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@ARTICLE{Chen:471514,
      author       = {Chen, Zhijiang and Na, X. and Curry, Chandra and Liang,
                      Suzhe and French, Martin and Descamps, Adrien and DePonte,
                      D. P. and Koralek, J. D. and Kim, Jongjin and Lebovitz, S.
                      and Nakatsutsumi, M. and Ofori-Okai, Benjamin and Redmer,
                      Ronald and Roedel, C. and Schörner, M. and Skruszewicz, S.
                      and Sperling, P. and Toleikis, S. and Mo, Mianzhen and
                      Glenzer, Siegfried},
      title        = {{O}bservation of a highly conductive warm dense state of
                      water with ultrafast pump–probe free-electron-laser
                      measurements},
      journal      = {Matter and radiation at extremes},
      volume       = {6},
      number       = {5},
      issn         = {2468-080X},
      address      = {Melville, NY},
      publisher    = {AIP Publishing},
      reportid     = {PUBDB-2021-04466},
      pages        = {054401},
      year         = {2021},
      abstract     = {The electrical conductivity of water under extreme
                      temperatures and densities plays a central role in modeling
                      planetary magnetic fields. Experimental data are vital to
                      test theories of high-energy-density water and assess the
                      possible development and presence of extraterrestrial life.
                      These states are also important in biology and chemistry
                      studies when specimens in water are confined and excited
                      using ultrafast optical or free-electron lasers (FELs). Here
                      we utilize femtosecond optical lasers to measure the
                      transient reflection and transmission of ultrathin water
                      sheet samples uniformly heated by a 13.6 nm FEL approaching
                      a highly conducting state at electron temperatures exceeding
                      20 000 K. The experiment probes the trajectory of water
                      through the high-energy-density phase space and provides
                      insights into changes in the index of refraction, charge
                      carrier densities, and AC electrical conductivity at optical
                      frequencies. At excitation energy densities exceeding 10
                      MJ/kg, the index of refraction falls to n = 0.7, and the
                      thermally excited free-carrier density reaches n$_e$ = 5 ×
                      10$^{27}$ m$^{−3}$, which is over an order of magnitude
                      higher than that of the electron carriers produced by direct
                      photoionization. Significant specular reflection is observed
                      owing to critical electron density shielding of
                      electromagnetic waves. The measured optical conductivity
                      reaches 2 × 10$^4$ S/m, a value that is one to two orders
                      of magnitude lower than those of simple metals in a liquid
                      state. At electron temperatures below 15 000 K, the
                      experimental results agree well with the theoretical
                      calculations using density-functional
                      theory/molecular-dynamics simulations. With increasing
                      temperature, the electron density increases and the system
                      approaches a Fermi distribution. In this regime, the
                      conductivities agree better with predictions from the Ziman
                      theory of liquid metals.},
      cin          = {DOOR ; HAS-User / FS-FLASH-O},
      ddc          = {530},
      cid          = {I:(DE-H253)HAS-User-20120731 /
                      I:(DE-H253)FS-FLASH-O-20160930},
      pnm          = {631 - Matter – Dynamics, Mechanisms and Control
                      (POF4-631) / 6G2 - FLASH (DESY) (POF4-6G2)},
      pid          = {G:(DE-HGF)POF4-631 / G:(DE-HGF)POF4-6G2},
      experiment   = {EXP:(DE-H253)F-BL3-20150101},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000681018600001},
      doi          = {10.1063/5.0043726},
      url          = {https://bib-pubdb1.desy.de/record/471514},
}