# During Spring 2021, 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.

## October 2020 – February 2021

**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

**Imaging nematic quantum Hall states and their interacting boundary modes **

Two-dimensional quantum Hall systems offer a versatile platform for realizing phenomena that can emerge from a confluence of electronic interactions, symmetry breaking and topology. In this talk, I will discuss scanning tunneling microscope experiments which explore the role of electron-electron interactions and their tunability on the surface of bismuth. Our spectroscopic measurements reveal a number of exotic ordered states that arise from spontaneous valley ordering of bismuth surface states in a large magnetic field. Specifically, we observe a nematic phase with broken rotational symmetry and a ferroelectric phase that carries a dipole moment. We use the scanning tunneling microscope to directly visualize the wavefunctions of these broken symmetry phases. Furthermore, we image local nematic domains and find counter-propagating one-dimensional quantum Hall edge modes at their boundaries. We can change the number of edge modes at the domain walls to realize strikingly different regimes where the boundary is either metallic or insulating, in accordance with theory for a new class of interacting Luttinger liquids.

Host: Jen

**Atomic-scale structure and electronic properties of twisted double bilayer graphene: topological edge states and nematic order**

Atomically thin van der Waals materials stacked with an interlayer twist are an excellent platform towards achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. Here we demonstrate the formation of emergent correlated phases in twisted double bilayer graphene (tDBG) in two regimes of twist angle: minimally twisted (<0.1°) and 1.1°. tDBG at a tiny twist angle, at which moiré physics do not play a role, host large regions of uniform rhombohedral four-layer (ABCA) graphene where scanning tunneling spectroscopy reveals unprecedentedly sharp flat band of 3-5 meV half-width. We demonstrate that, when this flat band straddles the Fermi level, a correlated many-body gap emerges. Moreover, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with Bernal graphene. On the other hand, scanning tunneling microscopy on tDBG at a regime of twist angles (~1.1°) at which moiré physics play an important role, reveals the presence of van Hove singularities whose spatial distribution within the moiré unit cell is determined by the inequivalent stacking sites. Tuning the electron filling as well as the displacement field reveals broken C_{3} symmetry that emerges when the Fermi level is brought in the flat band. This symmetry breaking is manifested as long-range commensurate stripes along a high-symmetry moiré crystallographic direction, distinctive of nematic correlations of electronic origin. Comparing our experimental data with a combination of microscopic and phenomenological modeling, we show that the nematic instability is not associated with the local scale of the graphene lattice, but is an emergent phenomenon at the scale of the moiré lattice, pointing to the universal caracter of this ordered state in flat band moiré materials.

Host: Jen

**X-ray vision of electrons in complex oxides: the case of magnetism in nickelate superconductors**

Many of the most remarkable properties of quantum materials come from the interplay of multiple charge, orbital and spin degrees of freedom. Probing all of these with a single technique is consequently highly desirable. In this talk, I will describe how resonant inelastic x-ray scattering (RIXS) opens up important new possibilities for measuring all these degrees of freedom. This will be illustrated by the specific case of probing the nature of magnetism in the newly discovered d^{9} superconducting nickelates.

**References**

- J. Q. Lin et al., arXiv:2008.08209 to appear in Physical Review Letters

Host: Qiang Li

**Title: A proposal for determination of polarity orientation in polar metals **

Host: Cyrus Dreyer

In typical ferroelectrics, which are polar insulators, switching of polarity is immediately manifested in a polarization switching current. By contrast, in a polar metal or semimetal, a corresponding experimental response is missing. In this talk, I will discuss that the nonlinear Hall effect (NLHE) can offer a way to detect the polarization orientation and polarization switching in polar metals and semimetals, as well as in narrow bandgap ferroelectric semiconductors, which often show conducting behavior due to the presence of defects and impurities. This effect is particularly enhanced in topological metals or topological semimetals due to the large concentrations in Berry curvature near the Fermi level, which is central to the description of the NLHE [1-2]. However, we find that NLHE can also be realized in topologically trivial materials. The nonlinear Hall response current, which appears as a second-order response to an external electric field, vanishes in the paraelectric phase and reverses its sign upon the polarity reversal in a polar metal [3]. The magnitude of this response current is large enough to be experimentally detected [4].

[1] I. Sodemann and L. Fu, Phys. Rev. Lett. 115, 216806 (2015).

[2] S. Singh, J. Kim, K. M. Rabe, and D. Vanderbilt, Phys. Rev. Lett. 125, 046402 (2020).

[3] R.-C. Xiao, D.-F. Shao, W. Huang, and H. Jiang, Phys. Rev. B 102, 024109 (2020).

[4] Ma et al., Nature 565, 337 (2019).