arXiv subjects

Di Xiao

Publications and source records attributed to Di Xiao.

At least 217 records · Page 12Linked to original sources

arXiv2016

Multiple Hot-Carrier Collection in Photo-Excited Graphene Moire Superlattices

In conventional light harvesting devices, the absorption of a single photon only excites one electron, which sets the standard limit of power-conversion efficiency, such as the Shockley-Queisser limit. In principle, generating and harnessing multiple carriers per absorbed photon can improve the efficiency and possibly overcome this limit. Here, we report the observation of multiple hot carrier collection in graphene-boron-nitride Moire superlattice structures. A record-high zero-bias photoresponsivity of 0.3 ampere per watt, equivalently, an external quantum efficiency exceeding 50 percent, is achieved utilizing graphene photo-Nernst effect, which demonstrates a collection of at least 5 carriers per absorbed photon. We reveal that this effect arises from the enhanced Nernst coefficient through Lifshtiz transition at low energy Van Hove singularities, which is an emergent phenomenon due to the formation of Moire minibands. Our observation points to a new means for extremely efficient and flexible optoelectronics based on van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2016

Raman scattering and anomalous Stokes anti-Stokes ratio in MoTe2 atomic layers

Stokes and anti-Stokes Raman scattering are performed on atomic layers of hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor. The data reveal all six types of zone center optical phonons, along with their corresponding Davydov splittings, which have been challenging to see in other TMDCs. We discover that the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, and find the effect to be tunable by excitation frequency and number of atomic layers. These observations are interpreted as a result of resonance effects arising from the C excitons in the vicinity of the Brillouin zone center in the photon-electron-phonon interaction process.

cond-mat.mtrl-sci↗

arXiv2016

Antiferromagnetic Spin Wave Field-Effect Transistor

In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field-effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its application in THz signal modulation. Our findings open up the exciting possibility of digital data processing utilizing antiferromagnetic spin waves and enable the direct projection of optical computing concepts onto the mesoscopic scale.

cond-mat.mes-hall↗

arXiv2016

Collusive Attacks to "Circle-Type" Multi-party Quantum Key Agreement Protocols

We find that existing multi-party quantum key agreement (MQKA) protocols designed for fairness of the key are, in fact, unfair. Our analysis shows that these protocols are sensitive to collusive attacks; that is, dishonest participants can collaborate to predetermine the key without being detected. In fact, the transmission structures of the quantum particles in those unfair MQKA protocols, three of which have already been analyzed, have much in common. We call these unfair MQKA protocols circle-type MQKA protocols. Likewise, the transmission structures of the quantum particles in MQKA protocols that can resist collusive attacks are also similar. We call such protocols complete-graph-type MQKA protocols. A MQKA protocol also exists that can resist the above attacks but is still not fair, and we call it the tree-type MQKA protocol. We first point out a common, easily missed loophole that severely compromises the fairness of present circle-type MQKA protocols. Then we show that two dishonest participants at special positions can totally predetermine the key generated by circle-type MQKA protocols. We anticipate that our observations will contribute to secure and fair MQKA protocols, especially circle-type protocols.

quant-ph↗

arXiv2015

RKKY interaction of magnetic impurities in Dirac and Weyl semimetals

We theoretically study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities in both Dirac and Weyl semimetals (SMs). We find that the internode process, as well as the unique three-dimensional spin-momentum locking, has significant influences on the RKKY interaction, resulting in both a Heisenberg and an Ising term, and an additional Dzyaloshinsky-Moriya term if the inversion symmetry is absent. These interactions can lead to rich spin textures and possible ferromagnetism in Dirac and time-reversal symmetry-invariant Weyl SMs. The effect of anisotropic Dirac and Weyl nodes on the RKKY interaction is also discussed. Our results provide an alternative scheme to engineer topological SMs and shed new light on the application of Dirac and Weyl SMs in spintronics.

cond-mat.mes-hall↗

arXiv2015

Electronic structures and theoretical modelling of two-dimensional group-VIB transition metal dichalcogenides

Atomically thin group-VIB transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional (2D) semiconductors with extraordinary properties including the direct band gap in the visible frequency range, the pronounced spin-orbit coupling, the ultra-strong Coulomb interaction, and the rich physics associated with the valley degree of freedom. These 2D TMDs exhibit great potentials for device applications and have attracted vast interest for the exploration of new physics. 2D TMDs have complex electronic structures which underlie their physical properties. Here we review the bulk electronic structures in these new 2D materials as well as the theoretical models developed at different levels, along which we sort out the understandings on the origins of a variety of properties observed or predicted.

cond-mat.mes-hall↗

arXiv2015

Berry phase modification to the energy spectrum of excitons

By quantizing the semiclassical motion of excitons, we show that the Berry curvature can cause an energy splitting between exciton states with opposite angular momentum. This splitting is determined by the Berry curvature flux through the $\bm k$-space area spanned by the relative motion of the electron-hole pair in the exciton wave function. Using the gapped two-dimensional Dirac equation as a model, we show that this splitting can be understood as an effective spin-orbit coupling effect. In addition, there is also an energy shift caused by other "relativistic" terms. Our result reveals the limitation of the venerable hydrogenic model of excitons, and highlights the importance of the Berry curvature in the effective mass approximation.

cond-mat.mes-hall↗

arXiv2015

Converting a topologically trivial superconductor into a topological superconductor via magnetic doping

We present a comparative theoretical study of the effects of standard Anderson and magnetic disorders on the topological phases of two-dimensional Rashba spin-orbit coupled superconductors, with the initial state to be either topologically trivial or nontrivial. Using the self-consistent Born approximation approach, we show that the presence of Anderson disorders will drive a topological superconductor into a topologically trivial superconductor in the weak coupling limit. Even more strikingly, a topologically trivial superconductor can be driven into a topological superconductor upon diluted doping of independent magnetic disorders, which gradually narrow, close, and reopen the quasi-particle gap in a nontrivial manner. These topological phase transitions are distinctly characterized by the changes in the corresponding topological invariants. The central findings made here are also confirmed using a complementary numerical approach by solving the Bogoliubov-de Gennes equations self-consistently within a tight-binding model. The present study offers appealing new schemes for potential experimental realization of topological superconductors.

cond-mat.supr-con↗

arXiv2015

Spin-transfer torque induced spin waves in antiferromagnetic insulators

We explore the possibility of exciting spin waves in insulating antiferromagnetic films by injecting spin current at the surface. We analyze both magnetically compensated and uncompensated interfaces. We find that the spin current induced spin-transfer torque can excite spin waves in insulating antiferromagnetic materials and that the chirality of the excited spin wave is determined by the polarization of the injected spin current. Furthermore, the presence of magnetic surface anisotropy can greatly increase the accessibility of these excitations.

cond-mat.mes-hall↗

arXiv2015

Magnetic ground state of semiconducting transition metal trichalcogenide monolayers

Layered transition metal trichalcogenides with the chemical formula $ABX_3$ have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides. We show that the second and third nearest-neighbor exchange interactions ($J_2$ and $J_3$) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer $\text{CrSiTe}_3$ is an antiferromagnet with a zigzag spin texture due to significant contribution from $J_3$, whereas $\text{CrGeTe}_3$ is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn-compounds ($\text{MnPS}_3$ and $\text{MnPSe}_3$) always show antiferromagnetic Neel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that $\text{ABX}_3$ can be a promising platform to explore 2D magnetic phenomena.

cond-mat.mtrl-sci↗

arXiv2015

Spin responses and effective Hamiltonian for the two dimensional electron gas at oxide interface {LaAlO}$_3$/{SrTiO}$_3$

Strong Rashba spin-orbit coupling (SOC) of the two-dimensional electron gas (2DEG) at the oxide interface $\mathrm{LaAlO_{3}/SrTiO_{3}}$ underlies a variety of exotic physics, but its nature is still under debate. We derive an effective Hamiltonian for the 2DEG at the oxide interface $\mathrm{LaAlO_{3}/SrTiO_{3}}$ and find a different anisotropic Rashba SOC for the $d_{xz}$ and $d_{yz}$ orbitals. This anisotropic Rashba SOC leads to anisotropic static spin susceptibilities and also distinctive behavior of the spin Hall conductivity. These unique spin responses may be used to determine the nature of the Rashba SOC experimentally and shed light on the orbital origin of the 2DEG.

cond-mat.mes-hall↗

arXiv2015

Ultrafast Switching of Antiferromagnets via Spin-transfer Torque

Picosecond switching of the staggered antiferromagnetic order is shown to be realizable through spin-transfer torques from a short current pulse. The coupled dynamics of sublattice magnetization is mapped onto a classical pendulum subject to gravity and a driving pulse, where switching occurs if the pendulum acquires sufficient kinetic energy during the pulse to overcome the maximum of the effective gravity potential. The optimal switching scheme is explored through the dependence of switch angle and magnetic loss on the duration and strength of the current pulse. The physics discussed here provides a general route towards multi-functional THz applications via the spin-transfer torque in antiferromagnetic materials.

cond-mat.mes-hall↗

arXiv2015

Gate-tunable Topological Valley Transport in Bilayer Graphene

Valley pseudospin, the quantum degree of freedom characterizing the degenerate valleys in energy bands, is a distinct feature of two-dimensional Dirac materials. Similar to spin, the valley pseudospin is spanned by a time reversal pair of states, though the two valley pseudospin states transform to each other under spatial inversion. The breaking of inversion symmetry induces various valley-contrasted physical properties; for instance, valley-dependent topological transport is of both scientific and technological interests. Bilayer graphene (BLG) is a unique system whose intrinsic inversion symmetry can be controllably broken by a perpendicular electric field, offering a rare possibility for continuously tunable valley-topological transport. Here, we used a perpendicular gate electric field to break the inversion symmetry in BLG, and a giant nonlocal response was observed as a result of the topological transport of the valley pseudospin. We further showed that the valley transport is fully tunable by external gates, and that the nonlocal signal persists up to room temperature and over long distances. These observations challenge contemporary understanding of topological transport in a gapped system, and the robust topological transport may lead to future valleytronic applications.

cond-mat.mes-hall↗

arXiv2015

Optical generation and detection of pure valley current in monolayer transition metal dichalcogenides

We propose a practical scheme to generate a pure valley current in monolayer transition metal dichalcogenides by one-photon absorption of linearly polarized light. We show that the pure valley current can be detected by either photoluminescence measurements or the ultrafast pump-probe technique. Our method, together with the previously demonstrated generation of valley polarization, opens up the exciting possibility of ultrafast optical-only manipulation of the valley index. The tilted field effect on the valley current in experiment is also discussed.

cond-mat.mes-hall↗

arXiv2015

Plasmon mode as a detection of the chiral anomaly in Weyl semimetals

Weyl semimetals are one kind of three-dimensional gapless semimetal with nontrivial topology in the momentum space. The chiral anomaly in Weyl semimetals manifests as a charge imbalance between the Weyl nodes of opposite chiralities induced by parallel electric and magnetic fields. We investigate the chiral anomaly effect on the plasmon mode in both intrinsic and doped Weyl semimetals within the random phase approximation. We prove that the chiral anomaly gives rise to a different plasmon mode in intrinsic Weyl semimetals. We also find the chiral anomaly leads to some exotic properties in the plasmon dispersion in doped Weyl semimetals. Consequently, the unconventional plasmon mode acts as a signature of the chiral anomaly in Weyl semimetals, by which the spectrum of plasmon provides a proper way to detect the Lifshitz transition.

cond-mat.str-el↗

arXiv2015

Oxygen Vacancies on SrO-terminated SrTiO3(001) Surfaces studied by Scanning Tunneling Spectroscopy

The electronic structure of SrTiO3(001) surfaces was studied using scanning tunneling spectroscopy and density-functional theory. With high dynamic range measurements, an in-gap transition level was observed on SrO-terminated surfaces, at 2.7 eV above the valence band maximum. The density of centers responsible for this level was found to increase with surface segregation of oxygen vacancies and decrease with exposure to molecular oxygen. Based on these finding, the level is attributed to surface O vacancies. A level at a similar energy is predicted theoretically on SrO-terminated surfaces. For TiO2-terminated surfaces, no discrete in-gap state was observed, although one is predicted theoretically. This lack of signal is believed to be due to the nature of defect wavefunction involved, as well as the possible influence of transport limitations in the tunneling spectroscopy measurements.

cond-mat.mtrl-sci↗

arXiv2014

Two-Dimensional Material Nanophotonics

The emerging two-dimensional (2D) materials exhibit a wide range of electronic properties, ranging from insulating hexagonal boron nitride, semiconducting transition metal dichalcogenides such as molybdenum disulfide, to semi-metallic graphene. Here, we first review the optical properties and applications of a variety of 2D materials, followed by two different approaches to enhance their interactions with light: through their integration with external photonic structures and through their intrinsic polaritonic resonances. Finally, we cover a narrow bandgap layered material, black phosphorus, which serendipitously bridges the zero gap graphene and the relatively large-bandgap TMDCs. The plethora of 2D materials and their heterostructures, together with the approaches for enhancing light-matter interaction offers the promise of scientific discoveries and nanophotonics technologies across a wide range of electromagnetic spectrum.

cond-mat.mes-hall↗

arXiv2014

Effect of modulations of doping and strain on the electron transport in monolayer MoS_2

The doping and strain effects on the electron transport of monolayer MoS_2 are systematically investigated using the first-principles calculations with Boltzmann transport theory. We estimate the mobility has a maximum 275 cm^2/(Vs) in the low doping level under the strain-free condition. The applying a small strain (3%) can improve the maximum mobility to 1150 cm^2/(Vs) and the strain effect is more significant in the high doping level. We demonstrate that the electric resistance mainly due to the electron transition between K and Q valleys scattered by the M momentum phonons. However, the strain can effectively suppress this type of electron-phonon coupling by changing the energy difference between the K and Q valleys. This sensitivity of mobility to the external strain may direct the improving electron transport of MoS_2.

cond-mat.mtrl-sci↗