October 2023
Abstracts of the Quantum Center Lunch Seminar
Date: Thursday, October 5, 2023
Place: ETH Zurich, Hönggerberg, HPF G 6
Time: 12:00 - 13:30
Ab initio quantum transport simulation of optoelectronic devices
Jiang Cao - Nano-TCAD (Luisier group), ETH Zurich
To accurately describe the many-body effects occurring in the advanced optoelectronic devices, two physical phenomena must be accounted for: light-matter and carrier-carrier interactions. Traditionally, the electron-electron interactions are investigated with the help of semi-classical Monte Carlo simulations. However, such a treatment becomes insufficient in devices with nano-scale dimensions where the influence of quantum mechanical effects can no longer be ignored. Moreover, many-electron interactions should be treated at the same level as quantum transport, which has never been achieved in ab initio quantum transport simulations due to the great computational challenges.
In this talk, I will report on the recent development of such an advanced framework combining many-body and quantum transport properties. I will show that the well-known band gap underestimation of density functional theory (DFT) can be corrected in device simulation within the so-called GW approximation. In addition, the latter allows us to take electron-electron interactions into account and to study, for example, impact ionization. The photo-avalanche effect is also captured: it requires to simultaneously take electron-electron and electron-photon interactions into account. The tool we developed can handle 1D/2D/3D system made of hundreds to thousands of atoms thanks to efficient parallel numerical algorithms. Its strength and usefulness will be demonstrated by examining bulk-like InAs, 2D monolayer MoS2, and 1D carbon nanotube devices. The predictive power of this simulator opens up the possibility to investigate different types of nanoscale optoelectronic devices and to provide design guidelines to experimentalists.
Spin liquids in 2D Materials
Clemens Kuhlenkamp - Quantum Photonics (Imamoglu group), ETH Zurich
Spin liquids are exotic phases of matter with fractionalized excitations and intrinsic topological order. While spin liquids have been predicted to appear in various frustrated magnetic systems, they usually occupy small regions of the phase diagram, making it challenging to find conclusive evidence for their existence. In this talk, I present a theoretical proposal aimed to circumvent the need to fine-tune experimental parameters by combining layer pseudo-spin with strong external magnetic fields in heterostructures of 2D materials. The external field breaks time-reversal symmetry and generates an exceptionally robust chiral spin liquid, which we understand in terms of simple analytical arguments. Crucially, by using the layer degree of freedom, one can access novel optical and transport probes, which can directly detect topological order. I will also discuss exotic quantum critical points between symmetry-protected states and phases with intrinsic topological order which emerge in these systems.