Abstract: When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moire pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moire patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moire pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moire super cell.

Abstract: The phase transition processes induced by ultrashort, 100 fs pulsed laser irradiation of Au, Cu, and Ni are studied by means of a combined atomistic-continuum approach. A moderately low absorbed laser fluence range, from 200 to 600 J/m2 is considered to study phase transitions by means of a local and a nonlocal order parameter. At low laser fluences, the occurrence of layer-by-layer evaporation has been observed, which suggests a direct solid to vapor transition. The calculated amount of molten material remains very limited under the conditions studied, especially for Ni. Therefore, our results show that a kinetic equation that describes a direct solid to vapor transition might be the best approach to model laser-induced phase transitions by continuum models. Furthermore, the results provide more insight into the applicability of analytical superheating theories that were implemented in continuum models and help the understanding of nonequilibrium phase transitions.

Abstract: Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS3) whiskers are made out of weakly interacting TiS3 layers, where each layer is made of weakly interacting quasi-one-dimensional chains extending along the b axis. Here we establish the unusual vibrational properties of TiS3 both experimentally and theoretically. Unlike other two-dimensional systems, the Raman active peaks of TiS3 have only out-of-plane vibrational modes, and interestingly some of these vibrations involve unique rigid-chain vibrations and S-S molecular oscillations. High-pressure Raman studies further reveal that the A(g)(S-S) S-S molecular mode has an unconventional negative pressure dependence, whereas other peaks stiffen as anticipated. Various vibrational modes are doubly degenerate at ambient pressure, but the degeneracy is lifted at high pressures. These results establish the unusual vibrational properties of TiS3 with strong in-plane anisotropy, and may have relevance to understanding of vibrational properties in other anisotropic two-dimensional material systems.

Abstract: We study electron tunneling through a planar magnetic and electric barrier on the surface of a three-dimensional topological insulator. For the double barrier structures, we find (i) a directional-dependent tunneling which is sensitive to the magnetic field configuration and the electric gate voltage, (ii) a spin rotation controlled by the magnetic field and the gate voltage, (iii) many Fabry-Pérot resonances in the transmission determined by the distance between the two barriers, and (iv) the electrostatic potential can enhance the difference in the transmission between the two magnetization configurations, and consequently lead to a giant magnetoresistance. Points (i), (iii), and (iv) are alike with that in graphene stemming from the same linear-dispersion relations.

Abstract: We demonstrate theoretically how local strains in graphene can be tailored to generate a valley-polarized current. By suitable engineering of local strain profiles, we find that electrons in opposite valleys (K or K′) show different Brewster-like angles and Goos-Hänchen shifts, exhibiting a close analogy with light propagating behavior. In a strain-induced waveguide, electrons in K and K′ valleys have different group velocities, which can be used to construct a valley filter in graphene without the need for any external fields.

Abstract: We find localized electron and hole states in a ring-shaped potential kink in biased bilayer graphene. Within the continuum description, we show that for sharp potential steps the Dirac equation describing carrier states close to the K (or K′) point of the first Brillouin zone can be solved analytically for a circular kink/antikink dot. The solutions exhibit interfacial states which exhibit AharonovBohm oscillations as functions of the height of the potential step and/or the radius of the ring.

Abstract: A systematic investigation is undertaken for studying the optoelectronic properties of single crystal diamond (SCD) grown by microwave plasma chemical vapor deposition (MPCVD). It is indicated that, without intentional doping and surface treatment during the sample growth, the terahertz (THz) optical conduction in SCD is mainly affected by surface H-terminations, -OH-, O- and N-based functional groups. By using THz time-domain spectroscopy (TDS), we measure the transmittance, the complex dielectric constant and optical conductivity σ(ω) of SCD. We find that SCD does not show typical semiconductor characteristics in THz regime, where σ(ω) cannot be described rightly by the conventional Drude formula. Via fitting the real and imaginary parts of σ(ω) to the Drude-Smith formula, the ratio of the average carrier density to the effective electron mass γ = ne/m*, the electronic relaxation time τ and the electronic backscattering or localization factor can be determined optically. The temperature dependence of these parameters is examined. From the temperature dependence of γ, a metallic to semiconductor transition is observed at about T = 10 K. The temperature dependence of τ is mainly induced by electron coupling with acoustic-phonons and there is a significant effect of photon-induced electron backscattering or localization in SCD. This work demonstrates that THz TDS is a powerful technique in studying SCD which contains H-, N- and O-based bonds and has low electron density and high dc resistivity. The results obtained from this study can benefit us to gain an in-depth understanding of SCD and may provide new guidance for the application of SCD as electronic, optical and optoelectronic materials.

Abstract: We present a theoretical study on optical properties such as optical conductance and light transmission coefficient for mono- and multi-layer graphene systems with AB- and ABC-stacking. Considering an air/graphene/dielectric-water structure, the optical coefficients for those graphene systems are examined and compared. The universal optical conductance sigma(N)(0)=N pi e(2)/(2h) for N layer graphene systems in the visible region is verified. For N 3 layer graphene, the mini-gap induced absorption edges can be observed in odd layers AB-stacked multilayer graphene, where the number and position of the absorption edges are decided by the layers number N. Meanwhile, we can observe the optical absorption windows for those graphene systems in the infrared to terahertz bandwidth (0.2-150 THz). The absorption window is induced by different transition energies required for inter- and intra-band optical absorption channels. We find that the depth and width of the absorption window can be tuned not only via varying temperature and electron density but also by changing the number of graphene layers and the stacking order. These theoretical findings demonstrate that mono- and multi-layer graphene systems can be applied as frequency tunable optoelectronic devices working in infrared to terahertz bandwidth. (C) 2014 Elsevier B.V. All rights reserved.

Abstract: We investigate the effect of Rashba spin-orbit coupling (SOC) on the optoelectronic properties of n- and p-type monolayer MoS2. The optical conductivity is calculated within the Kubo formalism. We find that the spin-flip transitions enabled by the Rashba SOC result in a wide absorption window in the optical spectrum. Furthermore, we evaluate the effects of the polarization direction of the radiation, temperature, carrier density, and the strength of the Rashba spin-orbit parameter on the optical conductivity. We find that the position, width, and shape of the absorption peak or absorption window can be tuned by varying these parameters. This study shows that monolayer MoS2 can be a promising tunable optical and optoelectronic material that is active in the infrared to terahertz spectral range.

Abstract: We report measurements which indicate the appearance of spontaneous vortices in lead superconducting films with embedded magnetic nanoparticles and a temperature-induced phase transition between different vortex phases. Unlike common vortices in superconductors, the vortex phase appears in the absence of applied magnetic field. The vortices nucleate exclusively due to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field and simultaneously acting as pinning centers. Transport measurements reveal dynamical phase transitions that depend on temperature (T) and applied field (H) and support the obtained H-T phase diagram.

Abstract: Because of strong flux confinement in mesoscopic superconductors, a giant vortex may appear in the ground state of the system in an applied magnetic field. This multiquanta vortex can then split into individual vortices (and vice versa) as a function of, e.g., applied current, magnetic field, or temperature. Here we show that such transitions can be identified by calorimetry, as the formation or splitting of a giant vortex results in a clear jump in measured heat capacity versus external drive. We attribute this phenomenon to an abrupt change in the density of states of the quasiparticle excitations in the vortex core(s), and further link it to a sharp change of the magnetic susceptibility at the transitionproving that the formation of a giant vortex can also be detected by magnetometry.