arXiv subjects

Yangyang Wang

Publications and source records attributed to Yangyang Wang.

At least 55 records · Page 3Linked to original sources

arXiv2018

Scaling Behavior of Anisotropy Relaxation in Deformed Polymers

Drawing an analogy to the paradigm of quasi-elastic neutron scattering, we present a general approach for quantitatively investigating the spatiotemporal dependence of structural anisotropy relaxation in deformed polymers by using small-angle neutron scattering. Experiments and non-equilibrium molecular dynamics simulations on polymer melts over a wide range of molecular weights reveal that their conformational relaxation at relatively high momentum transfer $Q$ and short time can be described by a simple scaling law, with the relaxation rate proportional to $Q$. This peculiar scaling behavior, which cannot be derived from the classical Rouse and tube models, is indicative of a surprisingly weak direct influence of entanglement on the microscopic mechanism of single-chain anisotropy relaxation.

cond-mat.soft↗

arXiv2018

Spontaneous Valley Splitting and Valley Pseudospin Field Effect Transistor of Monolayer VAgP2Se6

Valleytronics is a rising topic to explore the emergent degree of freedom for charge carriers in energy band edges and has attracted a great interest due to many intriguing quantum phenomena and potential application in information processing industry. Creation of permanent valley polarization, i.e. unbalanced occupation at different valleys, is a chief challenge and also urgent question to be solved in valleytronics. Here we predict that the spin-orbit coupling and magnetic ordering allow spontaneous valley Zeeman-type splitting in pristine monolayer of VAgP2Se6 by using first-principles calculations. The Zeeman-type valley splitting can lead to permanent valley polarization after suitable doping. The Zeeman-type valley splitting is similar to the role of spin polarization in spintronics and is a vital requirement for practical devices in valleytronics. The nonequivalent valleys of VAgP2Se6 monolayer can emit or absorb circularly polarized photons with opposite chirality, and thus this material shows a great potential to work as a photonic spin filter and circularly-polarized-light resource. A valley pseudospin field effect transistor (VPFET) is designed based on the monolayer VAgP2Se6 akin to the spin field effect transistors. Beyond common transistors, VPFETs carry information of not only the electrons but also the valley pseudospins.

cond-mat.mtrl-sci↗

arXiv2017

Strain Heterogeneity in Sheared Colloids Revealed by Neutron Scattering

Recent computational and theoretical studies have shown that the deformation of colloidal suspensions under a steady shear is highly heterogeneous at the particle level and demonstrate a critical influence on the macroscopic deformation behavior. Despite its relevance to a wide variety of industrial applications of colloidal suspensions, scattering studies focusing on addressing the heterogeneity of the non-equilibrium colloidal structure are scarce thus far. Here, we report the first experimental result using small-angle neutron scattering. From the evolution of strain heterogeneity, we conclude that the shear-induced deformation transforms from nearly affine behavior at low shear rates, to plastic rearrangements when the shear rate is high.

cond-mat.soft↗

arXiv2017

Dynamically Correlated Region in Sheared Colloidal Glasses Revealed by Neutron Scattering

The microscopic deformation mechanism of charged colloidal glasses with extended-range interactions under shear is investigated by in-situ small-angle neutron scattering, and a dynamically correlated region (DCR) is identified. This short-lived region provides the resistance to the configurational rearrangement imposed by the external deformation, as evidenced by the evolution of the size of DCR in the shear thinning regime and the quantitative agreement between the local stress sustained by DCR and the macroscopic stress from rheological measurements at low and mediate shear rates. This finding suggests that DCR is an important quantity for microscopically addressing the flow and deformation behavior of strongly interacting colloids.

cond-mat.soft↗

arXiv2017

Characterization of microscopic deformation through two-point spatial correlation function

The molecular rearrangements of most fluids under flow and deformation do not directly follow the macroscopic strain field. In this work, we describe a phenomenological method for characterizing such non-affine deformation via the anisotropic pair distribution function (PDF). We demonstrate now the microscopic strain can be calculated in both simple shear and uniaxial extension, by perturbation expansion of anisotropic PDF in terms of real spherical harmonics. Our results, given in the real as well as the reciprocal space, can be applied in spectrum analysis of small-angle scattering experiments and non-equilibrium molecular dynamics simulations of soft matter under flow.

cond-mat.soft↗

arXiv2017

Fingerprinting molecular relaxation in deformed polymers

The flow and deformation of macromolecules is ubiquitous in nature and industry, and an understanding of this phenomenon at both macroscopic and microscopic length scales is of fundamental and practical importance. Here we present the formulation of a general mathematical framework, which could be used to extract, from scattering experiments, the molecular relaxation of deformed polymers. By combining and modestly extending several key conceptual ingredients in the literature, we show how the anisotropic single-chain structure factor can be decomposed by spherical harmonics and experimentally reconstructed from its cross sections on the scattering planes. The resulting wavenumber-dependent expansion coefficients constitute a characteristic fingerprint of the macromolecular deformation, permitting detailed examinations of polymer dynamics at the microscopic level. We apply this approach to survey a long-standing problem in polymer physics regarding the molecular relaxation in entangled polymers after a large step deformation. The classical tube theory of Doi and Edwards predicts a fast chain retraction process immediately after the deformation, followed by a slow orientation relaxation through the reptation mechanism. This chain retraction hypothesis, which is the keystone of the tube theory for macromolecular flow and deformation, was critically examined by analyzing the fine features of the two-dimensional anisotropic spectra from small-angle neutron scattering by entangled polystyrenes. It is shown that the unique scattering patterns associated with the chain retraction mechanism were not experimentally observed. This result calls for a fundamental revision of the current theoretical picture for nonlinear rheological behavior of entangled polymeric liquids.

cond-mat.soft↗

arXiv2015

Does P-type Ohmic Contact Exist in WSe2-metal Interfaces?

Formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of 2D WSe2 devices. We present the first comparative study of the interfacial properties between ML/BL WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts by using ab initio energy band calculations with inclusion of the spin-orbital coupling (SOC) effects and quantum transport simulations. The interlayer coupling tends to reduce both the electron and hole Schottky barrier heights (SBHs) and alters the polarity for WSe2-Au contact, while the SOC chiefly reduces the hole SBH. In the absence of the SOC, Pd contact has the smallest hole SBH with a value no less than 0.22 eV. Dramatically, Pt contact surpasses Pd contact and becomes p-type Ohmic or quasi-Ohmic contact with inclusion of the SOC. Our study provides a theoretical foundation for the selection of favorable metal electrodes in ML/BL WSe2 devices.

cond-mat.mtrl-sci↗

arXiv2015

Monolayer Phosphorene-Metal Interfaces

Recently, phosphorene electronic and optoelectronic prototype devices have been fabricated with various metal electrodes. We systematically explore for the first time the contact properties of monolayer (ML) phosphorene with a series of commonly used metals (Al, Ag. Cu, Au, Cr, Ni, Ti, and Pd) via both ab initio electronic structure calculations and more reliable quantum transport simulations. Strong interactions are found between all the checked metals, with the energy band structure of ML phosphorene destroyed. In terms of the quantum transport simulations, ML phosphorene forms a n-type Schottky contact with Au, Cu, Cr, Al, and Ag electrodes, with electron Schottky barrier heights (SBHs) of 0.30, 0.34, 0.37, 0.51, and 0.52 eV, respectively, and p-type Schottky contact with Ti, Ni, and Pd electrodes, with hole SBHs of 0.30, 0.26, and 0.16 eV, respectively. These results are in good agreement with available experimental data. Our findings not only provide an insight into the ML phosphorene-metal interfaces but also help in ML phosphorene based device design.

cond-mat.mes-hall↗

arXiv2015

Silicene on Substrates: A Theoretical Perspective

Silicene, as the silicon analog of graphene, has been successfully fabricated by epitaxial growing on various substrates. Similar to free-standing graphene, free-standing silicene possesses a honeycomb structure and Dirac-cone-shaped energy band, resulting in many fascinating properties such as high carrier mobility, quantum spin Hall effect, quantum anomalous Hall effect, and quantum valley Hall effect. The maintenance of the honeycomb crystal structure and the Dirac cone of silicene is crucial for observation of its intrinsic properties. In this review, we systematically discuss the substrate effects on the atomic structure and electronic properties of silicene from a theoretical point of view, especially focusing on the changes of the Dirac cone.

cond-mat.mtrl-sci↗

arXiv2015

Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations

Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in the two types of contacts and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS2-metal contacts have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.

cond-mat.mes-hall↗

arXiv2015

Silicene Spintronics

Spintronics involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. The fascinating spin-resolved properties of graphene motivate numerous researchers into the studies of spintronics in graphene and other two-dimensional (2D) materials. Silicene, silicon analog of graphene, is considered as a promising material for spintronics. Here, we present a review on the theoretical advances about the spin-dependent properties including the electric field and exchange field tunable topological properties of silicene and the corresponding spintronic device simulations.

cond-mat.mtrl-sci↗

arXiv2015

All-Metallic Vertical Transistors Based on Stacked Dirac Materials

It is a persisting pursuit to use metal as a channel material in a field effect transistor. All metallic transistor can be fabricated from pristine semimetallic Dirac materials (such as graphene, silicene, and germanene), but the on/off current ratio is very low. In a vertical heterostructure composed by two Dirac materials, the Dirac cones of the two materials survive the weak interlayer van der Waals interaction based on density functional theory method, and electron transport from the Dirac cone of one material to the one of the other material is therefore forbidden without assistance of phonon because of momentum mismatch. First-principles quantum transport simulations of the all-metallic vertical Dirac material heterostructure devices confirm the existence of a transport gap of over 0.4 eV, accompanied by a switching ratio of over 104. Such a striking behavior is robust against the relative rotation between the two Dirac materials and can be extended to twisted bilayer graphene. Therefore, all-metallic junction can be a semiconductor and novel avenue is opened up for Dirac material vertical structures in high-performance devices without opening their band gaps.

cond-mat.mtrl-sci↗

arXiv2015

Silicene Nanomesh

Similar to graphene, zero band gap limits the application of silicene in nanoelectronics despite of its high carrier mobility. By using first-principles calculations, we reveal that a band gap is opened in silicene nanomesh (SNM) when the width W of the wall between the neighboring holes is even. The size of the band gap increases with the reduced W and has a simple relation with the ratio of the removed Si atom and the total Si atom numbers of silicene. Quantum transport simulation reveals that the sub-10 nm single-gated SNM field effect transistors show excellent performance at zero temperature but such a performance is greatly degraded at room temperature.

cond-mat.mtrl-sci↗

arXiv2014

Graphdiyne-metal contacts and graphdiyne transistors

Graphdiyne is prepared on metal surface, and making devices out of it also inevitably involves contact with metals. Using density functional theory with dispersion correction, we systematically studied for the first time the interfacial properties of graphdiyne contacting with a series of metals (Al, Ag, Cu, Au, Ir, Pt, Ni, and Pd). Graphdiyne is in an n-type Ohmic or quasi-Ohmic contact with Al, Ag, and Cu, while it is in a Schottky contact with Au (at source/drain interface), Pd, Pt, Ni, and Ir (at source/drain-channel interface), with high Schottky barrier heights of 0.39, 0.21 (n-type), 0.30, 0.41, and 0.45 (p-type) eV, respectively. A graphdiyne field effect transistor (FET) with Al electrodes is simulated by using quantum transport calculations. This device exhibits an on-off ratio up to 104 and a very large on-state current of 1.3 * 104 mA/mm in a 10 nm channel length. Thus, a new prospect is opened up for graphdiyne in high performance nanoscale devices.

cond-mat.mtrl-sci↗

arXiv2014

Does the Dirac Cone Exist in Silicene on Metal Substrates?

Absence of the Dirac cone due to a strong band hybridization is revealed to be a common feature for epitaxial silicene on metal substrates according to our first-principles calculations for silicene on Ir, Cu, Mg, Au, Pt, Al, and Ag substrates. The destroyed Dirac cone of silicene, however, can be effectively restored with linear or parabolic dispersion by intercalating alkali metal atoms between silicene and the metal substrates, offering an opportunity to study the intriguing properties of silicene without further transfer of silicene from the metal substrates.

cond-mat.mtrl-sci↗

arXiv2013

Tunable band gap in germanene by surface adsorption

Opening a sizable band gap in the zero-gap germanene without heavy loss of carrier mobility is a key issue for its application in nanoelectronic devices such as high-performance field effect transistors (FETs) operating at room temperature. Using the first-principles calculations, we find a band gap is opened at the Dirac point in germanene by single-side adsorption of alkali metal (AM) atoms. This band gap is tunable by varying the coverage and the species of AM atoms, ranging from 0.02 to 0.31 eV, and the maximum global band gap is 0.26 eV. Since the effective masses of electrons and holes in germanene near the Dirac point after surface adsorption (ranging from 0.005 to 0.106 me) are small, the carrier mobility is expected not to degrade much. Therefore germanene is a potential candidate of effective FET channel operating at room temperature upon surface adsorption.

cond-mat.mes-hall↗

arXiv2013

Tunable Band Gap and Doping Type in Silicene by Surface Adsorption: towards Tunneling Transistors

Structural and electronic properties of silicene adsorbed by five kinds of transition metal atoms (Cu, Ag, Au, Pt, and Ir) are systematically studied by using first-principles calculations. We find that such adsorption can induce a band gap at the Dirac point of doped silicene. Doped silicene can reach a band gap up to 0.23 eV while keeping a relatively small effective mass of around 0.1 me, thus having high carrier mobility estimated to be 50000 cm2/Vs. P-type doping and neutral state is realized in silicene by Ir and Pt adsorption, respectively, while n-type doping is done by Cu, Ag, and Au adsorption. Based on the knowledge above, a silicene p-i-n tunneling field effect transistor (TFET) is proposed and simulated by both first-principles and semi-empirical approaches. Silicene TFET shows high performance with an on-off ratio of 10^3, a sub-threshold swing of 90 mV/dec, and an on-state current of 1 mA/μm. Such an on-state current is even larger than that of most other existing TFETs.

cond-mat.mtrl-sci↗

arXiv2013

Strong Band Hybridization between Silicene and Ag(111)Substrate

By using first-principles calculations, we systematically investigated several observed phases of silicene on Ag(111) substrates and their electronic structures. We find that the original Dirac cone of silicene is about 1.5-1.7 eV deeply below the Fermi level and severely destroyed by the band hybridization between silicene and Ag in all the examined phases. Thus, silicene synthesized on Ag(111) substrates could not preserve its excellent electronic property and new method is needed to develop in synthesizing silicene with its Dirac cone surviving.

cond-mat.mes-hall↗