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We report low-temperature electronic transport measurements performed in two multi-terminal Corbino samples formed in GaAs/Al-GaAs two-dimensional electron gases (2DEG) with both ultra-high electron mobility ($\gtrsim 20\times 10^6$ $cm^2/Vs)$ and with distinct electron density of $1.7$ and $3.6\times 10^{11}~cm^{-2}$. In both Corbino samples, a non-monotonic behavior is observed in the temperature dependence of the resistance below 1~$K$. Surprisingly, a sharp {\it decrease} in resistance is observed with {\it increasing} temperature in the sample with lower electron density, whereas an opposite behavior is observed in the sample with higher density. To investigate further, transport measurements were performed in large van der Pauw samples having identical heterostructures, and as expected they exhibit resistivity that is monotonic with temperature. Finally, we discuss the results in terms of various lengthscales leading to ballistic and hydrodynamic electronic transport, as well as a possible Gurzhi effect.

The electronic transport properties of an SNS junction formed by an InN nanowire (N) and Al contacts (S) with a superconducting transition temperature T_c ~ 0.92 K were investigated. As a function of dc bias, superconducting quasiparticle transport resonance peaks at E=2\Delta were observed, in agreement with BCS theory with 2\Delta(T=0) \equiv \Delta_0=275\mueV. Several additional transport resonances scaling linearly in energy were observed at high-bias above 2\Delta, up to E\simeq 15\Delta_0, consistent with McMillan-Rowell oscillations. The persistence of McMillan-Rowell oscillations at high-bias and under applied magnetic field were investigated.

We report a highly unusual temperature dependence in the magnetoresistance of a weakly interacting high mobility 2D electron gas (2DEG) under a parallel magnetic field and when the current is perpendicular to the field. While the linear temperature dependence below 10 K and the exponential temperature dependence above 40 K agree with existing theory of electron-phonon scattering, a field induced resistivity saturation behaviour characterized by an almost complete suppression of the temperature dependence is observed from approximately 20 to 40 K, which is in sharp contrast to the phenomenology observed when the current is parallel to the field. Possible origins of this intriguing intermediate temperature phenomenon are discussed., Comment: 5 pages, 3 figures

International audience; When a strong magnetic field is applied perpendicularly (along z) to a sheet confining electrons to two dimensions (x-y), highly correlated states emerge as a result of the interplay between electron-electron interactions, confinement and disorder. These so-called fractional quantum Hall (FQH) liquids form a series of states which ultimately give way to a periodic electron solid that crystallizes at high magnetic fields. This quantum phase of electrons has been identified previously as a disorder-pinned two-dimensional Wigner crystal with broken translational symmetry in the x-y plane. Here, we report our discovery of a new insulating quantum phase of electrons when a very high magnetic field, up to 45T, is applied in a geometry parallel (y-direction) to the two-dimensional electron sheet. Our data point towards this new quantum phase being an electron solid in a "quasi-3D" configuration induced by orbital coupling with the parallel field.

A study of the resistively detected nuclear magnetic resonance (RDNMR) lineshape in the vicinity of $��=1$ was performed on a high-mobility 2D electron gas formed in GaAs/AlGaAs. In higher Landau levels, application of an RF field at the nuclear magnetic resonance frequency coincides with an observed minimum in the longitudinal resistance, as predicted by the simple hyperfine interaction picture. Near $��=1$ however, an anomalous dispersive lineshape is observed where a resistance peak follows the usual minimum. In an effort to understand the origin of this anomalous peak we have studied the resonance under various RF and sample conditions. Interestingly, we show that the lineshape can be completely inverted by simply applying a DC current. We interpret this as evidence that the minima and maxima in the lineshape originate from two distinct mechanisms., 5 pages, 3 figures, EP2DS 17, to be published in Physica E

The specific heat of superfluid $^{3}$He, disordered by a silica aerogel, is found to have a sharp discontinuity marking the thermodynamic transition to superfluidity at a temperature reduced from that of bulk $^{3}$He. The magnitude of the discontinuity is also suppressed. This disorder effect can be understood from the Ginzburg-Landau theory which takes into account elastic quasiparticle scattering suppressing both the transition temperature and the amplitude of the order parameter. We infer that the limiting temperature dependence of the specific heat is linear at low temperatures in the disordered superfluid state, consistent with predictions of gapless excitations everywhere on the Fermi surface., Comment: accpeted for publication in Physical Review Letters

The phase diagram of the superfluid phases of $^{3}$He in 98% aerogel was determined in the range of pressure from 15 to 33 bars and for fields up to 3 kG using high-frequency sound. The superfluid transition in aerogel at 33.4 bars is field independent from 0 to 5 kG and shows no evidence of an $A_{1}-A_{2}$ splitting. The first-order transition between the A and B-phases is suppressed by a magnetic field, and exhibits strong supercooling at high pressures. We show that the equilibrium phase in zero applied field is the B-phase with at most a region of A-phase {\small $\alt$} 20 $\mu$K just below T${_c}$ at a pressure of 33.4 bars. This is in contrast to pure $^{3}$He which has a large stable region of A-phase and a polycritical point. The quadratic coefficient for magnetic field suppression of the AB-transition, $g_{a}(\beta)$, was obtained. The pressure dependence of $g_{a}(\beta)$ is markedly different from that for the pure superfluid, $g_{0}(\beta)$, which diverges at a polycritical pressure of 21 bars. We compare our results with calculations from the homogeneous scattering model for $g_{a}(\beta)$, defined in a Ginzburg-Landau theory in terms of strong-coupling parameters $\beta$. We find qualitatively good agreement with the experiment if the strong-coupling corrections are rescaled from known values of the $\beta$'s for pure $^{3}$He, reduced by the suppression of the superfluid transition temperature. The calculations indicate that the polycritical pressure in the aerogel system is displaced well above the melting pressure and out of experimental reach. We cannot account for the puzzling supercooling of the aerogel AB-transition in zero applied field within the framework of known nucleation scenarios., Comment: 10 pages, 8 figures