To conduct, or to insulate? That is the question

July 2, 2015, University of Cambridge

A new study has discovered mysterious behaviour of a material that acts like an insulator in certain measurements, but simultaneously acts like a conductor in others. In an insulator, electrons are largely stuck in one place, while in a conductor, the electrons flow freely. The results, published today in the journal Science, challenge current understanding of how materials behave.

Conductors, such as metals, conduct electricity, while insulators, such as rubber or glass, prevent or block the flow of electricity. But by tracing the path that follow as they move through a material, researchers led by the University of Cambridge found that it is possible for a single material to display dual metal-insulator properties at once - although at the very lowest temperatures, it completely disobeys the rules that govern . While it's not known exactly what's causing this mysterious behaviour, one possibility is the existence of a potential third phase which is neither insulator nor .

The duelling metal-insulator properties were observed throughout the interior of the material, called samarium hexaboride (SmB6). There are other recently-discovered which behave both as a conductor and an insulator, but they are structured like a sandwich, so the surface behaves differently from the bulk. But the new study found that in SmB6, the bulk itself can be both conductor and insulator simultaneously.

"The discovery of dual metal-insulator behaviour 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's Cavendish Laboratory, who led the research.

In order to learn more about SmB6 and various other materials, Sebastian and her colleagues traced the path that the electrons take as they move through the material: the geometrical surface traced by the orbits of the electrons leads to a construction which is known as a Fermi surface. In order to find the Fermi surface, the researchers used a technique based on measurements of quantum oscillations, which measure various properties of a material in the presence of a high magnetic field to get an accurate 'fingerprint' of the material. For quantum oscillations to be observed, the materials must be as close to pure as possible, so that there are minimal defects for the electrons to bump into. Key experiments for the research were conducted at the National High Magnetic Field Laboratory in Tallahassee, Florida.

SmB6 belongs to the class of materials called Kondo insulators, which are close to the border between insulating and conducting behaviour. 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 behaviour.

"It's a dichotomy," said Sebastian. "The high electrical resistance of SmB6 reveals its insulating behaviour, but the Fermi surface we observed was that of a good metal."

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 behaviour: 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 behaviour, 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 and conductor, we found it displays some really strange properties - we're exploring the possibility that it's a new quantum phase."

Explore further: Spin-based electronics: New material successfully tested

More information: Tan, B. S. et al. Unconventional Fermi surface in an insulating state. Science (2015). DOI: 10.1126/science.aaa7974

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redge_little
not rated yet Jul 03, 2015
Such holographic excitation of insulator SmB6 (samarium boride) is explained by the relativisitic spinrevorbital of RBL and the Rules of Ferrochemistry as published in Jan 2015 and archived in Dec 2005 in sci.print and Dec 2012 in VixRa! In " A Theory of the Relativistic Fermionic Spinrevorbital" (2015) {http://www.academ...AB049585 (click 'Full Text PDF' for free copy)} the Rules of Ferrochemistry Pages 1-4 are outlined and for SmB6 at low enough temperature Little Rule 2 governs such that the SmB6 should be an insulator as it manifest the quantum discontinuum among its formula units. But by Little Rule 1, a sufficiently strong magnetic field can couple to the electrons near the Fermi Level in SmB6 and thereby excite the fermions from discontinuum into continuum states as by the Little Effect; wherein in the holographically excited continuum state of SmB6 the materials would behave as metal and obey Little Rule 3 !
redge_little
not rated yet Jul 03, 2015
Thereby the continuum as discovered by RBL is intervening forbidden states between discontinuum states wherein the fermions exhibit relativistic spinrevorbital motions. By such magnetic holography of the electron between discontinuum (insulating) (Rule 2) to continuum (metallic) (Rule 3) states the SmB6 at the low temperature can manifest both insulting and metal properties as observed by the researchers in July 2015! Although such holography was computed by Sean Hartnoll and Diego Hofman of Harvard in 2010, the prior theory of such continuum and discontinuum was first proposed by RBL in 2000 as for strong magnetic field in iron nanoparticles for holographically altering orbitals in hydrocarbons and surface carbon for rehybridizing the electron orbitals into graphene (and CNT) (metal or semimetal semiconductor) and stronger magnetic field rehybridizing more local orbitals into sp3 diamond (insulator) by Rule 2!
redge_little
not rated yet Jul 03, 2015
Yes RBL was by Little Effect magnetically altering electronic motions to transform a metal to an insulator (but in carbon allotropes as this 2015 press release involves SmB6) in 2000; So now fifteen years ago I did the same and proposed the same as these researchers in 2015 are using such magnetic field in 2015 to observed metal and insulating states in SmB6. Of course this is written for the intellectuals, many ears will not be able to handle it! This magnetic alteration of orbital dynamics in 2000 was named 'the Little Effect' and it is apparent that the change in orbit of electrons in SmB6 as apply magnetic field (as currently reported in July 2015) is a manifestation of 'the Little Effect' but by Rule 3!
redge_little
not rated yet Jul 03, 2015
Later computations of Hartnoll and Hofman in 2000 only substantiate 'the Little Effect' from 2000. Yes 'the Little Effect' is manifested in this observed behavior of SmB6 in 2015 and the RB Little Rules of Ferrochemistry clearly govern such interesting insulating to metal in SmB6. In this spinrevorbital manuscript, I go from length scales ranging the whole universe from subatomic and nuclear realms all the way to the sun, stars, supernovas and blackholes realms noting the mixed Rules of Little Rule 2 and Little Rule 3 for dynamics of macroscopic discontinuum and continuum in analog to submicroscopic discontinuum and continuum for extremely novel unification of quantum mechanics and relativity; which was originally done in 2005! My consideration was later computed by others such as Hartnoll and Hofman in 2010 as they apply blackhole holography to submicroscopic quantum realms! I thank GOD for the vision!

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