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Search Results: 1 - 10 of 334020 matches for " Patrick S. Kirchmann "
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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.
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.
Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological Insulator Bi$_2$Se$_3$
Jonathan A. Sobota,Shuolong Yang,Alexander F. Kemper,J. J. Lee,Felix T. Schmitt,Wei Li,Robert G. Moore,James G. Analytis,Ian R. Fisher,Patrick S. Kirchmann,Thomas P. Devereaux,Zhi-Xun Shen
Physics , 2013, DOI: 10.1103/PhysRevLett.111.136802
Abstract: We characterize the occupied and unoccupied electronic structure of the topological insulator Bi$_2$Se$_3$ by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:Sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.
High Resolution Angle Resolved Photoemission with Tabletop 11eV Laser
Yu He,Inna Vishik,Ming Yi,Shuolong Yang,Zhongkai Liu,James Lee,Sudi Chen,Slavko Rebec,Dominik Leuenberger,Alfred Zong,Michael Jefferson,Robert Moore,Patrick Kirchmann,Andrew Merriam,Zhixun Shen
Physics , 2015,
Abstract: We developed a table-top vacuum ultraviolet (VUV) laser with $113.778$nm wavelength (10.897eV) and demonstrated its viability as a photon source for high resolution angle-resolved photoemission spectroscopy (ARPES). This sub-nanosecond pulsed VUV laser operates at a repetition rate of 10MHz, provides a flux of 2$\times$10$^{12}$ photons/second, and enables photoemission with energy and momentum resolutions better than 2meV and 0.012\AA$^{-1}$, respectively. Space-charge induced energy shifts and spectral broadenings can be reduced below 2meV. The setup reaches electron momenta up to 1.2\AA$^{-1}$, granting full access to the first Brillouin zone of most materials. Control over the linear polarization, repetition rate, and photon flux of the VUV source facilitates ARPES investigations of a broad range of quantum materials, bridging the application gap between contemporary low energy laser-based ARPES and synchrotron-based ARPES. We describe the principles and operational characteristics of this source, and showcase its performance for rare earth metal tritellurides, high temperature cuprate superconductors and iron-based superconductors.
Entecavir for treatment of chronic hepatitis B: A clinical update for the treatment of patients with decompensated cirrhosis  [PDF]
P. Patrick Basu, Robert S. Brown Jr.
Open Journal of Internal Medicine (OJIM) , 2012, DOI: 10.4236/ojim.2012.22012
Abstract: The introduction of nucleos(t)ide analogues for the treatment of chronic hepatitis B virus (HBV) infection was transformative in reducing morbidity and mortality. Entecavir, a potent selective nucleoside analogue first approved in 2005 for treatment of chronic HBV, is associated with significant antiviral, biochemical, serologic, and histologic responses. Rapid reductions in HBV DNA levels, low risk of resistance development, and a favorable adverse event profile have contributed to its clinical usefulness. Re-cent developments in the use of entecavir have increased its utility in the management of difficult-to-treat patients with chronic HBV, including those patients with decompensated liver disease. Recent studies in this population have demonstrated that entecavir 1.0 mg/d given for up to 48 weeks had superior antiviral activity when compared with adefovir and was generally safe and well tolerated. Long-term outcomes of entecavir in difficult-to-treat populations are eagerly anticipated.
A Reinterpretation of Historic Aquifer Tests of Two Hydraulically Fractured Wells by Application of Inverse Analysis, Derivative Analysis, and Diagnostic Plots  [PDF]
Patrick A. Hammond, Malcolm S. Field
Journal of Water Resource and Protection (JWARP) , 2014, DOI: 10.4236/jwarp.2014.65048
Abstract:

Aquifer test methods have greatly improved in recent years with the advent of inverse analysis, derivative analysis, and diagnostic plots. Updated analyses of past aquifer tests allow for improved interpretations of the data to enhance the knowledge and the predictive capabilities of the flow system. This work thoroughly reanalyzes a series of pre- and post-hydraulic fracturing, single-well aquifer tests conducted in two crystalline rock wells in New Hampshire as part of an early 1970’s study. Previous analyses of the data had relied on older manual type-curve methods for predicting the possible effects of hydraulic fracturing. This work applies inverse analysis, derivative analysis, and diagnostic plots to reanalyze the 1970’s aquifer test data. Our results demonstrate that the aquifer tests were affected by changes in flow regimes, dewatering of the aquifer and discrete fractures, and changes due to well development. Increases in transmissivities are related to well development prior to hydraulic fracturing, propagation of a single, vertical fracture hydraulically connecting the two wells after stimulation and expansion of troughs of depression. After hydraulic fracturing, the estimated total yield of the individual wells increased by 2.5 times due to the hydraulic fracturing. However, the wells may be receiving water from the same source, and well interference may affect any significant increase in their combined yield. Our analyses demonstrate the value in applying inverse analysis, derivative analysis, and diagnostic plots over the conventional method of manual type-curve analysis. In addition, our improvement in the aquifer test interpretation of the 1970’s test data has implications for more reliable estimates of sustained well yields.

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