# 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.

## June – October 2020

**Unlocking the secrets of growth and switching in ferroelectric multilayers using synchrotron x-ray diffraction**

Powerful new capabilities at the NSLS-II synchrotron at Brookhaven National Laboratory have enabled us to perform a series of in-situ experiments that have allowed us to unlock important secrets about the growth and switching processes in ferroelectric multilayers. In one set of experiments, at 4-ID, the In-Situ and Resonant scattering beamline (ISR), we perform x-ray diffraction during growth of sputtered ferroelectric multilayers and reveal that not only is the polarization intricately linked to crystal structure, also plays an important role in the growth process, influencing growth rates, relaxation mechanisms, electrical properties and domain structures. In a complementary set of experiments on the 11-ID, Coherent Hard X-Ray scattering (CHX) beamline, the high coherent flux at this beamline allows us to perform X-Ray Photon Correlation Spectroscopy measurements that monitor correlations between ferroelectric domain configurations as they are influenced by temperature and electric field stimuli. This method gives us an unprecedented view of the nanoscale arrangement of domains as a statistical ensemble as they are driven through thermal relaxation or switching events. Taken together, and combined with the extensive complimentary characterization we perform in our laboratory these experiments provide an unprecedented end-to-end window on to how strain, polarization and domain structure evolve into the complex configurations that generate much of the interest in these exciting materials.

**Two talks!**

**1:30 pm -2:00 pm: Kevin Cremin (postdoc in Richard Averitt’s group at UCSD)**

**“Photo-enhanced metastable c-axis electrodynamics in stripe-ordered cuprate La_{1.885}Ba_{0.115}CuO_4″**

**2:00 pm -2:30 pm: Jeremy Lee-Hand (Ph.D. student in Cyrus Dreyer’s group)**

**First-principles study of molybdinate perovskite oxides**

**Quantum Coherence and Dynamics Controlled by Light: **

**From Higgs Bosons to Chiral Fermions**

** *** **Jigang Wang*

*Department of Physics & Astronomy and Ames Laboratory-U.S. DOE, Iowa State University, Ames, IA 50011, USA*

Revolutionary properties of quantum materials are often manifestations of coherence and entanglement, e.g. it exists between the Cooper pairs in high-temperature superconductors (SCs); it protects chiral charge transport from disorder scattering in topological states of matter (TSM). The recent development of ultrafast and terahertz (THz) spectroscopy tools facilitates discovering and understanding driven coherent systems involving SCs and TSM controllable by light. In this talk, I will discuss strategic advantages, with help of some recent examples, of implementing this approach to probe and control many-body quantum phases and collective modes by light-driven coherence, e.g., light-induced gapless superconductivity and high harmonics generation [1], forbidden Anderson pseudo-spin precessions [2], hidden gapless quantum fluid [3], hybrid Higgs modes [4], spin exciton modes [5], phonon-controlled topology switching [6, 7] and chiral charge pumping [8]…. We argue that the light-driven coherence and sub-cycle dynamic symmetry breaking demonstrated in these work revealed represent universal principles for emergent materials discovery and light-matter quantum control for quantum information science.

[1] X. Yang, et al., Nature Photonics. 13, 707 (2019)

[2] C. Vaswani et al., Phys. Rev. Lett. arXiv:1912.01676 (2020)

[3] X. Yang, et al., Nature Materials. 17, 586 (2018).

[4] C. Vaswani, et al., submitted (2020)

[5] X. Yang, et al., Phys. Rev. Lett. 121, 267001 (2018)

[6] C. Vaswani, et al., Phys Rev X 10, 021013 (2020).

[7] X. Yang, et al., npj Quantum Materials** **5, 13 (2020)**.**

[8] L. Luo et al., submitted (2020)

**Title: Robust Quantum information Processing with Bosonic Modes**

**Phononic Frequency Combs – A New Addition to Photonics, MEMS,**

**Quantum Information Science, Molecular Science & Material Science**

Phononic frequency combs (PFC) are the mechanical analogs of

celebrated photonic frequency combs. These represent a newly

documented physical phenomenon in the domain of vibrations [1]. The

emergence of PFC is mediated by the nonlinear coupling among phonon

modes. Through a series of experiments with mechanical resonators, the

fundamentals of PFC have been established. Phononic frequency combs

could find applications in photonics, quantum information science,

molecular science and material science. My talk will describe my

fundamental work on phononic frequency combs and my plan for future

research.

[1] Ganesan, A., Do, C. and Seshia, A., 2017. Phononic frequency comb

via intrinsic three-wave mixing. Physical review letters, 118(3),

p.033903.

Host: Phil

**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.