Substrate effects on quasiparticles and excitons in graphene nanoflakes

The effects of substrate on electronic and optical properties of triangular and hexagonal graphene nanoflakes with armchair edges are investigated by using a configuration interaction approach beyond double excitation scheme. The quasiparticle correction to the energy gap and exciton binding energy are found to be dominated by the long-range Coulomb interactions and exhibit similar dependence on the dielectric constant of the substrate, which leads to a cancellation of their contributions to the optical gap. As a result, the optical gaps are shown to be insensitive to the dielectric environment and unexpectedly close to the single-particle gaps.Comment: 4 pages, 4 figure

The infrared spectra of ABC-stacking tri- and tetra-layer graphenes studied by first-principles calculations

The infrared absorption spectra of ABC-stacking tri- and tetra-layer graphenes are studied using the density functional theory. It is found that they exhibit very different characteristic peaks compared with those of AB-stacking ones, caused by the different stacking sequence and interlayer coupling. The anisotropy of the spectra with respect to the direction of the light electric field is significant. The spectra are more sensitive to the stacking number when the electric field is perpendicular to the graphene plane due to the interlayer polarization. The high sensitivities make it possible to identify the stacking sequence and stacking number of samples by comparing theory and experiment.Comment: 7 pages, 5 figure

Bond algebraic liquid phase in strongly correlated multiflavor cold atom systems

When cold atoms are trapped in a square or cubic optical lattice, it should be possible to pump the atoms into excited $p-$level orbitals within each well. Following earlier work, we explore the metastable equilibrium that can be established before the atoms decay into the $s-$wave orbital ground state. We will discuss the situation with integer number of bosons on every site, and consider the strong correlation "insulating" regime. By employing a spin-wave analysis together with a new duality transformation, we establish the existence and stability of a novel gapless "critical phase", which we refer to as a "bond algebraic liquid". The gapless nature of this phase is stabilized due to the emergence of symmetries which lead to a quasi-one dimensional behavior. Within the algebraic liquid phase, both bond operators and particle flavor occupation number operators have correlations which decay algebraically in space and time. Upon varying parameters, the algebraic bond liquid can be unstable to either a Mott insulator phase which spontaneously breaks lattice symmetries, or a $\mathbb{Z}_2$ phase. The possibility of detecting the algebraic liquid phase in cold atom experiments is addressed. Although the momentum distribution function is insufficient to distinguish the algebraic bond liquid from other phases, the density correlation function can in principle be used to detect this new phase of matter.Comment: 15 pages, 10 figure

Internal Josephson-Like Tunneling in Two-Component Bose-Einstein Condensates Affected by Sign of the Atomic Interaction and External Trapping Potential

We study the Josephson-like tunneling in two-component Bose-Einstein condensates coupled with microwave field in respond to various attractive and repulsive atomic interaction under the various aspect ratio of trapping potential and the gravitational field. It is very interesting to find that the dynamic of Josephson-like tunneling can be controlled from fast damped oscillations and asymmetric occupation to nondamped oscillation and symmetric occupation.Comment: 4 pages, 5 figure

Odd-even mass staggering with Skyrme-Hartree-Fock-Bogoliubov theory

We have studied odd-even nuclear mass staggering with the Skyrme-Hartree-Fock-Bogoliubov theory by employing isoscalar and isovector contact pairing interactions. By reproducing the empirical odd-even mass differences of the Sn isotopic chain, the strengths of pairing interactions are determined. The optimal strengths adjusted in this work can give better description of odd-even mass differences than that fitted by reproducing the experimental neutron pairing gap of $^{120}$Sn.Comment: 9 pages, 3 figures, submitted to PRC Brief Repor

Impact of disorder on unconventional superconductors with competing ground states

Non-magnetic impurities are known as strong pair breakers in superconductors with pure d-wave pairing symmetry. Here we discuss d-wave states under the combined influence of impurities and competing instabilities, such as pairing in a secondary channel as well as lattice symmetry breaking. Using the self-consistent T-matrix formalism, we show that disorder can strongly modify the competition between different pairing states. For a d-wave superconductor in the presence of a subdominant local attraction, Anderson's theorem implies that disorder always generates an s-wave component in the gap at sufficiently low temperature, even if a pure d_{x^2-y^2} order parameter characterizes the clean system. In contrast, disorder is always detrimental to an additional d_{xy} component. This qualitative difference suggests that disorder can be used to discriminate among different mixed-gap structures in high-temperature superconductors. We also investigate superconducting phases with lattice symmetry breaking in the form of bond order, and show that the addition of impurities quickly leads to the restoration of translation invariance. Our results highlight the importance of controlling disorder for the observation of competing order parameters in cuprates.Comment: 13 pages, 10 figure

Scanning Photo-Induced Impedance Microscopy - Resolution studies and polymer characterization

Scanning Photo-Induced Impedance Microscopy (SPIM) is an impedance imaging technique that is based on photocurrent measurements at field-effect structures. The material under investigation is deposited onto a semiconductor-insulator substrate. A thin metal film or an electrolyte solution with an immersed electrode serves as the gate contact. A modulated light beam focused into the space charge region of the semiconductor produces a photocurrent, which is directly related to the local impedance of the material. The absolute impedance of a polymer film can be measured by calibrating photocurrents using a known impedance in series with the sample. Depending on the wavelength of light used, charge carriers are not only generated in the focus but also throughout the bulk of the semiconductor. This can have adverse effects on the lateral resolution. Two-photon experiments were carried out to confine charge carrier generation to the spacecharge layer. The lateral resolution of SPIM is also limited by the lateral diffusion of charge carriers in the semiconductor. This problem can be solved by using thin silicon layers as semiconductor substrates. A resolution of better than 1 mu m was achieved using silicon on sapphire (SOS) substrates with a I l.Lm thick silicon layer

Comparison of different measures for quantum discord under non-Markovian noise

Two geometric measures for quantum discord were recently proposed by Modi et al. [Phys. Rev. Lett. 104, 080501 (2010)] and Dakic et al. [Phys. Rev. Lett. 105, 190502 (2010)]. We study the similarities and differences for total quantum correlations of Bell-diagonal states using these two geometry-based quantum discord and the original quantum discord. We show that, under non-Markovian dephasing channels, quantum discord and one of the geometric measures stay constant for a finite amount of time, but not the other geometric measure. However, all the three measures share a common sudden change point. Our study on critical point of sudden transition might be useful for keeping long time total quantum correlations under decoherence.Comment: 10 pages, 3 figures submitted for publicatio