arXiv subjects

Chang-De Gong

Publications and source records attributed to Chang-De Gong.

At least 37 records · Page 2Linked to original sources

arXiv2011

Zigzag graphene nanoribbons without inversion symmetry

Graphene on a substrate will suffer an inversion-symmetry-breaking (ISB) lattice potential. Taking electron-electron interaction into account, we study in this paper the possibility of half-metallicity and noncollinear (NC) magnetic phase for graphene zigzag nanoribbons without inversion symmetry. At half-filling it is found that half-metallic(HM) state can be achieved at an intermediate value of the ISB potential due to its competition with the electron-electron interaction. Away from half-filling, the phase diagrams of doping versus ISB potential for different ribbon width are given, where the regimes for the HM states and NC magnetic state are clearly indicated and discussed. For ribbons with perfect edges, we predict a topological transition between two HM states with different magnetic structures, which is accompanied by an abrupt transition of electrical conductance along the ribbon from $2e^2/h$ to $e^2/h$.

cond-mat.mes-hall↗

arXiv2011

Fractional Quantum Hall Effect of Hard-Core Bosons in Topological Flat Bands

Recent proposals of topological flat band (TFB) models have provided a new route to realize the fractional quantum Hall effect (FQHE) without Landau levels. We study hard-core bosons with short-range interactions in two representative TFB models, one of which is the well known Haldane model (but with different parameters). We demonstrate that FQHE states emerge with signatures of even number of quasi-degenerate ground states on a torus and a robust spectrum gap separating these states from higher energy spectrum. We also establish quantum phase diagrams for the filling factor 1/2 and illustrate quantum phase transitions to other competing symmetry-breaking phases.

cond-mat.str-el↗

arXiv2011

Manipulating the fully spin-polarized edge currents in graphene ribbon

Electron fully spin-polarized edge states in graphene emerged at the interfaces of a nonuniform magnetic field are studied numerically in a tight-binding model, with both the orbital and Zeeman-splitting effects of magnetic field considered. We show that the fully spin-polarized currents can be manipulated by a gate voltage. In order to make use of the fully spin-polarized currents in the spin related transport, a three-terminal experiment is designed and expected to export the fully spin-polarized currents. This may have important applications in spin based nanodevices.

cond-mat.mtrl-sci↗

arXiv2010

Tunable striped-patterns by lattice anisotropy and magnetic impurities in d-wave superconductors

The pattern transition induced by lattice anisotropy (LA) and magnetic impurities is computationally observed in near-optimally doped d-wave superconductors (DSCs). For the single impurity case, a transition from the checkerboard to stripe pattern can be induced even with a very weak LA. Moreover, the modulation period of eight lattice constants (8$a$) in the spin order coincides with neutron scattering data. For the two-impurity case, an orientation transition from the longitudinal impurity-pinned stripe into the transverse pattern is observed when the LA ratio reaches some critical value. At the critical point, it is found that the structures around magnetic impurities could restore checkerboard patterns. These results indicate that the formation of stripes in DSCs might induced by various effects, and could be tunable experimentally.

cond-mat.supr-con↗

arXiv2009

Edge States and Quantum Hall Effect in Graphene under a Modulated Magnetic Field

Graphene properties can be manipulated by a periodic potential. Based on the tight-binding model, we study graphene under a one-dimensional (1D) modulated magnetic field which contains both a uniform and a staggered component. New chiral current-carrying edge states are generated at the interfaces where the staggered component changes direction. These edge states lead to an unusual integer quantum Hall effect (QHE) in graphene, which can be observed experimentally by a standard four-terminal Hall measurement. When Zeeman spin splitting is considered, a novel state is predicted where the electron edge currents with opposite polarization propagate in the opposite directions at one sample boundary, whereas propagate in the same directions at the other sample boundary.

cond-mat.str-el↗

arXiv2009

Induced chiral Dirac fermions in graphene by a periodically modulated magnetic field

The effect of a modulated magnetic field on the electronic structure of neutral graphene is examined in this paper. It is found that application of a small staggered modulated magnetic field does not destroy the Dirac-cone structure of graphene and so preserves its 4-fold zero-energy degeneracy. The original Dirac points (DPs) are just shifted to other positions in k space. By varying the staggered field gradually, new DPs with exactly the same electron-hole crossing energy as that of the original DPs, are generated, and both the new and original DPs are moving continuously. Once two DPs are shifted to the same position, they annihilate each other and vanish. The process of generation and evolution of these DPs with the staggered field is found to have a very interesting patten, which is examined carefully. Generally, there exists a corresponding branch of anisotropic massless fermions for each pair of DPs, resulting in that each Landau level (LL) is still 4-fold degenerate except the zeroth LL which has a robust $4n_t$-fold degeneracy with nt the number of pairs of DPs. As a result, the Hall conductivity $σ_{xy}$ shows a step of size $4n_te^2/h$ across zero energy.

cond-mat.mes-hall↗

arXiv2009

Topological-Fermi-Liquid to Quantum-Hall-Liquid Transitions: $p$-Band and $d$-Band Fermions in a Magnetic Field

We find that in a multi-orbital system with intraorbital and interorbital hopping integrals, the Hall conductance exhibits various topological quantum phase transitions (QPTs) induced by on-site orbital polarization: integer quantum Hall (IQH) plateau transitions, and topological Fermi liquid to IQH transitions. Such topological QPTs are demonstrated in two systems: a $p$-band spinless fermionic system realizable with ultracold atoms in optical lattice, and a $d$-band spinful fermionic system closely related to giant orbital Hall effects in transition metals and their compounds.

cond-mat.mes-hall↗

arXiv2009

Magnetic Flux Tuning of Spin Chirality in Mott Insulators with Ring Exchanges

A manifestation of the many-body Aharonov-Bohm effect in the magnetic-flux-tuned Mott insulators with three-spin and four-spin ring exchanges, presents as an effective tool to manipulate the ground-state spin chirality, such as, tune the magnitude continuously, switch an abrupt jump, or even reverse its sign. Such a mechanism is demonstrated explicitly in both quasi-one-dimensional ladders and two-dimensional lattices with triangles as elementary plaquettes.

cond-mat.str-el↗

arXiv2007

Four-Step Evolution of Spin-Hall Conductance: Tight-Binding Electrons with Rashba Coupling in a Magnetic Field

An intriguing magneto-transport property is demonstrated by tight-binding lattice electrons with Rashba spin-orbit coupling (SOC) in a magnetic field. With the flux strength $ϕ={2π/N}$ ($N$ is an integer) and the Zeeman splitting fixed, when increasing the Rashba SOC $λ$, the spin-Hall and charge-Hall conductances (SHC and CHC) undergo four-step evolutions: the SHC shows size-dependent resonances and jumps at three critical $λ_{c}$'s, and changes its sign at $λ_{c1}$ and $λ_{c3}$; while the CHC exhibits three quantum jumps by $-Ne^2/h$, $+2Ne^2/h$ and $-Ne^2/h$. Such four-step evolutions are also reflected in topological characters and spin polarizations of edge states of a cylindrical system, and are robust against weak disorder.

cond-mat.mes-hall↗

arXiv2007

Tuning Kinetic Magnetism of Strongly Correlated Electrons via Staggered Flux

We explore the kinetic magnetism of the infinite-$U$ repulsive Hubbard models at low hole densities on various lattices with nearest-neighbor hopping integrals modulated by a staggered magnetic flux $\pmϕ$. Tuning $ϕ$ from 0 to $π$ makes the ground state (GS) change from a Nagaoka-type ferromagnetic state to a Haerter-Shastry-type antiferromagnetic state at a critical $ϕ_c$, with both states being of kinetic origin. Intra-plaquette spin correlation, as well as the GS energy, signals such a quantum criticality. This tunable kinetic magnetism is generic, and appears in chains, ladders and two-dimensional lattices with squares or triangles as elementary constituents.

cond-mat.str-el↗

arXiv2007

Redistributing Chern numbers of Landau subbands: tight-binding electrons under staggered-modulated magnetic fields

We investigate the magneto-transport properties of electrons on a square lattice under a magnetic field with the alternate flux strength $ϕ\pmΔϕ$ in neighboring plaquettes. A new peculiar behavior of the Hall conductance has been found and is robust against weak disorder: if $ϕ={p\over{2N}}\times{2π}$ ($p$ and $2N$ are coprime integers) is fixed, the Chern numbers of Landau subbands will be redistributed between neighboring pairs and hence the total quantized Hall conductance exhibits a direct transition by $\pm{N}e^2/h$ at critical fillings when $Δϕ$ is increased from 0 up to a critical value $Δϕ_c$. This effect can be an experimental probe of the staggered-flux phase.

cond-mat.mes-hall↗

arXiv2006

Extended staggered-flux phases in two-dimensional lattices

Based on the so called $t$-$ϕ$ model in two-dimensional (2D) lattices, we investigate the stabilities of a class of extended staggered-flux (SF) phases (which are the extensions of the $\sqrt{2}\times\sqrt{2}$ SF phase to generalized spatial periods) against the Fermi-liquid phase. Surprisingly, when away from the nesting electron filling, some extended-SF phases take over the dominant SF phase (the $\sqrt{2}\times\sqrt{2}$ SF phase for the square lattice, a $1\times\sqrt{3}$ SF phase for the triangular one), compete with the Fermi-liquid phase in nontrivial patterns, and still occupy significant space in the phase diagram through the advantage in the total electronic kinetic energies. The results can be termed as the generalized Perierls orbital-antiferromagnetic instabilities of the Fermi-liquid phase in 2D lattice-electron models.

cond-mat.str-el↗

arXiv2006

Search for quantum dimer phases and transitions in a frustrated spin ladder

A two-leg spin-1/2 ladder with diagonal interactions is investigated numerically. We focus our attention on the possibility of columnar dimer phase, which was recently predicted based on a reformulated bosonization theory. By using density matrix renormalization group technique and exact diagonalization method, we calculate columnar dimer order parameter, spin correlation on a rung, string order parameters, and scaled excitation gaps. Carefully using various finite-size scaling techniques, our results show no support for the existence of columnar dimer phase in the spin ladder under consideration.

cond-mat.str-el↗

arXiv2004

Numerical evidences of spin-1/2 chain approaching spin-1 chain

In this article, we study the one dimensional Heisenberg spin-1/2 alternating bond chain in which the nearest neighbor exchange couplings are ferromagnetic (FM) and antiferromagnetic (AF) alternatively. By using exact diagonalization and density matrix renormalization groups (DMRG) method, we discuss how the system approaches to the AF uniform spin-1 chain under certain condition. When the ratio of AF to FM coupling strength}$α$ $(α=J_{AF}/J_{F})$ \textit{is very small, the physical quantities of the alternating bond chain such as the spin-spin correlation, the string correlation function and the spin density coincide with that of the AF uniform spin-1 chain. The edge state problem is discussed in the present model with small}$α$\textit{limit. In addition, the Haldane gap of the AF uniform spin-1 chain is 4-times of the gap of the system considered.

cond-mat.str-el↗

arXiv2003

Local enhancement of spin and orbital antiferromagnetism around a single vortex in high-Tc superconductors

In a simple model we investigate three competing orders: d-wave superconductivity, spin and orbital antiferromagnetism (AF) in the vicinity of a single vortex in a cuprate superconductor. We find that when the potential for the orbital AF $V_{d}$ is comparatively small, the spin-density-wave (SDW) order has an enhancement at the vortex core center, and the ''d-density-wave'' (DDW) order exhibits a rather weak behavior around the core and vanishing small away from the core. However, when $V_{d}$ becomes large, globally, the SDW order decreases and the DDW order increases; locally, not only the peak of the SDW order around the core still exists, though relatively suppressed, but also a local peak for the DDW order finally appears. Similar effects are also revealed for the features when varying doping. Comparisons with experiments are discussed.

cond-mat.supr-con↗

arXiv2001

The study of spin-spin correlations in quasi-one-dimensional Heisenberg antiferromagnetic clusters

For a class of quasi-one-dimensional clusters, by using exact diagonalization, we study the effect of side spins on the spin-spin correlations on chain. Our calculations show that the side spins added in the same sublattice can effectively strengthen the spin-spin correlations in large distance region and make the change tend to flat. It is exactly proved that periodically adding side spins can set up magnetic long-range orders in the ground state. Also we investigate the effect of the density of side spins on correlation strength. The case that two sublattices have different localized spins is discussed.

cond-mat.str-el↗

arXiv2001

Quantum Phase Transitions in Spin-1/2 Frustrated Molecular Cluster: the Numerical Evidence

By exact diagonalization, we investigate several spin-$\frac 12$ $J_1-J_2$ real clusters of molecular scale with different shapes. Our calculations show that when the ratio, $η$, of next nearest neighbor to nearest neighbor bonds is equal to 1, even the cluster of only 25 sites exhibits the bulk behaviors and has the quantum phase transitions. Two effective critical points are around $J_2/J_1=0.3762$ and 0.612 respectively. They are very close to those of the infinite $J_1-J_2$ square lattice. But, when $η\leq 0.85$, the quantum phase transition around $J_2/J_1=0.3762$ disappears. By calculating the distributions of the average values of $S_i^z$, the bulk behaviors are demonstrated graphically. In the intermediate phase, the sites on the corners have distinctly different character from the other sites. The distribution is obviously centralized on the corner sites.

cond-mat.str-el↗

arXiv2001

Doping dependence of the resonance peak and incommensuration in high-$T_{c}$ superconductors

The doping and frequency evolutions of the incommensurate spin response and the resonance mode are studied based on the scenario of the Fermi surface topology. We use the slave-boson mean-field approach to the $t-t^{\prime}-J$ model and including the antiferromagnetic fluctuation correction in the random-phase approximation. We find that the equality between the incommensurability and the hole concentration is reproduced at low frequencies in the underdoped regime. This equality observed in experiments was explained {\it only} based on the stripe model before. We also obtain the downward dispersion for the spin response and predict its doping dependence for further experimental testing, as well as a proportionality between the low-energy incommensurability and the resonance energy. Our results suggest a common origin for the incommensuration and the resonance peak based on the Fermi surface topology and the d-wave symmetry.

cond-mat.supr-con↗