G. R. Facer

Zero length quantum wires (or point contacts) exhibit unexplained conductance structure close to $0.7\times {2e}^2/h$ in the absence of an applied magnetic field. We have studied the density- and temperature-dependent conductance of ultra low-disorder $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wires with nominal lengths $l=0,$ 0.5, and 2 $\mu \mathrm{m},$ fabricated from structures free of the disorder associated with modulation doping. In a direct comparison in zero magnetic field we observe structure near $0.7\times {2e}^2/h$ for $l=0,$ whereas the $l=2\mathrm{}$ $\mu \mathrm{m}$ wires show structure evolving with increasing electron density to $0.5\times {2e}^2/h,$ the value expected for an ideal spin-split subband. For intermediate lengths $(l=0.5\mathrm{}$ $\mu \mathrm{m})$ the feature at $0.7\times {2e}^2/h$ evolves to $0.55\times {2e}^2/h$ with increasing density. Our results suggest the dominant mechanism through which electrons interact can be strongly affected by the length of the one-dimensional region.

We report measurements of ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}$As undoped field-effect transistors in which two-dimensional electron gases (2DEG's) of exceptional quality and versatility are induced without modulation doping. Electron mobilities at $T=4.2\mathrm{}$ K and density $3\times {10}^{11}\hspace{0.5em}{\mathrm{cm}}^{-2}$ exceed $4\times {10}^6\hspace{0.5em}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. At lower temperatures, there is an unusually large drop in scattering, such that the mobility becomes too high to measure in 100-μm samples. Below $T=2.5\mathrm{}$ K, clear signatures of ballistic travel over path lengths in excess of 160 $\mu$m are observed in magnetic-focusing experiments. Multiple reflections at the edges of the 2DEG indicate a high degree of specularity.

Thin-film ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ has been studied with in-plane magnetic fields $B$ to 850 T, with good signal-to-noise to 300 T, using a GHz-transmission-line technique for electrical transport measurements in explosive-driven microsecond pulsed magnetic fields. At an ambient temperature of 1.6 K with $B\perp c$ axis we find an onset of dissipation at 150 T, the highest field reported, with a saturation of resistivity at 240 T. Comparison with the Pauli limit $B_p=155\mathrm{}$ T expected for this material suggests that the critical field is limited by spin paramagnetism.