Our current research focuses on topological quantum matter.
We study the unusual electronic structure of these novel materials and explore pathways to tailor them for new applications.

The discovery of new materials with novel properties is one of the fascinating aspects of physics. Such findings will not only create and lead new research areas in science but also open the door for exciting future technologies. A very recent example of conceptually new materials is the topological insulator.

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Figure 1: (A) The spin of electrons on the surface is correlated with their direction of motion (B) The lattice structure of Bi2Te3 and the predicted relativistic "Dirac cone" like electronic structure formed by the surface electrons. (C) The electronic structure measured by angle-resolved photoemission (Figure generated from data in Ref [2]) that confirmed the theoretical prediction and the topological nature of Bi2Te3.

Topological insulators represent a new state of quantum matter with a bulk gap and an odd number of relativistic Dirac fermions on the surface (Fig. 1). The bulk of such materials is insulating while the surface can conduct electric current with well-defined spin texture. Moreover, these unusually robust surface states are protected by the time-reversal symmetry against all time-reversal-invariant perturbations, such as scattering by non-magnetic impurities, crystalline defects, and distortion of the surface itself. This unique surface state can lead to striking quantum phenomena such as:

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Figure 2. Unusual properties of topological insulators. (A) Spin-polarized edge channels in a 2D topological insulator. (B) Illustration of an image monopole induced by an electric charge. (C) Majorana fermions formed in a topological insulator-superconductor structure. (D) Topological contribution to Faraday effect at zero magnetic field.
  • 1) Quantum spin Hall effect (Fig. 2A)
  • 2) An image magnetic monopole induced by an electric charge (Fig. 2B)
  • 3) Majorana fermions (whose anti-particle is itself) induced by proximity effect from a superconductor (Fig. 2C)
  • 4) A topological contribution to Faraday and Kerr effects (Fig. 2D)
  • 5) A half quantum Hall effect on the surface with Hall conductance of e2/2h

The swift development of topological insulators has also inspired the study of other topological states, such as quantum anomalous Hall insulators, topological semi-metals, topological crystalline insulators and topological superconductors. Since the initial proposal of these novel topological states, they have galvanized much interest in physics.

Besides the great scientific interest inspired by these novel materials, they demonstrate great potential for energy and technology applications, such as:

  • 1) Ultra-low power electronics and novel spintronic devices
  • 2) Optoelectronic applications
  • 3) High efficiency thermoelectric materials

Our research group strive to advance the exciting and fast-developing field of topological quantum matter, focusing on investigating the electronic structures of these novel materials and exploring pathways to tailor them for potential applications. We are based at the University of Oxford, leveraging other local infrastructure (such as the quantum material research program in Oxford, the Diamond synchrotron and UK's central Laser facility in the proximity) and other user facilities across Europe worldwide.

References:

  1. Yulin Chen
    "Studies on the Electronic Structures of Three-dimensional Topological Insulators by Angle Resolved Photoemission Spectroscopy"
    Frontiers of Physics, 7, 175 (2012)
  2. Y. L. Chen, J. G. Analytis, J. H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D.H. Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain and Z.-X. Shen
    "Experimental Realization of a Three Dimensional Topological Insulator, Bi2Te3"
    Science, 325, 178 (2009)
  3. Y. L. Chen, J. H. Chu, J. G. Analytis, Z. K. Liu, K. Igarashi, H.-H. Kuo, X. L. Qi, S.-K. Mo, R. G. Moore, D. H. Lu, M. Hashimoto, T. Sasagawa, S. C. Zhang, I. R. Fisher, Z. Hussain and Z.-X. Shen
    "Massive Dirac Fermion on the Surface of a magnetically doped Topological Insulator"
    Science, 329, 659 (2010)
  4. Y. L. Chen, Z. K. Liu, J. G. Analytis, J. H. Chu, H. J. zhang, B. H. Yan, S.-K. Mo, R. G. Moore, D. H. Lu, S. C. Zhang, I. R. Fisher, Z. Hussain and Z.-X. Shen
    "Single Dirac Cone Topological Surface State and Unusual Thermoelectric Property of Compounds from a New Topological Insulator Family"
    Physical Review Letter, 105, 266401 (2010)
  5. Desheng Kong, Yulin Chen, Judy J. Cha, Qianfan Zhang, James G. Analytis, Keji Lai, Zhongkai Liu, Seung Sae Hong, Kristie K. Koski, Sung-Kwan Mo, Zahid Hussain, Ian R. Fisher, Zhi-Xun Shen, and Yi Cui
    "Ambipolar field effect in topological insulator nanoplates of (BixSb1-x)2Te3"
    Nature Nanotechnology, 6, 705 (2011)
  6. Bo Zhou, Z. K. Liu, J. G. Analytis, K. Igarashi, S. K. Mo, D. H. Lu, R. G. Moore, I. R. Fisher, T. Sasagawa, Z. X. Shen, Z. Hussain, and Y. L. Chen
    "Controlling the carriers of topological insulators by bulk and surface doping"
    Semicond. Sci. Technol. 27, 124002 (2012)
  7. S. H. Yao, B. Zhou, M. H. Lu, Z. K. Liu, Y. B. Chen, J. G. Analytis, C. Brüne, W. H.Dang, S.-K. Mo, Z.-X. Shen, I. R. Fisher, L. W. Molenkamp, H. L. Peng, Z. Hussain,and Yulin Chen
    "Observing electronic structures on ex-situ grown topological insulator thin films"
    Phys.Status Solidi RRL,7,130(2013)