# 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.

## September – November 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)

Host: Jen

**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

**Three-dimensional flat bands from inhomogeneous nodal-line semimetals**

In this talk I will discuss the effects of inhomogeneous strain or chemical composition on the nodal line semimetals (NLSM). I will start by showing that quite generically inhomogeneity leads to Landau-level-like behavior in crystalline systems. NLSM as a Dirac system has zeroth Landau level which will be massively degenerate in a 3D system, forming a 3D approximately flat band. I will show the connection of such a flat band to the drumhead surface state of NLSM. I will apply this to models of realistic materials and will show that interactions may lead to formation of a superconducting or magnetic state stabilized by the geometric contribution to the superfluid stiffness.

Host: Jen

**The boundary density profile of a Coulomb droplet: Freezing at the edge**

**Stripes, Antiferromagnetism, and the Pseudogap in the Doped Hubbard Model at Finite Temperature**

Host: Cyrus Dreyer

The phase diagram of the two-dimensional Hubbard model at finite temperature poses one of the most interesting conundrums in contemporary condensed matter physics. Tensor network techniques, such as matrix-product based approaches as well as 2D tensor networks, yield state-of-the-art unbiased simulations of the 2D Hubbard model at zero temperature and are capable of giving unbiased results at finite temperature as well. A promising approach for applying tensor networks to study finite-temperature quantum systems is the minimally entangled typical thermal state (METTS) algorithm, which is a Monte Carlo technique that samples from a family of entangled wavefunctions, and which offers favorable scaling and parallelism. In this talk I will present some of our recent results applying this technique in the strong coupling, low-temperature and finite hole-doping regime [1]. We discover that a novel phase characterized by commensurate short-range antiferromagnetic correlations and no charge ordering occurs at temperatures above the half-filled stripe phase extending to zero temperature. We find the single-particle gap to be smallest close to the nodal point and detect a maximum in the magnetic susceptibility. These features bear a strong resemblance to the pseudogap phase of high-temperature cuprate superconductors. The simulations are verified using a variety of different unbiased numerical methods in the three limiting cases of zero temperature, small lattice sizes, and half-filling.

**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).

**Title: Conical Intersections in Semiconductor Nanocrystals—How is Electronic Energy Converted to Heat?**

Host: Cyrus Dreyer

Non-radiative recombination limits the efficiencies of semiconductor-based optoelectronic devices and photocatalysts by converting useful electronic energy into heat. It has been known for more than half a century that such recombination is facilitated by defects, but theoretical prediction of exactly which defects promote non-radiative recombination remains a challenge. In order to develop a predictive understanding of the role specific defects play in semiconductor photophysics, we are investigating the hypothesis that conical intersections between potential energy surfaces introduced by defects form pathways for recombination. We will present recent developments in the computational identification of such defect-induced conical intersections. Fast and stable graphics processing unit accelerated multireference electronic structure codes enable the identification of these defects, and new nonadiabatic molecular dynamics methods allow us to model dynamics in their vicinities. These tools have enabled us to identify defect-induced conical intersections in silicon nanomaterials, lead-halide perovskites, and chalcogenide nanomaterials. Through analysis of these intersections, we can understand how the structures of these materials determine their photophysical properties.

**Title: Linear and non-linear THz spectroscopy of collective excitations in correlated materials**

Host: Cyrus Dreyer

In this talk, I will discuss two new aspects of time-domain THz spectroscopy on strongly correlated materials. In the first case, we combine THz spectroscopy and inelastic neutron scattering measurements on the quantum spin liquid candidate YbMgGaO4 to obtain better insight into its exchange interactions. THz spectroscopy in this fashion functions as high-field electron spin resonance and probes the spin-wave excitations at the Brillouin zone center, ideally complementing neutron scattering. This study strongly constrains possible mechanisms responsible for the observed spin-liquid phenomenology. In the second case, I will discuss how we apply the new technique of non-linear THz two-dimensional coherent spectroscopy to extract the first measurements of energy relaxation ( decoherence ( T2 ) times close to the insulating side of the 3D metal-insulator transition (MIT) in phosphorus doped silicon. I will argue how the observed features imply that a strongly disordered interacting insulator cannot be mapped to a system of effectively non-interacting localized excitations. We dub this new phenomenology a “marginal Fermi glass”.

Xinshu Zhang, Fahad Mahmood, et. al. Physical Review X 8, 031001 (2018)

Fahad Mahmood, et. al. arXiv:2005.10822 (2020)

**Vestigial electronic orders in quantum materials**

A hallmark of the phase diagrams of quantum materials is the existence of multiple electronic ordered states. In many cases, they cannot be simply described as independent competing phases, but instead display a complex intertwinement. In this talk, I will present a framework to describe intertwined phases in terms of a primary and a vestigial phase. While the former is characterized by a multi-component order parameter, the fluctuation-driven vestigial state is characterized by a composite order parameter formed by higher-order, symmetry-breaking combinations of the primary order parameter. Exotic electronic states with scalar and vector chiral order, spin-nematic order, Potts-nematic order, time-reversal symmetry-breaking order, and charge 4e superconductivity emerge from this simple underlying principle. I will present a rich variety of possible phase diagrams involving the primary and vestigial orders, and discuss possible realizations of these exotic composite orders in different quantum materials, such as high-temperature superconductors and twisted moiré systems.

Host: Jen

**Quantization of fractional corner charge in C**

_{n}-symmetric higher-order topological crystalline insulators_{n}-symmetric crystalline insulators with vanishing polarization, having as a consequence the existence of corner-localized charges quantized in multiples of e/n. I will discuss the relationship between fractional charge and topological states and show how symmetry-protected, corner-localized topological states can exist even in the absence of an energy gap.

Host: Jen