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Search Results: 1 - 10 of 327342 matches for " S. McKeown Walker "
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Quasiparticle dynamics and spin-orbital texture of the SrTiO3 two-dimensional electron gas
P. D. C. King,S. McKeown Walker,A. Tamai,A. de la Torre,T. Eknapakul,P. Buaphet,S. -K. Mo,W. Meevasana,M. S. Bahramy,F. Baumberger
Physics , 2014, DOI: 10.1038/ncomms4414
Abstract: Two-dimensional electron gases (2DEGs) in SrTiO$_3$ have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the $d$-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orbital, and lattice degrees of freedom in this system. We reveal how they lead to pronounced orbital ordering, mediate an orbitally-enhanced Rashba splitting with complex subband-dependent spin-orbital textures and markedly change the character of electron-phonon coupling, co-operatively shaping the low-energy electronic structure of the 2DEG. Our results allow for a unified understanding of spectroscopic and transport measurements across different classes of SrTiO$_3$-based 2DEGs, and yield new microscopic insights on their functional properties.
Control of a two-dimensional electron gas on SrTiO3(111) by atomic oxygen
S. McKeown Walker,A. de la Torre,F. Y. Bruno,A. Tamai,T. K. Kim,M. Hoesch,M. Shi,M. S. Bahramy,P. D. C. King,F. Baumberger
Physics , 2014, DOI: 10.1103/PhysRevLett.113.177601
Abstract: We report on the formation of a two-dimensional electron gas (2DEG) at the bare surface of (111) oriented SrTiO3. Angle resolved photoemission experiments reveal highly itinerant carriers with a 6-fold symmetric Fermi surface and strongly anisotropic effective masses. The electronic structure of the 2DEG is in good agreement with self-consistent tight-binding supercell calculations that incorporate a confinement potential due to surface band bending. We further demonstrate that alternate exposure of the surface to ultraviolet light and atomic oxygen allows tuning of the carrier density and the complete suppression of the 2DEG.
Controlling bulk conductivity in topological insulators: Key role of anti-site defects
D. O. Scanlon,P. D. C. King,R. P. Singh,A. de la Torre,S. McKeown Walker,G. Balakrishnan,F. Baumberger,C. R. A. Catlow
Physics , 2012, DOI: 10.1002/adma.201200187
Abstract: The binary Bi-chalchogenides, Bi2Ch3, are widely regarded as model examples of a recently discovered new form of quantum matter, the three-dimensional topological insulator (TI) [1-4]. These compounds host a single spin-helical surface state which is guaranteed to be metallic due to time reversal symmetry, and should be ideal materials with which to realize spintronic and quantum computing applications of TIs [5]. However, the vast majority of such compounds synthesized to date are not insulators at all, but rather have detrimental metallic bulk conductivity [2, 3]. This is generally accepted to result from unintentional doping by defects, although the nature of the defects responsible across different compounds, as well as strategies to minimize their detrimental role, are surprisingly poorly understood. Here, we present a comprehensive survey of the defect landscape of Bi-chalchogenide TIs from first-principles calculations. We find that fundamental differences in the energetics of native defect formation in Te- and Se-containing TIs enables precise control of the conductivity across the ternary Bi-Te-Se alloy system. From a systematic angle-resolved photoemission (ARPES) investigation of such ternary alloys, combined with bulk transport measurements, we demonstrate that this method can be utilized to achieve true topological insulators, with only a single Dirac cone surface state intersecting the chemical potential. Our microscopic calculations reveal the key role of anti-site defects for achieving this, and predict optimal growth conditions to realize maximally-resistive ternary TIs.
Collapse of the Mott gap and emergence of a nodal liquid in lightly doped Sr$_2$IrO$_4$
A. de la Torre,S. McKeown Walker,F. Y. Bruno,S. Ricco,Z. Wang,I. Gutierrez Lezama,G. Scheerer,G. Giriat,D. Jaccard,C. Berthod,T. K. Kim,M. Hoesch,E. C. Hunter,R. S. Perry,A. Tamai,F. Baumberger
Physics , 2015, DOI: 10.1103/PhysRevLett.115.176402
Abstract: Superconductivity in underdoped cuprates emerges from an unusual electronic state characterised by nodal quasiparticles and an antinodal pseudogap. The relation between this state and superconductivity is intensely studied but remains controversial. The discrimination between competing theoretical models is hindered by a lack of electronic structure data from related doped Mott insulators. Here we report the doping evolution of the Heisenberg antiferromagnet Sr$_2$IrO$_4$, a close analogue to underdoped cuprates. We demonstrate that metallicity emerges from a rapid collapse of the Mott gap with doping, resulting in lens-like Fermi contours rather than disconnected Fermi arcs as observed in cuprates. Intriguingly though, the emerging electron liquid shows nodal quasiparticles with an antinodal pseudogap and thus bares strong similarities with underdoped cuprates. We conclude that anisotropic pseudogaps are a generic property of two-dimensional doped Mott insulators rather than a unique hallmark of cuprate high-temperature superconductivity.
Spectroscopic evidence for polaronic behaviour of the strong spin-orbit insulator Sr$_3$Ir$_2$O$_7$
P. D. C. King,T. Takayama,A. Tamai,E. Rozbicki,S. McKeown Walker,M. Shi,L. Patthey,R. G. Moore,D. Lu,K. M. Shen,H. Takagi,F. Baumberger
Physics , 2013, DOI: 10.1103/PhysRevB.87.241106
Abstract: We investigate the bilayer Ruddlesden-Popper iridate Sr$_3$Ir$_2$O$_7$ by temperature-dependent angle-resolved photoemission. We find a narrow-gap correlated insulator, with spectral features indicative of a polaronic ground state, strikingly similar to that observed previously for the parent compounds of the cuprate superconductors. We additionally observe similar behaviour for the single-layer cousin Sr$_2$IrO$_4$, indicating that strong electron-boson coupling dominates the low-energy excitations of this exotic family of materials, and providing a microscopic link between the insulating ground states of the seemingly-disparate 3d cuprates and 5d iridates.
Tailoring the nature and strength of electron-phonon interactions in the SrTiO$_3$(001) two-dimensional electron liquid
Z. Wang,S. McKeown Walker,A. Tamai,Z. Ristic,F. Y. Bruno,A. de la Torre,S. Riccò,N. C. Plumb,M. Shi,P. Hlawenka,J. Sánchez-Barriga,A. Varykhalov,T. K. Kim,M. Hoesch,P. D. C. King,W. Meevasana,U. Diebold,J. Mesot,M. Radovic,F. Baumberger
Physics , 2015,
Abstract: Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO$_3$(001) surface to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO$_3$/SrTiO$_3$ interface, our angle-resolved photoemission data show replica bands separated by 100\,meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies strong long-range coupling to a single longitudinal optical phonon mode. In the overdoped regime the preferential coupling to this mode decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow for a unified understanding of the transport properties in SrTiO$_3$-based 2DELs.
Parity-Violating Electron Scattering and the Electric and Magnetic Strange Form Factors of the Nucleon
D. S. Armstrong,R. D. McKeown
Physics , 2012, DOI: 10.1146/annurev-nucl-102010-130419
Abstract: Measurement of the neutral weak vector form factors of the nucleon provides unique access to the strange quark content of the nucleon. These form factors can be studied using parity-violating electron scattering. A comprehensive program of experiments has been performed at three accelerator laboratories to determine the role of strange quarks in the electromagnetic form factors of the nucleon. This article reviews the remarkable technical progress associated with this program, describes the various methods used in the different experiments, and summarizes the physics results along with recent theoretical calculations.
Improved High Definition Multimedia Interface Authentication Mechanism  [PDF]
R. N. Iyare, S. D. Walker
Journal of Computer and Communications (JCC) , 2014, DOI: 10.4236/jcc.2014.212003

Security information has become so significant in transmission due to the rapid advancement in digital data exchange. Thus, it is necessary to protect the confidentiality and licensing of video content from illegal access. Currently, High-bandwidth Digital Content Protection (HDCP) provides the confidentiality and licensing of digital content for High Digital Multimedia Interface (HDMI). In this paper, we have been able to show how cryptanalysts have conducted attacks on the HDCP protocol showing its vulnerability in protecting digital contents. Therefore, the HDCP scheme is seriously flawed and compromised. Encryption and decryption of audio/video files were implemented in both Data Encryption Standard (DES) and Advanced Encryption Standard (AES) algorithms by means of Python Cryptography Toolkit (Pycrypto). Security analysis such as key space analysis and statistical analysis were carried out on the encrypted image. The result of the analysis indicates that AES algorithm is secure and robust; hence the authentication and licensing of HDMI can be improved by implementing HDCP design with AES algorithm.

Programming Protocol-Independent Packet Processors
Pat Bosshart,Dan Daly,Martin Izzard,Nick McKeown,Jennifer Rexford,Cole Schlesinger,Dan Talayco,Amin Vahdat,George Varghese,David Walker
Computer Science , 2013,
Abstract: P4 is a high-level language for programming protocol-independent packet processors. P4 works in conjunction with SDN control protocols like OpenFlow. In its current form, OpenFlow explicitly specifies protocol headers on which it operates. This set has grown from 12 to 41 fields in a few years, increasing the complexity of the specification while still not providing the flexibility to add new headers. In this paper we propose P4 as a strawman proposal for how OpenFlow should evolve in the future. We have three goals: (1) Reconfigurability in the field: Programmers should be able to change the way switches process packets once they are deployed. (2) Protocol independence: Switches should not be tied to any specific network protocols. (3) Target independence: Programmers should be able to describe packet-processing functionality independently of the specifics of the underlying hardware. As an example, we describe how to use P4 to configure a switch to add a new hierarchical label.
Rainy season characteristics of the Free State Province of South Africa with reference to rain-fed maize production
ME Moeletsi, S Walker
Water SA , 2012,
Abstract: The study assesses onset of rains, cessation of rains, duration of rainy season and seasonal rainfall at different probability levels. Daily rainfall data for 309 stations located in the Free State Province of South Africa were analysed from 1950 to 2008. The cumulative rainfall over 3 consecutive dekads (10-day periods) and cumulative rainfall over 1 dekad were used to determine onset of rains and cessation of rains respectively. Seasonal rainfall was determined as the accumulated rainfall from November to March. Rainbow statistical software was utilised to test for normality and determine probabilities at 20%, 50% and 80% risk levels. The other rainy season characteristics investigated were the probability of onset failure and probability of rainy season duration of less than 50, 100, 120 and 140 days. These rainy season indices were investigated in relation to maize production in the Free State. Rainfall behaviour during the growing period is one of the main limiting factors to rain-fed maize production, consequently influencing household food security. The results show that for onset of rains there is a large spatial variance over the Free State while cessation of rains shows small variance. There is also an east to west progression of onsets while the duration of the rainy season and seasonal rainfall also increased from west to east. Areas of low risk associated with rainy season characteristics are evident over the Thabo Mofutsanyane, eastern Motheo, eastern and northeastern Lejweleputswa and the Fezile Dabi districts, making these areas highly suitable for maize production. By contrast, high-risk areas are in the western and southern parts of the province and thus dryland maize production has low production potential in these areas.
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