Physicists carry out quantum simulation of the physical effects of disorder in topological insulators

Oct 23, 2018
Physicists carry out quantum simulation of the physical effects of disorder in topological insulators (Nanowerk News) A team of researchers from the University of Illinois at Urbana-Champaign, United States, the Institute of Photonic Sciences (ICFO) and the Universitat Politècnica de Catalunya (UPC) uses experiments with ultra-cold atoms trapped and driven by lasers to create a new disorder-induced topological insulator (Science, "Observation of the topological Anderson insulator in disordered atomic wires").
The unusual properties of this material open new technological applications in quantum computing, next-generation miniaturised data storage and spintronics.
Topological insulators (TI) host an exotic physics that could shed new light on the fundamental laws of nature. The unusual properties of TIs hold tremendous promise for technological applications, including in quantum computing, next-generation miniaturised data storage and spintronics.

Artist's depiction of a disorder-induced transition to the topological Anderson insulator phase. A river flowing along a straight path is altered by disorder in the underlying landscape. After going through a transition (waterfall), the river forms a closed loop—a shape with a different topology to that of the initially straight path. In the topological Anderson insulator phase, the trivial band structure of a normal material is transformed into a topologically non-trivial band structure due to disorder and disruptions in the tunnel couplings between lattice sites. The winding number in the topological Anderson insulator phase is distinct from that of the normal case without disorder. (Image: Lachina Creative, Bryce Gadway, University of Illinois at Urbana-Champaign)
Scientists around the globe are working to understand the microscopic properties of these materials, which freely conduct electricity along their edges even though their bulk is an insulator. An international team of theoretical and experimental physicists have made the first observation of a specific type of disorder-induced TI.
The team of experimental physicists at the University of Illinois at Urbana-Champaign, formed by Professor Bryce Gadway and postgraduate students Eric Meier and Alex An, used atomic quantum simulation, an experimental technique employing finely tuned lasers and ultra-cold atoms about a billion times colder than room temperature, to mimic the physical properties of one-dimensional electronic wires with precisely tunable disorder.
The system starts with trivial topology just outside the regime of a topological insulator; adding disorder nudges the system into the nontrivial topological phase. This type of topological insulator induced by disorder is called the topological Anderson insulator, named after the noted theoretical physicist and Nobel laureate Philip Anderson.
Surprisingly, while disorder typically inhibits transport and destroys non-trivial topology, in this system it helps to stabilise a topological phase.
The observation was made possible through close collaboration with an international team of theoretical physicists at the University of Urbana-Illinois, the Institute of Photonic Sciences (ICFO) and the Universitat Politècnica de Catalunya (UPC).The team of physicists elucidated the quantum physics at work and identified the key signature that the experimentalists should look for in the system. The team at ICFO and the UPC is made up of the theoretical physicists Pietro Massignan , Alexandre Dauphin and Maria Maffei.
Massignan, Ramón y Cajal researcher at the UPC’s Department of Physics and ICFO, comments: “Intuitively, one would think that disorder should play against conductance. For example, running is easy in an open field, but gets harder and harder as one moves through an increasingly dense forest. But here we show that suitably tailored disorder can actually trigger some peculiar conducting excitations, called topologically protected edge modes.”
Meier is lead author of the paper. “Interestingly,” he adds, “in a 3D or 2D topological system, those edge states would be characterised by freely flowing electrons. But in a 1D system like ours, the edge states simply sit there, at either end of the wire. In any TI, the boundary states have the dimensionality of your system minus one. In our 1D topological Anderson insulator, the boundary states are basically just points at either end of the wire. While the boundary physics is actually a bit boring in this system, there is rich dynamics going on in the bulk of the system that is directly related to the same topology–this is what we studied.”
The group’s experimental observation validates the concept of topological Anderson insulators that was worked out roughly a decade ago. The topological Anderson insulator phase was first discovered theoretically by J. Li et al. in 2009 (Physical Review Letters, "Topological Anderson Insulator"), and its origin was further explained by C. W. Groth, et al. that same year (Physical Review Letters, "Theory of the Topological Anderson Insulator").
Five years later, a pair of works, one by A. Altland et al. (Physical Review Letters, "Quantum Criticality of Quasi-One-Dimensional Topological Anderson Insulators") and one by Taylor Hughes et al. (Physical Review Letters, "Topological Criticality in the Chiral-Symmetric AIII Class at Strong Disorder") working with the group of Emil Prodan at Yeshiva University in New York, predicted the occurrence of the topological Anderson insulator in one-dimensional wires, as realised in the new experiments from the Gadway group.
Gadway emphasises. “Our taking on this research was really inspired by the 2014 prediction of Taylor Hughes and his graduate student Ian Mondragon-Shem. Taylor was a key collaborator. Likewise, our colleagues in Barcelona made a tremendous contribution in introducing the concept of mean chiral displacement, which allows the topology to be measured directly in the bulk of the material.”
“Working with Taylor,” Gadway adds, “our Spanish colleagues found that the mean chiral displacement is essentially equivalent to the topological invariant of such a one-dimensional system, something called the winding number. This was critical to our being able to take the data on the system and relate what we saw in the experiment to the system’s topology. This was one project where having a bevy of theorists around was a big help, both for performing the right measurements and for understanding what it all meant.”
“This is an exciting result in terms of potential applications,” says Gadway. “This suggests we may be able to find real materials that are almost topological that we could manipulate through doping to imbue them with these topological properties. This is where quantum simulation offers a tremendous advantage over real materials—it’s good for seeing physical effects that are very subtle. Our ‘designer disorder’ is precisely controllable, where in real materials, disorder is as messy as it sounds—it’s uncontrollable.”
“Gadway’s experimental setup is a theorist’s dream,” adds Massignan. “It was like playing with LEGO: the model we envisaged could be built step-by-step, in a real laboratory. Every single element of the Hamiltonian we had in mind could be implemented in a very careful way, and changed in real time.” ICFO researcher Alexandre Dauphin adds: “This platform is also very promising for studying the effects of both interaction and disorder in topological systems, which could lead to exciting new physics.”

Source: Universitat Politècnica de Catalunya (UPC)

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