Magnetoelectric materials are attractive for several applications, including actuators, switches, and magnetic field sensors. Typical mechanisms for achieving a strong magnetoelectric coupling are rooted in transition metal magnetism. In sharp contrast, here we identify CsEr(MoO4)2 as a magnetoelectric material without magnetic transition metal ions, thus ensuring that the Er ions play a key role in achieving this interesting property. Our detailed study includes measurements of the structural, magnetic, and electric properties of this material. Bulk characterization and neutron powder diffraction show no evidence for structural phase transitions down to 0.3 K and therefore CsEr(MoO4)2 maintains the room temperature P2/c space group over a wide temperature range without external magnetic field. These same measurements also identify collinear antiferromagnetic ordering of the Er3+ moments below TN = 0.87 K. Complementary dielectric constant and pyroelectric current measurements reveal that a ferroelectric phase (P ~ 0.5 nC/cm2) emerges when applying a modest external magnetic field, which indicates that this material has a strong magnetoelectric coupling. We argue that the magnetoelectric coupling in this system arises from a pseudo Jahn-Teller distortion induced by the magnetic field.
In developing physical theories analogical reasoning has been found to be very powerful, as attested by a number of important historical examples. An analogy between two apparently different phenomena, once established, allows one to transfer information and bring new concepts from one phenomenon to the other. Here we discuss an important analogy between two widely different physical problems, namely, the Jahn-Teller distortion in molecular physics and the Rashba spin splitting in condensed matter physics. By exploring their conceptual and mathematical features and by searching for the counter…
We present an ab-initio and analytical study of the Jahn-Teller effect in two diluted magnetic semiconductors (DMS) with d4 impurities, namely Mn-doped GaN and Cr-doped ZnS. We show that only the combined treatment of Jahn-Teller distortion and strong electron correlation in the 3d shell may lead to the correct insulating electronic structure. Using the LSDA+U approach we obtain the Jahn-Teller energy gain in reasonable agreement with the available experimental data. The ab-initio results are completed by a more phenomenological ligand field theory.
The Jahn-Teller distortion, by its very nature, is often at the heart of the various electronic properties displayed by perovskites and related materials. Despite the Jahn-Teller mode being non- polar in nature, we devise and demonstrate in the present letter an electric field control of Jahn-Teller distortions in bulk perovskites. The electric field control is enabled through an anharmonic lattice mode coupling between the Jahn-Teller distortion and a polar mode. We confirm this coupling, and explicitly an electric field effect, through first principles calculations. The coupling will always …
Three high-spin phases recently discovered in the spin-crossover system Mn(taa) are identified through analysis by a combination of first-principles calculations and Monte Carlo simulation as a low-temperature Jahn-Teller ordered (solid) phase, an intermediate-temperature dynamically correlated (liquid) phase, and an uncorrelated (gas) phase. In particular, the Jahn-Teller liquid phase arises from competition between mixing with low-spin impurities, which drive the disorder, and inter-molecular strain interactions. The latter are a key factor in both the spin-crossover phase transition and the…
The Jahn-Teller effect refers to the symmetry-lowering geometrical distortion in a crystal (or non-linear molecule) due to the presence of a degenerate electronic state. Usually, the Jahn-Teller distortion is not polar. Recently, GaV4S8 with the lacunar spinel structure was found to undergoes a Jahn-Teller distortion from cubic to ferroelectric rhombohedral structure at TJT = 38K. In this work, we carry out a general group theory analysis to show how and when the Jahn-Teller effect gives rise to ferroelectricity. On the basis of this theory, we find that the ferroelectric Jahn-Teller distort i…
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…
Berry Phase and Ground State Symmetry in H x h Dynamical Jahn-Teller Systems
由于Berry相的普遍存在，在大多数情况下，动力学Jahn-Teller系统的振动基态的对称性与原始简并电子态的对称性相同。作为一个例外，与C60阳离子物理相关的线性H x h二十面体模型由一个额外的自由参数确定，该参数可以连续调整以从低能闭合路径中消除Berry相：因此，基态改变为完全对称的非简并态。
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…
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…
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…
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''
在这篇评论中，[S。伦耶尔岛F. Schirmer，H. Hesse和Th. W. Kool，Phys. Rev. B { \bf 66}，165106（2002）]讨论了钙钛矿中氧空位的电子结构。第一原理计算支持这些空位中的深能级，而Lenjer等人认为电子的相互作用能是负的，但电导率的数据则相反。
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…
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
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…
In a linear magnetoelectric the lattice is coupled to electric and magnetic fields: both affect the longitudinal-transverse splitting of zone-center optical phonons on equal footing. A response matrix relates the macroscopic fields (D,B) to (E,H) at infrared frequencies. It is shown that the response matrices at frequencies 0 and \infty fulfill a generalized Lyddane-Sachs-Teller relationship. The rhs member of such relationship is expressed in terms of weighted averages over the longitudinal and transverse excitations of the medium, and assumes a simple form for an harmonic crystal.
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…
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.…
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.
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 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 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…
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 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…
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.
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…
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 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…
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…
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…
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+)
基于先前计算的电子-声子耦合和库仑交换参数，我们计算并分类了C60（n+）的静态Jahn-Teller畸变，即最低绝热势能面的最小值，对于所有的电荷1 <= n <= 9和自旋。我们计算的带内电子激发能在不同的最佳几何形状的突然近似，并找到一个大约1 eV的电子状态的传播。我们还得到了基态能量的主要振动量子修正，等于由于绝热Jahn-Teller极小点处声子软化而导致的零点能量降低：这些非绝热修正是如此之大，以至于对于4 <= n <= 6个不同自旋对称性的态，其能量低于高自旋绝热基态。
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…
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…
