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Unformatted text preview: fastijahuart/"physicists discovered that an to: the shelf foriidecades was, in fact, a record: y ' ng tn‘termetallicsuperconductor ” steal Q garnish; an in metal alloys and compounds ap- peared to remain trapped by a glass ceiling. Over the previous 10 years the temperature at which certain oxide— based compounds such as bismuth strontium calcium copper oxide and mercury barium calcium copper oxide lost their resistance to electric current had soared to well over 100 K. Mean— while, the transition temperature, 32, for carbonabased materials, including alkali—doped carbon-6O compounds, exposingboronfilamentto had risen close to the boiling point Of The wiresareshown hexttoaUS pennyforscale. liquid nitrogen <7 7 K) During the same period, however, the superconducting transition tempera- ture of intermetallic compounds (materials made solely of metals and metal-like elements) remained close to 20 K ~ as it had been since the mid- l9605. By February 2001 everything had totally changed. It was as if a firecracker had gone oil in the tidy little ant hill of superconductivity research. For the first few months of 2001, groups all over the world raced to understand the properties of a new intermetallic superconductor. The substance that everyone was scrambling to buy or make, the substance that was causing this grand commotion, was magnesium diboride (MgBZ). This seemingly innocuous binary compound, which had been present in many labs for over half a century, had been discovered to superconductjust below 40 K. 3o what? Before going into the detailed properties of magnesium di— boride; before presenting a brief history of our understand- ing of superconductivity; and before examining how we could miss superconductivity in l‘vlng for so long, we have to answer a question: so what? S uperconductors are not just strange compounds that only physicists play with. Superconducting materials are ideally suited to generating the high magnetic fields commonly required in research labs and in the magnetic resonance imaginrI {Mill} machines that are becoming so common in hospitals. The reason is that a solenoid made from supercon— Parsms Wests with tr 29522 physicsw Segments of Mng Wire that were synthesized by d gorges i gorillas ducting wire can carry large currents, and thus generate large magnetic fields, without any dissipation (Le. without any resistive heating). In addition, superconducting power cables can carry many times the current density of normal cables. This means that the power capacity of a city can be increased dramatically by simply re- placing copper cables with supercon— ducting ones, rather than digging up the roads to lay new cables. Indeed, a test length of superconducting power cable made from ribbons of high— TC oxide clad in silver was recently laid under the city of Detroit, and the installation of a second length is now being planned for Los Angeles. But superconductors have to be cooled well below the transition temperature — to roughly about half of TC ~ for use in applications. Typically, intermetallic superconductors op— erate in a bath of liquid helium (Le. at a temperature of about i K) while cables that are made from high-TC oxides are cooled by liquid nitrogen. During the past 20 years7 closed—cycle refrigerators 7* which are similar in principle to household refrigerators e have im— proved dramatically. In fact, it is now quite easy to cool objects to 20 K with no liquid cryogens. That said, the attractions of a new superconductor with a transition temperature of 40 K were clear to physicists. A material that could be cooled using a closed-cycle refrigerator would find many applications, pro— vided it had good superconducting‘and material properties. These considerations _ as well as the general enthusiasm of physicists with a new puzzle to solve A~ were the driving forces behind last year’s excitement. Many groups all over the world are also currently in the process of filing patents, but whether any of these will prove to be valuable will ultimately depend on the properties of h/lng. magnesium vapour. Basie ideas and one situation Supercond’uctivity was discovered in lgll by the Dutch phy— sicist licilte Kamcrlingh Onnes. Three years earlier, Unnes and colleagues had discoyercd how to liquefy helium, which shot; 29 "that had been sitting 0n ~> WaMWWWWmWMWW anism of superconductivity came over 5:0 years later thanks to a theory devised by John Bardeen, Leon Snorer and- Robert Schrieffer. The BCS theory, as it became know, explains how electrons form pairs, known as Cooper pairs, that act as the building blocks of the super— conducting state. This pairing takes place through an intermediary, namely a lattice vibration known as a phonon. W7 hat initially sounds like the imposs— ible attraction between two like—charged objects (ie. electrons) can be understood at some level via partially inaccurate, but useful, analogies. Picture two people bouncing on a bed or a trampoline. Even though there is no attraction be- tween these people on the ground, the depression left on the trampoline by one person can draw the other person closer. A microscopic example is an electron moving through a crystal lattice, drawing positively charged ions towards itself. This distortion ~— with its somewhat en» hanced positive charge " attracts a second electron, This par— ticular example is a somewhat static view of what is really a dynamic process, but it gives the basic picture. The 308 theory has essentially three parameters, as can be seen in the equation for the superconducting transition tem~ perature kBTC Z l . lBfiwDe"1/WlEF>, where kg is the Boltzmann constant, 7? is the Planck constant divided by 27:, (DD is the Debye frequency, V is the strength of the coupling between the electrons and the phonons, and N<Epl is the density of states at the Fermi level. The Debye frequency is the characteristic frequency of the lattice vibrations that couple the electrons in the super— conducting state. Given that lattice vibrations mediate the Cooper pairs, it is not surprising that TC is directly propor- tional to this characteristic vibrational frequency Now let us invoke a grossly simplified, mechanical model of a crystal that regards the atoms as masses that are coupled together with little springs (see figure 1). The characteristic frequency of this system is o) = Vic/m, where k is the spring constant and m is the mass of the atom. Using this simplification we can see that the value of TC should increase as the mass decreases. This gives rise to a prejudice that compounds containing lighter elements will have higher values of TC than those composed of heavier elements. The next parameter is V, the strength of the coupling between the electrons and the phonons, A high value of K can he achieved with large couplings as long as the crystal does not distort or loose stability. When the electron—phonon coupling becomes too strong, however, the structure of the crystal can distort to form a so-called charge—density wave at low temperatures. And for really large values of 1/: a given crystal structure may simply cease to exist in favour of a dif~ lercnt one, in either case, the new or distorted structure tends ”tot to be superconducting because it generally has fewer elec~ trons axr'ailable to participate in the superconducting ground state. For this reason, it was felt that higher transition tem- the isotope effect. 30 physiesweberg A ball and spring model of a hexagonal lattice. Approximatingthe vibrational spectra of a boron— atom lattice by a single springm mass system, with mass m and spring constant k; is a rather gross simplifieatiod But tt does capturethe basic physics of new the critical temperature depends on the mass of the atoms a rid, more specifically, dibe fo nd near‘striic: tions’. Here the coup— , ,_ ong: as possible while a '* suitable crystal structure is maintained. The final term in the BCS equation is VIN/"(E13, the density of states at the “Fermi surface”. Simply speaking, NiErl ' is a measure of the number of electrons that can take part in the superconduct— ing ground state. in general, compounds containing transition metals w elements that have a partially filled “d—shell” ~ have a larger density of states at the Fermi surface, and thus a higher trans— ition temperature, than non~transition— metal compounds. Before 2001 the reigning kings of the intermetallic super— conductors were niobium germanide, vanadium silicide, niobium nitride and other transition—metal compounds. This led many physicists to believe that a high value for TC could only be achieved in compounds that included transition metals to boost the den- sity of states. The 1308 equation and, to some extent, these prejudices have helped to define the search for new superconductors over the past decades. ‘While physicists and chemists have a rough idea of how to control the Debye frequency and the density of states, the electronephonon coupling has remained a somewhat elusive parameter. Much of the search for new intermetallic superconductors has therefore focused on com- pounds that contain light elements and / or compounds with transition metals. However, the term for the electronwphonon coupling re— mains important, and by noting that lead has one of the highest superconducting transition temperatures of all the ele— ments (7.2 K) _. even though it is very heavy and not a trans- ition metal v we are forced to acknowledge that electron~ phonon coupling plays an important role. And as we will see, the significance of this coupling is even more clearly demon— strated by MgBQ. hide and prejudice Summarizing all of the prejudices from our whirlwind tour of ROS theory: to find an intermetallic compound that loses its resistance at relatively high temperatures, we clearly need to look for something that is made of light elements, preferably containing a transition metal, and that has strong phonon coupling. lVl any groups and individuals have tried to find such compounds over the decades with varying degrees of success. A recent attempt to find new intermetallic superconductors has involved mixtures of titanium, magnesium and boron. Since physicists knew relatively little about this ternary sys— tem, they thought it would be a good place to fish for new superconductors. After all, magnesium and boron atoms are light, while titanium is not too heaxy and'also provides the transition-metal d—shell electrons that are considered vital for a large density of states and, thus, a high transition tempera— ture. it is a nice story with a good plot, but in this case the truth turned out to be stranger than fiction. l’llheri jun Akamitsuis group at Aoyama-Gakuin Univer— sity in Tokyo studied this ternary system, they observed small Parents Warts stuns” are: mm»wWm)V!MWmewmowmzwwwvmmmwmrmmmwmmwrmwmmmaww . nMWM‘WMMWWMMWWWWMWMMWWW .. . .. . . . hints of superc After more re work, they d , _ 1t Wa aetu ally the binary compound, magnesium diboride, that became superconducting (see Nagamatsu et (ii. in further readin, ) During a meeting in Sendai japan, in the second week of january 00l , super: conductivity in l‘vlgBQ was announced publicly The clock started ticking. The electronic grapevine started car- rying hints of excitement, but no details. ‘Nhen our group heard v within about a week of the Sendai conference ~ no information was available. On hearing of a superconductor with a transition temperature near 40 K, many theorists and experimentalists immediately con— cluded that some exotic (i.e not well understood) mechanism other than electron~phonon coupling must be at work. indeed, they thought the physics might even be similar to the high—TC oxide superconductors, which still lack an agreed theory. On the other hand, re— searchers familiar with intermetallie superconductors felt that high? was probably an extreme example of standard, old“ fashioned superconductivity Either way, superconductivity at 40 K in higiig looked like an exciting proposition. To give a measure ofjust how excited people were, our own group had posted its first paper on the Web by the end of January and had published three papers on lVlgBQ in Physical Review Letters by mid—March. And at the American Physical Society’s March meeting in Seattle, nearly 1000 physicists gathered late into the night to hear some 80 two—minute up— dates on the latest research. Shape am! size As soon as we heard about the report at the Sendai meeting, we decided to make magnesium diboride, to test its supercon— ducting transition temperature and, hopefully, to address some of the questions about the underlying mechanism in— volved. We emptied all of our furnaces and started trying to produce the compound 7 but making MgB2 is a tricky busi— ness. The simplest way of making intermctallic compounds by simply melting the elements together vwas not an option open to us because of the high decomposition temperature of Mng and the high vapour pressure ‘of magnesium. In other words, the magnesium would just evaporate before the com- pound could form. However, we realized that if exactly the right proportions of magnesium and boron were sealed in an inert tantalum vessel and reacted at a high enough temperature (950 0C), then polycrystalline pellets of MgBQ could be made in as little as two hours. While we use this method in the laboratory to make 5~~l0 gramme samples of MgBQ, industrial suppliers like Accumet Nlaterials use a similar technique to make l0~ l00 kg quantities of the compound. Within three days of hearing the rumours, we had made high—purity pellets of magnesium diboride and were able to confirm superconductivity near 40 K. Although the transition temperature can be measured on sintcred pellets of this kind, many other measurements and applications require the super— ?srstcs 32510519 Err-urge? 28:32 Cress as ions or a boron filament some i 109 mlferons in diameter, and the MgB2 wire segments thatWere produced from it, The boron filament expands to a diameter of 150 mierons as the magnesium vapour diffuses into the boron to make Mng. r defined geometry ltjthen suddenly wned n us that-we mtghtbe able to _ , m Mng wires by simply exposing boron filaments to magnesium vapour. The reason we believed that this ap— proach would work is because MgBQ is iiicomposEd of just maélemefits, and'be— cause magnesium has a relatively high vapour pressure (Le. it readily turns into a gas). Indeed, one third of an atmo~ sphere of magnesium vapour exists in equilibrium with the liquid metal at 950 0C. This simple idea was rapidly put to the test and we soon found that we could produce segments of l‘vlng wire up to 0.4 mm in diameter from lengths of boron filament (see figure 2 and Canfield elf al. in further reading). Such boron filaments are found in a variety of composite materials ranging from fibre in military garments to high—per— formance sports equipment. Nloreover, they can be up to several kilometres in length, which bodes well for future appli— cations. T he same technique is also being exploited by our group and others, including Hans Christen and co~workers at the Oak Ridge National Laboratory, to turn boron films into magnesium-diboride films. Starting with boron filament is one particularly elegant method of making wire—like samples, but there is another tried and trusted way to produce superconducting wires from a wide variety of materials —~ the “powder—in—a-tube” method. In this approach, magnesium-diboride powder is poured into a tube that is then made thinner and longer. This method has been used by a variety of groups around the world, including Sungho jin and co—workers at Lucent Technologies in the US and Edward Collings’ group at Ohio State University (see jin et all. in further reading). Already wires ranging in length from l0 m to 100 m have been made, or are in the process of being made. At this stage it is not clear which approach will ultimately produce the best results, but it is fairly clear that magnesium-diboride wires will be made and utilized in the foreseeable future. But this is putting the cart before the horse. First let us review some of the basic properties of Night2 and then return to the applications. What makes it tick? So is magnesium diboride an old—fashioned superconductor that can be explained by BCS theory or is it more exotic? Bardeen, Cooper and Schrieffer showed that the transition ‘ temperature of a superconductor is proportional to the fre~ quency of the lattice vibrations. And earlier in this article we showed that a simple model of the lattice predicts that higher transition temperatures can be achieved for com- pounds with lighter atoms. But how can we change the mass of the atoms without changing the compound itself '7? The answer is isotopes! Now we start to see just how important light elements are. Boron has two stable'naturally occurring isotopes: boron- l0 and boron— ll. The simple predictions of the BCS model can be tested by making two samples of high? with isotopically pure boron. indeed, the theory predicts a dillerence in the physicsweberg 3i e er, brim, with a memnvama Wmmwwwmm w «mum.» WmWWWWWwW MWW’ w «maximum swarm l l 40 42 temperature (K) ‘l’he magnetization Wig-3182 (blue) and MgmB2 (red) as a function of temporal re; The sudden change in magnetization (as well as resistivity and specific heat), which signals the onset of superconductivity, occurs 1 K higher in the lighter compound (see Bud’ko et at. in further reading). Later data from David Hlnks’s group at the Argonne National Lab confirmed these results and showed additionally that there is virtually no shift associated with magnesium isotopes {see Hinks er al. in further reading). The results are consistent with boron vibrations heingthe key to superconductivity in MgBQ. valuc of TC of 0.85 K bctwccn the two dillcrcnt compounds. With our first sintcrcd pcllcts of magnesium diboridc, wc discovered a shift of l K in the resistivity, magnetization and spccifiohcat measurements (scc figure 3 and Budjko er al. in further reading). Thcsc simplc measurements immediately changed the nature of the discussions about magnesium diboridc. Thcy rcvcalcd that 234ng is most likely an cxtrcmc cxamplc of a traditional supcrconductor with a low density of statcs, a high chyc frcqucncy, a large clcctronrwphonon coupling and a vcry high value of TC This was extremely good news. Stan- dard intormctallic superconductors are much easier to work with, and can form useful wires much more casily than the high—tcmpcraturc oxide supcrconductors. Sash: properties Having addrcsscd thc mechanism that underlies supcrcon~ ductivity in Mng (at least to some cxtcnt), and having devised a way to make samples in a variety of shapes and sizes, physi— cists starlcd to address the basic properties of MgBZ. By mid blanuary we know that magnesium diboridc lost its resistance below 40 K, but ovcr what range of temperatures and ap— plied magnetic fields would it superconduct? And, cvcn morc importantly for applications, under what conditions would it be a useful superconductor? At this point it is prudent to review some of the character— istic features of supcrconductors. Thcrc arc two basic types of superconductors: tych and typo—ll. Thc dillcrcncc, in pociic terms, is csscntially diplomatic, and refers to the way the superconducting state reacts to an applicd magnetic hold. Typecl superconductors simply rcfusc to compromisc with the applicd field in any way, shapc or form. Thcy only super— conduct in magnetic fields bclow a ccrtain critical valuc, HC. Above this critical ficld, supcrconducrivily is dcstroycd and thc samplc rcturns to its normal statc. 'l’hc situation is quitc dilicrcnr for typo—ll supcrconductors, which can still conduct witl‘iout rcsistancc in rclativcly large applicd magnctic holds. in this casc. thcrc arc two important 32 physicswebcrg aura l ” r bchavcsjust like a ty l c s , \ field, H62, abovc which _ nduc or. For fie...
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