P. Jiang

Spin blockade in the conduction of colloidal CdSe nanocrystal films

The conduction of thin films of n-type CdSe colloidal quantum dots is studied at low temperature and under magnetic field. At medium and high magnetic fields (10 T), the films exhibit positive magnetoresistance consistent with the variable range hopping model. At low magnetic field(<0.3 T) but in the strong electric field regime, there is a narrower magnetoresistance of order 10%-15%. The magnetoresistance shows a strong bias dependence, small and positive at low bias, increasing but still positive at higher bias, and turning negative at the highest bias. A similar behavior has been reported recently for thin film organics. Weak localization effects are ruled out. The explanation for the observations is based on spin blockade relaxed by the hyperfine interaction. The weak magnetoresistance at low bias is attributed to the diffusing paths taken by the hopping electrons. At higher bias, the more directed motion of electrons leads to increasingly positive magnetoresistance due to the more effective spin blockade. At the highest bias, the magnetoresistance becomes negative, which is attributed to the increased exchange interaction associated with the shorter tunneling distance.

Pinning-Mode Resonance of a Skyrme Crystal near Landau-Level Filling Factor ν = 1

Microwave pinning-mode resonances found around integer quantum Hall effects, are a signature of crystallized quasiparticles or holes. Application of in-plane magnetic field to these crystals, increasing the Zeeman energy, has negligible effect on the resonances just below Landau-level filling ν=2, but increases the pinning frequencies near ν=1, particularly for smaller quasiparticle or hole densities. The charge dynamics near ν=1, characteristic of a crystal order, are affected by spin, in a manner consistent with a Skyrme crystal.

Observation of a pinning mode in a Wigner solid with ν=1/3 fractional quantum Hall excitations.

We report the observation of a resonance in the microwave spectra of the real diagonal conductivities of a two-dimensional electron system within a range of ∼ ± 0.015 from filling factor ν = 1/3. The resonance is remarkably similar to resonances previously observed near integer ν, and is interpreted as the collective pinning mode of a disorder-pinned Wigner solid phase of e/3-charged carriers.

Zero-bias anomalies in narrow tunnel junctions in the quantum Hall regime

We report on the study of cleaved-edge-overgrown line junctions with a serendipitously created narrow opening in an otherwise thin, precise line barrier. Two sets of zero-bias anomalies are observed with an enhanced conductance for filling factors ν>1 and a strongly suppressed conductance for ν<1. A transition between the two behaviors is found near ν≈1. The zero-bias anomaly (ZBA) line shapes find explanation in Luttinger liquid models of tunneling between quantum Hall edge states. The ZBA for ν<1 occurs from strong backscattering induced by suppression of quasiparticle tunneling between the edge channels for the n=0 Landau levels. The ZBA for ν>1 arises from weak tunneling of quasiparticles between the n=1 edge channels.

Quantum oscillations observed in graphene at microwave frequencies

We have measured the microwave conductance of mechanically exfoliated graphene at frequencies up to 8.5 GHz. The conductance at 4.2 K exhibits quantum oscillations, and is independent of the frequency.

Interaction of water with clean and gallium precovered Fe(111) surfaces

The interaction of water with the clean Fe(111) surface and that surface modified by small coverages of Ga has been investigated. Both thermal energy atom scattering and Auger electron spectroscopy have been used to probe this interaction. Water adsorption occurs via a molecular precursor state on these surfaces, and a passivated overlayer is formed on the clean surface exposed to water at 423 K. Small amounts of Ga precoverage, from the decomposition of triethyl gallium, strongly affect the subsequent water adsorption. Preadsorbed gallium reduces the molecular water sticking coefficient and inhibits water dissociation. This effect is proportional to the Ga precoverage. Electron beam irradiation during Auger measurement also strongly affects the water adsorption on the clean and Ga modified surface, and this effect is discussed.

The adsorption of H2 on Fe(111) studied by thermal energy atom scattering

The interaction of H2 with Fe(111) has been studied by thermal energy atom scattering (TEAS). The specularly scattered He intensity as a function of hydrogen coverage exhibits a concave drop in scattered He intensity up to 30% coverage, followed by a plateau and another drop in intensity at 80% coverage. A model has been developed to account for this data which assumes three adsorption sites for H on Fe(111), in analogy with the three desorption peaks seen in temperature‐programmed desorption (TPD). The adsorption sites have not been definitely assigned, but are labeled ‘‘deep–hollow,’’ ‘‘shallow–hollow,’’ and ‘‘on‐top.’’ Competitive adsorption into the deep‐hollow and shallow‐hollow sites is assumed to account for the initial concavity of the data. Adsorption into on‐top sites only becomes significant at 80% coverage. Effective cross sections and reflectivities for the three sites have been obtained for θi=60° and 40°. When a H saturated surface is heated, the scattered He intensity decreases upon heatin...

Polymer-free patterning of graphene at sub-10-nm scale by low-energy repetitive electron beam.

A polymer-free technique for generating nanopatterns on both synthesized and exfoliated graphene sheets is proposed and demonstrated. A low-energy (5-30 keV) scanning electron beam with variable repetition rates is used to etch suspended and unsuspended graphene sheets on designed locations. The patterning mechanisms involve a defect-induced knockout process in the initial etching stage and a heat-induced curling process in a later stage. Rough pattern edges appear due to inevitable stochastic knockout of carbon atoms or graphene structure imperfection and can be smoothed by thermal annealing. By using this technique, the minimum feature sizes achieved are about 5 nm for suspended and 7 nm for unsuspended graphene. This study demonstrates a polymer-free direct nanopatterning approach for graphene.

Competing weak localization and weak antilocalization in amorphous indium–gallium–zinc-oxide thin-film transistors

We have investigated the gate-voltage dependence and the temperature dependence of the magnetoconductivity of amorphous indium–gallium–zinc-oxide thin-film transistors. A weak-localization feature is observed at small magnetic fields on top of an overall negative magnetoconductivity at higher fields. An intriguing controllable competition between weak localization and weak antilocalization is observed by tuning the gate voltage or by varying the temperature. Our findings reflect controllable quantum interference competition in the electron systems in amorphous indium–gallium–zinc-oxide thin-film transistors.

Aharonov–Bohm-like oscillations in Fabry–Perot interferometers

An experimental study of a Fabry-Perot interferometer in the quantum Hall regime reveals Aharonov-Bohm-like (ABL) oscillations. Unlike the Aharonov-Bohm effect, which has a period of one flux quantum, 80, ABL oscillations possess a flux period of 80/f , where f is the integral value of fully filled Landau levels in the constrictions. The detection of ABL oscillations is limited to the low magnetic field side of the c = 1,2,4,6,..., integer quantum Hall plateaus. These oscillations can be understood within the Coulomb- dominated model of quantum Hall interferometers as forward tunneling and backscattering, respectively, through the center of the interferometer from the bulk and the edge states.