Superconductors with split personalities

New superconductors could emerge from research by American and Japanese physicists who are investigating new ceramic materials with split personalities.

A class of materials known as cuprates have been found to switch between two different kinds of superconductivity under certain physical conditions and Thomas Lemberger (http://www.physics.ohio-state.edu/~trl/group/) of Ohio State University and colleagues at the NTT Basic Research Laboratories, in Kanagawa, Japan hope this dual character might be exploited in creating tough new superconductors as well as settling a debate among physicists about how cuprate superconductivity arises in the first place.

Cuprate semiconductor

Cuprate semiconductor

There are two camps among physicists studying cuprate superconductors. In one, they believe these materials display d-wave superconductivity but the other camp claims a mutually exclusive position citing cuprates as s-wave materials.

The difference depends on how the electrons are arranged within the material, explains Lemberger. Materials with s-wave behaviour could be more useful technologically as they may, for instance, perform better in high-frequency devices like mobile phone base stations. In order to exploit superconductivity, which is the loss of electrical resistance in a material, researchers need to find materials that do so at close to everyday temperatures rather than the chilly climes of liquid nitrogen and below.

Instrumentation in the Lemberger lab

Instrumentation in the Lemberger lab

Unfortunately, most high-temperature cuprates have been shown to exhibit the less desirable d-wave behaviour.

Lemberger

Lemberger

A glimmer of hope appeared on the horizon in 1991 when Bell Labs scientists observed s-wave superconductivity in the all-carbon soccerball molecule buckminsterfullerene when they added, or doped it with, potassium ions at a relatively balmy temperature. Researchers have been kicking around the idea of materials displaying s-wave superconductivity at higher temperatures ever since.

The familiar face of superconductors showing the Meissner effect

The familiar face of superconductors showing the Meissner effect

Now, Lemberger and his team say they can flip the behaviour of a certain type of cuprate from d-wave to s-wave if they dope it with enough cerium metal ions. The formulae for the materials they have studied are Pr(2-x)Ce(x)CuO(4) and La(2-x)Ce(x)CuO(4). It seems that the mechanisms for both kinds of behaviour are always present in these materials, Lemberger explains, So if you do something to suppress one behaviour, a cuprate will automatically switch to the other.

Lemberger explains that changing the composition of a cuprate just a little seems to turn it into a superconductor. With the tiniest wisp of voltage, you’ll get huge currents flowing, he enthuses.

In the cuprates, copper and some oxygen atoms (one Cu to two Os) form a one-atom-thick layer between the other atoms, which Lemberger describes as being like a slice of bologna between two crackers. The crackers comprise the rest of the oxygen atoms and the metal atoms, like Pr and Ce. And, like crackers, these compounds are brittle, so cannot be made into useful wires. Nevertheless, he told Spotlight, remarkable progress has been made and in five years superconducting wires will be routinely considered by companies as an option for power lines, e.g., between dams and cities tens of miles away.

Lemberger adds that the scientific controversy surrounding the nature of superconductivity in cuprates will come to a head this summer, as researchers gather in Taiwan to debate which of the two personalities, d-wave or s-wave, is the true state of the material. Our work bridges the gap between the two camps, Lemberger says, We propose that it’s just a matter of composition. Pushing the cuprates towards the s-wave character could ultimately lead to new high-temperature superconductors. Such materials hold the promise of much more efficient power generation because an electric current will flow through with zero energy losses due to heating caused by the material’s resistance.

Further reading

Phys. Rev. Lett. 88, 207005 (2002)
http://dx.doi.org/10.1103/PhysRevLett.88.207005

Suggested searches

High Temperature Superconductors
Superconductors

Sun and Bass

A new member of the stellar orchestra has been identified by astronomers and the star, xi Hya in the constellation of hydra 130 light-years away, plays a very different tune to our own star, the Sun.

The Sun resonates like a giant, spherical organ pipe. It produces well-defined notes although they are far too deep to be seriously described as music. The energy that produces these enormous waves of sound comes from the turbulent region just below the Sun’s visible surface. Solar scientists have been studying these sound waves with the art of helioseismology for about thirty years. As with the Earth’s own vibrations before and after an earthquake, the researchers can use the frequency and movement of the waves to indirectly explore the Sun’s interior.

Oscillation frequencies

Oscillation frequencies

Astronomers are now tuning into the pitch of stars elsewhere in the heavens and have applied the general variation on the theme – asteroseismology – to stars that resemble the sun. The first observations of a star very different from the sun have been made on xi Hya also known rather enigmatically as CD-31 9083 among some audiences.

An international team of astronomers has observed xi Hya with the Swiss 1.2 metre Euler telescope at the European Southern Observatory’s La Silla Observatory, in Chile, and discovered that the star behaves like a giant sub-ultra-bass instrument. Xi Hya is twenty times the diameter of our Sun and is about 60 times more luminous; it is also, sadly close to retirement from the firmament, so a very different instrument indeed. The team has measured the timbre of xi Hya and found that it oscillates with several periods of around 3 hours.

Views of the ESO La Silla Observatory

Views of the ESO La Silla Observatory

It is estimated that the star’s outer envelope will soon, within hundreds of thousands of years, anyway, expand and the star will take on the stature of a red giant. Asteroseismology hopes to become the scientific method of choice for reading the underlying score of such changes and providing a more detailed understanding of stellar interiors and the overall evolution of stars.

View of the ESO La Silla Observatory (2)

View of the ESO La Silla Observatory (2)

The team led by Conny Aerts of the Catholic University of Leuven, in Belgium, Soeren Frandsen of the University of Aarhus in Denmark, and Fabien Carrier of the Geneva Observatory in Sauverny, Switzerland and their colleagues used the telescope’s CORALIE spectrograph to measure the oscillation velocities of the stellar surface.

Soeren Frandsen

Soeren Frandsen

xi Hya is a giant star so the waves need more time to propagate through from deep within the star but the surface reveals a broad distribution of about a dozen different frequency sound waves. The team point out that it is more difficult to model the interior of a giant star than our familiar sun because the giant’s core has changed a lot during its evolution. The sound waves in the sun are mostly concentrated in the outer parts of the Sun but for xi Hya there are also gravity modes to take into account deep in the interior of the star.

Non-radial oscillations

Non-radial oscillations

The team is planning to use the CORALIE and the, soon to be installed, HARPS instruments to listen to the sound show of other stars at different stages in their evolution from those still tuning up to the middle-aged virtuosi.

Acoustic waves in a Solar-like star

Acoustic waves in a Solar-like star

The researchers have taken the bold step of announcing their results ahead of publication in Astronomy & Astrophysics Letters. We oppose that researchers keep their results for a long time before informing the press, hence we have given our first results free for the ESO publication relations office to give an example that this [technique] is feasible, Aerts told Spotlight, It is a little risky for us, but I do not like the chase for high impact factors and citations.

The MP3 file linked below raises the pitch of xi Hya’s voice 1 million-fold revealing what some have described as the thundering approach of the Four Horsemen of the Apocalypse rather than the music of the spheres.

Further reading

European Southern Observatory’s La Silla Observatory
http://www.eso.org/sci/facilities/lasilla/

Conny Aerts
http://www.ster.kuleuven.ac.be/~conny/mons/mons.html

Soeren Frandsen
http://www.phys.au.dk/~srf/

MP3 file
http://www.psigate.ac.uk/spotlight/images/PSI4-stellarsound.mp3

Suggested searches

Asteroseismology
Helioseismology
Stellar evolution

The Gem of Siberia

Lake Baikal is the deepest lake on Earth and is one of Asia’s largest bodies of fresh water. But, it seems to be getting bigger, faster than sediment supply can fill it. Russian scientists have taken a closer look at why.

For millions of years the chilling and crystal clear waters of Lake Baikal, the Gem of Siberia, have harboured a deep-water repository of unique and indigenous fauna, such as the Baikal seal, local forms of Arctic cisco and gobies, and a staggering one-fifth of the Earth’s fresh water. Alluvion constantly pours into the lake through landslides, mudflows and river flooding.

Lake Baikal

Lake Baikal

A Russian team has now shown that the volume of the incoming alluvion is four times less than the increase in size of the basin, which can only be explained by the changing crust of the Earth at the bottom of the lake and thankfully means that Baikal will still be on maps for years to come.

Boris Agafonov of the Institute of Earth’s Crust of the Siberian Branch of the Russian Academy of Sciences located in Irkutsk says that Lake Baikal is in a seismically active but comparatively young area. Geologically speaking, the elongated basin of the lake is a rift trough like those found at the bottom of the oceans.

Agafonov and his colleagues believe that the lake might be considered a prototype ocean. Satellite data have revealed that the Baikal basin is extending at a rate of 5 mm a year, which is equivalent to an increase in volume of about 20 million cubic metres but more than that Agafonov reckons the volume of the lake is also increasing as the original basin bed subsides through earthquake activity.

Agafonov’s calculations reveal that movements of the earth’s crust have resulted in the hollow of the lake having increased immensely since the massive earthquake of 1862. During the 139-year period, from 1862 to 2001, the volume of the lake increased by 3.95 billion cubic metres while the volume of water has increased by 2.9 billion cubic metres, Agafonov explained to Spotlight. The lake is, one would assume, likely to remain the Gem of Siberia for the foreseeable future.

Further reading

Doklady RAN (Reports of Russian Academy of science), V. 382, 4, pp. 540-542; (in Russian)

Suggested searches

Lake Baikal
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