Theoretical and Computational Nanoscience

Group Leader: Stephan Roche

Main Research Lines

  • Theoretical research on quantum transport phenomena in topological quantum matter (topological insulators, Weyl semimetals)

  • Spin dynamics and entanglement properties in Dirac matter (graphene, two-dimensional materials) and van der Waals heterostructures

  • Molecular dynamics, thermal transport properties and thermoelectricity in nanomaterials of interest for microelectronics (amorphous Graphene and boron nitride)

  • Predictive modelling and multiscale numerical simulation of complex nanomaterials and quantum nanodevices

In 2020 the group published the following relevant works:

Canted Spin Texture and Quantum Spin Hall Effect in WTe2
Together with the Center for Advanced 2D Materials from the National University of Singapore and the University of Grenoble Alpes (France), we have studied the transport properties of semimetal material (tungsten ditelluride monolayer, WTe2), and have predicted unique features of a low-symmetry structural phase of the tungsten ditelluride (WTe2) monolayer – a material of the transition metal dichalcogenide family – which suggests alternative ways to manipulate spin information. Our quantum transport simulations and modelling show that the structure of this material leads to an unconventional quantum spin Hall (QSH) effect, a mechanism which has been confirmed by two experimental groups in the US and Australia following our theory. These results are relevant to the development of new nanodevices based on all-electrical spin control.

Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN
In collaboration with various research groups in Korea, we have discovered that atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. In this new work, we demonstrated a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provide a future playground for optoelectronics.

Emergence of intraparticle entanglement and time-varying violation of Bell’s inequality in Dirac matter
We have discovered the emergence and dynamics of intraparticle entanglement in massless Dirac fermions. This entanglement, generated by spin-orbit coupling, arises between the spin and sublattice pseudospin of electrons in graphene. The entanglement is a complex dynamic quantity but is generally large and independent of the initial state. Its time dependence implies a dynamical violation of a Bell inequality, while its magnitude indicates that a large intraparticle entanglement is a general feature of graphene on a substrate. These features are also expected to impact entanglement between pairs of particles, and may be detectable in experiments that combine Cooper pair splitting with nonlocal measurements of spin-spin correlation in mesoscopic devices based on Dirac materials.

Ultralow-dielectric-constant amorphous boron nitride
In collaboration with the Ulsan National Institute of Science and Technology (UNIST, Republic of Korea) and the Samsung Advanced Institute of Technology (SAIT), we have reported in Nature the first synthesis of a thin film of amorphous Boron Nitride (a-BN) showing extremely low dielectric characteristics, high breakdown voltage and likely superior metal barrier properties, which represents a significant achievement for future electronics.

More specifically, a-BN as thin as 3 nm was synthesized on a silicon substrate (using low temperature inductively coupled plasma-chemical vapour deposition, ICP-CVD), which showed an exceptionally low dielectric constant of 1.78 at 100 kHz.

Tests of the diffusion barrier properties of this amorphous material, conducted in very harsh conditions, have also demonstrated that it is able to prevent metal atom migration from the interconnects into the insulator. Together with a high breakdown voltage, these characteristics makes a-BN very attractive for practical electronic applications.

To explain the correlation between the structural and morphological properties and the dielectric response of the a-BN film, our group has performed atomistic calculations on samples of a-BN generated “in silico”. By means of classical molecular dynamics, the chemical vapour deposition (CVD) method employed experimentally was simulated to build an a-BN film on Si having similar dimensions as the fabricated one. Ultimately, the calculations helped identifying the key factors for the excellent performances of a-BN: the nonpolar character of the BN bonds and the lack of order preventing dipoles alignment. The results of this simulation have contributed to understanding the structural morphology of this amorphous material as well as to explaining its superior dielectric performance.


Group Leader

Stephan Roche

ICREA Research Professor
stephan.roche@icn2.cat

Prof. Stephan Roche is a theoretician with more than 25 years’ experience in the study of transport theory in low-dimensional systems, including graphene, carbon nanotubes, semiconducting nanowires, organic materials and topological insulators.

He has published more than 250 papers in journals such as Nature, Review of Modern Physics, Nature Physics, Nano Letters and Physical Review Letters and he is the co-author of the book titled “Introduction to Graphene-Based Nanomaterials: From Electronic Structure to Quantum Transport” (Cambridge University Press, 2020-second edition).

He received the qualification to supervise PhD students from the Université Joseph Fourier (Grenoble, France) in 2004, and since then he has supervised more than ten PhD students and about 25 postdoctoral researchers in France, Germany and Spain. In 2009 Prof. Roche was awarded the Friedrich Wilhelm Bessel Research Award by the Alexander Von-Humboldt Foundation (Germany) and, since 2011, he has been actively involved in the European Graphene Flagship project as deputy leader of the Spintronics Work Package (WP). He is serving as leader of this WP since April 2020 and will continue until March 2023. He is also Division Leader of the Graphene Flagship.

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