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Qian Niu

Publications and source records attributed to Qian Niu.

At least 181 records · Page 10Linked to original sources
arXiv2012

Electron Dynamics in Slowly Varying Antiferromagnetic Texture

Effective dynamics of conduction electrons in antiferromagnetic (AFM) materials with slowly varying spin texture is developed via non-Abelian gauge theory. Quite different from the ferromagnetic (FM) case, the spin of a conduction electron does not follow the background texture even in the adiabatic limit due to the accumulation of a SU(2) non-Abelian Berry phase. Correspondingly, it is found that the orbital dynamics becomes spin-dependent and is affected by two emergent gauge fields. While one of them is the non-Abelian generalization of what has been discovered in FM systems, the other leads to an anomalous velocity that has no FM counterpart. Two examples are provided to illustrate the distinctive spin dynamics of a conduction electron.

cond-mat.mes-hall↗
arXiv2012

Microscopic theory of quantum anomalous Hall effect in graphene

We present a microscopic theory to give a physical picture of the formation of quantum anomalous Hall (QAH) effect in graphene due to a joint effect of Rashba spin-orbit coupling $λ_R$ and exchange field $M$. Based on a continuum model at valley $K$ or $K'$, we show that there exist two distinct physical origins of QAH effect at two different limits. For $M/λ_R\gg1$, the quantization of Hall conductance in the absence of Landau-level quantization can be regarded as a summation of the topological charges carried by Skyrmions from real spin textures and Merons from \emph{AB} sublattice pseudo-spin textures; while for $λ_R/M\gg1$, the four-band low-energy model Hamiltonian is reduced to a two-band extended Haldane's model, giving rise to a nonzero Chern number $\mathcal{C}=1$ at either $K$ or $K'$. In the presence of staggered \emph{AB} sublattice potential $U$, a topological phase transition occurs at $U=M$ from a QAH phase to a quantum valley-Hall phase. We further find that the band gap responses at $K$ and $K'$ are different when $λ_R$, $M$, and $U$ are simultaneously considered. We also show that the QAH phase is robust against weak intrinsic spin-orbit coupling $λ_{SO}$, and it transitions a trivial phase when $λ_{SO}>(\sqrt{M^2+λ^2_R}+M)/2$. Moreover, we use a tight-binding model to reproduce the ab-initio method obtained band structures through doping magnetic atoms on $3\times3$ and $4\times4$ supercells of graphene, and explain the physical mechanisms of opening a nontrivial bulk gap to realize the QAH effect in different supercells of graphene.

cond-mat.mtrl-sci↗
arXiv2011

MoS_2 as an ideal material for valleytronics: valley-selective circular dichroism and valley Hall effect

A two-dimensional honeycomb lattice harbors a pair of inequivalent valleys in the k-space electronic structure, in the vicinities of the vertices of a hexagonal Brillouin zone, K}$_{\pm}$. It is particularly appealing to exploit this emergent degree of freedom of charge carriers, in what is termed "valleytronics", if charge carrier imbalance between the valleys can be achieved. The physics of valley polarization will make possible electronic devices such as valley filter and valley valve, and optoelectronic Hall devices, all very promising for next-generation electronic and optoelectronic applications. The key challenge lies with achieving valley imbalance, of which a convincing demonstration in a two-dimensional honeycomb structure remains evasive, while there are only a handful of examples for other materials. We show here, using first principles calculations, that monolayer MoS_2, a novel two-dimensional semiconductor with a 1.8 eV direct band gap, is an ideal material for valleytronics by valley- selective circular dichroism, with ensuing valley polarization and valley Hall effect.

cond-mat.mtrl-sci↗
arXiv2011

Influence of Intrinsic Electronic Properties on Light Transmission through Subwavelength Holes on Gold and MgB2 Thin Films

We show how intrinsic material properties modify light transmission through subwavelength hole arrays on thin metallic films in the THz regime. We compare the temperature-dependent transmittance of Au films and MgB$_{2}$ films. The experimental data is consistent with analytical calculations, and is attributed to the temperature change of the conductivity of both films. The transmission versus conductivity is interpreted within the open resonator model when taking the skin depth into consideration. We also show that the efficiency of this temperature control depends on the ratio of the transmission peak frequency to the superconducting energy gap in MgB$_{2}$ films.

cond-mat.supr-con↗
arXiv2011

Quantum anomalous Hall effect with tunable Chern number in magnetic topological insulator film

We study the possibility of realizing quantum anomalous Hall effect (QAHE) with tunable Chern number through doping magnetic elements in the multi-layer topological insulator film. We find that high Chern number QAHE phases exist in the ideal neutral samples and it can make transition to another QAHE phase directly by means of tuning the exchange field strength or sample thickness. With the help of an extended Haldane model, we demonstrate the physical mechanism of the tunable Chern number QAHE phase. Further, we study the phenomena in the realistic samples with the Fermi energy being gating-modulated, where the multiple-integer QAHE plateaus remain and two separated QAHE phase are connected by metallic phase. The stability of the high Chern number QAHE phase is also discussed.

cond-mat.mes-hall↗
arXiv2011

Energy Magnetization and Thermal Hall Effect

We obtain a set of general formulae for determining magnetizations, including the usual electromagnetic magnetization as well as the gravitomagnetic energy magnetization. The magnetization corrections to the thermal transport coefficients are explicitly demonstrated. Our theory provides a systematic approach for properly evaluating the thermal transport coefficients of magnetic systems, eliminating the unphysical divergence from the direct application of the Kubo formula. For an anomalous Hall system, the corrected thermal Hall conductivity obeys the Wiedemann-Franz law.

cond-mat.stat-mech↗
arXiv2011

Two-Dimensional Topological Insulator State and Topological Phase Transition in Bilayer Graphene

We show that gated bilayer graphene hosts a strong topological insulator (TI) phase in the presence of Rashba spin-orbit (SO) coupling. We find that gated bilayer graphene under preserved time-reversal symmetry is a quantum valley Hall insulator for small Rashba SO coupling $λ_{\mathrm{R}}$, and transitions to a strong TI when $λ_{\mathrm{R}} > \sqrt{U^2+t_\bot^2}$, where $U$ and $t_\bot$ are respectively the interlayer potential and tunneling energy. Different from a conventional quantum spin Hall state, the edge modes of our strong TI phase exhibit both spin and valley filtering, and thus share the properties of both quantum spin Hall and quantum valley Hall insulators. The strong TI phase remains robust in the presence of weak graphene intrinsic SO coupling.

cond-mat.mtrl-sci↗
arXiv2011

Magnetic and Electronic Properties of Metal-Atom Adsorbed Graphene

We systematically investigate the magnetic and electronic properties of graphene adsorbed with diluted 3d-transition and noble metal atoms using first principles calculation methods. We find that most transition metal atoms (i.e. Sc, Ti, V, Mn, Fe) favor the hollow adsorption site, and the interaction between magnetic adatoms and π-orbital of graphene induces sizable exchange field and Rashba spin-orbit coupling, which together open a nontrivial bulk gap near the Dirac points leading to the quantum-anomalous Hall effect. We also find that the noble metal atoms (i.e. Cu, Ag, Au) prefer the top adsorption site, and the dominant inequality of the AB sublattice potential opens another kind of nontrivial bulk gap exhibiting the quantum-valley Hall effect.

cond-mat.mtrl-sci↗
arXiv2011

Singular Effects of Spin-Flip Scattering on Gapped Dirac Fermions

We investigate the effects of spin-flip scattering on the Hall transport and spectral properties of gapped Dirac fermions. We find that in the weak scattering regime, the Berry curvature distribution is dramatically compressed in the electronic energy spectrum, becoming singular at band edges. As a result the Hall conductivity has a sudden jump (or drop) of $e^2/2h$ when the Fermi energy sweeps across the band edges, and otherwise is a constant quantized in units of $e^2/2h$. In parallel, spectral properties such as the density of states and spin polarization are also greatly enhanced at band edges. Possible experimental methods to detect these effects are discussed.

cond-mat.mtrl-sci↗
arXiv2011

Brownian motion in superfluid $^4$He

We propose to study the Brownian motion of a classical microsphere submerged in superfluid $^4$He using the recent laser technology as a direct investigation of the thermal fluctuations of quasiparticles in the quantum fluid. By calculating the temperature dependence of both the friction coefficient and the strength of the random force, we show that the resonant mode of the fluctuational motion can be fully resolved by the present technology. Contrary to the previous work, it is found that the roton contribution is not negligible, and it even becomes dominant when the temperature is above 0.76\,K.

cond-mat.stat-mech↗
arXiv2011

Electronic Highways in Bilayer Graphene

Bilayer graphene with an interlayer potential difference has an energy gap and, when the potential difference varies spatially, topologically protected one-dimensional states localized along the difference's zero-lines. When disorder is absent, electronic travel directions along zero-line trajectories are fixed by valley Hall properties. Using the Landauer-Büttiker formula and the non-equilibrium Green's function technique we demonstrate numerically that collisions between electrons traveling in opposite directions, due to either disorder or changes in path direction, are strongly suppressed. We find that extremely long mean free paths of the order of hundreds of microns can be expected in relatively clean samples. This finding suggests the possibility of designing low power nanoscale electronic devices in which transport paths are controlled by gates which alter the inter-layer potential landscape.

cond-mat.mes-hall↗
arXiv2011

Quantum Anomalous Hall Effect in Single-layer and Bilayer Graphene

The quantum anomalous Hall effect can occur in single and few layer graphene systems that have both exchange fields and spin-orbit coupling. In this paper, we present a study of the quantum anomalous Hall effect in single-layer and gated bilayer graphene systems with Rashba spin-orbit coupling. We compute Berry curvatures at each valley point and find that for single-layer graphene the Hall conductivity is quantized at $σ_{xy} = 2e^2/h$, with each valley contributing a unit conductance and a corresponding chiral edge state. In bilayer graphene, we find that the quantized anomalous Hall conductivity is twice that of the single-layer case when the gate voltage $U$ is smaller than the exchange field $M$, and zero otherwise. Although the Chern number vanishes when $U > M$, the system still exhibits a quantized valley Hall effect, with the edge states in opposite valleys propagating in opposite directions. The possibility of tuning between different topological states with an external gate voltage suggests possible graphene-based spintronics applications.

cond-mat.mes-hall↗
arXiv2011

Anomalous Magnetic Susceptibility and Hall Effect from Valley Degrees of Freedom

We study the magnetic and transport properties of epitaxial graphene films in this letter. We predict enhanced signal of magnetic susceptibility and relate it to the intrinsic valley magnetic moments. There is also an anomalous contribution to the ordinary Hall effect, which is due to the valley dependent Berry phase or valley-orbit coupling.

cond-mat.mes-hall↗
arXiv2011

Quantum Size Effect and Electronic Stability of Freestanding Metal Atom Wires

Using DFT calculations, we present a thorough study of the quantum size effects on the stability of freestanding metal atom wires. Our systems include Na, Ag, Au, In, Ga and Pb atom wires, i.e. $s$, $sd$, and $sp$ electron prototypes. We found that the total energy always oscillates with the wire length, which clearly indicates the existence of preferred lengths. Increasing the length, the s-system exhibits even-odd oscillations following a $a/x +b/x^2$ decay law in the stability, which can be attributed to electron band filling and quantum confinement along the wire. The $sd$-system exhibits a similar oscillation pattern, even in the presence of $sd$ hybridization. In $sp$-system, the energy oscillations are beyond the simple even-odd nature, likely due to unpaired p orbitals and the corresponding nontrival band filling. Our findings clearly demonstrate that electronic contribution is quite critical to the stability of freestanding wires, and this stability may be important even when wires are deposited on substrates or strained. This study sheds light on the underlying formation mechanism of metal atom wires.

cond-mat.mtrl-sci↗
arXiv2011

Spin Polarized and Valley Helical Edge Modes in Graphene Nanoribbons

Inspired by recent progress in fabricating precisely zigzag-edged graphene nanoribbons and the observation of edge magnetism, we find that spin polarized edge modes with well-defined valley index can exist in a bulk energy gap opened by a staggered sublattice potential such as that provided by a hexagonal Boron-Nitride substrate. Our result is obtained by both tight-binding model and first principles calculations. These edge modes are helical with respect to the valley degree of freedom, and are robust against scattering, as long as the disorder potential is smooth over atomic scale, resulting from the protection of the large momentum separation of the valleys.

cond-mat.mtrl-sci↗
arXiv2011

Superconducting phase with a chiral $f$-wave pairing symmetry and Majorana fermions induced in a hole-doped semiconductor

We show that a chiral $f+if$-wave superconducting pairing may be induced in the lowest heavy hole band of a hole-doped semiconductor thin film through proximity contact with an \textit{s}-wave superconductor. The chirality of the pairing originates from the $3π$ Berry phase accumulated for a heavy hole moving along a close path on the Fermi surface. There exist three chiral gapless Majorana edge states, in consistence with the chiral $f+if$% -wave pairing. We show the existence of zero energy Majorana fermions in vortices in the semiconductor-superconductor heterostructure by solving the Bogoliubov-de-Gennes equations numerically as well as analytically in the strong confinement limit.

cond-mat.supr-con↗
arXiv2010

Scattering universality classes of side jump in anomalous Hall effect

The anomalous Hall conductivity has an important extrinsic contribution known as side jump contribution, which is independent of both scattering strength and disorder density. Nevertheless, we discover that side jump has strong dependence on the spin structure of the scattering potential. We propose three universality classes of scattering for the side jump contribution, having the characters of being spin-independent, spin-conserving and spin-flip respectively. For each individual class, the side jump contribution takes a different unique value. When two or more classes of scattering are present, the value of side jump is no longer fixed but varies as a function of their relative disorder strength. As system control parameter such as temperature changes, due to the competition between different classes of disorder scattering, the side jump Hall conductivity could flow from one class dominated limit to another class dominated limit. Our result indicates that magnon scattering plays a role distinct from normal impurity scattering and phonon scattering in the anomalous Hall effect because they belong to different scattering classes.

cond-mat.mes-hall↗