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Search Results: 1 - 10 of 470408 matches for " Jonathan A. Sobota "
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Electron propagation from a photo-excited surface: implications for time-resolved photoemission
Shuolong Yang,Jonathan A. Sobota,Patrick S. Kirchmann,Zhi-Xun Shen
Physics , 2013, DOI: 10.1007/s00339-013-8154-9
Abstract: We perform time- and angle-resolved photoelectron spectroscopy on p-type GaAs(110). We observe an optically excited population in the conduction band, from which the time scales of intraband relaxation and surface photovoltage decay are both extracted. Moreover, the photovoltage shift of the valence band intriguingly persists for hundreds of picoseconds at negative delays. By comparing to a recent theoretical study, we reveal that the negative-delay dynamics reflects the interaction of the photoelectrons with a photovoltage-induced electric field outside the sample surface. We develop a conceptual framework to disentangle the intrinsic electron dynamics from this long-range field effect, which sets the foundation for understanding time-resolved photoemission experiments on a broad range of materials in which poor electronic screening leads to surface photovoltage. Finally, we demonstrate how the long-lasting negative-delay dynamics in GaAs can be utilized to conveniently establish the temporal overlap of pump and probe pulses in a time-resolved photoemission setup.
Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3
Jonathan A. Sobota,Shuolong Yang,James G. Analytis,Yulin Chen,Ian R. Fisher,Patrick S. Kirchmann,Z. -X. Shen
Physics , 2012, DOI: 10.1103/PhysRevLett.108.117403
Abstract: Using femtosecond time- and angle- resolved photoemission spectroscopy, we investigated the nonequilibrium dynamics of the topological insulator Bi2Se3. We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a meta-stable population at the bulk conduction band edge, which feeds a nonequilibrium population of the surface state persisting for >10ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.
Study on the process of single-screw extrusion-cooking of mixtures with a content of pea hulls
Z. Rzedzicki,A. Sobota
International Agrophysics , 2006,
Abstract: A study was conducted on the possibility of application of pea hulls as a valuable source of dietary fibre and of biologically active compounds for the production of cereal extrudates. The research was performed using a single-screw extrusion-cooker with a compression ratio of 3:1 and L:D=12:1, using the following cereal materials: commercially available corn grits, pea hulls of cv. Opal, and oats bran. The aim of the study was to examine the effect of the leguminous material and of the process parameters (profile of barrel temperature distribution and raw material moisture) on the possibility of stabilization of extrusion conditions, physical properties of the extrudates, and on the sensory features of the product. The study showed that the technology of single-screw extrusion-cooking can be applied to process mixtures containing maximum 15% of pea hulls; higher levels of the pea hulls content caused a disturbance of the stability of the process and so-called 'material slippage'. Increase of the pea hulls content in the mixture above 10% caused a decrease in radial expansion and an increase in specific density even up to 140 kgm-3. Moderate introduction of pea hulls did not result in any deterioration in the texture of the extrudates. The physical properties of the extrudates were significantly affected by the temperature of extrusion and by the raw material moisture content. Increased process temperature resulted in a decrease in radial expansion with simultaneous decrease in specific density of products, increase in crispness and decrease in the WAI of the extrudates. Increase in the moisture of the processed material, on the other hand, resulted in lower radial expansion, increased specific density, and increased water absorption index (WAI) of grinded extrudates. Admixture of pea hulls in amounts up to 7.5% did not cause any deterioration in the sensory features of the extrudate.
Effect of the extrusion process of corn semolina and pea hulls blends on chemical composition and selected physical properties of the extrudate
A. Sobota,Z. Rzedzicki
International Agrophysics , 2009,
Abstract: A study was conducted on the process of extrusion of blends of corn semolina and pea hulls to determine the effect of the share of high-fibre material and of parameters of the process of extrusion eg extruder barrel temperature and material moisture content, on the solubility of dry matter, viscosity, and content of fundamental chemical components – proteins, fats, and dietary fibre in the extrudates. Detailed analysis were performed for changes taking place in the dietary fibre fractions. Also analyzed were changes in the content of total dietary fibre as determined with the enzymatic method, including the fractions of soluble dietary fibre and insoluble dietary fibre. The study showed that extrusion cooking leads to a reduction in the content of crude fibre and free fats, and to a slight reduction in the level of proteins in the processed material. No significant changes were observed in the content of ash. Extrusion cooking resulted in decreased content of neutral-detergent fibre, acid-detergent fibre and cellulose fractions; at the same time a reduction was observed in the content of insoluble dietary fibre and an increase in the content of soluble dietary fibre fractions. It was found that intensive parameters of the process of extrusion (low moisture content of material and high process temperature) resulted in increased water solubility index values for the extrudates. The highest viscosity (after heating and cooling of suspension to 20°C) was characteristic of products obtained with the application of process temperature of 125°C.
Model of urban public transport network for the analysis of punctuality
A. Sobota,R. ?ochowska
Journal of Achievements in Materials and Manufacturing Engineering , 2008,
Abstract: Purpose: The building of mathematic model, which enable description of urban public transport network is a purpose of the article. This model will make the estimation of public transport quality according to analysis of punctuality easier.Design/methodology/approach: The methodology of building the mathematical model of public transport network consists in application of the graph theory. The mathematical instruments make the description of real network easier.Findings: The mathematical model of public transport network on the proper level of aggregation may be effective instrument, which assists the decision process. It makes an estimation of chosen coefficients that describe public transport quality easier.Research limitations/implications: The research has been conducted only in the range of punctuality estimation for the sake of time-consuming and expenses. This limit also concerns database. It requires detailed description of public transport network to exact presentation in mathematic model.Originality/value: To make the calculations more efficient, the relative database has been built. It includes geometric parameters of the sections and junctions in network. It also covers all necessary detailed data of analysed public transport lines. The database is the ground for estimating punctuality in research area.
Electronic Structure of the Metallic Antiferromagnet PdCrO$_2$ Measured by Angle-Resolved Photoemission Spectroscopy
Jonathan A. Sobota,Kyoo Kim,Hiroshi Takatsu,Makoto Hashimoto,Sung-Kwan Mo,Zahid Hussain,Tamio Oguchi,Tatsuya Shishidou,Yoshiteru Maeno,Byung Il Min,Zhi-Xun Shen
Physics , 2013, DOI: 10.1103/PhysRevB.88.125109
Abstract: PdCrO$_2$ is material which has attracted interest due to the coexistence of metallic conductivity associated with itinerant Pd 4d electrons and antiferromagnetic order arising from localized Cr spins. A central issue is determining to what extent the magnetic order couples to the conduction electrons. Here we perform angle-resolved photoemission spectroscopy (ARPES) to experimentally characterize the electronic structure. We find that the Fermi surface has contributions from both bulk and surface states, which can be experimentally distinguished and theoretically verified by slab band structure calculations. The bulk Fermi surface shows no signature of electronic reconstruction in the antiferromagnetic state. This observation suggests that there is negligible interaction between the localized Cr spin structure and the itinerant Pd electrons measured by ARPES.
Distinguishing bulk and surface electron-phonon coupling in the topological insulator Bi2Se3 using time-resolved photoemission spectroscopy
Jonathan A. Sobota,Shuolong Yang,Dominik Leuenberger,Alexander F. Kemper,James G. Analytis,Ian R. Fisher,Patrick S. Kirchmann,Thomas P. Devereaux,Zhi-Xun Shen
Physics , 2014, DOI: 10.1103/PhysRevLett.113.157401
Abstract: We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi2Se3. We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A1g phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. DFT calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A1g phonon mode coupling to the surface state in a topological insulator.
Ultrafast Electron Dynamics in the Topological Insulator Bi2Se3 Studied by Time-Resolved Photoemission Spectroscopy
Jonathan A. Sobota,Shuolong Yang,Dominik Leuenberger,Alexander F. Kemper,James G. Analytis,Ian R. Fisher,Patrick S. Kirchmann,Thomas P. Devereaux,Zhi-Xun Shen
Physics , 2014, DOI: 10.1016/j.elspec.2014.01.005
Abstract: We characterize the topological insulator Bi$_2$Se$_3$ using time- and angle- resolved photoemission spectroscopy. By employing two-photon photoemission, a complete picture of the unoccupied electronic structure from the Fermi level up to the vacuum level is obtained. We demonstrate that the unoccupied states host a second, Dirac surface state which can be resonantly excited by 1.5 eV photons. We then study the ultrafast relaxation processes following optical excitation. We find that they culminate in a persistent non-equilibrium population of the first Dirac surface state, which is maintained by a meta-stable population of the bulk conduction band. Finally, we perform a temperature-dependent study of the electron-phonon scattering processes in the conduction band, and find the unexpected result that their rates decrease with increasing sample temperature. We develop a model of phonon emission and absorption from a population of electrons, and show that this counter-intuitive trend is the natural consequence of fundamental electron-phonon scattering processes. This analysis serves as an important reminder that the decay rates extracted by time-resolved photoemission are not in general equal to single electron scattering rates, but include contributions from filling and emptying processes from a continuum of states.
Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO$_{3}$ Films
Shuolong Yang,Jonathan A. Sobota,Dominik Leuenberger,Alexander F. Kemper,James J. Lee,Felix T. Schmitt,Wei Li,Rob G. Moore,Patrick S. Kirchmann,Zhi-Xun Shen
Physics , 2015, DOI: 10.1021/acs.nanolett.5b01274
Abstract: Ultrathin FeSe films grown on SrTiO$_{3}$ substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. Using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO$_{3}$ films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1 unit cell, 3 unit cell, and 60 unit cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00(2) to 5.25(2) THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A$_\textrm{1g}$ phonon. The dominant mechanism for the phonon softening in 1 unit cell thick FeSe films is a substrate-induced lattice strain. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.
Inequivalence of Single-Particle and Population Lifetimes in a Cuprate Superconductor
Shuolong Yang,Jonathan A. Sobota,Dominik Leuenberger,Yu He,Makoto Hashimoto,Donghui Lu,Hiroshi Eisaki,Patrick S. Kirchmann,Zhi-Xun Shen
Physics , 2015,
Abstract: We study optimally doped Bi-2212 ($T_\textrm{c}=96$~K) using femtosecond time- and angle-resolved photoelectron spectroscopy. Energy-resolved population lifetimes are extracted and compared with single-particle lifetimes measured by equilibrium photoemission. The population lifetimes deviate from the single-particle lifetimes in the low excitation limit by one to two orders of magnitude. Fundamental considerations of electron scattering unveil that these two lifetimes are in general distinct, yet for systems with only electron-phonon scattering they should converge in the low-temperature, low-fluence limit. The qualitative disparity in our data, even in this limit, suggests that scattering channels beyond electron-phonon interactions play a significant role in the electron dynamics of cuprate superconductors.
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