Jaime E. Santos

Optical bistability of graphene in the terahertz range

We are grateful to D. K. Ferry and A. A. Ignatov for sharing their insights on the early developments of Boltzmann transport theory for semiconductors, and we thank N. A. Mortensen for useful remarks. This work was partially supported by the FEDER COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) through Grant No. PEst-C/FIS/UI0607/2013. We acknowledge support from the EC under Graphene Flagship (Contract No. CNECT-ICT-604391). The Center for Nanostructured Graphene (CNG) is sponsored by the Danish National Research Foundation, Project No. DNRF58. The work contract of J.E.S. is financed within the framework of the Program of Recruitment of Post Doctoral Researchers for the Portuguese Scientific and Technological System, with the Operational Program Human Potential (POPH) of the QREN, participated in by the European Social Fund (ESF) and national funds of the Portuguese Ministry of Education and Science (MEC).

Electronic doping of graphene by deposited transition metal atoms

We perform a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters. The sample is placed in a capacitor device such that the electronic doping of graphene can be varied by the application of a gate voltage and such that transport measurements can be performed via the application of a (much smaller) voltage along the graphene sample, as reported in the work of 18 . The analysis allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionisation potential of the metal clusters in terms of the chemical interaction between graphene and the clusters. We are also able, by modeling the metallic clusters as perfect conducting spheres, to determine the scattering potential due to these clusters on the electronic carriers of graphene and hence the contribution of these clusters to the resistivity of the sample. The model presented is able to explain the measurements performed by 18 on Pt-covered graphene samples at the lowest metallic coverages measured and we also present a theoretical argument based on the above model that explains why significant deviations from such a theory are observed at higher levels of coverage.

Exact time-dependent correlation functions for the symmetric exclusion process with open boundary

As a simple model for single-file diffusion of hard core particles we investigate the one-dimensional symmetric exclusion process. We consider an open semi-infinite system where one end is coupled to an external reservoir of constant density rho(*) and which initially is in a nonequilibrium state with bulk density rho(0). We calculate the exact time-dependent two-point density correlation function C(k,l)(t) identical with- and the mean and variance of the integrated average net flux of particles N(t)-N(0) that have entered (or left) the system up to time t. We find that the boundary region of the semi-infinite relaxing system is in a state similar to the bulk state of a finite stationary system driven by a boundary gradient. The symmetric exclusion model provides a rare example where such behavior can be proved rigorously on the level of equal-time two-point correlation functions. Some implications for the relaxational dynamics of entangled polymers and for single-file diffusion in colloidal systems are discussed.

Scanning tunneling microscopy currents on locally disordered graphene

We study the local density of states at and around a substituting impurity, and use these results to compute current versus bias characteristic curves of scanning tunneling microscopy (STM) experiments done on the surface of graphene. This allows us to detect the presence of substituting impurities on graphene. The case of vacancies is also analyzed. We find that the shape and magnitude of the STM characteristic curves depend on the position of the tip and on the nature of the defect, with the strength of the binging between the impurity and the carbon atoms playing an important role. Also the nature of the last atom of the tip has an influence on the shape of the characteristic curve.

Renormalization of nanoparticle polarizability in the vicinity of a graphene-covered interface

Financial support from the Portuguese Foundation for Science and Technology (FCT) through Projects PTDC-FIS-113199-2009 and PEst-C/FIS/UI0607/2013 is acknowledged. We also acknowledge support from the European Commission under Graphene Flagship (Contract No. CNECT-ICT-604391). J.E.S.s work contract is financed in the framework of the Program of Recruitment of Post Doctoral Researchers for the Portuguese Scientific and Technological System, within the Operational Program Human Potential (POPH) of the QREN, participated by the European Social Fund (ESF) and national funds of the Portuguese Ministry of Education and Science (MEC).

Renewal processes and fluctuation analysis of molecular motor stepping

We present a systematic method of analysis of experiments performed with single motor proteins. The use of such a method should allow a more detailed description of the motors chemical cycle through the precise fitting of the experimental data. We model the dynamics of a processive or rotary molecular motor using a renewal process, in line with the work initiated by Svoboda, Mitra and Block. We apply a functional technique to compute different types of multiple-time correlation function of the renewal process, which have applications to bead-assay experiments performed both with processive molecular motors, such as myosin V and kinesin, and rotary motors, such as F1-ATPase.

Dipolar interactions in superconductor-ferromagnet heterostructures

We consider a simple model for a superlattice composed of a thin magnetic film placed between two bulk superconductors. The magnetic film is modeled by a planar but otherwise arbitrary distribution of magnetic dipoles and the superconductors are treated in the London approximation. Due to the linearity of the problem, we are able to compute the magnetic energy of the film in the presence of the superconductors. We show that for wave vectors that are much larger than the inverse London penetration depth, the magnetic energy is unchanged with respect to the film in free space, whereas in the case of small wave numbers compared to the inverse London penetration depth, the magnetic energy resembles the energy of a distribution of magnetization in a two-dimensional space. Possible experimental applications of these results are discussed.

Strong light-matter interaction in systems described by a modified Dirac equation

The bulk states of some materials, such as topological insulators, are described by a modified Dirac equation. Such systems may have trivial and non-trivial phases. In this article, we show that in the non-trivial phase a strong light-matter interaction exists in a two-dimensional system, which leads to an optical conductivity at least one order of magnitude larger than that of graphene.

Non-equilibrium behavior at a liquid-gas critical point

Abstract:Second-order phase transitions in a non-equilibrium liquid-gas model with reversible mode couplings, i.e., model H for binary-fluid critical dynamics, are studied using dynamic field theory and the renormalization group. The system is driven out of equilibrium either by considering different values for the noise strengths in the Langevin equations describing the evolution of the dynamic variables (effectively placing these at different temperatures), or more generally by allowing for anisotropic noise strengths, i.e., by constraining the dynamics to be at different temperatures in d || - and d⊥-dimensional subspaces, respectively. In the first, isotropic case, we find one infrared-stable and one unstable renormalization group fixed point. At the stable fixed point, detailed balance is dynamically restored, with the two noise strengths becoming asymptotically equal. The ensuing critical behavior is that of the standard equilibrium model H. At the novel unstable fixed point, the temperature ratio for the dynamic variables is renormalized to infinity, resulting in an effective decoupling between the two modes. We compute the critical exponents at this new fixed point to one-loop order. For model H with spatially anisotropic noise, we observe a critical softening only in the d⊥-dimensional sector in wave vector space with lower noise temperature. The ensuing effective two-temperature model H does not have any stable fixed point in any physical dimension, at least to one-loop order. We obtain formal expressions for the novel critical exponents in a double expansion about the upper critical dimension dc = 4 - d || and with respect to d || , i.e., about the equilibrium theory.

Assessment of shear modulus by different seismic wave-based techniques

This work was developed with the financial support provided by FCT (Portuguese Foundation for Science and Technology) under the research project WaveSoil - PTDC/ECM/122751/2010 from FCOMOP-01-0124-FEDER-020365 project.