SIMES Publications
Enter search terms and sorting preferences below:
"Ultrafast formation of domain walls of a charge density wave in SmTe3" — M.Trigo: P. Giraldo-Gallo, J. N. Clark, M. E. Kozina, T. Henighan, M. P. Jiang, M. Chollet, I. R. Fisher, J. M. Glownia, T. Katayama, P. S. Kirchmann, D. Leuenberger, H. Liu, D. A. Reis, Z. X. Shen, and D. Zhu; Physical Review B, 02/15/21.
Show Abstract » | Show Additional Authors »
Additional Authors
P. Giraldo-Gallo, J. N. Clark, M. E. Kozina, T. Henighan, M. P. Jiang, M. Chollet, I. R. Fisher, J. M. Glownia, T. Katayama, P. S. Kirchmann, D. Leuenberger, H. Liu, D. A. Reis, Z. X. Shen, and D. Zhu
Abstract
We study ultrafast x-ray diffraction on the charge density wave (CDW) of SmTe3 using an x-ray free-electron laser. The high momentum and time resolution afforded by the x-ray laser enabled capturing fine wave-vector and time-dependent features of the CDW that originate from fast (in time) and sharp (in real space) variations of the CDW lattice distortion, which we attribute to an inversion of the order parameter. These domain inversions occur near the surface and are caused by the short penetration depth of the near-infrared pump with the wavelength centered at 800 nm, resulting in CDW domain walls perpendicular to the sample surface. These domain walls break the CDW long-range order on the scale of the x-ray probe depth, controlled experimentally by the x-ray incidence angle and suppress the diffraction intensity of the CDW for times much longer than the ∼1−ps recovery of the electronic gap observed in time and angle-resolved photoemission spectroscopy. We model the spatial and temporal dependences of the order parameter using a simple Ginzburg-Landau model with all the parameters obtained from the published literature. We find reasonable agreement between the calculated and the measured diffraction across the momentum, time, fluence, and incidence angle dependence without adjusting any parameters. We reconstruct the spatial and temporal dependences of the lattice order parameter and find that at long times, depending on the pump fluence, multiple domain walls remain at distances of a few nanometers from the surface.
"Time-resolved RIXS experiment with pulse-by-pulse parallel readout data collection using X-ray free electron laser" — H Lu: A. Gauthier, M. Hepting, A. S. Tremsin, A. H. Reid, P. S. Kirchmann, Z. X. Shen, T. P. Devereaux, Y. C. Shao, X. Feng, G. Coslovich, Z. Hussain, G. L. Dakovski, Y. D. Chuang & W. S. Lee; Scientific Reports, 12/17/20.
Show Abstract » | Show Additional Authors »
Additional Authors
A. Gauthier, M. Hepting, A. S. Tremsin, A. H. Reid, P. S. Kirchmann, Z. X. Shen, T. P. Devereaux, Y. C. Shao, X. Feng, G. Coslovich, Z. Hussain, G. L. Dakovski, Y. D. Chuang & W. S. Lee
Abstract
Time-resolved resonant inelastic X-ray scattering (RIXS) is one of the developing techniques enabled by the advent of X-ray free electron laser (FEL). It is important to evaluate how the FEL jitter, which is inherent in the self-amplified spontaneous emission process, influences the RIXS measurement. Here, we use a microchannel plate (MCP) based Timepix soft X-ray detector to conduct a time-resolved RIXS measurement at the Ti L3-edge on a charge-density-wave material TiSe2 . The fast parallel Timepix readout and single photon sensitivity enable pulse-by-pulse data acquisition and analysis. Due to the FEL jitter, low detection efficiency of spectrometer, and low quantum yield of RIXS process, we find that less than 2% of the X-ray FEL pulses produce signals, preventing acquiring sufficient data statistics while maintaining temporal and energy resolution in this measurement. These limitations can be mitigated by using future X-ray FELs with high repetition rates, approaching MHz such as the European XFEL in Germany and LCLS-II in the USA, as well as by utilizing advanced detectors, such as the prototype used in this study.
"Quantum-well states in fractured crystals of the heavy-fermion material CeCoIn5" — Nicolas Gauthier: Jonathan A. Sobota, Makoto Hashimoto, Heike Pfau, Dong-Hui Lu, Eric D. Bauer, Filip Ronning, Patrick S. Kirchmann, and Zhi-Xun Shen; Physical Review B, 09/08/20.
Show Abstract » | Show Additional Authors »
Additional Authors
Jonathan A. Sobota, Makoto Hashimoto, Heike Pfau, Dong-Hui Lu, Eric D. Bauer, Filip Ronning, Patrick S. Kirchmann, and Zhi-Xun Shen
Abstract
Quantum-well states appear in metallic thin films due to the confinement of the wave function by the film interfaces. Using angle-resolved photoemission spectroscopy, we unexpectedly observe quantum-well states in fractured single crystals of CeCoIn5. We confirm that confinement occurs by showing that these states’ binding energies are photon-energy independent and are well described with a phase accumulation model, commonly applied to quantum-well states in thin films. This indicates that atomically flat thin films can be formed by fracturing hard single crystals. For the two samples studied, our observations are explained by free-standing flakes with thicknesses of 206 and 101 Å. We extend our analysis to extract bulk properties of CeCoIn5. Specifically, we obtain the dispersion of a three-dimensional band near the zone center along in-plane and out-of-plane momenta. We establish part of its Fermi surface, which corresponds to a hole pocket centered at Γ. We also reveal a change of its dispersion with temperature, a signature that may be caused by the Kondo hybridization.
"Tuning time and energy resolution in time-resolved photoemission spectroscopy with nonlinear crystals" — Alexandre Gauthier: Jonathan A. Sobota, Nicolas Gauthier, Ke-Jun Xu, Heike Pfau, Costel R. Rotundu, Zhi-Xun Shen, and Patrick S. Kirchmann; Journal of Applied Physics, 09/01/20.
Show Abstract » | Show Additional Authors »
Additional Authors
Jonathan A. Sobota, Nicolas Gauthier, Ke-Jun Xu, Heike Pfau, Costel R. Rotundu, Zhi-Xun Shen, and Patrick S. Kirchmann
Abstract
Time- and angle-resolved photoemission spectroscopy is a powerful probe of electronic band structures out of equilibrium. Tuning time and energy resolution to suit a particular scientific question has become an increasingly important experimental consideration. Many instruments use cascaded frequency doubling in nonlinear crystals to generate the required ultraviolet probe pulses. We demonstrate how calculations clarify the relationship between laser bandwidth and nonlinear crystal thickness contributing to experimental resolutions and place intrinsic limits on the achievable time-bandwidth product. Experimentally, we tune time and energy resolution by varying the thickness of nonlinear β-BaB2O4 crystals for frequency upconversion, providing a flexible experiment design. We achieve time resolutions of 58–103 fs and corresponding energy resolutions of 55–27 meV. We propose a method to select crystal thickness based on desired experimental resolutions.
"A high-power, high-repetition-rate THz source for pump-probe experiments at Linac Coherent Light Source II" — Z. Zhang: A. S. Fisher, M. C. Hoffmann, B. Jacobson, P. S. Kirchmann, W.-S. Lee, A. Lindenberg, A. Marinelli, E. Nanni, R. Schoenlein, M. Qian, S. Sasaki, J. Xu and Z. Huang; Journal of Synchrotron Radiation, 06/15/20.
Show Abstract » | Show Additional Authors »
Additional Authors
A. S. Fisher, M. C. Hoffmann, B. Jacobson, P. S. Kirchmann, W.-S. Lee, A. Lindenberg, A. Marinelli, E. Nanni, R. Schoenlein, M. Qian, S. Sasaki, J. Xu and Z. Huang
Abstract
Experiments using a THz pump and an X-ray probe at an X-ray free-electron laser (XFEL) facility like the Linac Coherent Light Source II (LCLS II) require frequency-tunable (3 to 20 THz), narrow bandwidth (~10%), carrier-envelopephase-stable THz pulses that produce high fields (>1 MV cm-1) at the repetition rate of the X-rays and are well synchronized with them. In this paper, a two-bunch scheme to generate THz radiation at LCLS II is studied: the first bunch produces THz radiation in an electromagnet wiggler immediately following the LCLS II undulator that produces X-rays from the second bunch. The initial time delay between the two bunches is optimized to compensate for the path difference in THz transport. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the experimental hall.
"Low work function in the 122-family of iron-based superconductors" — H. Pfau: H. Soifer, J. A. Sobota, A. Gauthier, C. R. Rotundu, J. C. Palmstrom, I. R. Fisher, G.-Y. Chen, H.-H. Wen, Z.-X. Shen, and P. S. Kirchmann; Physical Review Materials, 03/04/20.
Show Abstract » | Show Additional Authors »
Additional Authors
H. Soifer, J. A. Sobota, A. Gauthier, C. R. Rotundu, J. C. Palmstrom, I. R. Fisher, G.-Y. Chen, H.-H. Wen, Z.-X. Shen, and P. S. Kirchmann
Abstract
We determine the work functions of the iron arsenic compounds AFe2As2 (A=Ca,Ba,Cs) using photoemission spectroscopy to be 2.7 eV for CaFe2As2, 1.8 eV for BaFe2As2, and 1.3 eV for CsFe2As2. The work functions of these 122 iron-based superconductors track those of the elementary metal A but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an A-layer. The low work function and good photoemission cross section of BaFe2As2 and CsFe2As2 enable photoemission even from a common white LED light.
"High-Harmonic Generation from Topological Insulators" — Denitsa Baykusheva: Jian Lu, Jonathan A. Sobota, Hadas Soifer, Costel R. Rotundu, Patrick S. Kirchmann, David A. Reis, and Shambhu Ghimire; Optical Society of America, 05/05/19.
Show Abstract » | Show Additional Authors »
Additional Authors
Jian Lu, Jonathan A. Sobota, Hadas Soifer, Costel R. Rotundu, Patrick S. Kirchmann, David A. Reis, and Shambhu Ghimire
Abstract
We report the observation of efficient high-harmonic generation in the three-dimensional topological insulators Bi2Se3 and Bi2Te3 driven by mid-infrared laser pulses of peak intensities ~1012 W/cm2.
"Mode-Selective Coupling of Coherent Phonons to the Bi2212 Electronic Band Structure" — S.-L. Yang: J. A. Sobota, Y. He, D. Leuenberger, H. Soifer, H. Eisaki, P. S. Kirchmann, and Z.-X. Shen; Physical Review Letters, 05/03/19.
Show Abstract » | Show Additional Authors »
Additional Authors
J. A. Sobota, Y. He, D. Leuenberger, H. Soifer, H. Eisaki, P. S. Kirchmann, and Z.-X. Shen
Abstract
Cuprate superconductors host a multitude of low-energy optical phonons. Using time- and angleresolved photoemission spectroscopy, we study coherent phonons inBi2Sr2Ca0.92Y0.08Cu2O8+δ. Sub-meV modulations of the electronic band structure are observed at frequencies of 3.94±0.01 and 5.59 ±0.06 THz. For the dominant mode at 3.94 THz, the amplitude of the band energy oscillation weakly increases as a function of momentum away from the node. Theoretical calculations allow identifying the observed modes as CuO2-derived A1g phonons. The Bi- and Sr-derived A1g modes which dominate Raman spectra in the relevant frequency range are absent in our measurements. This highlights the mode selectivity for phonons coupled to the near-Fermi-level electrons, which originate from CuO2 planes and dictate thermodynamic properties.
"Band-Resolved Imaging of Photocurrent in a Topological Insulator" — H. Soifer: A. Gauthier, A. F. Kemper, C. R. Rotundu, S.-L. Yang, H. Xiong, D. Lu, M. Hashimoto, P. S. Kirchmann, J. A. Sobota, and Z.-X. Shen; Physical Review Letters, 04/24/19.
Show Abstract » | Show Additional Authors »
Additional Authors
A. Gauthier, A. F. Kemper, C. R. Rotundu, S.-L. Yang, H. Xiong, D. Lu, M. Hashimoto, P. S. Kirchmann, J. A. Sobota, and Z.-X. Shen
Abstract
We study the microscopic origins of photocurrent generation in the topological insulator Bi2Se3 via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.
"Coherent order parameter dynamics in SmTe3" — M. Trigo: P. Giraldo-Gallo, M. E. Kozina, T. Henighan, M. P. Jiang, H. Liu, J. N. Clark, M. Chollet, J. M. Glownia, D. Zhu, T. Katayama, D. Leuenberger, P. S. Kirchmann, I. R. Fisher, Z. X. Shen, and D. A. Reis; Physical Review B, 03/28/19.
Show Abstract » | Show Additional Authors »
Additional Authors
P. Giraldo-Gallo, M. E. Kozina, T. Henighan, M. P. Jiang, H. Liu, J. N. Clark, M. Chollet, J. M. Glownia, D. Zhu, T. Katayama, D. Leuenberger, P. S. Kirchmann, I. R. Fisher, Z. X. Shen, and D. A. Reis
Abstract
We present ultrafast optical pump-probe and ultrafast x-ray diffraction measurements of the charge density wave dynamics in SmTe3 at 300 K. We performed ultrafast x-ray diffraction measurements at the Linac Coherent Light Source to directly probe the dynamics of the finite-wave-vector order parameter. The dynamics reveal coherent oscillations at ∼1.6 THz that become overdamped with increasing fluence. We identify this oscillation with the lattice component of the amplitude mode. Furthermore, our data allow for a clear identification of the amplitude mode frequency in the optical pump-probe data. In both measurements, the system reaches the symmetric phase at high fluence, where the order parameter vanishes and the response (reflectivity and x-ray intensity) is quadratic in the order parameter. This is observed in the x-ray diffraction as a small overdamped modulation near zero intensity. Similar overdamped features are observed in the optical reflectivity at high fluence. A time-dependent Ginzburg-Landau model captures qualitatively the essential features of the experimental observations.