Michael C. Tringides

Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in Graphene

We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.

Adsorbate-adsorbate interaction effects in surface diffusion

The results of MC simulations of the fluctuation, LID and mean-square diffusion distance methods of determining D for circular probed regions are presented for attractive and repulsive nearest-neighbor interactions, respectively, over a wide temperature range. In the absence of interactions all three methods give identical D values. For attractive interactions D from mean-square dispacements is larger than that obtained from the LID method which in turn is bigger than the equilibrium value obtained from the fluctuation method, while the inverse order holds for repulsive interactions. The activation energies obtained from the LID method differ by several J (J being the nn interaction energy) from the equilibrium values, but, possibly fortuitously the prefactors D0 are very similar to the equilibrium values. The reason for the difference in E is that the LID method measures a complicated average over different coverages, while deviations from mean coverage are very small in the fluctuation method. It was also shown that Boltzmann-Matano analysis cannot be carried out for cylindrical geometry, because the relevant solutions do not take the form (cr,t) = f(rt12).

Quantum size effects in metallic nanostructures

Electrons confined in ultrathin metal films provide a window on the peculiar world of quantum mechanics.

Remarkable effects of disorder on superconductivity of single atomic layers of lead on silicon

In bulk materials, superconductivity is remarkably robust with respect to non-magnetic disorder. In the two-dimensional limit, however, disorder and electron correlations both tend to destroy the quantum condensate. Here we study, both experimentally and theoretically, the effect of structural disorder on the local spectral response of crystalline superconducting monolayers of lead on silicon. In a direct scanning tunnelling microscopy measurement, we reveal how the local superconducting spectra lose their conventional character and show variations at scales significantly shorter than the coherence length. We demonstrate that the precise atomic organization determines the robustness of the superconducting order with respect to structural defects, such as single atomic steps, which may disrupt superconductivity and act as native Josephson barriers. We expect that our results will improve the understanding of microscopic processes in surface and interface superconductivity, and will open a new way of engineering atomic-scale superconducting quantum devices.

Uniform island height selection in the low temperature growth of Pb/Si(111)-(7×7)

Self-organized, uniform-height, Pb islands with flat tops and steep edges form on Si(111)-(7 x 7) at low temperatures 120<T<250 K. Islands of heights differing by bilayer height increments are observed depending on growth conditions. The formation of these structures is highly unusual since at low temperatures thermal diffusion is suppressed. The origin of the regular structures is believed to be quantum size effects (i.e. effects related to the quantization of the electron energy levels in the islands). We have studied with two complementary techniques (i.e. high resolution spot profile analysis low energy electron diffraction and variable temperature scanning tunneling microscopy) how the preferred island heights depend on the growth parameters (i.e. temperature, coverage, kinetic pathway etc.). We have constructed a kinetic phase diagram in the coverage-temperature plane which indicates the type of islands formed under different growth conditions. The phase diagram can be used as a guide so the island height can be easily controlled.

Metal Nanostructure Formation on Graphene: Weak versus Strong Bonding

Graphene is an exciting material with numerous potential applications. To understand metal graphene interaction two different metals were studied. Two large Pb islands nucleate at 78K indicating fast diffusion and weak interaction(right). On the contrary, for Dysprosium a high island density is observed confirming slow diffusion and strong interaction(left).

Strong metal adatom-substrate interaction of Gd and Fe with graphene.

Graphene is a unique 2D system of confined electrons with an unusual electronic structure of two inverted Dirac cones touching at a single point, with high electron mobility and promising microelectronics applications. The clean system has been studied extensively, but metal adsorption studies in controlled experiments have been limited; such experiments are important to grow uniform metallic films, metal contacts, carrier doping, etc. Two non-free-electron-like metals (rare earth Gd and transition metal Fe) were grown epitaxially on graphene as a function of temperature T and coverage θ. By measuring the nucleated island density and its variation with growth conditions, information about the metal-graphene interaction (terrace diffusion, detachment energy) is extracted. The nucleated island densities at room temperature (RT) are stable and do not coarsen, at least up to 400 °C, which shows an unusually strong metal-graphene bond; most likely it is a result of C atom rebonding from the pure graphene sp(2) C-C configuration to one of lower energy.

On the theory of surface diffusion: kinetic versus lattice gas approach

Abstract The present paper extends the results of a recent analytic kinetic theory of particle-on-substrate diffusion. The approach treats explicitly the molecule–surface interaction and takes into account inter-molecular interaction within the hard particle approximation. The physics influencing the diffusion pre-exponential factor and mechanisms determining the density dependence of collective diffusivity are discussed. The kinetic results are compared with those of the traditional lattice gas hopping models. Analytical expressions for jump rates in the low density limit are derived, and the density dependence of effective jump rates at finite occupancy is discussed. It is shown how the traditional hopping model oversimplifies the picture of diffusion by neglecting the collision part of the hopping process.

Wetting-layer transformation for Pb nanocrystals grown on Si(111)

We present the results of in situ x-ray scattering experiments that investigate the growth of Pb nanocrystalline islands on Si(111). It is conclusively shown that the Pb nanocrystals do not reside on top of a Pb wetting layer. The nucleating Pb nanocrystals transform the highly disordered Pb wetting layer beneath the islands into well-ordered fcc Pb. The surface then consists of fcc Pb islands directly on top of the Si surface with the disordered wetting layer occupying the region between the islands. As the Pb nanocrystals coalesce at higher coverage we observe increasing disorder that is consistent with misfit strain relaxation. These results have important implications for predicting stable Pb island heights.

The use of the Laser-Induced Desorption (LID) method for measuring diffusion on interactive systems

Abstract The use of the LID method to measure diffusion coefficients on interactive systems is examined with Monte Carlo simulations. For site exclusion interactions the method gives coverage independent diffusion coefficients as the fluctuation method. Simulations on systems with nearest neighbor interactions, however, show that diffusion coefficients obtained with the LID method are different from the ones obtained by equilibrium methods. We identify the difference to be related to the strong non-equilibrium initial condition, of no particle occupation of the probe area, that removes the system from the linear hydrodynamic regime. Atomic interactions can be extracted with detailed modelling.