U.S., German and Austrian physicists studying the perplexing class of materials that includes high-temperature superconductors are reporting this week the unexpected discovery of a simple "scaling" behavior in the electronic excitations measured in a related material. The experiments, which were conducted on magnetic heavy-fermion metals, offer direct evidence of the large-scale electronic consequences of "quantum critical" effects.
"Our experiments clearly show that variables from classical physics cannot explain all of the observed macroscopic properties of materials at quantum critical points," said lead experimentalist Frank Steglich, director of the Max Planck Institute for Chemical Physics of Solids.
The experiments by Steglich's group were conducted on a heavy-fermion metal containing ytterbium, rhodium and silicon that is known as YbRh2Si2 (YRS). YRS is one of the best-characterized and most-studied quantum critical materials.
Quantum criticality refers to a phase transition, or tipping point, that marks an abrupt change in the physical properties of a material. The most common example of an everyday phase change would be the melting of ice, which marks the change of water from a solid to a liquid phase. The term "quantum critical matter" refers to any material that undergoes a phase transition due solely to the jittering of subatomic particles as described by Heisenberg’s uncertainty principle. Heavy-fermion metals like YRS are one such material class, and considerable evidence exists that high-temperature superconductors are another.
Scientists are keen to better understand high-temperature superconductivity because the technology could revolutionize electric generators, MRI scanners, high-speed trains and other devices.
High temperature superconductivity typically arises at the border of magnetism, and some physicists believe it originates in the fluctuations associated with magnetic quantum criticality. In magnetic systems such as YRS, traditional theories attempt to explain quantum criticality by considering magnetism alone. In this view, electrons – the carriers of electricity – are considered as microscopic details that play no role in quantum criticality.
In 2001, Si and colleagues proposed a new theory based upon a new type of quantum critical point. Their "local quantum criticality" incorporates both magnetism and charged electronic excitations. A key prediction of the theory is that Fermi volume collapses at a quantum critical point.
"Fermi volume" refers to the combined momenta, or wavelengths, of all the electrons in a crystalline solid. It exists because electrons -- part of the family of elementary particles called "fermions" – must occupy different quantum mechanical states.
The newly reported results about YRS are the culmination of more than seven years' worth of research by Si, Steglich and colleagues. In 2004, they reported the first evidence for the collapse of a Fermi volume in a quantum critical matter, and three years later they reported the first telltale signs of a link between the Fermi-volume collapse and thermodynamic properties in YRS.
In YRS, the transition from one quantum phase to another -- the tipping point -- is marked by a flip between magnetic and nonmagnetic states. By cooling YRS to a set temperature near absolute zero, and adjusting the magnetic field applied to the supercooled YRS, Steglich's team can mark the points along the magnetic continuum that mark both the onset and the end of the Fermi-volume collapse.
In the current study, this method was applied systematically, over a broad range of temperature and magnetic-field settings. To rule out the possibility that irregularities in a particular sample were influencing the results, Steglich's team studied two samples of different qualities and applied an identical set of tests to each. For each sample, the researchers measured the "crossover width," the distance between the beginning and ending points of the Fermi-volume change. The extensive experiments established that the Fermi-volume change is robust, or happens roughly the same way even in different types of samples. The experiments also revealed something entirely new.
"After hundreds of experiments, we plotted the crossover width as a function of temperature, and the plot formed a straight line that ran through the origin," Steglich said. "The effect was the same, regardless of differences between samples, so it is clearly not an artifact of the sample preparation."
"The linear dependence of the Fermi-volume crossover width on the temperature reveals particular quantum-critical scaling properties regarding the electronic excitations," said Si, Rice’s Harry C. and Olga K. Wiess Professor of Physics and Astronomy. "It is striking that the electronic scaling is so robust at a magnetic quantum critical point."
Scaling refers to the fact that the mathematics that describes the electronic relationship is similar to the math that describes fractals; the relationships it describes are the same, regardless of whether the scale is large or small. Si said scaling at a quantum critical point is also "dynamical," which means it occurs not only as a function of length scales but also in terms of time scales.
"The experiments provide, for the first time, the evidence for a salient property of local quantum criticality, namely the driving force for dynamical scaling is the Fermi-volume collapse, even though the quantum transition is magnetic," said co-author Silke Paschen, professor and head of the Institute of Solid State Physics at the Vienna University of Technology.
Additional co-authors include Sven Friedmann, Niels Oeschler, Steffen Wirth, Cornelius Krellner and Christoph Geibel, all of the Max Planck Institute for Chemical Physics of Solids, and Stefan Kirchner, a former postdoctoral fellow at Rice University who is now at the Max Planck Institute for the Physics of Complex Systems. The research was supported by the German Research Foundation, the European Research Council, the National Science Foundation and the Welch Foundation.
Jade Boyd | EurekAlert!
A 100-year-old physics problem has been solved at EPFL
23.06.2017 | Ecole Polytechnique Fédérale de Lausanne
Quantum thermometer or optical refrigerator?
23.06.2017 | National Institute of Standards and Technology (NIST)
An international team of scientists has proposed a new multi-disciplinary approach in which an array of new technologies will allow us to map biodiversity and the risks that wildlife is facing at the scale of whole landscapes. The findings are published in Nature Ecology and Evolution. This international research is led by the Kunming Institute of Zoology from China, University of East Anglia, University of Leicester and the Leibniz Institute for Zoo and Wildlife Research.
Using a combination of satellite and ground data, the team proposes that it is now possible to map biodiversity with an accuracy that has not been previously...
Heatwaves in the Arctic, longer periods of vegetation in Europe, severe floods in West Africa – starting in 2021, scientists want to explore the emissions of the greenhouse gas methane with the German-French satellite MERLIN. This is made possible by a new robust laser system of the Fraunhofer Institute for Laser Technology ILT in Aachen, which achieves unprecedented measurement accuracy.
Methane is primarily the result of the decomposition of organic matter. The gas has a 25 times greater warming potential than carbon dioxide, but is not as...
Hydrogen is regarded as the energy source of the future: It is produced with solar power and can be used to generate heat and electricity in fuel cells. Empa researchers have now succeeded in decoding the movement of hydrogen ions in crystals – a key step towards more efficient energy conversion in the hydrogen industry of tomorrow.
As charge carriers, electrons and ions play the leading role in electrochemical energy storage devices and converters such as batteries and fuel cells. Proton...
Scientists from the Excellence Cluster Universe at the Ludwig-Maximilians-Universität Munich have establised "Cosmowebportal", a unique data centre for cosmological simulations located at the Leibniz Supercomputing Centre (LRZ) of the Bavarian Academy of Sciences. The complete results of a series of large hydrodynamical cosmological simulations are available, with data volumes typically exceeding several hundred terabytes. Scientists worldwide can interactively explore these complex simulations via a web interface and directly access the results.
With current telescopes, scientists can observe our Universe’s galaxies and galaxy clusters and their distribution along an invisible cosmic web. From the...
Temperature measurements possible even on the smallest scale / Molecular ruby for use in material sciences, biology, and medicine
Chemists at Johannes Gutenberg University Mainz (JGU) in cooperation with researchers of the German Federal Institute for Materials Research and Testing (BAM)...
19.06.2017 | Event News
13.06.2017 | Event News
13.06.2017 | Event News
23.06.2017 | Physics and Astronomy
23.06.2017 | Physics and Astronomy
23.06.2017 | Information Technology