Condensed Matter Physics Colloquium UNSW – FLEET Seminar

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Broken symmetries in heterostructures based on 2D materials

Prof Enrique Diez. Group of Nanotechnology, Nanolab, University of Salamanca., Spain

ABSTRACT

Two-dimensional (2D) crystals are particularly well suited for studying the interplay between symmetry and nonlinearity due to their high level of ordering. Remarkably, electronic states in these systems display quantum effects that give rise to novel and intriguing nonlinear effects simplifying further symmetry analysis.

In addition to the spin degeneracy, charge carriers in graphene have an additional degree of freedom called valley pseudospin. At the corners of the Brillouin zone (K and K’ points), the electronic states on the two sublattices in pristine graphene are decoupled and have the same energy, giving rise to the so-called valley degeneracy. This degeneracy can be lifted, as for example, by stacking graphene with hexagonal boron nitride (hBN) and twisting properly the layers of the heterostructure leading to the appearance of an angle-dependent Moiré pattern. Such effect can break several symmetries and enhances collective interactions, providing the appearance of a plethora of exotic states of matter.

In our latest devices, the three different layers are aligned with a commensurate pattern of the natural edges far away from the magic angle. Multiple alignment possibilities could happen following this procedure due to the two different crystalline directions of graphene and hBN, zigzag and armchair. By combining different optical and electrical measurements we can determine their exact relative alignment. Low temperature electrical characterisation has determined, by local measurements, a high-quality sample with a sizeable mobility. The valley and spin degenerations are broken, and multiple plateaus appear when a magnetic field is applied. However, the most remarkable results of this device are not obtained with local measurements. Magnetotransport measurements on this device have revealed an intriguing non-local electrical signal, with a chiral flavour, when small external magnetic fields are applied. The non-local signal in graphene is widely studied and different effects causing it can be found in the literature. However, very few effects can explain this chirality in the results.

We will also talk about a few other  devices fabricated in our group, like  graphene FETs for THz detection and  about devices based in other 2D  materials beyond graphene with broken symmetries that exhibits unconventional photoresponse revealed by low-temperature photocurrent spectroscopy.

There is no doubt that the study of different alignments among 2D materials opens many new possibilities in Twistronics, which could include a better understanding of anomalous superconductivity, paving the way for new methods to obtain materials with novel quantum properties. In addition to their undoubted fundamental interest, these materials may also enable new quantum technologies that constitute a technological breakthrough.

BIO

Enrique Diez is a full professor of Theoretical Physics at the Department of Fundamental Physics at Salamanca University, Spain. He is the Director of the Nanotechnology Group and Head of the Clean Room and Low Temperature Laboratories, as well as the Director of the Fundamental Physics Department. He was born in Luzern (Swiss Confederation) in 1969 and obtained his Ph.D. in Physics at University Carlos III of Madrid in 1997. He continued his postdoctoral studies with Prof. Daniel Tsui in Princeton University. Prof.. Diez has focused his research in the transport and electronic properties of quantum nanosystems particularly 2D systems in the extreme quantum limit (high magnetic fields and few millikelvin temperatures).  He is currently researching unconventional 2D materials including graphene heterostructures and topological insulators looking for unique physical properties bearing in mind their uses in novel optoelectronic devices for the generation and detection of sub-THz and THz radiation to be used in a wide range of applications (Communications, Healthcare, Security, Life Sciences). More recently he devotes a large attention to the study of broken symmetry nanomaterials and the use of quantum point contacts for reveal exotic quantum features.   Web link to my research group: nanotech.usal.es