Transverse magnetoresistance

Fig. 50. Normalized transverse magnetoresistance, Ap(H)/p(0) — [p(H, T)-p(0, Dl/pfO, T), vs. the applied magnetic field at low temperatures for YbNijBjC. The held direction is perpendicular and parallel to the c axis in (a) and (b), respectively (Yatskar et al. 1999).

It is also possible to electrodeposit multilayers in cylindrical pores of a suitable etched polymer membrane. Typically, wires with diameters of about 100 nm and length of 5-10 fim can be obtained. The deposition cycles are similar to the ones described above. Magnetoresistance [this is a term describing the relative decrease (increase) in electrical resistance of a material when subjected to a magnetic field longitudinally (transversely) to the current flow] measurements with the current perpendicular to the planes are possible. In addition, giant magnetoresistance (GMR defined below) effects may be observed as well. 

A complicated behavior of the magnetoresistance MR was found by Lacerda et al. (1996), Yatskar et al. (1999) and Christianson et al. (2001). Above 5 K, the transverse MR is negative and approximately isotropic, whereas at low T it is strongly anisotropic with respect to the crystal axes and changes its sign below 1 K for H Lc. A strong temperature dependence of the Hall coefficient Rh was reported by Narozhnyi et al. (1999b), which is in contrast with the weakly temperature-dependent Rh observed for several other borocarbides (see Section 3.4.2). 

Needle-like coke has been widely used for the industrial production of graphite electrodes for steel refining. The changes in transverse magnetoresistance (Ap/p0) with the rotation angles 9 and 9 of the magnetic held along and perpendicular to the axis of a needle-like particle (TL and T... 

The classical transverse magnetoresistance (MR) is mainly due to the bending of the charge carrier trajectory by the Lorentz force. It is proportional to the square of the field, with the proportionality constant expressed as a function of charge transport scattering time [23]. In crystalline 3D metals the dominant... 

C104 salt did obey KR approximately. We concluded [23] that there was no qualitative change in magnetoresistance even though the (inelastic) mean free path became as low as 1/250 of a lattice parameter in the c direction. This is the experimental basis for the remarks in Section III about the possible applicability of the Boltzmann formulas even when the inelastic mean free path perpendicular to the chains is very small, and the apparent absence of a minimum metallic conductivity in the transverse directions. 

Fig. 4 Comparative study of transverse magnetoresistance at low temperature (as indicated in parentheses) for highly oriented pyrolytic graphite (the dot-dash curve), typical MPCFs heat treated at different temperatures (the solid curves), VGCFs (the dashed curve), and MWNTs (the dotted curve). Vapor-grown carbon fibers exhibit a similar magnetoresistance behavior to that of MWNTs.

The most useful experimental configuration for measuring the conductivity anisotropy was with the current flow perpendicular to the b-axis. The transverse magnetoresistance could then be measured for H//b and Hib where the greatest anisotropy is expected. The angular dependence of the observed transverse magnetoresistance follows a... 

With current flow parallel to the fibers (along b-axis) a transverse magnetoresistance of the same magnitude was also observed. For... 

Fig. 22. Relative transverse magnetoresistivity Ap(B) = Apjps.o = Pb pB o)/p8-o as a function of temperature. Inset temperature T, at which p T) changes sign, as a function of magnetic field B (Remenyi et al. 1983). Lines through data points are guides to the eye.

Fig. 30. Transverse magnetoresistivity Ap against T (on logarithmic scale) for CeCu at B = 7.5 T for different pressures. Inset shows magnetic-field dependence of T the temperature at which Ap changes sign, for p = 0 and 10 kbar, respectively (Onuki and Komatsubara 1987). Lines through data points are guides to the eye.

Shubnikov-de Haas Effect and Thermoelectric Power [2e.f.o1. Below 1 K and above 8 Tesla, Shubnikov-de Haas (SdH) oscillations were observed in the transverse magnetoresistance curve (Fig. 15). This is the first observation of a SdH signal in the organic superconductors, and is the conclusive evidence of the 2D nature of this compound. The oscillation of A(l/H) = 0.0015 T- (for both H and D salts) corresponds to the area of the extremal orbit of S = 6.37 x 10 cm- from ... 

Fig. 21. Transverse magnetoresistance of (BEDT-TTF)2[KHg(SCN)4]. An unusual resistance decrease can be seen above 10 T, with a kink structure at 22 T indicated by the large arrow. The small arrows indicate Shubnikov-de Haas oscillations [34].

The transverse magnetoresistance measurements on a Type A sample showed a saturation at around 10 Tesla, and a decrease up to about 20 Tesla [34]. SdH oscillations are superposed on this background magnetoresistance above 20 Tesla (Fig. 21). The oscillation period is 0.0015 T-i. The band calculation, based on the extended Hiickel MO, shows that the Fermi surface of this salt is composed of 2D closed (hole like) and ID surfaces (Fig. 9) [23]. The area of the extremal orbit calculated from the SdH oscillation (16.5% of the first Brillouin zone) is comparable to that of the closed Fermi surface in Fig. 9 (19%). More precise measurements on the magnetoresistance and other properties are underway. 

We have studied the transverse magnetoresistance of ET CuCNCS) in order to explore its H-T phase diagram to lower temperatures and higher fields than before and especially to test whether the "two-dimensional" structure in real space is reflected in the electronic structure. 

D peak almost vanishes for true graphite fibers. Similarly, transverse magnetoresistivity (Ap/po) is positive only for true graphite fibers, e.g., for P100 and P120 mesopitch based fibers made by Amoco, but negative for all the other carbon fibers. 

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