Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F15: Focus Session: Exciton and Electron Transport in Nanostructures |
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Sponsoring Units: DMP Chair: Richard Haglund, Vanderbilt University Room: 008B |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F15.00001: Exciton Transport in Nanostructured Solids Invited Speaker: Vladimir Bulovic Transport of nanoscale energy in the form of excitons is at the core of operation of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. The talk will present recent advancements in directly visualizing exciton transport, with spatial, temporal and spectral evolution recorded for molecular crystals, disordered thin films, and colloidal quantum dot solids. Our measurements demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology and the design of nanoscale building blocks. In addition, the talk will show that the excitonic energy landscape can be directly manipulated in solid-state thin films using dipole$-$dipole interactions, which can be increased under mechanical pressure, or molecular doping with polar molecules, leading to dramatic shifts in the exciton energy structure. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F15.00002: Spectral Road Map of Strain-Split Bulk GaAs Excitons: Evidence of Excitation-Induced Dephasing Daniel Webber, Kimberley Hall, Brian Wilmer, Alan Bristow, Xinyu Liu, Margaret Dobrowolska, Jacek Furdyna Thin films of bulk GaAs studied with two-dimensional coherent spectroscopy reveal strain-split heavy- (HH) and light-hole (LH) excitons with quantum interference. Excitation overlapping the HH continuum show strong excitation-induced dephasing (EID) and emission at the HH exciton for collinear polarization. The results are consistent with excitations beyond the perturbative $\chi^{(3)}$ regime. Cross-linear polarization suppresses the HH emission and enhances the LH exciton as well as HH biexciton emission. Two-quantum spectra at low excitation concentrations are also consistent with HH and LH exciton interference and EID. Results are compared to four-wave mixing transients that require modelling for interpretation. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F15.00003: Nonlinear, driven-dissipative hydrodynamics and effective chiral description of an exciton-polariton superfluid Manas Kulkarni, German Kolmakov Given recent remarkable experimental success on capturing hydrodynamic features of exciton-polariton condensates in optical microcavities and their potential implications for quantum and optical computing and information technologies, we present an effective chiral description for such systems. This description captures the fingerprints of hydrodynamics, namely, nonlinearity, dispersion and dissipation in the exciton-polariton system. The resulting chiral equation for the condensate perturbation wave dynamics is found to be of Burgers-type thereby providing a more transparent understanding of the complicated underlying coupled exciton-photon dynamics. By using analytical calculations and numerical simulations, we describe the phenomenon of polariton shock waves, solitons and defects in such systems. Our mapping is expected to have broad implications for other polariton and photon systems including dipolar exciton and magnon condensates. This mapping can further help one in engineering a delicate balance between the pump and damping to produce stable optical signals propagating in polariton circuits. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F15.00004: Recipe for Topological Polaritons Torsten Karzig, Charles-Edouard Bardyn, Netanel Lindner, Gil Refael The interaction between light and matter can give rise to novel topological states. This principle was recently exemplified in Floquet topological insulators, where \emph{classical} light was used to induce a topological electronic band structure. Here, in contrast, we show that mixing \emph{single} photons with excitons can result in new topological polaritonic states --- or ``topolaritons''. Taken separately, the underlying photons and excitons are topologically trivial. Combined appropriately, however, they give rise to non-trivial polaritonic bands with chiral edge modes allowing for unidirectional polariton propagation. The main ingredient in our construction is an exciton-photon coupling with a phase that winds in momentum space. We demonstrate how this winding emerges from spin-orbit coupling in the electronic system and an applied Zeeman field. We discuss the requirements for obtaining a sizable topological gap in the polariton spectrum. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F15.00005: Topological Polaritons and Excitons in Garden Variety Systems Charles-Edouard Bardyn, Torsten Karzig, Gil Refael, Tim Liew Topological polaritons (aka topolaritons) present a new frontier for topological behavior in solid-state systems. They combine light and matter, which allows to probe and manipulate them in a variety of ways. They can also be made strongly interacting, due to their excitonic component. Here we present a scheme which allows to realize topolaritons in garden variety zinc-blende quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as a particularly appealing alternative for topological states in exciton-based systems. Indirect excitons are robust and long lived (with lifetimes up to milliseconds), and, therefore, provide a flexible platform for the realization, probing, and utilization of topological coupled light-matter states. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F15.00006: Excitonic condensation in spatially separated 1D systems David Abergel We introduce the concept of excitonic condensation between spatially separated ground state populations of 1D electrons and holes mediated by their attractive direct Coulomb interaction. Candidate systems for observing this phenomenon include semiconductor quantum wires, core-shell nanowires, stacked graphene nanoribbons, and carbon nanotubes. We focus on the core-shell nanowire system and present calculations of the excitonic gap (which characterizes the stability of the condensate) and the critical temperature of the condensate. We also discuss additional effects such as the dependence on the material parameters, the presence of multiple bands, and spin or valley degeneracy. We show that 1D systems may have substantial improvement in the critical temperature of the condensate over comparable 2D systems because the screening of the inter-layer Coulomb interaction is weaker. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F15.00007: Kondo Physics in $4f$ metals: Gadolinium nanocontacts Bernat Olivera, Carlos Untiedt, Elke Scheer The study of electron transport in conducting materials at the nanoscale can be carried out by using Scanning Tunneling Microscope (STM) and Mechanically Controllable Break Junction techniques (MCBJ).\footnote{N. Agra\"{i}t, A. Levy-Yeyati, J.M. van Ruitenbeek. Phys. Rep. 377 (2003), 81.} At such scales, Kondo effect vanishes the magnetic properties of the $3d$ transition metals Fe, Co and Ni.\footnote{M. R. Calvo \textit{et al.}, Nature 458 (7242) (2009), 1150-1153.} The $4f$ rare earth metals are an interesting aim of study because of their strong magnetic properties among other things. At our laboratories we have measured gadolinium with both STM and MCBJ techniques. In the spectroscopy measurements of this material we perceive a set of features that could be related to its magnetic properties. The interplay between the $4f^{7}$ and $5d^{1}$ orbitals from Gd drives us to pose the mechanisms that are involved in the electronic transport properties of these systems. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F15.00008: Theory of space charge limited currents in films and nanowires with dopants Xiaoguang Zhang, Sokrates Pantelides We show that proper description of the space charge limited currents (SCLC) in a homogeneous bulk material must account fully for the effect of the dopants and the interplay between dopants and traps [1]. The sharp rise in the current at the trap-filled-limit (TFL) is partially mitigated by the dopant energy levels and the Frenkel effect, namely the lowering of the ionization energy by the electric field, which is screened by the free carriers. In nanowires, lack of effective screening causes the trap occupation at small biases to reach a high level comparable to the TFL in bulk. This explains the high current density in SCLCs observed in nanowires. [1] X.-G. Zhang and S. T. Pantelides, Phys. Rev. Lett. 108, 266602 (2012). [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F15.00009: Long Minority Carrier Diffusion Lengths in Bridged Silicon Nanowires Dong Yu, Mark Triplett, Yiming Yang, Francois Leonard, Alec Talin, Saif Islam Nanowires have large surface areas which create new challenges for their optoelectronic applications. Lithographic processes involved in device fabrication and substrate interfaces can lead to surface defects and substantially reduce charge carrier lifetimes and diffusion lengths. Here, we show that using a bridging method to suspend pristine nanowires allows for circumventing detrimental fabrication steps and interfacial effects associated with planar device architectures. We report electron diffusion lengths up to 2.7 $\mu $m in bridged silicon nanowire devices, much longer than previously reported values for silicon nanowires with a diameter of 100 nm. Strikingly, electron diffusion lengths are reduced to only 45 nm in planar devices incorporating nanowires grown under the same conditions. The highly scalable and low-cost silicon nano-bridge devices with the demonstrated long diffusion lengths may find exciting applications in photovoltaics, image sensing and photodetectors. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F15.00010: Analytical Perturbative Treatment of Multiterminal Nonequilibrium Anderson Impurity Models Nobuhiko Taniguchi We investigate analytically the nonequilibrium Anderson impurity model connecting with multiterminal leads\footnote{N.~Taniguchi, Phys. Rev. B \textbf{90} 115421 (2014).}. Within the validity of the second-order perturbation regarding the interaction strength, the full dependence on frequency and bias voltage of the nonequilibrium self-energy and spectral function is determined for a generic multiterminal setting where the current preservation has been an issue.\footnote{S.~Hershfield \textit{et al.}, Phys. Rev. B \textbf{46}, 7046 (1992).} Our analytical perturbative treatment respects the current conservation as well as the spectral sum rule, and it encompasses Fermi-liquid and non-Fermi liquid behaviors, showing that increasing finite-bias voltage leads to a crossover from the Kondo resonance to the Coulomb blockade phenomena. Analysis on two-terminal and multiterminal settings shows that finite-bias voltage does not split the Kondo resonance in this order; no specific structure due to multiple leads emerges in the spectral function. Overall bias dependence is quite similar to finite-temperature effect, which could be understood by help of the Ward identity and the limit of $N\gg 1$ terminals. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F15.00011: Kondo effects and interference in transport through single molecules Jens Paaske, Kim Georg Lind Pedersen, Per Hedegaard Quantum transport through single molecules or quantum dot arrays with spin-degenerate ground states can be dominated by Kondo effects at low temperatures. In contrast to the single impurity case, quantum interference plays a significant role in such `multi-orbital' systems and may have a strong influence on the possible Kondo physics: deciding between single- or multi-channel screening and even ferromagnetic Kondo effect. We investigate a range of smaller molecules with source, and drain electrodes attached in different specific contacting geometries. The interacting pi-electron system is treated by means of exact diagonalization, and combining with a perturbative treatment of molecule-lead tunnel couplings, we calculate the zero-bias cotunneling conductance as a function of a gate-voltage shifting the molecular levels. We show that interference nodes cannot occur simultaneously in potential, and exchange scattering terms, which means that interference causes no conductance nodes. Nevertheless, interference nodes in the exchange scattering term may lead to a non-standard gate dependence of the Kondo temperature, as indicated by experiments. We discuss the flow towards strong coupling and the possibilities for two-channel, and/or ferromagnetic Kondo effect. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F15.00012: Quantum transport in linear chains under periodic perturbations Daniel Thuberg, Sebasti\'an Reyes, Sebastian Eggert Quantum transport properties across a time dependent impurity in a linear chain are studied theoretically using the Floquet formalism. We obtain the exact reflection coefficient for a wide range of parameters. Our results compare well with known approximations in the high frequency regime. Furthermore, at lower frequencies we observe new features and a significant departure from the approximated predictions. Below a well defined frequency the impurity couples waves of differing $k$-values. As a result, the impurity not only splits the incident wave into a transmitting and a reflecting part of the same wave number $k$, but also waves of differing $k$ emerge from the perturbed site. The amplitude of these waves is also obtained. [Preview Abstract] |
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