CENTER FOR QUANTUM MATERIALS AND CONDENSED MATTER PHYSICS SEMINARS

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 name.lastname@stonybrook.edu)

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.

The lectures in this series will attract a broad audience of physicists from SBU and BNL,
and SBU graduate students.

September – October 2020

Sep
18
Fri
Berthold Jäck (Princeton/HKUST) **note the special time**
Sep 18 @ 10:00 am – 11:00 am

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)

Host: Jen

Sep
25
Fri
Shubhayu Chatterjee (Berkeley)
Sep 25 @ 1:30 pm – 2:30 pm

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.

Host: Jen

Oct
2
Fri
Dmitry Pikulin (Microsoft Station Q)
Oct 2 @ 1:30 pm – 2:30 pm

Title: TBD

Host: Jen

Oct
9
Fri
Gabriel Jose Goulart Cardoso (Stony Brook)
Oct 9 @ 1:30 pm – 2:30 pm
The boundary density profile of a Coulomb droplet: Freezing at the edge
We revisit the problem of computing the boundary density profile of a droplet of two-dimensional one-component plasma (2D OCP) with logarithmic interaction between particles in a confining harmonic potential. At a sufficiently low temperature but still in the liquid phase, the density exhibits oscillations as a function of the distance to the boundary of the droplet. We obtain the density profile numerically using Monte-Carlo simulations of the 2D OCP. We argue that the decay and period of those oscillations can be explained within a picture of the Wigner crystallization near the boundary, where the crystal is gradually melted with the increasing distance to the boundary.

Host: Sasha
Oct
16
Fri
Roni Ilan (Tel Aviv University)
Oct 16 @ 1:30 pm – 2:30 pm

Title: TBD

Host: Jen

Oct
23
Fri
Xie Zhang (UCSB)
Oct 23 @ 1:30 pm – 2:30 pm

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. 

[1] X. Zhang, J.-X. Shen, and C. G. Van de Walle, J. Phys. Chem. Lett. 9, 2903 (2018).

[2] X. Zhang, J.-X. Shen, W. Wang, and C. G. Van de Walle, ACS Energy Lett. 3, 2329 (2018).

[3] J.-X. Shen, X. Zhang, S. Das, E. Kioupakis, and C. G. Van de Walle, Adv. Energy Mater. 8, 1801027 (2018).

[4] X. Zhang, J.-X. Shen, and C. G. Van de Walle, Adv. Energy Mater. 10, 1902830 (2020).

[5] X. Zhang, M. E. Turiansky, J.-X. Shen, and C. G. Van de Walle, Phys. Rev. B 101, 140101 (2020).

[6] X. Zhang, M. E. Turiansky, and C. G. Van de Walle, J. Phys. Chem. C 124, 6022 (2020).

[7] X. Zhang, J.-X. Shen, M. E. Turiansky, and C. G. Van de Walle, J. Mater. Chem. A 8, 12964 (2020).