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Search Results: 1 - 10 of 200611 matches for " P. Hlawenka "
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Massless Dirac fermions in epitaxial graphene on Fe(110)
A. Varykhalov,J. Sánchez-Barriga,P. Hlawenka,O. Rader
Physics , 2012,
Abstract: Graphene grown on Fe(110)by chemical vapor deposition using propylene is investigated by means of angle-resolved photoemission. The presence of massless Dirac fermions is clearly evidenced by the observation of a fully intact Dirac cone. Unlike Ni(111) and Co(0001), the Fe(110) imposes a strongly anisotropic quasi-one-dimensional structure on the graphene. Certain signatures of a superlattice effect appear in the dispersion of its \sigma-bands but the Dirac cone does not reveal any detectable superlattice or quantum-size effects although the graphene corrugation is twice as large as in the established two-dimensional graphene superlattice on Ir(111).
Angle-resolved and core-level photoemission study of interfacing the topological insulator Bi1.5Sb0.5Te1.7Se1.3 with Ag, Nb and Fe
N. de Jong,E. Frantzeskakis,B. Zwartsenberg,Y. K. Huang,D. Wu,P. Hlawenka,J. Sánchez-Barriga,A. Varykhalov,E. van Heumen,M. S. Golden
Physics , 2015, DOI: 10.1103/PhysRevB.92.075127
Abstract: Interfaces between a bulk-insulating topological insulator (TI) and metallic adatoms have been studied using high-resolution, angle-resolved and core-level photoemission. Fe, Nb and Ag were evaporated onto Bi1.5Sb0.5Te1.7Se1.3 (BSTS) surfaces both at room temperature and 38K. The coverage- and temperature-dependence of the adsorption and interfacial formation process have been investigated, highlighting the effects of the overlayer growth on the occupied electronic structure of the TI. For all coverages at room temperature and for those equivalent to less than 0.1 monolayer at low temperature all three metals lead to a downward shift of the TI's bands with respect to the Fermi level. At room temperature Ag appears to intercalate efficiently into the van der Waals gap of BSTS, accompanied by low-level substitution of the Te/Se atoms of the termination layer of the crystal. This Te/Se substitution with silver increases significantly for low temperature adsorption, and can even dominate the electrostatic environment of the Bi/Sb atoms in the BSTS near-surface region. On the other hand, Fe and Nb evaporants remain close to the termination layer of the crystal. On room temperature deposition, they initially substitute isoelectronically for Bi as a function of coverage, before substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too immobile for substitution processes and show a behaviour consistent with clustering on the surface. For both Ag and Fe/Nb, these differing adsorption pathways leads to the qualitatively similar and remarkable behavior for low temperature deposition that the chemical potential first moves upward (n-type dopant behavior) and then downward (p-type behavior) on increasing coverage.
Electronic structure and quantum criticality in Ba(Fe$_{1-x-y}$Co$_{x}$Mn$_{y}$)$_{2}$As$_{2}$, an ARPES study
E. D. L. Rienks,T. Wolf,K. Koepernik,I. Avigo,P. Hlawenka,C. Lupulescu,T. Arion,F. Roth,W. Eberhardt,U. Bovensiepen,J. Fink
Physics , 2013, DOI: 10.1209/0295-5075/103/47004
Abstract: We used angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations to study the electronic structure of Ba(Fe1-x-yCoxMny)2As2 for x=0.06 and 0<=y <=0.07. From ARPES we derive that the substitution of Fe by Mn does not lead to hole doping, indicating a localization of the induced holes. An evaluation of the measured spectral function does not indicate a diverging effective mass or scattering rate near optimal doping. Thus the present ARPES results indicate a continuous evolution of the quasiparticle interaction and therefore question previous quantum critical scenarios.
Samarium hexaboride: A trivial surface conductor
P. Hlawenka,K. Siemensmeyer,E. Weschke,A. Varykhalov,J. Sánchez-Barriga,N. Y. Shitsevalova,A. V. Dukhnenko,V. B. Filipov,S. Gabáni,K. Flachbart,O. Rader,E. D. L. Rienks
Physics , 2015,
Abstract: Recent theoretical and experimental studies suggest that SmB$_6$ is the first topological Kondo insulator: A material in which the interaction between localized and itinerant electrons renders the bulk insulating at low temperature, while topological surface states leave the surface metallic. While this would elegantly explain the material's puzzling conductivity, we find the experimentally observed candidates for both predicted topological surface states to be of trivial character instead: The surface state at $\bar{\Gamma}$ is very heavy and shallow with a mere $\sim 2$ meV binding energy. It exhibits large Rashba splitting which excludes a topological nature. We further demonstrate that the other metallic surface state, located at $\bar{X}$, is not an independent in-gap state as supposed previously, but part of a massive band with much higher binding energy (1.7 eV). We show that it remains metallic down to 1 K due to reduced hybridization with the energy-shifted surface 4$f$ level.
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.
Hybrid Fuzzy Controller Based Frequency Regulation in Restructured Power System  [PDF]
P. Anitha, P. Subburaj
Circuits and Systems (CS) , 2016, DOI: 10.4236/cs.2016.76065
Abstract: This paper discusses the implementation of Load Frequency Control (LFC) in restructured power system using Hybrid Fuzzy controller. The formulation of LFC in open energy market is much more challenging; hence it needs an intelligent controller to adapt the changes imposed by the dynamics of restructured bilateral contracts. Fuzzy Logic Control deals well with uncertainty and indistinctness while Particle Swarm Optimization (PSO) is a well-known optimization tool. Abovementioned techniques are combined and called as Hybrid Fuzzy to improve the dynamic performance of the system. Frequency control of restructured system has been achieved by automatic Membership Function (MF) tuned fuzzy logic controller. The parameters defining membership function has been tuned and updated from time to time using Particle Swarm Optimization (PSO). The robustness of the proposed hybrid fuzzy controller has been compared with conventional fuzzy logic controller using performance measures like overshoot and settling time following a step load perturbation. The motivation for using membership function tuning using PSO is to show the behavior of the controller for a wide range of system parameters and load changes. Error based analysis with parametric uncertainties and load changes is tested on a two-area restructured power system.
Integral Performance Criteria Based Analysis of Load Frequency Control in Bilateral Based Market  [PDF]
P. Anitha, P. Subburaj
Circuits and Systems (CS) , 2016, DOI: 10.4236/cs.2016.76086
Abstract: Performance index based analysis is made to examine and highlight the effective application of Particle Swarm Optimization (PSO) to optimize the Proportional Integral gains for Load Frequency Control (LFC) in a restructured power system that operates under Bilateral based policy scheme. Various Integral Performance Criteria measures are taken as fitness function in PSO and are compared using overshoot, settling time and frequency and tie-line power deviation following a step load perturbation (SLP). The motivation for using different fitness technique in PSO is to show the behavior of the controller for a wide range of system parameters and load changes. Error based analysis with parametric uncertainties and load changes are tested on a two-area restructured power system. The results of the proposed PSO based controller show the better performance compared to the classical Ziegler-Nichols (Z-N) tuned PI andFuzzy Rule based PI controller.
Projection of the Semi-Axes of the Ellipse of Intersection  [PDF]
P. P. Klein
Applied Mathematics (AM) , 2017, DOI: 10.4236/am.2017.89097
It is well known that the line of intersection of an ellipsoid and a plane is an ellipse (see for instance [1]). In this note the semi-axes of the ellipse of intersection will be projected from 3d space onto a 2d plane. It is shown that the projected semi-axes agree with results of a method used by Bektas [2] and also with results obtained by Schrantz [3].
黄金科学技术 , 1991,
Land Use and Land Cover Changes in a Tropical River Basin: A Case from Bharathapuzha River Basin, Southern India  [PDF]
P. P. Nikhil Raj, P. A. Azeez
Journal of Geographic Information System (JGIS) , 2010, DOI: 10.4236/jgis.2010.24026
Abstract: A study of the spatial and temporal changes in land use and land cover (LULC) was conducted using Remote Sensing and GIS. We analyzed the LULC of Bharathapuzha river basin, south India using multispectral LANDSAT imageries of 1973-2005 time periods. 31% depletion in the natural vegetation cover and 8.7% depletion in wetland agriculture area were seen in the basin during the period. On the other hand the urban spread in the basin increased by 32%. The study highlights the need for a scientific management plan for the sustainability of the river basin, keeping in view the recent climatic anomalies and hydrological conditions of the basin.
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