Combining experiments under extreme conditions with theoretical analysis, researchers pursue knowledge that could be used in the future to create a new generation of sustainable functional materials for use in quantum information device or superconductor

The common phase transitions are those that occur as a function of temperature variation. Ice changes phase to become liquid water at 0 °C. Liquid water changes phase to become water vapor at 100 °C. Similarly, magnetic materials become nonmagnetic at critical temperatures. However, there are also phase transitions that do not depend on temperature. They occur in the vicinity of absolute zero [-273.15 °C] and are associated with quantum fluctuations.

A study involving experiments under extreme conditions, especially ultralow temperatures and intense magnetic fields, and accompanied by theoretical interpretation of the experimental results explored this type of situation and investigated the quantum critical point manifested in a highly unusual transition.

Italian researcher Valentina Martelli and Peruvian Julio Larrea, both professors at the University of São Paulo Physics Institute (IF-USP) in Brazil, participated in the study, which is published in Proceedings of The National Academy of Sciences of the United States of America (PNAS). {module INSIDE STORY}

The experimental part, led by Professor Silke Paschen, was conducted in the laboratories of the Vienna University of Technology (TUW) in Austria. The theoretical work was performed by a group led by Qimiao Si, Professor of Physics and Astronomy at Rice University in the United States.

"We found and interpreted evidence of two successive quantum critical points associated with a double breakdown of the Kondo effect," Larrea told.

Named for Japanese physicist Jun Kondo (born 1930), the Kondo effect explains the formation of heavy fermions in metal compounds based on rare-earth elements. In these compounds, the electrons behave collectively owing to their strong correlation, forming a singlet (a collective of distinct particles that behave as a single particle), which can be represented as the coupling of the localized magnetic moment of the rare-earth ion with the conduction electron around it. This quasi-particle can reach masses up to thousands of times the mass of a free electron.

In the study described here, the singlet was broken twice in two magnetic orders: one dipolar, resulting from the magnetic moment of the quasi-particle, and the other quadrupolar, resulting from the interaction between its electronic orbitals.

The experiment was performed with the heavy-fermion Ce3Pd20Si6, a compound of cerium (Ce), palladium (Pd) and silicon (Si). Larrea is set to continue the investigations, with support from São Paulo Research Foundation - FAPESP via the project "An investigation into topological and exotic quantum states under extreme conditions".

"The starting point for these transitions is the strong correlations between electrons and certain materials, which enable us to understand this type of state change," Larrea said.

"Various kinds of collective interaction can affect electrons. One possible state is what we call 'strange metal'. In heavy fermions, electron transport is analogous to that of ordinary metals, but the electrons are strongly correlated and behave collectively as if they formed a single quasi-particle, which transports the charge. This is not what happens in a quantum phase transition, so the state is called 'strange'. What we observed experimentally is that physical properties such as electrical resistance behave quite differently from classical electron transport in metals."

The phenomenon occurs at extremely low temperatures very close to absolute zero. When temperatures fall this low, thermodynamic fluctuations practically disappear, and quantum fluctuations are observed, constituting the "medium" in which interactions among electrons take place.

"Until the publication of our study, most experiments of this kind had focused on materials in which electron correlation leads to what is known as simultaneously itinerant and localized electron magnetism. These materials belong to the group of rare earths and include heavy fermions: 'fermions' because the electrons have fractionary spin and obey Fermi-Dirac statistics; 'heavy' because they correlate with a quasi-particle with large effective mass," Larrea said.

"These materials also have a magnetic moment, so in addition to a charge-carrying quasi-particle, they are also associated with a quasi-particle with a magnetic moment shielded or screened by the conduction electrons. Each screened magnetic moment can be coupled to its neighbor in the crystal lattice, producing a magnetic order throughout the material. In the case of Ce3Pd20Si6, this order is of the anti-ferromagnetic kind, which means that the magnetic moments in the lattice are coupled in an anti-parallel fashion. At the quantum critical point, this magnetic order can be suppressed without the influence of a thermodynamic control parameter but by applying a magnetic field. The Kondo singlet breaks down, and the electron that was coupled to this magnetic order simply separates."

This does not contradict the fundamentals of quantum mechanics, but it is very different from what is described in basic physics textbooks. Because the magnetic moment is defined relative to the spin, the suppression of the magnetic order creates a situation in which the electrons appear to lack spin.

"This quantum critical point based on a magnetic order had previously been reported in other articles," Larrea said. "The difference in our case was that besides the dipolar magnetic order, the material also exhibited a quadrupolar magnetic order generated by the electrons' orbitals. Our phase diagram, which is almost a graphical summary of the study, therefore shows two quantum critical points: one in which the dipolar order is disrupted, and the other in which the quadrupolar order is broken."

According to Larrea, apart from this discovery, the results of the study are also important insofar as they contribute to an understanding of other unsolved problems, such as how electrons are collectively organized to produce superconductivity. "A collective order is needed to produce long-range transport," he said. "Certain kinds of material with strong correlations among electrons can provide this. We now know that these strong correlations can be suppressed to favor the formation of new states with measurable physical properties, even at temperatures different from absolute zero."

The next step is to extend the investigation of changes in electron correlations using a different control parameter - pressure - so that it will be possible in the future to make technological use of this knowledge in areas such as quantum supercomputing.

Next year, NASA plans to launch a new Mars rover to search for signs of ancient life on the Red Planet. A new study shows that the rover's Jezero crater landing site is home to deposits of hydrated silica, a mineral that just happens to be particularly good at preserving biosignatures.

"Using a technique we developed that helps us find rare, hard-to-detect mineral phases in data taken from orbiting spacecraft, we found two outcrops of hydrated silica within Jezero crater," said Jesse Tarnas, a Ph.D. student at Brown University and the study's lead author. "We know from Earth that this mineral phase is exceptional at preserving microfossils and other biosignatures, so that makes these outcrops exciting targets for the rover to explore." {module INSIDE STORY}

The research is published in Geophysical Research Letters.

NASA announced late last year that its Mars 2020 rover would be headed to Jezero, which appears to have been home to an ancient lake. The star attraction at Jezero is a large delta deposit formed by ancient rivers that fed the lake. The delta would have concentrated the wealth of material from a vast watershed. Deltas on Earth are known to be good at preserving signs of life. Adding hydrated silica to the mix at Jezero increases that preservation potential, the researchers say. One of the silica deposits was found on the edge of the delta at low elevation. It's possible that the minerals formed in place and represent the bottom layer of the delta deposit, which is a great scenario for preserving signs of life.

"The material that forms the bottom layer of a delta is sometimes the most productive in terms of preserving biosignatures," said Jack Mustard, a professor at Brown and study co-author. "So if you can find that bottom set layer, and that layer has a lot of silica in it, that's a double bonus."

For the study, researchers used data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument that flies aboard NASA's Mars Reconnaissance Orbiter. The technique applied to the CRISM data used big data analysis methods to tease out the weak spectral signature of the silica deposits.

While the geologic context of the deposits suggests they could have formed at the base of the delta, it's not the only possibility, the researchers say. The minerals could have formed upstream in the watershed that fed Jezero and been washed subsequently into the crater, by volcanic activity or later episodes of water saturation in the Jezero crater lake. The rover should be able to isolate the real source, the researchers say.

"We can get amazing high-resolution images and compositional data from orbit, but there's a limit on what we can discern in terms of how these minerals formed," Tarnas said. "Given instruments on the rover, however, we should be able to constrain the origin of these deposits."

The rover will be able to perform fine-scale chemical analysis of the deposits and provide a close-up view of how the deposits are situated in relation to surrounding rock units. It will also have a sensor similar to CRISM to link orbital and lander data. That will go a long way to determining how the deposits formed. What's more, one instrument aboard the rover is able to look for complex organic material. If the silica deposits have high concentrations of organics, it would be an especially intriguing find, the researchers say.

And in addition to the work, the rover does on-site, it will also cache samples to be returned to Earth by future missions.

"If these deposits present themselves in the form of rocks that are big and competent enough to drill into, they could be put into the cache," Mustard said. "This work suggests that they'd be a great sample to have."