Showing papers on "Charge transfer insulators published in 2017"
TL;DR: In this paper, the evolution of the electronic states of (Sr_(1−x)La_x)_2IrO_4, which is an effective spin-1/2 Mott insulator like the cuprates, but is chemically radically different.
Abstract: It is widely believed that high-temperature superconductivity in the cuprates emerges from doped Mott insulators. When extra carriers are inserted into the parent state, the electrons become mobile but the strong correlations from the Mott state are thought to survive—inhomogeneous electronic order, a mysterious pseudogap and, eventually, superconductivity appear. How the insertion of dopant atoms drives this evolution is not known, nor is whether these phenomena are mere distractions specific to hole-doped cuprates or represent genuine physics of doped Mott insulators. Here we visualize the evolution of the electronic states of (Sr_(1−x)La_x)_2IrO_4, which is an effective spin-1/2 Mott insulator like the cuprates, but is chemically radically different. Using spectroscopic-imaging scanning tunnelling microscopy (SI-STM), we find that for a doping concentration of x ≈ 5%, an inhomogeneous, phase-separated state emerges, with the nucleation of pseudogap puddles around clusters of dopant atoms. Within these puddles, we observe the same iconic electronic order that is seen in underdoped cuprates. We investigate the genesis of this state and find evidence at low doping for deeply trapped carriers, leading to fully gapped spectra, which abruptly collapse at a threshold of x ≈ 4%. Our results clarify the melting of the Mott state, and establish phase separation and electronic order as generic features of doped Mott insulators.
93 citations
TL;DR: In this article, the heat transfer surface charge accumulation model of operating DC-GIL was developed, including the nonlinear relationship between volume current in gas and electric field, and the space charge was also considered in the model.
Abstract: With this expected future advance of HVDC, the use of gas insulated transmission lines (GIL) for dc application are getting increasingly interesting. For now, the problem of surface charge accumulation on gas-insulator interface is one of the critical factors for the development of DC-GIL. In many previous works, the model of surface charge accumulation on insulator was investigated. However, the quantitative relationship between temperature and surface charge accumulation on insulator was not exactly obtained since the lack of complicated heat transfer progress in the model. In this paper, the heat transfer surface charge accumulation model of operating DC-GIL was developed, including the nonlinear relationship between volume current in gas and electric field. Moreover, the space charge was also considered in the model. Based on the developed model, temperature distributions in DC-GIL insulator under different current are obtained. Afterwards, the temperature impact on space charge density in the insulator, the saturation time of surface charge accumulation, the surface charge on the insulator surface, and the electric field distribution on the insulator were investigated. It was proven that the tangential component of the electric field reaches to 5.3 kV/mm on lower interface and 5.0 kV/mm on upper interface for Ti=378 K. This value increase 17.8% on lower interface and 17.6% on upper interface along with the conductor temperature from 298 K to 378 K. The data can be referred in the insulation design of DC-GIL.
78 citations
TL;DR: The instability of the quantum anomalous Hall (QAH) effect has been studied as a function of the electric current and temperature in ferromagnetic topological insulator thin films and a characteristic current is found, indicating that the Hall electric field is relevant to the breakdown.
Abstract: The instability of the quantum anomalous Hall (QAH) effect has been studied as a function of the electric current and temperature in ferromagnetic topological insulator thin films We find that a characteristic current for the breakdown of the QAH effect is roughly proportional to the Hall-bar width, indicating that the Hall electric field is relevant to the breakdown We also find that electron transport is dominated by variable range hopping (VRH) at low temperatures Combining the current and temperature dependences of the conductivity in the VRH regime, the localization length of the QAH state is evaluated to be about $5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ The long localization length suggests a marginally insulating nature of the QAH state due to a large number of in-gap states
30 citations
TL;DR: In this paper, the authors provide a summary of charge distribution measurement within and on the surface of insulators along with a new approach to measure the space charge distribution on surface of glass insulator strings.
Abstract: A reliability of electrical energy supply could be disrupted by the build-up of space charge distribution within or on the surface of insulator. Preventive measures have been carried out since the earliest days of power transmission network invented till today to alleviate the space charge effect on electrical energy systems. Although the preventive measures have been taken greatly, there are inevitable factors especially environmental conditions that lead to the accumulation of space charge. Space charge which carries energy such as kinetic, heat and other forms is believed released these energies during the space charge accumulation process and subsequently distort the local electric field. Despite the numerous papers published on space charge investigation in polymeric insulation, there is an absence of thorough review papers on this topic as well as space charge effects on glass insulation. Hence, this paper provides a summary of charge distribution measurement within and on the surface of insulators along with a new approach to measure the space charge distribution on the surface of glass insulator strings. This knowledge will provide a contribution to the development of a new online space charge distribution measurement system for glass insulator string that will be more efficient, by utilizing the available technology for data transmission. This new approach also has the capability to monitor the condition of the insulator while energized and in service, thus the faulty insulators can be replaced without disrupting the distribution of electrical energy around the world.
9 citations
TL;DR: In this paper, the authors presented the charge distributions measurement on the surfaces of glass insulators string without grading ring subject to clean and contaminated conditions, and the experimental results revealed that the distribution of charge for glass insulator recently removed from service has negative distribution pattern.
7 citations
TL;DR: This work finds that there is a dynamical mass generation for the excitonic states that preserves time-reversal symmetry and is related to the dynamical chiral-symmetry breaking of the model.
Abstract: In this work, we analyze the excitonic gap generation in the strong-coupling regime of thin films of three-dimensional time-reversal-invariant topological insulators. We start by writing down the effective gauge theory in 2 + 1-dimensions from the projection of the 3 + 1-dimensional quantum electrodynamics. Within this method, we obtain a short-range interaction, which has the form of a Thirring-like term, and a long-range one. The interaction between the two surface states of the material induces an excitonic gap. By using the large-N approximation in the strong-coupling limit, we find that there is a dynamical mass generation for the excitonic states that preserves time-reversal symmetry and is related to the dynamical chiral-symmetry breaking of our model. This symmetry breaking occurs only for values of the fermion-flavor number smaller than [Formula: see text]. Our results show that the inclusion of full dynamical interaction strongly modifies the critical number of flavors for the occurrence of exciton condensation, and therefore cannot be neglected.
7 citations
TL;DR: Large tensile pressure applied to the putative topological Kondo insulator SmB6 results in an expansion of the lattice in all directions, and a huge increase in the temperature range over which surface-dominated conduction can be observed.
Abstract: Large tensile pressure applied to the putative topological Kondo insulator SmB6 results in an expansion of the lattice in all directions, and a huge increase in the temperature range over which surface-dominated conduction can be observed.
7 citations
TL;DR: In this article, the effect of a magnetic thin-film ribbon at the surface of a topological insulator on the charge and spin transport due to surface electrons was discussed, and the Seebeck coefficient was calculated when electron flux is due to a temperature gradient.
Abstract: We discuss the effect of a magnetic thin-film ribbon at the surface of a topological insulator on the charge and spin transport due to surface electrons.\\ If the magnetization in the magnetic ribbon is perpendicular to the surface of a topological insulator, it leads to a gap in the energy spectrum of surface electrons. As a result, the ribbon is a barrier for electrons, which leads to electrical resistance.\\ We have calculated conductance of such a structure. The conductance reveal some oscillations with the length of the magnetized region due to the interference of transmitted and reflected waves. We have also calculated the Seebeck coefficient when electron flux is due to a temperature gradient.
5 citations
Posted Content•
TL;DR: In this paper, the authors show that negative-U$U$ centers are responsible for widespread insulator-to-metal transitions in amorphous HfO$_2$ and Al$_ 2$ O$_3$ thin films with a 10$^9$-fold resistance drop.
Abstract: Attractive negative-$U$ interactions between electrons facilitated by strong electron-phonon interaction are common in highly polarizable and disordered materials such as amorphous chalcogenides, but there is no direct evidence for them in large-band-gap insulators. Here we report how such negative-$U$ centers are responsible for widespread insulator-to-metal transitions in amorphous HfO$_2$ and Al$_2$O$_3$ thin films with a 10$^9$-fold resistance drop. Triggered by a static hydraulic pressure or a 0.1 ps impulse of magnetic pressure, the transition can proceed at such low pressure that there is very little overall deformation (strain~10$^{-5}$). Absent a significant energy change overall, the transition is attributed to the reversal of localized electron-phonon interaction: By reversing the sign of $U$, trapped electrons are destabilized and released, thus clearing conduction paths previously blocked by charged traps. The results also suggest that Mott insulators when disordered may become Anderson insulators with strong electron-phonon interactions regulating incipient conduction paths, a novel finding of technological significance for electronic devices.
4 citations
TL;DR: In this article, the modified Dirac model was used to study magnetic quantum dots in two-dimensional band and topological insulators under nonuniform magnetic fields, and it was found that the ground states oscillate between the spin-up and spin-down states when the magnetic field or the dot size changes.
Abstract: Magnetic quantum dots in two-dimensional band and topological insulators are studied by solving the modified Dirac model under nonuniform magnetic fields. The Landau levels split into discrete states with certain angular momentum. The states splitting from the zero Landau levels lie in the energy gap for topological insulators but are out of the gap for band insulators. It is found that the ground states oscillate between the spin-up and spin-down states when the magnetic field or the dot size changes. The oscillation manifests itself as changes of sign and strength of charge currents near the dot's edge.
4 citations
Journal Article•
TL;DR: In this article, the topological states of matter are characterized by global topological invariants which change their value across a topological quantum phase transition, and it is commonly assumed that the transition be...
Abstract: Topological states of matter are characterized by global topological invariants which change their value across a topological quantum phase transition. It is commonly assumed that the transition be ...
TL;DR: Results reveal the formation of charge puddles due to the interplay of disorder and quantum interference at low temperatures.
Abstract: The Anderson insulating states in Au nanoparticle assembly are identified and studied under the application of magnetic fields and gate voltages. When the inter-nanoparticle tunneling resistance is smaller than the quantum resistance, the system showing zero Mott gap can be insulating at very low temperature. In contrast to Mott insulators, Anderson insulators exhibit great negative magnetoresistance, inferring charge delocalization in a strong magnetic field. When probed by the electrodes spaced by ~200 nm, they also exhibit interesting gate-modulated current similar to the multi-dot single electron transistors. These results reveal the formation of charge puddles due to the interplay of disorder and quantum interference at low temperatures.
TL;DR: In this article, the exact solutions for a Coulomb impurity in two-dimensional infinite and finite topological insulators were obtained by a powerful method and the level order and zero-energy degeneracy of the spectra were found to be quite different between topological trivial and nontrivial phases.
Abstract: Introducing a powerful method, we obtain the exact solutions for a Coulomb impurity in two-dimensional infinite and finite topological insulators. The level order and zero-energy degeneracy of the spectra are found to be quite different between topological trivial and nontrivial phases. For quantum dots of topological insulator, the variation of the edge and Coulomb states with dot size, Coulomb potential, and magnetic field are clearly shown. It is found that for small dots the edge states can be strongly coupled with the Coulomb states and for large dots the edge states are insensitive to the Coulomb fields but sensitive to the magnetic fields.
14 May 2017
TL;DR: In this article, two surface charge accumulation models of the post-insulator with particle pollution were established taking the particle pollution, as well as the generating, recombination, migration and diffusion of the positive and negative ions in the gas into account.
Abstract: Aiming at the problem of conducting particle pollution in DC-GIL, surface charge accumulation on the post-insulator is analyzed based on COMSOL and the platform for surface charge measurement. In this study, two surface charge accumulation models of the post-insulator with particle pollution were established taking the particle pollution, as well as the generating, recombination, migration and diffusion of the positive and negative ions in the gas into account. Based on these models, the impacts of attached particles on charge accumulation are explored by COMSOL, and the effectiveness and correctness of the simulations were verified through the experimental results from the platform. The results illustrate that, the accumulation of surface charge is affected by the conductive particles attached to the surface of the insulator greatly. In addition, the charge surge caused by the particles contact with the electrode is more remarkable, and the charge polarity at the ends of the particle which is located at the middle of the insulator is opposite to the polarity of the electrode. However, the charge polarity at the ends of the particle which is contact with the electrode is the same as the polarity of the electrode. The effect of micro-discharge on charge generation plays an important role on charge accumulation.
TL;DR: In this article, the surface properties of t -LiFeSO 4 F have been investigated under the GGA+U framework and the surface energies of (1, 0, 0), (0, 1, 0, 0), and (0, 0, 1) surfaces at the lowest energy termination have been obtained.
TL;DR: In this article, a simple model with a flexible Chern number is proposed to demonstrate that the bulk gapless states emerge at the phase boundary of topological phase transitions, despite the absence of a band inversion process.
Abstract: In commonly employed models for two-dimensional (2D) topological insulators, bulk gapless states are well known to form at the band inversion points where the degeneracy of the states is protected by symmetries. It is thus sometimes quite tempting to consider this feature, the occurrence of gapless states, a result of the band inversion process under protection of the symmetries. Similarly, the band inversion process might even be perceived as necessary to induce 2D topological phase transitions. To clarify these misleading perspectives, we propose a simple model with a flexible Chern number to demonstrate that the bulk gapless states emerge at the phase boundary of topological phase transitions, despite the absence of a band inversion process. Furthermore, the bulk gapless states do not need to occur at the special $k$ points protected by symmetries. Given the significance of these fundamental conceptual issues and their widespread influence, our clarification should generate strong general interests and significant impacts. Furthermore, the simplicity and flexibility of our general model with an arbitrary Chern number should prove useful in a wide range of future studies of topological states of matter.
01 May 2017
TL;DR: In this paper, the interface between the insulator body and the surrounding gas and its immediate vicinity appears to be the most vulnerable region for the occurrence of severe discharges leading to surface flashover.
Abstract: HVDC technology is increasingly adopted for long distance transmission of bulk power at reduced cost in comparison with HVAC systems. Epoxy is a popular material for insulators/spacers used in sealed space such as in GIS or wall bushings with selected fillers, providing excellent electrical insulation and mechanical support. Practical operation and previous research [1-5] has however suggested that insulators under HVDC stress exhibit very different long term behaviour from HVAC insulators due to charge accumulation under an electric field of single polarity. The interface between the insulator body and the surrounding gas and its immediate vicinity appears to be the most vulnerable region for the occurrence of severe discharges leading to surface flashover. There has been work on the modelling of charge transport processes [6-8] but most of them concerned applications at relatively low voltage levels, typically below 200 kV.
Patent•
15 Jun 2017
TL;DR: In this article, an electrical bias is formed for a period of time across the two or more spaced apart electrical contacts in the insulator structure to fill electron traps at the interface between the at least two insulator layers.
Abstract: A method for trapping electrons includes providing an insulator structure comprising at least two insulator layers. Two or more spaced apart electrical contacts to an interface between the at least two insulator layers are formed. An electrical bias is formed for a period of time across the two or more spaced apart electrical contacts in the insulator structure to fill electron traps at the interface between the at least two insulator layers.
01 Apr 2017
TL;DR: In this paper, the spin filter effect was used to generate spin-polarized electrical currents in ferromagnetic insulators, which are essential to spintronics. But the spin-filter effect was not applied to ferromagnetism insulators.
Abstract: Ferromagnetic (or ferrimagnetic) insulators attract much attention since highly spin-polarized electrical currents, which is essential to spintronics, are generated by the spin-filter effect.
15 Jun 2017
TL;DR: In this paper, a mathematical model to study the indirect interaction in topological insulators was constructed, and the dependence of the constants of the effective exchange interaction upon the λ parameter was shown.
Abstract: In this work, a mathematical model to study the indirect interaction in topological insulators was constructed. Analysis of the model was carried out numerically. We have calculated the indirect exchange interaction in the film of a topological insulator, for example Bi2Te3, within the s–d model. The calculations showed that the magnetic ordering of the impurity spins varies periodically with increasing distance between atoms, asymptotically decreasing to zero. λ is a parameter associated with hexagonal distortion and is a component of the dispersion relation. The dependence of the constants of the effective exchange interaction upon the λ parameter is shown; this parameter characterizes the crystal lattice geometry for a topological insulator.