Nonadiabatic superconductivity in K3C60 and Rb3C60

We discuss the validity of Migdal–Eliashberg theory applied to the superconductor fullerides K₃C₆₀ and Rb₃C₆₀. Recently, the relevant superconductor properties have been measured, like the isotope coefficient, the energy gap and critical temperatures for these compounds and compared with their optical properties. They all present a very disperse band of phonon frequencies, running from very small to very large energies, the latter being close to the Fermi edge. Therefore, these materials exceed the limit of validity of the adiabatic Migdal theorem, measured with a nonadiabatic parameter m=w₀/E_F, where w₀ is a characteristic phonon frequency and E_F=250 meV, the Fermi level. We examine previous theories incorporating vertex corrections into the Eliashberg equations to deal with such a situation. We compare these approaches by calculating the critical temperatures using a multimodal Eliashberg spectral function ²F(w) to study the contribution of the various phononic modes. We arrive at the conclusion that the optical modes, not the intramolecular ones, are among those which maximize T_c independently of including vertex corrections or not. This result goes in the direction to understand why doped fullerides A₃C₆₀ are superconductors.     

Microwave studies of the superconducting state in Rb3C60

Methods of electron-spin resonance (ESR) and direct, non-linear field-modulated microwave absorption (FMMA) were applied for the measurements in low- and high-purity samples of rubidium-doped fullerene, Rb₃C₆₀. The coexistence of the normal strong ESR signals and regular series of weak absorption lines similar to those seen in systems of Josephson junctions was observed in the high-purity sample. The possible influence of the vortex lattice on the ESR signals was also studied. We determined from FMMA investigations using the Portis model the critical field μ₀H^* = 40 μT, the depinning current density J_c^*(μ₀H₀ = 1 mT) 4 × 10⁸ A/m² in low magnetic field and J_c^*(μ₀H₀ > 100 mT) 1.6 × 10⁸ A/m² in higher fields. These values were generally one order of magnitude higher than the highest corresponding values previously observed in high-temperature superconductors (HTS's).     

Photoelectron spectroscopy measurements of the valence band structures of polymerized thin films of C60 and La0.1C60

The electronic valence band structures of polymerized thin films of C₆₀ and La_0.1C₆₀ have been studied by using ultra-violet photoelectron spectroscopy. Additionally, the films have been characterized by using Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The valence band of the C₆₀ film shows major peaks at binding energies of 2.6, 7.2, 10.3, and 12.6 eV. In the case of the doped film, we observe (i) an additional peak with a binding energy of 13.7 eV, (ii) evidence for redistribution of the density of electronic states due to hybridization between the 5d orbitals of La and the C₆₀ cage, and (iii) significantly higher density of the electronic states near the Fermi energy. The valence band spectra of the doped film are in good agreement with recent results of the density functional theory that support strong hybridization between the d-valence orbitals of La and the C₆₀ cage.     

Rb掺杂C60单晶的相衍变和电子态

在C₆₀单晶超高真空解理面上制备C₆₀的Rb填隙化合物薄膜.用同步辐射光电子能谱研究了相衍变过程.观察到对应于固溶相、Rb₁C₆₀和Rb₃C₆₀的电子态密度分布.当数纳米厚Rb₃C₆₀薄膜在C₆₀单晶（111）解理面形成后，室温条件下进一步沉积Rb至样品表面不产生fcc到bct或bcc结构相变.C_{60 关键词： 4}C₆₀和Rb₅C₆₀吸附相')" href="#">金属性Rb₄C₆₀和Rb₅C₆₀吸附相 60单晶')" href="#">C₆₀单晶 相衍变 同步辐射光电子能谱     

过渡金属硼碳化物TM_3B_3C和TM_4B_3C_2稳定性和性能的理论计算

在过渡金属轻元素化合物中,寻找新的硬质或者超硬材料是当前的一个研究热点.目前寻找范围多集中在过渡金属硼化物、碳化物和氮化物等二元体系,三元相的研究则相对较少.本文以已知Nb_3B_3C和Nb_4B_3C_2结构为模板,采用元素替代法构建了29种TM_3B_3C (TM为过渡金属元素)结构和29种TM_4B_3C_2结构,采用基于密度泛函理论的第一性原理计算方法,成功找到了热力学、动力学以及力学都稳定的Ta_3B_3C和Ta4B_3C2两种新相.结构搜索计算确认了这两相为全局能量最优结构.能带结构和态密度的计算显示这两相均为导体,导电性主要源于Ta原子的d电子.这两种新相的硬度大约为26 GPa,说明Ta_3B_3C和Ta_4B_3C_2属于高硬度材料,但不是超硬材料.     

Electronic conduction properties of Au/C60/p-Si and C60/Au/p-Si sandwich structures: I–V and transducer characteristics

Gold-fullerite [C₆₀]-silicon (p-type) sandwich structures have been fabricated in order to investigate intrinsic cross-sectional and planar electronic conductive properties, in particular the C₆₀/p-Si p–n heterojunction. The turn-on voltage of this p–n heterojunction lies in the range 0.25–0.27 V. The I–V characteristics of the Au/C₆₀/p-Si structure are mostly defined by the bulk specific resistance of the fullerite crystal film itself (6×10⁷ Ω cm). I–V curves in the C₆₀/Au/p-Si structure are shown to be ohmic. Au/C₆₀/p-Si sandwiches irradiated with swift (300 MeV) heavy ions, (⁸⁴Kr¹⁴⁺) to a total fluence 10¹⁰ ion/cm² yield structures which are sensitive to ambient air pressure, specifically in the case of a transverse contact configuration, and if one of the contacts is located on the irradiated part of the fullerite film. The sandwich-structure sensitivity to pressure is 5×10⁻⁶ Pa⁻¹. This exceeds the sensitivity of conventional silicon pressure transducers by almost three orders of magnitude.     

Superconductivity in Alkali-Doped C60

There ark three kinds of effective attractive interaction between C₆₀^2- and "nearly free" electron in A₃C₆₀: 1) the strong short-range attraction between the low energy unoccupied orbital of C₆₀^2- ions and the "nearly free" electrons; 2) the potential barrier due to large angular momentum of t_1u orbital; 3) the A⁺ crystal field forced "nearly free" electrons in C₆₀^2- neighborhood. All effective attraction coupling with the A⁺ optical modes and the vibration modes of C₆₀ mass center produces a strong electron-phonon interaction. It plays a principal role for A₃C₆₀ superconductivity. Based upon this mechanism critical temperature and other properties of superconductivity state can be explained by means of the BCS theory.     

The surface structure of the metallic sodium tungsten bronze Na0.667WO3(001)

Scanning tunnelling microscopy has been used to identify a number of surface reconstructions on the (001) surface of the cubic metallic sodium tungsten bronze, Na_0.667WO₃. Which is dominant has been found to depend critically on sample preparation. As well as a reconstruction that bears a striking similarity to that of the parent material, tungsten trioxide, regions of (2×1) periodicity are observed that can only be explained in terms of an Na_yO surface layer. In the current work, we relate the effect of sample preparation on the surface electronic structure of Na_0.667WO₃(001) with that on the atomic structure by comparing photoemission spectra with STM images. Particular interest is focused on band gap defect states in photoemission spectra which, in contrast to similar states in spectra from WO₃, do not appear to correlate with the appearance of localised defects or highly reduced terraces in STM images. The existence of peroxide-like oxygen dimers at the (2×2) reconstructed surface, on the other hand, is characterised by the appearance of identifiable states in the valence band spectrum.     

NMR study of solid C60(γ-cyclodextrin)2

A solid complex of C₆₀ with γ-cyclodextrin (γ-CyD) was examined with NMR spectroscopic methods in order to understand the dynamics of C₆₀, and the interaction between C₆₀ and γ-CyD. A ¹³C solid-state cross-polarization magic angle spinning (CP/MAS) NMR spectra shows C₆₀ resonance at 142.6 ppm. This provides the evidence of interaction between ¹³C spins in C₆₀ and ¹H spins in the γ-CyD host. Ambient temperature experiments on the ¹³C CP/MAS NMR, with varying contact time, shows that the water associated with γ-CyDs plays an important role in the nuclear relaxation processes. The dynamics of C₆₀ in γ-CyD was investigated using temperature and field-dependent ¹³C spin-lattice relaxation time measurements. The influence of water on the dynamics of C₆₀ was less significant below 250 K.     

EPR evidence of local lattice mode in K3H(SO4)2 and Rb3H(SO4)2 fast-proton conductors

The observation of an anomalous temperature dependence of Mn²⁺ EPR spectra linewidth and nonaxial crystal-field parameter in K₃H(SO₄)₂ and Rb₃H(SO₄)₂ allows one to suggest the presence of “local mode” predicted by Yamada (Ferroelectrics 170 (1995) 23). The activation energy for this kind of excitation was found and equals 11.3 (0.5) and 7.4 (0.3) meV for Mn²⁺ doped K₃H(SO₄)₂ and Rb₃H(SO₄)₂, respectively.