Evidence of a quantum phase transition without symmetry breaking in cerium-cobalt-indium 5

 

Credit: Shannon C. Haley

Many condensed matter physicists have spent the last few decades studying quantum phase transitions that aren't obviously related with a broken symmetry. These transitions are intriguing because they may provide insight into the process of high-temperature superconductivity.

Researchers at the University of California, Berkeley, recently discovered evidence of a quantum phase transition in cerium-cobalt-indium 5 (CeCoIn5), an unusual superconductor, that occurs without symmetry violation. Their work, which was published in Science, also provides a model that could be used to explain CeCoIn5's strange behavior.

Nikola Maksimovic, the paper's principal author told, "We initially started exploring this material with a very different focus, especially on critical scaling issues in resistivity measurements." "Over the course of about three years, we observed that our data seemed to point to little quantities of chemical substitution causing rapid changes in the material's properties. Previous measurements pointed at a transition like this."

Maksimovic and his colleagues proposed that the widely observed rapid changes in the characteristics of CeCoIn5 may be explained by a delocalization transition of the cerium f-orbital electron in the material, based on previous theoretical work. As a result, they opted to focus their research on characterization of f-electrons in the material rather than measuring low-temperature resistivity.

"We believed that our research would provide an answer to a basic question: whether the cerium atom's f-electron is limited to the cerium site or itinerant (i.e., free to roam around in the metal)," Maksimovic stated. "Previous experimental studies on other materials, such as Yb-based metals and copper-oxide ceramics, also influenced our work."

The researchers employed a well-known experimental technique called Hall Effect measurement to look at the f-electrons in CeCoIn5. A transverse voltage created by a magnetic field is applied to a sample in this approach.

This voltage can then be used to calculate the density of mobile electrons in a given material. Maksimovic and his colleagues utilized the Hall Effect as a probe to see if the f-electrons in a CeCoIn5 sample were mobile or bonded to their host atoms in their studies.

Maksimovic explained, "These measurements were performed in severe settings, at around half a kelvin above absolute zero and in magnetic fields up to 73 Tesla." "In order to create a meaningful signal, tiny electrical devices had to be patterned out of CeCoIn5 pieces."

The researchers' findings revealed a quick change in the material's low-temperature carrier density, which was triggered by the chemical substitution of CeCoIn5. The amount of the Hall coefficient rise they measured was found to be consistent with an f-electron localized-to-delocalized transition, which was surprising. These findings backed up their initial theory.

Maksimovic and his collaborators then set out to analyse the electronic energy spectrum in CeCoIn5 samples of various compositions. They did this by employing a variety of cutting-edge spectroscopy techniques, including quantum oscillation and angle-resolved photoemission.

"We presented a model that argues for the fractionalization of electrons in the metal into independent charge and spin carrying excitations towards the critical point when the f-electrons are close to delocalizing based on existing theories of f-electron metals," Maksimovic said. "Such a 'breaking up' of electrons is a very exotic phase of matter, made feasible only by the collective quantum characteristics of particular metals with highly correlated electrons."

The novel model proposed by this group of researchers, as well as their calculations, may be able to explain some of the material's electrical conductivity features. Furthermore, their discoveries provide new information that could improve our understanding of CeCoIn5 and other atypical superconductors.

Past research have revealed evidence of a qualitatively comparable delocalization transition in these materials, which is similarly driven by chemical substitution. This current discovery could motivate the creation of similar models that pertain to high-temperature superconductors in the future.

Maksimovic and his colleagues want to look for additional direct evidence of separate spin and charge carrying excitation in CeCoIn5 in the meantime. To do so, they'll take measurements of thermal conductivity and electrical conductivity at very low temperatures, because a substantial difference between the two could show that heat carriers vary from charge carriers.

"We also discovered that in select samples revealed indications of a transition triggered by very strong magnetic fields," Maksimovic noted. "At this time, its unclear how this relates to the zero-field transition we discovered recently. As a result, we intend to use the pulsed field equipment at Los Alamos National Lab to further investigate the high-field transition."

References:

• Nikola Maksimovic et al, Evidence for a delocalization quantum phase transition without symmetry breaking in CeCoIn5, Science (2021). DOI: 10.1126/science.aaz4566

• V. A. Sidorov et al, Superconductivity and Quantum Criticality inCeCoIn5, Physical Review Letters (2002). DOI: 10.1103/physrevlett.89.157004

• S. Paschen et al, Hall-effect evolution across a heavy-fermion quantum critical point, Nature (2004). DOI: 10.1038/nature03129