J Supercond Nov Magn (2010) 23: 873–875 DOI 10.1007/s10948-009-0620-4 ORIGINAL PAPER Crossover Behavior of Variable Range Hopping in Bi-Doped C 60 Jinke Tang · Corin Chepko · Wendong Wang · Xianjie Wang · Guang-Lin Zhao Received: 13 December 2009 / Accepted: 21 December 2009 / Published online: 5 January 2010 © Springer Science+Business Media, LLC 2010 Abstract Abstract: Mott variable-range hopping (VRH) is characterized by an exp[1/T 1/4 ] behavior in the temper- ature dependence of resistivity and is generally observed near the metal–insulator transition as electron screening in- creases and the Coulomb gap disappears near the metal- lic phase, while Efros–Shklovskii VRH, characterized by exp[1/T 1/2 ], is found deeper in the insulating region. We have investigated the transport properties of Bi-doped fullerene C 60 . Samples were prepared via solid state reac- tion in a sealed quartz tube near 600°C. The resistivity data can be fit with a single function exp[1/T υ ] (υ = 1/4 or 1/2, depending on the Bi concentration) over the entire temper- ature range below 300 K and over 5–6 orders of magnitude in resistivity. υ changes from 1/4 to 1/2 as the Bi concen- tration increases, suggesting a crossover from Mott VRH to intergranular tunneling at higher Bi concentration. The thermoelectric Seebeck coefficient was also measured and is about 20 μV/K at room temperature. It decreases with de- creasing temperature. The thermal conductivity of the doped samples is extremely low. Keywords Fullerene · Doped C 60 · Transport property Mott variable-range hopping (VRH) is characterized by an exp[1/T υ ] (υ = 1/4) behavior in the temperature depen- J. Tang · C. Chepko · W. Wang · X. Wang Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA G.-L. Zhao ( ) Department of Physics, Southern University and A & M College, Baton Rouge, LA 70813, USA e-mail: Guang-Lin_Zhao@subr.edu dence of resistivity and is generally observed near the metal– insulator transition as electron screening increases and the Coulomb gap disappears near the metallic phase [1], while Efros–Shklovskii VRH, characterized by exp[1/T υ ] (υ = 1/2), is found in the presence of Coulomb gap deeper in the insulating region [2]. Most of the reported experimental data fall in either of the two categories [3], although rather diverse values of υ have been found: from 0.18 to 0.7 [4–6]. Theoretically, depending on the density of states as a func- tion of energy, υ is given by υ = (n + 1)/(n + 4), where n is the exponent in the density of states and energy relationship [7, 8]. On the other hand, in a granular system where metal- lic nanoparticles are imbedded in an insulating matrix, in- tergranular tunneling leads to an exp[1/T 1/2 ] behavior in the resistance [9–11]. We have investigated the transport properties of Bi-doped fullerene C 60 . A switch-over from exp[1/T 1/4 ] to exp[1/T 1/2 ] as the Bi concentration in- creases is due to the increased amount of Bi granules formed in the C 60 matrix. C 60 samples doped with different amount of Bi were pre- pared by mixing powders of Bi and C 60 at atomic ratios of Bi:C = 1:60, 1:600 and 1:6000 and pressing them into pellets. The pellet samples were heated in a sealed quartz tube near 600°C for several days after they were preheated at about 400°C in the same sealed quartz tube to prevent the sudden increase of the vapor pressure in the quartz tube. The electrical resistivity, Seebeck coefficient and thermal con- ductivity of the samples were measured with a Quantum De- sign physical property measurement system (PPMS). Figure 1 shows the X-ray diffraction (XRD) pattern of the samples. It exhibits diffraction peaks from both C 60 and Bi. The amount of Bi phase decreases with the decreasing amount of Bi doping. It is noticed from the XRD that even at the lowest Bi-doping level (Bi:C = 1:6000) some Bi par-