New evidence this week supports a theory developed five years ago at Rice University to explain the electrical properties of several classes of materials -- including unconventional superconductors -- that have long vexed physicists.
The findings in this week's issue of Nature Materials uphold a theory first offered in 2006 by physicist Qimiao Si, Rice's Harry C. and Olga K. Wiess Professor of Physics and Astronomy. They represent an important step toward the ultimate goal of creating a unified theoretical description of the quantum behavior of high-temperature superconductors and related materials.
"We now have a materials-based global phase diagram for heavy-fermion systems -- a kind of road map that helps relate the predicted behavior of several different classes of materials," Si said. "This is an important step on the road to a unified theory."
High-temperature superconductivity is one of the greatest unsolved mysteries of modern physics. In the mid-1980s, experimental physicists discovered several compounds that could conduct electricity with zero resistance. The effect happens only when the materials are very cold, but still far above the temperatures required for the conventional superconductors that were discovered and explained earlier in the 20th century.
In searching for a way to explain high-temperature superconductivity, physicists discovered that the phenomenon was one of a larger family of behaviors called "correlated electron effects."
In correlated electron processes, the electrons in a superconductor behave in lockstep, as if they were a single entity rather than a large collection of individuals. These processes bring about tipping points called "quantum critical points" at which materials change phases. These phase changes are similar to thermodynamic phase changes that occur when ice melts or water boils, except they are governed by quantum mechanics.
Materials at the border of magnetism and superconductivity -- including heavy-fermion metals and high-temperature superconductors -- are the prototype systems for quantum critical points.
In 2001, Si and colleagues proposed what has now become the dominant theory to explain correlated electron effects in heavy-fermion systems. Their "local quantum critical" theory concluded that both magnetism and charged electron excitations play a role in bringing about quantum critical points.
Experiments over the past decade have provided overwhelming evidence for the role of both effects. In addition, experiments have shown that quantum critical points fall into different classes for different types of materials, including several nonsuperconductors.
"In light of the experimental evidence, an important question arose as to whether a unifying principle might exist that could explain the behavior of all the classes of quantum critical points that had been observed in heavy-fermion materials," Si said.
In 2006, Si put forward a new theory aimed at doing just that. Experiments two years ago confirmed that the theoretical global phase diagram could explain the quantum critical behavior of YRS -- composites of ytterbium, rhodium and silicon that are among the most-studied quantum critical materials.
In the new Nature Materials paper, a group led by experimental physicist Silke Paschen of Vienna University of Technology in Vienna examined a new material made of cerium, palladium and silicon (CPS). Both YRS and CPS are heavy-fermion compounds; however, YRS is a composite of stacked two-dimensional layers, and CPS has a three-dimensional crystalline structure.
"In YRS, the collapse of charged electronic excitations occurs at the onset of magnetic order," Paschen said. "In CPS, we established a similar collapse of the electronic excitations but inside an ordered phase."
To explain the difference between the observations in CPS and YRS, Si and co-author Rong Yu, a Rice postdoctoral researcher, invoked the effect of dimensionality.
"In systems like YRS, reduced dimensionality enhances the quantum fluctuations between the electrons, and that enhancement influences their collective behavior," Yu said. "In the three-dimensional material, we found that the quantum fluctuations were reduced, and this affected the quantum critical point and the correlated behavior in a way that was predicted by theory."
Si said the linkage between the quantum critical points of CPS and YRS is important for the ultimate question of how to classify and unify quantum criticality.
"Our study not only highlights a rich variety of quantum critical points but also indicates an underlying universality," he said.
Si said it is important to test the theory's ability to correctly predict the behavior of even more materials, and his group is working with Paschen and other experimentalists via the International Collaborative Center on Quantum Matter to carry out those tests.
Co-authors on the Nature Materials paper include J. Custers, K.-A. Lorenser, M. Müller, A. Prokofiev, A. Sidorenkio and H. Winkler, all of Vienna University of Technology; A.M. Strydom of the University of Johannesburg in South Africa; and Y. Shimura and T. Sakakibara, both of the University of Tokyo. The research was supported by the European Research Council, the Austrian Science Foundation, the National Science Foundation and the Welch Foundation.
A high-resolution image is available for download at: http://www.media.rice.edu/images/media/NewsRels/0104_lorenzer_sidorenko2.JPG
CAPTION: Physics graduate students Karl-Anton Lorenzer (left) and Andrey Sidorenko adjust equipment at Vienna University of Technology. CREDIT: F. Aigner/TU Wien
A high-resolution image is available for download at: http://www.media.rice.edu/images/media/NewsRels/0104_winkler_sidorenko.JPG
CAPTION: Vienna University of Technology graduate students Hannes Winkler (left) and Andrey Sidorenko are co-authors of a new paper that sheds light on "correlated electron effects" in heavy fermion materials. CREDIT: F. Aigner/TU Wien
The Nature Materials paper is available at: http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3214.html
Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation's top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is known for its "unconventional wisdom." With 3,708 undergraduates and 2,374 graduate students, Rice's undergraduate student-to-faculty ratio is less than 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice has been ranked No. 1 for best quality of life multiple times by the Princeton Review and No. 4 for "best value" among private universities by Kiplinger's Personal Finance. To read "What they're saying about Rice," go to www.rice.edu/nationalmedia/Rice.pdf
From ancient fossils to future cars
21.10.2016 | University of California - Riverside
Study explains strength gap between graphene, carbon fiber
20.10.2016 | Rice University
Ultrafast lasers have introduced new possibilities in engraving ultrafine structures, and scientists are now also investigating how to use them to etch microstructures into thin glass. There are possible applications in analytics (lab on a chip) and especially in electronics and the consumer sector, where great interest has been shown.
This new method was born of a surprising phenomenon: irradiating glass in a particular way with an ultrafast laser has the effect of making the glass up to a...
Terahertz excitation of selected crystal vibrations leads to an effective magnetic field that drives coherent spin motion
Controlling functional properties by light is one of the grand goals in modern condensed matter physics and materials science. A new study now demonstrates how...
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.
"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...
In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.
A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...
By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.
"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...
14.10.2016 | Event News
14.10.2016 | Event News
12.10.2016 | Event News
25.10.2016 | Earth Sciences
25.10.2016 | Power and Electrical Engineering
25.10.2016 | Process Engineering