G. Gervais

Two-dimensional magnetotransport in a black phosphorus naked quantum well

Black phosphorus (bP) is the second known elemental allotrope with a layered crystal structure that can be mechanically exfoliated to atomic layer thickness. Unlike metallic graphite and semi-metallic graphene, bP is a semiconductor in both bulk and few-layer form. Here we fabricate bP-naked quantum wells in a back-gated field effect transistor geometry with bP thicknesses ranging from 6±1 nm to 47±1 nm. Using a polymer encapsulant, we suppress bP oxidation and observe field effect mobilities up to 900 cm2 V−1 s−1 and on/off current ratios exceeding 105. Shubnikov-de Haas oscillations observed in magnetic fields up to 35 T reveal a 2D hole gas with Schrödinger fermion character in a surface accumulation layer. Our work demonstrates that 2D electronic structure and 2D atomic structure are independent. 2D carrier confinement can be achieved without approaching atomic layer thickness, advantageous for materials that become increasingly reactive in the few-layer limit such as bP.

Evidence for skyrmion crystallization from NMR relaxation experiments.

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate (1/T(1)) was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of (75)As is found to relax much more efficiently with T --> 0 and when a well developed quantum Hall state with R(xx) approximately 0 occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e., a Skyrmion crystal.

1D-1D Coulomb Drag Signature of a Luttinger Liquid

Observing the Upturn When two parallel conducting wires are separated by a small insulating barrier, a current in one wire can generate a net charge displacement in the other by virtue of electron-electron interactions. Some of the models for this process predict a nonmonotonic temperature dependence of the resulting Coulomb drag voltage, with an upturn occurring at a certain low temperature T*. Laroche et al. (p. 631, published online 23 January) observed this upturn in a pair of vertically integrated quantum wires separated by a 15-nanometer-wide barrier. An upturn in the temperature dependence of the drag resistance of two closely-spaced conducting wires is observed. One-dimensional (1D) interacting electronic systems exhibit distinct properties when compared to their counterparts in higher dimensions. We report Coulomb drag measurements between vertically integrated quantum wires separated by a barrier only 15 nanometers wide. The temperature dependence of the drag resistance is measured in the true 1D regime where both wires have less than one 1D subband occupied. As a function of temperature, an upturn in the drag resistance is observed below a temperature T*∼1.6 kelvin. This crossover in Coulomb drag behavior is consistent with Tomonaga-Luttinger liquid models for the 1D-1D drag between quantum wires.

Intrinsic gap of the nu=5/2 fractional quantum Hall state.

The fractional quantum Hall effect is observed at low magnetic field where the cyclotron energy is smaller than the Coulomb interaction energy. The nu=5/2 excitation gap at 2.63 T is measured to be 262+/-15 mK, similar to values obtained in samples with twice the electronic density. Examining the role of disorder on the 5/2 state, we find that a large discrepancy remains between theory and experiment for the intrinsic gap extrapolated from the infinite mobility limit. The observation of a 5/2 state in the low-field regime suggests that inclusion of nonperturbative Landau level mixing may be necessary to fully understand the energetics of half-filled fractional quantum Hall liquids.

Phase diagram of the superfluid phases of 3 He in 98% aerogel

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 ≤20 μK 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 (β), was obtained. The pressure dependence of g a (β) is markedly different from that to the pure superfluid, g 0 (β), which diverges at a polycritical pressure of 21 bars. We compare our results with calculations from the homogeneous scattering model for g a (β), defined in a Ginzburg-Landau theory in terms of strong-coupling parameters β. We find qualitatively good agreement with the experiment if the strong-coupling corrections are rescaled from known values of the βs 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.

Scattering mechanism in modulation-doped shallow two-dimensional electron gases

We report on a systematic investigation of the dominant scattering mechanism in shallow two-dimensional electron gases (2DEGs) formed in modulation-doped GaAs/AlxGa1−xAs heterostructures. The power-law exponent of the electron mobility versus density, μ∝nα, is extracted as a function of the 2DEG’s depth. When shallower than 130 nm from the surface, the power-law exponent of the 2DEG, as well as the mobility, drops from α≃1.65 (130 nm deep) to α≃1.3 (60 nm deep). Our results for shallow 2DEGs are consistent with theoretical expectations for scattering by remote dopants, in contrast to the mobility-limiting background charged impurities of deeper heterostructures.

The pathlength distribution of simulated aerogels

Abstract The diffusion limited cluster aggregation (DLCA) model is a simple algorithm that simulates the growth of silica aerogels and can be used to compare with scattering experiments. In order to study the properties of helium liquids within aerogel we have extended application of the DLCA model to lengthscales larger than the mean free path. Of particular importance are the effects of the models periodic boundary conditions. When these are accounted for the distribution function beyond a few hundred Angstroms develops into a simple exponential decay which can be extrapolated, allowing a precise determination of the mean free path, l. We find l is inversely proportional to the density, ρ , over the range investigated, 0.001 ρ

Flow Conductance of a Single Nanohole

The mass flow conductance of single nanoholes with a diameter ranging from 75 to 100 nm was measured using mass spectrometry. For all nanoholes, a smooth crossover is observed between single-particle statistical flow (effusion) and the collective viscous flow emanating from the formation of a continuum. This crossover is shown to occur when the gas mean free path matches the size of the nanohole diameter. As a consequence of the pinhole geometry, the breakdown of the Poiseuille approximation is observed in the power-law temperature exponent of the measured conductance.

Contrasting behavior of the 5/2 and 7/3 fractional quantum Hall effect in a tilted field.

Using a tilted-field geometry, the effect of an in-plane magnetic field on the even denominator nu=5/2 fractional quantum Hall state is studied. The energy gap of the nu=5/2 state is found to collapse linearly with the in-plane magnetic field above approximately 0.5 T. In contrast, a strong enhancement of the gap is observed for the nu=7/3 state. The radically distinct tilted-field behavior between the two states is discussed in terms of Zeeman and magneto-orbital coupling within the context of the proposed Moore-Read Pfaffian wave function for the 5/2 fractional quantum Hall effect.

Quantum hall effect in hydrogenated graphene

The quantum Hall effect is observed in a two-dimensional electron gas formed in millimeter-scale hydrogenated graphene, with a mobility less than 10  cm2/V·s and corresponding Ioffe-Regel disorder parameter (k(F)λ)(-1) ≫ 1. In a zero magnetic field and low temperatures, the hydrogenated graphene is insulating with a two-point resistance of the order of 250h/e2. The application of a strong magnetic field generates a negative colossal magnetoresistance, with the two-point resistance saturating within 0.5% of h/2e2 at 45 T. Our observations are consistent with the opening of an impurity-induced gap in the density of states of graphene. The interplay between electron localization by defect scattering and magnetic confinement in two-dimensional atomic crystals is discussed.