Pressure effects on the CDW transitions and magnetoresistances in NbSe3

Magnetoresistance (MR) of NbSe3 has been measured between 2 K and 150 K under high pressure including critical pressures (Pc2 and Pc1) where the lower and upper CDW phase are totally suppressed by pressure, respectively. At ambient pressure, a large MR (LMR) is observed just below Tc2, but no MR is detected above Tc2. Under high pressure, in addition to the LMR, we observe newly a MR above Tc2. Here we call it the pressure-induced MR (PIMR). The LMR always appears as long as the lower CDW phase exists, but it rapidly disappears above Pc2. Similarly, the PIMR disappears above Pc1. Furthermore, no anomalies associated with the field-induced CDW are observed within our experimental limits. From these findings, we claim that the LMR and PIMR are due to normal carriers in small pockets created by a pressure-dependent imperfect nesting of the Fermi surface in the CDW transitions. (orig.)