Fleeting fluctuations in superconductivity disappear close to transition temperature

Enlarge

"Our findings suggest that in cuprate superconductors, the transition to the non-superconducting state is driven by a loss of coherence among theelectron pairs,"said Brookhaven physicist Ivan Bozovic, a co-author on a paper describing the results inNature Physicsonline, February 13, 2011.

Scientists have been searching for an explanation of high-T_csuperconductivity in cuprates since these materials were discovered some 25 years ago. Because they can operate at temperatures much warmer than conventional superconductors, which must be cooled to nearabsolute zero(0 K or -273 degrees Celsius), high- Tc superconductors have the potential for real world applications. If scientists can unravel the current-carrying mechanism, they may even be able to discover or design versions that operate at room temperature for applications such as zero-loss power transmission lines. For this reason, many researchers believe that understanding how this transition to superconductivity occurs in cuprates is one of the most important open questions in physics today.

In conventional superconductors, electron pairs form at the transition temperature and condense into a collective, coherent state to carry current with no resistance. In high- Tc varieties, which can operate at temperatures as high as 165 K, there are some indications that electron pairs might form at temperatures 100-200 K higher, but only condense to become coherent when cooled to the transition temperature.

To explore the phase transition, the Johns Hopkins-BNL team sought evidence for superconducting fluctuations above T_c.

"These fluctuations are something like small islands or droplets of superconductivity, within which the electron pairs are coherent, which pop up here and there and live for a while and then evaporate to pop up again elsewhere,"Bozovic said."Such fluctuations occur in every superconductor,"he explained,"but in conventional ones only very, very close to T_c— the transition is in fact very sharp."

Some scientists have speculated that in cuprates, on the contrary, superconducting fluctuations might exist in an extremely broad region, all the way up to the temperature at which the electron pairs form. In the present study, the scientists tackle this question head-on, by measuring the conductivity as a function of temperature and frequency up to the terahertz range.

"With this technique, one can see superconducting fluctuations as short-lived as one billionth of one billionth of a second— the shortest possible— and over the entire phase diagram,"Bozovic said.

The scientists studied a superconductor containing variable amounts of lanthanum and strontium layered with copper oxide. The samples were fabricated at Brookhaven, using a unique atomic-layer molecular beam epitaxy system that allows for digital synthesis of atomically smooth and perfect thin films. Terahertz spectroscopy measurements were performed at Johns Hopkins.

The central finding was somewhat surprising: The scientists clearly observed superconducting fluctuations, but these fluctuations faded out relatively quickly, within about 10-15 K above T_c, regardless of the lanthanum/strontium ratio.

This implies that in cuprates at the transition temperature, electron pairs lose their coherence. This is in contrast to what happens in conventional superconductors, where theelectron pairsbreak apart at thetransition temperature.

"So, unlike in conventional superconductors, the transition in cuprates is not driven by electron (de)pairing but rather by loss of coherence between pairs— that is, by phasefluctuations,"Bozovic said."The hope is that understanding this process in full detail may bring us one step closer towards cracking the enigma of high-temperaturesuperconductivity."

Source

Compact high-temperature superconducting cables demonstrated at NIST

Enlarge

Described in a paper just published online,* the new method involves winding multiple HTS-coated conductors** around a multi-strand copper“former” or core. The superconducting layers are wound in spirals in alternating directions. One prototype cable is 6.5 millimeters (mm) in outer diameter and carries a current of 1,200 amperes; a second cable is 7.5 mm in diameter and carries a current as high as 2,800 amperes. They are roughly one-tenth the diameter of typical HTS cables used in the power grid. (Standard electrical transmission lines normally operate at currents below 1,000 amperes.)

HTS materials, which conduct electricity without resistance when cooled sufficiently (below 77 K, or minus 196 C/minus 321 F, for the new cables) with liquid nitrogen or helium gas, are used to boost efficiency in some power grids. The main innovation in the compact cables is the tolerance of newer HTS conductors to compressive strain that allows use of the unusually slendercopperformer, says developer Danko van der Laan, a University of Colorado scientist working at NIST.

“The knowledge I gained while working at NIST on electromechanical properties of high-temperature superconductors was very important for inventing the initial cable concept,” van der Laan says.“For instance, my discovery that the conductor surviveslarge compressive strainsmade me realize that wrapping the conductor around a small diameter former would most likely work.”

Van der Laan and NIST colleagues demonstrated the feasibility of the new concept by making several cables and testing their performance. They used an HTS material with a critical current that is less sensitive to strain than some other materials. Although the prototype cables are wound by hand, several manufacturers say mass production is feasible.

NIST researchers are now developing prototype compact HTS cables for the military, which requires small size and light weight as well as flexibility to pull transmission lines through conduits with tight bends. Beside power transmission, the flexible cabling concept could be used for superconducting transformers, generators, and magnetic energy storage devices that require high-current windings. The compact cables also could be used in high-field magnets for fusion and for medical applications such as next-generation magnetic resonance imaging and proton cancer treatment systems.

The work was supported in part by the U.S. Department of Energy.

Source

Neutron analysis reveals '2 doors down' superconductivity link

Enlarge

Researchers at the Department of Energy's Oak Ridge National Laboratory and the University of Tennessee, using the Spallation Neutron Source's ARCS Wide Angular Range ChopperSpectrometer, performed spin-wave studies of magnetically ordered iron chalcogenides. They based their conclusions on comparisons with previous spin-wave data on magnetically ordered pnictides, another class of iron-based superconductors.

"As we analyze the spectra, we find that even though the nearest neighbor exchange couplings between chalcogenide and pnictide atoms are different, the next nearest neighbor exchange couplings are closely similar,"said Pengcheng Dai, who has a joint appointment with ORNL's Neutron Sciences Directorate and the University of Tennessee.

Dai referred to theories that have suggested second-nearest-neighbor couplings could be responsible for the widely acclaimed but poorly understood properties ofhigh-temperature superconductors.

"There are theories suggesting that it's the second nearest neighbor that drives the superconductivity,"he said."Our discovery of similar next-nearest-neighbor couplings in these two iron-based systems suggests that superconductivity shares a common magnetic origin."

Oliver Lipscombe of the University of Tennessee, Dai and ORNL's Doug Abernathy used the ARCS time-of-flight instrument on the SNS to study spin waves of the chalcogenide iron-tellurium superconductor and compared these with iron pnictide superconductors.

Scientists have been studying the iron-based superconductors since their discovery in 2008 to see if the dynamics behind their high-temperature superconducting properties -- in which electricity flows without resistance at temperatures well above absolute zero -- could help explain what was until recently thought to be exclusive to copper-oxide-based superconductors.

"Finding commonalities is always a good step when you're looking for a very basic understanding of a phenomenon like high-temperature superconductivity,"said Abernathy, who is lead instrument scientist for the ARCS instrument.

The team's neutron scattering analysis of the materials was made possible by the high intensity of the neutron beams provided by the SNS, which is the world's most powerful pulsedneutronsource. Neutrons, which carry no electric charge but can act as subatomic magnets, are well suited for studying atom-scale spin characteristics.

"Since the interactions in the high-temperature superconductors are so strong, measurement of these materials' spin waves requires beams of energetic neutrons that were unavailable to the research community at this intensity before the SNS,"Abernathy said.

The work, which was funded by the DOE Office of Science, is published inPhysical Review Letters.

Source

Using complex electron systems to create green materials

Enlarge

Accordingly, the RIKEN Advanced Science Institute (Japan) established the Green-forefront Materials Department in April 2010, aiming to create‘green-forefront materials’, that is, new materials for helping to solve environmental and energy problems. In the same year, Hidenori Takagi set up and led the Complex Electrons and Functional Materials Research Group in the Green-forefront Materials Department. This group is striving to create high-temperaturesuperconductorsand thermoelectric materials using complex electron systems.

This year will mark the 100th anniversary of the discovery of the superconductor by the Dutch physicist Heike Kamerlingh Onnes. Superconductors are materials in which electrons can flow with no electrical resistance, and the first such materials displayed this behavior when cooled to near absolute zero. This year will also mark the 25th anniversary of the discovery of the first‘high-temperature’ cuprate superconductor—a material that makes the transition to the zero-resistance state at a temperature of 30 K. In that year, 1986, Takagi successfully conducted a follow-up experiment to verify the discovery of the high-temperature cuprate superconductor. A worldwide search for high-temperature superconductors then ensued with the hope of discovering new materials that could significantly contribute to solving environmental and energy problems as a medium for lossless electric power transmission and storage. Today, the highest transition temperature achieved to date for cuprate superconductors is 160 K, or about−113°C, under high pressure.

Cuprates are basically insulators, so it is one of the most intriguing questions in science how such materials can become superconductors. According to Takagi,“Metals contain unbound electrons that can move freely through the material, so when a voltage is applied to these electrons in this‘gaseous’ state, an electrical current flows.” Free electrons are also present in some transition-metal elements such as cuprates (metallic oxides including metals from group III to group II in the periodic table).“Their electron orbits, however, are so narrow that they cannot move to adjacent atom because they are electrically repelled by the existing electrons of the atom. This state can be compared to that of the solid state in which electrons are stuck tightly in place because they repel each other. The material in this state acts as an insulator. Systems composed of such strongly interacting electron populations are known as strongly correlated electron systems or complex electron systems, and they can change their electrical states or electron phases dramatically in response to small changes in conditions.”

For example, as electron vacancies, called holes, are created in such systems, the remaining electrons slowly begin to move, changing the electron state from tightly bound solid state to more fluid mobile state (Fig. 1).“In strongly correlated electron systems, the solid electron state first changes into a sticky liquid and then a gaseous state in which electrons can move freely. These systems change state in response to changes in conditions. That is where the fun is.”

High-temperature superconductivity occurs when complex electron systems change state. Electrons in a superconductor are known to form pairs of electrons called Cooper pairs. The mechanism by which electrons form Cooper pairs in cuprates when they usually repel each other under, however, remains a mystery. Takagi and his group have been conducting world-leading research into achieving direct observations of the electron state in high-temperature cuprate superconductors by scanning tunneling microscopy (STM).“We found that most electrons in high-temperature cuprate superconductors are stuck tightly in place except for a small number of electrons that are left free from atomic bonds. This state is considered to be what creates Cooper pairs and causes superconductivity.

Many unknown electron states lie hidden in complex electron systems.“We aim to discover new electron states and to use them to create new materials.”

The states of complex electron systems can also be changed by taking advantage of the property of electrons known as‘spin’. Spin describes the angular momentum of an electron, similar in nature to the rotation of Earth, and can be either‘up’ or‘down’. In the same was as two bar magnets arranged in the same direction repel each other yet when arranged in opposite directions are pulled together, a square lattice of atoms is most stable when the up and down spins of adjacent are arranged alternately (Fig. 1).“If the directions of electron spins are arranged in an orderly manner, spins are considered to form a spin solid. How are the spins aligned in a triangular atomic lattice? There are three electrons located at the vertices of a triangle, but only two spins can form a pair, leaving one unpaired spin. In such a lattice, the orientations of electron spins are unstable. This state, called‘spin frustration’, is considered to be responsible for s spin liquid state.”

Electron systems tend to become more stable at lower temperature. In 1973, however, it was theoretically predicted that a lattice with extremely strong spin frustration could adopt a special state called a‘quantum spin liquid’, in which the directions of electron spins do not become stable even when the temperature is lower to absolute zero.“This state, of course, is very hard to produce and has long been considered merely a theoretical dream.”

Enlarge

Figure 2: Na4Ir3O8, a transition-metal oxide that can turn into a quantum spin liquid. The transition-metal oxide Na4Ir3O8 has a triangular lattice that forms spirals. It exhibits strong spin frustration and becomes a quantum spin liquid at absolute zero.

Yet Takagi and his group were eventually successfully in creating a quantum spin liquid using the compound Na4Ir3O8, a transition metal oxide (Fig. 2) consisting of a triangular lattice of atoms in a spiral configuration. Takagi named this structure the‘hyper-kagome lattice’. Another research group also successfully created a quantum spin liquid using a different structure at around the same time, and the field now attracts a great deal of attention.

The possible uses of complex electron systems are actually more practical than might be expected.“One of the best examples is water ice that freezes at 10°C,” says Takagi.“Wine and sushi, for example can be overcooled if placed in normal ice at 0°C. Ice that freezes at 10°C would be very convenient in these types of situations. We have in fact created a coolant in which the electron state changes from solid to liquid at 10°C.”

Takagi and his co-workers have also taken advantage of spin frustration to successfully develop a new compound known as Mn3XN with a thermal expansion coefficient of zero (Fig. 3). Manganese nitride, the base material for the new compound, has a triangular lattice of manganese atoms. The compound adopts a liquid state at high temperature because of its electron state, and becomes a solid when the temperature is lowered to near room temperature. The electrons try to stay at the triangular lattice points, but the directions of electron spins remain unstable because of the strong spin frustration in this system. The surrounding lattice, however, expands when cooled to relax the spin frustration, resulting in a material that expands when cooled and contracts when heated—the opposite behavior to that of most natural compounds. For example, an iron bar increases in length by 0.0012% for every 1°C increase in temperature. Although small, such thermal expansion can sometimes causes problems in processing equipment and high-precision measuring devices, particularly in semiconductor circuits where nanometer-accuracy is required.

Enlarge

Figure 3: Mn3XN, a manganese nitride with negative thermal expansion coefficient. The manganese nitride Mn3XN has an antiperovskite structure. The symbol X represents copper or zinc. The component X can be replaced with germanium or tin atom, resulting in a material with a zero thermal expansion coefficient.

“Manganese nitride itself contracts discontinuously at a certain temperature. By introducing germanium or tin, we can create a substance that contracts gradually over a temperature range of about 100°C. In 2008, we successfully created a new material with a zero thermal expansion coefficient, meaning it does not change volume, over a temperature range of about 70°C including room temperature.”

Conventional substances with a zero thermal expansion coefficient are compound materials consisting of compounds with different expansion coefficients, which leads to deformation or cracking and low strength. Such materials are also usually expensive because they contain rare elements.“Manganese nitride is resistant to deformation and cracking, and is also less expensive than conventional substances because it is a single compound.”

In 2010, Takagi and his colleagues started the Complex Electrons and Functional Materials Research Group at the RIKEN Advanced Science Institute.“We aim to create‘green-forefront materials’ that help to solve environmental and energy problems by taking advantage of complex electron systems. A typical example of our work is the development of high-efficiency thermal conversion materials.”

Only 30% of the thermal energy produced by the combustion of gasoline in an automobile engine is used to power the vehicle; the other 70% is lost as waste heat. A large proportion of the thermal energy produced by burning fossil fuels in thermal power stations and many factories is also lost as waste heat. The key to reducing fossil fuel usage and carbon dioxide emissions is how effectively that waste heat can be harnessed. There are high hopes for thermal conversion materials, or thermoelectrics, which convert heat energy into electrical energy and vice versa.

“The currently used Bi2Te3-based thermoelectrics cannot be used at high temperatures and are not very efficient at converting heat to electricity. These include the Peltier elements used in some mobile refrigerators, which use electricity to cool beverages, for example, by the reverse thermoelectric effect, but which do not have sufficient conversion efficiency to make ice. We aim to develop efficient thermoelectric materials that could be used to freeze water.”

Spin frustration could be utilized to increase the thermal conversion efficiency of such materials.“The principle of heat-to-electricity conversion is based on temperature difference. A temperature difference causes electrons or holes to move and thus produce a voltage, generating electrical power. The higher the disorder in the electron state, the higher the conversion efficiency, and the more electricity is generated. For example, entangled electrons with strong spin frustration are unstable and move in a more random manner, increasing the amount of disorder, or entropy.” Takagi and his team are working to create new high-efficiency thermoelectrics based on a new principle such as spin frustration.

In order to efficiently use the waste heat generated by vehicles and factories, the thermoelectric materials must be able to withstand temperatures of more than several hundred degrees celsius.“We should be able to develop heat-resistant, high-efficiency thermoelectrics because the transition metal oxides we are dealing with are strong and heat-resistant.”

In 2008, Hideo Hosono and his research group at the Tokyo Institute of Technology in Japan discovered an iron-based superconductor that attracted worldwide attention.“This superconductor has a transition temperature of 55 K, which is currently the highest transition temperature for non-cuprates. It was previously considered impossible to produce magnetic superconducting materials such as those containing iron, cobalt or nickel, so these iron-based superconductors probably have a different superconducting mechanism to that in cuprates,” says Takagi.

In April 2010, Tetsuro Hanaguri, a senior research scientist in RIKEN’s Magnetic Materials Laboratory, successfully observed the structure of a Cooper pair ofelectronsin an iron-based superconductor by STM. Metallic superconductors are known to form a type of Cooper pair called an s wave, whereas cuprate superconductors form a Cooper pair called a d wave. Hanaguri demonstrated for the first time that iron-based superconductors formed a new type of Cooper pair called an s± wave. Spin fluctuation is considered to be the mechanism responsible for creating Cooper pairs in iron-based superconductors.

Recent advances in research on superconductivity have raised the expectations for creating room-temperature superconductors, which would be the ultimate green forefront material.“The highest-temperature superconductor produced so far was discovered in 1994, a record that has remained unbroken for 16 years. This is the longest period since 1911, when superconductivity was first discovered, that there has been no increase the highest recorded transition temperature. We are adrift in research on higher-temperature superconductors. To challenge the status quo, I would like to break the long-standing record of 160 K by discovering a new high-temperature superconductor based on a new principle, which would again trigger a worldwide race for higher-temperature superconductors.”

Source