The Quest for Room-Temperature Superconductivity in Hydrides Warren Pickett, and Mikhail Eremets
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The quest for room-temperature superconductivity in hydrides Warren Pickett, and Mikhail Eremets Citation: Physics Today 72, 5, 52 (2019); doi: 10.1063/PT.3.4204 View online: https://doi.org/10.1063/PT.3.4204 View Table of Contents: https://physicstoday.scitation.org/toc/pto/72/5 Published by the American Institute of Physics ARTICLES YOU MAY BE INTERESTED IN Tracking the journey of a uranium cube Physics Today 72, 36 (2019); https://doi.org/10.1063/PT.3.4202 Microswimmers with no moving parts Physics Today 72, 44 (2019); https://doi.org/10.1063/PT.3.4203 The hydrodynamics of a quantum fluid Physics Today 72, 22 (2019); https://doi.org/10.1063/PT.3.4199 The power of illustration Physics Today 72, 8 (2019); https://doi.org/10.1063/PT.3.4193 When condensed-matter physics became king Physics Today 72, 30 (2019); https://doi.org/10.1063/PT.3.4110 Machine learning meets quantum physics Physics Today 72, 48 (2019); https://doi.org/10.1063/PT.3.4164 THE QUEST FOR ROOM-TEMPERATURE SUPERCONDUCTIVITY IN HYDRIDES Warren Pickett and Mikhail Eremets Whereas previous discoveries of superconductors were largely serendipitous, the latest advances have emerged from the close coupling of theoretical predictions and high-pressure experiments. 52 PHYSICS TODAY | MAY 2019 Warren Pickett is a distinguished professor in the department of physics at the University of California, Davis. Mikhail Eremets is a research physicist at the Max Planck Institute for Chemistry in Mainz, Germany. hen a material becomes superconducting below some T critical temperature c, its electrical resistivity drops abruptly to zero. That complete loss of resistance may wseem impossible because current-carrying electrons scatter from atomic vibrations, impurities, and crystal imperfections of the lattice. Electrons also ordinarily repel each other because they have the same negative charge. But two electrons in a metallic lattice can experience an attractive interaction through polarization of the positive ionic lattice, as explained in box 1. Because of that interaction, pairs of elec- trons with opposite momenta and spin angular momenta can bind together. Those Cooper pairs coalesce into a coherent wavefunction known as the superconducting condensate. First observed soon after helium was lique- conduction of electric current without resis - fied in 1908 (see the article by Dirk van Delft tance—and thus without heat dissipation or and Peter Kes, PHYSICS TODAY, September 2010, power loss—forms the basis of most applica- page 38) and today open to simple tabletop tions developed for superconductors. Magnet- demonstrations, the superconducting condensate ically levitated trains, for instance, now operate holds a unique standing in physics. Rather than in a few parts of the world, and superconduct- being a normal metal in which even small elec- ing magnets are common in hospitals world- tric voltages drive conducting charge carriers, wide for MRI machines. a superconductor exhibits an energy gap—the A larger goal is long-range power transmis- energy 2Δ required to split a pair out of the con- sion without energy loss. If industrially appli- densate—that is explained in box 2. In BCS the- cable materials could be designed, discovered, ory, developed by John Bardeen, Leon Cooper, or engineered, the savings in electrical power and J. Robert Schrieffer in 1957, the gap directly would be tremendous. (See PHYSICS TODAY, T relates to the critical temperature c at which March 1996, page 48, and January 2008, page 30.) electrons no longer scatter from vibrations or The full story is more complicated, but super- k T k impurities: 2Δ = 3.52 B c, where B is Boltz- conducting power transmission, if it were pro- mann’s constant. duced at room temperature, would constitute An energy bandgap, which provides zero a transformative technology. The achievement conductivity, is the sine qua non that distin- would not be free of complications—even the guishes insulators such as silicon, diamond, and best material candidates would suffer some sodium chloride from normal metals. The su- dissipation under large current loads. The chal- perconducting gap is far different. It provides, lenge would be to design an optimal system. via the underlying condensate, an infinite con- For physicists, chemists, and materials sci- ductivity. That characteristic reflects the coun- entists, the challenge is to achieve the super- terintuitive property that superconductors con- conducting state at increasingly higher temper- duct electricity in the absence of any applied atures. The primary task in reaching that goal electric voltage. is to understand the theoretical and practical The tabletop demonstrations of a supercur- limitations of the superconducting critical tem- rent include zero resistance and the exclusion perature. That kind of materials challenge, of an applied or existing magnetic field. The broadened to applications in clean energy, MAY 2019 | PHYSICS TODAY 53 SUPERCONDUCTIVITY IN HYDRIDES national security, and human welfare, was behind the 2011 in- ago, one of us (Eremets) found that pressurized sulfur hydride troduction of the Obama administration’s Materials Genome superconducts at 203 K (see PHYSICS TODAY, July 2016, page 21). Initiative (www.mgi.gov). The motivating idea was to integrate And more recently, reports have emerged that lanthanum the rapidly expanding capability of computational simulations hydride superconducts at temperatures as high as 280 K (see of materials with experimental synthesis to speed products’ “Pressurized superconductors approach room-temperature time to market and to stimulate the design and discovery of new realm,” PHYSICS TODAY online, 23 August 2018.) materials—for instance, ones that will superconduct at higher temperatures. Therein lies a parable. Progress in context In June 1973 PHYSICS TODAY published a letter to the editor BCS theory provides an understanding of superconductivity T (page 11) that included the maximum known values of c versus as arising from the pairing of electrons via quantized lattice time. The data from 1911 to 1973, presented in graphical form vibrations, or phonons. A primary result was an expression T λ μ* (figure 1), fit well into a straight line of slope three degrees per for the critical temperature in the form c ~ Ω exp[−1/( − )], decade. Its author, Bruce Friday, joked that room-temperature where Ω is the characteristic phonon vibration frequency, λ superconductivity was within sight, as long as one’s sight ex- the electron–phonon coupling constant, and μ* the Coulomb tended to the year 2840. pseudopotential—a measure of the Coulomb repulsion be- The letter’s implied conclusion could hardly have been tween electrons. T λ more discouraging. In 1973 the maximum c was a mere 23 K. The BCS expression is valid for when the coupling is Fortunately, the outlook improved. Little more than a decade weak. In the late 1960s William McMillan of Bell Labs extended later came the announcement and rapid confirmation that lay- the BCS analysis to moderately strong coupling. His equation T ered copper oxide compounds provided superconductivity near for c was extrapolated beyond its regime of validity to fortify 100 K. With applied pressure, that critical value rose to 160 K. claims that 30 K would be the upper limit for electron–phonon The achievement vitalized the field. The 1987 March Meeting coupling. But rigorous analysis of strong-coupling theory in of the American Physical Society featured a marathon session— 1975 by Philip Allen of Stony Brook University and Robert T now known as “the Woodstock of physics”—of about 50 pre- Dynes of Bell Labs demonstrated that c continues to increase sentations on the superconducting cuprates. (rather strongly) with increasing coupling strength, everything Although some impressive and serendipitous developments else being equal. have emerged, the 160 K maximum stood for 25 years. A recent Although substantial experimental searches persisted, no Reviews of Modern Physics T article celebrating the history of ob- superconductors with c higher than 23 K were found over a served that “progress in discovering new superconductors has 15-year period. In 1986 Georg Bednorz and Alex Müller, of IBM always been linked to the clever performance of making the Zürich, discovered an entirely new class of superconductors, the T correct material” (Art Hebard and Greg Stewart, PHYSICS TODAY, magnetic copper oxides. The initial values of c were around 30 K, February 2019, page 44). but shortly thereafter they were extended by other researchers In this article we illustrate how that paradigm is shifting. to 160 K under pressure. What’s more, the magnetism found in Computational theorists and experimentalists are currently part- those cuprates rendered conventional BCS theory inapplicable. nering to design and discover new superconducting hydrides Aoyama Gakuin University’s Jun Akimitsu’s 2001 discovery with the highest critical temperatures ever found. Three years of BCS superconductivity at 40 K in magnesium diboride (MgB2) COURTESY OF AMBER CRENNA-ARMSTRONG Box 1. Superconducting pairing of electrons BCS theory—named after its authors netic fluctuations may become attrac- John Bardeen, Leon Cooper, and J. Robert tive. The electron–phonon coupling term Schrieffer—established the two features λ is then defined as positive, and larger λ underlying the superconducting state: leads to a higher critical temperature Tc, the pairing of electrons and subsequent at which a material starts superconduct- coalescence of those pairs into a coher- ing. (See the article by J. Robert Schrieffer, ent superconducting wavefunction. Each PHYSICS TODAY, July 1973, page 23.) pair has an energy within a range of 2Δ Fast-moving electrons become cou- of the Fermi energy (the highest energy pled to heavier, slow-moving atomic of occupied electronic states), has equal cores. As shown here, a negatively charged and oppositely directed momenta k, and electron (red) with quantum numbers k has oppositely directed spin angular mo- and σ careens through the lattice, distort- menta σ, conventionally called up and ing positively charged ions in its wake.