Keiko Matsubara

a- and c-axis resistivity and magnetoresistance in graphite intercalation compounds

The a-axis electrical resistivity and the transverse magnetoresistance (aT-MR ) of stage-2 to 6 graphite intercalation compounds (GICs) have been measured in the temperature (T) range between 4.2 and 300 K and magnetic field (H) range between 0 and 7 kOe. The data are analysed together with the results of c-axis resistivity and the longitudinal magnetoresistance (cL-MR ) reported in our previous work. For stage-2 to 5 GICs shows a metallic-like T dependence and exhibits no logarithmic behaviour, while shows a metallic-like behaviour for low stage (2), a logarithmic behaviour for the intermediate stages (3, 4) and a semiconductor-like behaviour for high stages (5, 6). For all stages the sign of aT-MR is positive, while the sign of cL-MR is negative for the intermediate stages (3-5) and positive for low stages in low T and weak H. The resistivity is formed of a series connection of G-I-G (G: graphite layer, I: intercalate layer) and G-G resistivity, while is formed of a parallel connection of each layer contribution. The behaviour difference between and is discussed in the light of the role of the interior G layer forming a bottleneck to the c-axis conduction. The logarithmic behaviour and negative magnetoresistance in arise from the two-dimensional weak localization occurring in these interior G layers.

Theoretical Analysis of the Electrical Resistivity of Graphite at Temperatures below 10 K

Temperature dependence of the in-plane electrical resistivity of graphite below 10 K is calculated using the Planck distribution of phonons. Theoretical curve well reproduces the observed result. Values of the electron-phonon coupling constants are re-determined so as to fit the theory to the experimental data.

Diamagnetism of Carbon Fibers

The diamagnetic susceptibilities of a polyacrylonitryle-based carbon fiber (PAN-CF) and benzene derived carbon fiber (BDF) stepwise heat-treated up to 3000°C were measured between liquid-nitrogen and room temperatures. Total susceptibility (χT) vs reciprocal temperature (1/T) plots for PAN-CF specimens heat-treated beyond 1300°C are entirely in accordance with McClure-Hickmans theory dealing with the transverse size effect on the orbital diamagnetism of π electrons in folded graphitic ribbons, while those of BDF are not for the specimens heat-treated above 2200°C. As the heat-treatment temperature is increased beyond 2200°C, the structure of BDF gradually becomes three-dimensional; finally, χT of a 3000°C-treated sample can be explained by a theory of Sharma, Johnson and McClure for singlecrystal graphite.

Theory of the a- and c-Axis Resistivity and Magnetoresistance in MoCl 5 Graphite Intercalation Compounds

The c -axis resistivity ρ c and longitudinal magnetoresistance Δρ c L /ρ 0 of MoCl 5 GICs (stage-2 to 6) have been extensively studied theoretically in terms of the following series-resistance model: \(\rho _{c}\cong \{d_{I}\rho _{c}(GIG)+d_{G}(n-1)\rho _{c}(GG)\}/I_{c}\), ρ c ( G G )=∑ i =1 n -1 ρ G G ( i , i +1)/( n -1), where n is the stage number, ρ c ( G I G ) is the partial resistivity related to graphite-intercalant-graphite sandwich layer and ρ G G ( i , i +1) corresponds to the resistivity associated with the i ↔ i +1 transition in an interior layer region without intervening intercalate layers. ρ G G ( i , i +1) makes a bottleneck in the conduction process along the c -axis, and plays an essential role in the longitudinal negative magnetoresistance for stage-4 to 6 and in the stage dependence of ρ c vs. T curve. By using the same model we can explain the metallic temperature dependence and positive magnetoresistance in the in-plane conduction which is independent of the stage number.

Theory of the g -Factor in Graphite Intercalation Compounds

Based on Blinowski-Rigauxs band model, an expression for the g -shift ( Δ g ) in stage-4 donor graphite intercalation compound has been derived by using the Dresselhaus-Dresselhaus spin-orbit Hamiltonian. If the bands are labeled as B - , A - , B + and A + in order of energy E s , g -shifts for A ± - and B ± -states are of opposite sign to each other, and in most cases | Δ g (A - )|>| Δ g (B - )|>| Δ g (B + )|>| Δ g (A + )|. Since the higher the energy E s the less the state density, the total g -shift Δ g =Σ s Σ E s Δ g ( E s )δ( E s - E F )/Σ s Σ E s δ( E s - E F ) is very small in agreement with the observations: If the Fermi level crosses the four bands Δ g becomes essentially zero, while the crossing with the three bands A - , B + and A + makes Δ g finite but still small in the magnitude. Each Δ g ( E s ) is very sensitive to the potential difference between the bounding and interior layers.

Diamagnetism of Dilute Acceptor Compounds of Graphite

Diamagnetism study on graphite-acceptor compounds has been further extended over dilute nitrates and slightly neutron-irradiated graphite. An analysis was made on the basis of Sharma-Johnson-McClures theory taking into account the trigonal warping of the Fermi surface. The susceptibility vs reciprocal termperature plots, including those for IC l compounds not completely interpreted in the previous study, have been theoretically reproduced by assuming a mixed phase structure of orderly intercalated and purely graphitic domains in each sample. The hole production rate (ionization efficiency) of acceptors was evaluated by using Ono-Sugiharas formulae which take into account the trigonal warping effect on the carrier density. It has been found that 5% of intercalated IC l is ionized in the doped regions irrespectively of the total content.

c-axis resistivity of graphite intercalation compounds

The c-axis resistivity of stage-2, 3, 4, 5, 6, 7, and 9 graphite intercalation compounds (GICs) has been measured in the temperature range between 4.2 and 300 K with and without an external magnetic field along the c-axis. In these compounds the c-axis conduction is dominated by the in-plane conduction because of the highly anisotropic resistivity. The temperature dependence of strongly depends on the stage number. The stage-2 and 3 GICs show a metallic behaviour: increasing with increasing temperature. The logarithmic behaviour in is observed in a limited temperature range for stages 5 and 6, and negative longitudinal magnetoresistance is observed for stages 3 to 7, indicating that the two-dimensional weak localization effect may occur mainly in the interior graphite layers with a small charge transfer. The c-axis resistivity of stage-3 to 9 GICs shows a unusual thermal hysteresis in the temperature range between 180 and 240 K. It has two local minima at critical temperatures - 210 K) and (= 223 - 231 K) depending on the stage number. The carriers in the bounding graphite layers with a large charge transfer are scattered by enhanced fluctuations due to electric dipole moments of molecules. There may occur a two-dimensional- (2D-) like dipole ordered phase below and a 3D-like dipole ordered phase below .

Two-Dimensional Weak Localization Effect in Stage-4 MoCl5 Graphite Intercalation Compound

Abstract The c-axis resistivity ρc(H, T) of stage-4 MoCl5 GIC between 1.9 and 50 K has been measured with and without an external magnetic field along the c axis (0≤H≤44 kOe). The interior G layers form a bottleneck to the c-axis conduction. The T and H dependence of ρc is mainly determined from that of the bottleneck resistivity proportional to the in-plane resistivity of interior graphite layers. A logarithmic behavior of ρc in the form of In(T) and In(H) indicates that the two-dimensional weak localization occurs in the interior graphite layers. The T and H dependence of the in-plane conductivity derived from ρc is discussed in terms of scaling relation predicted from the theory of two-dimensional weak localization.

Theory of the c-axis conduction and longitudinal negative magnetoresistance in MoCl5 GICs

Abstract A general theory on the c-axis resistivity ρc and longitudinal magneto-resistance δρc /ρ 0 in graphite intercalation compounds (GICs) is presented and theory provides a reasonable explanation for the observed results of MoCl5 GICs.