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Series

Physics Publications

2005

Articles 1 - 17 of 17

Full-Text Articles in Physics

Control Of Finite-Dimensional Quantum Systems: Application To A Spin-1/2 Particle Coupled With A Finite Quantum Harmonic Oscillator, Chitra Rangan, A.M. Bloch Jan 2005

Control Of Finite-Dimensional Quantum Systems: Application To A Spin-1/2 Particle Coupled With A Finite Quantum Harmonic Oscillator, Chitra Rangan, A.M. Bloch

Physics Publications

In this paper, we consider the problem of the controllability of a finite-dimensional quantum system in both the Schrödinger and interaction pictures. Introducing a Quantum Transfer Graph, we elucidate the role of Lie algebra rank conditions and the complex nature of the control matrices. We analyze the example of a sequentially coupled N-level system: a spin-1/2 particle coupled to a finite quantum harmonic oscillator. This models an important physical paradigm of quantum computers - the trapped ion. We describe the control of the finite model obtained, under the right conditions, from the original infinite-dimensional system.

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Absolute Wave-Number Measurements In 130te2: Reference Lines Spanning The 420.9-464.6-Nm Region, T.J. Scholl, Steven J. Rehse, R.A. Holt, S.D. Rosner Jan 2005

Absolute Wave-Number Measurements In 130te2: Reference Lines Spanning The 420.9-464.6-Nm Region, T.J. Scholl, Steven J. Rehse, R.A. Holt, S.D. Rosner

Physics Publications

We have measured the absolute wave numbers of 39 transitions of 130Te2 spanning the spectral region of 420.9-464.6 nm to an accuracy of better than 2 parts in 109 by use of saturation spectroscopy and Fabry-Pérot interferometry. These measurements provide a set of convenient and accurate transfer standards for laser wavelength calibration spanning the entire Stilbene-420 dye-tuning curve.

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Theory Of Detection Of Angular Momentum States In Rydberg Atoms Using Half-Cycle Pulses, Chitra Rangan, R.J.A. Murray Jan 2005

Theory Of Detection Of Angular Momentum States In Rydberg Atoms Using Half-Cycle Pulses, Chitra Rangan, R.J.A. Murray

Physics Publications

The creation and detection of specific angular momentum states in Rydberg atoms is a significant advance in quantum-information processing. Detection of high angular momentum states in alkali-metal atoms is difficult because they are nearly-degenerate in energy. We present theoretical results on the detection of such states using terahertz-frequency half-cycle pulses (HCPs). Calculations are done for n=15,=0,...,14 states in hydrogen and cesium using a grid-based pseudopotential method. Our results indicate that HCP redistribution is a reliable method of distinguishing angular momentum Rydberg states.

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Binding Energy Of The Positronium Negative Ion: Relativistic And Qed Energy Shifts, Gordon W. F. Drake, M. Grigorescu Jan 2005

Binding Energy Of The Positronium Negative Ion: Relativistic And Qed Energy Shifts, Gordon W. F. Drake, M. Grigorescu

Physics Publications

The leading relativistic and QED corrections to the ground-state energy of the three-body system e-e+e- are calculated numerically using a Hylleraas correlated basis set. The accuracy of the nonrelativistic variational ground state is discussed with respect to the convergence of the energy with increasing size of the basis set, and also with respect to the variance of the Hamiltonian. The corrections to this energy include the lowest order Breit interaction, the vacuum polarization potential, one and two photon exchange contributions, the annihilation interaction and spin-spin contact terms. The relativistic effects and the residual interactions considered here decrease the one-electron binding …

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Vacancy-Mediated Hydrogen Desorption In Naalh4, C. Moysés Araújo, S. Li, R. Ahuja, Puru Jena Jan 2005

Vacancy-Mediated Hydrogen Desorption In Naalh4, C. Moysés Araújo, S. Li, R. Ahuja, Puru Jena

Physics Publications

First-principles calculations based on density functional theory are carried out to understand the mechanisms responsible for hydrogen desorption from Ti-doped sodium-alanate (NaAlH4). While the energy needed to remove a hydrogen atom from NaAlH4 with Ti substituted either at the Na site or at Al site is found to be significantly lower than that from the pristine NaAlH4, the presence of Na vacancies is shown to play an even larger role: It is not only an order of magnitude smaller than that from Ti-doped sodium-alanate, but the removal of hydrogen associated with a Na vacancy is exothermic with respect to formation …

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Structural Growth In Iron Oxide Clusters: Rings, Towers, And Hollow Drums, N. O. Jones, B. V. Reddy, F. Rasouli, Shiv N. Khanna Jan 2005

Structural Growth In Iron Oxide Clusters: Rings, Towers, And Hollow Drums, N. O. Jones, B. V. Reddy, F. Rasouli, Shiv N. Khanna

Physics Publications

It is shown that the transition from an elementary FeO molecule to the bulk rock-salt FeO proceeds via hollow rings, towers, and drums. Our first-principles electronic structure calculations carried out within a gradient-corrected density functional framework show that small FenOn (n=2,3,4,5) clusters form single, highly stable rings. Starting at Fe6O6, these elementary rings begin to assemble into nano columnar structures to form stable Fe6O6, Fe7O7, Fe8O8, Fe9O9, Fe10O10, and Fe12O12 towers. The rings and the empty towers can be further stabilized by capping O atoms at the ends, leading to FenOn+1 and FenOn+2 sequences. The theoretical results provide insight into …

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Nearly-Free-Electron Gas In A Silicon Cage, J. Ulises Reveles, Shiv N. Khanna Jan 2005

Nearly-Free-Electron Gas In A Silicon Cage, J. Ulises Reveles, Shiv N. Khanna

Physics Publications

A systematic study of the ground state geometries, electronic structure, and stability of the metal (M) encapsulated MSi12 (M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni) clusters has been carried out within a gradient-corrected density functional formalism. It is shown that the ground state of most MSi12 clusters has the lowest spin multiplicity as opposed to the high spin multiplicity in free transition metal atoms. Consequently, a proper inclusion of the spin conservation rules is needed to understand the variation of the binding energy of M to Si12 clusters. Using such rules, CrSi12 and FeSi12 are found to exhibit the …

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First-Principles Study Of The Onset Of Noncollinearity In Mnn Clusters: Magnetic Arrangements In Mn5 And Mn6, T. Morisato, Shiv N. Khanna, Y. Kawazoe Jan 2005

First-Principles Study Of The Onset Of Noncollinearity In Mnn Clusters: Magnetic Arrangements In Mn5 And Mn6, T. Morisato, Shiv N. Khanna, Y. Kawazoe

Physics Publications

First-principles theoretical investigations of the noncollinearity of atomic spin moments in manganese clusters have been carried out within a gradient-corrected density-functional approach. Our studies on Mn5 and Mn6 include investigation of both collinear and noncollinear arrangements. It is shown that while the atomic structure of the ground state of Mn5 is a triangular bipyramid, the collinear and noncollinear arrangements have comparable energies and hence are degenerate. For Mn6, while the ground state has a square bipyramid arrangement, the noncollinear configuration is most stable making it the smallest cluster to feature a noncollinear ground state. The results are discussed in view …

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Electroreflectance Spectroscopy In Self-Assembled Quantum Dots: Lens Symmetry, A. H. Rodríguez, C. Trallero-Giner, Martín Muñoz, María C. Tamargo Jan 2005

Electroreflectance Spectroscopy In Self-Assembled Quantum Dots: Lens Symmetry, A. H. Rodríguez, C. Trallero-Giner, Martín Muñoz, María C. Tamargo

Physics Publications

Modulated electroreflectance spectroscopy ΔR∕R of semiconductor self-assembled quantum dots is investigated. The structure is modeled as dots with lens shape geometry and circular cross section. A microscopic description of the electroreflectance spectrum and optical response in terms of an external electric field (F) and lens geometry have been considered. The field and lens symmetry dependence of all experimental parameters involved in the ΔR∕Rspectrum have been considered. Using the effective mass formalism the energies and the electronic states as a function of F and dot parameters are calculated. Also, in the framework of the strongly confined regime general expressions for the …

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Ab Initio Study Of Ferromagnetism In Ga1−Xcrxn Thin Films, Qin Wang, Q. Sun, Puru Jena, J. Z. Yu, R. Note, Y. Kawazoe Jan 2005

Ab Initio Study Of Ferromagnetism In Ga1−Xcrxn Thin Films, Qin Wang, Q. Sun, Puru Jena, J. Z. Yu, R. Note, Y. Kawazoe

Physics Publications

Electronic structure and magnetic properties of Ga1-xCrxN thin films are studied using the gradient corrected density functional method and a supercell slab model. Calculations are carried out by varying the concentration of doped Cr atoms and the sites they occupy. Cr atoms are found to prefer to reside on the surface sites and cluster around N as Mn atoms do. However, unlike Mn-doped GaN, Cr-doped GaN is found to be ferromagnetic for all concentrations studied. The calculated ferromagnetism is in agreement with recent experimental observations.

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First-Principles Study Of Ferromagnetic Coupling In Zn1-Xcrxte Thin Film, Q. Wang, Q. Sun, P. Jena, Y. Kawazoe Jan 2005

First-Principles Study Of Ferromagnetic Coupling In Zn1-Xcrxte Thin Film, Q. Wang, Q. Sun, P. Jena, Y. Kawazoe

Physics Publications

Using gradient-corrected density functional theory and supercell technique, we have calculated total energies, electronic structure, and magnetic properties of Cr-doped ZnTe in both bulk and thin-film configurations. Calculations with full geometry optimization for a Zn1−xCrxTesupercell were carried out for different Cr concentrations (x=0.095, 0.143, and 0.19) and by varying the sites Cr atoms occupy. We show that the ferromagnetic phase of Zn1−xCrxTe in both bulk and thin film is energetically the most preferable state irrespective of the concentration and∕or site occupation of the Cr atom. The strong hybridization between Cr3d and Te5p states is found to be responsible for the …

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Evolution Of The Electronic Structure Of Be Clusters, V. Cerowski, B. K. Rao, S. K. Khanna, P. Jena, Soh Ishii, Kaoru Ohno, Yoshiyuki Kawazoe Jan 2005

Evolution Of The Electronic Structure Of Be Clusters, V. Cerowski, B. K. Rao, S. K. Khanna, P. Jena, Soh Ishii, Kaoru Ohno, Yoshiyuki Kawazoe

Physics Publications

Using a modified symbiotic genetic algorithm approach and many-body interatomic potential derived from first principles, we have calculated equilibrium geometries and binding energies of the ground-state and low-lying isomers of Be clusters containing up to 41 atoms. Molecular-dynamics study was also carried out to study the frequency of occurrence of the various geometrical isomers as these clusters are annealed during the simulation process. For a selected group of these clusters, higher-energy isomers were more often found than their ground-statestructures due to large catchment areas. The accuracy of the above ground-stategeometries and their corresponding binding energies were verified by carrying out …

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Magnetic Coupling Between Cr Atoms Doped At Bulk And Surface Sites Of Zno, Q. Wang, Q. Sun, Puru Jena, Y. Kawazoe Jan 2005

Magnetic Coupling Between Cr Atoms Doped At Bulk And Surface Sites Of Zno, Q. Wang, Q. Sun, Puru Jena, Y. Kawazoe

Physics Publications

Contrary to theoretical prediction that Cr-doped bulk ZnO is ferromagnetic, recent experiments on Cr-doped ZnOthin film reveal the coupling to be antiferromagnetic. Using first-principles calculations based on gradient corrected density functional theory, we show that a possible origin of this disagreement may be associated with the site preference of Cr. In bulk, when Cr substitutes Zn,bond contraction occurs and Cr atoms prefer to cluster around O atoms. The ferromagnetic coupling among Cr atoms is driven by Cr 3d and O 2p exchange interactions as in Cr2O cluster. However, when Cr atoms replace Zn on the surface, due to the different …

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Role Of Titanium In Hydrogen Desorption In Crystalline Sodium Alanate, C. Moysés Araújo, Rajeev Ahuja, J. M. Osorio Guillén, Puru Jena Jan 2005

Role Of Titanium In Hydrogen Desorption In Crystalline Sodium Alanate, C. Moysés Araújo, Rajeev Ahuja, J. M. Osorio Guillén, Puru Jena

Physics Publications

The role of Ti in improving the thermodynamics of hydrogen desorption in crystalline sodium alanate (NaAlH4) has been investigated by using the density functional theory. The total energy calculations reveal that Ti prefers to occupy the Na site over that of the Al site when the atomic energies are used as the reference. However, the use of the cohesive energies of Al,Na, and Ti leads to the Al site being the least unfavorable one. Irrespective of whether Ti occupies the Na or the Al site, the energy necessary to remove a hydrogen atom from Ti substituted sodium alanate is significantly …

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Effects Of Hydrogen On The Morphology And Electrical Properties Of Gan Grown By Plasma-Assisted Molecular-Beam Epitaxy, Y. Dong, R. M. Feenstra, D. W. Greve, J. C. Moore, M. D. Sievert, A. A. Baski Jan 2005

Effects Of Hydrogen On The Morphology And Electrical Properties Of Gan Grown By Plasma-Assisted Molecular-Beam Epitaxy, Y. Dong, R. M. Feenstra, D. W. Greve, J. C. Moore, M. D. Sievert, A. A. Baski

Physics Publications

We study the effect of introducing hydrogen gas through the rf-plasma source during plasma-assisted molecular-beam epitaxy of GaN(0001). The well-known smooth-to-rough transition that occurs for this surface as a function of decreasing Ga flux in the absence of H is found to persist even with H present, although the critical Ga flux for this transition increases. Under Ga-rich conditions, the presence of hydrogen is found to induce step bunching (facetting) on the surface. Conductive atomic force microscopy reveals that leakage current through dislocation cores is significantly reduced when hydrogen is present during the growth.

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Spectroscopy And Carrier Dynamics In Cdse Self-Assembled Quantum Dots Embedded In Znxcdymg1−X−Yse, G. Comanescu, W. B. Wang, S. Gundry, B. Das, R. R. Alfano, M. N. Perez-Paz, M. C. Tamargo, M. Muñoz, I. Popov, L. L. Isaacs Jan 2005

Spectroscopy And Carrier Dynamics In Cdse Self-Assembled Quantum Dots Embedded In Znxcdymg1−X−Yse, G. Comanescu, W. B. Wang, S. Gundry, B. Das, R. R. Alfano, M. N. Perez-Paz, M. C. Tamargo, M. Muñoz, I. Popov, L. L. Isaacs

Physics Publications

Time-resolved and steady-state photoluminescence,reflectivity, and absorption experiments were performed on CdSequantum dots in ZnxCdyMg1−x−ySe barriers. Studies of the capture times of the photoexcited carriers into the quantum dots and of electron-hole recombination times inside the dots were performed. Photoluminescence rise time yielded capture times from 20 ps to 30 ps. All samples exhibit fast and slow photoluminescence decays, consistent with observing two independent but energetically overlapping decays. The faster relaxation times for the sample emitting in the blue range is 90 ps, whereas for the two samples emitting in the green it is 345 ps and 480 ps. The slower …

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Electroreflectance Spectroscopy In Self-Assembled Quantum Dots: Lens Symmetry, A. H. Rodríguez, C. Trallero-Giner, Martín Muñoz, María C. Tamargo Jan 2005

Electroreflectance Spectroscopy In Self-Assembled Quantum Dots: Lens Symmetry, A. H. Rodríguez, C. Trallero-Giner, Martín Muñoz, María C. Tamargo

Physics Publications

Modulated electroreflectance spectroscopy Delta R/R of semiconductor self-assembled quantum dots is investigated. The structure is modeled as dots with lens shape geometry and circular cross section. A microscopic description of the electroreflectance spectrum and optical response in terms of an external electric field (F) and lens geometry have been considered. The field and lens symmetry dependence of all experimental parameters involved in the Delta R/R spectrum have been considered. Using the effective mass formalism the energies and the electronic states as a function of F and dot parameters are calculated. Also, in the framework of the strongly confined regime general …

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