of the vibronic state a…

Cooperative Jahn-Teller transition and resonant x-ray scattering in thin film ${\rm LaMnO_3}$

Epitaxial thin films of stoichiometric ${\rm LaMnO_3}$ were grown on ${\rm SrTiO_3(110)}$ substrates using the pulsed laser deposition technique. From the high resolution x-ray diffraction measurements, the lattice parameters were determined as a function of temperature and the cooperative Jahn-Teller transition was found to occur at $T_{JT}$=573.0 K. Also measured was resonant x-ray scattering intensity of the orthorhombic (100) peak of ${\rm LaMnO_3}$ near the Mn K edge from low temperatures to above $T_{JT}$. We demonstrate that the integrated intensity of the (100) peak is proportional to …

Exchange and spin Jahn-Teller distortions for a triangular cluster of spin-1/2

We study the effects of magnetoelastic coupling on the degenerate ground state of the spin-1/2 antiferromagnetic Heisenberg model for the regular triangular spin cluster. Static displacement of spins spontaneously lifts the degeneracy of the ground state through the distance dependence of exchange coupling, i.e., a spin Jahn-Teller mechanism takes place. On the other hand, dynamical displacement does not lift the degeneracy, though the cluster distorts spontaneously. The energy decrease obtained by dynamical theory is twice as large as that obtained by static theory because of quantum fluctuat…

Magnetoelastic effects in Jahn-Teller distorted CrF$_2$ and CuF$_2$ studied by neutron powder diffraction

We have studied the temperature dependence of crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF$_2$ and CuF$_2$ by neutron powder diffraction in the temperature range 2-280 K. The lattice parameters and the unit cell volume show magnetoelastic effects below the Néel temperature. The lattice strain due to the magnetostriction effect couples with the square of the order parameter of the antiferromagnetic phase transition. We also investigated the temperature dependence of the Jahn-Teller distortion which does not show any significant effect at the anti…

Origin of Jahn-Teller distortion and orbital-order in LaMnO3

The origin of the cooperative Jahn-Teller distortion and orbital-order in LaMnO3 is central to the physics of the manganites. The question is complicated by the simultaneous presence of tetragonal and GdFeO3-type distortions and the strong Hund's rule coupling between e_g and t_2g electrons. To clarify the situation we calculate the transition temperature for the Kugel-Khomskii superexchange mechanism by using the local density approximation+dynamical mean-field method, and disentangle the effects of super-exchange from those of lattice distortions. We find that super-exchange alone would yiel…

Jahn-Teller distortions and excitation energies in C60(n+)

Based on previously computed parameters for the electron-phonon couplings and the Coulomb exchange, we compute and classify the static Jahn-Teller distortions, i.e. the minima of the lowest adiabatic potential energy surface, of C60(n+), for all values of charge 1 <= n <= 9 and spin. We compute the intra-band electronic excitation energies in the different optimal geometries in the sudden approximation, and find a spread of the electronic states of roughly 1 eV. We also obtain the leading vibronic quantum corrections to the ground-state energy, equal to zero-point energy lowering due to the so…

Electrons doped in cubic perovskite SrMnO3: isotropic metal versus chainlike ordering of Jahn-Teller polarons

Single crystals of electron-doped SrMnO3 with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only 1-2% electron doping by Ce substitution for Sr, a G-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distorti…

The high temperature Jahn-Teller transition in LaMn7O12

A first order structural Jahn-Teller transition at T_{JT} = 650 K has been recently reported for the quadruple perovskite LaMn7O12 . We have carried out magnetization and transport measurements below and above T_{JT} in order to investigate the effect of the transition. Electrical conduction turns out to be polaronic, changing from non-adiabatic to adiabatic through the transition. Magnetic behavior can be described by non-interacting Mn3+ ions below T_{JT}, while above T_{JT} it is of questionable interpretation. The effect of thermal cycling on as grown samples of different purity degree als…

Unraveling the Jahn-Teller effect in Mn doped GaN using the Heyd-Scuseria-Ernzerhof hybrid functional

We present an ab-initio study of the Mn substitution for Ga in GaN using the Heyd-Scuseria-Ernzerhof hybrid functional (HSE). Contrary to semi-local functionals, the majority Mn t$_{2}$ manifold splits into an occupied doublet and an unoccupied singlet well above the Fermi-level resulting in an insulating groundstate, which is further stabilized by a sizeable Jahn-Teller distortion. The predictions are confirmed using $GW$ calculations and are in agreement with experiment. A transition from a localized to a delocalized Mn hole state is predicted from GaN to GaAs.

Superconductivity in molecular solids with Jahn-Teller phonons

We analyze fulleride superconductivity at experimental doping levels, treating the electron-electron and electron-phonon interactions on an equal footing, and establish the existence of novel physics which helps explain the unusually high superconducting transition temperatures in these systems. The Jahn-Teller phonons create a local (intramolecular) pairing that is surprisingly resistant to the Coulomb repulsion, despite the weakness of retardation in these low-bandwidth systems. The requirement for coherence throughout the solid to establish superconductivity then yields a very strong doping…