Linnaeus Physics Colloquium: Topological quantum matter

Lecturer: Professor Carlo Canali, Linnaeus University
Title: Topological quantum matter
Place: Kalmar: room Ma402 (Skeppet), building Magna. Växjö: through link, room D0073, building D. Live: https://lnu-se.zoom.us/j/335602333.

Coffee and buns at 13.45 in room Ma402 (Skeppet), building Magna, Kalmar.

Calculated band structure for a thin film of the three-dimensional topological insulator Bi2Se3. Inside the bulk band gap the conical linear dispersion of the Dirac surface states is visible. The inset of the figure shows the spin-momentum locking of the Dirac surface states. Courtesy of Anna Pertsova and C.M. Canali, Linnaeus University & Nordita

Scanning tunneling microscopy and spectroscopy data obtained on magnetic V-doped Sb2Te3 topological insulator. The dopants’ concentration amounts to 1.5% V in each Sb layer. Courtesy of Paolo Sessi, Max Planck Institute, Halle, Germany

Abstract

In recent years, a new paradigm has arisen in condensed matter physics, in which different non-trivial phases and phase transitions of condensed matter systems are characterized not by a local order parameter, as Lev Landau taught us, but rather by a discrete topological index that describes the global, or topological, properties of the system. One early example of this is the quantum Hall effect, where the topological index directly gives the quantized Hall conductance. More recently, a new class of materials, known as topological insulators, was found to be characterized by a topological number signaling that these insulators are fundamentally different from the ordinary insulators. One of the most striking consequences of this topological index is the existence of metallic states on the surfaces of the material, despite the bulk being an insulator. These electronic surface states are described by a massless Dirac equation, with the spin direction locked to the direction of the momentum. As a result, their conducting properties are topologically protected against scattering off impurities that do not break time-reversal symmetry, making these materials particularly promising for applications in low-energy electronic devices and nanospintronics.

In this colloquium, I will present an elementary introduction of some of the basic concepts undelaying the physical properties of these topological materials. I will then discuss some of the remarkable quantum phenomena that emerge in topological insulator thin films when time-reversal symmetry is broken, such as the quantum anomalous Hall effect and the topological magneto-electro effect, which are among the present research interests of the Condensed Matter Physics research group at Linnaeus University.