Yong Xu

Intrinsic anisotropy of thermal conductance in graphene nanoribbons

Thermal conductance of graphene nanoribbons (GNRs) with the width varying from 0.5 to 35 nm is systematically investigated using nonequilibrium Green’s function method. Anisotropic thermal conductance is observed with the room temperature thermal conductance of zigzag GNRs up to ∼30% larger than that of armchair GNRs. At room temperature, the anisotropy is found to disappear when the width is larger than 100 nm. This intrinsic anisotropy originate from different boundary condition at ribbon edges, and can be used to tune thermal conductance, which have important implications for the applications of GNRs in nanoelectronics and thermoelectricity.

Nonequilibrium Green’s function method for phonon-phonon interactions and ballistic-diffusive thermal transport

Phonon-phonon interaction is systematically studied by nonequilibrium Greens function (NEGF) formulism in momentum space at finite temperatures. Within the quasi-particle approximation, phonon frequency shift and lifetime are obtained from the retarded self-energy. The lowest order NEGF provides the same phonon lifetime as Fermis golden rule. Thermal conductance is predicted by the Landauer formula with a phenomenological transmission function. The main advantage of our method is that it covers both ballistic and diffusive limits and thermal conductance of different system sizes can be easily obtained once the mode-dependent phonon mean free path is calculated by NEGF. As an illustration, the method is applied to two one-dimensional atom chain models (the FPU-beta model and the phi^4 model) with an additional harmonic on-site potential. The obtained thermal conductance is compared with that from a quasi-classical molecular dynamics method. The harmonic on-site potential is shown to remove the divergence of thermal conductivity in the FPU-beta model.

Robust linear dependence of thermal conductance on radial strain in carbon nanotubes

Nanotubes have recently been experimentally demonstrated to be perfect phonon waveguides. To explore the underlying physics, we present atomic scale calculations of thermal transport in carbon nanotubes under radial strain using the nonequilibrium Greens function method. It is found that the thermal conductance exhibits a robust linear response behavior to radial strain over the whole elastic range. A detailed analysis of phonon transmission reveals that an elastic radial strain can be viewed as a perturbation of the transport of most of the low-frequency phonons. This is attributed to the unique bonding configuration of nanotubes, which can be well preserved even under severe deformation. Such a structural response to deformation, which is rare in other systems, explains the robust thermal transport in nanotubes against severe radial deformation.

Superconductivity in Few-layer Stanene

A single atomic slice of α-tin—stanene—has been predicted to host the quantum spin Hall effect at room temperature, offering an ideal platform to study low-dimensional and topological physics. Although recent research has focused on monolayer stanene, the quantum size effect in few-layer stanene could profoundly change material properties, but remains unexplored. By exploring the layer degree of freedom, we discover superconductivity in few-layer stanene down to a bilayer grown on PbTe, while bulk α-tin is not superconductive. Through substrate engineering, we further realize a transition from a single-band to a two-band superconductor with a doubling of the transition temperature. In situ angle-resolved photoemission spectroscopy (ARPES) together with first-principles calculations elucidate the corresponding band structure. The theory also indicates the existence of a topologically non-trivial band. Our experimental findings open up novel strategies for constructing two-dimensional topological superconductors.Stanene is a single sheet of tin atoms. Here, it is shown that a few stacked layers of stanene can be a superconductor. Changing the thickness of the substrate modifies the form of superconductivity and critical temperature.

Tuning thermoelectricity in a Bi2Se3 topological insulator via varied film thickness

We report thermoelectric transport studies on Bi2Se3 topological insulator thin films with varied thickness grown by molecular beam epitaxy. We find that the Seebeck coefficient and thermoelectric power factor decrease systematically with the reduction of film thickness. These experimental observations can be explained quantitatively by theoretical calculations based on realistic electronic band structure of the Bi2Se3 thin films. Lastly, this work illustrates the crucial role played by the topological surface states on the thermoelectric transport of topological insulators, and sheds new light on further improvement of their thermoelectric performance.

Manipulating topological phase transition by strain

First-principles calculations show that strain-induced topological phase transition is a universal phenomenon in those narrow-gap semiconductors for which the valence band maximum (VBM) and conduction band minimum (CBM) have different parities. The transition originates from the opposite responses of the VBM and CBM, whose magnitudes depend critically on the direction of the applied strain. Our work suggests that strain can play a unique role in tuning the electronic properties of topological insulators for device applications, as well as in the achievement of new topological insulators.

Emerging topological states in quasi-two-dimensional materials

Inspired by the discovery of graphene, various two‐dimensional (2D) materials have been experimentally realized, which exhibit novel physical properties and support promising applications. Exotic topological states in 2D materials (including quantum spin Hall and quantum anomalous Hall insulators), which are characterized by nontrivial metallic edge states within the insulating bulk gap, have attracted considerable attentions in the past decade due to their great importance for fundamental research and practical applications. They also create a surge of research activities and attract extensive efforts to search for new topological materials in realistic 2D/quasi‐2D systems. This review presents a comprehensive survey of recent progress in designing of topological states in quasi‐2D materials, including various quantum well heterostructures and 2D atomic lattice structures. In particular, the possibilities of constructing topological nontrivial states from commonly used materials are discussed and the ways of enlarging energy gaps of topological states and realizing different topological states in a single material are presented. WIREs Comput Mol Sci 2017, 7:e1296. doi: 10.1002/wcms.1296

Structures of 1,1'‐diphenyl‐1,1'‐bicyclopentyl, 1,1'‐diphenyl‐1,1'‐bicyclohexyl and 1,1'‐diphenyl‐1,1'‐bicycloheptyl. Erratum

Of the title compounds, 1, 1-diphenyl-1,1-bicyclopentyl, C 22 H 26 (1), 1, 1-diphenyl-1, 1-bicyclohexyl, C 24 H 30 (2) and 1,1-diphenyl-1,1-bicycloheptyl C 26 H 34 (3)> (1) adopts the gauche conformation whereas (2) and (3) are in the trans conformation. The cyclopentyl ring is in envelope form and the cyclohexyl ring in chair form, while the cycloheptyl ring is in a twisted-chair configuration. The influence of increasing ring size on the central inter-ring C-C bond length is discussed as well as the effects of steric interaction within the molecules

Theoretical studies on tunable electronic structures and potential applications of two-dimensional arsenene-based materials

Research efforts in the area of two‐dimensional (2D) arsenene‐based materials have been fueled up recently due to similarities in honeycomb atomic structures and differences in physical and chemical properties between arsenene and graphene. The pioneering prediction of monolayered arsenene in 2015 and successful synthesis of multilayered arsenene nanoribbons in 2016 have promoted intensive subsequent studies, especially in the theoretical aspect. Density functional theory computations not only revealed desirable fundamental band gap, structural stability, and high carrier mobility of various arsenene‐based materials but also suggested promising applications in future optoelectronic and thermoelectric devices, as well as in the quantum spin Hall devices via surface functionalization and modulation of interlayer interactions. With an aim to present a comprehensive review on the tunable electronic structures of 2D arsenene‐based materials, our focus is placed on the tailoring routes through surface functionalization to modify the electronic and optoelectronic properties of the arsenenes. An emphasis is also given to recent progress in designing topological states in arsenene monolayers. The challenges and outlooks are also laid out in aspects of experimental fabrication, device performance, and arsenene‐based chemical reactions.

Experimental evidence of the thickness- and electric-field-dependent topological phase transitions in topological crystalline insulator SnTe(111) thin films

Using in situ angle-resolved photoemission spectroscopy, we systematically studied molecular-beam-epitaxy-grown topological crystalline insulator SnTe(111) thin films with varied thicknesses and substrate conditions. An oscillation in the band gap size with an increase in thickness was observed to depend on the electric field perpendicular to the films. The observations are consistent with the theoretically predicted thickness- and electric-field-dependent topological phase transitions between the normal insulator and the quantum spin Hall insulator phases in SnTe(111) films and demonstrate them to be an excellent and electrically tunable quantum spin Hall system.