Energy-Momentum dispersion relation of plasmarons in bilayer graphene

The relation between the energy and momentum of plasmarons in bilayer graphene is investigated within the Overhauser approach, where the electron-plasmon interaction is described as a field theoretical problem. We find that the Dirac-like spectrum is shifted by $\Delta E(\mathbf{k})\sim 100\div150\,{\rm meV}$ depending on the electron concentration $n_{e}$ and electron momentum. The shift increases with electron concentration as the energy of plasmons becomes larger. The dispersion of plasmarons is more pronounced than in the case of single layer graphene, which is explained by the fact that the energy dispersion of electrons is quadratic and not linear. We expect that these predictions can be verified using angle-resolved photoemission spectroscopy (ARPES).Comment: 4 pages, 3 figure

Plasmons and their interaction with electrons in trilayer graphene

The interaction between electrons and plasmons in trilayer graphene is investigated within the Overhauser approach resulting in the 'plasmaron' quasi-particle. This interaction is cast into a field theoretical problem, nd its effect on the energy spectrum is calculated using improved Wigner-Brillouin perturbation theory. The plasmaron spectrum is shifted with respect to the bare electron spectrum by $\Delta E(\mathbf{k})\sim 50\div200\,{\rm meV}$ for ABC stacked trilayer graphene and for ABA trilayer graphene by $\Delta E(\mathbf{k})\sim 30\div150\,{\rm meV}$ ($\Delta E(\mathbf{k})\sim 1\div5\,{\rm meV}$) for the hyperbolic linear) part of the spectrum. The shift in general increases with the electron concentration $n_{e}$ and electron momentum. The dispersion of plasmarons is more pronounced in \textit{ABC} stacked than in ABA tacked trilayer graphene, because of the different energy band structure and their different plasmon dispersion.Comment: arXiv admin note: substantial text overlap with arXiv:1310.623

Electron transport in waveguides with spatially modulated strengths of the Rashba and Dresselhaus terms of the spin-orbit interaction

We study electron transport through waveguides (WGs) in which the strengths of the Rashba ($\alpha$) and Dresselhaus ($\beta$) terms of the spin-orbit interaction (SOI) vary in space. Subband mixing, due to lateral confinement, is taken into account only between the two first subbands. For sufficiently narrow WGs the transmission $T$ exhibits a square-like shape as a function of $\alpha$ or $\beta$. Particular attention is paid to the case of equal SOI strengths, $\alpha=\beta$, for which spin-flip processes are expected to decrease. The transmission exhibits resonances as a function of the length of a SOI-free region separating two regions with SOI present, that are most pronounced for $\alpha=\beta$. The sign of $\alpha$ strongly affects the spin-up and spin-down transmissions. The results show that the main effect of subband mixing is to shift the transmission resonances and to decrease the transmission from one spin state to another. The effect of possible band offsets between regions that have different SOI strengths and effective masses is also discussed

Ballistic transport through graphene nanostructures of velocity and potential barriers

We investigate the electronic properties of graphene nanostructures when the Fermi velocity and the electrostatic potential vary in space. First, we consider the transmission T and conductance G through single and double barriers. We show that G for velocity barriers differs markedly from that for potential barriers for energies below the height of the latter and it exhibits periodic oscillations as a function of the energy for strong velocity modulation. Special attention is given to superlattices (SLs). It is shown that an applied bias can efficiently widen or shrink the allowed minibands of velocity-modulated SLs. The spectrum in the Kronig–Penney limit is periodic in the strength of the barriers. Collimation of an electron beam incident on an SL with velocity and potential barriers is present but it disappears when the potential barriers are absent. The number of additional Dirac points may change considerably if barriers and wells have sufficiently different Fermi velocities

Integral quantum Hall effect in graphene: Zero and finite Hall field

We study the influence of a finite Hall field EH on the Hall conductivity σyx in graphene. Analytical expressions are derived for σyx using the Kubo-Greenwood formula. For vanishing EH, we obtain the well-known expression σyx=4(N+1/2)e2/h. The inclusion of the dispersion of the energy levels, previously not considered, and their width, due to scattering by impurities, produces the plateau of the n=0 Landau level. Further, we evaluate the longitudinal resistivity ρxx and show that it exhibits an oscillatory behavior with the electron concentration. The peak values of ρxx depend strongly on the impurity concentration and their potential. For a finite EH, the result for σyx is the same as that for EH=0, provided EH is not strong, but the values and positions of the resistivity maxima are modified due to the EH-dependent dispersion of the energy levels

How to realise a homogeneous dipolar Bose gas in the roton regime

Homogeneous quantum gases open up new possibilities for studying many-body phenomena and have now been realised for a variety of systems. For gases with short-range interactions the way to make the cloud homogeneous is, predictably, to trap it in an ideal (homogeneous) box potential. We show that creating a close to homogeneous dipolar gas in the roton regime, when long-range interactions are important, actually requires trapping particles in soft-walled (inhomogeneous) box-like potentials. In particular, we numerically explore a dipolar gas confined in a pancake trap which is harmonic along the polarisation axis and a cylindrically symmetric power-law potential $r^p$ radially. We find that intermediate $p$'s maximise the proportion of the sample that can be brought close to the critical density required to reach the roton regime, whereas higher $p$'s trigger density oscillations near the wall even when the bulk of the system is not in the roton regime. We characterise how the optimum density distribution depends on the shape of the trapping potential and find it is controlled by the trap wall steepness.Comment: 7 pages, 3 figure

Transport properties of low-dimensional semiconductor structures in the presence of spin–orbit interaction

Transport properties of a two-dimensional electron gas (2DEG) and of quantum wires are theoretically studied in the presence of both Rashba and Dresselhaus terms of the spin–orbit interaction (SOI). Fully quantum mechanical expressions for the conductivity are evaluated for very low temperatures and the differences between them and previous semiclassical results are highlighted. Two kinds of confining potentials in quantum wires are considered, square-type and parabolic. Various cases depending on the relative strengths of two different SOI terms are discussed and the relaxation times for various impurity potentials are evaluated. In addition, the spin accumulation in a 2DEG and in a quantum wire (QW) is evaluated semiclassically and its dependence on the Fermi energy and the SOI strengths is discussed. A nearly saw-tooth dependence on the electron concentration is obtained for a QW with parabolic confinement

Combined approach of density functional theory and quantum Monte Carlo method to electron correlation in dilute magnetic semiconductors

We present a realistic study for electronic and magnetic properties in dilute magnetic semiconductor (Ga,Mn)As. A multi-orbital Haldane-Anderson model parameterized by density-functional calculations is presented and solved with the Hirsch-Fye quantum Monte Carlo algorithm. Results well reproduce experimental results in the dilute limit. When the chemical potential is located between the top of the valence band and an impurity bound state, a long-range ferromagnetic correlations between the impurities, mediated by antiferromagnetic impurity-host couplings, are drastically developed. We observe an anisotropic character in local density of states at the impurity-bound-state energy, which is consistent with the STM measurements. The presented combined approach thus offers a firm starting point for realistic calculations of the various family of dilute magnetic semiconductors.Comment: 5 pages, 4 figure

Atom cloud detection and segmentation using a deep neural network

Funder: Royal Society; doi: http://dx.doi.org/10.13039/501100000288Funder: Trinity College, University of Cambridge; doi: http://dx.doi.org/10.13039/501100000727Funder: John Fell Fund, University of Oxford; doi: http://dx.doi.org/10.13039/501100004789Abstract: We use a deep neural network (NN) to detect and place region-of-interest (ROI) boxes around ultracold atom clouds in absorption and fluorescence images—with the ability to identify and bound multiple clouds within a single image. The NN also outputs segmentation masks that identify the size, shape and orientation of each cloud from which we extract the clouds’ Gaussian parameters. This allows 2D Gaussian fits to be reliably seeded thereby enabling fully automatic image processing. The method developed performs significantly better than a more conventional method based on a standardized image analysis library (Scikit-image) both for identifying ROI and extracting Gaussian parameters

New high-speed centre of mass method incorporating background subtraction for accurate determination of fluorescence lifetime

We demonstrate an implementation of a centre-of-mass method (CMM) incorporating background subtraction for use in multifocal fluorescence lifetime imaging microscopy to accurately determine fluorescence lifetime in live cell imaging using the Megaframe camera. The inclusion of background subtraction solves one of the major issues associated with centre-of-mass approaches, namely the sensitivity of the algorithm to background signal. The algorithm, which is predominantly implemented in hardware, provides real-time lifetime output and allows the user to effectively condense large amounts of photon data. Instead of requiring the transfer of thousands of photon arrival times, the lifetime is simply represented by one value which allows the system to collect data up to limit of pulse pile-up without any limitations on data transfer rates. In order to evaluate the performance of this new CMM algorithm with existing techniques (i.e. Rapid lifetime determination and Levenburg-Marquardt), we imaged live MCF-7 human breast carcinoma cells transiently transfected with FRET standards. We show that, it offers significant advantages in terms of lifetime accuracy and insensitivity to variability in dark count rate (DCR) between Megaframe camera pixels. Unlike other algorithms no prior knowledge of the expected lifetime is required to perform lifetime determination. The ability of this technique to provide real-time lifetime readout makes it extremely useful for a number of applications