Professor Pengcheng Dai

Department of Physics and Astronomy

Rice University

Host: Gannon

Title:  Magnetic order and spin excitations in two-dimensional honeycomb lattice van der Waals ferromagnetic CrI3

Abstract:  We use neutron scattering to study magnetic order and spin excitations in honeycomb lattice van der Waals ferromagnet CrI3. We show that ferromagnetic phase transition is first order in nature, and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. We discuss the role of the next-nearest-neighbor Dzyaloshinskii-Moriya interaction and Kitaev’s interaction plays on the spin-wave spectra and the possible presence of topological spin excitations in CrI3.

Professor Erik Henriksen

Department of Physics

Washington University in St. Louis

Host: Kaul

Title:  a-RuCl3 as a 2d crystalline acceptor: modulation doping, pn junctions, and the pursuit of Veselago's lens

Abstract: a-RuCl3 is a layered antiferromagnetic Mott insulator widely thought to host a quantum spin liquid state related to the Kitaev QSL. It can be exfoliated down to monolayer thicknesses and incorporated into van der Waals heterostructures along with graphene and myriad other atomically thin materials. Proximity of a-RuCl3 to graphene leads to a significant charge transfer between the two that, surprisingly, persists even when a thin insulating layer is inserted between them, a phenomenon analogous to modulation doping in epitaxially-grown semiconductors. In addition to constituting a new method of charge control in van der Waals stacks, we are using this to generate near-atomically-sharp pn junctions in graphene that may enable a realization of the electron-optic version of Veselago's lens. 

Professor Alannah Hallas

Department of Physics and Astronomy

Stewart Blusson Quantum Matter Institute

University of British Columbia

Host:  Gannon

Title: Intertwined Dipolar and Quadrupolar Correlations in the Pyrochlore Tb2Ge2O7

Abstract:  Rare earth pyrochlores are model systems for the study of highly frustrated magnetism, in large part, because the majority of these materials have a clean separation of energy scales between their single-ion properties and their collective interactions. Terbium pyrochlores represent the exception, where a low energy crystal field excitation profoundly affects the spin interactions and imbues these materials with especially complex phase behavior. We find that the magnetic states of the terbium pyrochlores evolve over three distinct temperature regimes, strongly suggesting a universality to their low temperature phase behavior. In this talk, I will present a comprehensive heat capacity and neutron scattering study of Tb2Ge2O7 where, for the first time, we have been able to elucidate the nature of each of these three regimes. We find that the complex phase behavior of the terbium pyrochlores originates from not only the usual geometric frustration but also frustration due to the competition between magnetic dipoles and electric quadrupoles.

Professor Hitesh Changlani

Department of Physics 

Florida State University

and The National High Magnetic Field Laboratory

Host:  Kaul

Abstract:

Geometrically frustrated magnets harbor novel phases of strongly interacting quantum matter, including those with fractionalized excitations, such as quantum spin liquids.  While there is tremendous progress on understanding their ground state properties, I will primarily focus on their dynamics by highlighting two directions that my group is pursuing. First, I will present our work (done in collaboration with experimentalists) on a newly synthesized pyrochlore, NaCaNi2F7, which we find to be an almost ideal realization of a spin-1 three-dimensional highly frustrated antiferromagnet with no magnetic order and a continuum of excitations, as seen in inelastic neutron scattering [1]. We determine its effective Hamiltonian and show the presence of characteristic "pinch points”, along with good quantitative agreement at intermediate energy scales, from three different theoretical techniques [2]. In the second part of my talk, I switch my focus to the dynamical non-equilibrium effect of ``quantum scarring” which was first reported in a one dimensional Rydberg atom setup [3], with no known real material analog. I demonstrate that this effect is not restricted to 1D, and can be realized in higher dimensional systems [4], which I explain with the help of an exactly solvable point (that we recently discovered) in the XXZ-Heisenberg model on the frustrated kagome lattice [5]. Within the framework of this proposal, I suggest what would be needed to realize scarring in real materials.

[1] K. W. Plumb, H.J. Changlani, A. Scheie, S. Zhang, J. Krizan, J. A. Rodriguez-Rivera, Y.Qiu, B.Winn, R.J. Cava, C.L. Broholm, Nature Physics, 15, 54-59 (2019)

[2] S. Zhang, H.J. Changlani, K. Plumb, O. Tchernyshyov, R. Moessner, Phys. Rev. Lett. 122, 167203 (2019)

[3] H. Bernien et al., Nature 551, 579–584 (2017); C. Turner et al., Nature Physics 14, 745-749 (2018)

[4] K. Lee, R. Melendrez, A. Pal, H.J. Changlani, Phys. Rev. B 101, 241111(R) (2020)

[5] H.J. Changlani, D. Kochkov, K. Kumar, B. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018)

Professor Chad Risko

Department of Chemistry

University of Kentucky

Host:  Brill

Abstract:

Organic semiconductors are derived from π-conjugated molecules or polymers, whose chemical composition and structure are only limited by the imagination of the synthetic chemist – or, perhaps in the future, are determined through machine-learned paradigms. The structure of organic semiconductors are also determined by their processing environment. Currently, how molecular design and processing are interwoven to result in semiconducting materials with optimal performance remains a mixture of art and science. Here we will discuss the development of atomistic-scale models that bring together molecular topology and processing conditions to provide first-principles insight into the physicochemical connections that are required to be controlled.

Professor Kenneth R. Graham

Department of Chemistry

University of Kentucky

Host: Brill

Organic thermoelectrics and perovskite solar cells are both promising technologies for generating electricity in a more sustainable manner.  Organic thermoelectrics, which are typically based on doped π-conjugated polymers, provide a means of converting waste heat to electrical energy using low-cost and mechanically flexible devices.  On the other hand, perovskite solar cells rely on low-cost and solution processable organic metal halide perovskites to efficiently convert solar energy to electrical energy.  For both material classes and device types, ultraviolet, inverse, and x-ray photoelectron spectroscopy (UPS, IPES, and XPS, respectively) provide an experimental means to measure the energy of electronic states that help determine material and device performance.  We have developed low-energy UPS and IPES systems that minimize sample damage in sensitive materials, such as organic semiconductors and organic metal halide perovskites. In this talk I will discuss how UPS, IPES, and XPS are applied to better understand the thermoelectric properties of organic semiconductors and interfacial chemistry and energetics within organic metal halide perovskite solar cells.  In the area of thermoelectrics, I will discuss how material blends can be used to manipulate the energy dependence of charge transport and improve the power factor.  Furthermore, I will discuss our recent finding that high levels of p-type doping of π-conjugated polymers can lead to n-type thermoelectric behavior, i.e., negative Seebeck coefficients, and a negative Hall voltage indicative of electrons as the dominant delocalized charge carriers.  IPES and UPS measurements of these doped polymers indicates that the transport gap decreases with increasing doping concentration and the density of states appears semi-metallic at high doping concentrations..  In the area of perovskite solar cells, I will discuss surface ligand binding, the influence of surface ligands on interfacial energetics, and how these energetics impact solar cell performance in both Pb- and Sn-based perovskites.

I will describe recent progress in our understanding of unusual quantum critical phenomena that lie outside the standard Landau paradigm.  Crucial related input into these quantum critical points has come from the study of gapped topological phases of matter and from the understanding of dualities of quantum field theories. I will highlight these connections and describe several new results on quantum critical points in 3+1-dimensional systems.  Based on this understanding, I will describe an interesting possible 3+1-D field theory duality.

Recently graphene moire structures have been shown to realize novel highly tunable platforms for correlated electron physics. I will discuss the theory of these systems. I will show that in many such systems the appropriate theoretical model involves tacking the problem of strong correlations in a partially filled topologically non-trivial band.

The physics of a square lattice of pseudospin-half electrons in layered iridates has been shown to be particularly rich, giving rise to a novel playground for some of the most outstanding and challenging problems in condensed matter physics, such as metal-insulator transition and quantum magnetism. Significant interests have been focused on the analogy with high-Tc cuprates due to the appealing electronic and magnetic similarities with the CuO2 plane despite the much larger spin-orbit coupling (SOC) of Ir. However, unlike the large material family of cuprates, studies on the layered iridates have been limited to a few Ruddlesden-Popper (RP) compounds. This talk will discuss our recent work on overcoming this bottleneck by constructing different artificial variants of the two-dimensional (2D) lattice with heteroepitaxial growth of perovskite iridate. By tuning the layer dimension and the quantum confinement structure, our results show that the antiferromagnetic order and the magnetic interactions are highly sensitive to the lattice degrees of freedom. By leveraging with this structural control, we demonstrate a giant response of the 2D antiferromagnetic order to a sub-Tesla external field. This effect manifests a hidden spin rotational symmetry of the pseudospin-half lattice that was originally proposed for cuprates but never realized due to the small SOC of Cu, illustrating the power of atomic layering in exploring and revealing the intriguing SOC-driven emergent behavior beyond the cuprate phenomenology.

Exploring the physics of the iridates is expected to shed light onto high-temperature superconductivity as well as provide valuable insights into the interplay of spin- orbit coupling (SOC), Hunds and Coulomb interactions. It was shown that this 5d family of transition metal oxides has strong structural and electronic similarities to the famous 3d family of copper oxides - however, no superconductivity has been reported in quasi-two-dimensional square lattice iridium oxides so far. I will present our recent theoretical advances in understanding charge and magnetic excitations in these strongly correlated compounds, as revealed by Photoemission Spectroscopy (ARPES) and Resonant Inelastic X-ray scattering (RIXS). Since SOC mediated magnetic momenta locking to the lattice has been reported in iridates in 2018, tuning local crystal environment emerged as a new route to control structural and physical properties. I will discuss how magnetic and electronic degrees of freedom can be manipulated by applying external pressure and strain, as discovered recently using RIXS and two-magnon Raman scattering.

Host: Ambrose Seo