Multicarrier Transport and Quantum Effect in Charge-Density-Wave Material of γ-Mo_4O_(11)

摘要

Electronic properties of the orthorhombic γ-type MO_4O_(11) crystal that shows a charge-density-wave (CDW) transition at T_c = 100 K have been studied by a pulsed-laser induced "transient thermoelectric effect" (TTE) measurements under a static electric field and a magnetic field over the time range 50 ns-500 ms and temperature range 4.2-300 K. Upon application of a static electric field, the TTE voltages are enhanced and most of these signals change their sign for reversal of the field direction, which is attributable to the change in photoinduced "ambipolar states". The TTE voltages are characterized by photogenerated carrier diffusions with four relaxation times τ~i (i = 1-4), from which we have evaluated the carrier mobilities μ_i. Using our dynamic data and existing block-band model, we have self-consistently calculated the temperature dependence of the dc transport quantities, in satisfactory agreement with the experiments. When a static magnetic field is applied in the CDW state, the relaxation times τ_2 and τ_3 are found to oscillate periodically with the applied magnetic field, whose periods increase with increasing temperature. Based on the combined theories of Aharonov-Bohm (AB) and Al'tshuler-Aronov-Spivak (AAS) effects for carriers found in various "mesoscopic" systems, we have attributed the oscillatory behaviors to the quantum interference effect on photoinduced carriers passing through pinned CDW regions, which is the first evidence of the AB effect occurring in "macroscopic" material system of γ-MO_4O_(11). The difference in the electronic properties between γ- and its similar family η-type MO_4O_(11) is also discussed.