Jonathan Sobota

All Publications

• Screening of Polar Electron-Phonon Interactions near the Surface of the Rashba Semiconductor BiTeCl PHYSICAL REVIEW LETTERS Qu, J., Cuddy, E. F., Han, X., Liu, J., Li, H., Zeng, Y., Moritz, B., Devereaux, T. P., Kirchmann, P. S., Shen, Z., Sobota, J. A. 2024; 133 (10): 106401

  Abstract

  Understanding electron-phonon coupling in noncentrosymmetric materials is critical for controlling the internal fields which give rise to Rashba interactions. We apply time- and angle-resolved photoemission spectroscopy (trARPES) to study coherent phonons in the surface and bulk regions of the polar semiconductor BiTeCl. Aided by ab initio calculations, our measurements reveal the coupling of out-of-plane A_{1} modes and an in-plane E_{2} mode. By considering how these modes modulate the electric dipole moment in each unit cell, we show that the polar A_{1} modes are more effectively screened in the metallic surface region, while the nonpolar E_{2} mode couples in both regions. In addition to informing strategies to optically manipulate Rashba interactions, this Letter has broader implications for the behavior of electron-phonon coupling in systems characterized by inhomogeneous dielectric environments.

  View details for DOI 10.1103/PhysRevLett.133.106401

  View details for Web of Science ID 001308077700003

  View details for PubMedID 39303246

• Orbital Ingredients and Persistent Dirac Surface State for the Topological Band Structure in FeTe_{0 . 55} Se_{0 . 45} PHYSICAL REVIEW X Li, Y., Chen, S., Garcia-diez, M., Iraola, M. I., Pfau, H., Zhu, Y., Mao, Z., Chen, T., Yi, M., Dai, P., Sobota, J. A., Hashimoto, M., Vergniory, M. G., Lu, D., Shen, Z. 2024; 14 (2)

  View details for DOI 10.1103/PhysRevX.14.021043

  View details for Web of Science ID 001245175600001

• Band structure of Bi surfaces formed on Bi2Se3 upon exposure to air PHYSICAL REVIEW MATERIALS Gauthier, A., Sobota, J. A., Gauthier, N., Rotundu, C. R., Shen, Z., Kirchmann, P. S. 2024; 8 (5)

  View details for DOI 10.1103/PhysRevMaterials.8.054201

  View details for Web of Science ID 001231926400007

• Coherent light control of a metastable hidden state. Science advances Maklar, J., Sarkar, J., Dong, S., Gerasimenko, Y. A., Pincelli, T., Beaulieu, S., Kirchmann, P. S., Sobota, J. A., Yang, S., Leuenberger, D., Moore, R. G., Shen, Z. X., Wolf, M., Mihailovic, D., Ernstorfer, R., Rettig, L. 2023; 9 (47): eadi4661

  Abstract

  Metastable phases present a promising route to expand the functionality of complex materials. Of particular interest are light-induced metastable phases that are inaccessible under equilibrium conditions, as they often host new, emergent properties switchable on ultrafast timescales. However, the processes governing the trajectories to such hidden phases remain largely unexplored. Here, using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast dynamics of the formation of a hidden quantum state in the layered dichalcogenide 1T-TaS2 upon photoexcitation. Our results reveal the nonthermal character of the transition governed by a collective charge-density-wave excitation. Using a double-pulse excitation of the structural mode, we show vibrational coherent control of the phase-transition efficiency. Our demonstration of exceptional control, switching speed, and stability of the hidden state are key for device applications at the nexus of electronics and photonics.

  View details for DOI 10.1126/sciadv.adi4661

  View details for PubMedID 38000022

• Development of deflector mode for spin-resolved time-of-flight photoemission spectroscopy. The Review of scientific instruments Han, X., Qu, J., Sakamoto, S., Liu, D., Guan, D., Liu, J., Li, H., Rotundu, C. R., Andresen, N., Jozwiak, C., Hussain, Z., Shen, Z. X., Sobota, J. A. 2023; 94 (10)

  Abstract

  Spin- and angle-resolved photoemission spectroscopy ("spin-ARPES") is a powerful technique for probing the spin degree-of-freedom in materials with nontrivial topology, magnetism, and strong correlations. Spin-ARPES faces severe experimental challenges compared to conventional ARPES attributed to the dramatically lower efficiency of its detection mechanism, making it crucial for instrumentation developments that improve the overall performance of the technique. In this paper, we demonstrate the functionality of our spin-ARPES setup based on time-of-flight spectroscopy and introduce our recent development of an electrostatic deflector mode to map out spin-resolved band structures without sample rotation. We demonstrate the functionality by presenting the spin-resolved spectra of the topological insulator Bi2Te3 and describe in detail the spectrum calibrations based on numerical simulations. By implementing the deflector mode, we minimize the need for sample rotation during measurements, hence improving the overall efficiency of experiments on small or inhomogeneous samples.

  View details for DOI 10.1063/5.0168447

  View details for PubMedID 37850856

• Reversal of spin-polarization near the Fermi level of the Rashba semiconductor BiTeCl NPJ QUANTUM MATERIALS Qu, J., Han, X., Sakamoto, S., Jia, C. J., Liu, J., Li, H., Guan, D., Zeng, Y., Schuler, M., Kirchmann, P. S., Moritz, B., Hussain, Z., Devereaux, T. P., Shen, Z., Sobota, J. A. 2023; 8 (1)

  View details for DOI 10.1038/s41535-023-00546-x

  View details for Web of Science ID 000941936700001

• Signatures of the exciton gas phase and its condensation in monolayer 1T-ZrTe2. Nature communications Song, Y., Jia, C., Xiong, H., Wang, B., Jiang, Z., Huang, K., Hwang, J., Li, Z., Hwang, C., Liu, Z., Shen, D., Sobota, J. A., Kirchmann, P., Xue, J., Devereaux, T. P., Mo, S. K., Shen, Z. X., Tang, S. 2023; 14 (1): 1116

  Abstract

  The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The material realization of EI has been challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the preformed exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking. Here we report a distinct correlated phase beyond the 2×2 CDW ground state emerging in monolayer 1T-ZrTe2 and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band- and energy-dependent folding behavior in a two-step process, which is the signatures of an exciton gas phase prior to its condensation into the final CDW state. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.

  View details for DOI 10.1038/s41467-023-36857-7

  View details for PubMedID 36849499

  View details for PubMedCentralID PMC9971207

• Influence of local symmetry on lattice dynamics coupled to topological surface states PHYSICAL REVIEW B Sobota, J. A., Teitelbaum, S. W., Huang, Y., Querales-Flores, J. D., Power, R., Allen, M., Rotundu, C. R., Bailey, T. P., Uher, C., Henighan, T., Jiang, M., Zhu, D., Chollet, M., Sato, T., Trigo, M., Murray, e. D., Savic, I., Kirchmann, P. S., Fahy, S., Reis, D. A., Shen, Z. 2023; 107 (1)

  View details for DOI 10.1103/PhysRevB.107.014305

  View details for Web of Science ID 000921610800004

• Probing topological phase transitions using high-harmonic generation NATURE PHOTONICS Heide, C., Kobayashi, Y., Baykusheva, D. R., Jain, D., Sobota, J. A., Hashimoto, M., Kirchmann, P. S., Oh, S., Heinz, T. F., Reis, D. A., Ghimire, S. 2022

  View details for DOI 10.1038/s41566-022-01050-7

  View details for Web of Science ID 000841689800001

• Connection between coherent phonons and electron-phonon coupling in Sb (111) PHYSICAL REVIEW B Sakamoto, S., Gauthier, N., Kirchmann, P. S., Sobota, J. A., Shen, Z. 2022; 105 (16)

  View details for DOI 10.1103/PhysRevB.105.L161107

  View details for Web of Science ID 000806812400002

• Expanding the momentum field of view in angle-resolved photoemission systems with hemispherical analyzers. The Review of scientific instruments Gauthier, N., Sobota, J. A., Pfau, H., Gauthier, A., Soifer, H., Bachmann, M. D., Fisher, I. R., Shen, Z. X., Kirchmann, P. S. 2021; 92 (12): 123907

  Abstract

  In photoelectron spectroscopy, the measured electron momentum range is intrinsically related to the excitation photon energy. Low photon energies <10 eV are commonly encountered in laser-based photoemission and lead to a momentum range that is smaller than the Brillouin zones of most materials. This can become a limiting factor when studying condensed matter with laser-based photoemission. An additional restriction is introduced by widely used hemispherical analyzers that record only electrons photoemitted in a solid angle set by the aperture size at the analyzer entrance. Here, we present an upgrade to increase the effective solid angle that is measured with a hemispherical analyzer. We achieve this by accelerating the photoelectrons toward the analyzer with an electric field that is generated by a bias voltage on the sample. Our experimental geometry is comparable to a parallel plate capacitor, and therefore, we approximate the electric field to be uniform along the photoelectron trajectory. With this assumption, we developed an analytic, parameter-free model that relates the measured angles to the electron momenta in the solid and verify its validity by comparing with experimental results on the charge density wave material TbTe3. By providing a larger field of view in momentum space, our approach using a bias potential considerably expands the flexibility of laser-based photoemission setups.

  View details for DOI 10.1063/5.0053479

  View details for PubMedID 34972440

• Expanding the momentum field of view in angle-resolved photoemission systems with hemispherical analyzers REVIEW OF SCIENTIFIC INSTRUMENTS Gauthier, N., Sobota, J. A., Pfau, H., Gauthier, A., Soifer, H., Bachmann, M. D., Fisher, I. R., Shen, Z., Kirchmann, P. S. 2021; 92 (12)

  View details for DOI 10.1063/5.0053479

  View details for Web of Science ID 000731435700002

• All-Optical Probe of Three-Dimensional Topological Insulators Based on High-Harmonic Generation by Circularly Polarized Laser Fields. Nano letters Baykusheva, D., Chacon, A., Lu, J., Bailey, T. P., Sobota, J. A., Soifer, H., Kirchmann, P. S., Rotundu, C., Uher, C., Heinz, T. F., Reis, D. A., Ghimire, S. 2021

  Abstract

  We report the observation of an anomalous nonlinear optical response of the prototypical three-dimensional topological insulator bismuth selenide through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation.

  View details for DOI 10.1021/acs.nanolett.1c02145

  View details for PubMedID 34676752

• Strong correlations and orbital texture in single-layer 1T-TaSe₂ (vol 16, pg 218, 2020) NATURE PHYSICS Chen, Y., Ruan, W., Wu, M., Tang, S., Ryu, H., Tsai, H., Lee, R. L., Kahn, S., Liou, F., Jia, C., Albertini, O. R., Xiong, H., Jia, T., Liu, Z., Sobota, J. A., Liu, A. Y., Moore, J. E., Shen, Z., Louie, S. G., Mo, S., Crommie, M. F. 2021; 17 (7): 867

  View details for DOI 10.1038/s41567-021-01283-3

  View details for Web of Science ID 000657589300001

• Angle-resolved photoemission studies of quantum materials REVIEWS OF MODERN PHYSICS Sobota, J. A., He, Y., Shen, Z. 2021; 93 (2)

  View details for DOI 10.1103/RevModPhys.93.025006

  View details for Web of Science ID 000655978600001

• Quantum-well states in fractured crystals of the heavy-fermion material CeCoIn5 PHYSICAL REVIEW B Gauthier, N., Sobota, J. A., Hashimoto, M., Pfau, H., Lu, D., Bauer, E. D., Ronning, F., Kirchmann, P. S., Shen, Z. 2020; 102 (12)

  View details for DOI 10.1103/PhysRevB.102.125111

  View details for Web of Science ID 000566891800007

• Tuning time and energy resolution in time-resolved photoemission spectroscopy with nonlinear crystals JOURNAL OF APPLIED PHYSICS Gauthier, A., Sobota, J. A., Gauthier, N., Xu, K., Pfau, H., Rotundu, C. R., Shen, Z., Kirchmann, P. S. 2020; 128 (9)

  View details for DOI 10.1063/5.0018834

  View details for Web of Science ID 000567597300001

• Low work function in the 122-family of iron-based superconductors PHYSICAL REVIEW MATERIALS Pfau, H., Soifer, H., Sobota, J. A., Gauthier, A., Rotundu, C. R., Palmstrom, J. C., Fisher, I. R., Chen, G., Wen, H., Shen, Z., Kirchmann, P. S. 2020; 4 (3)

  View details for DOI 10.1103/PhysRevMaterials.4.034801

  View details for Web of Science ID 000517972500004

• Strong correlations and orbital texture in single-layer 1T-TaSe2 NATURE PHYSICS Chen, Y., Ruan, W., Wu, M., Tang, S., Ryu, H., Tsai, H., Lee, R., Kahn, S., Liou, F., Jia, C., Albertini, O. R., Xiong, H., Jia, T., Liu, Z., Sobota, J. A., Liu, A. Y., Moore, J. E., Shen, Z., Louie, S. G., Mo, S., Crommie, M. F. 2020

  View details for DOI 10.1038/s41567-019-0744-9

  View details for Web of Science ID 000508145800003

• Mode-Selective Coupling of Coherent Phonons to the Bi2212 Electronic Band Structure PHYSICAL REVIEW LETTERS Yang, S., Sobota, J. A., He, Y., Leuenberger, D., Soifer, H., Eisaki, H., Kirchmann, P. S., Shen, Z. 2019; 122 (17): 176403

  Abstract

  Cuprate superconductors host a multitude of low-energy optical phonons. Using time- and angle-resolved photoemission spectroscopy, we study coherent phonons in Bi_{2}Sr_{2}Ca_{0.92}Y_{0.08}Cu_{2}O_{8+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 CuO_{2}-derived A_{1g} phonons. The Bi- and Sr-derived A_{1g} 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 CuO_{2} planes and dictate thermodynamic properties.

  View details for DOI 10.1103/PhysRevLett.122.176403

  View details for Web of Science ID 000467042000022

  View details for PubMedID 31107058

• Band-Resolved Imaging of Photocurrent in a Topological Insulator PHYSICAL REVIEW LETTERS Soifer, H., Gauthier, A., Kemper, A. F., Rotundu, C. R., Yang, S., Xiong, H., Lu, D., Hashimoto, M., Kirchmann, P. S., Sobota, J. A., Shen, Z. 2019; 122 (16): 167401

  Abstract

  We study the microscopic origins of photocurrent generation in the topological insulator Bi_{2}Se_{3} 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.

  View details for DOI 10.1103/PhysRevLett.122.167401

  View details for Web of Science ID 000466440000009

  View details for PubMedID 31075004

• High-Harmonic Generation from Topological Insulators Baykusheva, D., Lu, J., Sobota, J. A., Soifer, H., Rotundu, C. R., Kirchmann, P. S., Reis, D. A., Ghimire, S., IEEE IEEE. 2019

  View details for Web of Science ID 000482226301135

• Anomalous Hall effect in ZrTe5 NATURE PHYSICS Liang, T., Lin, J., Gibson, Q., Kushwaha, S., Liu, M., Wang, W., Xiong, H., Sobota, J. A., Hashimoto, M., Kirchmann, P. S., Shen, Z., Cava, R. J., Ong, N. P. 2018; 14 (5): 451-+

  View details for DOI 10.1038/s41567-018-0078-z

  View details for Web of Science ID 000431301800016

• Revealing the Coulomb interaction strength in a cuprate superconductor PHYSICAL REVIEW B Yang, S., Sobota, J. A., He, Y., Wang, Y., Leuenberger, D., Soifer, H., Hashimoto, M., Lu, D. H., Eisaki, H., Moritz, B., Devereaux, T. P., Kirchmann, P. S., Shen, Z. 2017; 96 (24)

  View details for DOI 10.1103/PhysRevB.96.245112

  View details for Web of Science ID 000417487400004

• Taking control of spin currents NATURE Shen, Z., Sobota, J. 2017; 549 (7673): 464–65

  View details for PubMedID 28959972

• Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser SCIENCE Gerber, S., Yang, S., Zhu, D., Soifer, H., Sobota, J. A., Rebec, S., Lee, J. J., Jia, T., Moritz, B., Jia, C., Gauthier, A., Li, Y., Leuenberger, D., Zhang, Y., Chaix, L., Li, W., Jang, H., Lee, J., Yi, M., Dakovski, G. L., Song, S., Glownia, J. M., Nelson, S., Kim, K. W., Chuang, Y., Hussain, Z., Moore, R. G., Devereaux, T. P., Lee, W., Kirchmann, P. S., Shen, Z. 2017; 357 (6346): 71–74

  Abstract

  The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a "coherent lock-in" measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.

  View details for PubMedID 28684521

• Three-dimensional nature of the band structure of ZrTe5 measured by high-momentum-resolution photoemission spectroscopy PHYSICAL REVIEW B Xiong, H., Sobota, J. A., Yang, S., Soifer, H., Gauthier, A., Lu, M., Lv, Y., Yao, S., Lu, D., Hashimoto, M., Kirchmann, P. S., Chen, Y., Shen, Z. 2017; 95 (19)

  View details for DOI 10.1103/PhysRevB.95.195119

  View details for Web of Science ID 000401229800001

• Spin-polarized surface resonances accompanying topological surface state formation NATURE COMMUNICATIONS Jozwiak, C., Sobota, J. A., Gotlieb, K., Kemper, A. F., Rotundu, C. R., Birgeneau, R. J., Hussain, Z., Lee, D., Shen, Z., Lanzara, A. 2016; 7

  Abstract

  Topological insulators host spin-polarized surface states born out of the energetic inversion of bulk bands driven by the spin-orbit interaction. Here we discover previously unidentified consequences of band-inversion on the surface electronic structure of the topological insulator Bi2Se3. By performing simultaneous spin, time, and angle-resolved photoemission spectroscopy, we map the spin-polarized unoccupied electronic structure and identify a surface resonance which is distinct from the topological surface state, yet shares a similar spin-orbital texture with opposite orientation. Its momentum dependence and spin texture imply an intimate connection with the topological surface state. Calculations show these two distinct states can emerge from trivial Rashba-like states that change topology through the spin-orbit-induced band inversion. This work thus provides a compelling view of the coevolution of surface states through a topological phase transition, enabled by the unique capability of directly measuring the spin-polarized unoccupied band structure.

  View details for DOI 10.1038/ncomms13143

  View details for Web of Science ID 000385548800001

  View details for PubMedID 27739428

  View details for PubMedCentralID PMC5067600

• Inequivalence of Single-Particle and Population Lifetimes in a Cuprate Superconductor PHYSICAL REVIEW LETTERS Yang, S., Sobota, J. A., Leuenberger, D., He, Y., Hashimoto, M., Lu, D. H., Eisaki, H., Kirchmann, P. S., Shen, Z. 2015; 114 (24)

  Abstract

  We study optimally doped Bi-2212 (T(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 1-2 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.

  View details for DOI 10.1103/PhysRevLett.114.247001

  View details for Web of Science ID 000356132700010

• Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO3 Films NANO LETTERS Yang, S., Sobota, J. A., Leuenberger, D., Kemper, A. F., Lee, J. J., Schmitt, F. T., Li, W., Moore, R. G., Kirchmann, P. S., Shen, Z. 2015; 15 (6): 4150-4154

  Abstract

  Ultrathin FeSe films grown on SrTiO3 substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO3 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 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A1g 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.

  View details for DOI 10.1021/acs.nanolett.5b01274

  View details for Web of Science ID 000356316900074

  View details for PubMedID 26027951

• Classification of collective modes in a charge density wave by momentum-dependent modulation of the electronic band structure PHYSICAL REVIEW B Leuenberger, D., Sobota, J. A., Yang, S., Kemper, A. F., Giraldo-Gallo, P., Moore, R. G., Fisher, I. R., Kirchmann, P. S., Devereaux, T. P., Shen, Z. 2015; 91 (20)

  View details for DOI 10.1103/PhysRevB.91.201106

  View details for Web of Science ID 000354985300001

• Distinguishing Bulk and Surface Electron-Phonon Coupling in the Topological Insulator Bi2Se3 Using Time-Resolved Photoemission Spectroscopy PHYSICAL REVIEW LETTERS Sobota, J. A., Yang, S., Leuenberger, D., Kemper, A. F., Analytis, J. G., Fisher, I. R., Kirchmann, P. S., Devereaux, T. P., Shen, Z. 2014; 113 (15)

  Abstract

  We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi(2)Se(3). 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 A(1g) 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. Density functional theory calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A(1g) phonon mode coupling to the surface state in a topological insulator.

  View details for DOI 10.1103/PhysRevLett.113.157401

  View details for Web of Science ID 000344052200011

• Ultrafast electron dynamics in the topological insulator Bi2Se3 studied by time-resolved photoemission spectroscopy JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA Sobota, J. A., Yang, S., Leuenberger, D., Kemper, A. F., Analytis, J. G., Fisher, I. R., Kirchmann, P. S., Devereaux, T. P., Shen, Z. 2014; 195: 249-257

  View details for DOI 10.1016/j.elspec.2014.01.005

  View details for Web of Science ID 000342872800036

• Electron propagation from a photo-excited surface: implications for time-resolved photoemission APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING Yang, S., Sobota, J. A., Kirchmann, P. S., Shen, Z. 2014; 116 (1): 85-90

  View details for DOI 10.1007/s00339-013-8154-9

  View details for Web of Science ID 000338214300012

• Superconducting graphene sheets in CaC6 enabled by phonon-mediated interband interactions NATURE COMMUNICATIONS Yang, S., Sobota, J. A., Howard, C. A., Pickard, C. J., Hashimoto, M., Lu, D. H., Mo, S., Kirchmann, P. S., Shen, Z. 2014; 5

  Abstract

  There is a great deal of fundamental and practical interest in the possibility of inducing superconductivity in a monolayer of graphene. But while bulk graphite can be made to superconduct when certain metal atoms are intercalated between its graphene sheets, the same has not been achieved in a single layer. Moreover, there is a considerable debate about the precise mechanism of superconductivity in intercalated graphite. Here we report angle-resolved photoelectron spectroscopy measurements of the superconducting graphite intercalation compound CaC6 that distinctly resolve both its intercalant-derived interlayer band and its graphene-derived π* band. Our results indicate the opening of a superconducting gap in the π* band and reveal a substantial contribution to the total electron-phonon-coupling strength from the π*-interlayer interband interaction. Combined with theoretical predictions, these results provide a complete account for the superconducting mechanism in graphite intercalation compounds and lend support to the idea of realizing superconducting graphene by creating an adatom superlattice.

  View details for DOI 10.1038/ncomms4493

  View details for Web of Science ID 000334301700004

  View details for PubMedID 24651261

  View details for PubMedCentralID PMC3973042

• Discovery of a single topological Dirac fermion in the strong inversion asymmetric compound BiTeCl NATURE PHYSICS Chen, Y. L., Kanou, M., Liu, Z. K., Zhang, H. J., Sobota, J. A., Leuenberger, D., Mo, S. K., Zhou, B., Yang, S., Kirchmann, P. S., Lu, D. H., Moore, R. G., Hussain, Z., Shen, Z. X., Qi, X. L., Sasagawa, T. 2013; 9 (11): 704-708

  View details for DOI 10.1038/NPHYS2768

  View details for Web of Science ID 000326685000010

• Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological Insulator Bi2Se3 PHYSICAL REVIEW LETTERS Sobota, J. A., Yang, S., Kemper, A. F., Lee, J. J., Schmitt, F. T., Li, W., Moore, R. G., Analytis, J. G., Fisher, I. R., Kirchmann, P. S., Devereaux, T. P., Shen, Z. 2013; 111 (13)

  Abstract

  We characterize the occupied and unoccupied electronic structure of the topological insulator Bi2Se3 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.

  View details for DOI 10.1103/PhysRevLett.111.136802

  View details for PubMedID 24116801

• Electronic structure of the metallic antiferromagnet PdCrO2 measured by angle-resolved photoemission spectroscopy PHYSICAL REVIEW B Sobota, J. A., Kim, K., Takatsu, H., Hashimoto, M., Mo, S., Hussain, Z., Oguchi, T., Shishidou, T., Maeno, Y., Min, B. I., Shen, Z. 2013; 88 (12)

  View details for DOI 10.1103/PhysRevB.88.125109

  View details for Web of Science ID 000324051700006

• Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3 PHYSICAL REVIEW LETTERS Sobota, J. A., Yang, S., Analytis, J. G., Chen, Y. L., Fisher, I. R., Kirchmann, P. S., Shen, Z. 2012; 108 (11)

  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 metastable population at the bulk conduction band edge, which feeds a nonequilibrium population of the surface state persisting for >10 ps. 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.

  View details for DOI 10.1103/PhysRevLett.108.117403

  View details for Web of Science ID 000301478400014

  View details for PubMedID 22540508

• RKKY interactions in the regime of strong localization PHYSICAL REVIEW B Sobota, J. A., Tanaskovic, D., Dobrosavljevic, V. 2007; 76 (24)

  View details for DOI 10.1103/PhysRevB.76.245106

  View details for Web of Science ID 000251986600022