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Magnesium Diboride: Better Late Than Never

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Magnesium Diboride: Better Late Than Never Magnesium Diboride: Better Late than Never With a superconducting transition temperature of 40 K and simple compound of two abundant, in- two superconducting gaps, MgB is full of surprises for both expensive elements. The long sought- 2 after, high-temperature, intermetallic experimentalists and theorists. superconductor had finally made its appearance—better late than never. Although the initial interest in Paul C. Canfield and George W. Crabtree MgB2 arose solely from its high Tc, fur- ther work revealed that MgB2 breaks he heyday of research into the basic properties of in- new ground for superconductivity based on the well-known Ttermetallic superconductors took place between 1950 electron–phonon interaction. The material displays a and 1980. During those years, the number of known su- plethora of remarkable features. MgB2 not only has im- perconducting intermetallic compounds (consisting of sev- portant technological potential (figure 1 and the cover of eral metallic and metalloid elements) grew explosively, this issue show images of MgB2 wire), but also will have a lasting impact on how the research community looks at and superconducting transition temperatures Tc were and looks for superconductors. pushed to just over 23 K (Nb3Ge). (In comparison, the first superconductor, discovered by Heike Kamerlingh Onnes in The understanding of this material has grown at a whirlwind rate, driven by the immediacy of preprint post- 1911, was mercury, with Tc = 4.15 K). Research groups all ings on the Internet. The daily appearance of new research over the world searched for higher and higher Tc values. The researchers were motivated by a basic desire to find results brought theory and experiment into resonance and an intrinsic limiting temperature for this intriguing quan- dramatically accelerated progress in the international tum phase and by a very applied interest in making use- community. Thanks to that positive feedback, the basic ful superconducting devices. physical properties of MgB2 were delineated within a year 1 By the 1970s, an empirical glass ceiling seemed to and a half of its discovery. have been hit. Transition temperatures were stuck at 23 K, and some theorists proclaimed that higher transition Background and foreshadowing temperatures were not possible. The basic research com- For decades after the discovery of superconductivity, the munity became more interested in other issues in super- experimental data puzzled theorists. One of the key ex- conductivity, such as the interaction of local magnetic periments that pointed toward the correct approach to un- moments with superconductivity, heavy-fermion super- derstanding the phenomenon was the isotope effect, which conductors, organic superconductors, and, more recently, showed that the transition temperature depends on the the high-temperature, copper oxide–based ceramic super- mass of the superconducting atoms. Motivated in part by those experiments, theorists recognized that lattice vi- conductors. With the discovery of high-Tc superconductiv- brations (phonons) could produce an attractive interaction ity over a decade ago, the 23-K ceiling for Tc was shat- tered, giving us not only new horizons in transition between the like-charged electrons. The 1957 theory of temperature (the current record is about 160 K under John Bardeen, Leon Cooper, and J. Robert Schrieffer pressure) but also a profusion of interesting phenomena (BCS) explained superconducting properties quantita- like d-wave pairing symmetry, pseudogaps, stripes, and tively using an elegant microscopic theory and the elec- exotic pairing mechanisms. Conventional wisdom holds tron–phonon interaction as the mechanism for pairing that these new horizons are enabled by the complexity of electrons. multielement compounds. These complex materials, in At the most basic level, the superconducting ground turn, have required complex processing to achieve the in- state is a coherent superposition of Cooper pairs—pairs of dustrial production of superconducting wires and devices electrons coupled by an attractive force. The attraction can that is now taking place. arise from phonons in the following way. As a negatively The January 2001 discovery that magnesium diboride charged electron moves through a lattice of positive ions, becomes superconducting at about 40 K produced an ex- it attracts and locally distorts the lattice in its neighbor- plosion of enthusiasm and excitement. Although 40 K is hood. The residual distortion left in its wake produces a indeed much cooler than 160 K, it represents a near dou- net positive charge that attracts a second electron. bling of the previous record intermetallic T and means The key feature of the superconducting state is the en- c ergy gap in the excitation spectrum. Below T , this gap pre- that MgB can be cooled to an operational temperature by c 2 vents scattering of the electrons and produces zero elec- either liquid hydrogen or readily available, fairly inex- trical resistance. The gap also manifests itself in tunneling pensive, closed-cycle refrigerators. In addition, MgB is a 2 experiments (in a manner similar to a semiconducting gap) that map its energy dependence and in thermodynamic ex- Paul Canfield is a senior physicist at Ames Laboratory and a periments, such as specific heat measurements, that probe professor in the department of physics and astronomy at Iowa thermal excitations across the gap. State University in Ames, Iowa. George Crabtree is a senior One of the most significant and influential predictions scientist and the director of the materials science division at of the BCS theory is the description of Tc in terms of two Argonne National Laboratory in Argonne, Illinois. fundamental constants (Boltzmann’s constant kB and 34 March 2003 Physics Today © 2003 American Institute of Physics, S-0031-9228-0303-010-2 Figure 1. Magnesium diboride was discov- ered in 2001 to be- come superconduct- ing below about 40 K, a remarkably high transition tem- perature for an inter- metallic compound. This image from an optical microscope shows a cross sec- tion of a MgB2 wire, about 150 mm in di- ameter, composed of fine grains with a wide distribution of orientations. (From ref. 4.) Planck’s constant \) and three basic materials parameters: searches for new superconductors often placed a much larger emphasis on N(E ) and w than on V, because the k T = 1.13 \w exp[–1/VN(E )]. F D B c D F electron–phonon coupling—and hence V—is hard to pre- The Debye frequency wD is the characteristic frequency of dict or control. the acoustic lattice vibrations. It sets the energy scale of the attractive interaction between Cooper pairs. In the MgB2: Discovery and isotope effect simplest model of a solid, the atoms can be viewed as Based on these experiments, theories, and prejudices, masses m connected by springs with spring constant k. groups around the world searched for ternary or quater- From high-school physics (or what should be high-school nary compounds that were rich in light elements such as physics), the characteristic frequency of such an idealized lithium, boron, carbon, and magnesium. Jun Akimitsu’s model is w = (k/m)1/2. This simple observation, combined group at Aoyama Gakuin University in Japan explored the with the BCS formula above, is the essence of the isotope titanium-boron-magnesium ternary phase diagram. That effect. Within this framework, Tc is inversely proportional system has two light elements as well as the transition to the square root of the masses of the constituent ele- metal Ti, which could provide 3d electrons to boost N(EF). ments. So, to first order, compounds with lighter elements Two years ago,2 the researchers indeed found a remark- have a better shot at higher values of Tc. ably high Tc, but in a simple binary compound: MgB2 en- The electron–electron attractive interaction V is a tered the superconducting state just below 40 K (see fig- caliper of the strength of the electron–phonon interaction. ure 2). In early January 2001, Akimitsu reported their If the electron–phonon interaction is increased, V and Tc results at a meeting in Sendai, Japan. His announcement rise, but if it becomes too large, it can induce a phase tran- set off a rush of experimental and theoretical work to con- sition to a different structure, often with vastly inferior su- firm and explore that remarkably high transition temper- perconducting properties. For that reason, researchers ature in a binary intermetallic compound (see PHYSICS have often sought high values of Tc near, but just short of, TODAY, April 2001, page 17). structural phase transitions; that approach is the most Whereas MgB2 is a simple binary compound (only three pragmatic method of maximizing V for a given structure. atoms per hexagonal unit cell), it is hard to make by con- The density of itinerant electron states N(EF), where ventional methods. For starters, elemental Mg has a high EF is the Fermi energy, provides a caliper of the number of vapor pressure. Furthermore, MgB2 decomposes rather electrons that can participate in the superconducting than melts and does not have any accessible liquid–solid 1,3 state. The larger N(EF), the larger Tc. Given that transi- transitions at ambient pressures. Simple crystal growth tion metals with partially occupied d shells tend to have of MgB2 thus appears to be intractable. (This difficulty is large values of N(EF), conventional wisdom held that high one of the reasons that superconductivity in MgB2 was not values of Tc could be found in transition-metal compounds. discovered 40 years ago.) Fortunately, MgB2 can be synthe- Experimentally, it is possible to tune N(EF) by varying sized by a simple, alternate route: reaction of B (in any num- the chemical composition in an isostructural series of com- ber of forms) with Mg vapor, generally at a temperature of pounds. Using light elements can raise wD. Experimental about 900°C for as little as a couple of hours.
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