Topological insulators take two steps forward

Aug 10, 2010
Topological Insulators Take Two Steps Forward
This small gap indicates a big change in the properties of a topological insulator. (Image courtesy Yulin Chen.)

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 collaborators have pushed research into topological insulators not just one, but two steps forward.

Taken together, both steps bring the unique material closer to use in the nascent technology of , with the potential to result in, among other advances, smaller, more efficient transistors and memory devices. Both steps could also give physicists from fields as varied as condensed matter physics, cosmology, and glimpses into such exotic phenomena as magnetic monopoles, the fractional quantum Hall effect and axion fields, for starters.

As explained in a paper appearing online in Science, in step 1, Chen and his collaborators took the marvelous property of a three-dimensional topological insulator—in which electrons flow inexorably along the surface in the same direction, slaloming around defects instead of scattering off them in all directions—and broke it.

In Step 2, the team demonstrated the ability to "tune" the topological insulator's Fermi energy level, the amount of energy in the most energetic electrons still occupying a specific .

Essentially, the team completely redrew the energy portrait for the topological insulator. Initially the portrait showed two cones, called Dirac cones, with their apexes touching at the so-called Dirac point, where electrons can cruise serenely along the insulator's surface. Introducing magnetic impurities into the system turns electrons into bumbling pedestrians that scatter and bounce off the impurities, in effect creating a gap separating the upper and lower Dirac cones. While tuned into that gap, the usually free flow of electrons across the material's surface remains "broken."

Chen and his collaborators accomplished this feat by first adding tiny amounts of magnetic materials to their topological insulator of choice, di-bismuth tri-selenide, and then adding materials that contributed either extra electrons or extra "holes"—places electrons already in the material might want to settle. Chen likened the process to having two control knobs, the B knob and the E knob. ("B" is a standard variable for magnetic fields in equations.)

"The B knob opens the gap (between Dirac cones) and the E knob adjusts the Fermi energy of the material," Chen said. "You need both."

The team spent months tuning the B and E knobs. On the practical side, they were able to transform the topological insulator into different varieties of semiconductors, an accomplishment that points the way toward semiconductor-based functional devices. On the research side, the team's most immediate contribution was to confirm theoretical predictions, but according to Chen their efforts have a more long-lasting benefit.

"This is a material base for many exciting future investigations," he said. And on this base, "Many exotic phenomena may be realized." How exotic? "In this condensed matter system, one electric charge can induce a magnetic monopole. If a future experiment confirms this, it'll be fascinating."

That's not all. The system might also provide a sort of analogy for an axion field, something cosmologists are on the hunt for, as well as demonstrating what's known as the fractional quantum Hall effect, in which the electrons as a group behave like quasiparticles with fractional charges.

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More information: "Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator", Science 6 August 2010: Vol. 329. no. 5992, pp. 659 - 662, DOI: 10.1126/science.1189924

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Jigga
1.6 / 5 (7) Aug 10, 2010
I presume, PO readers should understand well the conceptual difference between superconductors, heavy electron metals, ballistic conductors (with Dirac electrons like graphene) and topological insulators.

All these four categories of solid state physics are closely related each other, although their properties differ significantly. Without it you'll get lost in increasing number of articles about these materials.
Jigga
1.6 / 5 (7) Aug 10, 2010
Because movable electrons in atom lattice are both attracted by atom nuclei, both repulsed mutually by another electrons, they're penetrating the atom lattice like fluid, which roughly follows the virtual surface between atoms, where attractive and repulsive forces are balanced mutually - so called the Fermi surface.

We can model this situation by porous material, filled with fluid with strong surface tension. The surface of material can be hydrophobic or hydrophilic with respect to this fluid.

In addition, this porous surface can be formed by wire mesh with positive curvature, or by mutually connected holes like sponge or cheese. Thus results in four possible combinations of electron fluid behavior within atom lattice.
Jigga
1.6 / 5 (7) Aug 10, 2010
The topological insulators correspond the situation, where surface of material is repulsing the electrons and its formed with holes of negative curvature. As the result, the electron fluid is expelled from space between atoms to the surface of material in similar way, like sponge soaked with mercury. The interior of sponge is filled with separated droplets, so it remains non-permeable insulator. Most of its conductivity is concentrated on its surface.

For example, one of interesting aspects of these insulators is, the random scratching of their surface increases their conductivity. Whereas the surface scratching of normal conductors usually increases their resistance. Topological insulators are formed by large atoms with only few movable electrons per atoms. These electrons are forced to revolve atoms with relativistic speed and they tend to fill up the free space between atoms in hexagonal honeycomb lattice.
Jigga
1.6 / 5 (7) Aug 10, 2010
Because electrons at the surface of topological insulator are insensitive to mechanical obstacles, they cannot be blocked and/or scattered by common impurities, like at the case of common insulators.

Instead of it, these electrons could be scattered with atoms, which are containing unpaired electrons, forming strong magnetic field around them. The electrons moving in parallel direction in magnetic field are separated mutually by Lens law in similar way, like parallel wires loaded with current in magnetic field. Therefore magnetic impurities are forming holes in flow of surface electrons like boulders at river stream.

The incorporation of electropositive or electronegative atoms at the surface of topological insulator makes this surface less or more "hydrophobic" with respect to electron fluid, which can modulate the effect of magnetic impurities in even more complex, still imaginable way.
Auxon
3 / 5 (2) Aug 11, 2010
@Jigga, I have to admit that, I have no idea if it's right, but it sounds reasonable.
MustaI
Aug 11, 2010
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Tahoma
Aug 11, 2010
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