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Q. W. Shi

Publications and source records attributed to Q. W. Shi.

At least 19 recordsLinked to original sources
arXiv2020

Anisotropic behavior of TiB2 nanoparticles reinforced 2024Al composites rolling sheet

The in-plane anisotropy (IPA) of mechanical properties existed in plate products is often undesired since it limits the industrial application and increases the difficulty of material processing. It is thus one of the important factors which requires strict control during the manufacture of high-performance aluminum alloy plates. The anisotropic behavior of TiB2/2024Al composite rolling sheet under different heat treatment is studied in this article. T4 heat treated specimen shows good isotropy of yield strength and its texture components are significantly weakened compared with traditional Al alloys. The evolution of microstructures for T4 specimen was studied. The submicron TiB2 particles increase the driving force of recrystallization during rolling process and static recrystallization (SRX) is stimulated, thus induces texture randomization. Anisotropic behavior of yield strength occurs in specimen after T3 heat treatment. It is supposed that the difference in the activation of slip system under different loading direction gives rise to this anisotropy.

cond-mat.mtrl-sci↗
arXiv2011

Vacancy Induced Splitting of Dirac Nodal Point in Graphene

We investigate the vacancy effects on quasiparticle band structure of graphene near the Dirac point. It is found that each Dirac nodal point splits into two new nodal points due to the coherent scattering among vacancies. The splitting energy between the two nodal points is proportional to the square root of vacancy concentration. In addition, an extra dispersionless impurity band of zero energy due to particle-hole symmetry is found. Our theory offers an excellent explanation to the recent experiments.

cond-mat.quant-gas↗
arXiv2010

Topological Transition of Graphene from Quantum Hall Metal to Quantum Hall Insulator at $ν=0$

The puzzle of recently observed insulating phase of graphene at filling factor $ν=0$ in high magnetic field quantum Hall (QH) experiments is investigated. We show that the magnetic field driven Peierls-type lattice distortion (due to the Landau level degeneracy) and random bond fluctuations compete with each other, resulting in a transition from a QH-metal state at relative low field to a QH-insulator state at high enough field at $ν=0$. The critical field that separates QH-metal from QH-insulator depends on the bond fluctuation. The picture explains well why the field required for observing the insulating phase is lower for a cleaner sample.

cond-mat.dis-nn↗
arXiv2010

Accurate evaluation of the Green's function of disordered graphenes

An accurate simulation of Green's function and self-energy function of non-interacting electrons in disordered graphenes are performed. Fundamental physical quantities such as the elastic relaxation time τe, the phase velocity vp, and the group velocity vg are evaluated. New features around the Dirac point are revealed, showing hints that multi-scattering induced hybridization of Bloch states plays an important role in the vicinity of the Dirac point.

cond-mat.mes-hall↗
arXiv2008

The shape of disorder broadened Landau subbands in graphene

Density of states (DOS) of graphene under a high uniform magnetic field and white-noise random potential is numerically calculated. The disorder broadened zero-energy Landau band has a Gaussian shape whose width is proportional to the random potential variance and the square root of magnetic field. Wegner-type calculation is used to justify the results.

cond-mat.mes-hall↗
arXiv2008

Electronic Structure in Gapped Graphene with Coulomb Potential

In this paper, we numerically study the bound electron states induced by long range Coulomb impurity in gapped graphene and the quasi-bound states in supercritical region based on the lattice model. We present a detailed comparison between our numerical simulations and the prediction of the continuum model which is described by the Dirac equation in (2+1)-dimensional Quantum Electrodynamics (QED). We also use the Fano's formalism to investigate the quasi-bound state development and design an accessible experiments to test the decay of the supercritical vacuum in the gapped graphene.

cond-mat.mes-hall↗
arXiv2007

Edge Effects on the Electronic Structures of Chemically Modified Armchair Graphene Nanoribbons

In this paper, we apply the first-principle theory to explore how the electronic structures of armchair graphene nanoribbons (AGNRs) are affected by chemical modifications. The edge addends include H, F, N, NH$_{2}$, and NO$_{2}$. Our theoretical results show that the energy gaps are highly tunable by controlling the widths of AGNRs and addends. The most interesting finding is that N-passivated AGNRs with various widths are metallic due to the unique electronic features of N-N bonds. This property change of AGNRs (from semiconducting to metallic) is important in developing graphene-based devices.

cond-mat.mtrl-sci↗
arXiv2007

Average Density of States in Disordered Graphene systems

In this paper, the average density of states (ADOS) with a binary alloy disorder in disordered graphene systems are calculated based on the recursion method. We observe an obvious resonant peak caused by interactions with surrounding impurities and an anti-resonance dip in ADOS curves near the Dirac point. We also find that the resonance energy (Er) and the dip position are sensitive to the concentration of disorders (x) and their on-site potentials (v). An linear relation, not only holds when the impurity concentration is low but this relation can be further extended to high impurity concentration regime with certain constraints. We also calculate the ADOS with a finite density of vacancies and compare our results with the previous theoretical results.

cond-mat.mes-hall↗
arXiv2007

Quantum Dot in Z-shaped Graphene Nanoribbon

Stimulated by recent advances in isolating graphene, we discovered that quantum dot can be trapped in Z-shaped graphene nanoribbon junciton. The topological structure of the junction can confine electronic states completely. By varying junction length, we can alter the spatial confinement and the number of discrete levels within the junction. In addition, quantum dot can be realized regardless of substrate induced static disorder or irregular edges of the junction. This device can be used to easily design quantum dot devices. This platform can also be used to design zero-dimensional functional nanoscale electronic devices using graphene ribbons.

cond-mat.mes-hall↗
arXiv2007

Switching mechanism of photochromic diarylethene derivatives molecular junctions

The electronic transport properties and switching mechanism of single photochromic diarylethene derivatives sandwiched between two gold surfaces with closed and open configurations are investigated by a fully self-consistent nonequilibrium Green's function method combined with density functional theory. The calculated transmission spectra of two configurations are strikingly distinctive. The open form lacks any significant transmission peak within a wide energy window, while the closed structure has two significant transmission peaks on the both sides of the Fermi level. The electronic transport properties of the molecular junction with closed structure under a small bias voltage are mainly determined by the tail of the transmission peak contributed unusually by the perturbed lowest perturbed unoccupied molecular orbital. The calculated on-off ratio of currents between the closed and open configurations is about two orders of magnitude, which reproduces the essential features of the experimental measured results. Moreover, we find that the switching behavior within a wide bias voltage window is extremely robust to both substituting F or S for H or O and varying end anchoring atoms from S to Se and Te.

cond-mat.other↗
arXiv2007

Strain Effect on Energy Gaps of Armchair Graphene Nanoribbons

We report a first-principles study on electronic structures of the deformed armchair graphene nanoribbons (AGNRs). The variation of the energy gap of AGNRs as a function of uniaxial strain displays a zigzag pattern, which indicates that the energy gaps of AGNRs can be effectively tuned. The spatial distributions of two occupied and two empty subbands close to the Fermi level are swapped under different strains. The tunable width of energy gaps becomes narrower as increasing the width of AGNRs. Our simulations with tight binding approximation, including the nearest neighbor hopping integrals between $π$- orbitals of carbon atoms, reproduce these results by first-principles calculations. One simple empirical formula is obtained to describe the scaling behavior of the maximal value of energy gap as a function of the width of AGNRs.

cond-mat.mtrl-sci↗
arXiv2007

Tuning the electronic structures of armchair graphene nanoribbons through chemical edge modification: A theoretical study

We report combined first-principle and tight-binding (TB) calculations to simulate the effects of chemical edge modifications on structural and electronic properties. The C-C bond lengths and bond angles near the GNR edge have considerable changes when edge carbon atoms are bounded to different atoms. By introducing a phenomenological hopping parameter $t_{1}$ for nearest-neighboring hopping to represent various chemical edge modifications, we investigated the electronic structural changes of nanoribbons with different widths based on the tight-binding scheme. Theoretical results show that addends can change the band structures of armchair GNRs and even result in observable metal-to-insulator transition.

cond-mat.mtrl-sci↗
arXiv2007

An insight into the electronic structure of graphene: from monolayer to multi-layer

In this paper, we analytically investigate the electronic structure of Bernal stacking (AB stacking) graphene evolving from monolayer (a zero-gap semiconductor with a linear Dirac-like spectrum around the Fermi energy) to multi-layer (semi-metal bulk graphite). We firstly derive a real space analytical expression for the free Green's function (propagator) of multi-layer graphene based on the effective-mass approximation. The simulation results exhibit highly spatial anisotropy with three-fold rotational symmetry. By combining with the STM measurement of d2I/dV2 (the second derivative of current), we also provide a clear high-throughput and non-destructive method to identify graphene layers. Such a method is lacking in the emerging graphene research.

cond-mat.mes-hall↗
arXiv2006

Electronic Structure of Bilayer Graphene: A Real-space Green's Function Study

In this paper, a real-space analytical expression for the free Green's function (propagator) of bilayer graphene is derived based on the effective-mass approximation. Green's function displays highly spatial anisotropy with three-fold rotational symmetry. The calculated local density of states (LDOS) of a perfect bilayer graphene produces the main features of the observed scanning tunneling microscopy (STM) images of graphite at low bias voltage. Some predicted features of the LDOS can be verified by STM measurements. In addition, we also calculate the LDOS of bilayer graphene with vacancies by using the multiple-scattering theory (scatterings are localized around the vacancy of bilayer graphene). We observe that the interference patterns are determined mainly by the intrinsic properties of the propagator and the symmetry of the vacancies.

cond-mat.mtrl-sci↗
arXiv2006

Graphene switch design: an illustration of the Klein paradox

An armchair graphene ribbon switch has been designed based on the principle of the Klein paradox. The resulting switch displays an excellent on-off ratio performance. Anomalous tunneling phenomena are observed in our numerical simulations. According to our analytical results, selective tunneling rule is proposed to explain this interesting transport behavior. The switch design enriches the phenomenon of the Klein paradox and also provides a platform to study the paradox.

cond-mat.mes-hall↗
arXiv2004

Spin filtering through a double-bend structure

We propose a simple scheme for the spin filter by studying the coherent transport of electrons through a double-bend structure in a quantum wire with a weak lateral magnetic potential which is much weaker than the Fermi energy of the leads. {\em Extremely large} spin polarized current in the order of {\em micro-Ampere} can be obtained because of the strong resonant behavior from the double bends. Further study suggests the roubustness of this spin filter.

cond-mat.mes-hall↗
arXiv2003

Spin oscillations in transient diffusion of a spin pulse in n-type semiconductor quantum wells

By studying the time and spatial evolution of a pulse of the spin polarization in $n$-type semiconductor quantum wells, we highlight the importance of the off-diagonal spin coherence in spin diffusion and transport. Spin oscillations and spin polarization reverse along the the direction of spin diffusion in the absence of the applied magnetic field are predicted from our investigation.

cond-mat↗