Finite-temperature conductivity and magnetoconductivity of topological insulators

The electronic transport experiments on topological insulators exhibit a dilemma. A negative cusp in magnetoconductivity is widely believed as a quantum transport signature of the topological surface states, which are immune from localization and exhibit the weak antilocalization. However, the measured conductivity drops logarithmically when lowering temperature, showing a typical feature of the weak localization as in ordinary disordered metals. Here, we present a conductivity formula for massless and massive Dirac fermions as a function of magnetic field and temperature, by taking into account the electron-electron interaction and quantum interference simultaneously. The formula reconciles the dilemma by explicitly clarifying that the temperature dependence of the conductivity is dominated by the interaction while the magnetoconductivity is mainly contributed by the quantum interference. The theory paves the road to quantitatively study the transport in topological insulators and other two-dimensional Dirac-like systems, such as graphene, transition metal dichalcogenides, and silicene.Comment: 5 pages, 5 figure

Enhanced current noise correlations in a Coulomb-Majorana device

Majorana bound states (MBSs) nested in a topological nanowire are predicted to manifest nonlocal correlations in the presence of a finite energy splitting between the MBSs. However, the signal of the nonlocal correlations has not yet been detected in experiments. A possible reason is that the energy splitting is too weak and seriously affected by many system parameters. Here we investigate the charging energy induced nonlocal correlations in a hybrid device of MBSs and quantum dots. The nanowire that hosts the MBSs is assumed in proximity to a mesoscopic superconducting island with a finite charging energy. Each end of the nanowire is coupled to one lead via a quantum dot with resonant levels. With a floating superconducting island, the devices shows a negative differential conductance and giant super-Poissonian shot noise, due to the interplay between the nonlocality of the MBSs and dynamical Coulomb blockade effect. When the island is strongly coupled to a bulk superconductor, the current cross correlations at small lead chemical potentials are negative by tuning the dot energy levels. In contrast, the cross correlation is always positive in a non-Majorana setup. This difference may provide a signature for the existence of the MBSs.Comment: 11 pages, 10 figure

Extrinsic anomalous Hall conductivity of a topologically nontrivial conduction band

A key step towards dissipationless transport devices is the quantum anomalous Hall effect, which is characterized by an integer quantized Hall conductance in a ferromagnetic insulator with strong spin-orbit coupling. In this work, the anomalous Hall effect due to the impurity scattering, namely the extrinsic anomalous Hall effect, is studied when the Fermi energy crosses with the topologically nontrivial conduction band of a quantum anomalous Hall system. Two major extrinsic contributions, the side-jump and skew-scattering Hall conductivities, are calculated using the diagrammatic techniques in which both nonmagnetic and magnetic scattering are taken into account simultaneously. The side-jump Hall conductivity changes its sign at a critical sheet carrier density for the nontrivial phase, while it remains sign unchanged for the trivial phase. The critical sheet carrier densities estimated with realistic parameters lie in an experimentally accessible range. The results imply that a quantum anomalous Hall system could be identified in the good-metal regime.Comment: 5 pages, 4 figure

Detecting and Switching Magnetization of Stoner Nanograin in Non-local Spin Valve

The magnetization detection and switching of an ultrasmall Stoner nanograin in a non-local spin valve (NLSV) device is studied theoretically. With the help of the rate equations, a unified description can be presented on the same footing for the NLSV signal that reads out the magnetization, and for the switching process. The setup can be viewed as that the grain is connected to two non-magnetic leads via sequential tunneling. In one lead, the chemical potentials for spin-up and -down electrons are split due to the spin injection in the NLSV. This splitting (or the spin bias) is crucial to the NLSV signal and the critical condition to the magnetization switching. By using the standard spin diffusion equation and parameters from recent NLSV device, the magnitude of the spin bias is estimated, and found large enough to drive the magnetization switching of the cobalt nanograin reported in earlier experiments. A microscopic interpretation of NLSV signal in the sequential tunneling regime is thereby raised, which show properties due to the ultrasmall size of the grain. The dynamics at the reversal point shows that there may be a spin-polarized current instead of the anticipated pure spin current flowing during the reversal due to the electron accumulation in the floating lead used for the readout of NLSV signal.Comment: 15 pages, 11 figure

Rational Design of Advanced Functional Materials for Electrochemical Devices

In recent years, there has been a fast-growing trend in developing urea (CO(NH2)2) as a substitute H2 carrier in energy conversion due to its high energy density, nontoxicity, stability, and nonflammability. Urea, a byproduct in the metabolism of proteins and a frequent contaminant in wastewater, is an abundant compound that has demonstrated favorable characteristics as a hydrogen-rich fuel source with 6.7 wt % gravimetric hydrogen content. Also, there is 2-2.5 wt % urea from mammal urine; therefore, 0.5 million ton of additional fuels will be produced per year just from human urine (240 million ton each year). Electrochemical oxidation has been recognized as an efficient strategy for urea conversion and wastewater remediation. Thus, the chemical energy harvested from urea/urine can be converted to electricity via urea oxidation reaction (UOR). Moreover, the removal of urea from water is a priority for improving drinking water quality and presents an opportunity for UOR. However, the transition of UOR from theory and laboratory experiments to real-world applications is largely limited by the conversion efficiency, catalyst cost, and feasibility of wide-spread usage. Therefore, utilization of urea using electrochemical method is a ‘two birds with one stone’ strategy which convert wastewater to electricity via anodic urea oxidation reaction (Seen in Chapter 2). Developing efficient and low-cost urea oxidation reaction (UOR) catalysts is a promising but still challenging task for environment and energy conversion technologies such as wastewater remediation and urea electrolysis. NiO nanoparticles that incorporated graphene as the NiO@Graphene composite were constructed to study the UOR process in terms of density functional theory. The single-atom model, which differed from the previous used heterojunction model (Chapter 2), was employed for the adsorption/desorption of urea and CO2 in the alkaline media. As demonstrated from the calculated results, NiO@Graphene prefers to adsorb the hydroxyl group than urea in the initial stage due to the stronger adsorption energy of the hydroxyl group. After NiOOH@Graphene was formed in the alkaline electrolyte, it presents excellent desorption energy of CO2 in the rate-determining step. Electronic density difference and the d band center diagram further confirmed that the Ni(III) species is the most favorable site for urea oxidation while facilitating charge transfer between urea and NiO@Graphene. Moreover, graphene provides a large surface for the incorporation of NiO nanoparticles, enhancing the electron transfer between NiOOH and graphene and promoting the mass transport in the alkaline electrolyte. Notably, this work provides theoretical guidance for the electrochemical urea oxidation work (As presented in Chapter 3). In addition, urea oxidation reaction (UOR) has been known as a typical energy conversion reaction but is also a viable method for renal/liver disease diagnostic detection. Here, we reported the three-dimensional nickel oxide nanoparticles decorated on the carbonized eggshell membrane (3D NiO/c-ESM) as a modified electrode toward urea detection. The electrocatalysts are characterized by XRD, SEM, and EDX to confirm its structural and morphological information. NiO/c-ESM modified electrode exhibits an outstanding performance for urea determination with a linear range from 0.05 to 2.5 mM, and limit detection of ~20 μM (3σ). This work offered a green approach for introducing 3D nanostructure through employing biowaste ESMs as templates, providing a typical example for producing new value-added nanomaterials with urea detection (Presented in Chapter 4). Generally, urea oxidation reaction happens on the anode, less attention is paid on the cathode. In fact, hydrogen evolution reaction happens on cathode during water/urea electrolysis. Therefore, in this chapter (Chapter 5), we focus our attention on the cathodic reaction, as follows: Transition metal oxides (TMOs), especially nickel oxide (NiO), are environmentally benign and cost-effective materials, and have recently emerged as potential hydrogen evolution reaction (HER) electrocatalysts for future industrial scale water splitting in alkaline environment. However, their applications in HER electrocatalysts remain challenging because of poor electronic conductivity and unsatisfactory activity. Besides, the disposal of eggshell waste is also an environmentally and economically challenging problem because of food industry. Here, we report the synthesis of NiO nanoparticles (NPs) encapsulated in the carbonization of eggshell membrane via a green and facile approach for HER application. Noteworthy to mention here that the active carbon was made from the waste, eggshell membrane (ESM), meanwhile, the eggshell was used as a micro-reactor for preparation of electrocatalyst, NiO/C nanocomposite. Then, the as-prepared NiO/C nanocomposite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS). The SEM, EDS and TEM images reveal that NiO nanoparticles distributed on the carbon support, and XRD patterns confirm the presence of the nanoparticles are NiO and C hybrids. The catalytic activity and durability of NiO/C nanocomposite was examined for HER in 1 M KOH solution. It has been observed that NiO/C nanocomposite showed the better catalytic activity with the smallest Tafel slope of 77.8 mV dec−1 than single component\u27s result, NiO particles (112.6 mV dec−1) and carbonization of ESM (94.4 mV dec−1). It indicates that the HER performance of electrocatalyst can be enhanced by synergistic effect between NiO particles and carbonization of ESM, with better durability after 500 CV cycles. Furthermore, such design principle for developing interfaces between TMOs and C by a green and facile method can offer a new approach for preparing more efficient electrocatalysts (Seen in Chapter 5). Differed from other chapters, Chapter 4 focuses on the electroanalytical application of advanced nanomaterials. In this chapter, the sweep wave voltammetry (SWV) method was used for molecule detection. It is noted that we also developed several methods to detect small molecules, including differential pulse voltammetry (DPV) and chronopotentiometry (i-t). Therefore, several novel nanomaterials like gold nanoparticles and ZIF-8, two-dimensional nickel phthalocyanine-based metal-organic framework compounds were synthesized, respectively, and then used for the electroanalytical application, listed as Appendix A and B avoiding breaking the logistic of the whole manuscript

Quantum percolation in quantum spin Hall antidot systems

We study the influences of antidot-induced bound states on transport properties of two- dimensional quantum spin Hall insulators. The bound statesare found able to induce quantum percolation in the originally insulating bulk. At some critical antidot densities, the quantum spin Hall phase can be completely destroyed due to the maximum quantum percolation. For systems with periodic boundaries, the maximum quantum percolationbetween the bound states creates intermediate extended states in the bulk which is originally gapped and insulating. The antidot in- duced bound states plays the same role as the magnetic field inthe quantum Hall effect, both makes electrons go into cyclotron motions. We also draw an analogy between the quantum percolation phenomena in this system and that in the network models of quantum Hall effect

Quantum impurity in the bulk of topological insulator

We investigate physical properties of an Anderson impurity embedded in the bulk of a topological insulator. The slave-boson mean-field approximation is used to account for the strong electron correlation at the impurity. Different from the results of a quantum impurity on the surface of a topological insulator, we find for the band-inverted case, a Kondo resonant peak and in-gap bound states can be produced simultaneously. However, only one type of them appears for the normal case. It is shown that the mixed-valence regime is much broader in the band-inverted case, while it shrinks to a very narrow regime in the normal case. Furthermore, a self-screening of the Kondo effect may appear when the interaction between the bound-state spin and impurity spin is taken into account.Comment: 11 pages, 8 figure

Quantum Transport in Magnetic Topological Insulator Thin Films

The experimental observation of the long-sought quantum anomalous Hall effect was recently reported in magnetically doped topological insulator thin films [Chang et al., Science 340, 167 (2013)]. An intriguing observation is a rapid decrease from the quantized plateau in the Hall conductance, accompanied by a peak in the longitudinal conductance as a function of the gate voltage. Here, we present a quantum transport theory with an effective model for magnetic topological insulator thin films. The good agreement between theory and experiment reveals that the measured transport originates from a topologically nontrivial conduction band which, near its band edge, has concentrated Berry curvature and a local maximum in group velocity. The indispensable roles of the broken structure inversion and particle-hole symmetries are also revealed. The results are instructive for future experiments and transport studies based on first-principles calculations.Comment: 5 pages, 4 figure