Self-assembly of the simple cubic lattice with an isotropic potential

Conventional wisdom presumes that low-coordinated crystal ground states require directional interactions. Using our recently introduced optimization procedure to achieve self-assembly of targeted structures (Phys. Rev. Lett. 95, 228301 (2005), Phys. Rev. E 73, 011406 (2006)), we present an isotropic pair potential $V(r)$ for a three-dimensional many-particle system whose classical ground state is the low-coordinated simple cubic (SC) lattice. This result is part of an ongoing pursuit by the authors to develop analytical and computational tools to solve statistical-mechanical inverse problems for the purpose of achieving targeted self-assembly. The purpose of these methods is to design interparticle interactions that cause self-assembly of technologically important target structures for applications in photonics, catalysis, separation, sensors and electronics. We also show that standard approximate integral-equation theories of the liquid state that utilize pair correlation function information cannot be used in the reverse mode to predict the correct simple cubic potential. We report in passing optimized isotropic potentials that yield the body-centered cubic and simple hexagonal lattices, which provide other examples of non-close-packed structures that can be assembled using isotropic pair interactions.Comment: 16 pages, 12 figures. Accepted for publication in Physical Review

The formation of an energy gap in graphene on ruthenium by controlling the interface

In this work, we have investigated the spectral function of graphene on a monolayer of intercalated gold on Ru(0001) using angle-resolved photoemission spectroscopy (ARPES). The intercalation leads to a decoupling of the graphene film, as documented by emergence of the characteristic linear π-bands near the Fermi level. However, a band gap at the band crossing is observed. We relate this gap opening to the broken symmetry of the two carbon sublattices, induced by the special lattice mismatch of the graphene and the intercalated gold monolayer

Influence of static electric fields on an optical ion trap

We recently reported on a proof-of-principle experiment demonstrating optical trapping of an ion in a single-beam dipole trap superimposed by a static electric potential [Nat. Photonics 4, 772--775 (2010)]. Here, we first discuss the experimental procedures focussing on the influence and consequences of the static electric potential. These potentials can easily prevent successful optical trapping, if their configuration is not chosen carefully. Afterwards, we analyse the dipole trap experiments with different analytic models, in which different approximations are applied. According to these models the experimental results agree with recoil heating as the relevant heating effect. In addition, a Monte-Carlo simulation has been developed to refine the analysis. It reveals a large impact of the static electric potential on the dipole trap experiments in general. While it supports the results of the analytic models for the parameters used in the experiments, the analytic models cease their validity for significantly different parameters. Finally, we propose technical improvements for future realizations of experiments with optically trapped ions.Comment: 16 pages, 16 figure

Charge and spin distributions in GaMnAs/GaAs Ferromagnetic Multilayers

A self-consistent electronic structure calculation based on the Luttinger-Kohn model is performed on GaMnAs/GaAs multilayers. The Diluted Magnetic Semiconductor layers are assumed to be metallic and ferromagnetic. The high Mn concentration (considered as 5% in our calculation) makes it possible to assume the density of magnetic moments as a continuous distribution, when treating the magnetic interaction between holes and the localized moment on the Mn(++) sites. Our calculation shows the distribution of heavy holes and light holes in the structure. A strong spin-polarization is observed, and the charge is concentrated mostly on the GaMnAs layers, due to heavy and light holes with their total angular momentum aligned anti-parallel to the average magnetization. The charge and spin distributions are analyzed in terms of their dependence on the number of multilayers, the widths of the GaMnAs and GaAs layers, and the width of lateral GaAs layers at the borders of the structure.Comment: 12 pages,7 figure

Gap modification of atomically thin boron nitride by phonon mediated interactions

A theory is presented for the modification of bandgaps in atomically thin boron nitride (BN) by attractive interactions mediated through phonons in a polarizable substrate, or in the BN plane. Gap equations are solved, and gap enhancements are found to range up to 70% for dimensionless electron-phonon coupling \lambda=1, indicating that a proportion of the measured BN bandgap may have a phonon origin

Electron-Phonon Coupling in Highly-Screened Graphene

Photoemission studies of graphene have resulted in a long-standing controversy concerning the strength of the experimental electron-phonon interaction in comparison with theoretical calculations. Using high-resolution angle-resolved photoemission spectroscopy we study graphene grown on a copper substrate, where the metallic screening of the substrate substantially reduces the electron-electron interaction, simplifying the comparison of the electron-phonon interaction between theory and experiment. By taking the nonlinear bare bandstructure into account, we are able to show that the strength of the electron-phonon interaction does indeed agree with theoretical calculations. In addition, we observe a significant bandgap at the Dirac point of graphene.Comment: Submitted to Phys. Rev. Lett. on July 20, 201

On the Application of a Monolithic Array for Detecting Intensity-Correlated Photons Emitted by Different Source Types

It is not widely appreciated that many subtleties are involved in the accurate measurement of intensity-correlated photons; even for the original experiments of Hanbury Brown and Twiss (HBT). Using a monolithic 4x4 array of single-photon avalanche diodes (SPADs), together with an off-chip algorithm for processing streaming data, we investigate the difficulties of measuring second-order photon correlations g2 in a wide variety of light fields that exhibit dramatically different correlation statistics: a multimode He-Ne laser, an incoherent intensity-modulated lamp-light source and a thermal light source. Our off-chip algorithm treats multiple photon-arrivals at pixel-array pairs, in any observation interval, with photon fluxes limited by detector saturation, in such a way that a correctly normalized g2 function is guaranteed. The impact of detector background correlations between SPAD pixels and afterpulsing effects on second-order coherence measurements is discussed. These results demonstrate that our monolithic SPAD array enables access to effects that are otherwise impossible to measure with stand-alone detectors.Comment: 17 pages, 6 figure

Optical Trapping of an Ion

For several decades, ions have been trapped by radio frequency (RF) and neutral particles by optical fields. We implement the experimental proof-of-principle for trapping an ion in an optical dipole trap. While loading, initialization and final detection are performed in a RF trap, in between, this RF trap is completely disabled and substituted by the optical trap. The measured lifetime of milliseconds allows for hundreds of oscillations within the optical potential. It is mainly limited by heating due to photon scattering. In future experiments the lifetime may be increased by further detuning the laser and cooling the ion. We demonstrate the prerequisite to merge both trapping techniques in hybrid setups to the point of trapping ions and atoms in the same optical potential.Comment: 5 pages, 3 figure

Quantum dots and spin qubits in graphene

This is a review on graphene quantum dots and their use as a host for spin qubits. We discuss the advantages but also the challenges to use graphene quantum dots for spin qubits as compared to the more standard materials like GaAs. We start with an overview of this young and fascinating field and will then discuss gate-tunable quantum dots in detail. We calculate the bound states for three different quantum dot architectures where a bulk gap allows for confinement via electrostatic fields: (i) graphene nanoribbons with armchair boundary, (ii) a disc in single-layer graphene, and (iii) a disc in bilayer graphene. In order for graphene quantum dots to be useful in the context of spin qubits, one needs to find reliable ways to break the valley-degeneracy. This is achieved here, either by a specific termination of graphene in (i) or in (ii) and (iii) by a magnetic field, without the need of a specific boundary. We further discuss how to manipulate spin in these quantum dots and explain the mechanism of spin decoherence and relaxation caused by spin-orbit interaction in combination with electron-phonon coupling, and by hyperfine interaction with the nuclear spin system.Comment: 23 pages, 10 figures, topical review prepared for Nanotechnolog

Multi-phonon Raman scattering in semiconductor nanocrystals: importance of non-adiabatic transitions

Multi-phonon Raman scattering in semiconductor nanocrystals is treated taking into account both adiabatic and non-adiabatic phonon-assisted optical transitions. Because phonons of various symmetries are involved in scattering processes, there is a considerable enhancement of intensities of multi-phonon peaks in nanocrystal Raman spectra. Cases of strong and weak band mixing are considered in detail. In the first case, fundamental scattering takes place via internal electron-hole states and is participated by s- and d-phonons, while in the second case, when the intensity of the one-phonon Raman peak is strongly influenced by the interaction of an electron and of a hole with interface imperfections (e. g., with trapped charge), p-phonons are most active. Calculations of Raman scattering spectra for CdSe and PbS nanocrystals give a good quantitative agreement with recent experimental results.Comment: 16 pages, 2 figures, E-mail addresses: [email protected], [email protected], [email protected], accepted for publication in Physical Review