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Kosmas Prassides - One of the best experts on this subject based on the ideXlab platform.
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Chemical tuning of samarium valence in mixed valence (Sm1-xCax)2.75C60 Fullerides
Journal of Physics and Chemistry of Solids, 2021Co-Authors: Naoya Yoshikane, J Arvanitidis, Takeshi Nakagawa, Keisuke Matsui, Hitoshi Yamaoka, Nozomu Hiraoka, Hirofumi Ishii, Kosmas PrassidesAbstract:Abstract Rare-earth (RE) Fullerides are an intriguing family of materials in which electronic instabilities on the RE sublattice couple to the electronic and lattice degrees-of-freedom of the strongly-correlated fulleride sublattice. In particular, insulating Sm2.75C60 adopts an orthorhombic superstructure, arising from long-range ordering of Sm partial vacancies, and can be driven to a lattice-collapsed metallic state due to a valence transition towards Sm3+ upon pressurization. Here we use synchrotron X-ray absorption spectroscopy (XAS) at the Sm L3 edge at ambient conditions to authenticate the mixed valence character of the material by identifying two features due to distinct Sm2+ and Sm3+ components – their relative intensity allows a direct measure of the average Sm valence in Sm2.75C60 at +2.07(3). We then attempt to mimic the physical pressure effect on the electronic properties by co-intercalation of the smaller-size valence-precise Ca2+ ion to form the series of ternary solid solutions, (Sm1-xCax)2.75C60 (0 ≤ x ≤ ⅔). XAS measurements in their high-resolution partial fluorescence yield (PFY) variance find that chemical pressure leads to an increase of the Sm3+ contribution to the average valence by >10% in the most contracted member, (Sm⅓Ca⅔)2.75C60 of the present study with the average valence reaching a value of +2.33(2). Assuming full charge transfer between the metal ions and C60, the charge on the C60 units remains invariant throughout at approximately −5.78. This opens the possibility that the system can be tuned further towards the higher average valences needed for the high-pressure insulator-to-metal transition in Sm2.75C60 to be shifted to ambient pressure with a single-valence Sm3+ state accessible at a Ca content of x ~0.9.
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structural and electronic response of overexpanded superconducting Fullerides close to the mott insulator boundary
International Journal of Modern Physics B, 2018Co-Authors: Melita Menelaou, Yasuhiro Takabayashi, H E Okur, Ruth H Zadik, Kosmas PrassidesAbstract:The ternary fulleride, Rb0.25Cs2.75C60, is the most expanded member of the family of face-centered cubic (fcc) structured superconducting Fullerides ever accessed with superconductivity surviving a...
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unconventional high tc superconductivity in Fullerides
Philosophical Transactions of the Royal Society A, 2016Co-Authors: Yasuhiro Takabayashi, Kosmas PrassidesAbstract:A3C60 molecular superconductors share a common electronic phase diagram with unconventional high-temperature superconductors such as the cuprates: superconductivity emerges from an antiferromagnetic strongly correlated Mott-insulating state upon tuning a parameter such as pressure (bandwidth control) accompanied by a dome-shaped dependence of the critical temperature, Tc However, unlike atom-based superconductors, the parent state from which superconductivity emerges solely by changing an electronic parameter-the overlap between the outer wave functions of the constituent molecules-is controlled by the C60 (3-) molecular electronic structure via the on-molecule Jahn-Teller effect influence of molecular geometry and spin state. Destruction of the parent Mott-Jahn-Teller state through chemical or physical pressurization yields an unconventional Jahn-Teller metal, where quasi-localized and itinerant electron behaviours coexist. Localized features gradually disappear with lattice contraction and conventional Fermi liquid behaviour is recovered. The nature of the underlying (correlated versus weak-coupling Bardeen-Cooper-Schrieffer theory) s-wave superconducting states mirrors the unconventional/conventional metal dichotomy: the highest superconducting critical temperature occurs at the crossover between Jahn-Teller and Fermi liquid metal when the Jahn-Teller distortion melts.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.
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raman spectroscopic study of the rare earth Fullerides eu6 xsrxc60
Nanoscale, 2011Co-Authors: J Arvanitidis, K Papagelis, Yoshihiro Iwasa, D Christofilos, G A Kourouklis, A Paloumpi, S Ves, Kosmas PrassidesAbstract:We present Raman spectroscopic studies of the isostructural and isoelectronic Eu6−xSrxC60 (x = 0, 3, 5, 6) and Ba6C60 compounds. The Raman spectra of the Eu-based Fullerides show dramatic changes compared to the pure alkaline-earth systems, including significant broadening, splitting and frequency shifts of the fivefold degenerate Hg intramolecular modes of C60. Moreover, the Ag(2) mode exhibits an even larger downshift and a remarkable broadening. These findings are consistent with distortions of the C60 molecular cages and a considerable electron-phonon coupling strength—strongly enhanced in the Eu containing systems—originating from the strong orbital hybridization between the metal atom and the C60 molecule.
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mixed valency in rare earth Fullerides
Philosophical Transactions of the Royal Society A, 2008Co-Authors: Kosmas Prassides, Yasuhiro Takabayashi, Takeshi NakagawaAbstract:Mixed-valence phenomena associated with the highly correlated narrow-band behaviour of the 4f electrons in rare earths are well documented for a variety of rare-earth chalcogenides, borides and intermetallics (Kondo insulators and heavy fermions). The family of rare-earth Fullerides with stoichiometry RE2.75C60 (REZSm, Yb, Eu) also displays an analogous phenomenology and a remarkable sensitivity of the rare-earth valency to external stimuli (temperature and pressure) making them the first known molecular-based members of this fascinating class of materials. Using powerful crystallographic and spectroscopic techniques which provide direct indications of what is happening in these materials at the microscopic level, we find a rich variety of temperature- and pressure-driven abrupt or continuous valence transitions—the electronically active fulleride sublattice acts as an electron reservoir that can accept electrons from or donate electrons to the rare-earth 4f/5d bands, thereby sensitively modulating the valence of the rare-earth sublattice.
Liviu F Chibotaru - One of the best experts on this subject based on the ideXlab platform.
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jahn teller effect in the cubic Fullerides a 3 c 60
Physical Review B, 2021Co-Authors: Zhishuo Huang, Naoya Iwahara, Munirah D Albaqami, Tohru Sato, Liviu F ChibotaruAbstract:Compared to isolated ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ ions, characterized by a three-dimensional equipotential trough at the bottom of the lowest adiabatic potential energy surface (APES), the Jahn-Teller (JT) effect in cubic Fullerides is additionally influenced by the interaction of JT distortions at ${\mathrm{C}}_{60}$ sites with vibrational modes of the lattice. This leads to modification of JT stabilization energy and to the warping of the trough at each fullerene site, as well as to the interaction of JT distortions at different sites. Here we investigate these effects in three fcc Fullerides with $A=\mathrm{K}$, Rb, Cs and in ${\mathrm{Cs}}_{3}{\mathrm{C}}_{60}$ with bcc (A15) structure. DFT calculations of orbital vibronic coupling constants at ${\mathrm{C}}_{60}$ sites and of phonon spectra have been done for fully ordered lattices (1 ${\mathrm{C}}_{60}/\mathrm{u}.\mathrm{c}.$). Based on them the elastic response function for local JT distortions has been evaluated and the lowest APES investigated. To this end an expression for the latter as a function of trough coordinates of all sites has been derived. The results show that the JT stabilization energy slightly increases compared to an isolated ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ and a warping of the trough of a few meV occurs. The interaction of JT distortions on nearest- and next-nearest-neighbor fullerene sites is of similar order of magnitude. These effects arise first of all due to the interaction of ${\mathrm{C}}_{60}$ sites with the displacements of neighbor alkali atoms and are more pronounced in fcc Fullerides than in the A15 compound. The results of this study support the picture of weakly hindered independent rotations of JT deformations at ${\mathrm{C}}_{60}$ sites in cubic ${A}_{3}{\mathrm{C}}_{60}$.
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orbital disproportionation of electronic density is a universal feature of alkali doped Fullerides
Nature Communications, 2016Co-Authors: Naoya Iwahara, Liviu F ChibotaruAbstract:Alkali-doped Fullerides show a wide range of electronic phases in function of alkali atoms and the degree of doping. Although the presence of strong electron correlations is well established, recent investigations also give evidence for dynamical Jahn–Teller instability in the insulating and the metallic trivalent Fullerides. In this work, to reveal the interplay of these interactions in Fullerides with even electrons, we address the electronic phase of tetravalent fulleride with accurate many-body calculations within a realistic electronic model including all basic interactions extracted from first principles. We find that the Jahn–Teller instability is always realized in these materials too. In sharp contrast to the correlated metals, tetravalent system displays uncorrelated band-insulating state despite similar interactions present in both Fullerides. Our results show that the Jahn–Teller instability and the accompanying orbital disproportionation of electronic density in the degenerate lowest unoccupied molecular orbital band is a universal feature of Fullerides. Understanding the electronic phases of alkali-doped Fullerides is a long-standing and challenging task for material scientists. Here the authors show that Jahn-Teller instability and orbital disproportionation of electronic density in the lowest unoccupied molecular orbital band is universal in these systems.
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dynamical jahn teller instability in metallic Fullerides
Bulletin of the American Physical Society, 2015Co-Authors: Naoya Iwahara, Liviu F ChibotaruAbstract: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 lower than the current estimates of $U$, meaning that dynamical Jahn-Teller effect takes place in all cubic Fullerides. This leads to strong splitting of LUMO orbitals on C$_{60}$ sites and calls for reconsideration of the role of orbital degeneracy in the Mott-Hubbard transition in Fullerides.
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spin vibronic superexchange in mott hubbard Fullerides
Physical Review Letters, 2005Co-Authors: Liviu F ChibotaruAbstract:In the Mott-Hubbard cubic fulleride Li3(NH3)6C60 the superexchange energy is found to be much smaller than the rotational quantum for Jahn-Teller deformations at fullerene sites. This gives rise to a new type of superexchange interaction involving threefold degenerate vibronic ground states of C3-60 ions. In contrast with spin-orbital models, the spin-vibronic superexchange can be only antiferromagnetic and shows a significant vibronic reduction of the superexchange amplitude, in agreement with magnetic susceptibility data. As a function of the transfer parameters, two quadrupolar fully dynamical vibronic orders with quenched vibronic moments on sites develop in the ground state.
Qikun Xue - One of the best experts on this subject based on the ideXlab platform.
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direct observation of full gap superconductivity and pseudogap in two dimensional Fullerides
Physical Review Letters, 2020Co-Authors: Mingqiang Ren, Sha Han, Shuze Wang, Jiaqi Fan, Canli Song, Qikun XueAbstract:Alkali-fulleride superconductors with a maximum critical temperature T_{c}∼40 K exhibit a similar electronic phase diagram to that of unconventional high-T_{c} superconductors. Here we employ cryogenic scanning tunneling microscopy to show that trilayer K_{3}C_{60} displays fully gapped strong coupling s-wave superconductivity, accompanied by a pseudogap above T_{c}∼22 K and within vortices. A precise control of the electronic correlations and potassium doping enables us to reveal that superconductivity occurs near a superconductor-Mott-insulator transition and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of the superconductivity below half-filling, indicates that alkali Fullerides are predominantly phonon-mediated superconductors, although the electronic correlations also come into play.
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visualizing molecular orientational ordering and electronic structure in cs n c 60 fulleride films
Physical Review B, 2020Co-Authors: Sha Han, Mingqiang Ren, Canli Song, Mengxue Guan, Yilin Wang, Sheng Meng, Qikun XueAbstract:Alkali-doped Fullerides exhibit a wealth of unusual phases that remain controversial by nature. Here we report a cryogenic scanning tunneling microscopy study of the submolecular structural and electronic properties of expanded fullerene ${\mathrm{C}}_{60}^{n\ensuremath{-}}$ films with various cesium dopings. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of ${\mathrm{C}}_{60}^{n\ensuremath{-}}$ anions, yet the tunneling conductance spectra are all robustly characteristic of energy gaps, hallmarks of Jahn-Teller instability and electronic correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of $n\phantom{\rule{4pt}{0ex}}=1$, 2, 3, and 4, apart from which it moves toward band edges with concomitant electronic states within the energy gap. Our findings establish the universality of Jahn-Teller instability and clarify the relationship between the doping and structural and electronic properties in ${\mathrm{Cs}}_{n}{\mathrm{C}}_{60}$ Fullerides.
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coexistence of full gap superconductivity and pseudogap in two dimensional Fullerides
arXiv: Superconductivity, 2019Co-Authors: Mingqiang Ren, Sha Han, Shuze Wang, Jiaqi Fan, Canli Song, Qikun XueAbstract:Alkali-fulleride superconductors with a maximum critical temperature Tc of 40 K exhibit similar electronic phase diagram with unconventional high-Tc superconductors where the superconductivity resides proximate to a magnetic Mott-insulating state. However, distinct from cuprate compounds, which superconduct through two-dimensional (2D) CuO2 planes, alkali Fullerides are attributed to the three-dimensional (3D) members of high-Tc family. Here, we employ scanning tunneling microscopy to show that trilayer K3C60 displays fully gapped strong coupling s-wave superconductivity that coexists spatially with a cuprate-like pseudogap state above Tc = 22 K and within vortices. A precise control of electronic correlations and doping reveals that superconductivity occurs near a superconductor-Mott insulator transition and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of superconductivity below half-filling, demonstrates that alkali Fullerides are predominantly phonon-mediated superconductors, although the multiorbital electronic correlations also come into play.
J. H. Weaver - One of the best experts on this subject based on the ideXlab platform.
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electrical transport in na k rb and cs Fullerides phase formation microstructure and metallicity
Physical Review B, 1993Co-Authors: F Stepniak, P J Benning, D. M. Poirier, J. H. WeaverAbstract:The electrical properties of the alkali-metal Fullerides have been investigated via parallel resistivity measurements and photoelectron spectroscopy. Phase formation associated with the incorporation of Na, K, Rb, and Cs into ${\mathrm{C}}_{60}$ films is reflected by the dependence of resistivity on temperature and alkali-metal concentration. Photoemission spectra show the details of the filling of states derived from the lowest unoccupied molecular level and the consequences of electron correlation. For K-${\mathrm{C}}_{60}$ and Rb-${\mathrm{C}}_{60}$ films, the resistivity and photoemission results indicate metallic character only for the ${\mathit{A}}_{3}$${\mathrm{C}}_{60}$ phase. Changes in the temperature coefficient of resistivity for Rb-${\mathrm{C}}_{60}$ as a function of doping reveal the granular nature of the film and show the transition from insulating to metallic character. In contrast, the Na and Cs Fullerides are insulators for all concentrations. The temperature-dependent resistivity results for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{C}}_{60}$ show a phase transformation at 226 K for x between two and three, but neither phase is metallic.
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x ray photoemission investigations of binary and ternary c 60 Fullerides of na k rb and cs
Physical Review B, 1993Co-Authors: D. M. Poirier, T R Ohno, G H Kroll, P J Benning, L. P.f. Chibante, J. H. Weaver, F Stepniak, R E SmalleyAbstract:Core-level photoemission studies of alkali fulleride ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films indicate alkali bonding configurations that can be associated with octahedral and tetrahedral interstitial sites of the ${\mathrm{C}}_{60}$ lattice. For ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$, the K 2p core-level results suggest ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ nucleation for K concentration as low as x\ensuremath{\sim}0.1 with occupation of both octahedral and tetrahedral sites. In contrast, the Rb 3d and Cs 4d core-level spectra show filling of only octahedral sites until x\ensuremath{\sim}1, forming ${\mathit{A}}_{1}$${\mathrm{C}}_{60}$ phases. For intercalation beyond x\ensuremath{\sim}1, the fcc tetrahedral sites of Rb are occupied as ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ forms. Intercalation with K or Rb beyond x\ensuremath{\sim}3, or with Cs beyond x\ensuremath{\sim}1, results in a spectral feature that is consistent with formation of a body-centered phase with equivalent tetrahedral interstitial sites. Results for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{C}}_{60}$ suggest that Na ions occupy off-center positions in the large octahedral sites. Studies of ternary Fullerides ${\mathrm{K}}_{\mathit{x}}$${\mathit{A}}_{\mathit{y}}$${\mathrm{C}}_{60}$ (x+y3) demonstrate preferential filling of the octahedral sites by the larger ion.
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electronic structure of ca x c 60 Fullerides
Physical Review B, 1992Co-Authors: Yu Chen, T R Ohno, L. P.f. Chibante, J. H. Weaver, D. M. Poirier, M B Jost, Jose Luriaas Martins, R E SmalleyAbstract:Photoemission and inverse photoemission studies of ${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{C}}_{60}$ films show insulating character for x3\ifmmode\pm\else\textpm\fi{}0.3 as states derived primarily from the LUMO orbitals of ${\mathrm{C}}_{60}$ are occupied. Metallic character is observed at higher Ca content as a second band of states is partially occupied. This band has mixed character since it is derived from the LUMO+1 orbitals of ${\mathrm{C}}_{60}$ and the Ca s states. These states are responsible for superconductivity in ${\mathrm{Ca}}_{5}$${\mathrm{C}}_{60}$.
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Fullerides of alkaline earth metals
Physical Review B, 1992Co-Authors: Yu Chen, L. P.f. Chibante, J. H. Weaver, F Stepniak, R E SmalleyAbstract:Synchrotron radiation photoemission studies show that the mixing of Mg, Sr, and Ba with ${\mathrm{C}}_{60}$ yields metallic (Sr and Ba) and nonmetallic (Mg) Fullerides. In all cases, fulleride formation leads to the occupation of hybrid bands near the Fermi level that are derived from ${\mathrm{C}}_{60}$ states and the s states of the alkaline-earth atoms. The nonmetallic character of ${\mathrm{Mg}}_{\mathit{x}}$${\mathrm{C}}_{60}$ probably reflects electron correlation effects, and metallic ${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{C}}_{60}$ and ${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{C}}_{60}$ Fullerides are candidates for superconductivity.
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c 60 and c 70 fullerenes and potassium Fullerides
Physical Review B, 1992Co-Authors: P J Benning, T R Ohno, G H Kroll, J. H. Weaver, D. M. Poirier, Yu Chen, F Stepniak, M B Jost, J Fure, R E SmalleyAbstract:Photoemission and inverse photoemission studies of thin films of ${\mathrm{C}}_{60}$ and ${\mathit{C}}_{70}$ reveal the distribution of occupied and empty electronic states of these molecular solids. X-ray photoemission results also show the C 1s main line and features related to \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ shakeups, electron energy losses, and plasmons. Potassium doping produces changes that can be related to the occupation of states derived from the lowest unoccupied molecular orbitals of the fullerenes and band-structure effects. Important differences are observed upon K doping of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, particularly in states near the Fermi level, and these would be reflected in the electron-phonon coupling, superconductivity, and the phase diagram. Resistivity measurements for ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ show a resistivity minimum for ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and a dependence on stoichiometry that is indicative of dispersed conducting micrograins in an insulating medium. Oxygen-exposure studies demonstrate that ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films are unstable.
Naoya Iwahara - One of the best experts on this subject based on the ideXlab platform.
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jahn teller effect in the cubic Fullerides a 3 c 60
Physical Review B, 2021Co-Authors: Zhishuo Huang, Naoya Iwahara, Munirah D Albaqami, Tohru Sato, Liviu F ChibotaruAbstract:Compared to isolated ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ ions, characterized by a three-dimensional equipotential trough at the bottom of the lowest adiabatic potential energy surface (APES), the Jahn-Teller (JT) effect in cubic Fullerides is additionally influenced by the interaction of JT distortions at ${\mathrm{C}}_{60}$ sites with vibrational modes of the lattice. This leads to modification of JT stabilization energy and to the warping of the trough at each fullerene site, as well as to the interaction of JT distortions at different sites. Here we investigate these effects in three fcc Fullerides with $A=\mathrm{K}$, Rb, Cs and in ${\mathrm{Cs}}_{3}{\mathrm{C}}_{60}$ with bcc (A15) structure. DFT calculations of orbital vibronic coupling constants at ${\mathrm{C}}_{60}$ sites and of phonon spectra have been done for fully ordered lattices (1 ${\mathrm{C}}_{60}/\mathrm{u}.\mathrm{c}.$). Based on them the elastic response function for local JT distortions has been evaluated and the lowest APES investigated. To this end an expression for the latter as a function of trough coordinates of all sites has been derived. The results show that the JT stabilization energy slightly increases compared to an isolated ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ and a warping of the trough of a few meV occurs. The interaction of JT distortions on nearest- and next-nearest-neighbor fullerene sites is of similar order of magnitude. These effects arise first of all due to the interaction of ${\mathrm{C}}_{60}$ sites with the displacements of neighbor alkali atoms and are more pronounced in fcc Fullerides than in the A15 compound. The results of this study support the picture of weakly hindered independent rotations of JT deformations at ${\mathrm{C}}_{60}$ sites in cubic ${A}_{3}{\mathrm{C}}_{60}$.
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orbital disproportionation of electronic density is a universal feature of alkali doped Fullerides
Nature Communications, 2016Co-Authors: Naoya Iwahara, Liviu F ChibotaruAbstract:Alkali-doped Fullerides show a wide range of electronic phases in function of alkali atoms and the degree of doping. Although the presence of strong electron correlations is well established, recent investigations also give evidence for dynamical Jahn–Teller instability in the insulating and the metallic trivalent Fullerides. In this work, to reveal the interplay of these interactions in Fullerides with even electrons, we address the electronic phase of tetravalent fulleride with accurate many-body calculations within a realistic electronic model including all basic interactions extracted from first principles. We find that the Jahn–Teller instability is always realized in these materials too. In sharp contrast to the correlated metals, tetravalent system displays uncorrelated band-insulating state despite similar interactions present in both Fullerides. Our results show that the Jahn–Teller instability and the accompanying orbital disproportionation of electronic density in the degenerate lowest unoccupied molecular orbital band is a universal feature of Fullerides. Understanding the electronic phases of alkali-doped Fullerides is a long-standing and challenging task for material scientists. Here the authors show that Jahn-Teller instability and orbital disproportionation of electronic density in the lowest unoccupied molecular orbital band is universal in these systems.
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dynamical jahn teller instability in metallic Fullerides
Bulletin of the American Physical Society, 2015Co-Authors: Naoya Iwahara, Liviu F ChibotaruAbstract: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 lower than the current estimates of $U$, meaning that dynamical Jahn-Teller effect takes place in all cubic Fullerides. This leads to strong splitting of LUMO orbitals on C$_{60}$ sites and calls for reconsideration of the role of orbital degeneracy in the Mott-Hubbard transition in Fullerides.
