Simulation and detection of Dirac fermions with cold atoms in an optical lattice

We propose an experimental scheme to simulate and observe relativistic Dirac fermions with cold atoms in a hexagonal optical lattice. By controlling the lattice anisotropy, one can realize both massive and massless Dirac fermions and observe the phase transition between them. Through explicit calculations, we show that both the Bragg spectroscopy and the atomic density profile in a trap can be used to demonstrate the Dirac fermions and the associated phase transition.Comment: 4 pages; Published versio

Development of variable renewable energy policy in developing countries: A case study of Sri Lanka

Copyright © 2018 Inderscience Enterprises Ltd. Development of policy for variable renewable energy (VRE), such as wind and solar power, could be difficult in developing countries that have limited capacity, limited technical, and fiscal resources. Sri Lanka is such a case. Although Sri Lanka has a potential to develop VRE, the development was neither sufficient nor smooth. This study explores the development of VRE in the 2000s and analyses the impacts of the cost reflective feed-in-tariffs (FITs). The study finds that VRE public policy in Sri Lanka can be improved in a number of ways: better coordination among government agencies; technology specific policy and accommodation of cost dynamics; and redesign of subsidy policies. Generally, the governance mechanism suggested in this research is an innovated structure to coordinate multiple government agencies by specifying policies according to technologies and redesigning the subsidy structures by accommodating the cost dynamics

Scaling of geometric phases close to quantum phase transition in the XY chain

We show that geometric phase of the ground state in the XY model obeys scaling behavior in the vicinity of a quantum phase transition. In particular we find that geometric phase is non-analytical and its derivative with respect to the field strength diverges at the critical magnetic field. Furthermore, universality in the critical properties of the geometric phase in a family of models is verified. In addition, since quantum phase transition occurs at a level crossing or avoided level crossing and these level structures can be captured by Berry curvature, the established relation between geometric phase and quantum phase transitions is not a specific property of the XY model, but a very general result of many-body systems.Comment: 4 page

Topological quantum phase transition in an extended Kitaev spin model

We study the quantum phase transition between Abelian and non-Abelian phases in an extended Kitaev spin model on the honeycomb lattice, where the periodic boundary condition is applied by placing the lattice on a torus. Our analytical results show that this spin model exhibits a continuous quantum phase transition. Also, we reveal the relationship between bipartite entanglement and the ground-state energy. Our approach directly shows that both the entanglement and the ground-state energy can be used to characterize the topological quantum phase transition in the extended Kitaev spin model.Comment: 9 Pages, 4 figure

Implementing topological quantum manipulation with superconducting circuits

A two-component fermion model with conventional two-body interactions was recently shown to have anyonic excitations. We here propose a scheme to physically implement this model by transforming each chain of two two-component fermions to the two capacitively coupled chains of superconducting devices. In particular, we elaborate how to achieve the wanted operations to create and manipulate the topological quantum states, providing an experimentally feasible scenario to access the topological memory and to build the anyonic interferometry.Comment: 4 pages with 3 figures; V2: published version with minor updation

Seed mediated one-pot growth of versatile heterogeneous upconversion nanocrystals for multimodal bioimaging

© 2016 SPIE. The rapid development of a variety of molecular contrast agents makes the multimodality bioimaging highly attractive towards higher resolution, more sensitive, informative diagnosis. The key lies in the development of facile material synthesis that allows the integration of multiple contrast agents, ideally in a way that each of the components should be logically assembled to maximize their performances. Here, we report the one-pot programmable growth of multifunctional heterogeneous nanocrystal with tunable size, shape, composition, and properties. We demonstrated a facile one-pot hot-injection method to enable the highly selectively controlled growth of different sodium lanthanide fluoride nanomaterials in either longitudinal or transversal directions with atomic scale precision. This technique allows the upconversion luminescence signal, MRI signal and x-ray signal logically integrated and optimized within one single versatile nanoplatform for multimode bioimaging. These findings suggest that the facile strategy developed here have the promising to get the desired heterogeneous nanocrystals as an all-in-one contrast agent for integrated and self-correlative multimodal bioimaging

Fidelity susceptibility and long-range correlation in the Kitaev honeycomb model

We study exactly both the ground-state fidelity susceptibility and bond-bond correlation function in the Kitaev honeycomb model. Our results show that the fidelity susceptibility can be used to identify the topological phase transition from a gapped A phase with Abelian anyon excitations to a gapless B phase with non-Abelian anyon excitations. We also find that the bond-bond correlation function decays exponentially in the gapped phase, but algebraically in the gapless phase. For the former case, the correlation length is found to be $1/\xi=2\sinh^{-1}[\sqrt{2J_z -1}/(1-J_z)]$, which diverges around the critical point $J_z=(1/2)^+$.Comment: 7 pages, 6 figure

ARPES observation of isotropic superconducting gaps in isovalent Ru-substituted Ba(Fe0.75_{0.75}Ru0.25_{0.25})2_2As2_2

We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ ($T_c = 15$ K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 meV and 4.5 meV superconducting gaps on the inner and outer $\Gamma$-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the M-centered electron Fermi surface pockets. Our results are consistent with the $J_1-J_2$ model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.Comment: 5 pages, 4 figure

Highly Doped Upconversion Nanoparticles for In Vivo Applications Under Mild Excitation Power.

One of the major challenges in using upconversion nanoparticles (UCNPs) is to improve their brightness. This is particularly true for in vivo studies, as the low power excitation is required to prevent the potential photo toxicity to live cells and tissues. Here, we report that the typical NaYF4:Yb0.2,Er0.02 nanoparticles can be highly doped, and the formula of NaYF4:Yb0.8,Er0.06 can gain orders of magnitude more brightness, which is applicable to a range of mild 980 nm excitation power densities, from 0.005 W/cm2 to 0.5 W/cm2. Our results reveal that the concentration of Yb3+ sensitizer ions plays an essential role, while increasing the doping concentration of Er3+ activator ions to 6 mol % only has incremental effect. We further demonstrated a type of bright UCNPs 12 nm in total diameter for in vivo tumor imaging at a power density as low as 0.0027 W/cm2, bringing down the excitation power requirement by 42 times. This work redefines the doping concentrations to fight for the issue of concentration quenching, so that ultrasmall and bright nanoparticles can be used to further improve the performance of upconversion nanotechnology in photodynamic therapy, light-triggered drug release, optogenetics, and night vision enhancement

Failure analysis of a steel motorcycle kickstand

Copyright @ 2009 Springer US.A fractured steel motorcycle kickstand was metallurgically investigated using a range of failure analysis tools [visual examination, energy dispersive X-ray (EDX) analysis, electron microprobe analysis (EPMA), scanning electron microscopy (SEM), fractography, optical microscopy, hardness testing and non-destructive testing (NDT)]. The steel kickstand’s composition, its microstructure, electron fractographs, and mechanical test results have been critically interpreted. Some evidence of wear damage, in the failed kickstand, was observed. The microstructural and fractographic analyses showed pre-existing micro-cracks which were believed to have grown to result in ductile failure followed by acceleration of corrosion. Recommendations have been made to avoid the failure of the motorcycle kickstand