arXiv subjects

Xiangang Wan

Publications and source records attributed to Xiangang Wan.

At least 127 records · Page 7Linked to original sources

arXiv2015

Integrated Digital Inverters Based on Two-dimensional Anisotropic ReS2 Field-effect Transistors

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as top candidates for post-silicon electronics. While most of 2D TMDs exhibit isotropic behavior, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here, we present atomically thin rhenium disulfide (ReS2) flakes with a unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated mono- and few-layer ReS2 field effect transistors (FETs), which exhibit competitive performance with large current on/off ratios (~107) and low subthreshold swings (100 mV dec-1). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known 2D semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic FETs, suggesting the promising implementation of large-scale 2D logic circuits. Our results underscore the unique properties of 2D semiconducting materials with low crystal symmetry for future electronic applications.

cond-mat.mtrl-sci↗

arXiv2015

Enhanced quantum coherence in graphene caused by Pd cluster deposition

We report on the unexpected increase in the dephasing lengths of a graphene sheet caused by the deposition of Pd nanoclusters, as demonstrated by weak localization measurements. The dephasing lengths reached saturated values at low temperatures. Theoretical calculations indicate the p-type charge transfer from the Pd clusters, which contributes more carriers. The saturated values of dephasing lengths often depend on both the carrier concentration and mean free path. Although some impurities are increased as revealed by decreased mobilities, the intense charge transfer leads to the improved saturated values and subsequent improved dephasing lengths.

cond-mat.mes-hall↗

arXiv2014

Dirac and Weyl Semimetal in XYBi (X=Ba, Eu; Y=Cu, Ag and Au)

Weyl and Dirac semimetals recently stimulate intense research activities due to their novel properties. Combining first-principles calculations and effective model analysis, we predict that nonmagnetic compounds BaYBi (Y=Au, Ag and Cu) are Dirac semimetals. As for the magnetic compound EuYBi, although the time reversal symmetry is broken, their long-range magnetic ordering cannot split the Dirac point into pairs of Weyl points. However, we propose that partially substitute Eu ions by Ba ions will realize the Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2014

Metallic ferroelectricity induced by anisotropic unscreened coulomb interaction in LiOsO3

As the first well-documented example of the ferroelectric metal, LiOsO3 has received extensive research attention recently. Using density-functional calculations, we perform a systematic study for LiOsO3. We address the controversy about the depth of the double well in the potential surface, and propose that the ferroelectric transition is order-disorder like. Moreover, we unambiguously demonstrate that the electric screening in this compound is highly anisotropic, and there is still unscreened dipole-dipole interaction in one special direction which results in the long range ferroelectric order despite the metallic nature of LiOsO3.

cond-mat.mtrl-sci↗

arXiv2014

Tunable Magnetic Semiconductor Behavior Driven by Half-Filled One Dimensional Band in Zigzag Phosphorene Nanoribbons

An antiferromagnetic insulating state has been found in the zigzag phosphorene nanoribbons (ZPNRs) from a comprehensive density functional theory calculations. Comparing with other one-dimensional systems, the magnetism in ZPNRs display several surprising characteristics: (i) the magnetic moments are antiparallel arranged at each zigzag edge; (ii) the magnetism is quite stable in energy (about 29 meV/magnetic-ion) and the band gap is big (about 0.7 eV); (iii) a moderate compressive strain will induce a magnetic to nonmagnetic as well as semiconductor to metal transition. All of these phenomena arise naturally due to one unique mechanism, namely the electronic instability induced by the half-filled one dimensional bands which cross the Fermi level at around π/2a. The unusual electronic and magnetic properties in ZPNRs endow them great potential for the applications in nanoelectronic devices.

cond-mat.mes-hall↗

arXiv2014

Experimental evidence and control of the bulk-mediated intersurface coupling in topological insulator Bi2Te2Se nanoribbons

Nearly a decade after the discovery of topological insulators (TIs), the important task of identifying and characterizing their topological surface states through electrical transport experiments remains incomplete. The interpretation of these experiments is made difficult by the presence of residual bulk carriers and their coupling to surface states, which is not yet well understood. In this work, we present the first evidence for the existence and control of bulk-surface coupling in Bi2Te2Se nanoribbons, which are promising platforms for future TI-based devices. Our magnetoresistance measurements reveal that the number of coherent channels contributing to quantum interference in the nanoribbons changes abruptly when the film thickness exceeds the bulk phase relaxation length. We interpret this observation as an evidence for bulk-mediated coupling between metallic states located on opposite surfaces. This hypothesis is supported by additional magnetoresistance measurements conducted under a set of gate voltages and in a parallel magnetic field, the latter of which alters the intersurface coupling in a controllable way.

cond-mat.mes-hall↗

arXiv2014

Turning a Band Insulator Into an Exotic Superconductor

Understanding exotic, non s--wave--like states of Cooper pairs is important and may lead to new superconductors with higher critical temperatures and novel properties. Their existence is known to be possible but has always been thought to be associated with non--traditional mechanisms of superconductivity where electronic correlations play an important role. Here we use a first principles linear response calculation to show that in doped Bi$_{2}$Se$_{3}$ an unconventional p--wave--like state can be favored via a conventional phonon--mediated mechanism, as driven by an unusual, almost singular behavior of the electron--phonon interaction at long wavelengths. This may provide a new platform for our understanding superconductivity phenomena in doped band insulators.

cond-mat.supr-con↗

arXiv2014

Orbital Dependent Electronic Masses in Ce Heavy Fermion Materials studied via Gutzwiller Density Functional Theory

A series of Cerium based heavy fermion materials is studied using a combination of local density functional theory and many-body Gutzwiller approximation. Computed orbial dependent electronic mass enhancements parameters are compared with available data extracted from measured values of the Sommerfeld coefficient. The Gutzwiller density functional theory is shown to remarkably follow the trends across a variety of Ce compounds, and to give important insights on orbital selective mass renormalizations that allows a better understanding of wide spread of data.

cond-mat.str-el↗

arXiv2014

Electron-Phonon Superconductivity in LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$

We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum and electron--phonon coupling for newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$. Significant similarity between LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is found, i.e. there is a strong Fermi surface nesting at ($π$,$π$,0), which results in unstable phonon branches. Combining the frozen phonon total energy calculations and an anharmonic oscillator model, we find that the quantum fluctuation prevents the appearance of static long--range order. The calculation shows that LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is highly anisotropic, and same as LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, this compound is also a conventional electron-phonon coupling induced superconductor.

cond-mat.supr-con↗

arXiv2014

Spin-dependent optical response of multiferroic EuO

Using first-principles density functional calculations, electronic and optical properties of ferromagnetic semiconductor EuO are investigated. In particular, we have developed a way to obtain the spin-dependent optical response of the magnetic materials, which is helpful to verify the spin-dependent band structure of EuO. Significantly different optical responses from spin-up and spin-down channels are obtained in both linear and nonlinear cases, making it possible to distinguish contributions from different spin-channels in the optical absorption spectra if spin-flip process can be neglected. In addition, the red-shift of the absorption edge from paramagnetic to ferromagnetic ordering is explained by exchange interactions. Using such method, we have also compared the optical properties of multiferroic EuO which is induced by strong epitaxial strain. Our results show that from tensile to compressive strain, the blue-shift of the leading absorption peaks in the optical spectra, the red-shift of the optical band gap in spin-up state can be observed, consistent to the energy difference between spin-splitting orbits. The spin-dependent nonlinear optical properties reveal that in the infrared and visible light region, the contributions to second-harmonic generation (SHG) susceptibilities are mainly from spin-majority channels. In addition, the strain effect is also discussed. With the increase of epitaxial strain, the larger energy shift of the leading absorption peaks, and the more remarkable nonlinear optical response can be obtained.

cond-mat.mtrl-sci↗

arXiv2013

Microscopic Mechanism of Stereochemically Active Lone Pair Studied from Orbital Selective External Potential Calculation

The nature of the stereochemically active lone pair has long been in debate. Here, by application of our recently developed orbital selective external potential (OSEP) method, we have studied the microscopic mechanism of stereochemically active lone pairs in various compounds. The OSEP method allows us to shift the energy level of specific atomic orbital, therefore is helpful to identify unambiguously the role of this orbital to the chemical and physical properties of the system we are interested in. Our numerical results, with compelling proofs, demonstrate that the on-site mixing of cation valence s orbital with the nominally empty p orbitals of the same subshell is crucial to the formation of lone pair, whereas the anion p orbital has only small effect. Our detailed investigation of Sn and Pb monochalcogenides show that structures of these systems have significant effects on lone pairs. In return, the formation of lone pair, which can be controlled by our OSEP method, could result in structural instabilities of Sn and Pb monochalcogenides.

cond-mat.mtrl-sci↗

arXiv2012

Epitaxial strain induced magnetic transitions and phonon instabilities of the tetragonal SrRuO3

Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both the tensile and compressive strain in the xy-plane could induce the abrupt change in the magnetic moment of Ru atom. In particular, under the in-plane ~4% compressive strain, a ferromagnetic to nonmagnetic transition is induced. Whereas for the tensile strain larger than 3%, the Ru magnetic moment drops gradually with the increase of the strain, exhibiting a weak ferromagnetic state. We find that such magnetic transitions could be qualitatively explained by the Stoner model. In addition, frozen phonon calculations at Γ point reveal structural instabilities could occur under both compressive and tensile strains. Such instabilities are very similar to those of the ferroelectric perovskite oxides, even though SRO remains to be metallic in the range we studied. These might have influence on the physical properties of oxide supercells taking SRO as constituent.

cond-mat.str-el↗

arXiv2012

Density-functional calculations of the electronic structure and lattice dynamics of superconducting LaO$_{0.5}$F$_{0.5}$BiS$_{2}$: Evidence for an electron-phonon interaction near the charge-density-wave instability

We discuss the electronic structure, lattice dynamics and electron-phonon interaction of newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ using density functional based calculations. A strong Fermi surface nesting at $\mathbf{k}$=($π$,$π$,0) suggests a proximity to charge density wave instability and leads to imaginary harmonic phonons at this $\mathbf{k}$ point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant $λ\simeq 0.85$ prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials.

cond-mat.supr-con↗

arXiv2012

Giant exchange bias and ferromagnetism in the CoO shell of Co/CoO-MgO core-shell nanoparticles

Using magnetron sputtering, we produced a series of Co/CoO-MgO nanoparticles on Si(100) substrates. High-resolution transmission electron microscopy (HRTEM) image shows that small isolated Co-clusters (core) covered with CoO (shells) with a size of a few nm embedded in a MgO matrix. Resistivity as a function of Co atomic ratio exhibits a distinct percolation threshold with a sharp decrease around 69% Co content. Across the threshold, the resistivity drops about 7 orders of magnitude. For a sample at this percolation critical threshold, we have observed a giant exchange bias field HE=2460 Oe at T= 2K, and using soft x-ray magnetic circular dichroism at the Co-L2,3 edge, we have detected a ferromagnetic (FM) signal originating from the antiferromagnetic CoO shell. Moreover, decreasing the Mg-impurities will reduce the FM signal from CoO shell (namely the uncompensated spin density) and the size of HE, thus directly support the uncompensated spin model.

cond-mat.mtrl-sci↗

arXiv2012

BaFe2Se2O as an Iron-Based Mott Insulator with Antiferromagnetic Order

A new compound with a quasi-two-dimensional array of FeSe3O tetrahedra and an orthorombic structure, namely BaFe2Se2O, has been successfully fabricated. Experimental results show that this compound is an insulator and has an antiferromagnetic (AF) transition at 240 K. Band structure calculation reveals the narrowing of Fe 3d bands near the Fermi energy, which leads to the localization of magnetism and the Mott insulating behavior. The large distances between the Fe atoms perhaps are responsible for the characters. Linear response calculation further indicates a strong in-plane AF exchange $J$, this can account for the enhanced magnetic susceptibility (which has a maximum at about 450 K) above the Neel temperature.

cond-mat.str-el↗

arXiv2012

Spin-orbit tuned metal-insulator transitions in single-crystal Sr2Ir1-xRhxO4 (0\leqx\leq1)

Sr2IrO4 is a magnetic insulator driven by spin-orbit interaction (SOI) whereas the isoelectronic and isostructural Sr2RhO4 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. Our investigation of structural, transport, magnetic and thermal properties reveals that substituting 4d Rh4+ (4d5) ions for 5d Ir4+(5d5) ions in Sr2IrO4 directly reduces the SOI and rebalances the competing energies so profoundly that it generates a rich phase diagram for Sr2Ir1-xRhxO4 featuring two major effects: (1) Light Rh doping (0\leqx\leq0.16) prompts a simultaneous and precipitous drop in both the electrical resistivity and the magnetic ordering temperature TC, which is suppressed to zero at x = 0.16 from 240 K at x=0. (2) However, with heavier Rh doping (0.24< x<0.85 (\pm0.05)) disorder scattering leads to localized states and a return to an insulating state with spin frustration and exotic magnetic behavior that only disappears near x=1. The intricacy of Sr2Ir1-xRhxO4 is further highlighted by comparison with Sr2Ir1-xRuxO4 where Ru4+(4d4) drives a direct crossover from the insulating to metallic states.

cond-mat.str-el↗

arXiv2012

The mechanism of anisotropic exchange interaction in superconducting iron arsenides

Using a combination of linear response theory and constrained orbital hybridization approach, we study the mechanism of magnetic exchange interaction of iron-based superconductor. We reproduce the observed highly anisotropic exchange interaction, and our constrain-orbital calculation unambiguously identifies that the anisotropic feature of exchange interaction is not sensitive to the unequal d$_{xz}$/d$_{yz}$ orbital population.

cond-mat.supr-con↗

arXiv2012

Electronic structure and magnetic properties of NaOsO$_{3}$

A comprehensive investigation of the electronic and magnetic properties of NaOsO3 has been made using the first principle calculations, in order to understand the importance of Coulomb interaction, spin-orbit coupling and magnetic order in its temperature-induced and magnetic-related metal-insulator transition. It is found that its electronic structure near the Fermi energy is dominated by strongly hybridized Os 5d and O 2p states. Despite of the large strength of spin-orbit coupling, it has only small effect on the electronic and magnetic properties of NaOsO3. On the other hand, the on-site Coulomb repulsion affects the band structure significantly, but, a reasonable U alone cannot open a band gap. Its magnetism is itinerant, and the magnetic configuration plays an important role in determining the electronic structure. Its ground state is of a G-type antiferromagnet, and it is the combined effect of U and magnetic configuration that results in the insulating behavior of NaOsO3.

cond-mat.str-el↗