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Studies Bolster Promise of Topological Insulators Print

A few years ago, a strange new material began to drive research in condensed-matter physics around the world. First theorized and then discovered by researchers at Berkeley Lab and their colleagues in other institutions, these "strong 3D topological insulators"—TIs for short—are seemingly mundane semiconductors with startling properties. Not only are they promising materials for energy-conserving electronic applications, they provide a fascinating medium for possibly observing still-theoretical particles that could play a major role in quantum computing. Two angle-resolved photoemission spectroscopy (ARPES) studies recently performed at ALS Beamline 12.0.1 improve the prospects for the practical application of TIs in advanced devices.

No U-Turns Allowed

It is no exaggeration to state that technological progress in our society depends on finding ways to better manipulate electricity. One goal is to develop devices that consume less energy as electrons and ions flow through their circuits. Normally, these charge carriers encounter various forms of resistance, generating heat and wasting energy. This happens because electrons scatter as they interact with imperfections in the crystalline structure of the conducting material. In topological insulators, however, surface electrons are "protected" from this kind of scattering: when electrons hit an impurity, they don't bounce back but run through it, so that there is no degradation of the electrical current.

This protection comes from a property called "spin," an intrinsic property like a particle's mass and charge. On the surface of a topological insulator, electrons moving in opposite directions have opposite spins. If they hit a defect, they cannot just bounce back, as that would require also flipping the spin to match the spin of the other electrons flowing in the opposite direction. Flipping the spin would require a change in the magnetic moment at the barrier; without a magnetic change, there can be no flip of spin, and a U-turn is forbidden. Beamline 12.0.1 at the ALS is extremely well suited to examining the electronic properties of this strange new material in great detail, as the results reported here demonstrate.

Topological insulators conduct electricity only on the surface; inside, they are good insulators. The direction and spin of the surface electrons are locked together and can only change in concert. Perhaps the most surprising consequence of this is that the surface electrons cannot be scattered by defects or other perturbations and thus meet little or no resistance as they travel. In the jargon, the surface states remain "topologically protected"—they can't scatter without breaking the rules of quantum mechanics.

Electrons on the surface of a topological insulator can flow with little resistance. Their spin and direction are intimately related; the direction of the electron determines its spin and in turn is determined by it.

This resistance to backscattering could potentially make these materials ideal candidates for electronic applications—except that at higher temperatures, where real-world devices would be used, scientists expected that increasing vibrations in the crystalline lattice (atoms vibrate as they warm up) would cause a different kind of electron scattering. Collisions between electrons and the lattice vibrations, known as phonons, would cause energy dissipation and losses in the conventional way. But no one had studied these higher-temperature electronic interactions, until now.

To study a TI's surface conductivity, electron transport on its surface has to be separated from total conductivity, including in the poorly conducting bulk, something for which ARPES is well suited. ALS Beamline 12.0.1 seems to have an ideal balance of energy, resolution, and flux for ARPES research on topological insulators. This beamline was used for some of the first experiments establishing that 3D TIs actually occur in nature, and several teams have worked there, validating the characteristics of TIs. By recording the angle and energy of photoemitted electrons on a CCD detector, ARPES gradually builds up a direct graphic visualization of the sample's electronic structure. Properties, including electron–phonon coupling, can be calculated from the diagrams.

ARPES maps the electronic properties, including the band structure and Fermi surface, of the TI Bi2Se3 (left). Like graphene, the lower-energy valence band of a TI meets the higher-energy conduction band at a point, the Dirac point, with no gap between the bands (center). Unlike graphene, however, the Fermi surface of a TI does not usually pass through the Dirac point. For surface electrons, distinct spin states (red arrows) are associated with each different orientation in momentum space (right).

Two recent studies were done at Beamline 12.0.1 on a particularly promising topological insulator, Bi2Se3, on whose surface electrons can flow at room temperature, making it an attractive candidate for practical applications, like spintronic devices, or more speculative ones like quantum computers.

In one experiment, ARPES measurements showed that the surface electrons barely couple with phonons at all, even as the temperature approached room temperature. Although there's still a long way to go, experimental confirmation that electron–phonon coupling is very small underlines Bi2Se3's practical potential and eliminates one potential impediment to TI-based spintronic devices that efficiently conduct electricity while consuming less energy.

The spin-locked electronic states of room-temperature TIs could also open a gateway for more exotic possibilities as well. For example, by layering a superconducting material onto the surface of a TI—a feat recently achieved by a group of Chinese scientists working at Beamline 12.0.1—it may be possible to create a theoretical but yet unseen particle that is its own antiparticle, one that could persist in the material undisturbed for long periods. Discovery of these so-called Majorana fermions would be an achievement in itself and could also provide a way of overcoming the main obstacle to realizing a working quantum computer, a method of indefinitely storing data as "qubits."

The experimental examination of strong, 3D topological insulators is a field hardly more than five years old, and the potential rewards, both for fundamental and applied science, have only begun to be explored.



Research conducted by: Z.-H. Pan and T. Valla (Brookhaven National Laboratory); A.V. Fedorov (ALS); D. Gardner, Y.S. Lee, and S. Chu (Massachusetts Institute of Technology); M.-X. Wang, C. Liu, J.-P. Xu, F. Yang, L. Miao, M.-Y. Yao, C.L. Gao, D. Qian, and J.-F. Jia (Shanghai Jiao Tong University, China); C. Shen and Z.-A. Xu (Zhejiang University, China); X. Ma  (Chinese Academy of Sciences); X. Chen and Q.-K. Xue (Tsinghua University, China); Y. Liu (Pennsylvania State University); and S.-C. Zhang (Stanford University and Tsinghua University, China).

Research funding: National Basic Research Program of China; National Natural Science Foundation of China; Shanghai Committee of Science and Technology, China; Chinese Academy of Sciences; Program for New Century Excellent Talents in University, China; Shanghai Municipal Education Commission; Shanghai Education Development Foundation; Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning; the U.S. National Science Foundation; and the U.S. Department of Energy (DOE). Operation of the ALS is supported by the DOE Office of Basic Energy Sciences.

Publications about this research: Z.-H. Pan, A.V. Fedorov, D. Gardner, Y.S. Lee, S. Chu, and T. Valla, "Measurement of an Exceptionally Weak Electron-Phonon Coupling on the Surface of the Topological Insulator Bi2Se3 Using Angle-Resolved Photoemission Spectroscopy," Phys. Rev. Lett. 108, 187001 (2012); and M.-X. Wang, C. Liu, J.-P. Xu, F. Yang, L. Miao, M.-Y. Yao, C.L. Gao, C. Shen, X. Ma, X. Chen, Z.-A. Xu, Y. Liu, S.-C. Zhang, D. Qian, J.-F. Jia, Q.-K. Xue, "The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films," Science 336, 52 (2012).

ALS Science Highlight #260


ALSNews Vol. 337