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Register a new userThree Jahn-Teller states of matter in the spin-crossover system Mn(taa)
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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 magnetoelectric coupling. Jahn-Teller liquids may exist in other spin-crossover materials and materials that have multiple equivalent Jahn-Teller axes.
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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 other hand, the substitution of Jahn-Teller active Cu$^{2+}$ for Mg$^{2+}$ and Zn$^{2+}$ in Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ where 0,$<$,$x$,$leq$,0.2 induce Jahn-Teller ordering at temperatures well above the Neel temperatures of these solid solutions, and yet spin interactions remain frustrated with long-range magnetic ordering occurring below 20,K without any further lattice distortion. The Jahn-Teller distorted solid solutions Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ adopt the orthorhombic $Fddd$ structure of ferrimagnetic CuCr$_2$O$_4$. Total neutron scattering studies of Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ suggest that there are local $A$O$_4$ distortions in these Cu$^{2+}$-containing solid solutions at room temperature and that these distortions become cooperative when average structure distortions occur. Magnetism evolves from compensated antiferromagnetism in MgCr$_2$O$_4$ and ZnCr$_2$O$_4$ to uncompensated antiferromagnetism with substitution of magnetic cations on the non-magnetic cation sites of these frustrated compounds.
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.
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 exist within the P b2 1 m space group, which is found to be the favoured ground state for various perovskites under sufficient tensile epitaxial strain. Intriguingly, the calculations reveal that this mechanism is not only restricted to Jahn-Teller active systems, promising a general route to tune or induce novel electronic functionality in perovskites as a whole.
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.
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 electronic state with substantial magnetic anisotropy. The Jahn-Teller distorted Ca3CoMnO6 in the magnetic ground state with up-up-down-down spin arrangement is predicted to have electric polarizations much greater than experimentally observed. Implications of the discrepancy between theory and experiment were discussed.
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