Merohedral Disorder Research Articles

Merohedral disorder and impurity impacts on superconductivity of fullerenes

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb3C60 films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes.

Isotropic Three‐Dimensional Molecular Conductor Based on the Coronene Radical Cation

AbstractIn this study, we obtained the first cation radical solid of a highly symmetric (D6h) polyaromatic hydrocarbon, coronene, by electrooxidation. The (coronene)3Mo6Cl14 salt, which is formed with an Oh‐symmetric molybdenum cluster unit Mo6Cl142–, has an isotropic cubic structure with Pm$\bar {3}$m symmetry. The presence of two orientations for the coronene molecules related by an in‐plane 90° rotation (merohedral disorder) allows for fourfold symmetry along the <100> direction. The disorder has dynamic features because 2H NMR spectroscopic studies revealed that the coronene molecules undergo an in‐plane flipping motion. The observation of two motional sites with significantly different rotational rates (300 Hz and 5 MHz at 103 K) in an approximate 2:1 ratio appears to be consistent with the splitting of a Raman‐active A1g mode, confirming a random charge‐disproportionated state instead of a uniform partially‐charged state. The slower‐ and faster‐rotating species are assigned to charge‐rich and charge‐poor coronenes, respectively, with respect to C–H···Cl hydrogen bonds with neighboring Mo6Cl142– cluster units. The electrical conductivity of the salt is rather high but is well‐described by a three‐dimensional (3D) variable‐range hopping mechanism, which is possibly associated with the random charge disproportionation. These results provide a significant step forward in developing an isotropic 3D π‐conducting system composed of planar π‐conjugated molecules.

Jahn–Teller orbital glass state in the expanded fcc Cs3C60 fulleride

Magic-angle-spinning NMR measurements on fcc Cs3C60 to cryogenic temperatures reveal freezing-out of the C603¬ Jahn–Teller dynamics and emergence of an electronic orbital glass state guided by the C603¬ merohedral disorder.

Longitudinal and Hall conductances in model alkali fulleridesA3C60

We have calculated the low-temperature, low-field longitudinal and transverse conductivities for various tight-binding models intended to represent the conduction band in ${A}_{3}{\mathrm{C}}_{60}$ compounds, by directly applying the Kubo-Greenwood formula to finite clusters. It turns out that the ``universal'' dependence of Hall coefficient on lattice constant found for ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}{\mathrm{C}}_{60}$ [L. Lu et al., Phys. Rev. Lett. 74, 1637 (1995)] cannot be accounted for by appealing to two types of disorder, one of which (merohedral disorder) has an energy scale that varies strongly with lattice constant and another of which (that we model as Anderson disorder) does not. The calculations also reveal enormous violations of Matthiessen's rule: it is even possible to decrease the resistivity by introducing merohedral disorder into a system which had only Anderson disorder.

Jahn–Teller distortion and merohedral disorder of C 60−· as observed by ESR

We used single crystals of [A +(C 6H 5) 4] 2C 60 −·B − as an ideal model system to determine accurately principal values and principal axes of the g-tensor of the C 60 mono radical anion embeded in a crystal field. The experimentally observed non axial g-tensor which corresponds to the molecular Jahn–Teller distortion of C 60 − shows that the crystal field has stabilized the Jahn–Teller distortion of D 2 h symmetry along a specific molecular C 2 axis. These stabilization occurs below a characteristic temperature T s for two C 60 −· orientations (merohedral disorder) and depends on the cations as we could show by the substitution of P + for As +.

Electronic structure ofA4C60:Joint effect of electron correlation and vibronic interactions

Effects of electron correlation, intrasite vibronic interaction, and merohedral disorder on the electronic structure of ${\mathrm{K}}_{4}{\mathrm{C}}_{60}$ are investigated with a model approach taking into account all essential interactions in the lowest unoccupied molecular orbital (LUMO) band. The self-energy was calculated within the GW approximation with self-consistency after the quasiparticle Green function starting from Hartree-Fock band structure. The insulating state arises due to interorbital charge disproportionation within the LUMO band while the band gap is strongly reduced by effects of long-range electron correlation. The results of the calculations are in reasonable agreement with experiment, providing evidence for a Jahn-Teller induced transition from a Mott-Hubbard to a band insulator state.

The MEM Charge Density Study of Rb2CsC60, K2RbC60, and Li2CsC60 by Synchrotron Radiation Powder Method

A charge density study of alkali metal doped fullerenes, Rb2CsC60, K2RbC60 and Li2CsC60, was carried out at room temperature by the Maximum Entropy Method (MEM) combined with the Rietveld analysis using synchrotron powder data. The MEM charge densities of the C60 molecules show distinct differences related to the superconducting properties. For both K2RbC60 and Rb2CsC60, which are superconductors at low temperature, the C60 molecules were in a merohedral disorder. On the other hand, the C60 molecules in Li2CsC60; non-superconductor even at very low temperature, were in an orientational disorder. In the interatomic region between the C60 molecules and doped metal atoms, any significant overlapped charge densities related to covalent bond were not found for all three materials. This is the clear evidence for that superconductivity of these materials and covalency of doped metals have no correlation at all. Instead, the charge deficiencies of the doped metal atoms which should be associated with charge transfer from the metal atoms to the C60 molecule seems to have a strong correlation to the superconducting transition temperature, Tc, i.e. the higher the Tc, the more the deficiencies.

Change in band structure of orthorhombic Rb 1C 60 with the chain rotation modeling merohedral disorder

Change in the band structure of orthorhombic Rb 1C 60 (o-Rb 1C 60)) caused by the rotation of one of the two kinds of C 60 polymer chains has been examined on the basis of the tight-binding calculation including both σ and π electrons. This rotation models merohedral disorder in this substance and the band-structure analysis has indicated that the chain rotation causes drastic variation of the highest occupied (HO) bandwidth. It has been suggested that such change influences on the dimensionality of the electronic properties of o-Rb 1 C 60.

Ferromagnetic and merohedral ordering in powder and crystalline tetrakis (dimethylamino) ethylene (TDAE)-C 60

A.c. susceptometry and electron spin resonance (ESR) measurements on crystals and powders show direct evidence of coupling between spin and molecular merohedral degrees of freedom in tetrakis(dimethylamino)ethylene (TDAE)-C 60 (16 < T c < 26 K). Ordering the spins using an external field in the magnetic phase is shown to imprint a merohedral order. Conversely, merohedrally ordering the C 60 molecules at high temperature influences the low-temperature magnetic transition temperature. The merohedral disorder gives rise to a distribution of π-electron exchange interactions between spins on neighbouring C 60 molecules, suggesting a microscopic origin for the observed spin-glass-like behaviour in the magnetic state. ESR measurements on single crystals of TDAE-C 60 show a frequency shift which is maximum when the absolute value of applied field ¦ H ext¦ is along the crystallographic a axis. On the basis of the present data we suggest the material to be a ferromagnet, with the magnetic axis along a and with the dipole-dipole interactions being responsible for the large ESR line shifts and line broadening in the magnetic phase. New IR reflectivity data suggest a low-frequency excitation unobserved to date, whose origin could be a Drude peak, indicating a metallic ground state or a possible low-frequency charge density wave (CDW) excitation.

Effect of isotopic disorder on the Fu modes in crystalline C60.

The reflectance of undoped single crystals of natural ${\mathrm{C}}_{60}$ and isotopically pure $^{12}\mathrm{C}_{60}$ and $^{13}\mathrm{C}_{60}$ has been measured from 400 to 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at high resolution (0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) above and below the orientational-ordering transition. In natural ${\mathrm{C}}_{60}$, the ${\mathit{F}}_{\mathit{u}}$(1) mode shows evidence for isotopic splitting at 295 K; all the other modes are singlets. Below the orientational-ordering transition, the ${\mathit{F}}_{\mathit{u}}$(1) mode splits into five resolvable components, the ${\mathit{F}}_{\mathit{u}}$(3) and ${\mathit{F}}_{\mathit{u}}$(4) modes split into quartets, and the ${\mathit{F}}_{\mathit{u}}$(2) mode appears to remain a singlet. In the isotopically pure materials all the modes are singlets at 295 K; below the orientational-ordering transition, the ${\mathit{F}}_{\mathit{u}}$(1) mode splits into only four components; the ${\mathit{F}}_{\mathit{u}}$(3) and ${\mathit{F}}_{\mathit{u}}$(4) modes split into triplets; the ${\mathit{F}}_{\mathit{u}}$(2) mode shows only weak evidence for splitting. In those modes which display crystal-field splitting, isotopic disorder generates only a weak side band. The ${\mathit{F}}_{\mathit{u}}$(1) mode displays fine structure which may be related to the intrinsic merohedral disorder in the ground state. It is proposed that the anomalous increase in the oscillator strength of the ${\mathit{F}}_{\mathit{u}}$(4) mode is due to a modulation of the charge density along the 〈110〉 intermolecular bonding directions.

Orientational and Magnetic Ordering of Buckyballs in TDAE-C 60

Spin ordering in the low-temperature magnetic phase is directly linked to the orientational ordering of C(60) molecules in organically doped fullerene derivatives. Electron spin resonance and alternating current susceptometry measurements on tetrakis(dimethylamino)ethylene-C(60) (TDAE-C(60)) (Curie temperature T(c) = 16 kelvin) show a direct coupling between spin and merohedral degrees of freedom. This coupling was experimentally demonstrated by showing that ordering the spins in the magnetic phase imprints a merohedral order on the solid or, conversely, that merohedrally ordering the C(60) molecules influences the spin order at low temperature. The merohedral disorder gives rise to a distribution of pi-lectron exchange interactions between spins on neighboring C(60) molecules, suggesting a microscopic origin for the observed spinglass behavior of the magnetic state.

Absorption at the dipole-forbidden optical gap of crystalline C 60

We report detailed optical studies of crystalline C 60 applying several optical spectroscopic techniques such as absorption, photoluminescence, and one-photon and two-photon photoluminescence excitation spectroscopy. The energetically lowest purely electronic 0′-0 transition is found at 1.846 eV. At low temperatures, the absorption spectrum systematically exhibits doublet features with the same energetic splitting as observed for the low-temperature fluorescence spectrum. These transition pairs either represent two distinct vibration-induced transitions or only one-vibration-induced transition and the splitting is then due to the two distinct alignments of neighbouring C 60 molecules (merohedral disorder). Temperature-dependent absorption measurements show that the two molecular alignments might indeed be responsible for the double resonances.

Short-range antiferromagnetic orientational correlations in Rb3C60.

The neutron pair distribution function (PDF) of ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ at 10 K reveals local deviations from the Stephens model of merohedral disorder. The experimental PDF in the 3--7 \AA{} range is best represented by a greater fraction of near-neighbor pairs with opposite standard orientations than would be expected from purely random decoration, indicating the presence of short-range ``antiferromagnetic'' orientational correlations.

Raman fine structure in crystalline C60: The effects of merohedral disorder, isotopic substitution, and crystal field.

Raman fine structure in crystalline C60: The effects of merohedral disorder, isotopic substitution, and crystal field.

Phase transition in RbC60: Vibrational and electronic properties.

In situ Raman scattering, ultraviolet-photoemission-spectroscopy (UPS), and electron-energy-loss-spectroscopy (EELS) measurements have been performed on crystalline films of ${\mathrm{RbC}}_{60}$ through the fcc-to-orthorhombic structural phase transition. The Raman spectra exhibit large changes in intensity of modes not active for isolated icosahedra, including ungerade vibrations. This is attributed to a substantial increase in interfullerene coupling in the presence of ${\mathrm{C}}_{60}$ merohedral disorder. UPS measurements indicate a finite density of states at the Fermi energy for both phases with small changes in the ${\mathrm{C}}_{60}$ derived states through the transition. Low-energy metalliclike excitations are observed in EELS.

Orientational Disorder of C 60 in Li 2 CsC 60

The x-ray diffraction of the nonsuperconducting ternary fulleride Li 2 CsC 60 reveals at room temperature a face-centered-cubic (Fm3̅m) disordered structure that persists to a temperature of 13 Kelvin. The crystal structure is best modeled as containing quasispherical [radius of 3.556(4) angstroms] C 60 3- ions, in sharp contrast to their orientational state in superconducting face-centered-cubic K 3 C 60 (merohedral disorder) and primitive cubic Na 2 CsC 60 (orientational order). The orientational disorder of the carbon atoms on the C 60 3- sphere was analyzed with symmetry-adapted spherical-harmonic functions. Excess atomic density is evident in the 〈111〉 directions, indicating strong bonding Li + —C interactions, not encountered before in any of the superconducting alkali fullerides. The intercalate-carbon interactions and the orientational state of the fullerenes have evidently affected the superconducting pair-binding mechanism in this material.

Anomalous splitting of the F1u(-->3Fu) vibrations in single-crystal C60 below the orientational-ordering transition.

The reflectance of undoped single-crystal ${\mathrm{C}}_{60}$ has been measured from 100 to 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at high resolution above and below the orientational-ordering transition. All of the ${\mathit{F}}_{1\mathit{u}}$ modes allowed in the free molecule are observed in the solid at \ensuremath{\approxeq}526, 576, 1183, and 1430 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Below the orientational-ordering transition, all the lines split into quartets, with the exception of the 576 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ line, which narrows and remains unsplit. The appearance of quartets is attributed to the possibility of triplet splitting of the ${\mathit{F}}_{1\mathit{u}}$(-&gt;3${\mathit{F}}_{\mathit{u}}$) modes due to the local crystal field in the low-temperature phase, as well as the perturbation of the local crystal field due to the merohedral disorder, that may lead to further line splitting.

C 60 orientational ordering in superconducting Na 2RbC 60

Rietveld refinements of x-ray powder profiles show that the C 60 molecules in Na 2RbC 60 are orientationally ordered in a manner essentially the same as in pure C 60, the only fullerene-based superconductor thus far to do so. Fits to data at both 300 K and 27 K are significantly better in space group Pa 3 (simple cubic) than in Fm 3 m (merohedral disorder) or by assuming uniform spherical shells of charge (complete disorder). The anomalously low T c of this compound cannot be attributed to excess orientational disorder, and therefore remains a mystery.