Specific features of transport phenomena in gapless semiconductors which are due to both peculiarities of the electron spectrum of an ideal crystal and those of the impurity states in these semiconductors, are discussed. A detailed critical analysis is made of a large number of experimental papers dealing with studies of the conductivity, Hall effect, and magnetores]stance in HgCdTe crystals as a function of temperature, pressure, and magnetic field. On the strength of this analysis it is concluded that the perturbing action of the potential of acceptor-type impurities leads to effective overlap of the valence band with the conduction band. As a result, when the impurity concentration is sufficiently high, and exceeds that at which the Mott-Anderson transition occurs, the density of states of a p-type zero-gap semiconductor differs substantially from that of an ideal crystal, and is similar to that of a semimetal.

We have measured the temperature (2.9 K < T < 50 K) and magnetic field (0 T < B < 9 T) dependences of longitudinal and Hall resistivities for HgCdTe/HgTe/HgCdTe system with HgTe quantum well width of 20.3 nm. The activation analysis of the experimental magnetoresistivity traces has been used as a quantitative tool to probe inter-Landau level distances. The activation energies were determined from the temperature dependence of the longitudinal resistivity in the regions of quantized Hall plateaus (for the filling factors ν of 1, 2 and 3) and the indications of the large values of the g factor ≅ 30-75 were found.

The 2D weak localization effects at low temperatures T = (0.2 ÷ 4.2) K have been investigated in nonsuperconducting sample Nd1.88Ce0.12CuO 4−δ and in the normal state of the superconducting sample Nd1.82Ce0.18CuO 4−δ for B > Bc2 ≃ 3 T . The phase coherence time τϕ(≃ 5 · 10 −11 s at 1.9 K) and the effective thickness of a conducting CuO2 layer d(≃ 1.5Å) have been estimated by the fitting of 2D weak localization theory expressions to the magnetoresistivity data for the normal to plane and the in-plane magnetic fields. The estimation of the parameter d ensures the condition of a strong carrier confinement and makes a basis to the model of almost decoupled 2D metallic sheets for the Nd2−xCexCuO 4−δ single crystals.

Transport properties of Nd 2−x Ce x CuO 4+δ single crystal films (B c, J ab) are investigated in magnetic fields B up to 9T at T =(0.4-4.2)K. An analysis of normal state (at B > B c2 ) Hall coefficient R H n dependence on Ce doping takes us to a conclusion about the existence both of electron-like and hole-like contributions to transport in nominally electron-doped system. In accordance with R H n (x) analysis an anomalous sign reversal of Hall effect in mixed state at B < B c2may be ascribed to a flux-flow regime for two types of carriers with opposite charges.