2 July 2015

To conduct, or to insulate? That is the question

Researchers working at the National MagLab have identified a material that behaves as a conductor and an insulator at the same time, challenging current understanding of how materials behave, and pointing to a new type of insulating state.

Students with a crystal of SmB6.

Students with a crystal of SmB6

News release courtesy of Cambridge University

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TALLAHASSEE, Fla. — A new study led by researchers from the University of Cambridge, has discovered mysterious behavior of a material that acts like an insulator in certain measurements, but simultaneously acts like a conductor in others. The results, published today (2 July) in the journal Science, challenge current understanding of how materials behave.

"The discovery of dual metal-insulator behavior in a single material has the potential to overturn decades of conventional wisdom regarding the fundamental dichotomy between metals and insulators," said Dr. Suchitra Sebastian of the University of Cambridge's Cavendish Laboratory, who led the research.

Conductors, such as metals, conduct electricity, while insulators, such as rubber or glass, prevent or block the flow of electricity. Working with samarium hexaboride (SmB6) at two unique facilities of the National High Magnetic Field Laboratory, the researchers found that it is possible for this one material to display dual metal-insulator properties at once — although at the very lowest temperatures, completely disobeying the rules that govern conventional metals.

While it's not known exactly what's causing this mysterious behavior, one possibility is the existence of a potential third phase that is neither insulator nor conductor.

There are other recently discovered materials known as topological insulators that carry properties of both a conductor and an insulator. However, these materials are structured like a sandwich with the surface behaving differently from the bulk. The new study found that in SmB6, the bulk itself can be both conductor and insulator simultaneously.

To understand more about SmB6, the research team conducted experiments on the lab's 45 tesla (T) hybrid and 35 T resistive magnet systems at the National MagLab's DC Field Facility and on the 60 T controlled waveform pulsed magnet located at the Pulsed Field Facility in Los Alamos. These high fields coupled with low temperatures and a unique measurement technique based on quantum oscillations allowed the research team to find the Fermi surface of SmB6, a critical discovery because it revealed a three dimensional component to the Fermi surface that extends into the bulk beyond the conductive surface state. This is the classic signature of a bulk metal despite the fact that we know the bulk of SmB6 is insulating. The observation of quantum oscillations reveal the true nature of the fundamental metallic state properties and rule out impurity states that otherwise would explain the behavior.

SmB6 belongs to the class of materials called Kondo insulators, which are close to the border between insulating and conducting behavior. Kondo insulators are part of a larger group of materials called heavy fermion materials, in which complex physics arises from an interplay of two types of electrons: highly localised 'f' electrons, and 'd' electrons, which have larger orbits. In the case of SmB6, correlations between these two types of electrons result in insulating behavior.

"This work on SmB6 provides a vivid and exciting illustration of emergent physics resulting from MagLab researchers refining the quality of the materials they study and pushing the sample environment to the extremes of high magnetic fields and low temperatures," said Tim Murphy, head of the MagLab's DC Field Facility where some of the research was conducted.

But the mystery didn't end there. At the very lowest temperatures, approaching 0 degrees Kelvin (-273 Celsius), it became clear that the quantum oscillations for SmB6 are not characteristic of a conventional metal.

In metals, the amplitude of quantum oscillations grows and then levels off as the temperature is lowered. Strangely, in the case of SmB6, the amplitude of quantum oscillations continues to grow dramatically as the temperature is lowered, violating the rules that govern conventional metals.

The researchers considered several reasons for this peculiar behavior: it could be a novel phase, neither insulator nor conductor; it could be fluctuating back and forth between the two; or because SmB6 has a very small 'gap' between insulating and conducting behavior, perhaps the electrons are capable of jumping that gap.

"The crossover region between two different phases — magnetic and non-magnetic, for example — is where the really interesting physics happens," said Sebastian. "Because this material is close to the crossover region between insulator and conductor, we found it displays some really strange properties - we're exploring the possibility that it's a new quantum phase."

"These are the kinds of fundamental discoveries made at the National MagLab that will enable us to someday achieve controlled functionality in materials and may well save lives in the future," said Associate Lab Director Chuck Mielke.

Collaborators on this work include Beng Tan, Mate Hartstein, Yu-Te Hsu, Maria Kiourlappou, Anand Srivastava and Gilbert Lonzarich from the Cavendish Lab at Cambridge University; Bin Zeng, Tim Murphy, Ju-Hyun Park, Luis Balicas from the MagLab's DC Field Facility; Neil Harrison and Zengwei Zhu (now at Wuhan Pulsed magnet lab in China) from the MagLab's Pulsed Field Facility; Monica Ciomaga Hatnean and Geetha Balakrishnan from University of Warwick; and Michelle Johannes from the Center for Computational Materials Science at the Naval Research Laboratory.

The research was supported by the National Science Foundation, the Department of Energy, the Royal Society, the Winton Programme for the Physics of Sustainability, the European Research Council and the Engineering and Physical Sciences Research Council (UK), the Office of Naval Research (ONR) through the Naval Research Laboratory’s Basic Research Program and the State of Florida.

The National High Magnetic Field Laboratory is the world’s largest and highest-powered magnet facility. Located at Florida State University, the University of Florida and Los Alamos National Laboratory, the interdisciplinary National MagLab hosts scientists from around the world to perform basic research in high magnetic fields, advancing our understanding of materials, energy and life. The lab is funded by the National Science Foundation (DMR-1157490) and the state of Florida. For more information, visit us online at nationalmaglab.org or follow us on Facebook, Twitter, Instagram and Pinterest at NationalMagLab.