2016 - 2017

Academic year 2016 - 2017

23 March A.B. Shvartsburg, Russian Academy of Sciences, Moscow

"All-dielectric metamaterials and displacement currents: from paradoxes to devices"

This presentation is devoted to the physical fundamentals, mathematical basis, and the first applications of the newly developing branch of applied electromagnetics, based on displacement currents in metamaterials. Three topics are considered:

1. Tunneling of light in gradient nanofilms (non Fresnel reflection, frequency–selective interfaces, broadband antireflection coatings)

2. All–dielectric magnetic dipoles (optical magnetism of dielectric circuits)

3. Negative magnetic inductance (metacrystals for information processing)

The mathematical fundamentals for the unified optimization of physical properties and fine spatial structures of metacrystal elements with sizes smaller then the wavelengths are needed; this approach based on new exact analytical solutions of Maxwell equations for artificial media and computational programs is in progress.

31 January Anton Ramsak, University of Ljubljana, Slovenia

"Magic Ratios of Linear Conductances of Gated Graphene Structures"

Recently a new magic ratio rule that captures the contribution of the connectivity to the conductance ratios of graphene-like cores, when the coupling to the electrodes is weak and the Fermi energy coincides with the center of the HOMO-LUMO gap, has been identified [1]. The rule is exact for a tight-binding bipartite lattice of identical sites with identical couplings when the Fermi energy is located at the gap center. It states that the connectivity-driven conductance ratio is simply the square of the ratio of two “magic integers” whose values depend only on the connectivities to the electrodes.

A comparison with available experimental values confirms the magic ratio rule qualitatively. There are several possible sources generating quantitative deviations from the magic ratio rule. We analysed the deviations due to top gate voltage pushing on-site energies away from the center of the HOMO-LUMO gap, due to the coupling to leads being non-negligible and due to the electrodes causing on-site energies on atoms to which leads are attached to deviate from on-site energies on other atoms [2]. The deviation from the ratio given by magic integers was found to become important when those parameters become of the order of the hopping integral of the molecule. For small values of those parameters, the deviation was found analytically. Furthermore, if the top gate voltage is non-zero, the molecule conducts even when both leads are attached to sites in the same sublattice, which for other perturbations is not the case. Effects due to the Coulomb electron-electron interaction and the effect of screening due to the gates seem to be most relevant for understanding of the deviation of the experimental results from the magic ratio rule [3].

[1] Y. Geng et al., J. Am. Chem. Soc. 137, 4469 (2015).

[2] L. Ulcakar, T. Rejec, and A. Ramsak, Acta Chim. Slov. 63, 583 (2016).

[3] T. Rejec, L. Ulcakar, A. Ramsak, J. Kokalj et al., to be submitted.

12 January Benjamin Villalonga, Univ. of Illinois at Urbana-Champaign

"Exploring one particle orbitals in Many-Body Localized systems"

Strong disorder in interacting quantum systems can give rise to the phenomenon of Many-Body Localization (MBL), which can be studied from a single particle perspective as recently proposed by Bera et al. [1]. We study the properties of the natural single particle orbitals defined by the eigenvectors of the one particle density matrix of many-body eigenstates of an MBL system in one dimension, subject to a random potential.

[1] S. Bera, H. Schomerus, F. Heidrich-Meisner, and J. H. Bardarson, Phys. Rev. Lett. 115, 046603 (2015).

3 October H. R. Sadeghpour, ITAMP - Harvard-Smithsonian

"Rydberg polarons in a Bose gas"

Polarons are fundamentally crucial to understanding many-body behavior in physical systems. The introduction of a foreign species in a quantum gas naturally perturbs and modifies the collective nature of the system. In the last few years, there has been an uptake in activity to realize "clean" polaronic effects in atomic quantum gases. In this work, we will explore the possibility of realizing yet another class of polarons in an atomic BEC by injecting the gas with an excitation impurity (a Rydberg atom). A couple of experimental realizations and the theory are described.