Anisotropic low-temperature piezoresistance in (311)A GaAs two-dimensional holes

We report low-temperature resistance measurements in a modulation-doped, (311)A GaAs two-dimensional hole system as a function of applied in-plane strain. The data reveal a strong but anisotropic piezoresistance whose magnitude depends on the density as well as the direction along which the resistance is measured. At a density of $1.6\times10^{11}$ cm$^{-2}$ and for a strain of about $2\times10^{-4}$ applied along [01$\bar{1}$], e.g., the resistance measured along this direction changes by nearly a factor of two while the resistance change in the [$\bar{2}$33] direction is less than 10% and has the opposite sign. Our accurate energy band calculations indicate a pronounced and anisotropic deformation of the heavy-hole dispersion with strain, qualitatively consistent with the experimental data. The extremely anisotropic magnitude of the piezoresistance, however, lacks a quantitative explanation.Comment: 4 pages. Submitted to Applied Physics Letter

Flexible Buffer Materials to Reduce Contact Resistance in Thermal Insulation Measurements

Thermal insulation test methods approach their lower limits as thermal resistance falls below 0.1 m2⋅K/W. This is the minimum value specified in ASTM C 518 (ASTM International, 2010b) while ASTM C 177 (ASTM International, 2010a) proposes about 0.06 m2⋅K/W. Nevertheless these are the test methods, along with their ISO equivalents, required by Australasian building codes and directed at many products and materials with thermal resistance on the low side of 0.1 m2⋅K/W. Alternatives, such as ASTM E 1530 (ASTM International, 2011), cover much lower resistances but require carefully prepared small specimens and very-high contact pressures and are therefore largely unsuitable for both technical and compliance reasons. For these low resistances, the insulation test methods face large errors because of interface resistance between specimen and the apparatus hot and cold plates. Staying with C 518, the problem can be avoided by using direct measurement of the test specimen surface temperatures, but this is difficult, has its own accuracy issues, and is often impractical for commercial laboratories. This technique is generally used in conjunction with interface materials such as flexible foam between the specimen and the hot and cold plates, to enhance contact and also provide an access path for temperature sensors. The alternative prospect of using these interface materials to ensure good specimen contact has been studied, in conjunction with a simple two-step thermal resistance determination based on the difference between presence and absence of the test specimen. This article presents results of a study using this difference approach for the measurement of 12 highly conducting materials, including sheets of aluminum, phenolic, HDPE, MgO, bonded rubber and cork granules, PMMA, and compressed wood fiber. For each material, repeated measurements have been performed with four different interface or “buffer” materials: PVC, silicone, EVA, and nitrile. Silicone sponge provides the most uniform results, consistent with a measurably lower hysteresis. The difference technique yielded a lower indicated thermal resistance than direct measurement by between 0.003 and 0.01 m2⋅K/W, with some variation depending on the specimen surface characteristics and to a lesser extent on the choice of buffer. Larger differences were associated with bowed, uneven or roughly surfaced specimens. The difference-technique results have greater variability, but they may be seen as better estimates of the actual specimen resistance, as contact resistance is much lower for soft-surface interfaces. An interface resistance of up to 0.01 m2⋅K/W is large enough to be of significance in many thermal measurements

Superconducting, Insulating, and Anomalous Metallic Regimes in a Gated Two-Dimensional Semiconductor-Superconductor Array

The superconductor-insulator transition in two dimensions has been widely investigated as a paradigmatic quantum phase transition. The topic remains controversial, however, because many experiments exhibit a metallic regime with saturating low-temperature resistance, at odds with conventional theory. Here, we explore this transition in a novel, highly controllable system, a semiconductor heterostructure with epitaxial Al, patterned to form a regular array of superconducting islands connected by a gateable quantum well. Spanning nine orders of magnitude in resistance, the system exhibits regimes of superconducting, metallic, and insulating behavior, along with signatures of flux commensurability and vortex penetration. An in-plane magnetic field eliminates the metallic regime, restoring the direct superconductor-insulator transition, and improves scaling, while strongly altering the scaling exponent

Centre bow and wet-deck design for motion and load reductions in wave piercing catamarans at medium speed

For wave piercing catamarans, the centre bow length and tunnel clearance are important design factors for slamming, passenger comfort and deck diving. This experimental study determined the influence of centre bow (CB) and wet-deck geometry on their motions and loads at reduced speed using five configurations. A 2.5 m hydroelastic segmented catamaran model was tested in regular head seas in wave heights equivalent to 2.7 m, 4.0 m and 5.4 m at full scale. Higher wet-decks had higher vertical accelerations but reduced slamming loads. The greatest peak vertical CB loads ranged between 18–105% of the total hull weight. Regression models were obtained for the vertical loads and bending moments. A reduction of speed from 38 knots to 20 knots reduces the maximum slam loads by approximately 30% in regular waves. Considering both low and high speeds, the Short CB was found to be a consistent design for slamming reduction

Efficient estimation of nearly sparse many-body quantum Hamiltonians

We develop an efficient and robust approach to Hamiltonian identification for multipartite quantum systems based on the method of compressed sensing. This work demonstrates that with only O(s log(d)) experimental configurations, consisting of random local preparations and measurements, one can estimate the Hamiltonian of a d-dimensional system, provided that the Hamiltonian is nearly s-sparse in a known basis. We numerically simulate the performance of this algorithm for three- and four-body interactions in spin-coupled quantum dots and atoms in optical lattices. Furthermore, we apply the algorithm to characterize Hamiltonian fine structure and unknown system-bath interactions.Comment: 8 pages, 2 figures. Title is changed. Detailed error analysis is added. Figures are updated with additional clarifying discussion

Efficient measurement of quantum dynamics via compressive sensing

The resources required to characterise the dynamics of engineered quantum systems-such as quantum computers and quantum sensors-grow exponentially with system size. Here we adapt techniques from compressive sensing to exponentially reduce the experimental configurations required for quantum process tomography. Our method is applicable to dynamical processes that are known to be nearly-sparse in a certain basis and it can be implemented using only single-body preparations and measurements. We perform efficient, high-fidelity estimation of process matrices on an experiment attempting to implement a photonic two-qubit logic-gate. The data base is obtained under various decoherence strengths. We find that our technique is both accurate and noise robust, thus removing a key roadblock to the development and scaling of quantum technologies.Comment: New title and authors. A new experimental section. Significant rewrite of the theor

Strain-induced Fermi contour anisotropy of GaAs 2D holes

We report measurements of magneto-resistance commensurability peaks, induced by a square array of anti-dots, in GaAs (311)A two-dimensional holes as a function of applied in-plane strain. The data directly probe the shapes of the Fermi contours of the two spin subbands that are split thanks to the spin-orbit interaction and strain. The experimental results are in quantitative agreement with the predictions of accurate energy band calculations, and reveal that the majority spin-subband has a severely distorted Fermi contour whose anisotropy can be tuned with strain.Comment: Accepted for publication in Phys. Rev. Let