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Dipolar and charged localized excitons in carbon nanotubes. (arXiv:1706.08347v1 [cond-mat.mes-hall])

June 27th, 2017|Publications|

By Jan T. Glückert, Lyudmyla Adamska, Wolfgang Schinner, Matthias S. Hofmann, Stephen K. Doorn, Sergei Tretiak, Alexander Högele

We study both experimentally and theoretically the fundamental interplay of exciton localization and polarization in semiconducting single-walled carbon nanotubes. From Stark spectroscopy of individual carbon nanotubes at cryogenic temperatures we identify localized excitons as permanent electric dipoles with dipole moments of up to 1e{AA}. Moreover, we demonstrate field-effect doping of localized excitons with an additional charge which results in defect-localized trions. Our findings provide not only fundamental insight into the microscopic nature of localized excitons in carbon nanotubes, they also signify their potential for sensing applications and may serve as guidelines for molecular engineering of exciton-localizing quantum dots in other atomically thin semiconductors including transition metal dichalcogenides.

Published : "arXiv Mesoscale and Nanoscale Physics".

Quantum Emitters in Hexagonal Boron Nitride Have Spectrally Tunable Quantum Efficiency. (arXiv:1706.08303v1 [cond-mat.mes-hall])

June 27th, 2017|Publications|

By Andreas W. Schell, Mikael Svedendahl, Romain Quidant

Understanding the properties of novel solid-state quantum emitters is pivotal for a variety of applications in field ranging from quantum optics to biology. Recently discovered defects in hexagonal boron nitride are especially interesting, as they offer much desired characteristics such as narrow emission lines and photostability. Here, we study the dependence of the emission on the excitation wavelength. We find that, in order to achieve bright single photon emission with high quantum efficiency, the excitation wavelength has to be matched to the emitter. This is a strong indication that the emitters possess a complex level scheme and cannot be described by a simple two or three level system. Using this excitation dependence of the emission, we thus gain further insight to the internal level scheme and demonstrate how to distinguish different emitters both spatially as well as in terms of their photon correlations.

Published : "arXiv Mesoscale and Nanoscale Physics".

Spatial structure of correlations around a quantum impurity at the edge of a two-dimensional topological insulator. (arXiv:1706.08179v1 [cond-mat.str-el])

June 27th, 2017|Publications|

By Andrew Allerdt (Northeastern University), Adrian E. Feiguin (Northeastern University), George B. Martins (Oakland University and Universidade Federal Fluminense)

We calculate exact zero-temperature real space properties of a substitutional magnetic impurity coupled to the edge of a zigzag silicene-like nanoribbon. Using a Lanczos transformation [Phys. Rev. B 91, 085101 (2015)] and the density matrix renormalization group method, we obtain a realistic description of stanene and germanene that includes the bulk and the edges as boundary one-dimensional helical metallic states. Our results for substitutional impurities indicate that the development of a Kondo state and the structure of the spin correlations between the impurity and the electron spins in the metallic edge state depend considerably on the location of the impurity. More specifically, our real space resolution allows us to conclude that there is a sharp distinction between the impurity being located at a crest or a trough site at the zigzag edge. We also observe, as expected, that the spin correlations are anisotropic due to an emerging Dzyaloshinskii-Moriya interaction with the conduction electrons, and that the edges scatter from the impurity and “snake” or circle around it. Our estimates for the Kondo temperature indicate that there is a very weak enhancement due to the presence of spin-orbit coupling.

Published : "arXiv Mesoscale and Nanoscale Physics".

Coherent Interlayer Tunneling and Negative Differential Resistance with High Current Density in Double Bilayer Graphene-WSe2 Heterostructures. (arXiv:1706.08034v1 [cond-mat.mes-hall])

June 27th, 2017|Publications|

By G. William Burg, Nitin Prasad, Babak Fallahazad, Amithraj Valsaraj, Kyounghwan Kim, Takashi Taniguchi, Kenji Watanabe, Qingxiao Wang, Moon J. Kim, Leonard F. Register, Emanuel Tutuc

We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe2. We observe large interlayer current densities of 2 uA/um2 and 2.5 uA/um2, and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 K and 300 K, respectively.

Published : "arXiv Mesoscale and Nanoscale Physics".

Exciton Trapping Is Responsible for the Long Apparent Lifetime in Acid-Treated MoS2. (arXiv:1706.08005v1 [cond-mat.mes-hall])

June 27th, 2017|Publications|

By Aaron J. Goodman, Adam P. Willard, William A. Tisdale

Here, we show that deep trapped “dark” exciton states are responsible for the surprisingly long lifetime of band-edge photoluminescence in acid-treated single-layer MoS2. Temperature-dependent transient photoluminescence spectroscopy reveals an exponential tail of long-lived states extending hundreds of meV into the band gap. These sub-band states, which are characterized by a 4 microsecond radiative lifetime, quickly capture and store photogenerated excitons before subsequent thermalization up to the band edge where fast radiative recombination occurs. By intentionally saturating these trap states, we are able to measure the “true” 150 ps radiative lifetime of the band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature. These experiments reveal the dominant role of dark exciton states in acid-treated MoS2, and suggest that excitons spend > 95% of their lifetime at room temperature in trap states below the band edge. We hypothesize that these states are associated with native structural defects, which are not introduced by the superacid treatment; rather, the superacid treatment dramatically reduces non-radiative recombination through these states, extending the exciton lifetime and increasing the likelihood of eventual radiative recombination.

Published : "arXiv Mesoscale and Nanoscale Physics".

Optical identification using imperfections in 2D materials. (arXiv:1706.07949v1 [cond-mat.mes-hall])

June 27th, 2017|Publications|

By Yameng Cao, Alexander J. Robson, Abdullah Alharbi, Jonathan Roberts, Christopher S. Woodhead, Yasir J. Noori, Ramón Bernardo-Gavito, Davood Shahrjerdi, Utz Roedig, Vladimir I. Falko, Robert J. Young

The ability to uniquely identify an object or device is important for authentication. Imperfections, locked into structures during fabrication, can be used to provide a fingerprint that is challenging to reproduce. In this paper, we propose a simple optical technique to read unique information from nanometer-scale defects in 2D materials. Flaws created during crystal growth or fabrication lead to spatial variations in the bandgap of 2D materials that can be characterized through photoluminescence measurements. We show a simple setup involving an angle-adjustable transmission filter, simple optics and a CCD camera can capture spatially-dependent photoluminescence to produce complex maps of unique information from 2D monolayers. Atomic force microscopy is used to verify the origin of the optical signature measured, demonstrating that it results from nanometer-scale imperfections. This solution to optical identification with 2D materials could be employed as a robust security measure to prevent counterfeiting.

Published : "arXiv Mesoscale and Nanoscale Physics".

Probing nanocrystalline grain dynamics in nanodevices. (arXiv:1706.07887v1 [cond-mat.mes-hall])

June 27th, 2017|Publications|

By Sheng-Shiuan Yeh, Wen-Yao Chang, Juhn-Jong Lin

Dynamical structural defects exist naturally in a wide variety of solids. They fluctuate temporally, and hence can deteriorate the performance of many electronic devices. Thus far, the entities of such dynamic objects have been identified to be individual atoms. On the other hand, it is a long-standing question whether a nanocrystalline grain constituted of a large number of atoms can switch, as a whole, reversibly like a dynamical atomic defect (i.e., a two-level system). This is an emergent issue considering the current development of nanodevices with ultralow electrical noise, qubits with long quantum coherence time, and nanoelectromechanical system (NEMS) sensors with ultrahigh resolution. Here we demonstrate experimental observations of dynamic nanocrystalline grains which repeatedly switch between two or more metastable coordinate states. We study temporal resistance fluctuations in thin ruthenium dioxide (RuO2) metal nanowires and extract microscopic parameters including relaxation time scales, mobile grain sizes, and the bonding strengths of nanograin boundaries. Such material parameters are not obtainable by other experimental approaches. When combined with previous in-situ high-resolution transmission electron microscopy (HRTEM), our electrical method can be used to infer rich information about the structural dynamics of a wide variety of nanodevices and new 2D materials.

Published : "arXiv Mesoscale and Nanoscale Physics".

Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons

June 26th, 2017|Publications|

Nature Nanotechnology. doi:10.1038/nnano.2017.106 Authors: You Zhou, Giovanni Scuri, Dominik S. Wild, Alexander A. High, Alan Dibos, Luis A. Jauregui, Chi Shu, Kristiaan De Greve, Kateryna Pistunova, Andrew Y. Joe, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Mikhail D. Lukin & Hongkun Park Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin–orbit coupling and spin–valley degrees of freedom. Depending on the spin configuration of the electron–hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication.

Published in: "Nature Nanotechnology (Advanced Online Publication)".