the quantum tunnel vol. 01no. 04

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The Quantum TunnelScience and the World around Us Volume 1, Number 4, May, 8 2011

The Road to the Room Temperature

SuperconductorThis article first appeared in the January 2011 issueof The Next Door Magazine

David S. Latchman

If you have ever had, or know someone who has,a Magnetic Resonance Imaging (MRI) scan or heardabout experiments to accelerate sub-atomic particlesto near light speeds and possibly create mini-blackholes, you may have heard something about super-

conductors in the conversation. Superconductors area type of material that has zero resistance at very coldtemperatures and also exhibit several other proper-ties. In the realm of science fiction, superconduc-tors have been used to open up wormholes that al-low people to travel almost instantaneously from oneplanet to the next and even other galaxies, as seen onthe Star Gate movies and TV series. In Larry NivensRingworld series, the City Builders energy collec-tion devices were built using room-temperature su-perconductors. While these materials remain largelyhidden from the general public, they do play an im-

portant role in our society, as we will soon learn,and though they dont exhibit the properties of theirscience fiction counterparts, they are, none the less,some very exciting materials.

There is one property of superconductors thatis both easy and dazzling to demonstrate, makingmagnets float in midair. The demonstrator startsby showing us a black ceramic disc; something thatlooks quite ordinary. When a magnet is placed ontop of this disc nothing happens but when the disc is

cooled with liquid nitrogen, bringing it down to tem-peratures of -320.42F (-195.79C), something mag-ical occurs; the magnet floats and is suspended inmidair.

This feat of levitation is not a trick at all, it isfirmly rooted in physics, and is known as the Meiss-ner Effect. To explain this phenomenon we mustdelve into the realm of quantum mechanics. Butthese materials arent just the toys of idle scientistsbut scientific marvels that already have an impact onour lives and will continue to do so in the future.

A Theory of Conductivity

All matter is made of atoms, tiny indivisible particlesmade up of a central positively charged nucleus sur-rounded by negatively charged electrons. In metals,these atoms are arranged in a lattice where the outer-most electrons have been separated from their parentatoms, creating a sea of electrons in which the pos-itively charged atoms float. This allows the elec-trons to freely move about and what makes metals

good conductors. When a voltage is applied acrossthe ends of the metal, electrons drift from one end ofthe conductor to the other thereby creating an elec-trical current.

Within the metals lattice, the atoms in the lat-tice vibrate with thermal energy. As a current flowsthrough the conductor electrons collide with themetals atoms and lose energy in the form of heat,the prime cause for resistance in metals. As the tem-perature increases the atoms vibrate more produc-

Newsletter Contents:The Road to the Room Temperature Supercon-

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The Quantum Tunnel Newsletter Vol. 1, May, 8 2011

Figure 1: A magnet levitating above a superconductor that has been cooled with liquid nitrogen.

ing more frequent collisions between atoms and elec-trons; resistivity increases with increasing tempera-ture.

The History of Super-conductivity

The history of superconductivity is both a rich andfascinating one and one of which many books havebeen written. But there are some key events that havecharacterized the fields history. Unlike many dis-coveries, superconductivity was discovered neitherby chance nor accident and while it could not havebeen predicted, the idea that resistivity drops withtemperature was postulated as far back as 1864. Itwas one of the driving reasons for developing themeans to reach very low temperatures by liquefyinggases.

In 1908, Heike Onnes reached a major milestone

by liquefying helium. In 1911, along with his doc-toral student Jacob Clay, he continued the experi-ments started by other scientists to investigate thereduction of resistance at low temperatures by us-ing his new liquid helium as a coolant. After some

initial failures, he eventually saw success with mer-cury and much to his surprise the resistance abruptlydisappeared at -451.11F (-268.95C). Onnes initiallythought that something was wrong with his appara-tus but after some methodical tests, he observed theeffect was very real. Onnes had discovered some-thing new superconductivity and in recognitionof this discovery was awarded the 1913 Nobel Prizein Physics.

Superconductors are not Perfect

Conductors

Superconductivity describes the effect of a mate-rials resistance falling to zero when the temperaturereaches or falls below a critical temperature. This isvery different from what we expect to happen. Asa materials temperature drops we expect the atomic

vibrations to slow down and eventually stop. Elec-trons can then flow without collision or resistanceand become a perfect conductor, the point at whichthis happens is called absolute zero. Physicists definethis temperature as zero Kelvin (0K) on the Kelvin

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scale or -459.67F (-273.15C) but the laws of thermo-dynamics tell us that this temperature is impossibleto attain; perfect conductors can not and should not

exist.But there is another curious effect that is char-

acteristic of superconductors, one we saw earlier,the Meissner effect. As a superconductor makes thetransition to its superconducting state it also can-cels any magnetic fields inside it, a phenomenonknown as perfect diamagnetism. This property wasfirst discovered in 1933 by Walther Meissner andRobert Ochsenfeld and indicated for the first timethat superconductors werent perfect conductors atall. As diamagnetism is a quantum mechanical phe-nomenon, superconductors needed to be described

quantum mechanically.

Every Theory of Superconductiv-

ity Can Be Disproved

This tongue-in-cheek theorem was first said bySwiss physicist Fleix Bloch and highlighted the chal-lenges that physicists of the early 1930s faced. Everymajor physicists of the time had tried and failed toexplain how certain metals lost all their electrical re-sistance when chilled to ultra-cold temperatures. Atthe time, physicists had neither the experience northe evidence to fully understand the problem beforethem; they needed to develop a much deeper un-derstanding of quantum mechanics and solid statephysics to fully appreciate a solution.

In 1933, John Bardeen became interested in theproblem of superconductivity as a graduate studentat Princeton University and while there is no indica-tion that he tackled the problem during this period,it is quite possible that he though about it. It wouldtake Bardeen several decades to fully realize a solu-tion to the problem of superconductivity, a solution

that would come while working with his graduatestudent Leon Cooper and post-doc J. Robert Schreif-fer in 1957. All three men were awarded the NobelPrize in Physics in 1972 for the BCS Theory of Su-perconductivity, named after the initials of all threegentlemen.

Electrons are negatively charged and thus repeleach other, a phenomena known as Coulomb repul-sion. Bardeen, along with his colleagues, postulatedthat at some attraction also exists between electronsand at low temperatures and this attraction will over-come the Coulomb repulsion and allow electrons to

bind and move together in pairs called Cooper pairs.These pairs of electrons combine in such a way thatallow them to glide effortlessly through a metals lat-tice structure and lose no energy thus moving with-out resistance.

High-Temperature Superconduc-

tors

When Onnes first discovered superconductivity, heenvisioned tremendous practical applications for theworld at large, especially in the transmission of elec-trical power. It is not difficult to imagine why thiswould be so important. As electricity is distributedthrough the electrical grid, as much as 10% of thisenergy is lost before it finds its way to the consumer.Some engineering solutions exist which may reducethis to 7% but there are limits and as the worlds de-mand for electrical energy increases these resistivelosses add up.

Unfortunately for Onnes, his vision of a worldwhere electricity could be transmitted over long dis-tances without loss could not be realized. He woulddiscover that superconductivity was very fragile an-imal that could be destroyed by sufficiently strongmagnetic fields or electric currents. When no mag-netic fields are present, superconductivity starts at acritical temperature but as the external magnetic fieldincreases this critical temperature decreases. Even-tually, strong enough magnetic fields, those above acritical field, will completely destroy a substancesability to super-conduct. Similarly, any currentsabove a critical current will also destroy any super-

conductivity. These low temperature superconduc-tors (LTC), typically made of metals, would not seethe applications Onnes had hoped for.

The BCS theory predicted small critical tempera-tures for metals with the highest theoretical criticaltemperature being 30K or -405.67F (243.20C). Itwould take an expensive and massive refrigerationsystem to maintain these temperatures thus limitingany practical use of superconductivity. Scientists,by then, had long given up on any widespread usefor superconductivity but the field would see sev-eral changes in the coming decades. In 1986 two IBM

researchers, Karl Mller and Johannes Bednorz, dis-covered a material, lanthanum barium copper oxide(LaBaCuO), that super-conducted above the theoret-ical 30K limit. These cuprate superconductors gen-erated a lot of excitement and re-energized the fieldamongst scientists and more high-temperature su-perconductors(HTC) would soon be discovered.

The Limitations of BCS Theory

As remarkable as the BCS theory was in explaining

superconductivity in metals, it failed to explain thesame phenomenon in these new materials. Cuprateor copper oxide superconductors (and other uncon-ventional superconductors) differ in many importantways from conventional LTC superconductors, such



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as mercury, which are adequately explained by BCStheory; their structure is more complex than a metalssimple lattice structure and does not allow for the

formation of Cooper pairs. One of the more excit-ing discoveries came from University of Alabama re-searchers who discovered a material, Yttrium bariumcopper oxide (YBCO), with a critical temperature of93K or -292.27F (-180.15C); exciting because it wasthe first superconducting material to achieve super-conductivity above the boiling point of nitrogen.

The discovery of YBCO allowed superconductiv-ity to be put into practical use as the cost of liquidnitrogen is cheaper than milk or, depending on whoyou ask, beer. The discovery of high-temperaturesuperconductivity in copper oxides also astonished

scientists as copper oxides are generally bad conduc-tors. This further lead scientists to believe that an-other mechanism was taking place. While researchhas indicated the materials and in what combina-tions may lead to high temperature superconductors(HTC), a theoretical understanding is a mystery andstill some distance away.

The Applications of Superconduc-

tors

Superconductors are ideal for producing strong elec-tromagnets and find their biggest application inmedical imaging devices, such as Magnetic Reso-nance Imaging (MRI) and Nuclear Magnetic Reso-nance (NMR). Magnetic-levitation is another appli-cation where superconductors will see future use.Like the magicians trick at the start of this arti-cle, engineers can use the Meissner effect to levitatetrains thereby virtually eliminating friction betweenthe train and its tracks to allow high-speed travel andtransportation.

But the most recent and interesting application

for superconductors concerns Onnes original vision;electrical power transmission. As a significant por-tion of energy is wasted as electricity travels alongcopper wires, the idea of loss-less power transmis-sion has its appeal but the high costs and imprac-ticality of cooling miles of superconducting wire tocryogenic temperatures still limits this application toshort distances. In 2001, three 400-foot HTC cableswere installed in Albany, New York capable of deliv-ering 100 million watts of power to consumers. Thismarked the first time commercial power has been de-livered to US consumers via superconducting wires.

Despite the challenges and costs of burying the HTCcables underground to keep them cool, as well as thecost and energy needed to maintain the liquid nitro-gen cooling system, the cost in energy saved still out-weigh the overall cost of the system. But energy sav-

ings isnt the only reason superconductors are gar-nering interest from utility companies.

The Northeast Blackout of 2003 affected manyparts of the Northeastern and Midwestern UnitedStates as well as Ontario, Canada. At the time, it wasonly the second most widespread blackout in history,after the 1999 Southern Brazil blackout. The cause ofboth blackouts was due to overloaded power lines.As overloaded power-lines can result in a difficultand costly repair, they are automatically shut downwhen detected. Power is then redistributed to otherareas to be compensated by other power lines which,in turn, must have the handling capacity to carry thisexcess load. If these power-lines also become over-loaded, they too can trip and be shut down. This can

result in the overload moving to other parts in thenetwork and may result in a cascade failure of theentire system.

Superconductors can alleviate this problem to adegree. As any currents above the critical current de-stroys superconductivity, the wires become normalconductors, damping and dissipating the extra en-ergy as heat. As this is a built-in current limitingfeature of superconductors it obviates the need forthe bulky circuit breakers placed in a citys powersub-stations. This redundancy can not only translateto a more robust system but also space savings for

cities with limited real estate.

The Search for the Room Tempera-

ture Superconductor

The dream of practical applications for superconduc-tors is an old one, as old as the dream for a room-temperature superconductor. Such a material thatcan maintain its superconducting state well above90F (30C) would be a huge technological boon asthere wont be need for a cryogenic cooling system

to maintain its superconducting state. This could al-low longer runs of superconducting cables and morewidespread distribution. Higher critical tempera-tures also mean higher critical fields and currentswhich will allow for smaller and more powerful elec-tromagnets. This in turn could also revolutionizehigh-speed transportation.

Since the discovery of high-temperature super-conductors, there have been several scientists whohave claimed to discover room-temperature super-conducting materials but so far all independent in-vestigation has proven these claims to be false. J.

Prins, a physicist from the University of Pretoria,South Africa, claimed to have discovered a room-temperature superconducting material in 2001, aclaim that has remained unverified to this day. In2010, D. Das Gupa, a physicist from the University of



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West Bengal, India, submitted a pre-print article witha room temperature superconductor claim. (Scien-tists submit pre-print articles to get comments from

peers in the field and to determine if there are anymerits to their findings before an article is formallysubmitted for publication and peer-review.) So whileDas Gupta has made no formal claim, it is certainlyan interesting one; it not only highlights the needfor these materials but that research is actively tak-ing place.

Scientists are constantly improving their under-standing of these materials and it is only a matter of

time before the secrets of room-temperature super-conductivity are cracked. Room-temperature super-conductivity isnt going to solve all our problems

but as the worlds energy demands continue to growand the need for more efficient high-speed travel in-creases, the need to make better use of our resourcesbecomes an important one. The technological solu-tion that a room temperature superconductor mayone day provide does not mitigate our personal re-sponsibility to not waste energy but one fact remains,the energy wasted during normal power transmis-sion is energy that the world can certainly use.

This newsletter was created with the use of a LATEX style template by David S. Latchman. If you are in needof your own specialized LATEX class or style files, a Beamer Presentation or any other LATEX typsetting task

performed I can be found on Elance.My Elance Page: http://www.elance.com/s/dlatchman/My Home Page: http://thequantumtunnel.wordpress.com/

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