During Fall 2020, seminars will be posted on this calendar and take place via Zoom. To join the mailing list and get the link, email Jennifer Cano (first email@example.com)
The CQM Distinguished Lecture series has been established in the Fall of 2015 to bring to Stony Brook University the renowned experts in the physics of quantum matter.
September – October 2020
Observation of a Majorana zero mode in a topologically protected edge state
Superconducting pairing in the helical edge state of topological insulators is predicted to provide a unique platform for realizing Majorana zero modes (MZMs). We use (spin-polarized) scanning tunneling microscopy measurements to probe the influence of proximity induced superconductivity and local magnetism on the helical edge states of bismuth(111) thin films, which are grown on a superconducting niobium substrate and decorated with iron clusters. Consistent with model calculations, our measurements reveal the emergence of a localized MZM at the interface between the superconducting helical edge state and the ferromagnetic iron clusters with strong magnetization component along the edge (1). Our experiments also resolve the MZM’s unique spin signature by which it can be distinguished from trivial in-gap states that may accidently occur at zero energy in a superconductor. High-resolution spectroscopic mapping of quasiparticle interference further demonstrates quasiparticle backscattering inside the one-dimensional helical edge state, which is induced by the ferromagnetic iron clusters that locally break time-reversal symmetry (2).
(1) B. Jäck, Y. Xie, J. Li, S. Jeon, B.A. Bernevig, A. Yazdani, Science 364, 1255-1259 (2019)
(2) B. Jäck, Y. Xie, B.A. Bernevig, A. Yazdani, PNAS, DOI:10.1073/pnas.2005071117 (2020)
Superconductivity from skyrmion condensation in magic angle graphene
We propose and analyze a strong-coupling route to superconductivity in twisted bilayer graphene near the magic angle (MATBG). Starting from a promising ordered insulating state featuring Chern bands, we show that topological textures/skyrmions of the order parameter carry electric charge due to band topology. Subsequently, we find a natural all-electronic mechanism of attraction between two such charge e textures. This leads to pairing into charge 2e bosons, whose condensation can trigger superconductivity on doping away from the insulating state. We discuss microscopic aspects of this scenario, including energetics and an estimate of the effective mass which yields Tc for the superconductor, within the framework of an effective field theory. We back up our analytical calculations by large-scale DMRG numerics on a related model that captures the relevant symmetry and topology of the flat bands in MATBG. In DMRG, we find clear evidence for superconductivity driven by the binding of electrons into charge-2e skyrmions, even when Coulomb repulsion is by far the largest energy scale.
Unveiling carrier recombination mechanisms in halide perovskites from first-principles calculations
Host: Cyrus Dreyer
Halide perovskites are highly efficient optoelectronic materials; the power conversion efficiency of perovskite solar cells has reached 25.2%, being already comparable with that of single-crystalline silicon cells (26.1%). To understand the fundamental physics behind the superior performance, carrier recombination mechanisms are crucial. In recent years, we have developed a full set of first-principles approaches that allow to quantitatively compute the carrier recombination rates based on density functional theory and to understand the underlying recombination mechanisms. I will present a number of critical insights into the radiative [1-2] and nonradiative [3-7] recombination mechanisms in halide perovskites obtained by applying our methodology to this technologically important system.
 X. Zhang, J.-X. Shen, and C. G. Van de Walle, J. Phys. Chem. Lett. 9, 2903 (2018).
 X. Zhang, J.-X. Shen, W. Wang, and C. G. Van de Walle, ACS Energy Lett. 3, 2329 (2018).
 J.-X. Shen, X. Zhang, S. Das, E. Kioupakis, and C. G. Van de Walle, Adv. Energy Mater. 8, 1801027 (2018).
 X. Zhang, J.-X. Shen, and C. G. Van de Walle, Adv. Energy Mater. 10, 1902830 (2020).
 X. Zhang, M. E. Turiansky, J.-X. Shen, and C. G. Van de Walle, Phys. Rev. B 101, 140101 (2020).
 X. Zhang, M. E. Turiansky, and C. G. Van de Walle, J. Phys. Chem. C 124, 6022 (2020).
 X. Zhang, J.-X. Shen, M. E. Turiansky, and C. G. Van de Walle, J. Mater. Chem. A 8, 12964 (2020).