Daniel Koller

INFRARED AND RAMAN EVIDENCE FOR DIMERS AND POLYMERS IN RBC60

The infrared- and Raman-active vibrational modes of C60 were measured in the various structural states of Rb1C60. According to earlier studies, Rb1C60 has an fcc structure at temperatures above � 100 ◦ C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled (“quenched”). We show that the spectra obtained in the polymer state are consistent with each C60 molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with Rb2(C60)2, a dimerization of C60.

Insulating and conducting phases of RbC60.

Optical measurements were performed on thin films of Rb[sub [ital x]]C[sub 60], identified by x-ray diffraction as mostly [ital x]=1 material. The samples were subjected to various heat treatments, including quenching and slow cooling from 400 K. The dramatic increase in the transmission of the quenched samples, and the relaxation towards the transmission observed in slow-cooled samples, provides direct evidence for the existence of a metastable insulating phase. Slow cooling results in a phase transition between two electrically conducting phases.

In situ infrared transmission study of Rb- and K-doped fullerenes

We have measured the four IR-active C[sub 60] molecular vibrations in [ital M][sub [ital x]]C[sub 60] ([ital M]= K,Rb) as a function of doping [ital x]. We observe discontinuous changes in the vibrational spectra showing four distinct phases (presumably [ital x]=0,3,4, and 6). The 1427-cm[sup [minus]1] and 576-cm[sup [minus]1] modes show the largest changes shifting downward in frequency in four steps as the doping increases. Several very weak modes are visible in the [ital x]=6 phase and are possibly Raman modes becoming weakly optically active. We present quantitative fits of data and calculate the electron-phonon coupling of the 1427-cm[sup [minus]1] IR mode.

Polymeric alkali fullerides are stable in air

Infrared transmission, electron spin resonance, and x‐ray diffraction measurements show unambiguously that RbC60 and KC60 are stable in air, in contrast to Rb6C60 which decomposes rapidly upon exposure. The specimens studied transform to pure C60 and other by‐products when heated above 100 °C, approximately the temperature of the orthorhombic‐fcc phase transition. The stability of these compounds raises the possibility of applying them as protective layers for the superconducting fullerides.

ENERGY GAP IN SUPERCONDUCTING FULLERIDES : OPTICAL AND TUNNELING STUDIES

Tunneling and optical transmission studies have been performed on superconducting samples of Rb{sub 3}C{sub 60}. At temperatures much below the superconducting transition temperature {ital T}{sub c}, the energy gap is 2{Delta}=5.2{plus_minus}0.2 meV, corresponding to 2{Delta}/{ital k}{sub B}{ital T}{sub c}=4.2. The low temperature density of states and the temperature dependence of the optical conductivity resemble BCS behavior, although there is an enhanced {open_quote}{open_quote}normal state{close_quote}{close_quote} contribution. The results indicate that this fulleride material is an {ital s}-wave superconductor, but the superconductivity cannot be described in the weak coupling limit. {copyright} {ital 1996 The American Physical Society.}

Infrared Spectroscopic Study of Phase Transisions in A1C60 Compounds (A = K,Rb,Cs)

Abstract Optical measurements on doped C60 films provide evidence for a transition from a high temperature conducting phase to a low temperatcure insulating phase in quenched Rb1 C60 and K1C60 compounds, while CS1C60 did not exhibit this behavior. For slow cooled samples our study confirms earlier results indicating phase separation of K1C60 to K3C60 and C60, and the formation of a new Rb1C60 phase at low temperatures. Upon slow cooling CS1 C60 behaves similar to Rb1 C60.

A chamber for the in‐situ IR measurement of C60 thin films while doping with alkali metals

A sample chamber has been designed to simultaneously measure the DC resistivity and IR transmission of C60 thin films while the films are doped with alkali metals in‐situ. The chamber construction allows a choice of windows to cover the entire IR and visible range, while x‐ray diffraction studies are also possible to determine the sample stoichiometry.