Strong Surface Disorder Research Articles

Observation and control of the anomalous Aharonov-Bohm oscillation in enhanced-mode topological insulator nanowire field-effect transistors

Aharonov-Bohm (AB) oscillation is a quantum mechanical phenomenon which reveals the coupling of electromagnetic potentials with the electron wave function, affecting the phase of the wave function. Such a quantum interference effect can be demonstrated through the magnetotransport measurement focusing on low-dimensional electronic states. Here, we report the experimental observation of anomalous AB oscillation in an enhanced-mode topological insulator Bi2Se3 nanowire field-effect transistor (FET) under strong surface disorder, which is different from the reported AB oscillation in topological insulator nanostructures. The surrounding gate of the nanowire FET gives rise to tunability of the chemical potential and introduces strong disorder on the surface states, leading to primary oscillation with an anomalous h/e period. Furthermore, the oscillation exhibits a significant dependence on the gate voltage which has been preliminary explained with the quantization of the surface conduction channel. The experimental demonstration can be very attractive for further exploration of quantum phase interference through electrical approaches, enabling applications in future information and electromagnetic sensing technology.

High surface conductivity of Fermi-arc electrons in Weyl semimetals

Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semi--quantized anomalous Hall effect and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi arcs surface states. Here we analyze electron--phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight binding model with the presence of quenched surface vacancies using the Coherent Potential Approximation (CPA) and Kubo--Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is a factor of 50 larger of a comparable set up with surface states of TI. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material.

The Surface of Hybrid Perovskite Crystals: A Boon or Bane

Hybrid perovskite single crystals have garnered tremendous research attention and are expected to be next-generation materials for high-efficiency photoactive devices. Therefore, it is fundamentally important to understand the relationship between the optoelectronic properties of these materials and the marginally exploited surface chemistry in ambient air. For instance, a strong surface disorder, including hydration and ion migration, can possibly lead to extremely different optical and electronic properties at the surface compared to the bulk of the single crystal (SC). From this perspective, we evaluate the key variables that underlie the perovskite SC surface restructuring in ambient air and discuss their merits and limitations. In addition, a comprehensive picture of surface disordering, the remarkable change in the charge carrier dynamics and carrier mobility, surface hydration, and the effect of ion migration on the surface behavior will be discussed. Finally, surface passivation methods are highli...

Holographic treatment of boundary disorder in a topological insulator

The effect of surface disorder on electronic systems is particularly interesting for topological phases with surface and edge states. Using exact diagonalization, it has been demonstrated that the surface states of a 3D topological insulator survive strong surface disorder, and simply get pushed to a clean part of the bulk. Here we explore a new method which analytically eliminates the clean bulk, and reduces a $D$-dimensional problem to a Hamiltonian-diagonalization problem within the $(D-1)$-dimensional disordered surface. This dramatic reduction in complexity allows the analysis of significantly bigger systems than is possible with exact diagonalization. We use our method to analyze a 2D topological spin-Hall insulator with non-magnetic and magnetic edge impurities, and we calculate the probability density (or local density of states) of the zero-energy eigenstates as a function of edge-parallel momentum and layer index. Our analysis reveals that the system size needed to reach behavior in the thermodynamic limit increases with disorder. We also compute the edge conductance as a function of disorder strength, and chart a lower bound for the length scale marking the crossover to the thermodynamic limit.

Helical Majorana surface states of strongly disordered topological superconductors with time-reversal symmetry

Noncentrosymmetric superconductors with strong spin-orbit coupling and the B phase of $^{3}\mathrm{He}$ are possible realizations of topological superconductors with time-reversal symmetry. The nontrivial topology of these time-reversal invariant superconductors manifests itself at the material surface in the form of helical Majorana modes. In this paper, using extensive numerical simulations, we investigate the stability and properties of these Majorana states under strong surface disorder, which influences both bulk and surface states. To characterize the effects of strong disorder, we compute the level spacing statistics and the local density of states of both two- and three-dimensional topological superconductors. The Majorana surface states, which are located in the outermost layers of the superconductor, are protected against weak disorder due to their topological characteristic. Sufficiently strong disorder, on the other hand, partially localizes the surface layers, with a more pronounced effect on states with energies close to the gap than on those with energies close to zero. In particular, we observe that for all disorder strengths and configurations there always exist two extended states at zero energy that can carry thermal current. At the crossover from weak to strong disorder the surface state wave functions and the local density of states show signs of critical delocalization. We find that at this crossover the edge density of states of two-dimensional topological superconductors exhibits a zero-energy divergence, reminiscent of the Dyson singularity of quasi-one-dimensional dirty superconductors.

Topological Insulator Bi2Se3 Nanowire Field Effect Transistors

Topological insulator is characterized as a new class of materials which have an insulating band gap in the bulk and gapless surface state which is protected by its intrinsic time-reversal symmetry. Here, we report high-performance topological insulator nanowire field effect transistors (FETs) with sharp turn-on, clear cutoff current, large On/Off current ratio (~ 108) and well-saturated, strong inversion-mode output current-voltage characteristics. The metallic electron transport at surface, with effective mobility of 200 cm2V-1s-1 ~ 1000 cm2V-1s-1, and bulk conduction, with activation energy close to band gap of bulk Bi2Se3 can be effectively separated and tuned by gated field effect. We also report the first experimental observation of anomalous Aharonov-Bohm oscillation in Bi2Se3 nanowire FET under strong surface disorder. The magnetotransport properties demonstrated here of topological insulator nanostructure provide effective approaches of surface states modulations, and can enable more spintronics device applications and sensing applications.

Strong side of weak topological insulators

Three-dimensional topological insulators are classified into "strong" (STI) and "weak" (WTI) according to the nature of their surface states. While the surface states of the STI are topologically protected from localization, this does not hold for the WTI. In this work we show that the surface states of the WTI are actually protected from any random perturbation that does not break time-reversal symmetry, and does not close the bulk energy gap. Consequently, the conductivity of metallic surfaces in the clean system remains finite even in the presence of strong disorder of this type. In the weak disorder limit the surfaces are found to be perfect metals, and strong surface disorder only acts to push the metallic surfaces inwards. We find that the WTI differs from the STI primarily in its anisotropy, and that the anisotropy is not a sign of its weakness but rather of its richness.

Fate of topological-insulator surface states under strong disorder

Three-dimensional topological insulators feature Dirac-like surface states which are topologically protected against the influence of weak quenched disorder. Here we investigate the effect of surface disorder beyond the weak-disorder limit using large-scale numerical simulations. We find two qualitatively distinct regimes: Moderate disorder destroys the Dirac cone and induces diffusive metallic behavior at the surface. Even more remarkably, for strong surface disorder a Dirac cone reappears, as new weakly disordered ``surface'' states emerge in the sample beneath the disordered surface layer, which can be understood in terms of an interface between a topological and an Anderson insulator. Together, this demonstrates the drastic effect of disorder on topological surface states, which cannot be captured within effective two-dimensional models for the surface states alone.

Anomalous Aharonov-Bohm Conductance Oscillations from Topological Insulator Surface States

We study Aharonov-Bohm (AB) conductance oscillations arising from the surface states of a topological insulator nanowire, when a magnetic field is applied along its length. With strong surface disorder, these oscillations are predicted to have a component with anomalous period Φ(0)=hc/e, twice the conventional period. The conductance maxima are achieved at odd multiples of 1/2Φ(0), implying that a π AB phase for electrons strengthens the metallic nature of surface states. This effect is special to topological insulators, and serves as a defining transport property. A key ingredient, the surface curvature induced Berry phase, is emphasized here. We discuss similarities and differences from recent experiments on Bi2Se3 nanoribbons, and optimal conditions for observing this effect.

Strong surface disorder and loss of N produced by ion bombardment of GaN

The damage buildup in wurtzite GaN films under light (12C) and heavy (197Au) ion bombardment at temperatures from −196 to 550 °C is studied by Rutherford backscattering/channeling spectrometry. A strong surface peak of lattice disorder in addition to the expected damage peak in the region of the maximum of nuclear energy loss has been observed for all implant conditions of this study. Capping of GaN with SiOx and SixNy layers prior to implantation somewhat reduces but does not eliminate surface disordering. This suggests that nitrogen loss is not the main reason for the observed enhanced surface disorder, but, rather, the GaN surface acts as a strong sink for migrating point defects. However, pronounced loss of N during ion bombardment is observed for high dose implantation when the near-surface region is amorphized. Moreover, after amorphization, annealing at temperatures above about 400 °C leads to complete decomposition of the near-surface layer.

Basic aspects regarding irregular metal surfaces and their application in electrochemistry

The topography and other properties related to disordered metal surfaces play an important role in deposition, corrosion and growth phenomena. Metal surfaces resulting from columnar or dendritic metal deposits can be taken as models of anisotropic and isotropic strong surface disorder, respectively. The topography of rough columnar surfaces can be determined by scanning tunneling microscopy at different scales and described by means of the dynamic scaling theory. Roughness relaxation kinetics can be followed by electrochemical techniques and interpreted through a coalescence-type mechanism. Typical examples of electrochemical reactions at this type of electrodes are presented. The study of the electrochemical formation of 3D or 2D metal dendrites provides basic information about the development of either diffusion limiting aggregation-type or dense radial patterns and the design and working condition ranges. In this case, the influence of convection and, particularly, that of the relative contribution of migration and diffusion on determining the topography of the object has been established. A generalized model based upon a Monte Carlo simulation has accounted for the experimental results

EVANESCENT X-RAYS PROBING SURFACE-DOMINATED PHASE TRANSITIONS

This article reviews the scattering of evanescent x-ray waves as they occur inside a solid in the regime of total external reflection. The theoretical and experimental details of glancing angle scattering and the use of synchrotron radiation are discussed. It is shown how strong surface disorder and surface roughness affect the Bragg scattering of these exponentially damped waves and how these scattering signals can be interpreted in a straightforward way within the distorted wave Born approximation (DWBA). The application of this novel surface technique to bulk phase transitions allows the observation of surface-dominated behaviour. This is illustrated via two examples:. 1. The discontinuous order-disorder transition in Cu 3 Au has been studied near the (001) and (111) surface by evanescent x-rays. It turns out that the order parameter close to the free surface of the alloy decays in a very pronounced way upon approaching the transition temperature. The quantitative analysis of the evanescent superlattice intensity is compatible with a wetting phenomenon. 2. The critical phenomena associated with the continuous order-disorder transition in Fe 3 Al become distinctly modified due to the presence of a free surface. By applying the evanescent wave method three new universal surface exponents β1, γ11 and η‖ could be determined. This allows for the first time a critical exprimental test of surface scaling relations which turn out to be in good agreement with the experimental findings. At the surface of binary alloys surface segregation phenomena occur which complicate the experimental observation of near-surface ordering and disordering phenomena as well as the theoretical models which have to account for these effects. Some implications of these surface effects is briefly presented.

Dynamics of roughness and surface reactions at solid electrodes

Surface disorder and random roughness of electrode surfaces are discussed in terms of Euclidean and fractal models. Roughness development at different metals results from two contributions operating in opposite directions, namely shadowing between growing clusters which leads to rough surfaces, and surface diffusion phenomena which tend to smooth surface irregularities. The second contribution depends on the specific mobility of the deposited metal atoms, the temperature, and the electrolyte composition. Monte Carlo simulations based upon nucleation and growth models show that the growth processes controlled by either surface reactions or mass transport of reacting species, including surface diffusion, produce compact deposits with a weak surface disorder. Conversely, the growth process under mass transport control involving a negligible surface diffusion generates open structures with a strong surface disorder. Rough metal deposits exhibiting a fractal behaviour are considered. The fractal dimension provides a quantitative description of the degree of disorder, the growth mechanism and the reactivity of those surfaces in relation to electrocatalysis.