Publications (346)1313.23 Total impact
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ABSTRACT: Onsager relations permit linear response of the resistivity to an external magnetic field only when time reversal symmetry is broken. By employing semiclassical Boltzmann transport theory generalized to account for anomalous velocities and orbital and spin magnetic moments, we obtain an explicit expression for the linear magnetoresistance of a crystalline conductor. Like the anomalous Hall effect, also permitted only when timereversal symmetry is broken, it includes both intrinsic and extrinsic contributions. The intrinsic linear magnetoresistance, which is dominant in strongly disordered systems, is due to the influence on transport of momentumspace Berry curvatures and momentumdependent Bloch state magnetic moments. For the case of surface state transport in magnetically ordered topological insulators we predict positive magnetoresistance that is linear in field but independent of field direction.  [Show abstract] [Hide abstract]
ABSTRACT: Topological phases with insulating bulk and gapless surface or edge modes have attracted much attention because of their fundamental physics implications and potential applications in dissipationless electronics and spintronics. In this review, we mainly focus on the recent progress in the engineering of topologically nontrivial phases (such as $\mathbb{Z}_2$ topological insulators, quantum anomalous Hall effects, quantum valley Hall effects \textit{etc.}) in twodimensional material systems, including quantum wells, atomic crystal layers of elements from group III to group VII, and the transition metal compounds. 
Article: Large magnetooptical Kerr effect in noncollinear antiferromagnets Mn$_{3}X$ ($X$ = Rh, Ir, or Pt)
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ABSTRACT: Magnetooptical Kerr effect, normally found in magnetic materials with nonzero magnetization such as ferromagnets and ferrimagnets, has been known for more than a century. Here, using firstprinciples density functional theory, we demonstrate large magnetooptical Kerr effect in high temperature noncollinear antiferromagnets Mn$_{3}X$ ($X$ = Rh, Ir, or Pt), in contrast to usual wisdom. The calculated Kerr rotation angles are large, being comparable to that of transition metal magnets such as bcc Fe. The large Kerr rotation angles and ellipticities are found to originate from the lifting of the band doubledegeneracy due to the absence of spatial symmetry in the Mn$_{3}X$ noncollinear antiferromagnets which together with the timereversal symmetry would preserve the Kramers theorem. Our results indicate that Mn$_{3}X$ would provide a rare material platform for exploration of subtle magnetooptical phenomena in noncollinear magnetic materials without net magnetization.  [Show abstract] [Hide abstract]
ABSTRACT: Within the wavepacket semiclassical approach, the Bloch electron energy is derived to second order in the magnetic field and classified into gaugeinvariant terms with clear physical meaning, yielding a fresh understanding of the complex behavior of orbital magnetic susceptibility. The Berry curvature and quantum metric of the Bloch states give rise to a geometrical magnetic susceptibility, which can be dominant when bands are filled up to a small energy gap. There is also an energy polarization term, which can compete with the PeierlsLandau and Pauli magnetism on a Fermi surface. All these, and an additional Langevin susceptibility, can be calculated from each single band, leaving the Van Vleck susceptibility as the only term truly from interband coupling.  [Show abstract] [Hide abstract]
ABSTRACT: Monolayer transitionmetal dichalcogenides possess a pair of degenerate helical valleys in the band structure that exhibit fascinating optical valley polarization. Optical valley polarization, however, is limited by carrier lifetimes of these materials. Lifting the valley degeneracy is therefore an attractive route for achieving valley polarization. It is very challenging to achieve appreciable valley degeneracy splitting with applied magnetic field. We propose a strategy to create giant splitting of the valley degeneracy by proximityinduced Zeeman effect. As a demonstration, our first principles calculations of monolayer MoTe$_2$ on a EuO substrate show that valley splitting over 300 meV can be generated. The proximity coupling also makes interband transition energies valley dependent, enabling valley selection by optical frequency tuning in addition to circular polarization. The valley splitting in the heterostructure is also continuously tunable by rotating substrate magnetization. The giant and tunable valley splitting adds a readily accessible dimension to the valleyspin physics with rich and interesting experimental consequences, and offers a practical avenue for exploring device paradigms based on the intrinsic degrees of freedom of electrons.  [Show abstract] [Hide abstract]
ABSTRACT: We derive a general scaling relation for the anomalous Hall effect in ferromagnetic metals involving multiple competing scattering mechanisms, described by a quadratic hypersurface in the space spanned by the partial resistivities. We also present experimental findings, which show strong deviation from previously found scaling forms when different scattering mechanism compete in strength but can be nicely explained by our theory.  [Show abstract] [Hide abstract]
ABSTRACT: The two inequivalent valleys in graphene preclude the protection against intervalley scattering offered by an oddnumber of Dirac cones characteristic of Z2 topological insulator phases. Here we propose a way to engineer a chiral singlevalley metallic phase with quadratic crossover in a honeycomb lattice through tailored \sqrt{3}N *\sqrt{3}N or 3N *3N superlattices. The possibility of tuning valleypolarization via pseudoZeeman field and the emergence of Dresselhaustype valleyorbit coupling are proposed in adatom decorated graphene superlattices. Such valley manipulation mechanisms and metallic phase can also find applications in honeycomb photonic crystals.  [Show abstract] [Hide abstract]
ABSTRACT: The dynamics of the nonlinear generalized quantum deltakicked rotator is investigated with different initial states, under quantum antiresonance (AR) and quantum resonance (QR) conditions. With the help of an analytical stationary solution of nonlinear Schrodinger equation with periodic condition, the relationship between the different stationary solutions obtained by Carr et al. [Phys. Rev. A 62, 063610 (2000)] is presented. We find that for weak nonlinearity, the quantum beating does not depend on the initial states except the beating frequency for the AR case, whereas the different rates of suppression depend on the initial states for the QR case. It is interesting to note that the energy of the system evolving in a truly irregular manner for large nonlinearity is bounded within some limits for both AR and QR conditions.  [Show abstract] [Hide abstract]
ABSTRACT: Within the wavepacket semiclassical approach, the Bloch electron energy is derived to second order in the magnetic field and classified into gaugeinvariant terms with clear physical meaning, yielding a fresh understanding of the complex behavior of orbital magnetism. The Berry curvature and quantum metric of the Bloch states give rise to a geometrical magnetic susceptibility, which can be dominant when bands are filled up to a small energy gap. There is also an energy polarization term, which can compete with the PeierlsLandau and Pauli magnetism on a Fermi surface. All these, and an additional Langevin susceptibility, can be calculated from each single band, leaving the Van Vleck susceptibility as the only term truly from interband coupling.  [Show abstract] [Hide abstract]
ABSTRACT: A topological insulator is a novel state of quantum matter, characterized by symmetryprotected Dirac interfacial states within its bulk gap. Tremendous effort has been invested into the search for topological insulators. To date, the discovery of topological insulators has been largely limited to natural crystalline solids. Therefore, it is highly desirable to tailormake various topological states of matter by design, starting with but a few accessible materials or elements. Here, we establish that valleydependent dimerization of Dirac surface states can be exploited to induce topological quantum phase transitions, in a binary superlattice bearing symmetryunrelated interfacial Dirac states. This mechanism leads to a rich phase diagram and allows for rational design of strong topological insulators, weak topological insulators, and topological crystalline insulators. Our ab initio simulations further demonstrate this mechanism in [111] and [110] superlattices of calcium and tin tellurides. While our results reveal a remarkable phase diagram for the binary superlattice, the mechanism is a general route to design various topological states.  [Show abstract] [Hide abstract]
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 orderinduced topological characters. We use our results to estimate GinzburgLandau model parameters and transition temperatures for the ordered states of bilayer graphene. Introduction.— Neutral bilayer graphene (BLG) [1, 2] and its ABCstacked multilayer cousins [3–6], are attractive platforms for unconventional twodimensional electron systems physics because they have flat band contact near their Fermi levels, and because order induces large momentumspace Berry curvatures [6] in their quasiparticle 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 localized in each layer, and have an interlayer phase that is strongly wavevector dependent. When latticescale corrections to bilayer graphene's massive Dirac model [1, 2] are neglected, the broken symmetry states predicted by meanfield theory have a charged quasiparticle energy gap [6, 7, 10–12] and spontaneous layer polarization within each of the four spinvalley flavors. Recent experiments [30–38] appear to rule out a competing family of nematic states [13–15], which do not have a quasiparticle gap and break rotational symmetry [39].  [Show abstract] [Hide abstract]
ABSTRACT: The valley dependent optical selection rules in recently discovered monolayer groupVI 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 inversionsymmetry breaking results in unusual valley dependent interLL 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 magnetooptical Faraday rotations.  [Show abstract] [Hide abstract]
ABSTRACT: We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in proximityinduced ferromagnetic palladium and platinum which is widely used in spintronics, within the Berry phase formalism based on the relativistic band structure calculations. We find that both the anomalous Hall ($\sigma_{xy}^A$) and Nernst ($\alpha_{xy}^A$) conductivities can be related to the spin Hall conductivity ($\sigma_{xy}^S$) and band exchangesplitting ($\Delta_{ex}$) by relations $\sigma_{xy}^A =\Delta_{ex}\frac{e}{\hbar}\sigma_{xy}^S(E_F)'$ and $\alpha_{xy}^A = \frac{\pi^2}{3}\frac{k_B^2T\Delta_{ex}}{\hbar}\sigma_{xy}^s(\mu)"$, respectively. In particular, these relations would predict that the $\sigma_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since the $\sigma_{xy}^S(E_F)'$ is positive (negative). Furthermore, both $\sigma_{xy}^A$ and $\alpha_{xy}^A$ are approximately proportional to the induced spin magnetic moment ($m_s$) because the $\Delta_{ex}$ is a linear function of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we predict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic platinum and palladium would be gigantic, being up to ten times larger than, e.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be significant.  [Show abstract] [Hide abstract]
ABSTRACT: An intersection between onedimensional chiral acts as a topological current splitter. We find that the splitting of a chiral zeroline mode obeys very simple, yet highly counterintuitive, partition laws which relate current paths to the geometry of the intersection. Our results have far reaching implications for device proposals based on chiral zeroline transport in the design of electron beam splitters and interferometers, and for understanding transport properties in systems where multiple topological domains lead to a statistical network of chiral channels. A massive chiral twodimensional electron gas (C2DEG) has a valley Hall conductivity that has the same sign as its mass. The valley Hall effect leads to conducting edge states and also, when the mass parameter varies spatially, to conducting states localized along mass zerolines. 1–3 Provided that intervalley scattering is weak, zeroline state properties are closely analogous 1–3  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: Spin pumping and spintransfer torques are two widely studied reciprocal phenomena in ferromagnets. However, pumping phenomena in homogeneous antiferromagnets and their relations to currentinduced torques have not been explored. By calculating how electrons scatter off a normal metalantiferromagnetic 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 currentinduced torques via Onsager reciprocity relations.  [Show abstract] [Hide abstract]
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. 
Article: Quantum Anomalous Hall Effect in Graphene Proximity Coupled to an Antiferromagnetic Insulator
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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 timereversal symmetry and spinorbit 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 proximityinduced exchange field in graphene is about 70 meV, and that a topologically nontrivial band gap is opened by Rashba spinorbit 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.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum transport measurements including the AltshulerAronovSpivak (AAS) and AharonovBohm (AB) effects, universal conductance fluctuations (UCF), and weak antilocalization (WAL) have been carried out on epitaxial Bi thin films ($1070$ 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 nontrivial 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.  [Show abstract] [Hide abstract]
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 firstprinciples 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 giantgaps are entirely due to the result of strong spinorbit 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 giantgap Tls BiX/SbX an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT.
Publication Stats
14k  Citations  
1,313.23  Total Impact Points  
Top Journals
 Physical Review Letters (54)
 Physical review. B, Condensed matter (42)
 Physical Review B (29)
 Physical Review Letters (26)
 Physical Review A (12)
Institutions

19922015

University of Texas at Austin
 Department of Physics
Austin, Texas, United States


20112014

Peking University
 • International Center for Quantum Materials
 • Department of Physics
Peping, Beijing, China 
University of Science and Technology of China
 Department of Physics
Luchow, Anhui Sheng, China


1998

Umeå University
Umeå, Västerbotten, Sweden 
University of Cambridge
 Department of Physics: Cavendish Laboratory
Cambridge, ENG, United Kingdom


19961998

University of Washington Seattle
 Department of Physics
Seattle, WA, United States


1994

Texas A&M University
College Station, Texas, United States


19881991

University of California, Santa Barbara
 Department of Physics
Santa Barbara, California, United States


19861988

University of Illinois, UrbanaChampaign
 Department of Physics
Urbana, Illinois, United States
