We report on a colossal negative magnetoresistance (MR) in GaAs/AlGaAs quantum well which, at low temperatures, is manifested by a drop of the resistivity by more than an order of magnitude at a magnetic field B ≈ 1 kG. In contrast to MR effects discussed earlier, the MR reported here is not parabolic, even at small B, and persists to much higher in-plane magnetic fields and temperatures. Remarkably, the temperature dependence of the resistivity at B ≈ 1 kG is linear over the entire temperature range studied (from 1 to 30 K) and appears to coincide with the high-temperature limit of the zero-field resistivity, hinting on the important role of acoustic phonons.

. (2014) Observation of microwave-induced resistance oscillations in a highmobility two-dimensional hole gas in a strained Ge/SiGe quantum well. Physical Review

We report on Hall field-induced resistance oscillations (HIRO) in a 60 nm-wide GaAs/AlGaAs quantum well with an in situ grown back gate, which allows tuning the carrier density n. At low n, when all electrons are confined to the lowest subband (SB1), the HIRO frequency, proportional to the product of the cyclotron diameter and the Hall field, scales with n −1/2 , as expected. Remarkably, population of the second subband (SB2) significantly enhances HIRO, while their frequency now scales as n −1. We demonstrate that in this two-subband regime HIRO still originate solely from backscattering of SB1 electrons. The unusual density dependence occurs because the population of SB2 steadily increases, while that of SB1 remains essentially unchanged. The enhancement of HIRO manifests an unexpected, step-like increase of the quantum lifetime of SB1 electrons, which reaches a record value of 52 ps in the two-subband regime. Continuous developments [1-8] in the molecular beam epitaxy and heterostructure design of 2D electron systems (2DES) have led to discoveries of a plethora of novel phenomena , especially in the field of low-temperature magnetotrans-port. Apart from the extremely rich quantum Hall physics in strong magnetic fields [9, 10], high-mobility 2DES display many prominent transport phenomena in low fields. Two salient examples of such phenomena are microwave-(MIRO) [11-16] and Hall field-induced resistance oscillations (HIRO) [17-26] which emerge when a 2DES is driven by microwave radiation and direct current, respectively. HIRO emerge due to elastic electron transitions between Landau levels, tilted by the Hall field, as a result of backscat-tering off short-range impurities [17, 27, 28]. The probability of these transitions is maximized each time the Hall voltage drop across the cyclotron diameter matches an integer multiple of the cyclotron energy. As a result, the differential re-sistivity acquires a 1/B-periodic correction δr which can be described by [27] δr/ρ 0 ≈ (16τ /πτ π)λ 2 cos(2πB 1 /B) , (1) where ρ 0 = m ⋆ /e 2 nτ is the resistivity at zero magnetic field B, m ⋆ ≈ 0.07m 0 is the effective mass, n is the electron density , λ = exp(−π/ω c τ q) is the Dingle factor, ω c = eB/m ⋆ is the cyclotron frequency, and τ, τ π , τ q are transport, backscat-tering, and quantum lifetimes, respectively [29]. The HIRO frequency (inverse period) B 1 is given by B 1 B ≡ eE(2R c) ω c ⇒ B 1 = 8π n m ⋆ e 2 j , (2) where E = Bj/ne is the Hall field and R c = √ 2πn/eB is the cyclotron radius. It is well known that in systems with several populated subbands, MIRO and HIRO often mix with magneto-intersubband oscillations (MISO) [30-36]. However, it is also important to examine how MIRO and HIRO are affected by the population of the second subband in the absence of such mixing. For example, capacitance measurements in a wide quantum well provided direct evidence of microwave-induced non-equilibrium redistribution of electrons between two subbands but no significant change in MIRO upon second subband population [37]. However, ...

We report on the effective mass measurements employing microwave-induced resistance oscillation in a tunable-density GaAs/AlGaAs quantum well. Our main result is a clear observation of the effective mass increase with decreasing density, in general agreement with earlier studies which investigated the density dependence of the effective mass employing Shubnikov-de Haas oscillations. This finding provides further evidence that microwave-induced resistance oscillations are sensitive to electron-electron interactions and offer a convenient and accurate way to obtain the effective mass.