New quantum state of matter discovered in meta-cinnabar (HgS)

Metacinnabar (Photo: Lou Perrloff, Photo-Atlas of Minerals

Researchers from University of Marseille and IFW Dresden propose new material for low-power consumption electronics devices.

Calculations have uncovered that the electrons inside crystals of the natural mineral meta-cinnabar organize themselves into an exotic quantum state. This state allows for electric current to flow on the surface of meta-cinnabar without power losses.

The team from Marseille and Dresden has discovered that mercury-sulfide (HgS) is a so-called topological insulator. These new materials act as both insulators and conductors, with their interior preventing the flow of electrical currents while their edges or surfaces allow the movement of charge.

Perhaps most importantly, the surfaces of topological insulators enable the transport of electrons while preventing the "scattering" typically associated with power consumption, in which electrons deviate from their trajectory, resulting in energy losses.

In recent years, topological insulators have become one of the hottest topics in physics. About ten candidate topological insulator materials have been identified so far.

Newly discovered mercury-sulfide (HgS) stands out among these because its surface spontaneously forms a pattern of parallel channels that transport charge. Within the channels electric current flows rapidly and unimpeded while charge can barely move from one channel to another. In this way the surface acts as a set of parallel electric wires.

"Interestingly, mercury-sulfide crystals have been known since time immemorial", says Prof. Jeroen van den Brink from the IFW Dresden, "known as cinnabar to the Romans and as vermillion in the Middle Ages, these crystals have a wonderful orange-red hue. We have investigated their grey-black phase, known as meta-cinnabar. Somewhat duller in color perhaps, but harboring an intriguing new quantum state of matter." 

Electrons traveling through a surface channel of meta-cinnabar (HgS) do not only carry an electrical charge but also a magnetic moment, the electron spin. On each electron the spin tags along for the ride.

In a normal metal such as copper, spins point in an arbitrary direction. But in meta-cinnabar the spins of electrons that move in one particular direction have identically the same orientation. Electrons that travel in the reverse direction have exactly the opposite spin orientation.

Having the alignment of a spin locked to the direction of motion causes a pure spin current to go hand in hand with a charge current. This is a promising effect for spintronics, an emerging information-processing technology that relies on the control of the intrinsic spin of electrons to build efficient transistors and memory devices.

Conventional scattering is capable of changing an electron's direction of travel, but cannot overturn its spin. Since in meta-cinnabar an alteration of the trajectory of an electron is fundamentally tied to a re-orientation of its spin, regular scattering becomes ineffective and cannot cause dissipation. Waves of electrons can flow through the surface channels without scattering backwards.

Because of these characteristics, topological insulators hold great potential for use in future transistors, memory devices and magnetic sensors that are highly energy efficient and require less power.

"Several experimental groups are actively exploring these promises in meta-cinnabar", says Prof. Bernd Büchner from the IFW Dresden, "the problem we anticipate is that the interior of HgS is still conductive when crystals are not pure enough. This spoils the more subtle surface effects. The controlled growth of pristine meta-cinnabar crystals, without impurities or defects, is a top priority."

These results will appear in the journal Physical Review Letters, PRL 106, 237001 (2011). F. Virot, R. Hayn, M. Richter and J. van den Brink, “Metacinnabar (beta-HgS): a strong 3D topological insulator with highly anisotropic surface states”.

URL: http://link.aps.org/doi/10.1103/PhysRevLett.106.236806

DOI: 10.1103/PhysRevLett.106.236806

Contact:

Prof. Dr. Jeroen van den Brink
j.van-den-brink@ifw-dresden.de
Tel. +49 351 4659 400

Dr. Manuel Richter
m.richter@ifw-dresden.de
Tel. +49 3514659 360

Dr. Carola Langer
Public Relations
c.langer@ifw-dresden.de
Tel.: +49 351 4659 234