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Search Results: 1 - 10 of 401236 matches for " M. Scheibner "
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Thermopower of a Kondo-correlated quantum dot
R. Scheibner,H. Buhmann,D. Reuter,M. N. Kiselev,L. W. Molenkamp
Physics , 2004, DOI: 10.1103/PhysRevLett.95.176602
Abstract: The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model. Keywords: Thermoelectric and thermomagnetic effects, Coulomb blockade, single electron tunneling, Kondo-effect PACS Numbers: 72.20.Pa, 73.23.Hk
Quantum dot as thermal rectifier
R. Scheibner,M. Koenig,D. Reuter,A. D. Wieck,H. Buhmann,L. W. Molenkamp
Physics , 2007, DOI: 10.1088/1367-2630/10/8/083016
Abstract: We report the observation of thermal rectification in a semiconductor quantum dot, as inferred from the asymmetric line shape of the thermopower oscillations. The asymmetry is observed at high in-plane magnetic fields and caused by the presence of a high orbital momentum state in the dot.
Entanglement Dynamics of Molecular Exciton States in Coupled Quantum Dots
Cameron Jennings,Michael Scheibner
Physics , 2015,
Abstract: We theoretically model the electronic dynamics of a coupled quantum dot pair in a static electric field. We then investigate the possibility of polarization-entangled photon emission from the radiative cascade of the molecular biexciton state. Through numerical simulations, we analyze the dependence of entanglement fidelity on temperature and electric field, as well as tunnel coupling. We establish a regime of direct-indirect exciton detunings for which coupled quantum dots are superior to single dots for entangled photon generation, yielding near-unit fidelity over a larger range of exchange splittings.
Antibonding ground states in semiconductor artificial molecules
M. F. Doty,J. I. Climente,M. Korkusinski,M. Scheibner,A. S. Bracker,P. Hawrylak,D. Gammon
Physics , 2008,
Abstract: The spin-orbit interaction is a crucial element of many semiconductor spintronic technologies. Here we report the first experimental observation, by magneto-optical spectroscopy, of a remarkable consequence of the spin-orbit interaction for holes confined in the molecular states of coupled quantum dots. As the thickness of the barrier separating two coupled quantum dots is increased, the molecular ground state changes character from a bonding orbital to an antibonding orbital. This result is counterintuitive, and antibonding molecular ground states are never observed in natural diatomic molecules. We explain the origin of the reversal using a four band k.p model that has been validated by numerical calculations that account for strain. The discovery of antibonding molecular ground states provides new opportunities for the design of artificially structured materials with complex molecular properties that cannot be achieved in natural systems.
Characterization of the Shell Structure in Coupled Quantum Dots through Resonant Optical Probing
Mauricio Garrido,Kushal C. Wijesundara,Swati Ramanathan,E. A. Stinaff,M. Scheibner,A. S. Bracker,D. Gammon
Physics , 2009,
Abstract: Excited states in single quantum dots (QDs) have been shown to be useful for spin state initialization and manipulation. For scalable quantum information processing it is necessary to have multiple spins interacting. Therefore, we present initial results from photoluminescence excitation studies of excited states in coupled quantum dots (CQDs). Due to the rich set of possible excitation and recombination possibilities, a technique for visualizing photoluminescence excitation in coupled quantum dots is discussed, by which both the interaction between the dots and the type of absorption and emission that generated the photoluminescence is easily and clearly revealed. As an example, this technique is applied to characterize the shell structure of the hole in the top dot and the results are compared with those using Level Anti-Crossing Spectroscopy (LACS).
Electric field tunable exchange interaction in InAs/GaAs coupled quantum dots
Kushal C. Wijesundara,Mauricio Garrido,Swati Ramanathan,E. A. Stinaff,M. Scheibner,A. S. Bracker,D. Gammon
Physics , 2009,
Abstract: Spin manipulation in coupled quantum dots is of interest for quantum information applications. Control of the exchange interaction between electrons and holes via an applied electric field may provide a promising technique for such spin control. Polarization dependent photoluminescence (PL) spectra were used to investigate the spin dependent interactions in coupled quantum dot systems and by varying an electric field, the ground state hole energy levels are brought into resonance, resulting in the formation of molecular orbitals observed as anticrossings between the direct and indirect transitions in the spectra. The indirect and direct transitions of the neutral exciton demonstrate high and low circular polarization memory respectively due to variation in the exchange interaction. The ratio between the polarization values as a function of electric field, and the barrier height was measured. These results indicate a possible method of tuning between indirect and direct configurations to control the degree of exchange interaction.
Sequential and co-tunneling behavior in the temperature-dependent thermopower of few-electron quantum dots
R. Scheibner,E. G. Novik,T. Borzenko,M. Koenig,D. Reuter,A. D. Wieck,H. Buhmann,L. W. Molenkamp
Physics , 2006, DOI: 10.1103/PhysRevB.75.041301
Abstract: We have studied the temperature dependent thermopower of gate-defined, lateral quantum dots in the Coulomb blockade regime using an electron heating technique. The line shape of the thermopower oscillations depends strongly on the contributing tunneling processes. Between 1.5 K and 40 mK a crossover from a pure sawtooth- to an intermitted sawtooth-like line shape is observed. The latter is attributed to the increasing dominance of cotunneling processes in the Coulomb blockade regime at low temperatures.
Decline of coral reefs during late Paleocene to early Eocene global warming
C. Scheibner,R. P. Speijer
eEarth (eE) & Discussions (eED) , 2008, DOI: 10.5194/ee-3-19-2008
Abstract: Since the 1980s the frequency of warming events has intensified and simultaneously widespread coral bleaching, and enhanced coral mortality have been observed. Yet, it remains unpredictable how tropical coral reef communities will react to prolonged adverse conditions. Possibly, coral reef systems are sufficiently robust to withstand continued environmental pressures. But if coral mortality increases, what will platform communities of the future look like? The co-evolution of early Paleogene carbonate platforms and palaeoclimate may provide insight. Here we document the impact of early Paleogene global warming on shallow-water carbonate platforms in the Tethys. Between 59 and 55 Ma, three discrete stages in platform development can be identified Tethys-wide: during the first stage carbonate platforms mainly consisted of coralgal reefs; during the second – transitional – stage coralgal reefs thrived only at middle latitudes and gave way to larger foraminifera as dominant carbonate producer in low latitudes; finally, during the third stage, newly developing larger foraminifera lineages completely took over the role as main carbonate-producing organisms in low to middle latitudes. We postulate that rising temperatures led to a stepwise demise of Paleocene coral reefs, giving way to an unprecedented expansion of larger foraminifera, dominating Tethyan platforms during the early Eocene.
Decline of coral reefs during late Paleocene to early Eocene global warming
C. Scheibner,R. P. Speijer
eEarth Discussions , 2007,
Abstract: Since the 1980s the frequency of warming events has intensified and simultaneously widespread coral bleaching, and enhanced coral mortality have been observed. Yet, it remains unpredictable how tropical coral reef communities will react to prolonged adverse conditions. Possibly, coral reef systems are sufficiently robust to withstand continued environmental pressures. But if coral mortality increases, what will platform communities of the future look like? The co-evolution of early Paleogene carbonate platforms and palaeoclimate may provide insight. Here we document the impact of early Paleogene global warming on shallow-water carbonate platforms in the Tethys. Between 59 and 55 Ma, three discrete stages in platform development can be identified Tethys-wide: during the first stage carbonate platforms mainly consisted of coralgal reefs; during the second – transitional – stage coralgal reefs thrived only at middle latitudes and gave way to larger foraminifera as dominant carbonate producer in low latitudes; finally, during the third stage, newly developing larger foraminifera lineages completely took over the role as main carbonate-producing organisms in low to middle latitudes. We postulate that rising temperatures led to a stepwise demise of Paleocene coral reefs, giving way to an unprecedented expansion of larger foraminifera, dominating Tethyan platforms during the early Eocene.
Antibonding hole ground state in artificial molecules
J.I. Climente, M. Korkusinski, M.F. Doty, M. Scheibner, A.S. Bracker, G. Goldoni, D. Gammon, and P. Hawrylak
OAtube Nanotechnology , 2008,
Abstract: Resonant tunneling of carriers between vertically coupled quantum dots enables the formation of hybridized, molecular-like orbitals which are important in many quantum dot-based devices, including those aiming at optically-controlled quantum information storage.[1] The differences in size and composition of quantum dots is overcome by the application of the vertical electric field, which brings the two quantum dot levels into resonance and induces either electron or hole tunneling.[2] The tunneling of electrons is now well understood[1-4], it leads to the formation of bonding molecular ground states in analogy to natural diatomic molecules. However, tunneling of holes does not have a counterpart in diatomic molecules and is less understood. In fact, previous atomistic calculations suggested a reversal of bonding and antibonding hole molecular ground states as the interdot barrier distance increases.[5-7]In this work, we present theory and experimental observation of the formation of the antibonding hole molecular ground state. Using a 4-band k·p approximation, the hole states are described as Luttinger spinors[8], which contain all the relevant symmetries. It is shown that the strong spin-orbit interaction in the valence band breaks the parity in the growth direction, mixing bonding and antibonding heavy- and light-hole components of the spinor. This mixing destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal. Molecular ground states are then found to have up to ~95% antibonding character. These conclusions are reproduced by numerical, atomistic multi-million-atom calculations using a sp^3d^5s* tight-binding model applied to the realistic self-assembled InGaAs/GaAs double quantum dot structures, including strain, structural asymmetries and vertical electric fields. The results are in qualitative agreement with the k·p theory and predict a bonding-to-antibonding ground state reversal at interdot distances of d 2 nm. Clear experimental evidence of this peculiar hole behavior is found in magneto-photoluminescence experiments of double dots. The character of the hole molecular orbitals is identified from the electric field dependence of the Zeeman splitting of the neutral exciton when resonant hole tunneling is induced[9]. Comparison of samples with different inter-dot separation shows the bonding-to-antibonding ground state reversal in agreement with theory [10].
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