Microscopic Structure of the Metal-Insulator Transition in Two Dimensions

Edge Reconstruction of a Time-Reversal Invariant Insulator: Compressible-Incompressible Stripes

Two-dimensional (2D) topological electronic insulators are known to give rise to gapless edge modes, which underlie low energy dynamics, including electrical and thermal transport. This has been thoroughly investigated in the context of quantum Hall phases, and time-reversal invariant topological insulators. Here we study the edge of a 2D, topologically trivial insulating phase, as a function of the strength of the electronic interactions and the steepness of the confining potential. For sufficiently smooth confining potentials, alternating compressible and incompressible stripes appear at the edge. Our findings signal the emergence of gapless edge modes which may give rise to finite conductance at the edge. This would suggest a novel scenario of a nontopological metal-insulator transition in clean 2D systems. The incompressible stripes appear at commensurate fillings and may exhibit broken translational invariance along the edge in the form of charge density wave ordering. These are separated by structureless compressible stripes.

The Role of N-Methyl-D-Aspartate Receptor Neurotransmission and Precision Medicine in Behavioral and Psychological Symptoms of Dementia

While the world’s population is aging, the prevalence of dementia and the associated behavioral and psychological symptoms of dementia (BPSD) rises rapidly. BPSD are associated with worsening of cognitive function and poorer prognosis. No pharmacological treatment has been approved to be beneficial for BPSD to date. Dysfunction of the N-methyl-D-aspartate receptor (NMDAR)-related neurotransmission leads to cognitive impairment and behavioral changes, both of which are core symptoms of BPSD. Memantine, an NMDAR partial antagonist, is used to treat moderate to severe Alzheimer’s disease (AD). On the other hand, a D-amino acid oxidase inhibitor improved early-phase AD. Whether to enhance or to attenuate the NMDAR may depend on the phases of dementia. It will be valuable to develop biomarkers indicating the activity of NMDAR, particularly in BPSD. In addition, recent reports suggest that gender difference exists in the treatment of dementia. Selecting subpopulations of patients with BPSD who are prone to improvement with treatment would be important. We reviewed literatures regarding the treatment of BPSD, focusing on the NMDAR-related modulation and precision medicine. Future studies examining the NMDAR modulators with the aid of potential biomarkers to tailor the treatment for individualized patients with BPSD are warranted.

Long spin lifetime and large barrier polarisation in single electron transport through a CoFe nanoparticle

We have investigated single electron spin transport in individual single crystal bcc Co30Fe70 nanoparticles using scanning tunnelling microscopy with a standard tungsten tip. Particles were deposited using a gas-aggregation nanoparticle source and individually addressed as asymmetric double tunnel junctions with both a vacuum and a MgO tunnel barrier. Spectroscopy measurements on the particles show a Coulomb staircase that is correlated with the measured particle size. Field emission tunnelling effects are incorporated into standard single electron theory to model the data. This formalism allows spin-dependent parameters to be determined even though the tip is not spin-polarised. The barrier spin polarisation is very high, in excess of 84%. By variation of the resistance, several orders of magnitude of the system timescale are probed, enabling us to determine the spin relaxation time on the island. It is found to be close to 10 μs, a value much longer than previously reported.

Transport in disordered monolayer MoS2 nanoflakes--evidence for inhomogeneous charge transport

We study charge transport in a monolayer MoS2 nanoflake over a wide range of carrier density, temperature and electric bias. We find that the transport is best described by a percolating picture in which the disorder breaks translational invariance, breaking the system up into a series of puddles, rather than previous pictures in which the disorder is treated as homogeneous and uniform. Our work provides insight to a unified picture of charge transport in monolayer MoS2 nanoflakes and contributes to the development of next-generation MoS2-based devices.

Spin-droplet state of an interacting 2D electron system

We report thermodynamic magnetization measurements of two-dimensional electrons in several high-mobility Si metal-oxide-semiconductor field-effect transistors. We provide evidence for an easily polarizable electron state in a wide density range from insulating to deep into the metallic phase. The temperature and magnetic field dependence of the magnetization is consistent with the formation of large-spin droplets in the insulating phase. These droplets melt in the metallic phase with increasing density and temperature, though they survive up to large densities.

Intrinsic instability of electronic interfaces with strong Rashba coupling

We consider a model for the two-dimensional electron gas formed at the interface of oxide heterostructures, which includes a Rashba spin-orbit coupling proportional to the electric field perpendicular to the interface. Based on the standard mechanism of polarity catastrophe, we assume that the electric field has a contribution proportional to the electron density. Under these simple and general assumptions, we show that a phase separation instability (signaled by a negative compressibility) occurs for realistic values of the spin-orbit coupling and of the electronic band-structure parameters. This provides an intrinsic mechanism for the inhomogeneous phases observed at the LaAlO(3)/SrTiO(3) or LaTiO(3)/SrTiO(3) interfaces.

Inhomogenous electronic structure, transport gap, and percolation threshold in disordered bilayer graphene

The inhomogenous real-space electronic structure of gapless and gapped disordered bilayer graphene is calculated in the presence of quenched charge impurities. For gapped bilayer graphene, we find that for current experimental conditions the amplitude of the fluctuations of the screened disorder potential is of the order of (or often larger than) the intrinsic gap Δ induced by the application of a perpendicular electric field. We calculate the crossover chemical potential Δ(cr), separating the insulating regime from a percolative regime in which less than half of the area of the bilayer graphene sample is insulating. We find that most of the current experiments are in the percolative regime with Δ(cr)≪Δ. The huge suppression of Δ(cr) compared with Δ provides a possible explanation for the large difference between the theoretical band gap Δ and the experimentally extracted transport gap.

Local charge of the ν = 5/2 fractional quantum Hall state

Electrons moving in two dimensions under the influence of strong magnetic fields effectively lose their kinetic energy and display exotic behaviour dominated by Coulomb forces. When the ratio of electrons to magnetic flux quanta in the system (ν) is near 5/2, the electrons are predicted to condense into a correlated phase with fractionally charged quasiparticles and a ground-state degeneracy that grows exponentially as these quasiparticles are introduced. The only way for electrons to transform between the many ground states would be to braid the fractional excitations around each other. This property has been proposed as the basis of a fault-tolerant quantum computer. Here we present observations of localized quasiparticles at ν = 5/2, confined to puddles by disorder. Using a local electrometer to compare how quasiparticles at ν = 5/2 and ν = 7/3 charge these puddles, we were able to extract the ratio of local charges for these states. Averaged over several disorder configurations and samples, we found the ratio to be 4/3, suggesting that the local charges are = e/3 and = e/4, where e is the charge of an electron. This is in agreement with theoretical predictions for a paired state at ν = 5/2. Confirming the existence of localized e/4 quasiparticles shows that proposed interferometry experiments to test statistics and computational ability of the state at ν = 5/2 would be possible.

Evidence of a transition from nonlinear to linear screening of a two-dimensional electron system detected by photoluminescence spectroscopy

We clearly identify single-electron-localization (SEL), nonlinear screening (NLS), and linear screening (LS) regimes of gate induced electrons in a GaAs quantum well from photoluminescence spectra and intergate capacitance. Neutral and charged excitons observed in the SEL regime rapidly lose their oscillator strength when electron puddles are formed, which mark the onset of NLS. A further increase in the density of the electrons induces the transition from the NLS to LS, where the emission of a charged exciton changes to the recombination of two-dimensional electron gas and a hole.

Metal-Insulator Transition in Disordered Two-Dimensional Electron Systems

We present a theory of the metal-insulator transition in a disordered two-dimensional electron gas. A quantum critical point, separating the metallic phase, which is stabilized by electronic interactions, from the insulating phase, where disorder prevails over the electronic interactions, has been identified. The existence of the quantum critical point leads to a divergence in the density of states of the underlying collective modes at the transition, causing the thermodynamic properties to behave critically as the transition is approached. We show that the interplay of electron-electron interactions and disorder can explain the observed transport properties and the anomalous enhancement of the spin susceptibility near the metal-insulator transition.

Local electronic structure of single-walled carbon nanotubes from electrostatic force microscopy

An atomic force microscope was used to locally perturb and detect the charge density in carbon nanotubes. Changing the tip voltage varied the Fermi level in the nanotube. The local charge density increased abruptly whenever the Fermi level was swept through a van Hove singularity in the density of states, thereby coupling the cantilever's mechanical oscillations to the nanotube's local electronic properties. By using our technique to measure the local band gap of an intratube quantum-well structure, created by a nonuniform uniaxial strain, we have estimated the nanotube chiral angle. Our technique does not require attached electrodes or a specialized substrate, yielding a unique high-resolution spectroscopic tool that facilitates the comparison between local electronic structure of nanomaterials and further transport, optical, or sensing experiments.

Two-dimensional metal-insulator transition as a percolation transition in a high-mobility electron system

By carefully analyzing the low temperature density dependence of 2D conductivity in undoped high-mobility n-GaAs heterostructures, we conclude that the 2D metal-insulator transition in this 2D electron system is a density inhomogeneity driven percolation transition due to the breakdown of screening in the random charged impurity disorder background. In particular, our measured conductivity exponent of approximately 1.4 approaches the 2D percolation exponent value of 4/3 at low temperatures and our experimental data are inconsistent with there being a zero-temperature quantum critical point in our system.

Localization of fractionally charged quasi-particles

An outstanding question pertaining to the microscopic properties of the fractional quantum Hall effect is understanding the nature of the particles that participate in the localization but that do not contribute to electronic transport. By using a scanning single electron transistor, we imaged the individual localized states in the fractional quantum Hall regime and determined the charge of the localizing particles. Highlighting the symmetry between filling factors 1/3 and 2/3, our measurements show that quasi-particles with fractional charge e* = e/3 localize in space to submicrometer dimensions, where e is the electron charge.

The microscopic nature of localization in the quantum Hall effect

The quantum Hall effect arises from the interplay between localized and extended states that form when electrons, confined to two dimensions, are subject to a perpendicular magnetic field. The effect involves exact quantization of all the electronic transport properties owing to particle localization. In the conventional theory of the quantum Hall effect, strong-field localization is associated with a single-particle drift motion of electrons along contours of constant disorder potential. Transport experiments that probe the extended states in the transition regions between quantum Hall phases have been used to test both the theory and its implications for quantum Hall phase transitions. Although several experiments on highly disordered samples have affirmed the validity of the single-particle picture, other experiments and some recent theories have found deviations from the predicted universal behaviour. Here we use a scanning single-electron transistor to probe the individual localized states, which we find to be strikingly different from the predictions of single-particle theory. The states are mainly determined by Coulomb interactions, and appear only when quantization of kinetic energy limits the screening ability of electrons. We conclude that the quantum Hall effect has a greater diversity of regimes and phase transitions than predicted by the single-particle framework. Our experiments suggest a unified picture of localization in which the single-particle model is valid only in the limit of strong disorder.

Resistance noise scaling in a dilute two-dimensional hole system in GaAs

We have measured the resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well, around the 2D metal-insulator transition (MIT) at zero magnetic field. The normalized noise power S(R)/R(2) increases strongly when the hole density p(s) is decreased, increases slightly with temperature (T) at the largest densities, and decreases strongly with T at low p(s). The noise scales with the resistance, S(R)/R(2) approximately R2.4, as for a second order phase transition such as a percolation transition. The p(s) dependence of the conductivity is consistent with a critical behavior for such a transition, near a density p(*) which is lower than the observed MIT critical density p(c).

Weak-localization-like temperature-dependent conductivity of a dilute two-dimensional hole gas in a parallel magnetic field

We have studied the magnetotransport properties of a high mobility two-dimensional hole gas (2DHG) in a 10 nm GaAs quantum well with densities in the range of (0.7-1.6) x 10(10) cm(-2) on the metallic side of the zero-field "metal-insulator transition." In a parallel field well above B(c) that suppresses the metallic conductivity, the 2DHG exhibits a conductivity Delta(g)(T) approximately (1/pi) (e(2)/h)lnT reminiscent of weak localization for Fermi liquids. The experiments are consistent with the coexistence of two phases in our system: a metallic phase and a weakly insulating Fermi liquid phase.

Two-dimensional percolation and cluster structure of the random packing of binary disks

In this paper we study the short-range correlated percolation and the cluster structure of two-dimensional (2D) random packing of binary disks with size ratio lambda in the range of 1-5. A Monte Carlo simulation model is used to generate the configuration of random packing first. Then a from-neighbor-to-neighbor propagation method is used to identify the number and sizes of the clusters. Results show that for lambda=1 the percolation threshold p(c) lies between the square and triangular site percolation thresholds. As lambda increases the percolation threshold p(c) (the area fraction of small disks) decreases. To characterize the cluster structure at the percolation threshold, we scale the cluster size s(c) with the cluster radius R as s(c) proportional, variant R(D). The fractal dimension D obtained lies between 1.86 and 1.88 and is independent of the size ratio lambda. This value is in good agreement with the 2D theoretical fractal dimension which is equal to 91/48.

Droplet state and the compressibility anomaly in dilute 2D electron systems

We investigate the space distribution of carrier density and the compressibility of two-dimensional (2D) electron systems by using the local density approximation. The strong correlation is simulated by the local exchange and correlation energies. A slowly varied disorder potential is applied to simulate the disorder effect. We show that the compressibility anomaly observed in 2D systems which accompanies the metal-insulator transition can be attributed to the formation of the droplet state due to a disorder effect at low carrier densities.