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Enhanced: Superconductivity in a Grain of
Salt Russell J. Hemley*
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.
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.
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).
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.
- See footnote in E. Wigner and H. B.
Huntington, J. Chem. Phys. 3, 764 (1935).
- A. A. Abrikosov, Sov. Phys.
JETP 18, 1399 (1963).
- See news article by A. L. Robinson, Science
236, 671 (1987).
- M. Eremets et al., ibid.
281, 1333 (1998).
- R. Reichlin et al., Phys.
Rev. Lett. 56, 2858 (1986).
- Q. Williams and R. Jeanloz, ibid., p. 163 [ADS].
- ------, ibid.
59, 1132 (1987) [ADS].
- H. K. Mao et al.,
Science 246, 649 (1989) [ADS];
H. K. Mao et al., Phys. Rev. Lett.
64, 1749 (1990) [ADS].
- K. Shimizu et al., J.
Supercond. 7, 921 (1994).
- N. Takeshita et al., Rev. High Pressure
Sci. Technol. 7, 595 (1998).
- K. Shimizu et al.,
Nature 393, 767 (1998).
- S. Kometani et al., J. Phys. Soc.
Jpn. 66, 2564 (1997).
- V. V. Struzhkin et al., Nature
390, 382 (1997).
- 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: firstname.lastname@example.org
Related Resources on the World Wide
- An article
about high pressure research by R. Hazen titled "The new alchemy"
appeared in the November-December 1994 issue of
- 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."
Gordon Research Conference Research at High
Pressure was held in June 1998.
Internet Pilot to Physics provides annotated links for
condensed matter physics resources on the Web.
guide to superconductivity is provided by D. Reimer, Physics
Department, University of Hamburg, Germany.
McDevitt group at the University of Texas, Austin, provides
superconductivity links and a
- The Applied
Superconductivity Center at the University of Wisconsin, Madison,
links to superconductivity resources.
- SciCentral collects
superconductivity resources on the Web.
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.
The Nobel Prize in Physics was awarded for superconductivity research
1972 to John Bardeen, Leon N. Cooper, and J. Robert Schrieffer, and in
1987 to J. Georg Bednorz and K. Alexander M½ller.
American Superconductor Corporation provides an introduction to
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.
McDevitt group at the University of Texas offers an illustrated guide to
making a superconductor.
Chemistry WebBook from the National Institute of Standards and Technology provides reference data for
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.
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
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.
diamond-anvil cell is described and illustrated on a Web page from the High-Pressure Mineral Physics Laboratory at the University of Washington, Seattle.
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
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]."
WebElements includes an entry for
SnI4 in a section listing tin compounds.
The WWWebster Dictionary defines
Chalcogen is defined in the
glossary of the NT Curriculum Project at the University of Wisconsin, Madison.
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.
Carnegie Institution of Washington issued a
press release about the transformation of the chalcogenide sulfur into a superconductor at 93 GPa.
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
Spectroscopy Group at the Institute for Solid State Research, Dresden, Germany, presents information on
Laser Processing Group at University College London discuss their research with
high-Tc cuprate superconductors.
Applied Superconductivity Center at the University of Wisconsin provides
images of high-temperature superconductors.
MEMS Material Properties database has an entry for
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.
High-Tc Update is a newsletter, available in print and electronic form, that provides rapid access to information about international superconductivity research.
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.