Peculiarities of current–voltage characteristics of bismuth under elastic wave generation

Steady-state IVC of a monocrystalline bismuth sample of thickness d ≃ 2.7 mm in strong crossed electric and magnetic fields (E ∼ Hs/c, Ωτl = (eH/m*c) τl ≫ 1, s being the velocity of sound and τl the relaxation time for mobile charge carriers) reveal two inflections, the first one towards increasing resistance (downwards) and the second one towards decreasing resistance (upwards). The first inflection (from the smaller current side) is observed only at helium temperatures and can be related to phonon generation along the current direction I(I ‖ y, H ‖ z), and the second one to the bulk generation of phonons in a plane normal to the current (Esaki effect). This interpretation is supported by the recordings of nonsteady effects typical of electroacoustics. The longitudinal phonon generation is attributed both to high values of current density in sample areas where the electron mean free path ls ∼ υF/Ω (e.g., in the vicinity of slip and twinning planes), and to a nonuniform concentration of conduction electrons which, in turn, facilitates an increase in the local current density. The characteristic size of the current concentration region h, defined by the relation ⟨jk1⟩ ≃ nesh/d (⟨jk1⟩ is the average current density corresponding to the downward inflection) is about 2·10−3 cm and is practically independent of the magnetic field in the interval 25–140 kOe. Nonlinear IVC for Hall contacts (nondiagonal IVC) have been studied for the first time. The Ux(I) dependences measured under steady-state conditions contain sections with negative differential resistance (NDR). For T = 4.2 K, the transition to NDR correlates with the downward inflection on the diagonal IVC, while for T = 20.4, it correlates with the upward inflection. The observed phenomena are attributed to the phonon electron drag before the Esaki effect and to an increase in the magnetoresistance after the realization of this effect, respectively. At T = 4.2 K, the nonlinear component of the nondiagonal IVC shows a reversal of the sign of the Hall emf under static and dynamic conditions, which is interpreted as a manifestation of the drag effect for I>Ik1. At T = 20.4 K, acoustoelectronic vibrations of Ux with a period T ≃ d/s are recorded. Signals of the nonlinear response to two successive current pulses with a controllable relative pulse duration are studied. The first pulse leads to nonequilibrium while the second marks its state in time.

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