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Enhanced: Superconductivity in a Grain of Salt Russell J. Hemley*

Seventy years ago, Bernal [HN1] proposed that all materials would become metals if compressed under sufficiently high pressure (1). Taking this idea one step further, Abrikosov predicted that electron pairing, the mechanism that creates superconductivity [HN2], would be enhanced in metals at high density (2). For years, these tenets remained untested and generated controversy (3) owing to a lack of adequate experimental techniques. A series of breakthroughs in the past year have not only placed this field on a sound experimental footing but are revealing new and surprising phenomena. The most recent example, reported on page 1333 of this issue by Eremets et al., is a study of the rather ordinary ionic solid CsI (4) [HN3]. When compressed to more than 200 GPa (2 megabars), CsI is not only a metal but also a superconductor in its very high density state (4).

Every field has its benchmark organism; in the field of high-pressure research, CsI is the equivalent of the molecular geneticist's Drosophila or Escherichia coli. Study of its transformations and transmutations under pressure--phase transitions, equations of state, optical spectra, soft modes, disproportionation, and metallization--by static compression, shock waves [HN4], and theory has served as a testing ground both for new ideas about compressed matter and for new techniques to study it. The new report is another chapter in the not-so-simple story of this simple salt. Eremets and colleagues from Osaka University (4) pressed CsI into the metallic state (5, 6) and directly measured the temperature and pressure dependence of the electrical resistance of the sample. They first provide direct evidence for metallization [HN5], at 115 GPa. Upon further increase in pressure a characteristic drop in resistance was found at 2 K near 180 GPa. Moreover, application of a magnetic field caused the resistance to reappear, a convincing sign of superconductivity.

Figure 1
Super under pressure. Cesium iodide, a white crystalline salt (left) with a cubic structure in its low-pressure "normal" state (yellow, cesium; dark red, iodine), transforms to a "hexagonal close-packed" metal on compression (atoms have similar electronic cores, orange) and is a superconductor (4). At right are the superconducting elements found by pressure application.

Many advances in high-pressure techniques have been made in recent years, but probing the electronic transport properties of materials at megabar pressures has been a major challenge: It is difficult to run current through microscopic samples as small as 1/10th the diameter of a human hair inside the pressure cell and to accurately measure it. Moreover, the necessary complementary magnetic measurements were equally difficult because of the small sample size. These obstacles were overcome during the past year with the development of ultrasensitive techniques in diamond anvil cells [HN6]. Amaya's group from Osaka University has pioneered the development of electrical techniques and tested them on a growing number of materials under pressure. The latest milestone is the first report of the application of the technique to above 200 GPa--nearly doubling their previous record pressure.

Experiments on CsI inevitably seem to generate new questions, and with this new report some earlier issues return. Disproportionation of the material to form elemental Cs and I was proposed to explain an apparent difference in the static and shock-wave equations of state (7) [HN7]. This discrepancy was finally resolved with the identification (8) of the high-pressure crystal structure (see figure). But the volumes of the elements are in fact lower as phase-separated components than in the compound, thereby imparting a driving force for dissociation, and this could be promoted by shear stresses. So is CsI the superconductor, or is one of the possible breakdown products responsible? Pure iodine has a transition temperature (Tc) of ~1 K measured at 28 to 74 GPa; the differences could be due to different states of crystallization or a pressure effect on Tc (9). Notably, SnI4) [HN8] is also a superconductor with a maximum Tc of 2 K in a crystalline state formed from its amorphous form (10), suggesting that a related transformation might also be considered for this material.

But the alternative interpretation of elemental superconductivity is in itself significant. Already 20 new elemental superconductors have been found at high pressure, increasing the number of known superconducting elements to at least 49 (see figure). The Osaka group has also made the extraordinary finding of superconductivity in oxygen at 100 GPa with a Tc of 0.6 K (11). This followed observations of superconductivity in sulfur at a similar pressure (12, 13); higher pressure studies of sulfur by magnetic susceptibility showed that Tc reaches 17 K at 160 GPa, the record Tc for an element (13). The chalcogenide family of elements [HN9] is in fact a family of superconductors with a wide range of Tc values. Notably, some of the results confirm reports of metallization and Tc from the early, crude diamond-indentor studies, which were discredited because of the lack of sample control and measurement of pressure (3).

With these recent experiments, more and more substances are giving up their "normal" state status as gases and liquids, and as insulators and semiconductors, to join the growing list of superconducting materials under pressure. Moreover, these "new" high-pressure superconductors may embody new mechanisms (14). Thus, understanding the origin of Tc in these materials, together with other new, but more well-known, ambient pressure superconductors--high Tc cuprates, fullerites, and borocarbides [HN10]--represents a serious challenge to theory.


  1. See footnote in E. Wigner and H. B. Huntington, J. Chem. Phys. 3, 764 (1935).
  2. A. A. Abrikosov, Sov. Phys. JETP 18, 1399 (1963).
  3. See news article by A. L. Robinson, Science 236, 671 (1987).
  4. M. Eremets et al., ibid. 281, 1333 (1998).
  5. R. Reichlin et al., Phys. Rev. Lett. 56, 2858 (1986).
  6. Q. Williams and R. Jeanloz, ibid., p. 163 [ADS].
  7. ------, ibid. 59, 1132 (1987) [ADS].
  8. H. K. Mao et al., Science 246, 649 (1989) [ADS]; H. K. Mao et al., Phys. Rev. Lett. 64, 1749 (1990) [ADS].
  9. K. Shimizu et al., J. Supercond. 7, 921 (1994).
  10. N. Takeshita et al., Rev. High Pressure Sci. Technol. 7, 595 (1998).
  11. K. Shimizu et al., Nature 393, 767 (1998).
  12. S. Kometani et al., J. Phys. Soc. Jpn. 66, 2564 (1997).
  13. V. V. Struzhkin et al., Nature 390, 382 (1997).
  14. C. F. Richardson and N. W. Ashcroft, Phys. Rev. Lett. 78, 118 (1997).

The author [HN11] is at the Geophysical Laboratory and Center for High Pressure Research, Carnegie Institution of Washington, Washington, DC 20015, USA. E-mail: hemley@gl.ciw.edu

Related Resources on the World Wide Web

General Hypernotes

An article about high pressure research by R. Hazen titled "The new alchemy" appeared in the November-December 1994 issue of Technology Review.

The July 1997 newsletter of the National Synchrotron Light Source at Brookhaven National Laboratory features an article by R. Hemley and H. K. Mao titled "High pressure and synchrotron radiation: The new era of megabar research."

The Gordon Research Conference Research at High Pressure was held in June 1998.

The Internet Pilot to Physics provides annotated links for condensed matter physics resources on the Web.

A guide to superconductivity is provided by D. Reimer, Physics Department, University of Hamburg, Germany.

The McDevitt group at the University of Texas, Austin, provides superconductivity links and a glossary.

The Applied Superconductivity Center at the University of Wisconsin, Madison, provides links to superconductivity resources.

SciCentral collects links to superconductivity resources on the Web.

Numbered Hypernotes

  1. H. Sheehan, School of Communications, Dublin City University, presents a Web page about John Desmond Bernal, based on material from her book Marxism and the Philosophy of Science.

  2. The Nobel Prize in Physics was awarded for superconductivity research in 1972 to John Bardeen, Leon N. Cooper, and J. Robert Schrieffer, and in 1987 to J. Georg Bednorz and K. Alexander M½ller. The American Superconductor Corporation provides an introduction to superconductivity. The Oak Ridge National Laboratory (ORNL) offers a Web site on high-temperature superconductors that includes a virtual poster session on applications of superconductors; ORNL also provides a teacher's guide to superconductivity. The McDevitt group at the University of Texas offers an illustrated guide to making a superconductor.

  3. The Chemistry WebBook from the National Institute of Standards and Technology provides reference data for cesium iodide.

  4. The Lawrence Livermore National Laboratory (LLNL) describes the two-stage gas gun used in shock compression experiments. Eric's Treasure Trove of Physics describes a high-pressure shock wave experiment.

  5. The Nelson group at the Department of Chemistry, Massachusetts Institute of Technology, presents a Web page about their work involving soft modes in ferroelectric phase transitions. B. Ravel et al. discuss soft mode behavior in phase transitions in the introduction to a paper titled "Order-disorder behavior in the phase transition of PbTiO." S. Williamson, Department of Chemistry, University of California, Santa Cruz, presents an introduction to disproportionation. The LLNL Public Affairs Office provides a fact sheet about the metallization of hydrogen achieved with shock-compression technology. More information about the metallic hydrogen experiment is provided by W. Nellis in an article that appeared in the September 1996 issue of LLNL's Science & Technology Review; a FAQ about metallization and metallic hydrogen is also available.

  6. The diamond-anvil cell is described and illustrated on a Web page from the High-Pressure Mineral Physics Laboratory at the University of Washington, Seattle. The National High Magnetic Field Laboratory (NHMFL) provides information about the diamond-anvil cell technology available at its facilities; S. Tozer of the NHMFL discusses high-pressure research that uses diamond-anvil cells.

  7. The usefulness of high-pressure research in studying equations of state is discussed in this LLNL document titled "Science on high-energy lasers: From today to the NIF [National Ignition Facility]."

  8. WebElements includes an entry for SnI4 in a section listing tin compounds.

  9. The WWWebster Dictionary defines chalcogen and chalcogenide. Chalcogen is defined in the glossary of the NT Curriculum Project at the University of Wisconsin, Madison. The chalcogenides are listed with their electron binding energies on the atomic data Web site maintained by the University of Guelph Department of Chemistry and Biochemistry. The Carnegie Institution of Washington issued a press release about the transformation of the chalcogenide sulfur into a superconductor at 93 GPa.

  10. A table of high-Tc cuprates and oxycarbonates is provided by the Superconducting Materials Research Group at the University of Wisconsin, Madison. The McDevitt group at the University of Texas offers a catalog of crystal structures of cuprates. The Spectroscopy Group at the Institute for Solid State Research, Dresden, Germany, presents information on cuprates and borocarbides. The Laser Processing Group at University College London discuss their research with high-Tc cuprate superconductors. The Applied Superconductivity Center at the University of Wisconsin provides images of high-temperature superconductors. The MEMS Material Properties database has an entry for fullerite. The 1 August 1998 issue of New Scientist reports on J.-P. Locquet's research that involved the use of a special compression technique to achieve a higher superconducting temperature; a press release about the research, which was published in the 30 July 1998 issue of Nature, is available from the IBM Zurich Research Laboratory. The High-Tc Update is a newsletter, available in print and electronic form, that provides rapid access to information about international superconductivity research.

  11. R. J. Hemley is at the Geophysical Laboratory of the Carnegie Institution of Washington.

Related articles in Science:

Metallic CsI at Pressures of up to 220 Gigapascals.
M. I. Eremets, K. Shimizu, T. C. Kobayashi, and K. Amaya
Science 1998 281: 1333-1335. (in Reports)

Volume 281, Number 5381 Issue of 28 Aug 1998, pp. 1296 - 1297
©1998 by The American Association for the Advancement of Science.