Strongly enhanced hole-phonon coupling in the metallic state of the dilute two-dimensional hole gas

Imaging the breaking of electrostatic dams in graphene for ballistic and viscous fluids

The charge carriers in a material can, under special circumstances, behave as a viscous fluid. In this work, we investigated such behavior by using scanning tunneling potentiometry to probe the nanometer-scale flow of electron fluids in graphene as they pass through channels defined by smooth and tunable in-plane p-n junction barriers. We observed that as the sample temperature and channel widths are increased, the electron fluid flow undergoes a Knudsen-to-Gurzhi transition from the ballistic to the viscous regime characterized by a channel conductance that exceeds the ballistic limit, as well as suppressed charge accumulation against the barriers. Our results are well modeled by finite element simulations of two-dimensional viscous current flow, and they illustrate how Fermi liquid flow evolves with carrier density, channel width, and temperature.

Phase-coherent transport in trigonal gallium nitride nanowires

Gallium nitride nanowires (GaN NWs) with triangular cross-section exhibit universal conductance fluctuations (UCF) originating from the quantum interference of electron wave functions in the NWs. The amplitude of UCF is inversely proportional to the applied bias current. The bias dependence of UCF, combined with temperature dependence of the resistance suggests that phase coherent transport dominates over normal transport in GaN NWs. A unique temperature dependence of phase-coherent length and fluctuation amplitude is associated with inelastic electron-electron scattering in NWs. The phase-coherence length extracted from the UCF is as large as 400 nm at 1.8 K, and gradually decreases as temperature increases up to 60 K.

Low temperature injected-caused charge carrier instability in n-type silicon below insulator-to-metal transition

We report on the electric transport properties of Si heavily doped with Sb at concentration just below the insulator-to-metal transition in the temperature range 1.9-3.0 K for current density J  <  0.2 A cm^-2. The change in the sign of the temperature dependence of the differential resistivity [Formula: see text] was observed: the d[Formula: see text]/dT is positive if J  <  0.045 A cm^-2 whereas it becomes negative at J  >  0.045 A cm^-2. The effect is explained assuming the exchange by electrons between the upper Hubbard band (UHB) and the conduction band. The obtained J dependencies of the activation energy, nonequilibrium concentration, mobility and scattering time of the conduction electrons correspond well to this hypothesis. The reason for charge instability is the Coulomb repulsion between electrons occupying states both in the UHB and conduction band. The estimated J dependencies of the conduction electrons lifetime and concentration of the D^- states in the UHB strongly supports this assumption.

Anisotropic Pauli Spin Blockade of Holes in a GaAs Double Quantum Dot

Electrically defined semiconductor quantum dots are attractive systems for spin manipulation and quantum information processing. Heavy-holes in both Si and GaAs are promising candidates for all-electrical spin manipulation, owing to the weak hyperfine interaction and strong spin-orbit interaction. However, it has only recently become possible to make stable quantum dots in these systems, mainly due to difficulties in device fabrication and stability. Here, we present electrical transport measurements on holes in a gate-defined double quantum dot in a GaAs/Al_xGa_1-xAs heterostructure. We observe clear Pauli spin blockade and demonstrate that the lifting of this spin blockade by an external magnetic field is highly anisotropic. Numerical calculations of heavy-hole transport through a double quantum dot in the presence of strong spin-orbit coupling show quantitative agreement with experimental results and suggest that the observed anisotropy can be explained by both the anisotropic effective hole g-factor and the surface Dresselhaus spin-orbit interaction.

Hot carriers in epitaxial graphene sheets with and without hydrogen intercalation: role of substrate coupling

The development of graphene electronic devices produced by industry relies on efficient control of heat transfer from the graphene sheet to its environment. In nanoscale devices, heat is one of the major obstacles to the operation of such devices at high frequencies. Here we have studied the transport of hot carriers in epitaxial graphene sheets on 6H-SiC (0001) substrates with and without hydrogen intercalation by driving the device into the non-equilibrium regime. Interestingly, we have demonstrated that the energy relaxation time of the device without hydrogen intercalation is two orders of magnitude shorter than that with hydrogen intercalation, suggesting application of epitaxial graphene in high-frequency devices which require outstanding heat exchange with an outside cooling source.

Qiu and Gao reply

A reply to the comment by A. Yu. Kuntsevich and V. M. Pudalov.

Connecting the reentrant insulating phase and the zero-field metal-insulator transition in a 2D hole system

We present the transport and capacitance measurements of 10 nm wide GaAs quantum wells with hole densities around the critical point of the 2D metal-insulator transition (critical density p(c) down to 0.8 × 10(10)/cm2, r(s) ∼ 36). For metallic hole density p(c) < p < p(c) + 0.15 × 10(10)/cm2, a reentrant insulating phase (RIP) is observed between the ν = 1 quantum Hall state and the zero-field metallic state and it is attributed to the formation of pinned Wigner crystal. Through studying the evolution of the RIP versus 2D hole density, we show that the RIP is incompressible and continuously connected to the zero-field insulator, suggesting a similar origin for these two phases.