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Search Results: 1 - 10 of 31017 matches for " Thomas Brabec "
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Performance engineering of semiconductor spin qubit systems
Ramin M. Abolfath,Thomas Brabec
Physics , 2010, DOI: 10.1103/PhysRevB.82.075311
Abstract: The performance of a quantum computation system is investigated, with qubits represented by magnetic impurities in coupled quantum dots filled with two electrons. Magnetic impurities are electrically manipulated by electrons. The dominant noise source is the electron mediated indirect coupling between magnetic impurities and host spin bath. As a result of the electron mediated coupling, both noise properties and the time needed for elementary gate operations, depend on controllable system parameters, such as size and geometry of the quantum dot, and external electric and magnetic fields. We find that the maximum number of quantum operations per coherence time for magnetic impurities increases as electron spin singlet triplet energy gap decreases. The advantage of magnetic impurities over electrons for weak coupling and large magnetic fields will be illustrated.
Spin-blockade effect and coherent control of DNA-damage by free radicals: a proposal on bio-spintronics
Ramin M. Abolfath,Thomas Brabec
Physics , 2011,
Abstract: Coherent control of OH-free radicals interacting with the spin-triplet state of a DNA molecule is investigated. A model Hamiltonian for molecular spin singlet-triplet resonance is developed. We illustrate that the spin-triplet state in DNA molecules can be efficiently populated, as the spin-injection rate can be tuned to be orders of magnitudes greater than the decay rate due to small spin-orbit coupling in organic molecules. Owing to the nano-second life-time of OH free radicals, a non-equilibrium free energy barrier induced by the injected spin triplet state that lasts approximately longer than one-micro second in room temperature can efficiently block the initial Hydrogen abstraction and DNA damage. For a direct demonstration of the spin-blockade effect, a molecular simulation based on an {\em ab-initio} Car-Parrinello molecular dynamics is deployed.
Transport properties of nano-devices: One-dimensional model study
Zhongxi Zhang,C. F. Destefani,Chris McDonald,Thomas Brabec
Physics , 2005, DOI: 10.1103/PhysRevB.72.161309
Abstract: A 1D model study of charge transport in nano-devices is made by comparing multi-configuration time dependent Hartree-Fock and frozen core calculations. The influence of exchange and Coulomb correlation on the tunneling current is determined. We identify the shape of the tunneling barrier and the resonance structure of the nano-device as the two dominant parameters determining the electron transport. Whereas the barrier shape determines the size of the tunneling current, the resonances determine the structure of the current.
Saturable Lorentz model for fully explicit three-dimensional modeling of nonlinear optics
Charles Varin,Graeme Bart,Rhys Emms,Thomas Brabec
Physics , 2014, DOI: 10.1364/OE.23.002686
Abstract: Inclusion of the instantaneous Kerr nonlinearity in the FDTD framework leads to implicit equations that have to be solved iteratively. In principle, explicit integration can be achieved with the use of anharmonic oscillator equations, but it tends to be unstable and inappropriate for studying strong-field phenomena like laser filamentation. In this paper, we show that nonlinear susceptibility can be provided instead by a harmonic oscillator driven by a nonlinear force, chosen in a way to reproduce the polarization obtained from the solution of the quantum mechanical two level equations. The resulting saturable, nonlinearly-driven, harmonic oscillator model reproduces quantitatively the quantum mechanical solutions of harmonic generation in the under-resonant limit, up to the 9th harmonic. Finally, we demonstrate that fully explicit leapfrog integration of the saturable harmonic oscillator is stable, even for the intense laser fields that characterize laser filamentation and high harmonic generation.
Femtosecond 240-keV electron pulses from direct laser acceleration in a low-density gas
Vincent Marceau,Charles Varin,Thomas Brabec,Michel Piché
Physics , 2013, DOI: 10.1103/PhysRevLett.111.224801
Abstract: We propose a simple laser-driven electron acceleration scheme based on tightly focused radially polarized laser pulses for the production of femtosecond electron bunches with energies in the few-hundreds-of-keV range. In this method, the electrons are accelerated forward in the focal volume by the longitudinal electric field component of the laser pulse. Three-dimensional test-particle and particle-in-cell simulations reveal the feasibility of generating well-collimated electron bunches with an energy spread of 5% and a temporal duration of the order of 1 fs. These results offer a route towards unprecedented time resolution in ultrafast electron diffraction experiments.
Reactive Molecular Dynamics study on the first steps of DNA-damage by free hydroxyl radicals
Ramin M. Abolfath,A. C. T. van Duin,Thomas Brabec
Physics , 2011,
Abstract: We employ a large scale molecular simulation based on bond-order ReaxFF to simulate the chemical reaction and study the damage to a large fragment of DNA-molecule in the solution by ionizing radiation. We illustrate that the randomly distributed clusters of diatomic OH-radicals that are primary products of megavoltage ionizing radiation in water-based systems are the main source of hydrogen-abstraction as well as formation of carbonyl- and hydroxyl-groups in the sugar-moiety that create holes in the sugar-rings. These holes grow up slowly between DNA-bases and DNA-backbone and the damage collectively propagate to DNA single and double strand break.
Spin textures in strongly coupled electron spin and magnetic or nuclear spin systems in quantum dots
Ramin M. Abolfath,Marek Korkusinski,Thomas Brabec,Pawel Hawrylak
Physics , 2011, DOI: 10.1103/PhysRevLett.108.247203
Abstract: Controlling electron spins strongly coupled to magnetic and nuclear spins in solid state systems is an important challenege in the field of spintronics and quantum computation. We show here that electron droplets with no net spin in semiconductor quantum dots strongly coupled with magnetic ion/nuclear spin systems break down at low temperature and form a non-trivial anti-ferromagnetic spatially ordered spin-texture of magneto-polarons. The spatially ordered combined electron-magnetic ion spin-texture, associated with spontaneous symmetry-breaking in the parity of electronic charge density and magnetization of magnetic ions, emerge from both ab-initio density functional approach to the electronic system coupled with mean-field approximation for the magnetic/nuclear spin system and fully mircoscopic exact diagonalization of small systems. The predicted phase diagram determines the critical temperature as a function of coupling strength and identifies possible phases of the strongly coupled spin system. This prediction may arrest fluctuations in spin system and open the way to control, manipulate and prepare magnetic and nuclear spin ensembles in semiconductor nanostructures.
MeV femtosecond electron pulses from direct-field acceleration in low density atomic gases
Charles Varin,Vincent Marceau,Pascal Hogan-Lamarre,Thomas Fennel,Michel Piché,Thomas Brabec
Physics , 2015,
Abstract: Using three-dimensional particle-in-cell simulations, we show that few-MeV electrons can be produced by focusing tightly few-cycle radially-polarized laser pulses in a low-density atomic gas. In particular, it is observed that for the few-TW laser power needed to reach relativistic electron energies, longitudinal attosecond microbunching occurs naturally, resulting in femtosecond structures with high-contrast attosecond density modulations. The three-dimensional particle-in-cell simulations show that in the relativistic regime the leading pulse of these attosecond substructures survives to propagation over extended distances, suggesting that it could be delivered to a distant target, with the help of a properly designed transport beamline.
Multiscale QM/MM Molecular Dynamics Study on the First Steps of Guanine-Damage by Free Hydroxyl Radicals in Solution
Ramin M. Abolfath,P. K. Biswas,R. Rajnarayanam,Thomas Brabec,Reinhard Kodym,Lech Papiez
Physics , 2012,
Abstract: Understanding the damage of DNA bases from hydrogen abstraction by free OH radicals is of particular importance to reveal the effect of hydroxyl radicals produced by the secondary effect of radiation. Previous studies address the problem with truncated DNA bases as ab-initio quantum simulation required to study such electronic spin dependent processes are computationally expensive. Here, for the first time, we employ a multiscale and hybrid Quantum-Mechanical-Molecular-Mechanical simulation to study the interaction of OH radicals with guanine-deoxyribose-phosphate DNA molecular unit in the presence of water where all the water molecules and the deoxyribose-phosphate fragment are treated with the simplistic classical Molecular-Mechanical scheme. Our result illustrates that the presence of water strongly alters the hydrogen-abstraction reaction as the hydrogen bonding of OH radicals with water restricts the relative orientation of the OH-radicals with respective to the the DNA base (here guanine). This results in an angular anisotropy in the chemical pathway and a lower efficiency in the hydrogen abstraction mechanisms than previously anticipated for identical system in vacuum. The method can easily be extended to single and double stranded DNA without any appreciable computational cost as these molecular units can be treated in the classical subsystem as has been demonstrated here.
Dynamical Magnetic and Nuclear Polarization in Complex Spin Systems: Semi-magnetic II-VI Quantum Dots
Ramin M. Abolfath,Anna Trojnar,Bahman Roostaei,Thomas Brabec,Pawel Hawrylak
Physics , 2012, DOI: 10.1088/1367-2630/15/6/063039
Abstract: Dynamical magnetic and nuclear polarization in complex spin systems is discussed on the example of transfer of spin from exciton to the central spin of magnetic impurity in a quantum dot in the presence of a finite number of nuclear spins. The exciton is described in terms of the electron and heavy hole spins interacting via exchange interaction with magnetic impurity, via hypeprfine interaction with a finite number of nuclear spins and via dipole interaction with photons. The time-evolution of the exciton, magnetic impurity and nuclear spins is calculated exactly between quantum jumps corresponding to exciton radiative recombination. The collapse of the wavefunction and the refilling of the quantum dot with new spin polarized exciton is shown to lead to build up of magnetization of the magnetic impurity as well as nuclear spin polarization. The competition between electron spin transfer to magnetic impurity and to nuclear spins simultaneous with the creation of dark excitons is elucidated. The technique presented here opens up the possibility of studying optically induced Dynamical Magnetic and Nuclear Polarization in Complex Spin Systems.
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