technology brief 3 superconductivityc3.eecs.umich.edu/techbriefs/tb03.pdf · 2015-06-25 ·...

“book” — 2015/5/4 — 6:57 — page 57 — #8

TECHNOLOGY BRIEF 3: SUPERCONDUCTIVITY 57

Technology Brief 3Superconductivity

When an electric voltage is applied across two points in aconductor, such as copper or silver, current flows betweenthem. The relationship between the voltage difference Vand the current I is given by Ohm’s law, V = IR, where Ris the resistance of the conducting material between thetwo points. It is helpful to visualize the electric currentas a fluid of electrons flowing through a dense forestof sturdy metal atoms, called the lattice. Under theinfluence of the electric field (induced by the appliedvoltage), the electrons can attain very high instantaneousvelocities, but their overall forward progress is impededby the frequent collisions with the lattice atoms. Everytime an electron collides and bounces off an atom, someof that electron’s kinetic energy is transferred to theatom, causing the atom to vibrate—which heats up thematerial—and causing the electron to slow down. Theresistance R is a measure of how much of an obstacle theresistor poses to the flow of current, as well as a measureof how much heat it generates for a given current. Thepower dissipated in R is I 2R if I is a dc current, and it is12 I 2R if the current is ac with an amplitude I .

Can a conductor ever have zero resistance? Theanswer is most definitely yes! In 1911, the Dutch physicistHeike Kamerlingh Onnes developed a refrigerationtechnique so powerful that it could cool helium downlow enough to condense it into liquid form at 4.2 K(0 kelvin = −373 ◦C). Into his new liquid helium container,he immersed (among other things) mercury; he soondiscovered that the resistance of a solid piece ofmercury at 4.2 K was zero! The phenomenon, which wascompletely unexpected and not predicted by classicalphysics, was coined superconductivity. According toquantum physics, many materials experience an abruptchange in behavior (called a phase transition) whencooled below a certain critical temperature TC.

Superconductors have some amazing properties. Thecurrent in a superconductor can persist with no externalvoltage applied. Even more interesting, currents havebeen observed to persist in superconductors for manyyears without decaying. When a magnet is broughtclose to the surface of a superconductor, the currentsinduced by the magnetic field are mirrored exactlyby the superconductor (because the superconductor’sresistance is zero), and consequently the magnet isrepelled (Fig. TF3-1). This property has been usedto demonstrate magnetic levitation and is the basisof some super-fast maglev trains (Fig. TF3-2) being

Figure TF3-1: The Meissner effect, or strong diamag-netism, seen between a high-temperature superconductorand a rare earth magnet. (Courtesy of Pacific NorthwestNational Laboratory.)

Figure TF3-2: Maglev train. (Courtesy of Central JapanRailway Company.)

used around the world. The same phenomenon is usedin the Magnetic Resonance Imaging (MRI) machinesthat hospitals use to perform 3-D scans of organs andtissues (Fig. TF3-3) and in Superconducting QuantumInterference Devices (SQUIDs) to examine brain activityat high resolution.

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58 TECHNOLOGY BRIEF 3: SUPERCONDUCTIVITY

Figure TF3-3: Magnetic Resonance Imaging machine. (Courtesy GE Healthcare.)

Superconductivity is one of the last frontiers in solid-state physics (see Table TT3-1). Even though the physicsof low-temperature superconductors (like mercury, lead,niobium nitride, and others) is now fairly well understood,a different class of high-temperature superconductorsstill defies complete theoretical explanation. This classof materials was discovered in 1986 when Alex Mullerand Georg Bednorz, at IBM Research Laboratoryin Switzerland, created a ceramic compound thatsuperconducted at 30 K. This discovery was followedby the discovery of other ceramics with even higherTC values; the now-famous YBCO ceramic discoveredat the University of Alabama-Huntsville (1987) has

a TC of 92 K, and the world record holder is agroup of mercury-cuprate compounds with a TC of138 K (1993). New superconducting materials andconditions are still being found; carbon nanotubes,for example, were recently shown to have a TC of15 K (Hong Kong University, 2001). Are there higher-temperature superconductors? What theory will explainthis higher-temperature phenomenon? Can so-calledroom-temperature superconductors exist? For engineers(like you) the challenges are just beginning: How canthese materials be made into useful circuits, devices, andmachines? What new designs will emerge? The race ison!

Table TT3-1: Critical temperatures.

Critical Temperature Tc [K] Material Type

138 HgBa2Ca2Cu3Ox

138 Bi2Sr2Ca2Cu3O10 (BSCCO) Copper-oxide superconductors92 YBa2Cu3O7 (YBCO)

55 SmFeAs41 CeFeAs Iron-based superconductors26 LaFeAs

18 Nb3Sn10 NbTi Metallic low-temperature9.2 Nb superconductors4.2 Hg (mercury)