Showing that Surfaces Alone Can Conduct Electricity

Short pulses of light reveal an intrinsic conducting surface that is different from the bulk.

Image courtesy of Peter Armitage, Johns Hopkins University,
Electric field amplitude for terahertz light transmitted through bismuth selenide (Bi2Se3). The large polarization rotation angle (arced scale) is direct experimental evidence that a highly conductive surface layer has spontaneously appeared on the topological insulator.

The Science

In 2007, an exotic new class of materials called “topological insulators” was first predicted; these materials are insulators on the inside but can conduct electricity along the surface, a very difficult property to measure. A new, highly sensitive, time-resolved spectroscopic technique exploits the strong effect of the surface properties on short, terahertz pulses of infrared light to distinguish the surface and bulk properties of bismuth selenide (Bi2Se3) thin films. The pulses allow the infrared signals to echo through the film and amplify the effect of the film’s electrons on the probing light.

The Impact

Thin conducting layers are ubiquitous in electronic and optical engineering so their spontaneous appearance on certain classes of insulators, as here confirmed experimentally, has profound scientific and technological implications. The novel high sensitivity technique used to detect the thin conducting layers will serve as a critical tool to probe and perfect the properties of this exotic class of materials that has the potential for wide-ranging technological applications.


For an exotic new class of materials called “topological insulators,” an electrically conducting skin covering the insulating interior of the materials was detected, confirming predictions made in 2007.  To do this research, a highly sensitive technique that can distinguish between the electrically conducting skin and the insulating bulk was developed by scientists at the Johns Hopkins Institute of Quantum Matter. The experiment employed short pulses of terahertz light (used in airport scanners) with linear polarization – analogous to what happens to sunlight as it passes through polarized sunglass lenses. The polarization of the transmitted light was found to have rotated over 65 degrees when compared to the incident light. This “Colossal Terahertz Kerr rotation” furnishes the first direct evidence for an electrically conducting surface layer on a topological insulator. This probe will be a critical tool to understand and perfect the material’s properties. The high electrical conductivity might contribute to a new generation of high performance low loss processors and the unique character of the surface states may play a key role in realizing the promise of quantum computing.


Peter Armitage
Institute of Quantum Matter
Johns Hopkins University

Collin Broholm
Institute of Quantum Matter
Johns Hopkins University


Department of Energy, Office of Science, Basic Energy Sciences program (John Hopkins University, THz experiments); National Science Foundation (University of Buffalo, magneto-optics experiments; Rutgers, development of thin film growth facility), and the Office of Naval Research (Rutgers, growth and characterization of thin films).


R Valdés Aguilar, A.V Stier, W Liu, L.S Bilbro, D.K George, N Bansal, L Wu, J Cerne, A.G Markelz, S Oh, N.P Armitage, “Terahertz Response and Colossal Kerr Rotation from the Surface States of the Topological Insulator Bi2Se3“, Phys. Rev. Lett. 108, 087403 (2012).

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