3,097 research outputs found
Aharonov-Bohm oscillations and magnetic focusing in ballistic graphene rings
We present low-temperature magnetotransport measurements on graphene rings
encapsulated in hexagonal boron nitride. We investigate phase-coherent
transport and show Aharonov-Bohm (AB) oscillations in quasi-ballistic graphene
rings with hard confinement. In particular, we report on the observation of
, and conductance oscillations. Moreover we show signatures
of magnetic focusing effects at small magnetic fields confirming ballistic
transport. We perform tight binding calculations which allow to reproduce all
significant features of our experimental findings and enable a deeper
understanding of the underlying physics. Finally, we report on the observation
of the AB conductance oscillations in the quantum Hall regime at reasonable
high magnetic fields, where we find regions with enhanced AB oscillation
visibility with values up to %. These oscillations are well explained by
taking disorder into account allowing for a coexistence of hard and soft-wall
confinement.Comment: 28 pages, 8 figure
Electro-optically tunable microring resonators in lithium niobate
Optical microresonators have recently attracted a growing attention in the
photonics community. Their applications range from quantum electro-dynamics to
sensors and filtering devices for optical telecommunication systems, where they
are likely to become an essential building block. The integration of nonlinear
and electro-optical properties in the resonators represents a very stimulating
challenge, as it would incorporate new and more advanced functionality. Lithium
niobate is an excellent candidate material, being an established choice for
electro-optic and nonlinear optical applications. Here we report on the first
realization of optical microring resonators in submicrometric thin films of
lithium niobate. The high index contrast films are produced by an improved
crystal ion slicing and bonding technique using benzocyclobutene. The rings
have radius R=100 um and their transmission spectrum has been tuned using the
electro-optic effect. These results open new perspectives for the use of
lithium niobate in chip-scale integrated optical devices and nonlinear optical
microcavities.Comment: 15 pages, 8 figure
Finite Size Effects on the Optical Transitions in Quantum Rings under a Magnetic Field
We present a theoretical study of the energy spectrum of single electron and
hole states in quantum dots of annular geometry under a high magnetic field
along the ring axis in the frame of uncorrelated electron-hole theory. We
predict the periodic disappearance of the optical emission of the electron-hole
pair as the magnetic field increases, as a consequence of the finite height of
the barriers. The model has been applied to semiconductor rings of various
internal and external radii, giving as limiting cases the disk and antidot.Comment: 22 pages, 7 figures, LaTeX, published in Eur. Phys. J. B 53, 99-108
(2006
Unusual nanostructures of "lattice matched" InP on AlInAs
We show that the morphology of the initial monolayers of InP on
Al0.48In0.52As grown by metalorganic vapor-phase epitaxy does not follow the
expected layer-by-layer growth mode of lattice-matched systems, but instead
develops a number of low-dimensional structures, e.g., quantum dots and wires.
We discuss how the macroscopically strain-free heteroepitaxy might be strongly
affected by local phase separation/alloying-induced strain and that the
preferred aggregation of adatom species on the substrate surface and reduced
wettability of InP on AlInAs surfaces might be the cause of the unusual (step)
organization and morpholog
Is it the boundaries or disorder that dominates electron transport in semiconductor `billiards'?
Semiconductor billiards are often considered as ideal systems for studying
dynamical chaos in the quantum mechanical limit. In the traditional picture,
once the electron's mean free path, as determined by the mobility, becomes
larger than the device, disorder is negligible and electron trajectories are
shaped by specular reflection from the billiard walls alone. Experimental
insight into the electron dynamics is normally obtained by magnetoconductance
measurements. A number of recent experimental studies have shown these
measurements to be largely independent of the billiards exact shape, and highly
dependent on sample-to-sample variations in disorder. In this paper, we discuss
these more recent findings within the full historical context of work on
semiconductor billiards, and offer strong evidence that small-angle scattering
at the sub-100 nm length-scale dominates transport in these devices, with
important implications for the role these devices can play for experimental
tests of ideas in quantum chaos.Comment: Submitted to Fortschritte der Physik for special issue on Quantum
Physics with Non-Hermitian Operator
Quantum Mechanics of Semiconductor Quantum Dots and Rings
We consider the several phenomena which are taking place in Quantum Dots (QD)
and Quantum Rings (QR): The connection of the Quantum Chaos (QC) with the
reflection symmetry of the QD, Disappearance of the QC in the tunnel coupled
chaotic QD, electron localization and transition between Double Concentric QR
in the transverse magnetic field, transition of electron from QR to the QD
located in the center of QR. Basis of this consideration is the effective
Schr\"odinger equation for the corresponding systems.Comment: 32 pages, 26 figures; This paper is the basis of two talks of Sergei
Matinyan presented at the workshop "Low Dimension Systems & Gauge principle"
(Yerevan, Tbilisi, 2011
Doctor of Philosophy
dissertationIn Part 1, we demonstrate the fabrication of organic light-emitting devices (OLEDs) with precisely patterned pixels by the spin-casting of Alq3 and rubrene thin films with dimensions as small as 10 μm. The solution-based patterning technique produces pixels via the segregation of organic molecules into microfabricated channels or wells. Segregation is controlled by a combination of weak adsorbing characteristics of aliphatic terminated self-assembled monolayers (SAMs) and by centrifugal force, which directs the organic solution into the channel or well. This novel patterning technique may resolve the limitations of pixel resolution in the method of thermal evaporation using shadow masks, and is applicable to the fabrication of large area displays. Furthermore, the patterning technique has the potential to produce pixel sizes down to the limitation of photolithography and micromachining techniques, thereby enabling the fabrication of high-resolution microdisplays. The patterned OLEDs, based upon a confined structure with low refractive index of SiO2, exhibited higher current density than an unpatterned OLED, which results in higher electroluminescence intensity and eventually more efficient device operation at low applied voltages. We discuss the patterning method and device fabrication, and characterize the morphological, optical, and electrical properties of the organic pixels. In part 2, we demonstrate a new growth technique for organic single crystals based on solvent vapor assisted recrystallization. We show that, by controlling the polarity of the solvent vapor and the exposure time in a closed system, we obtain rubrene in orthorhombic to monoclinic crystal structures. This novel technique for growing single crystals can induce phase shifting and alteration of crystal structure and lattice parameters. The organic molecules showed structural change from orthorhombic to monoclinic, which also provided additional optical transition of hypsochromic shift from that of the orthorhombic form. An intermediate form of the crystal exhibits an optical transition to the lowest vibrational energy level that is otherwise disallowed in the single-crystal orthorhombic form. The monoclinic form exhibits entirely new optical transitions and showed a possible structural rearrangement for increasing charge carrier mobility, making it promising for organic devices. These phenomena can be explained and proved by the chemical structure and molecular packing of the monoclinic form, transformed from orthorhombic crystalline structure
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