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The contactless surface acoustic wave technique is implemented to probe the high-frequency conductivity of a high-mobility p-Ge/GeSi quantum well structure in the regime of integer quantum Hall effect (IQHE) at temperatures 0.3-5.8 K and magnetic fields up to 18 T. It is shown that, in the IQHE regime at the minima of conductivity, holes are localized and ac conductivity is of hopping nature and can be described within the "two-site" model. The analysis of the temperature and magnetic-field-orientation dependence of the ac conductivity at odd filing factors enables us to determine the effective hole g-factor, |gzz| ≈4.5. It is shown that the in-plane component of the magnetic field leads to a decrease in the g-factor as well as increase in the cyclotron mass, which is explained by orbital effects in the complex valence band of germanium. [ABSTRACT FROM AUTHOR]

We report the results of a study of the magnetoresistance ρxx in dilute p-Si/SiGe/Si quantum well in the magnetic fields of up to 18 T, parallel to the two-dimensional channel plane. The observed effects can be explained by the influence of the in-plane magnetic field on the orbital motion of the charge carriers in the quasi-2D system. [ABSTRACT FROM AUTHOR]

The interaction of surface acoustic waves (SAW) with the Si “underdoped” n-GaAs/AlGaAs heterostructure with high photosensitivity has been studied. In the unexposed state, the 2D channel conductivity at T=4.2 K was less than 10-8 Ohm-1. Under the exposure of the LED light it was possible to increase the conductivity by 5 orders of magnitude, up to 10-3 Ohm-1. Conductivity dependence studies of the acousto-electric effects in the system allowed to elucidate the percolation nature of metal-insulator transition. An unusual property of the system was also its room-temperature sensitivity to the light. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Low-temperature conductance mechanism in low-density Si/SiGe heterostructures in high magnetic fields (ultra-quantum limit) is studied using AC and DC transport measurements. Evidence of the Wigner crystal in this regime is given. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Complex AC-conductance, σAC, in the systems with dense Ge0.7Si0.3 quantum dot (QD) arrays in Si has been determined from simultaneous measurements of attenuation, ΔΓ=Γ(H)-Γ(0), and velocity, ΔV/V(H)≡[V(H)-V(0)]/V(0), of surface acoustic waves (SAW) with frequencies f = 30–300 MHz as functions of transverse magnetic field H ≤ 18 T in the temperature range T = 1–20 K. It has been shown that in the sample with dopant (B) concentration 8.2×1011 cm-2 at temperatures T ≤ 4 K the AC conductivity is dominated by hopping between states localized in different QDs. The observed power-law temperature dependence, σ1(H=0)∼T2.4, and weak frequency dependence, σ1(H=0)∼ω0, of the AC conductivity are consistent with predictions of the two-site model for AC hopping conductivity for the case of ωτ0>>1, where ω=2πf is the SAW angular frequency and τ0 is the typical population relaxation time. At T > 7 K the AC conductivity is due to thermal activation of the carriers (holes) to the mobility edge. In intermediate temperature region 4 < T < 7 K, where AC conductivity is due to a combination of hops between QDs and diffusion on the mobility edge, one succeeded to separate both contributions. Temperature dependence of hopping contribution to the conductivity above T*∼4.5 K saturates, evidencing crossover to the regime where ωτ0 < 1. From crossover condition, ωτ0(T*) = 1, the typical value, τ0, of the relaxation time has been determined. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Dense (n = 3 × 1011 cm-2) arrays of Ge quantum dots (QDs) in Si host were studied using surface acoustic wave (SAW) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ΔΓ = Γ(ω, H) - Γ(ω, 0), and change of the SAW velocity, ΔV/V = [V(H) - V(0)]/V(0), were measured in the temperature interval T = 2–4.2 K as functions of magnetic field H up to 6 T for the waves in the frequency range f = 30 – 300 MHz. Basing on the dependences of ΔΓ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is hopping between the states localized in different QDs. The measured magnetic field dependence of the SAW attenuation is discussed basing on existing theoretical concepts. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]