Linear Temperature Dependent Resistivity at Constant Volume inRb3C60

Abstract

The normal-state electrical resistivity p of the superconductor Rb3C60 has been measured as a function of temperature and hydrostatic pressure. Under conditions of constant sample volume we find a linear temperature dependence, p -T, in sharp contrast to p -T observed under conditions of constant sample pressure. The pressure dependence of p at fixed temperature is exponential, with a reduction in p by more than 50% for 8 kbar at room temperature. Implications for the normal-state scattering mechanism are discussed. PACS numbers: 72.15.Eb, 62.50.+p, 74.70.Wz The discovery of superconductivity in the alkalidoped fullerenes [1,2] has resulted in materials with the highest T, 's known for any three dimensional superconductors. Consequently, there has been an extensive theoretical and experimental effort to understand both the superconducting and normal-state properties of these compounds.As of yet, however, many fundamental questions about the nature of these materials remain unanswered.In particular, the role of electron-phonon scattering versus electron-electron correlation effects in determining the normal-state electrical transport behavior has not yet been resolved.Previous studies [3 -7] have shown that the temperature dependence of the resistivity for the doped fullerenes varies as p(T) =a+ bT from T = 100 K to well above room temperature. For a typical metal, the electron-phonon (el-ph) scattering mechanism is expected to dominate for temperatures Tẽ z (the Debye temperature) resulting in a linear temperature dependence of the resistivity. The presence of a T -like dependence could be evidence for some other scattering mechanism, e. g., electron-electron scattering. However, the combination of high and low frequency phonons may also mimic this behavior [8,9]. An important observation is that all measurements so far reported for the resistivity of doped fullerenes have been performed at constant sample pressure, p~(T), whereas most theoretical treatments assume constant sample volume, p"(T). Differences between pp(T) and p"(T) have been shown to be important in other compounds including the high T, oxide superconductors [10, 11]and organic conductors.Given that the compressibility~of the representative material Rb3C60 is comparable to the c-axis compressibility of graphite [12], one is naturally led to the question of how important the volume dependence of the resistivity in this material is.In this Letter we show that external hydrostatic pressure has a significant influence on the normal-state resistivity of Rb3C60. At fixed temperature, the resistivity decreases exponentially with increasing pressure. Remarkably, this sensitive lattice constant dependence leads to a constantvolume resistivity that varies linearly with temperature.The single-crystal R13C6p samples used in this study were prepared following the method outlined in Ref. [3].Previous studies have demonstrated [5] that the overall normalized constant-pressure resistivity p~( T) of Rb3C6p crystals near optimal doping is very reproducible from...

Colloquium: Saturation of electrical resistivity

“…Actually, the corrected data show signs of saturation. A similar correction of the data for Rb 3 C 60 (Vareka and Zettl, 1994) changed the approximately quadratic T dependence found for constant pressure (see Fig. 3) to an approximately linear dependence for constant volume.…”

Simple Band Model of Doped Fullerene Crystal

“…Experiments show that for a wide range of temperatures T the resistivity depends linearly on T, particularly in Á3C60 (cf. [5]), which cannot be attributed to scattering on classical phonons. For the same reason status of superconductivity phonon mechanism is unclear in these crystals [4].…”

Superconductivity in fullerides

“…Therefore we focus on this mechanism below. More recent experimental work (Vareka and Zettl, 1994) has suggested that ρ(T ) at constant volume is linear in T down to relatively low T (100-200 K). This raises new questions, and the resistivity can not be considered as fully understood.…”

“…9 are for constant pressure. The experimental data have also been converted to constant volume, by measuring the pressure dependence (Vareka and Zettl, 1994). The resistivity then shows an approximately linear behavior for temperatures larger than about 100 to 200 K. This linear behavior appears not to have been explained theoretically, and it is an interesting problem.…”