Spectroscopy of soft modes and quantum phase transitions in coupled electron bilayers

Strongly-correlated two-dimensional electrons in coupled semiconductor bilayers display remarkable broken symmetry many-body states under accessible and controllable experimental conditions. In the cases of continuous quantum phase transitions soft collective modes drive the transformations that link distinct ground states of the electron double layers. In this paper we consider results showing that resonant inelastic light scattering methods detect soft collective modes of the double layers and probe their evolution with temperature and magnetic field. The light scattering experiments offer venues of research of fundamental interactions and continuous quantum phase transitions in low-dimensional electron liquids.Comment: 10 pages, 7 figure

Counting molecular-beam grown graphene layers

Copyright © 2013 American Institute of PhysicsWe have used the ratio of the integrated intensity of graphene's Raman G peak to that of the silicon substrate's first-order optical phonon peak, accurately to determine the number of graphene layers across our molecular-beam (MB) grown graphene films. We find that these results agree well both, with those from our own exfoliated single and few-layer graphene flakes, and with the results of Koh et al. [ACS Nano 5, 269 (2011)]. We hence distinguish regions of single-, bi-, tri-, four-layer, etc., graphene, consecutively, as we scan coarsely across our MB-grown graphene. This is the first, but crucial, step to being able to grow, by such molecular-beam-techniques, a specified number of large-area graphene layers, to order.Work supported by ONR (N000140610138 and Graphene Muri), EFRC Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control (award DE-SC0001085), NSF (CHE-0641523), NYSTAR, CSIC-PIF (200950I154), Spanish CAM (Q&C Light (S2009ESP-1503), Numancia 2 (S2009/ENE-1477)), and Spanish MEC (ENE2009-14481-C02-02, TEC2011-29120-C05-04, MAT2011-26534)

MBE growth of Quantum nanostructures for optoelectronics

Ponencia presemtada en el Workshop on Frontier Photonic and Electronic Materials and Devices - German-Japanese-Spanish Joint Workshop, celebrado en Kyoto del 11 al 14 de julio de 2015.Molecular Beam Epitaxy (MBE) is a powerful technique for the fabrication of several self-assembled III-V nanostructures such as quantum rings, quantum dots, and quantum wires that can cover a wide range of the spectrum from 0.98 μm to 1.6 μm. The possibility of performing in-situ, real-time, measurements of accumulated stress (Σσ) during growth of these nanostructures enables to achieve a deep understanding of the growth processes. For example, whereas quantum rings (QR) formation is crucially linked to the presence of liquid indium on the surface, quantum wires (QWR) are produced as an effective way of relaxing a large asymmetrical accumulated stress present on the sample. This information allows a fine-tuning of the optoelectronic properties by controlling their size and shape. Furthermore, the capability of tracking Σσ during growth is used to engineer strain compensated structures like multilayer quantum dot solar cells.CHE-0641523, CSIC-PIF200950I154,S2009ESP-1503, S2009ENE-1477 and AIC-B-2011-0806, MAT2011-26534.Peer Reviewe

Graphene growth on h-BN by Molecular Beam Epitaxy

The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.Comment: Solid State Communications, 201

Molecular beam growth of graphene nanocrystals on dielectric substrates

We demonstrate the growth of graphene nanocrystals by molecular beam methods that employ a solid carbon source, and that can be used on a diverse class of large area dielectric substrates. Characterization by Raman and Near Edge X-ray Absorption Fine Structure spectroscopies reveal a sp2 hybridized hexagonal carbon lattice in the nanocrystals. Lower growth rates favor the formation of higher quality, larger size multi-layer graphene crystallites on all investigated substrates. The surface morphology is determined by the roughness of the underlying substrate and graphitic monolayer steps are observed by ambient scanning tunneling microscopy.Comment: Accepted in Carbon; Discussion section added; 20 pages, 6 figures (1 updated

Optical detection of charge redistribution in a delta modulation-doped GaAs-AlxGa1-xAs heterojunction

We have investigated magnetically-induced charge redistribution within a delta modulation-doped GaAs-AlxGa1-xAs heterojunction structure by studying the photoluminescence due to electrons from the two-dimensional(2D) electron system recombining with photoexcited holes. At well defined values of magnetic field, charge transfer occurs between this 2D electron system and the V-shaped potential well formed in the AlxGa1-xAs by Si delta modulation-doping. This redistribution of charge is observed as discontinuities in the photoluminescence energies. From these measurements we have derived the characteristic transfer time for electrons to move between these two wells

Exceptionally large migration length of carbon and topographically-facilitated self-limiting molecular beam epitaxial growth of graphene on hexagonal boron nitride

We demonstrate growth of single-layer graphene (SLG) on hexagonal boron nitride (h-BN) by molecular beam epitaxy (MBE), only limited in area by the finite size of the h-BN flakes. Using atomic force microscopy and micro-Raman spectroscopy, we show that for growth over a wide range of temperatures (500 C e 1000 C) the deposited carbon atoms spill off the edge of the h-BN flakes. We attribute this spillage to the very high mobility of the carbon atoms on the BN basal plane, consistent with van der Waals MBE. The h-BN flakes vary in size from 30 mm to 100 mm, thus demonstrating that the migration length of carbon atoms on h-BN is greater than 100 mm. When sufficient carbon is supplied to compensate for this loss, which is largely due to this fast migration of the carbon atoms to and off the edges of the h-BN flake, we find that the best growth temperature for MBE SLG on h-BN is ~950 C. Selflimiting graphene growth appears to be facilitated by topographic h-BN surface features: We have thereby grown MBE self-limited SLG on an h-BN ridge. This opens up future avenues for precisely tailored fabrication of nano- and hetero-structures on pre-patterned h-BN surfaces for device applications.This work is supported by ONR (N000140610138 and Graphene MURI), AFOSR (FA9550-11-1-0010), EFRC Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control (award DE-SC0001085), NSF (CHE-0641523), NYSTAR and Spanish Government (AIC-B-2011-0806, MAT2014-54231, MAT2015-67021-R). S.W. and A.P. were supported by the US Department of Energy Office of Science, Division of Materials Science and Engineering (award DE-SC0010695).Peer reviewe