A unique superconductor

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2011

YBAL exhibits a special property known as quantum criticality in its natural state. 

KEY CONCEPTS
• A new superconductor known as YBAL exhibits a special property known as quantum criticality.
• YBAL displays quantum criticality in its natural state, which is unusual for superconductors.
• The researchers used experimental data to pinpoint the position of the quantum critical point in YBAL’s phase diagram.

As those in the lubricant industry struggle to minimize friction, there is one class of materials that has already reached this goal, superconductors. They are generally metallic alloys that conduct electricity without the dissipation of any heat. This phenomenon occurs at low temperatures, and means that electrons move through the material without friction. The result is that power loss is effectively eliminated.

Piers Coleman, professor of physics and astronomy at Rutgers University in Piscataway, N.J., says, “Classic metals form a Fermi liquid named after the Italian physicist Enrico Fermi. It is a kind of incompressible quantum mechanical fluid in which the electrons are very individualistic, and none of them like to move at the same velocity or occupy the same space as others.” 

Coleman continues, “Superconductivity was first discovered 100 years ago when mercury was cooled to a critical temperature at which friction-to-electron motion vanished altogether. In a superconductor, electrons develop a new type of behavior, where they all move together in the same direction in a similar manner to a flock of birds. When they move collectively, they do so without dissipation (friction).” 

Another important feature is that the collective flow of electrons in a superconducting material leads to the expulsion of magnetic fields from its interior. It is this property that leads to the levitation of superconductors in a magnetic field—a property of vital importance in the development of the Maglev train.

Coleman indicates that many low-temperature superconductors were discovered in the past century, but in the late 1980s a new class of superconductors that develop the property at higher temperatures was discovered. These materials, of the copper oxide family, hold the current record of the superconducting transition temperature of 135 K at ambient pressure. Recently, a new class of iron-based, high-temperature superconductors was discovered, and the search for novel superconducting materials continues.

A previous TLT article focuses on opportunities for these high-temperature superconductors (HTS) (1). included is the use of HTS in such applications as military power systems, electric vehicles and generation systems.

QUANTUM CRITICAL POINT
One of the issues facing researchers is to explain how materials can be made superconducting. A recent study of a new superconductor based on the element ytterbium (Yb) has led to better understanding of these materials and may also reveal an exotic new phase of matter realized at low temperatures.

Superconductivity in this material, known as beta-Ybaib4 (YBAL), occurs at very low temperature (below 0.1 K), yet it is hoped that understanding the mechanism of superconductivity will help shed light on the origin of this phenomenon in technologically important HTS materials.

Coleman and his associate, Andriy Nevidomskyy (now assistant professor of physics and astronomy at Rice University in Houston), working with colleagues from the University of Tokyo, have determined that YBAL exhibits a special property known as quantum criticality. At a quantum critical point, electrons no longer follow the classic Fermi liquid rules but instead exhibit a new type of “strange metal” behavior that is not well understood at present. Quantum critical metals show a marked tendency to develop superconductivity and, for this reason, they are of great interest.

At the heart of the team’s finding, is the unconventional quantum criticality in YBAL at ambient pressure and zero magnetic field. Nevidomskyy explains, “Most metals need to be tuned in order to reach a quantum critical point. Typically, this step is achieved by exposure to high magnetic fields and pressures or adding chemical impurities. YBAL is unusual because this superconductor’s quantum critical point occurs in its natural state without any tuning.” 

The structure of YBAL is furnished in Figure 2. Ytterbium atoms are shown in purple, aluminum atoms are in green, and boron atoms are gray.


Figure 2. YBAL, a new superconductor displays a special property known as quantum criticality at ambient pressure and zero magnetic field. (Courtesy of Science/AAAS)

Coleman and Nevidomskyy are very surprised by their finding. Coleman says, “We have found a material that is intrinsically quantum critical with very simple behavior. This is puzzling because there is nothing simple about the material’s structure. Nevidomskyy adds, “Typically, one would need to turn an experimental knob in order to reach quantum criticality in a material. For YBAL, we did not need to do anything.”

YBAL was synthesized by the University of Tokyo researchers, who also generated a considerable amount of data measuring the material’s response to the magnetic field. Theoretical physicists Coleman and Neivodmskyy took this data and conducted computational analyses over a wide range of temperatures and magnetic field strengths to pinpoint the position of the quantum critical point in the material’s phase diagram.

From their analysis, Coleman and Nevidomskyy concluded that the quantum critical point in YBAL must lie within a hair’s breadth of zero magnetic field. This critical point cannot be observed directly since it is obscured by the superconductivity but is reflected in the magnetic properties of this superconductor.

Nevidomskyy likens this finding to determining the location of a black hole in space. He says, “A black hole in space cannot be observed by direct means because light cannot escape from it. But its location can be determined due to the gravitational pull it exerts on neighboring stars. Just as black holes form a singulatory in space-time, YBAL exhibits a singular behavior at a quantum critical point in its phase diagram.”

The finding of an intrinsic quantum criticality in YBAL may be suggestive of a new phase of matter known as the critical strange metal phase. The existence of a critical phase rather than a critical point has been debated for years by physicists, according to Coleman.

Future work that may shed light on this issue will involve learning more about the properties of YBAL through analysis of this material under applied pressure. Additional information can be obtained in a recent article (2) or by contacting Carl Blesch of Rutgers University at cblesch@ur.rutgers.edu. 

REFERENCES
1. Kohanloo, B. (2009), “Why Superconductivity=Super Opportunity,” TLT, 65 (9), pp. 34–38.
2. Matsumoto, Y., Nakatsuji, S., Kuga, K., Karaki, Y., Horie, N., Shimura, Y., Sakakibara, T., Nevidomskyy, A. and Coleman, P. (2011), “Quantum Criticality Without Tuning in the Mixed Valence Compound Beta-YbAIB4,” Science, 331 (6015), pp.316–319.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.