Qian Niu

Peking University, Peping, Beijing, China

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Publications (314)1151.32 Total impact

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    ABSTRACT: We derive the field correction to the Berry curvature of Bloch electrons, which can be traced back to a positional shift due to the interband mixing induced by external electromagnetic fields. The resulting semiclassical dynamics is accurate to second order in the fields, in the same form as before, provided that the wave packet energy is derived up to the same order. As applications, we discuss the orbital magnetoelectric polarizability and predict nonlinear anomalous Hall effects.
    Physical Review Letters 04/2014; 112(16):166601. · 7.94 Impact Factor
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    ABSTRACT: Spin pumping and spin-transfer torques are two widely studied reciprocal phenomena in ferromagnets. However, pumping phenomena in homogeneous antiferromagnets and their relations to current-induced torques have not been explored. By calculating how electrons scatter off a normal metal-antiferromagnetic interface, we derive pumped spin and staggered spin currents in terms of the staggered field, the magnetization, and their rates of change. For both compensated and uncompensated interfaces, spin pumping is large and of a similar magnitude with a direction controlled by the microwave polarization. The pumped currents are connected to current-induced torques via Onsager reciprocity relations.
    04/2014;
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    ABSTRACT: Bilayer graphene is susceptible to a family of unusual broken symmetry states with spin and valley dependent layer polarization. We report on a microscopic study of the domain walls in these systems, demonstrating that they have interesting microscopic structures related to order-induced topological characters. We use our results to estimate Ginzburg-Landau model parameters and transition temperatures for the ordered states of bilayer graphene. Introduction.— Neutral bilayer graphene (BLG) [1, 2] and its ABC-stacked multilayer cousins [3–6], are attrac-tive platforms for unconventional two-dimensional elec-tron systems physics because they have flat band contact near their Fermi levels, and because order induces large momentum-space Berry curvatures [6] in their quasiparti-cle bands. Theoretical studies have identified a variety of potential broken symmetry states in neutral suspended BLG [6–29]. The band eigenstates in bilayer graphene are equal weight coherent sums of components local-ized in each layer, and have an interlayer phase that is strongly wavevector dependent. When lattice-scale cor-rections to bilayer graphene's massive Dirac model [1, 2] are neglected, the broken symmetry states predicted by mean-field theory have a charged quasiparticle energy gap [6, 7, 10–12] and spontaneous layer polarization within each of the four spin-valley flavors. Recent ex-periments [30–38] appear to rule out a competing family of nematic states [13–15], which do not have a quasipar-ticle gap and break rotational symmetry [39].
    arxiv. 04/2014; 1404.3607.
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    ABSTRACT: We derive the field correction to the Berry curvature of Bloch electrons, which can be traced back to a positional shift due to the interband mixing induced by external electromagnetic fields. The resulting semiclassical dynamics is accurate to second order in the fields, in the same form as before, provided that the wave packet energy is derived up to the same order. As applications, we discuss the orbital magnetoelectric polarizability and predict nonlinear anomalous Hall effects.
    03/2014; 112(16).
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    ABSTRACT: We propose realizing the quantum anomalous Hall effect by proximity coupling graphene to an antiferromagnetic insulator that provides both broken time-reversal symmetry and spin-orbit coupling. We illustrate our idea by performing ab initio calculations for graphene adsorbed on the (111) surface of BiFeO3. In this case, we find that the proximity-induced exchange field in graphene is about 70 meV, and that a topologically nontrivial band gap is opened by Rashba spin-orbit coupling. The size of the gap depends on the separation between the graphene and the thin film substrate, which can be tuned experimentally by applying external pressure.
    Physical Review Letters 03/2014; 112(11):116404. · 7.94 Impact Factor
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    ABSTRACT: Quantum transport measurements including the Altshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) effects, universal conductance fluctuations (UCF), and weak anti-localization (WAL) have been carried out on epitaxial Bi thin films ($10-70$ bilayers) on Si(111). The results show that while the film interior is insulating all six surfaces of the Bi thin films are robustly metallic. We propose that these properties are the manifestation of a novel phenomenon, namely, a topologically trivial bulk system can become topologically non-trivial when it is made into a thin film. We stress that what's observed here is entirely different from the predicted 2D topological insulating state in a single bilayer Bi where only the four side surfaces should possess topologically protected gapless states.
    03/2014;
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    ABSTRACT: Large bulk band gap is critical for the application of the quantum spin Hall (QSH) insulator or two dimensional (2D) Z2 topological insulator (TI) in spintronic device operating at room temperature (RT). Based on the first-principles calculations, here we predict a group of 2D topological insulators BiX/SbX (X = H, F, Cl, and Br) with extraordinarily large bulk gaps from 0.32 to a record value of 1.08 eV. These giant-gaps are entirely due to the result of strong spin-orbit interaction being related to the px and py orbitals of the Bi/Sb atoms around the two valley K and K' of honeycomb lattice, which is different significantly from the one consisted of pz orbital just like in graphene and silicene. The topological characteristic of BiX/SbX monolayers is confirmed by the calculated nontrivial Z2 index and an explicit construction of the low energy effective Hamiltonian in these systems. We show that the honeycomb structure of BiX remains stable even at a temperature of 600 K. These features make the giant-gap Tls BiX/SbX an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT.
    02/2014;
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    Ran Cheng, Qian Niu
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    ABSTRACT: When a spin-polarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the magnetization via spin transfer torques. In antiferromagnetic (AFM) materials, however, the corresponding problem is unsolved. We derive microscopically the dynamics of an AFM system driven by spin current generated by an attached FM polarizer, and find that the spin current exerts a driving force on the local staggered order parameter. The mechanism does not rely on the conservation of spin angular momentum, nor does it depend on the induced FM moments on top the AFM background. Two examples are studied: (i) A domain wall is accelerated to a terminal velocity by purely adiabatic effect where the Walker's break-down is avoided. (ii) Spin injection modifies the AFM resonance frequency, and spin current injection triggers spin wave instability of local moments above a threshold.
    Physical Review B 02/2014; 89(081105(R)). · 3.77 Impact Factor
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    ABSTRACT: The valley dependent optical selection rules in recently discovered monolayer group-VI transition metal dichalcogenides (TMDs) make possible optical control of valley polarization, a crucial step towards valleytronic applications. However, in presence of Landaul level(LL) quantization such selection rules are taken over by selection rules between the LLs, which are not necessarily valley contrasting. Using MoS$_{2}$ as an example we show that the spatial inversion-symmetry breaking results in unusual valley dependent inter-LL selection rules, which directly locks polarization to valley. We find a systematic valley splitting for all Landau levels (LLs) in the quantum Hall regime, whose magnitude is linearly proportional to the magnetic field and in comparable with the LL spacing. Consequently, unique plateau structures are found in the optical Hall conductivity, which can be measured by the magneto-optical Faraday rotations.
    01/2014;
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    Hua Chen, Qian Niu, A H Macdonald
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    ABSTRACT: As established in the very early work of Edwin Hall, ferromagnetic conductors have an anomalous Hall conductivity contribution that cannot be attributed to Lorentz forces and therefore survives in the absence of a magnetic field. These anomalous Hall conductivities are normally assumed to be proportional to magnetization. We use symmetry arguments and first-principles electronic structure calculations to counter this assumption and to predict that Mn_{3}Ir, a high-temperature antiferromagnet that is commonly employed in spin-valve devices, has a large anomalous Hall conductivity.
    Physical Review Letters 01/2014; 112(1):017205. · 7.94 Impact Factor
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    Hua Chen, Qian Niu, A H Macdonald
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    ABSTRACT: As established in the very early work of Edwin Hall, ferromagnetic conductors have an anomalous Hall conductivity contribution that cannot be attributed to Lorentz forces and therefore survives in the absence of a magnetic field. These anomalous Hall conductivities are normally assumed to be proportional to magnetization. We use symmetry arguments and first-principles electronic structure calculations to counter this assumption and to predict that Mn 3 Ir, a high-temperature antiferromagnet that is commonly employed in spin-valve devices, has a large anomalous Hall conductivity. In the presence of an external magnetic field, the Hall effect (current flow perpendicular to electric field) is present in all conductors. In ferromagnetic metals like Fe, Co, and Ni, however, the Hall effect is anomalous and controlled more by magnetization than by Lorentz forces [1,2]. Although the anomalous Hall effect is exper-imentally strong, it has stood alone among metallic trans-port effects for much of the last century because it lacked a usefully predictive, generally accepted theory. Progress over the past decade has explained why. It is now clear that the anomalous Hall effect in ferromagnets has contributions from both extrinsic scattering mechanisms similar to those that determine most transport coefficients, and from an intrinsic mechanism that is independent of scattering [2]. The intrinsic mechanism, first proposed by Karplus and Luttinger [3], has recently been reformulated in the lan-guage of Berry phases as a momentum-space geometrical effect [4] related to the quantum Hall effect and to topologi-cal insulators. In the quantized anomalous Hall effect [5] only the intrinsic component survives. The anomalous Hall effect is also observed in paramagnets, which have nonzero magnetization induced by an external magnetic field. Separately, studies of the anomalous Hall effect in frustrated and noncollinear magnetic systems have also been very active [2,6–13]. In these systems magnetic frus-tration may be removed by magnetocrystalline anisotropy or Dzyloshinskii-Moriya interactions, resulting in stable noncollinear magnetic order. For noncoplanar spin textures, the anomalous Hall effect in the strong local exchange coupling limit can then often be explained as a real-space Berry phase effect [2,6,9]. Although no explicit relationship has been established, the anomalous Hall effect in a particular material is usually assumed to be proportional to its magnetization. In this Letter, we point out that it is possible to have an anomalous Hall effect in a noncollinear antiferromagnet with zero net magnetization provided that certain common symmetries are absent, and predict that Mn 3 Ir, a technologically impor-tant antiferromagnetic material with noncollinear order that survives to very high temperatures, has a surprisingly large anomalous Hall effect comparable in size to those of the elemental transition metal ferromagnets. Mn 3 Ir can be viewed as an fcc crystal with Mn atoms on three of the four cubic sublattices [Fig. 1(a)], and is widely used as an exchange-bias material in spin-valve devices. Its Mn sublattice can be viewed as consisting of two-dimensional kagome lattices [Fig. 1(b)] stacked along the (111) direction. Although isolated two-dimensional kagome lattices order weakly because of strong magnetic frustration [14–18], the Mn moments in Mn 3 Ir have strong three-sublattice triangular (T1) magnetic order [Fig. 1(a)] [19,20] and a high magnetic transition temperature ∼950 K. The surprising stability is provided by inter-kagome coupling [21] combined with strong magnetic anisotropy [22,23]. Because the kagome structure, with appropriate exchange interactions, can support relatively simple noncollinear antiferromagnetic states similar to those of Mn 3 Ir, we begin our discussion of the anomalous Hall effect in noncollinear antiferromagnets by focusing on a simple two-dimensional kagome model. The itinerant electron Hamiltonian for an s-d model on the kagome lattice is
    Phys. Rev. Lett. 112, 017205. 01/2014; 11250.
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    Xiao Li, Fan Zhang, Qian Niu, Ji Feng
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    ABSTRACT: Gapless Dirac surface states are protected at the interface of topological and normal band insulators. In a binary superlattice bearing such interfaces, we establish that valley-dependent dimerization of symmetry-unrelated Dirac surface states can be exploited to induce topological quantum phase transitions. This mechanism leads to a rich phase diagram that allows us to design strong, weak, and crystalline topological insulators. Our ab initio simulations further demonstrate this mechanism in [111] and [110] superlattices of calcium and tin tellurides.
    10/2013;
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    ABSTRACT: We study the valley-dependent magnetic and transport properties of massive Dirac fermions in multivalley systems such as the transition metal dichalcogenides. The asymmetry of the zeroth Landau level between valleys and the enhanced magnetic susceptibility can be attributed to the different orbital magnetic moment tied with each valley. This allows the valley polarization to be controlled by tuning the external magnetic field and the doping level. As a result of this magnetic field induced valley polarization, there exists an extra contribution to the ordinary Hall effect. All these effects can be captured by a low energy effective theory with a valley-orbit coupling term.
    Physical Review B 09/2013; 88:115140. · 3.77 Impact Factor
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    Ran Cheng, Qian Niu
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    ABSTRACT: Spin-transfer torque (STT) provides key mechanisms for current-induced phenomena in ferromagnets. While it is widely accepted that STT involves both adiabatic and non-adiabatic contributions, their underlying physics and range of validity are quite controversial. By computing microscopically the response of conduction electron spins to a time varying and spatially inhomogeneous magnetic background, we derive the adiabatic and non-adiabatic STT in a unified fashion. Our result confirms the macroscopic theory [Phys.~Rev.~Lett.~\textbf{93},~127204~(2004)] with all coefficients matched exactly. Our derivation also reveals a benchmark on the validity of the result, which is used to explain three recent measurements of the non-adiabatic STT in quite different settings.
    Physical Review B 07/2013; 88:024422. · 3.77 Impact Factor
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    ABSTRACT: We report results from scanning tunneling microscopy and transport measurements on a series of crystalline lead films containing an integer number of atomic layers, and find that the observed features in sufficiently thin films are consistent with Berezinskii–Kosterlitz–Thouless (BKT) physics. Specifically, Cooper pairing and superconductivity disappear at two distinct temperatures; the current–voltage characteristics in the intermediate phase are non-Ohmic; and the temperature and current dependences of resistance agree with the expectation from the BKT theory.
    Solid State Communications 07/2013; 165:59–63. · 1.53 Impact Factor
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    ABSTRACT: We use spin density functional theory ab initio calculations to theoretically explore the possibility of achieving useful gate control over exchange coupling between cobalt clusters placed on a graphene sheet. By applying an electric field across supercells, we demonstrate that the exchange interaction is strongly dependent on gate voltage, and find that it is also sensitive to the relative sublattice registration of the cobalt clusters. We use our results to discuss strategies for achieving strong and reproducible magnetoelectric effects in graphene/transition-metal hybrid systems.
    Physical Review B 04/2013; 87:144410. · 3.77 Impact Factor
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    ABSTRACT: Conventional electronics are based invariably on the intrinsic degrees of freedom of an electron, namely its charge and spin. The exploration of novel electronic degrees of freedom has important implications in both basic quantum physics and advanced information technology. Valley, as a new electronic degree of freedom, has received considerable attention in recent years. In this paper, we develop the theory of spin and valley physics of an antiferromagnetic honeycomb lattice. We show that by coupling the valley degree of freedom to antiferromagnetic order, there is an emergent electronic degree of freedom characterized by the product of spin and valley indices, which leads to spin-valley-dependent optical selection rule and Berry curvature-induced topological quantum transport. These properties will enable optical polarization in the spin-valley space, and electrical detection/manipulation through the induced spin, valley, and charge fluxes. The domain walls of an antiferromagnetic honeycomb lattice harbors valley-protected edge states that support spin-dependent transport. Finally, we use first-principles calculations to show that the proposed optoelectronic properties may be realized in antiferromagnetic manganese chalcogenophosphates (MnPX, X = S, Se) in monolayer form.
    Proceedings of the National Academy of Sciences 03/2013; 110(10):3738-42. · 9.74 Impact Factor
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    ABSTRACT: In the first order effective theory of Bloch electrons in electromagnetic fields, the Berry curvature is introduced to yield an anomalous velocity term, which results in profound modification of the phase space density of states. Here we derive the second order single band effective theory, finding that the semiclassical dynamics of physical variables still follows the same structure as before, but with additional field corrections in the Berry curvature and band energy. We also discuss applications of our theory and its extension to multiple band case.
    03/2013;
  • Ran Cheng, Qian Niu
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    ABSTRACT: When a spin-polarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the magnetization via spin transfer torque. However, corresponding theory is absent in antiferromagnetic (AFM) metals due to the absence of spin conservation. We solve this problem via effective gauge theory without the necessity of spin conservation. By identifying the adiabatic dynamics of conduction electrons as a non-Abelian gauge theory on degenerate band, we derive the AFM version of Landau-Lifshitz-Gilbert equation with current-induced dynamics from a microscopic point of view. Quite different from its FM counterpart, current-induced dynamics in AFM materials does not behave as a torque, but a driving force triggering second order derivative of local staggered order with respect to time. Its physical consequences are studied in two examples: 1. A domain wall is accelerated to a terminal velocity without a Walker's threshold; 2. A sufficiently large spin current will generate spin wave excitation.
    03/2013;
  • Xiao Li, Fan Zhang, Qian Niu
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    ABSTRACT: We use a low energy effective model to analyze the optical responses of trilayer graphene samples. We first show that optical absorption of the ABA-stacked trilayer has strong dependence on both the Fermi energy and optical frequency, which is in sharp contrast to that of ABC-stacked trilayer graphene. Secondly, we are able to determine the possible existence of trigonal warping effects in the bandstructure of ABC-stacked trilayer graphene by a divergence in the absorption spectra at around 10 meV. In addition, we can partially distinguish the vairious broken symmetry states driven by electron-electron interactions in ABC-stacked trilayer graphene. In particular, the quantum anomalous Hall (QAH) state is sensitive to the polarization of the incident light, giving a way to detect its possible existence.
    03/2013;

Publication Stats

8k Citations
1,151.32 Total Impact Points

Institutions

  • 2011–2014
    • Peking University
      • International Center for Quantum Materials
      Peping, Beijing, China
    • University of Science and Technology of China
      Luchow, Anhui Sheng, China
  • 1992–2014
    • University of Texas at Austin
      • • Department of Physics
      • • Center for Nonlinear Dynamics
      Austin, Texas, United States
  • 2009
    • The University of Hong Kong
      • Department of Physics
      Hong Kong, Hong Kong
  • 2005–2007
    • Northeast Institute of Geography and Agroecology
      • Institute of Physics
      Beijing, Beijing Shi, China
    • National Taiwan University
      • Department of Physics
      Taipei, Taipei, Taiwan
  • 2006
    • Fudan University
      • Department of Physics
      Shanghai, Shanghai Shi, China
  • 2004
    • Texas A&M University
      College Station, Texas, United States
  • 2000
    • University of Tennessee
      • Department of Physics & Astronomy
      Knoxville, Tennessee, United States
  • 1999
    • Oak Ridge National Laboratory
      • Solid State Division
      Oak Ridge, Florida, United States
  • 1998
    • University of Cambridge
      • Department of Physics: Cavendish Laboratory
      Cambridge, ENG, United Kingdom
  • 1996–1998
    • University of Washington Seattle
      • Department of Physics
      Seattle, WA, United States
  • 1993
    • University of Everett Washington
      Austin, Texas, United States