G.V.Tikhomirova, A.N.Babushkin
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Low Temperature Physics Dept, Ural State University, Ekaterinburg
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Resistivity of fullerene and graphite in the temperature range from 77 to 450 K were studied at pressures up to 50 GPa.
The measurements were carried out in a high pressure cell with "rounded cone-plane" type diamond dies produced from a synthetic polycrystalline black diamond "carbonado". The estimation of pressure was proved for different materials in a wide range of temperatures and pressures.
A hysteresis in the pressure dependence of resistivity of C 60 was observed at room temperature during successive cycles of increasing and decreasing pressure in the range from 15 to 50 GPa, which is likely a result of metastable states arised due to stress. The hysteresis was not observed in graphite.
The character of the temperature dependences of resistivity of fullerene depends on a preceding stress applications. At initial putting samples under stress, the conductivity of C 60 at pressure up to 44 GPa is of activation character. At the pressure of 45.5 GPa, a decrease in the resistivity by two orders of magnitude was observed. At the temperatures T above 115 to 125 K (for different pressures), the temperature dependence of conductivity becomes metallike. After the next application of stress, the temperature dependence of conductivity turns out to be of activation character in all range of temperatures used showing two different activation energies, below 325 K and above 345 K, respectively.
The temperature dependences of resistivity of graphite under high pressures at the temperatures below 340 K are of the same character as for fullerene. There are two different activation energies, which change one to another at some T. However, the resistivity value for both materials is different by many orders of magnitude. This shows that both fulleren and graphite keep some features of their original microscopic structure even at high pressures.
This work was supported, in part, by CRDF grant N REC-005 for Ural Center of Research and Education "Perspective Materials".

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