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Displaying Results 126 - 150 of 1489 on page 6 of 60
Marked
Mark
Using surface and interface optics to probe the capping, with amorphous Si, of Au atom chains grown on vicinal Si(111)
(2009)
MC GILP, JOHN FINLAY
Using surface and interface optics to probe the capping, with amorphous Si, of Au atom chains grown on vicinal Si(111)
(2009)
MC GILP, JOHN FINLAY
Abstract:
The distinct optical signatures of aligned single and double Au atom chain structures, grown on vicinal Si(111) substrates, have been identified using reflectance anisotropy spectroscopy (RAS). Deposition of 0.04 monolayers (ML) of amorphous Si (a-Si) at room temperature perturbs the anisotropic optical response of the double chain structure. By one third of a monolayer, no significant optical anisotropy associated with the chains remains. No anisotropic response re-emerges at higher coverages, up to 4.6 nm (14.5 ML) where there is recent evidence that the crystal structure of the double chain phase is maintained under the cap. The RAS results show that the anisotropic properties of the phase are quenched by a-Si adsorption, even though the crystal structure of the capped phase appears to be preserved.
http://hdl.handle.net/2262/41123
Marked
Mark
Extracting the hysteresis loops of magnetic interfaces from optical second-harmonic intensity measurements
(2007)
FLEISCHER, KARSTEN; MC GILP, JOHN FINLAY
Extracting the hysteresis loops of magnetic interfaces from optical second-harmonic intensity measurements
(2007)
FLEISCHER, KARSTEN; MC GILP, JOHN FINLAY
Abstract:
Surface- and interface-sensitive optical techniques, such as optical second-harmonic generation (SHG), allow the buried interfacial structure of centrosymmetric materials to be explored through thin capping layers, and magnetic SHG (MSHG) extends this to magnetic interfaces. However, the variation of the MSHG intensity with magnetic field does not measure hysteresis loops directly, because the loops are displaced by an amount dependent on the crystallographic response and its phase difference with respect to the magnetic response, and also because there is a quadratic magnetization contribution to the SH intensity that may be significant. Two new procedures are reported for extracting hysteresis loops directly from the MSHG intensity. The first is applicable to all magnetic interfaces, including exchange-biased structures, where the saturation magnetization for positive and negative magnetic fields is equal and opposite. The second applies to all centrosymmetric hysteresis loops. Th...
http://hdl.handle.net/2262/41121
Marked
Mark
Probing surface and interface structure using optics
(2010)
MC GILP, JOHN FINLAY
Probing surface and interface structure using optics
(2010)
MC GILP, JOHN FINLAY
Abstract:
Optical techniques for probing surface and interface structure are introduced and recent developments in the field are discussed. These techniques offer significant advantages over conventional surface probes: all pressure ranges of gas–condensed matter interfaces are accessible and liquid–liquid, liquid–solid and solid–solid interfaces can be probed, due to the large penetration depth of the optical radiation. Sensitivity and discrimination from the bulk are the two challenges facing optical techniques in probing surface and interface structure. Where instrumental improvements have resulted in enhanced sensitivity, conventional optical techniques can be used to characterize heterogeneous adsorbed layers on a substrate, often with sub-monolayer resolution. Nanoscale lateral resolution is possible using scanning near-field optics. A separate class of techniques, which includes reflection anisotropy spectroscopy, and nonlinear optical probes such as second-harmonic and sum-frequency g...
http://hdl.handle.net/2262/41124
Marked
Mark
Using steps at the Si-SiO2 interface to test simple bond models of the optical second-harmonic response
(2007)
MC GILP, JOHN FINLAY
Using steps at the Si-SiO2 interface to test simple bond models of the optical second-harmonic response
(2007)
MC GILP, JOHN FINLAY
Abstract:
A simple bond model of the optical second-harmonic (SH) response from interfaces, which assumes a single axial component for each bond, is tested by comparing the response from different step structures at the Si–SiO2 interface. The response of native-oxide-covered vicinal Si(111), offcut by 3° towards , is compared with previous work on similar samples, but offcut by 5° in the opposite, , direction. The two offcuts have different step geometries, but the same terrace structure at the Si–SiO2 interface, while the bulk quadrupolar contributions from the two offcuts are related by simple geometric factors, thus significantly constraining the number of adjustable parameters. The bond model is tested against experiment for the eight sample rotation plots obtained by permuting s- and p-polarization combinations. The constrained fit is found to be too poor to justify the extraction of quantitative information. Kleinman symmetry is shown to be a necessary, but not sufficient, condition for...
http://hdl.handle.net/2262/41119
Marked
Mark
Electron-electron scattering in a double quantum dot: Effective mass approach
(2010)
PEROVA, TATIANA
Electron-electron scattering in a double quantum dot: Effective mass approach
(2010)
PEROVA, TATIANA
Abstract:
We present a theoretical description of the first-order scattering of interacting electrons and holes in a double quantum dot. Assuming infinitely high walls, strong confinement, and a two-band approximation, we derive general expressions for the two-particle matrix elements of the screened Coulomb potential. We also determine the selection rules for different scattering channels and consider special cases where the corresponding matrix elements can be represented by simple analytical expressions. Numerical calculations of the matrix elements and an analysis of their dependence on the geometrical and material parameters of the double quantum dot have also been performed.
http://hdl.handle.net/2262/41172
Marked
Mark
Raman and Fourier transform infrared study of substitutional carbon incorporation in rapid thermal chemical vapor deposited Si 1-x-yGexCy on (1 0 0) Si
(2010)
PEROVA, TATIANA; WASYLUK, JOANNA
Raman and Fourier transform infrared study of substitutional carbon incorporation in rapid thermal chemical vapor deposited Si 1-x-yGexCy on (1 0 0) Si
(2010)
PEROVA, TATIANA; WASYLUK, JOANNA
Abstract:
We report on a detailed study of the dependence of the vibrational modes in rapid thermal chemical vapor deposited Si1−x−yGexCy films on the substitutional carbon concentration. Si1−x−yGexCy films were investigated using Raman and infrared spectroscopy with x varying in the range of 10%–16% and y in the range of 0%–1.8%. The introduction of C into thin SiGe layers reduces the average lattice constant. It has been shown that the integrated infrared intensity of the Si–C peak and the ratio of both the Raman integrated and peak intensities of the Si–C peak at 605 cm−1 to the Si–Si peak of SiGeC layer, increase linearly with C content and are independent of the Ge content. This leads to the conclusion that infrared absorption and Raman scattering data can be used to determine the fraction of substitutional carbon content in Si1−x−yGexCy layers with a Ge content of up to 16%. It is also shown that the intensity ratio of the carbon satellite peak to the local carbon mode increases line...
http://hdl.handle.net/2262/41173
Marked
Mark
All optical, high contrast absorptive modulation in an asymmetric Fabry-Perot etalon
(1991)
HEGARTY, JOHN
All optical, high contrast absorptive modulation in an asymmetric Fabry-Perot etalon
(1991)
HEGARTY, JOHN
Abstract:
We report a 27:l switching contrast ratio with 2.5 mW of power in an asymmetric Fabry- Perot Ctalon. The modulation is achieved by optical saturation of the excitonic absorption profile of a 95 A GaAs/AlGaAs multiple quantum well structure grown on a high-reflectivity dielectric stack mirror.
http://hdl.handle.net/2262/38790
Marked
Mark
Demonstration of high-brightness-mode propagation in a compound waveguide structure
(1999)
HEGARTY, JOHN
Demonstration of high-brightness-mode propagation in a compound waveguide structure
(1999)
HEGARTY, JOHN
Abstract:
We demonstrate the existence of an unusual and useful high-brightness guided mode of a multimode AlGaAs/GaAs heterostructure compound slab waveguide. This mode has a narrow near-field single lobe confined to the low-index regions of the waveguide. This mode was selectively probed by optically exciting quantum wells optimally placed in the waveguide. By pumping in a stripe geometry, lasing is observed above a threshold of 80 kW/cm2 indicating efficient lasing in the highest-order waveguide mode. The near-field emission pattern of the waveguide was imaged to provide a direct measurement of the intensity profile of the higher-order mode.
http://hdl.handle.net/2262/38050
Marked
Mark
Multidiagnostic analysis of ultrafast laser ablation of metals with pulse pair
(2010)
LUNNEY, JAMES GERARD
Multidiagnostic analysis of ultrafast laser ablation of metals with pulse pair
(2010)
LUNNEY, JAMES GERARD
Abstract:
Copper targets are irradiated in the ablation regime by pairs of equal, time-delayed collinear laser pulses separated on a timescale going from 2 ps to 2 ns. The ablation plume is characterized by ion probe diagnostic, fast imaging, and temporally and spatially resolved optical emission spectroscopy. The variation in the ablation efficiency with the delay between the pulses is analyzed by measuring the ablation crater profile with a contact profilometer. The second laser pulse modifies the characteristics of the plasma plume produced by the first pulse and the ablation efficiency. The different mechanisms involved in double pulse ultrafast laser ablation are identified and discussed. The experimental findings are interpreted in the frame of a simple model of the interaction of the second pulse with the nascent ablation plume produced by the first pulse. This model yields consistent and quantitative agreement with the experimental findings predicting the observed experimental trend...
http://hdl.handle.net/2262/41263
Marked
Mark
Conformal anomaly of (2,0) tensor multiplet in six dimensions and AdS/CFT correspondence
(2000)
FROLOV, SERGEY
Conformal anomaly of (2,0) tensor multiplet in six dimensions and AdS/CFT correspondence
(2000)
FROLOV, SERGEY
Abstract:
We compute the conformal anomaly in the free d = 6 superconformal (2,0) tensor multiplet theory on generic curved background. Up to a trivial covariant totalderivative term, it is given by the sum of the type A part proportional to the 6-d Euler density, and the type B part containing three independent conformal invariants: two CCC contractions of Weyl tensors and a C∇2C + ... term. Multiplied by the factor 4N3, the latter Weyl-invariant part of the anomaly reproduces exactly the corresponding part of the conformal anomaly of large N multiple M5-brane (2,0) theory as predicted (hep-th/9806087) by AdS7 supergravity on the basis of AdS/CFT correspondence. The coefficients of the type A anomaly differ by the factor 4 7 × 4N3, so that the free tensor multiplet anomaly does not vanish on a Ricci-flat background. The coefficient 4N3 is the same as found (hep-th/9703040) in the comparison of the tensor multiplet theory and the d = 11 supergravity predictions for the absorption crosssection...
http://hdl.handle.net/2262/49076
Marked
Mark
Tachyon condensation and universality of DBI action
(2001)
FROLOV, SERGEY
Tachyon condensation and universality of DBI action
(2001)
FROLOV, SERGEY
Abstract:
We show that a low-energy action for massless fluctuations around a tachyonic soliton background representing a codimension one D-brane coincides with the Dirac-Born-Infeld action. The scalar modes which describe transverse oscillations of the D-brane are translational collective coordinates of the soliton. The appearance of the DBI action is a universal feature independent of details of a tachyon effective action, provided it has the structure implied by the open string sigma model partition function.
http://hdl.handle.net/2262/49328
Marked
Mark
Models of coherent exciton condensation
(2004)
EASTHAM, PAUL
Models of coherent exciton condensation
(2004)
EASTHAM, PAUL
Abstract:
That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focussing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.
http://hdl.handle.net/2262/49386
Marked
Mark
Konishi operator at intermediate coupling.
(2011)
FROLOV, SERGEY
Konishi operator at intermediate coupling.
(2011)
FROLOV, SERGEY
Abstract:
TBA equations for two-particle states from the sl(2) sector proposed by Arutyunov, Suzuki and the author are solved numerically for the Konishi operator descendent up to 't Hooft's coupling 2046. The data obtained is used to analyze the properties of Y-functions and address the issue of the existence of the critical values of the coupling. In addition we nd a new integral representation for the BES dressing phase which substantially reduces the computational time. Email:
http://hdl.handle.net/2262/49374
Marked
Mark
Microcavity quantum-dot systems for non-equilibrium Bose-Einstein condensation
(2010)
EASTHAM, PAUL
Microcavity quantum-dot systems for non-equilibrium Bose-Einstein condensation
(2010)
EASTHAM, PAUL
Abstract:
We review the practical conditions required to achieve a non-equilibrium BEC driven by quantum dynamics in a system comprising a microcavity field mode and a distribution of localised two-level systems driven to a step-like population inversion profile. A candidate system based on eight 3.8nm layers of In0.23Ga0.77As in GaAs shows promising characteristics with regard to the total dipole strength which can be coupled to the field mode.
http://hdl.handle.net/2262/49586
Marked
Mark
Stability, geometry, and electronic structure of an alternative I-III-VI2 material, CuScS2: A hybrid density functional theory analysis
(2010)
WATSON, GRAEME WILLIAM; SCANLON, DAVID
Stability, geometry, and electronic structure of an alternative I-III-VI2 material, CuScS2: A hybrid density functional theory analysis
(2010)
WATSON, GRAEME WILLIAM; SCANLON, DAVID
Abstract:
Cu based I-III-VI2 materials have received much attention due to their utility in solar cell applications. The vast majority these studies have focused on materials with group IIIA cations as the trivalent metal. In this study we utilize the screened hybrid exchange functional, HSE06, to investigate the stability of CuScS2 in the crystal structures of all the other I-III-VI2 materials, and find that it preferentially forms in its own unique structure. We analyze the electronic structure and optical properties of CuScS2 and in light of this discuss its semiconducting ability.
http://hdl.handle.net/2262/50588
Marked
Mark
Composition and strain in thin Si1-xGex virtual substrates measured by micro-Raman spectroscopy and X-ray diffraction
(2011)
PEROVA, TATIANA; WASYLUK, JOANNA
Composition and strain in thin Si1-xGex virtual substrates measured by micro-Raman spectroscopy and X-ray diffraction
(2011)
PEROVA, TATIANA; WASYLUK, JOANNA
Abstract:
Micro-Raman spectroscopy was employed for the determination of the germanium content, x and strain, , in ultrathin SiGe virtual substrates grown directly on Si by molecular beam epitaxy. The growth of highly relaxed SiGe layers was achieved by the introduction of point defects at a very low temperature during the initial stage of growth. SiGe virtual substrates with thicknesses in the range 40–200 nm with a high Ge content up to 50% and degree of relaxation, r, in the range 20%–100% were investigated using micro-Raman spectroscopy and x-ray diffraction XRD techniques. The Ge content, x, and strain, , were estimated from equations describing Si–Si, Si–Ge, and Ge–Ge Raman vibrational modes, modified in this study for application to thin SiGe layers. The alteration of the experimentally derived equations from previous studies was performed using independent data for x and r obtained from XRD reciprocal space maps. A number of samples consisting of a strained-silicon s-Si layer ...
http://hdl.handle.net/2262/54844
Marked
Mark
Expansion dynamics of laser produced plasma
(2011)
LUNNEY, JAMES GERARD
Expansion dynamics of laser produced plasma
(2011)
LUNNEY, JAMES GERARD
Abstract:
We consider the applicability of the isentropic, adiabatic gas dynamical model of plume expansion for laser ablation in vacuum. We show that the model can be applied to ionized plumes and estimate the upper electron temperature limit on the applicability of the isentropic approximation. The model predictions are compared with Langmuir ion probe measurements and deposition profiles obtained for excimer laser ablation of silver.
http://hdl.handle.net/2262/57336
Marked
Mark
AdS/QHE: towards a holographic description of quantum Hall experiments
(2011)
Bayntun, Allan; Burgess, C. P.; Dolan, Brian P.; Lee, Sung-Sik
AdS/QHE: towards a holographic description of quantum Hall experiments
(2011)
Bayntun, Allan; Burgess, C. P.; Dolan, Brian P.; Lee, Sung-Sik
Abstract:
Transitions among quantum Hall plateaux share a suite of remarkable experimental features, such as semicircle laws and duality relations, whose accuracy and robustness are difficult to explain directly in terms of the detailed dynamics of the microscopic electrons. They would naturally follow if the low-energy transport properties were governed by an emergent discrete duality group relating the different plateaux, but no explicit examples of interacting systems having such a group are known. Recent progress using the AdS/CFT correspondence has identified examples with similar duality groups, but without the dc ohmic conductivity characteristic of quantum Hall experiments. We use this to propose a simple holographic model for low-energy quantum Hall systems, with a nonzero dc conductivity that automatically exhibits all of the observed consequences of duality, including the existence of the plateaux and the semicircle transitions between them. The model can be regarded as a strongly ...
http://eprints.nuim.ie/2609/
Marked
Mark
Nitrogen assisted etching of graphene layers in a scanning electron microscope
(2011)
COLEMAN, JONATHAN NESBIT; FOX, DANIEL; ZHANG, HONGZHOU; ZHOU, DAN; O'NEILL, ARLENE
Nitrogen assisted etching of graphene layers in a scanning electron microscope
(2011)
COLEMAN, JONATHAN NESBIT; FOX, DANIEL; ZHANG, HONGZHOU; ZHOU, DAN; O'NEILL, ARLENE
Abstract:
We describe the controlled patterning of nanopores in graphene layers by using the low-energy (<10 keV) focused electron beam in a scanning electron microscope. Regular nanometer-sized holes can be fabricated with the presence of nitrogen gas. The effect of the gas pressure, beam current, and energy on the etching process are investigated. Transmission electron microscopy, coupled with plasmon energy loss imaging, reveals the microstructure modification of the etched graphene. A nitrogen-ion assisted etching mechanism is proposed for the controlled patterning.
http://hdl.handle.net/2262/57526
Marked
Mark
Influence of an Au capping layer on the magnetic properties of CoPt nanowires
(2011)
MONZON, LORENA; VENKATESAN, MUNUSWAMY; COEY, JOHN MICHAEL DAVID; STAMENOV, PLAMEN; BYRN...
Influence of an Au capping layer on the magnetic properties of CoPt nanowires
(2011)
MONZON, LORENA; VENKATESAN, MUNUSWAMY; COEY, JOHN MICHAEL DAVID; STAMENOV, PLAMEN; BYRNE, FIONA
Abstract:
Equiatomic CoPt nanowires with a gold cap layer have been fabricated by electrochemical template synthesis. Au improves the structural and magnetic properties of the L10 ordered CoPt, leading to coercivities of up to 1.43 T. The anisotropy constant K1 was estimated as 3.9(9) MJ m−3 from the approach to saturation, and four effective anisotropy constants were determined from analysis of torque curves in a 14 T field. Multisegmented magnetic nanowires are produced using gold interlayers.
http://hdl.handle.net/2262/57527
Marked
Mark
Magnetic properties of the magnetoelectric compound Cu2OSeO3: Magnetization and 77Se NMR study
(2011)
SHVETS, IGOR; WU, HAN-CHUN
Magnetic properties of the magnetoelectric compound Cu2OSeO3: Magnetization and 77Se NMR study
(2011)
SHVETS, IGOR; WU, HAN-CHUN
Abstract:
We present magnetization and 77Se Nuclear Magnetic Resonance (NMR) measurements in single crystals of the magneteoelectric compound Cu2OSeO3. The temperature and field dependence of the magnetization suggest a ferrimagnetic ordering at Tc 60 K in a 3up-1down configuration. The easy axis of the magnetization is along the [100] crystallographic direction. The 77Se NMR line shape data collected at 14.09 T are consistent with the symmetries imposed by the cubic P213 space group in the temperature range 20–290 K and confirm that the magnetic transition is not accompanied by any lowering of the crystal symmetry as has recently been proposed by Bos et al. [Phys. Rev. B 78 094416 (2008)].
http://hdl.handle.net/2262/57578
Marked
Mark
Holomorphic and anti-holomorphic conductivity flows in the quantum Hall effect
(2011)
Dolan, Brian P.
Holomorphic and anti-holomorphic conductivity flows in the quantum Hall effect
(2011)
Dolan, Brian P.
Abstract:
It is shown that the flow diagrams for the conductivities in the quantum Hall effect, arising from two ostensibly very different proposals based on modular symmetry, are in fact identical. The β-functions are different, the rate at which the flow lines are traversed are different, but the tangents to the flow lines are the same in both cases, hence the flow diagrams are same in all aspects.
http://eprints.nuim.ie/2608/
Marked
Mark
A universal Dirac operator and noncommutative spin bundles over fuzzy complex projective spaces
(2008)
Dolan, Brian P.; Huet, Idrish; Murray, Sean; O’Connor, Denjoe
A universal Dirac operator and noncommutative spin bundles over fuzzy complex projective spaces
(2008)
Dolan, Brian P.; Huet, Idrish; Murray, Sean; O’Connor, Denjoe
Abstract:
We present a universal Dirac operator for noncommutative spin and spinc bundles over fuzzy complex projective spaces. We give an explicit construction of these bundles, which are described in terms of finite dimensional matrices, calculate the spectrum and explicitly exhibit the Dirac eigenspinors. To our knowledge the spinc spectrum for CPn with n ≥ 3 is new.
http://eprints.nuim.ie/2677/
Marked
Mark
Dimensional Reduction and Vacuum Structure of Quiver Gauge Theory
(2009)
Dolan, Brian P.; Szabo, Richard J.
Dimensional Reduction and Vacuum Structure of Quiver Gauge Theory
(2009)
Dolan, Brian P.; Szabo, Richard J.
Abstract:
We describe the structure of the vacuum states of quiver gauge theories obtained via dimensional reduction over homogeneous spaces, in the explicit example of SU(3)-equivariant dimensional reduction of Yang-Mills-Dirac theory on manifolds of the form M × CP2. We pay particular attention to the role of topology of background gauge fields on the internal coset spaces, in this case U(1) magnetic monopoles and SU(2) instantons on CP2. The reduction of Yang-Mills theory induces a quiver gauge theory involving coupled Yang-Mills-Higgs systems on M with a Higgs potential leading to dynamical symmetry breaking. The criterion for a ground state of the Higgs potential can be written as the vanishing of a non-abelian Yang-Mills flux on the quiver diagram, regarded as a lattice with group elements attached to the links. The reduction of SU(3)-symmetric fermions yields Dirac fermions on M transforming under the low-energy gauge group with Yukawa couplings. The fermionic zero modes on CP2 yield e...
http://eprints.nuim.ie/2676/
Marked
Mark
Temperature dependent coercivity crossover in pseudo-spin-valve magnetic tunnel junctions with perpendicular anisotropy
(2011)
COEY, JOHN MICHAEL DAVID; WU, HAN-CHUN
Temperature dependent coercivity crossover in pseudo-spin-valve magnetic tunnel junctions with perpendicular anisotropy
(2011)
COEY, JOHN MICHAEL DAVID; WU, HAN-CHUN
Abstract:
We report the temperature dependent collapse of tunnel magnetoresistance (TMR) in perpendicular anisotropy magnetic tunnel junctions (pMTJs) with AlO(x) barriers and (Co/Pt)(3) multilayer electrodes, due to the coercivity crossover of the top and bottom (Co/Pt)(3) stacks. The different temperature dependence of two (Co/Pt)(3) stacks in pMTJs is mainly caused by the additional perpendicular anisotropy created at interface between the ferromagnetic electrode and the AlO(x) barrier.
http://hdl.handle.net/2262/59262
Displaying Results 126 - 150 of 1489 on page 6 of 60
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