We study conductance fluctuations in low-temperature, vertical transport through quantum Hall multilayers. The mesas studied are fabricated from a 50-period multilayer in which 150 Å GaAs wells alternate with 150 Å AlGaAs barriers that are delta-doped at their centers. We find qualitatively different temperature dependences of the variance, ␦G 2 , measured near the centers of the = 2 and = 1 quantum Hall states, with non-monotonic variation in ␦G 2 ͑T͒ at = 1. The observed temperature dependence of the correlation field is also surprising in light of theoretical predictions for fluctuations of the edge state sheath. Including the temperature dependence of the mean conductance and the effects of flux cancellation improves agreement between observed and predicted temperature dependences.The phenomenon of "universal" conductance fluctuations 1 is a well-known tool for studying electron-dephasing processes in metallic systems. These reproducible fluctuations arise in disordered systems, where electrons can take multiple paths through scattering sites, resulting in quantum interference. Varying an applied magnetic field changes the flux linked through the interfering paths. The result of this change in their phase differences is a reproducible pattern of conductance fluctuations unique to the disorder potential in the sample. The designation "universal" refers to the zerotemperature variance, ␦G 2 , of the fluctuations, which is on the order of ͑e 2 / h͒ 2 in metals. At finite temperature ͑T͒, phase coherence lengths shorten and quantum interference effects are muted as electrons dephase through inelastic phonon and electron-electron scattering. As a result, ␦G 2 in metallic systems decrease as T increases. Characterizing the behavior of ␦G 2 as a function of temperature, then, provides information on electron dephasing.Here we study the temperature dependence of reproducible conductance fluctuations in quantum Hall multilayers. The vertical transport mesas we study are bounded by an unusual 2D system, a chiral sheath of coupled edge states [ Fig. 1(a)]. This surface phase dominates vertical transport at low temperatures that freeze out parallel bulk transport. 2,3 While other groups 4-9 studied conductance fluctuations in in-plane transport in single-layer quantum Hall (QH) samples, these earlier experiments generally focused on transition regions between QH states. In contrast, our vertical transport samples allow us to study conductance fluctuations within QH states. We study smaller samples than reported on previously 10 to amplify the fluctuations relative to the mean and to move closer to the coherent limit at low T. We find a striking difference in behavior between the = 1 and =2 QH states and from the behavior of conventional metallic systems.We use 5 m, 10 m, 150 m, and 1 mm-per-side square mesas to study conductance fluctuations in the vertical conductance, G zz , at the centers of the = 1 and =2 quantum Hall states. Here, = 1 refers to the QH state for which the Fermi energy lies between the spin-split exte...
Uniaxial consolidation experiments conducted on porous A12O3 specimens with fine particle size have allowed the constitutive law to be determined. These experiments involved simultaneous measurements of load and strain, coupled with grain size determinations. The data over a wide range of stress and temperature are consistent with a power law, grain‐boundary‐climb mechanism of consolidation, at relative densities up to at least 0.9. Consolidation experiments on composite specimens have identified a spatial variation in matrix density near either a reinforcement or a hole. A finite element calculation performed using the matrix constitutive law compares well with a density map of the matrix made using image analysis.
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