Unfazed by imperfections

Jul 08, 2011
The introduction of magnetic impurities into a topological insulator causes a gap to open in the characteristic double-cone energy structure of the material (the x-axis shows electron momentum, the y-axis shows the energy). Credit: 2010 AAAS

While insulating against electrical currents in their interior, the surface of materials called topological insulators permits the flow of electron spins relatively unhindered. The almost lossless flow of spin information makes topological insulators a promising new class of materials for electronic applications: the electron spins could be harnessed to transmit information in the same way that electrical charges are used in conventional electronics. Electron spins are also susceptible to magnetic fields, so electrical control of the magnetic fields of these materials would offer further control over the properties of electronic devices. Magnetic impurities in these materials, however, have thwarted attempts by experimental physicists to fabricate topological insulators, because they destroy the characteristic energy structure of a topological insulator (Fig. 1).

In a  theoretical study, Kentaro Nomura and Naoto Nagaosa from the RIKEN Advanced Science Institute, Wako, have unexpectedly discovered that the electrical control of magnetization in topological insulators is actually enhanced by the presence of impurities. It may be possible, therefore, to develop novel devices from topological insulators by creating magnetization with electrical fields. 

Topological insulators owe their unique properties to time-reversal symmetry: if the flow of time were reversed, the material would behave in the same way. Magnetic impurities break this symmetry, as magnetism is sensitive to time reversal; electrical currents flowing forward and backward in time create magnetic fields pointing in opposite directions. Physicists therefore expected that magnetic impurities would disrupt the magnetization generated by electrical currents on the surface of a

Nomura and Nagaosa’s calculations, however, showed that randomly distributed magnetic impurities do not influence the strong coupling between electrical currents and magnetic fields. Electrical currents at the surface are quantized, which means that they change only in steps. Therefore, a change in the energy structure of the material would not affect the electric current and magnetization. The randomness of the impurities increases the usable energy range, says Nomura. “Usually impurities and disorder smear desired effects. In this case, imperfections enhance them.”

This finding is welcome news for experimental physicists working on topological insulators. All samples fabricated to date contain so many impurities that observing spin currents at their surface is almost impossible. The discovery that magnetic impurities should have no detrimental effect improves the likelihood of observing the proposed control of magnetization. Consequently, says Nomura, “a number of experimental groups are already working on this issue. I think this effect will be observed, hopefully soon.”

Explore further: Neutron tomography technique reveals phase fractions of crystalline materials in 3-dimensions

More information: Nomura, K. & Nagaosa, N. Surface-quantized anomalous Hall current and the magnetoelectric effect in magnetically disordered topological insulators. Physical Review Letters 106, 166802 (2011).

Chen, Y.L., et al. Massive Dirac fermion on the surface of a magnetically doped topological insulator. Science 329, 659–662 (2010).

add to favorites email to friend print save as pdf

Related Stories

Topological insulators take two steps forward

Aug 10, 2010

A team of researchers from the Stanford Institute of Materials and Energy Science, a joint institute of the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, and their international ...

A New Path of Conduction for Future Electronics

Jul 22, 2009

(PhysOrg.com) -- Last month, researchers from SLAC National Accelerator Laboratory made headlines when they revealed experimental evidence of a topological insulator: a material that could revolutionize computer ...

A remarkable step toward next-generation energy-conservation

Jun 29, 2011

Tohoku University, Osaka University and Japan Science and Technology Agency (JST) announced that they succeeded in directly observing electron spins in a topological insulator. The work has been published in Physical Review Le ...

Recommended for you

50-foot-wide Muon g-2 electromagnet installed at Fermilab

2 hours ago

One year ago, the 50-foot-wide Muon g-2 electromagnet arrived at the U.S. Department of Energy's Fermi National Accelerator Laboratory in Illinois after traveling 3,200 miles over land and sea from Long Island, ...

Spin-based electronics: New material successfully tested

Jul 30, 2014

Spintronics is an emerging field of electronics, where devices work by manipulating the spin of electrons rather than the current generated by their motion. This field can offer significant advantages to computer technology. ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jul 09, 2011
Electrons in topological insulator are similar to mercury fluid filling the pores of sponge - the repulsive forces of electrons are prohibiting them in flowing through pores (in real porous materials this property is used for determination of their porosity), but they form a continuous layer above surface of solid, thus enabling the charge transfer through it.

If you understood this explanation, try to estimate with using it, what will happen, if we scratch the surface of the topological insulator. How it will change its properties?