- Physical Review Journals
All JournalsPhysics Magazine

Physical Review X

Reuse & Permissions

It is not necessary to obtain permission to reuse this article or its components as it is available under the terms of the Creative Commons Attribution 4.0 International license. This license permits unrestricted use, distribution, and reproduction in any medium, provided attribution to the author(s) and the published article's title, journal citation, and DOI are maintained. Please note that some figures may have been included with permission from other third parties. It is your responsibility to obtain the proper permission from the rights holder directly for these figures.

Export citation

Export citation

Choose format for download:

Download Citation
  • Open Access

Phonon-Activated Collective Hopping in a Superionic Phase

Hung-Min Lin1, Mayanak K. Gupta2,3, Niuchang Ouyang4, Tyler Wilson5, Artem Pogodin6, Md Towhidur Rahman7, Alexandra Zevalkink8, J. Ross Stewart9, Fanni Juranyi10 et al.

Tao Hong11, Yan Wu11, Songxue Chi11, Douglas L. Abernathy11, and Olivier Delaire1,4,5

PDF

Phys. Rev. X 16, 031046 – Published 21 August, 2026

DOI: https://doi.org/10.1103/lx5w-x6wl

Abstract

Superionic conductors are fascinating systems representing a state of matter intermediate between solids and liquids, yet their underlying physics remain poorly understood. Correlations in jump dynamics have been theoretically proposed as a key mechanism underlying superionic diffusion, but a direct experimental demonstration remains elusive. Here, we report the direct observation of collective cation hopping in the argyrodite compound Cu7PS6, using single-crystal neutron scattering to access the full four-dimensional space-time correlations of ionic motion. By leveraging the rarely used coherent scattering channel in quasielastic neutron scattering, together with diffuse scattering, we reveal pronounced short-range dynamical correlations between diffusing Cu ions, providing direct experimental evidence of collective hopping in a superionic phase. Machine-learned molecular dynamics simulations quantitatively reproduce all our measurements and pinpoint the dominant collective hopping mechanism. Furthermore, we establish through both experiments and simulations that the collective diffusive hops originate from the breakdown of low-energy phonons dominated by the Cu motions in the parent ordered phase, demonstrating a key link between phonon precursors and the activation of facile collective hops. This work presents the first direct experimental evidence of collective hopping in superionic argyrodites and identifies wave-vector-resolved soft phonons as its physical origin, advancing the microscopic understanding of ionic transport in superionic solids.

View figure in article

Physics Subject Headings (PhySH)

Popular Summary

Superionic conductors are unique materials in which ions diffuse through a crystalline framework as fast as in a liquid, making them attractive for applications such as solid-state batteries, fuel cells, iontronics, and thermoelectrics. Theory has long suggested that fast ionic transport in these materials arises from collective diffusion, but direct experimental evidence has been lacking. We provide observations of collective ionic hopping in a superionic phase and show that it is driven by specific vibration modes of the parent phase. By experimentally establishing collective hopping and identifying its vibrational origin, we uncover a key mechanism of superionic conduction. This provides a direct path for designing and optimizing superionic materials for a variety of applications.

Article Text

Supplemental Material

References (82)

  1. James B. Boyce and Bernardo A. Huberman, Superionic conductors: Transitions, structures, dynamics, Phys. Rep. 51, 189 (1979).
  2. Stephen Hull, Superionics: Crystal structures and conduction processes, Rep. Prog. Phys. 67, 1233 (2004).
  3. Noriaki Kamaya, Kenji Homma, Yuichiro Yamakawa, Masaaki Hirayama, Ryoji Kanno, Masao Yonemura, Takashi Kamiyama, Yuki Kato, Shigenori Hama, Koji Kawamoto et al., A lithium superionic conductor, Nat. Mater. 10, 682 (2011).
  4. Sossina M. Haile, Dane A. Boysen, Calum R. I. Chisholm, and Ryan B. Merle, Solid acids as fuel cell electrolytes, Nature (London) 410, 910 (2001).
  5. Yuntong Zhu, Juan Carlos Gonzalez-Rosillo, Moran Balaish, Zachary D. Hood, Kun Joong Kim, and Jennifer L. M. Rupp, Lithium-film ceramics for solid-state lithionic devices, Nat. Rev. Mater. 6, 313 (2021).
  6. KyuJung Jun, Yu Chen, Grace Wei, Xiaochen Yang, and Gerbrand Ceder, Diffusion mechanisms of fast lithium-ion conductors, Nat. Rev. Mater. 9, 887 (2024).
  7. Guopeng Han, Andrij Vasylenko, Luke M. Daniels, Chris M. Collins, Lucia Corti, Ruiyong Chen, Hongjun Niu, Troy D. Manning, Dmytro Antypov, Matthew S. Dyer et al., Superionic lithium transport via multiple coordination environments defined by two-anion packing, Science 383, 739 (2024).
  8. Zhantao Liu, Po-Hsiu Chien, Shuo Wang, Shaowei Song, Mu Lu, Shuo Chen, Shuman Xia, Jue Liu, Yifei Mo, and Hailong Chen, Tuning collective anion motion enables superionic conductivity in solid-state halide electrolytes, Nat. Chem. 16, 1584 (2024).
  9. M Xu, J. Ding, and E. Ma, One-dimensional stringlike cooperative migration of lithium ions in an ultrafast ionic conductor, Appl. Phys. Lett. 101, 031901 (2012).
  10. Xingfeng He, Yizhou Zhu, and Yifei Mo, Origin of fast ion diffusion in super-ionic conductors, Nat. Commun. 8, 15893 (2017).
  11. Bingkai Zhang, Luyi Yang, Lin-Wang Wang, and Feng Pan, Cooperative transport enabling fast Li-ion diffusion in Thio-LISICON Li10SiP2S12 solid electrolyte, Nano Energy 62, 844 (2019).
  12. Zheyi Zou, Nan Ma, Aiping Wang, Yunbing Ran, Tao Song, Yao Jiao, Jinping Liu, Hang Zhou, Wei Shi, Bing He et al., Relationships between Na+ distribution, concerted migration, and diffusion properties in rhombohedral NASICON, Adv. Energy Mater. 10, 2001486 (2020).
  13. Aris Marcolongo and Nicola Marzari, Ionic correlations and failure of Nernst-Einstein relation in solid-state electrolytes, Phys. Rev. Mater. 1, 025402 (2017).
  14. Cibrán López, Riccardo Rurali, and Claudio Cazorla, How concerted are ionic hops in inorganic solid-state electrolytes?, J. Am. Chem. Soc. 146, 8269 (2024).
  15. Yirong Gao, Adelaide M. Nolan, Peng Du, Yifan Wu, Chao Yang, Qianli Chen, Yifei Mo, and Shou-Hang Bo, Classical and emerging characterization techniques for investigation of ion transport mechanisms in crystalline fast ionic conductors, Chem. Rev. 120, 5954 (2020).
  16. K Funke, Ion dynamics and correlations: Translational and localized ionic hopping motion in solid electrolytes, Philos. Mag. A 68, 711 (1993).
  17. Andrey D. Poletayev, Matthias C. Hoffmann, James A. Dawson, Samuel W. Teitelbaum, Mariano Trigo, M. Saiful Islam, and Aaron M. Lindenberg, The persistence of memory in ionic conduction probed by nonlinear optics, Nature (London) 625, 691 (2024).
  18. Léon Van Hove, Correlations in space and time and born approximation scattering in systems of interacting particles, Phys. Rev. 95, 249 (1954).
  19. Rolf Hempelmann, Quasielastic Neutron Scattering and Solid State Diffusion (Oxford University Press, New York, 2000).
  20. Tasso Springer, Quasielastic Neutron Scattering for the Investigation of Diffusive Motions in Solids and Liquids, Springer Tracts in Modern Physics Volume 64 (Springer, Berlin, Heidelberg, 2006), pp. 1–100.
  21. Naresh C. Osti, Niina Jalarvo, and Eugene Mamontov, Backscattering silicon spectrometer (BASIS): Sixteen years in advanced materials characterization, Mater. Horiz. 11, 4535 (2024).
  22. Bettina Schwaighofer, Miguel A. Gonzalez, Mark R. Johnson, John S. O. Evans, and Ivana Radosavljević Evans, Ionic mobility in energy materials: Through the lens of quasielastic neutron scattering, Chem. Mater. 37, 3575 (2025).
  23. J. M. Rowe, J. J. Rush, L. A. De Graaf, and G. A. Ferguson, Neutron quasielastic scattering study of hydrogen diffusion in a single crystal of palladium, Phys. Rev. Lett. 29, 1250 (1972).
  24. Masato Matsuura, Yasuyuki Fujiwara, Hiroki Moriwake, Koji Ohara, and Yukinobu Kawakita, Microscopic dynamics of lithium diffusion in single crystal of the solid-state electrolyte La2/3−xLi3×TiO3 (x=0.13) studied by quasielastic neutron scattering, Phys. Rev. B 104, 094305 (2021).
  25. Sawankumar V. Patel, Swastika Banerjee, Haoyu Liu, Pengbo Wang, Po-Hsiu Chien, Xuyong Feng, Jue Liu, Shyue Ping Ong, and Yan-Yan Hu, Tunable lithium-ion transport in mixed-halide argyrodites Li6−xPs5−xClBrx: An unusual compositional space, Chem. Mater. 33, 1435 (2021).
  26. Qingyong Ren, Mayanak K. Gupta, Min Jin, Jingxuan Ding, Jiangtao Wu, Zhiwei Chen, Siqi Lin, Oscar Fabelo, Jose Alberto Rodríguez-Velamazán, Maiko Kofu et al., Extreme phonon anharmonicity underpins superionic diffusion and ultralow thermal conductivity in argyrodite Ag8SnSe6, Nat. Mater. 22, 999 (2023).
  27. Jingxuan Ding, Mayanak K. Gupta, Carolin Rosenbach, Hung-Min Lin, Naresh C. Osti, Douglas L. Abernathy, Wolfgang G. Zeier, and Olivier Delaire, Liquid-like dynamics in a solid-state lithium electrolyte, Nat. Phys. 21, 118 (2025).
  28. Mayanak K. Gupta, Jingxuan Ding, Dipanshu Bansal, Douglas L. Abernathy, Georg Ehlers, Naresh C. Osti, Wolfgang G. Zeier, and Olivier Delaire, Strongly anharmonic phonons and their role in superionic diffusion and ultralow thermal conductivity of Cu7PSe6, Adv. Energy Mater. 12, 2200596 (2022).
  29. Xingchen Shen, Michael Marek Koza, Yung-Hsiang Tung, Niuchang Ouyang, Chun-Chuen Yang, Chen Wang, Yue Chen, Kristin Willa, Rolf Heid, Xiaoyuan Zhou et al., Soft phonon mode triggering fast Ag diffusion in superionic argyrodite Ag8GeSe6, Small 19, 2305048 (2023).
  30. Benjamin J. Morgan, Mechanistic origin of superionic lithium diffusion in anion-disordered Li6PS5x argyrodites, Chem. Mater. 33, 2004 (2021).
  31. Marcel Sadowski and Karsten Albe, Influence of Br-/S2-site-exchange on li diffusion mechanism in Li6PS5Br: A computational study, Phil. Trans. R. Soc. A 379, 20190458 (2021).
  32. Byeongsun Jun and Sang Uck Lee, Designing a descriptor for the computational screening of argyrodite-based solid-state superionic conductors: Uniformity of ion-cage size, J. Mater. Chem. A 10, 7888 (2022).
  33. Sokseiha Muy, Roman Schlem, Yang Shao-Horn, and Wolfgang G. Zeier, Phonon–ion interactions: Designing ion mobility based on lattice dynamics, Adv. Energy Mater. 11, 2002787 (2021).
  34. Tao Song, Yuxiao Lin, Da Wang, Qianli Chen, Chen Ling, and Siqi Shi, Renewing fundamental understanding of ionic transport in inorganic crystalline solid-state electrolytes from the perspective of lattice dynamics, Adv. Energy Mater. 2302440 (2024).
  35. Marvin A. Kraft, Sean P. Culver, Mario Calderon, Felix Bocher, Thorben Krauskopf, Anatoliy Senyshyn, Christian Dietrich, Alexandra Zevalkink, Jurgen Janek, and Wolfgang G. Zeier, Influence of lattice polarizability on the ionic conductivity in the lithium superionic argyrodites Li6PS5x (x=Cl, Br, i), J. Am. Chem. Soc. 139, 10909 (2017).
  36. Sokseiha Muy, John C. Bachman, Livia Giordano, Hao-Hsun Chang, Douglas L. Abernathy, Dipanshu Bansal, Olivier Delaire, Satoshi Hori, Ryoji Kanno, Filippo Maglia et al., Tuning mobility and stability of lithium ion conductors based on lattice dynamics, Energy Environ. Sci. 11, 850 (2018).
  37. Jennifer L. Niedziela, Dipanshu Bansal, Andrew F. May, Jingxuan Ding, Tyson Lanigan-Atkins, Georg Ehlers, Douglas L. Abernathy, Ayman Said, and Olivier Delaire, Selective breakdown of phonon quasiparticles across superionic transition in CuCrSe2, Nat. Phys. 15, 73 (2019).
  38. Mayanak Kumar Gupta, Sajan Kumar, Ranjan Mittal, and Samrath L. Chaplot, Soft-phonon anharmonicity, Floppy modes, and Na diffusion in Na3Fy (Y=S, Se, Te): Ab initio and machine-learned molecular dynamics simulations, Phys. Rev. B 106, 014311 (2022).
  39. Cibrán López, Agustí Emperador, Edgardo Saucedo, Riccardo Rurali, and Claudio Cazorla, Universal ion-transport descriptors and classes of inorganic solid-state electrolytes, Mater. Horiz. 10, 1757 (2023).
  40. Zhizhen Zhang and Linda F. Nazar, Exploiting the paddle-wheel mechanism for the design of fast ion conductors, Nat. Rev. Mater. 7, 389 (2022).
  41. KyuJung Jun, Byungju Lee, Ronald L. Kam, and Gerbrand Ceder, The nonexistence of a paddlewheel effect in superionic conductors, Proc. Natl. Acad. Sci. U.S.A. 121, e2316493121 (2024).
  42. Jeffrey G. Smith and Donald J. Siegel, A proper definition of the paddlewheel effect affirms its existence, Proc. Natl. Acad. Sci. U.S.A. 122, e2419892122 (2025).
  43. Alessio Zaccone and Matteo Baggioli, Universal law for the vibrational density of states of liquids, Proc. Natl. Acad. Sci. U.S.A. 118, e2022303118 (2021).
  44. Caleb Stamper, David Cortie, Zengji Yue, Xiaolin Wang, and Dehong Yu, Experimental confirmation of the universal law for the vibrational density of states of liquids, J. Phys. Chem. Lett. 13, 3105 (2022).
  45. T. Keyes, Unstable instantaneous normal modes and diffusion in supercooled liquids, J. Appl. Phys. 137, 074702 (2025).
  46. Kunio Wakamura, Roles of phonon amplitude and low-energy optical phonons on superionic conduction, Phys. Rev. B 56, 11593 (1997).
  47. Samuel M. Greene and Donald J. Siegel, Assessing correlations between phonon features and cation migration barriers in multivalent solid electrolytes, Chem. Mater. 36, 7476 (2024).
  48. D. G. Sangiovanni, J. Klarbring, D. Smirnova, N. V. Skripnyak, D. Gambino, M. Mrovec, S. I. Simak, and I. A. Abrikosov, Superioniclike diffusion in an elemental crystal: BCC titanium, Phys. Rev. Lett. 123, 105501 (2019).
  49. Ihor P. Studenyak, V. Yu Izai, Artem I. Pogodin, Oleksandr P. Kokhan, Vasil I. Sidey, M. Yu Sabov, Algimantas Kežionis, Tomas Šalkus, and Jūras Banys, Structural and electrical properties of argyrodite-type Cu7PS6 crystals, Lith. J. Phys. 57, 243 (2017).
  50. Xingchen Shen, Niuchang Ouyang, Yuling Huang, Yung-Hsiang Tung, Chun-Chuen Yang, Muhammad Faizan, Nicolas Perez, Ran He, Andrei Sotnikov, Kristin Willa et al., Amorphous-like ultralow thermal transport in crystalline argyrodite Cu7PS6, Adv. Sci. 11, 2400258 (2024).
  51. Varley F. Sears, Neutron scattering lengths and cross sections, Neutron News 3, 26 (1992).
  52. See Supplemental Material at http://link.aps.org/supplemental/10.1103/lx5w-x6wl for experimental and computational details, additional discussion, and supporting figures, which includes Refs. [10,13,16,26–28,49–50,53–62].
  53. Owen Arnold, Jean-Christophe Bilheux, J. M. Borreguero, Alex Buts, Stuart I. Campbell, L. Chapon, Mathieu Doucet, N. Draper, R. Ferraz Leal, M. A. Gigg et al., mantid—Data analysis and visualization package for neutron scattering and μ sr experiments, Nucl. Instrum. Methods Phys. Res., Sect. A 764, 156 (2014).
  54. R. A. Ewings, A. Buts, M. D. Le, J. Van Duijn, I. Bustinduy, and T. G. Perring, horace: Software for the analysis of data from single crystal spectroscopy experiments at time-of-flight neutron instruments, Nucl. Instrum. Methods Phys. Res., Sect. A 834, 132 (2016).
  55. Georg Kresse and Jürgen Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci. 6, 15 (1996).
  56. Georg Kresse and Daniel Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B 59, 1758 (1999).
  57. John P. Perdew, Kieron Burke, and Matthias Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77, 3865 (1996).
  58. Han Wang, Linfeng Zhang, Jiequn Han, and E. Weinan, deepmd-kit: A deep learning package for many-body potential energy representation and molecular dynamics, Comput. Phys. Commun. 228, 178 (2018).
  59. James Weng, Eric D. Dill, James D. Martin, Ross Whitfield, Christina Hoffmann, and Feng Ye, K-space algorithmic reconstruction (KAREN): A robust statistical methodology to separate Bragg and diffuse scattering, Appl. Crystallogr. 53, 159 (2020).
  60. Niek J. J. De Klerk, Irek Roslon, and Marnix Wagemaker, Diffusion mechanism of Li argyrodite solid electrolytes for Li-ion batteries and prediction of optimized halogen doping: The effect of Li vacancies, halogens, and halogen disorder, Chem. Mater. 28, 7955 (2016).
  61. I Hanghofer, M. Brinek, S. L. Eisbacher, B. Bitschnau, M. Volck, V. Hennige, I. Hanzu, D. Rettenwander, and H. M. R. Wilkening, Substitutional disorder: Structure and ion dynamics of the argyrodites Li6Ps5Cl, Li6Ps5Br and Li6Ps5I, Phys. Chem. Chem. Phys. 21, 8489 (2019).
  62. Jiho Lee, Suyeon Ju, Seungwoo Hwang, Jinmu You, Jisu Jung, Youngho Kang, and Seungwu Han, Disorder-dependent Li diffusion in Li6Ps5Cl investigated by machine-learning potential, ACS Appl. Mater. Interfaces 16, 46442 (2024).
  63. Gordon Leslie Squires, Introduction to the Theory of Thermal Neutron Scattering (Courier Corporation, North Chelmsford, MA, 1996).
  64. Margarita Russina, Gerrit Gunther, Bela Farago, Earl Babcock, Zahir Salhi, Alexander Ioffe, and Ferenc Mezei, Structural fluctuations at nanoscale and cooperative molecular dynamics in bulk water, J. Phys. Chem. Lett. 16, 5835 (2025).
  65. Holger Euchner, Tsunetomo Yamada, Helmut Schober, Stephane Rols, Marek Mihalkovič, Ryuji Tamura, Tsutomu Ishimasa, and Marc de Boissieu, Ordering and dynamics of the central tetrahedron in the 1/1 Zn6Sc periodic approximant to quasicrystal, J. Phys. Condens. Matter 24, 415403 (2012).
  66. Holger Euchner, Tsunetomo Yamada, S. Rols, Tsutomu Ishimasa, Y. Kaneko, Jacques Ollivier, H. Schober, M. Mihalkovic, and M. De Boissieu, Tetrahedron dynamics in the icosahedral quasicrystals i-ZnMgSc and i-ZnAgSc and the cubic 1/1-approximant Zn6Sc, J. Phys. Condens. Matter 25, 115405 (2013).
  67. Tatsiana Burankova, Rolf Hempelmann, Andrew Wildes, and Jan P. Embs, Collective ion diffusion and localized single particle dynamics in pyridinium-based ionic liquids, J. Phys. Chem. B 118, 14452 (2014).
  68. Jared Coles, Omar Chmaissem, Matthew Krogstad, Daniel M. Pajerowski, Feng Ye, Duck Young Chung, Mercouri G. Kanatzidis, Stephan Rosenkranz, and Raymond Osborn, Four-dimensional quasielastic neutron scattering (4D-QENS) analysis of correlated ionic conduction in SrCl2, PRX Energy 5, 023002 (2026).
  69. R. M. Moon, T. Riste, and W. C. Koehler, Polarization analysis of thermal-neutron scattering, Phys. Rev. 181, 920 (1969).
  70. C. T. Chudley and R. J. Elliott, Neutron scattering from a liquid on a jump diffusion model, Proc. Phys. Soc. London 77, 353 (1961).
  71. Shriparna Mukherjee, David J. Voneshen, Andrew Duff, Pooja Goddard, Anthony V. Powell, and Paz Vaqueiro, Beyond rattling: Tetrahedrites as incipient ionic conductors, Adv. Mater. 35, 2306088 (2023).
  72. P. G. De Gennes, Liquid dynamics and inelastic scattering of neutrons, Physica (Amsterdam) 25, 825 (1959).
  73. S. K. Sinha and D. K. Ross, Self-consistent density response function method for dynamics of light interstitials in crystals, Physica (Amsterdam) 149B+C, 51 (1988).
  74. Thomas Weber and Arkadiy Simonov, The three-dimensional pair distribution function analysis of disordered single crystals: Basic concepts, Z. Kristallographie 227, 238 (2012)..
  75. Matthew J. Krogstad, Stephan Rosenkranz, Justin M. Wozniak, Guy Jennings, Jacob P. C. Ruff, John T. Vaughey, and Raymond Osborn, Reciprocal space imaging of ionic correlations in intercalation Compounds, Nat. Mater. 19, 63 (2020).
  76. Thomas Richard Welberry, Diffuse X-Ray Scattering and Models of Disorder (Oxford University Press, New York, 2022), Vol. 31.
  77. Kim H. Pham, Vijaya Begum-Hudde, Amy K. Lin, Natan A. Spear, Jackson McClellan, Michael W. Zuerch, Andre Schleife, Kimberly A. See, and Scott K. Cushing, The dynamical role of optical phonons and sublattice screening in a solid-state ion conductor, J. Am. Chem. Soc. 147, 26456 (2025).
  78. Tyson Lanigan-Atkins, Xing He, M. J. Krogstad, D. M. Pajerowski, Douglas L. Abernathy, Guangyong N. M. N. Xu, Zhijun Xu, D.-Y. Chung, M. G. Kanatzidis, Stephan Rosenkranz et al., Two-dimensional overdamped fluctuations of the soft perovskite lattice in CsPbBr3, Nat. Mater. 20, 977 (2021).
  79. Chengjie Mao, Xing He, Hung-Min Lin, Mayanak K. Gupta, Patrick Postec, Tyson Lanigan-Atkins, Matthew Krogstad, Daniel M. Pajerowski, Tao Hong, Travis J. Williams et al., Correlated dynamic disorder, octahedral tilts, and acoustic phonon softening in CsSnBr3 and CsPbBr3, Phys. Rev. Mater. 9, 065401 (2025).
  80. Yu Chen, Caleb Ramette, Matthew Krogstad, Maxim Avdeev, Erick Lawrence, Satya Kushwaha, Chao-Chun Wei, Rajavallipuram Sankaran Subramanian, Birender Singh, Bin Liu et al., Framework short-range order observed in a spinel-type Li superionic conductor, J. Am. Chem. Soc. 148, 9244 (2026).
  81. 10.7924/r4r540.
  82. 10.5286/ISIS.E.RB2320140.

Outline

Information

Phys. Rev. X 16, 031046– Published 21 August, 2026
  • Received 16 December 2025
  • Accepted 29 June 2026
Crossmark - Check for updates button
Creative Commons Logo

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Sign In to Your Journals Account

Filter

Filter

Article Lookup

Enter a citation