“Our findings represent a clear-cut advance in the understanding of the electron’s organizing principle in quantum-critical matters,” said theoretical physist Qimiao Si, a paper co-author and professor of physics and astronomy at Rice. “The work could be important to the physics of a broad range of materials, including high-temperature superconductors and carbon nanotubes. In addition, it provides new insight for the field of phase transformations of matter, which is of interest in physics, chemistry and other disciplines.”
The new research bolsters the growing body of theoretical and experimental work in a new subfield of condensed matter physics known as “correlated electron physics,” a discipline that’s grown up in the past decade with the aim of understanding all the electronic processes governing both natural and man-made materials.
The impetus for correlated electron physics is the fact that the standard theory of metals cannot explain the electronic workings of materials that contain “correlated,” or strongly interacting electrons. Correlated systems include radioactive metals, such as plutonium, and compounds based on so-called rare earth elements and transition metals, such as cerium, ytterbium and copper. All strongly correlated materials contain electrons whose influence on one another is so pronounced that they cannot be explained by theoretical description of the independent electrons themselves but instead require an understanding of their dynamic interaction.
Electrons are a type of quantum particle called a “fermion.” Like all quantum particles, electrons can be considered both a particle and a wave, and quantum mechanics dictates that electron waves possess a definite momentum and that no two electrons can have the same momentum. What follows is the notion of “Fermi volume,” a volume in the momentum-space made up of all the combined momenta of all the electrons in a wire, a resistor or another solid-state structure.
The latest research offers the most significant body of experimental evidence aimed at answering the theoretical questions about changes in Fermi volume in quantum critical matters. Si, Coleman and Steglich, director at the Max-Planck Institute in Dresden, teamed with Max-Planck experimentalists Silke Paschen, an associate professor of physics, Thomas Lühmann and Steffen Wirth, to measure something called “the Hall effect.” The experiment included an ingenious setup designed to separate the various roles played by magnetic fields. Other members of the Max-Planck group are Octavio Trovarelli and Christoph Geibel, who synthesized extremely high-quality samples, as well as Philipp Gegenwart, who performed resistivity measurements necessary to analyze the Hall-effect data.
The theoretical study of quantum criticality is still in flux. Critical points governed by classical physics have been known for fifty years, and the conventional wisdom thinks of their quantum mechanical cousins as a kind of classical phase transition in higher dimensions. This traditional way of thinking has held sway in metal physics for the past half century, but it would predict a smooth evolution of the Fermi volume.
Instead, the results are more consistent with a local quantum critical point, a new class of quantum phase transition advanced by Si and colleagues in Nature in 2001. Another possible explanation favored by Coleman and colleagues is that electrons are actually breaking apart inside the quantum critical matter – a phenomenon known as spin-charge separation.
“This is the most direct evidence for a collapse of a Fermi volume in any quantum critical matter,” says Steglich. “We expect this new insight to have broad implications for other strongly correlated electron systems.”
Taken together, the experimental and theoretical works point toward fluctuations of the Fermi surface (the enclosure of the Fermi volume) as being responsible for the exotic physical properties of quantum critical matter.
The real-world effect of electron correlations on material properties can be profound. The effects are widely believed to be a key element behind the mechanism of high-temperature superconductivity, and a better understanding of electron correlations may answer questions arising from a host of other mysterious experimental observations such as: Why do the mobile electrons in some extremely cold exotic metals behave as if their masses were a thousand times that of free electrons in simple metals? Why do some strongly correlated materials display a very large thermoelectric response? Why do others display “colossal magnetoresistance,” or extreme sensitivity to magnetic changes?
Text for this article comes from a Rice press release.
Hmm, I used to work in the theory of electrons in metals, and studied classical critical phenomena, and I still can’t say I quite understand what they’re talking about! Some of this does seem to be stuff I’ve just missed out on the last few years. Ricky, thanks for the links, they help a lot – especially the “ScienceWeek” one, though it’s itself rather technical.
The odd thing is, at least from a brief look here, they don’t seem to be agreeing on what characterizes “quantum critical points”, other than a phase transition (like water changing to ice) that somehow involves the low-temperature quantum wave-like behavior of electrons.
For classical critical phenomena, as you change temperature or density or some other parameter of the material close to a critical point, you find that various properties of the material (specific heat is a common example) change in a way that is some strange power law of the “distance” (in temperature, density, etc.) from the critical point. And that some of these power laws are related to one another in odd and profound ways, in wide categories the power laws are “universal” – all materials undergoing the same transition exhibit the same behavior with the same (non-integer) power law. The behavior of a material in quite wide parameter regions can be governed by the critical-point.
But the “quantum critical” stuff sounds like something quite different – they mention an actual “abrupt change” (in the Fermi surface), and also the presence of new phenomena close to the critical point. Anybody have a clearer explanation of it all?