AlGaN ∕ GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source∕drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN∕GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of −(+)6.4×10−2mS∕bar for application of compressive (tensile) strain. The AlGaN∕GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.

The changes in the conductance of the channel of Al0.3Ga0.7N/GaN high-electron-mobility transistor structures during the application of both tensile and compressive strain were measured. For a fixed Al mole fraction, the changes in the conductance were roughly linear over the range up to 2.7×108 N cm−2, with coefficients for planar devices of −6.0+/−2.5×10−10 S N−1 m−2 for tensile strain and +9.5+/−3.5×10−10 S N−1 m−2 for compressive strain. For mesa-isolated structures, the coefficients were smaller due to the reduced effect of the AlGaN strain, with values of 5.5+/−1.1×10−13 S N−1 m−2 for tensile strain and 4.8×10−13 S N−1 m−2 for compressive strain. The large changes in the conductance demonstrate that simple AlGaN/GaN heterostructures are promising for pressure and strain sensor applications.

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We present results of magnetisation and electron paramagnetic resonance experiments on the spin-dimer system BaCuSi2O6. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian is from magnetic dipolar interactions. Axial symmetry-breaking is on a very small energy scale of ≤ 11 mK, confirming Bose Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point.