New Look of Superconductivity

NEWS=20 RELEASE
Office of Public Affairs
111 = TASF
Ames, IA=20 50011-3020
http://www.external.ameslab.gov

For release: July 5 , 2007=20

Contacts:
Ruslan Prozorov, = Condensed=20 Matter Physics (515) 294-9901
Saren Johnston , = Public=20 Affairs, (515) 294-3474
Kerry Gibson, Public = Affairs,=20 (515) 294-1405

THE NEW "LOOK" OF=20 SUPERCONDUCTIVITY

Ames Laboratory Discovery Sheds New Light = on 70=20 Years of Textbook Physics

AMES, Iowa =96 Like the = surface motif=20 of a bubble bath, the spatial distribution of a magnetic = field=20 penetrating a superconductor can exhibit an intricate, = foam-like=20 structure. Ruslan Prozorov at the U.S. Department of = Energy=92s=20 Ames Laboratory has observed these mystifying, = two-dimensional=20 equilibrium patterns in lead samples when the material is in = its=20 superconducting state, below 7.2 Kelvin, or minus 446.71 = degrees=20 Fahrenheit.

Through innovative research to relate the = complex=20 geometry of the equilibrium patterns to the macroscopic = physical=20 properties, such as magnetism, Prozorov has shown that the = shape of=20 the entire sample determines the pattern topology and = overall=20 magnetic behavior of the system =96 a significant finding = that=20 represents a major contribution to the field of=20 superconductivity. =93You can have the same volume and = same=20 mass, but if you just change the shape, you get a different = type of=20 response from the sample and a different type of geometry of = the=20 equilibrium field pattern,=94 he said. =93The = discovery has=20 reopened the whole field of equilibrium in type-I = superconductors,=20 which had gone dormant because it was considered=20 = closed.=94 &nb= sp;

Prozorov=92s discovery of the = complex patterns=20 in superconducting lead marks a noteworthy departure = from the=20 model first proposed by Russian physicist Lev Landau = in the=20 1930s. Landau=92s model, which resembles a = labyrinth or=20 laminar pattern, has been the unchallenged standard in = physics=20 textbooks for 70 years.

But Prozorov questions the Landau = model and=20 maintains that it=92s impossible to deduce the = equilibrium=20 patterns of superconductors from global energy = minimization =96=20 an established law of physics. =93You can assume = a certain=20 geometry or pattern and work with it to find an = optimal=20 configuration, but that doesn=92t guarantee that the = pattern=20 you=92ve assumed is the one that will turn out as the = absolute=20 minimum energy state in nature,=94 he = explained.

3D"Superconducting

Equilibrium = patterns in=20 superconducting lead: left, Prozorov=92s = =93soap-foam=94 pattern;=20 and right, the Landau laminar pattern. Both images are = obtained at the same temperature and magnetic field. = The only=20 difference is how the magnetic field was increased or=20 decreased to reach = equilibrium.

Offering an example of the problem = he sees=20 with the Landau model, Prozorov said, =93If you = assume two=20 patterns, you can calculate the total energy for each = of them,=20 and the one with the lowest energy may be the = equilibrium=20 pattern. Of course, you can=92t prove that there = isn=92t=20 another pattern that has even lower energy. You = need to,=20 in point of fact, observe the patterns and relate them = to the=20 actual measured physical properties.=94

Over the years there have been = observations of=20 equilibrium patterns in superconductors that differ = from the=20 labyrinth model proposed by Landau. However, the = unusual=20 patterns were considered to be defects or fluctuations = due to=20 imperfections in the material under study. No = one=20 bothered to relate the patterns they were observing to = macroscopic properties. No one, that is, until=20 Prozorov.

=93It all started with an accidental = finding,=94=20 he said. I was trying to calibrate a thermometer = in my=20 magneto-optical cryostat, so I put in a very clean,=20 stress-free piece of lead. This is an easy way = to=20 calibrate because lead becomes superconducting at 7.2 = Kelvin,=20 so when I looked at my sample and saw = superconductivity, I=20 knew my thermometer was correct.=94

But something else wasn=92t correct, at = least not=20 textbook correct. When Prozorov applied a sufficiently = large=20 magnetic field and looked at the lead sample in the = magneto-optics=20 system, he was surprised to see not the Landau labyrinth = pattern=20 but, rather, a pattern of two-dimensional tube shapes that = he=20 describes as looking like soap foam. =93I was shocked = because=20 this was totally unexpected,=94 he said. =93So now the = big=20 question was which pattern represents equilibrium?=94

Prozorov=92s experiments showed that, = depending on its=20 purity and macroscopic physical shape, the sample under=20 investigation displayed either the soap-foam pattern or the = Landau=20 laminar pattern. He knew that samples like disks or = slabs that=20 have two parallel surfaces also have a property known as a = geometric=20 barrier. Only those sample shapes exhibited the Landau = pattern, and only when the magnetic field was reduced. = However, Prozorov discovered that shapes without two flat = surfaces,=20 such as spheres, hemispheres, pyramids and cones, don=92t = exhibit the=20 Landau behavior. =93We observed the foam, or tubular, = phase in=20 all of these sample shapes, and we don=92t have the Landau = phase at=20 all,=94 he said. =93So it=92s the foam phase that=92s = the equilibrium=20 state of the system. Most of the past studies were = done on=20 samples with flat surfaces, that=92s why people never = observed this=20 previously for decreasing magnetic field.=94

Emphasizing the difficulty involved in = creating=20 these less common sample shapes, Prozorov said, =93To = observe this=20 soap-foam phenomenon, the samples must be very clean and = defect-free=20 with a uniformity of crystal structure. We spent a lot = of time=20 trying to make lead samples in the shapes of hemispheres, = cones and=20 pyramids and finally succeeded. Having access to the = materials=20 expertise available at Ames Laboratory has been a tremendous = benefit=20 in our efforts,=94 he added.

The DOE Office of Science, Basic Energy = Sciences=20 Office and the National Science Foundation funded the above = work on=20 equilibrium patterns in superconductors.

Ames=20 Laboratory, celebrating its 60th anniversary in 2007, is = operated for the Department = of=20 Energy by Iowa State = University. The Lab conducts research into various areas = of=20 national concern, including energy resources, high-speed = computer=20 design, environmental cleanup and restoration, and the = synthesis and=20 study of new materials.

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Note to Editors: A=20 professional journal article, =93Equili= brium=20 Topology of the Intermediate State in Type-I Superconductors = of=20 Different Shapes,=94 by Ruslan Prozorov appears in = Physical=20 Review Letters, June 22, 2007.

Real time visualization of the =93soap = foam=94=20 equilibrium pattern in a single crystal of lead is available = at http:/= /www.cmpgroup.ameslab.gov/supermaglab/video/Pb.html.

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