Small-molecule organic semiconductors form the basis for the emerging field of organic optoelectronics. In order to better understand the intrinsic photo-physical and transport phenomena in this important class of materials, it is necessary to study samples of very high structural order and chemical purity. Such materials exist in the form of molecular single crystals that can be used for fabrication of high-performance prototype devices, such as field-effect transistors, photo-conductors and photo-voltaic cells, in which intrinsic properties of organic semiconductors can be investigated without parasitic effects of disorder (see, e.g., [1,2,3]). This talk will overview some of the main achievements in the area of organic single-crystal devices, present resent progress and discuss novel methods of surface functionalization that result in an extremely low-noise charge transport regime at the surface of molecular crystals, leading to an observation of unprecedentedly clean and quiet (low-noise) Hall effect [4].  In addition, very interesting non-linear effects in photoconductivity originated from long-range exciton diffusion and multi-particle interactions will be discussed [5].     

References:

1. V. Podzorov, MRS Bulletin themed Issue: “Organic Single Crystals: Addressing fundamentals

   of organic electronics” introductory paper, MRS Bulletin 38, 15-24 (Jan. 2013).

2. V. Podzorov et al., "Hall effect in the accumulation layers on the surface of organic

    semiconductors", Phys. Rev. Lett. 95, 226601 (2005).

3. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, V. Podzorov, "Observation of long-range exciton

    diffusion in highly ordered organic semiconductors", Nature Mater. 9, 938 (2010).

4.  B. Lee, Y. Chen, D. Fu, H. T. Yi, K. Czelen, H. Najafov, V. Podzorov, “Trap healing and

     ultra low-noise Hall effect at the surface of organic semiconductors”, Nature Mater. 12, 1125

     (2013).

5. P. Irkhin, H. Najafov, V. Podzorov, submitted (2014).

Despite its apparent simplicity, the idealized model of a particle constrained to move on a circle has intriguing dynamic properties and immediate experimental relevance. While a rotor is rather easy to set up classically, the quantum regime is harder to realize and investigate. Here we demonstrate that the quantum dynamics of quasiparticles in certain classes of nanostructured superconductors can be mapped onto a quantum rotor. Furthermore, we provide a straightforward experimental procedure to convert this nanoscale superconducting rotor into a regular or inverted quantum pendulum with tunable gravitational field, inertia, and drive. We detail how these novel states can be detected via scanning tunneling spectroscopy. The proposed experiments will provide insights into quantum dynamics and quantum chaology.

Reference: S.H. Lin, M. Milosevic, L. Covaci, F. Peeters, B. Janko, Nature Sci. Rep. 4, 4542 (2014).

The encoding of SU(N) symmetric spin degrees of freedom in ultra-cold atom systems has recently brought the realization of antiferromagnetic order in optical lattices into close reach. We explore the quantum phases and phase transitions emerging in the most fundamental interacting models on the lattice, the SU(N)-symmetric Hubbard and Heisenberg model, by means of projective quantum Monte Carlo simulations from SU(2) to the large-N limit. We discuss the instabilities upon the introduction of local Coulomb repulsion and explicit SU(N)-symmetric Heisenberg-like spin exchange to different Fermi-surfaces. The extension to higher symmetries allows us to study the melting of phases as a function of correlations as well as symmetry.

The iridates have become fertile ground for studies of new physics driven by spin-orbit coupling (SOC) that is comparable to the on-site Coulomb and other relevant  interactions.  This unique circumstance creates a delicate balance between interactions that drives complex magnetic and dielectric behaviors and exotic states seldom or never seen in other materials. A profound manifestation of this competition is the novel Jeff = 1/2 Mott state that was observed in the layered iridates with tetravalent Ir4+(5d5) ions. On the other hand, very little attention has been drawn to iridates having pentavalent Ir5+(5d4) ions, primarily because the strong SOC limit is expected to impose a nonmagnetic singlet ground state (Jeff  = 0). In this talk, we review the underlying physical properties of the iridates including perovskites, honeycomb lattices and double perovskites with pentavalent Ir5+ ions, and report results of our recent studies that emphasize spin-orbit-tuned ground states stabilized by chemical doping, application of pressure and magnetic field. In addition, we address the urgent question that the Jeff states may not survive in the presence of strong non-cubic crystal fields and/or exchange interactions.