arXiv subjects

Hua Jiang

Publications and source records attributed to Hua Jiang.

At least 109 records · Page 6Linked to original sources

arXiv2018

Giant Spin-valley Polarization and Multiple Hall Effect in Functionalized Bi Monolayers

Valleytronic materials, characterized by local extrema (valley) in their bands, and topological insulators have separately attracted great interest recently. However, the interplay between valleytronic and topological properties in one single system, likely to enable important unexplored phenomena and applications, has been largely overlooked so far. Here, by combining a tight-binding model with first-principles calculations, we find the large-band-gap quantum spin Hall effects (QSHEs) and valley Hall effects (VHEs) appear simultaneously in the Bi monolayers decorated with halogen elements, denoted as Bi2XY (X, Y = H, F, Cl, Br, or I). A staggered exchange field is introduced into the Bi2XY monolayers by transition metal atom (Cr, Mo, or W) doping or LaFeO3 magnetic substrates, which together with the strong SOC of Bi atoms generates a time-reversal-symmetry-broken QSHE and a huge valley splitting (up to 513 meV) in the system. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system. These predicted multiple and exotic Hall effects, associated with various degrees of freedom of electrons, could enable applications of the functionalized Bi monolayers in electronics, spintronics, and valleytronics.

cond-mat.mes-hall↗

arXiv2018

Discrete Scale Invariance in Topological Semimetals

The discovery of Weyl and Dirac semimetals has produced a number of dramatic physical effects, including the chiral anomaly and topological Fermi arc surface states. We point out that a very different but no less dramatic physical effect is also to be found in these materials: discrete scale invariance. This invariance leads to bound state spectra for Coulomb impurities that repeat when the binding energy is changed by a fixed factor, reminiscent of fractal behavior. We show that this effect follows from the peculiar dispersion relation in Weyl and Dirac semimetals. It is observed when such a material is placed in very strong magnetic field B: there are oscillations in the magnetoresistivity somewhat similar to Shubnikov-de Haas oscillations but with a periodicity in ln B rather than 1/B. These oscillations should be present in other thermodynamic and transport properties. The oscillations have now been seen in three topological semimetals: ZrTe$_{5}$, TaAs, and Bi.

cond-mat.mtrl-sci↗

arXiv2018

Manipulation and Characterization of the Valley Polarized Topological Kink States in Graphene Based Interferometers

Valley polarized topological kink states, existing broadly in the domain wall of hexagonal lattices systems, are identified in experiments, unfortunately, only very limited physical properties being given. Using an Aharanov-Bohm interferometer composed of domain walls in graphene systems, we study the periodical modulation of pure valley current in a large range by tuning the magnetic field or the Fermi level. For monolayer graphene device, there exists one topological kink state, and the oscillation of transmission coefficients have single period. The $π$ Berry phase and the linear dispersion relation of kink states can be extracted from the transmission data. For bilayer graphene device, there are two topological kink states with two oscillation periods. Our proposal provides an experimental feasible route to manipulate and characterize the valley polarized topological kink states in classical wave and electronic graphene-type crystalline systems.

cond-mat.mes-hall↗

arXiv2018

Numerical study of Klein quantum dots in graphene system

Klein quantum dot (KQD) refers to a QD with quasi-bound states and a finite trapping time, which has been observed in experiments focused on graphene recently. In this paper, we develop a numerical method to calculate local density of states (LDOS) of KQD and apply it to monolayer graphene. By investigating the variation of LDOS in a circular quantum dot, we obtain the dependence of the quasi-bound states on the quantum dot parameters (e.g. the electron energy, radius, confined potential, etc). Based on these results, not only can we well explain the experimental phenomena, but also demonstrate how quasi-bound states turn to real bound states when intervalley scattering is taken into considered. We further study the evolution of the LDOS for KQD varying from a circle shape to a semicircle shape, which reveals the mechanism of whispering gallery mode on the quasi-bound states.

cond-mat.mes-hall↗

arXiv2018

Goos-Hänchen-like shifts at metal/superconductor interface

At a normal-metal/superconductor interface, an incident electron from the normal-metal (N) side can be normally reflected as an electron or Andreev reflected as a hole. We show that pronounced lateral shifts along the interface between the incident and the reflected quasiparticles can happen in both reflection processes, which are analogous to the Goos-Hänchen effect in optics. Two concrete model systems are considered. For the simplest model in which the N side is of the two-dimensional electron gas, we find that while the shift in Andreev reflection stays positive, the shift in normal reflection can be made either positive or negative, depending on the excitation energy. For the second model with the N side taken by graphene, the shift in Andreev reflection can also be made negative, and the shifts have rich behavior due to the additional sublattice pseudospin degree of freedom. We show that the shift strongly modifies the dispersion for the confined waveguide modes in an SNS structure. We also suggest a possible experimental setup for detecting the shift.

cond-mat.mes-hall↗

arXiv2018

Doubled Shapiro Steps in a Topological Josephson Junction

We study the transport properties of a superconductor-quantum spin Hall insulator-superconductor (S-QSHI-S) hybrid system in the presence of a microwave radiation. Instead of adiabatic analysis or using the resistively shunted junction model, we start from the microscopic Hamiltonian and calculate the DC current directly with the help of the non-equilibrium Green's Functions method. The numerical results show that (i) the I-V curves of background current due to multiple Andreev reflections (MAR) exhibit a different structure with that in the conventional junctions, (ii) all Shapiro steps are visible and appear one by one at high frequency, while at low frequency, the steps evolve exactly as the Bessel functions and the odd steps are completely suppressed, implying a fractional Josephson effect.

cond-mat.mes-hall↗

arXiv2018

Growth Modes and Chiral Selectivity of Single-Walled Carbon Nanotubes

Chemical vapor deposition synthesis of single-walled carbon nanotubes experiments, using Fe catalyst, and alternating methane and carbon monoxide as carbon feedstocks, lead to the reversible formation of junctions between tubes of different diameters. Combined with an atomistic modeling of the tube / catalyst interface, this shows that the ratio of diameters of the tube and its seeding particle, denoting the growth mode, depends on the carbon fraction inside the catalyst. With carbon monoxide, nanoparticles are strongly carbon enriched, and tend to dewet the tube, in a perpendicular growth mode. Cross-checking our results with available reports from the literature of the last decade strongly suggests that these latter conditions should favor the near armchair chiral selectivity observed empirically.

physics.chem-ph↗

arXiv2017

Magnetic-field-controlled negative differential conductance in graphene npn junction resonators

Negative differential conductance (NDC), characterized by the decreasing current with increasing voltage, has attracted continuous attention for its various novel applications. The NDC typically exists in a certain range of bias voltages for a selected system and controlling the regions of NDC in curves of current versus voltage (I-V) is experimentally challenging. Here, we demonstrate an unusual magnetic-field-controlled NDC in graphene npn junction resonators. The magnetic field not only can switch on and off the NDC, but also can continuously tune the regions of the NDC in the I-V curves. In the graphene npn junction resonators, magnetic fields generate sharp and pronounced Landau-level peaks with the help of the Klein tunneling of massless Dirac fermions. A tip of scanning tunneling microscope induces a relatively shift of the Landau levels in graphene beneath the tip. Tunneling between the misaligned Landau levels results in the magnetic-field-controlled NDC that may have potential applications for future graphene-based technology.

cond-mat.mes-hall↗

arXiv2017

Disorder Effects in Topological States -- Brief Review of the Recent Developments

Disorder inevitably exists in realistic samples, manifesting itself in various exotic properties for the topological states. In this paper, we summarize and briefly review work completed over the last few years, including our own, regarding recent developments in several topics about disorder effects in topological states. For weak disorder, the robustness of topological states is demonstrated, especially for both quantum spin Hall states with $Z_2=1$ and size induced nontrivial topological insulators with $Z_2=0$. For moderate disorder, by increasing the randomness of both the impurity distribution and the impurity induced potential, the topological insulator states can be created from normal metallic or insulating states. These phenomena and their mechanisms are summarized. For strong disorder, the disorder causes a metal-insulator transition. Due to their topological nature, the phase diagrams are much richer in topological state systems. Finally, the trends in these areas of disorder research are discussed.

cond-mat.mes-hall↗

arXiv2017

The valley filter efficiency of monolayer graphene and bilayer graphene line defect model

In addition to electron charge and spin, novel materials host another degree of freedom, the valley. For a junction composed of valley filter sandwiched by two normal terminals, we focus on the valley efficiency under disorder with two valley filter models based on monolayer and bilayer graphene. Applying the transfer matrix method, valley resolved transmission coefficients are obtained. We find that: i) under weak disorder, when the line defect length is over about $15\rm nm$, it functions as a perfect channel (quantized conductance) and valley filter (totally polarized); ii) in the diffusive regime, combination effects of backscattering and bulk states assisted intervalley transmission enhance the conductance and suppress the valley polarization; iii) for very long line defect, though the conductance is small, polarization is indifferent to length. Under perpendicular magnetics field, the characters of charge and valley transport are only slightly affected. Finally we discuss the efficiency of transport valley polarized current in a hybrid system.

cond-mat.mes-hall↗

arXiv2017

Perfect valley filter based on topological phase in disordered $\rm{Sb}$ Monolayer Heterostructure

The hydrogenated $\rm{Sb}$ monolayer epitaxially grown on a $\rm{LaFeO_3}$ substrate is a novel type of two-dimensional material hosting quantum spin-quantum anomalous Hall (QS-QAH) states. For a device formed by $\rm{Sb}$ monolayer ribbon, the QAH edge states, belong to a single valley, are located at opposite edges of the ribbon. The QSH edge states, on the other hand, belong to the other valley and are distributed in a very narrow region at the same edge. In this paper, we find such material can be used to fabricate perfect valley filter. Adopting scattering matrix method and Green's function method, the valley resolved transport and spatial distribution of local current are calculated, in the present of Anderson disorder, edge defects and edge deformations. The numerical results demonstrate that, in the presence of above three types of disorder with moderate strength, the carriers can flow disspationless with nearly perfect valley polarization. Moreover, when the device becomes longer, the transport current does not decrease while the valley filter works even better. The origin is that the disorder can destroy the QSH edge states, but the valley-polarized QAH edge states can well hold.

cond-mat.mes-hall↗

arXiv2017

Numerical Study of Universal Conductance Fluctuation in Three-dimensional Topological Semimetals

We study the conductance fluctuation in topological semimetals. Through statistic distribution of energy levels of topological semimetals, we determine the dominant parameters of universal conductance fluctuation (UCF), i.e., the number of uncorrelated bands $k$, the level degeneracy $s$, and the symmetry parameter $β$. These parameters allow us to predict the zero-temperature intrinsic UCF of topological semimetals by the Altshuler-Lee-Stone theory. Then, we obtain numerically the conductance fluctuations for topological semimetals of quasi-1D geometry. We find that for Dirac/Weyl semimetals, the theoretical prediction coincides with the numerical results. However, a non-universal conductance fluctuation behavior is found for topological nodal line semimetals, i.e., the conductance fluctuation amplitude increases with the enlargement of SOC strength. We find that such unexpected parameter-dependent phenomena of conductance fluctuation are related to Fermi surface shape of 3D topological semimetals. These results will help us to understand the existing and future experimental results of UCF in 3D topological semimetals.

cond-mat.dis-nn↗

arXiv2017

Geometric influences of a particle confined to a curved surface embedded in three-dimensional Euclidean space

In the spirit of the thin-layer quantization approach, we give the formula of the geometric influences of a particle confined to a curved surface embedded in three-dimensional Euclidean space. The geometric contributions can result from the reduced commutation relation between the acted function depending on normal variable and the normal derivative. According to the formula, we obtain the geometric potential, geometric momentum, geometric orbital angular momentum, geometric linear Rashba and cubic Dresselhaus spin-orbit couplings. As an example, a truncated cone surface is considered. We find that the geometric orbital angular momentum can provide an azimuthal polarization for spin, and the sign of the geometric Dresselhaus spin-orbit coupling can be flipped through the inclination angle of generatrix.

quant-ph↗

arXiv2017

Scanning tunneling microscopy and spectroscopy of nanoscale twisted bilayer graphene

Nanoscale twisted bilayer graphene (TBG) is quite instable and will change its structure to Bernal (or AB-stacking) bilayer with a much lower energy. Therefore, the lack of nanoscale TBG makes its electronic properties not accessible in experiment up to now. In this work, a special confined TBG is obtained in the overlaid area of two continuous misoriented graphene sheets. The width of the confined region of the TBG changes gradually from about 22 nm to 0 nm. By using scanning tunnelling microscopy, we studied carefully the structure and the electronic properties of the nanoscale TBG. Our results indicate that the low-energy electronic properties, including twist-induced van Hove singularities (VHSs) and spatial modulation of local density-of-state, are strongly affected by the translational symmetry breaking of the nanoscale TBG. Whereas, the electronic properties above the energy of the VHSs are almost not influenced by the quantum confinement even when the width of the TBG is reduced to only a single moire spot.

cond-mat.mes-hall↗

arXiv2017

Generating nanoscale and atomically-sharp p-n junctions in graphene via monolayer-vacancy-island engineering of Cu surface

Creation of high quality p-n junctions in graphene monolayer is vital in studying many exotic phenomena of massless Dirac fermions. However, even with the fast progress of graphene technology for more than ten years, it remains conspicuously difficult to generate nanoscale and atomically-sharp p-n junctions in graphene. Here, we employ monolayer-vacancy-island engineering of Cu surface to realize nanoscale p-n junctions with atomically-sharp boundaries in graphene monolayer. The variation of graphene-Cu separations around the edges of the Cu monolayer-vacancy-island affects the positions of the Dirac point in graphene, which consequently lead to atomically-sharp p-n junctions with the height as high as 660 meV in graphene. The generated sharp p-n junctions isolate the graphene above the Cu monolayer-vacancy-island as nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. Massless Dirac fermions are confined by the p-n junctions for a finite time to form quasi-bound states in the GQDs. By using scanning tunneling microscopy, we observe resonances of quasi-bound states in the GQDs with various sizes and directly visualize effects of geometries of the GQDs on the quantum interference patterns of the quasi-bound states, which allow us to test the quantum electron optics based on graphene in atomic scale.

cond-mat.mes-hall↗

arXiv2017

Evidence of electron-electron interactions around Van Hove singularities of a graphene Moiré superlattice

A variety of new and interesting correlated states have been predicted in graphene monolayer doped to Van Hove singularities (VHSs) of its density-of-state (DOS). However, tuning the Fermi energy to reach a VHS of graphene by either gating or chemical doping is prohibitively difficult, owning to their large energy distance (3 eV). Therefore, these correlated states, which arise from effects of strong electron-electron interactions at the VHSs, have remained experimentally elusive. Here, we report experimental evidences of electron-electron interactions around the VHSs of a twisted bilayer graphene (TBG) through scanning tunneling microscopy measurements. By introducing a small twisted angle between two adjacent graphene sheets, we are able to generate low-energy VHSs arbitrarily approaching the Fermi energy. The split of the VHSs are observed and the symmetry breaking of electronic states around the VHSs are directly visualized. These results experimentally demonstrate the important effects of electron-electron interactions on electronic properties around the VHSs of the TBG, therefore providing motivation for further theoretical and experimental studies in graphene systems with considering many-body interactions.

cond-mat.mes-hall↗

arXiv2017

Two-Dimensional Lattice Model for the Surface States of Topological Insulators

The surface states in three-dimensional (3D) topological insulators (TIs) can be described by a two-dimensional (2D) continuous Dirac Hamiltonian. However, there exists the Fermion doubling problem when putting the continuous 2D Dirac equation into a lattice model. In this letter, we introduce a Wilson term with a zero bare mass into the 2D lattice model to overcome the difficulty. By comparing with a 3D Hamiltonian, we show that the modified 2D lattice model can faithfully describe the low-energy electrical and transport properties of surface states of 3D TIs. So this 2D lattice model provides a simple and cheap way to numerically simulate the surface states of 3D TI nanostructures. Based on the 2D lattice model, we also establish the wormhole effect in a TI nanowire by a magnetic field along the wire and show the surface states being robust against disorder. The proposed 2D lattice model can be extensively applied to study the various properties and effects, such as the transport properties, Hall effect, universal conductance fluctuations, localization effect, etc.. So it paves a new way to study the surface states of the 3D topological insulators.

cond-mat.mes-hall↗

arXiv2017

Global Phase Diagram of Disordered Type-II Weyl Semimetals

With electron and hole pockets touching at the Weyl node, type-II Weyl semimetal is a newly proposed topological state distinct from its type-I cousin. We numerically study the localization effect for tilted type-I as well as type-II Weyl semimetals and give the global phase diagram. For dis- ordered type-I Weyl semimetal, an intermediate three-dimensional quantum anomalous Hall phase is confirmed between Weyl semimetal phase and diffusive metal phase. However, this intermediate phase is absent for disordered type-II Weyl semimetal. Besides, near the Weyl nodes, comparing to its type-I cousin, type-II Weyl semimetal possesses even larger ratio between the transport lifetime along the direction of tilt and the quantum lifetime. Near the phase boundary between the type-I and the type-II Weyl semimetals, infinitesimal disorder will induce an insulating phase so that in this region, the concept of Weyl semimetal is meaningless for real materials.

cond-mat.mes-hall↗