Pressure effect on large magnetoresistance in the lower charge-density-wave transition of ${\mathrm{NbSe}}_3$

A large magnetoresistance appearing below the $T_{c2\mathrm{}}-\mathrm{CDW}$ (charge-density wave) transition temperature in ${\mathrm{NbSe}}_3$ has been studied under various pressures including pressure above the critical pressure where the $T_{c2\mathrm{}}-\mathrm{CDW}$ phase is suppressed totally. The large magnetoresistance appears as long as the $T_{c2\mathrm{}}-\mathrm{CDW}$ phase exists. However, when the phase is suppressed totally by high pressure, neither large magnetoresistance nor any anomalies are induced by the magnetic field up to 12 T, suggesting that it is negative for the formation of a magnetic-field-induced CDW. The origin of the large magnetoresistance is discussed in terms of normal carriers on the ungapped Fermi surface created on the CDW transition compared with the pressure-induced magnetoresistance observed newly above $T_{c2\mathrm{}}.$