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

The spin-1/2 triangular-lattice antiferromagnet is a central model in frustrated quantum magnetism: it is the first two-dimensional magnet proposed to host a SU(2) symmetric resonating valence-bond ground-state and its fractionalized magnetic excitations. Although it is now accepted that the model, at least in its simplest Heisenberg form, orders magnetically, it remains intimately associated with the concepts of quantum spin-liquid and exotic magnetic excitations. In the last few years, advances in materials discovery, crystal growth, neutron spectroscopy and theory have fueled a lively triangular-lattice antiferromagnet “renaissance”. In this talk, I will describe recent neutron scattering investigations on two realizations of this model: the transition metal compound Ba3CoSb2O9 and the rare-earth system YbMgGaO4. Experimental results elucidate the role of quantum fluctuations, spin-orbit coupling, chemical disorder, and non-linear effects in generating anomalous spin dynamics in these materials. This project is supported by the NSF under grant DMR-1750186.

Host: Ribhu Kaul

One of the promising routes towards creating novel topological states and excitations is to combine superconductivity with quantum Hall (QH) effect. However, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has so far eluded experimental observation. Here, we explore high mobility graphene/boron nitride heterostructures contacted by type II superconducting electrodes that could withstand  magnetic fields of a few Tesla. At low magnetic fields, our devices demonstrate the Fraunhoffer pattern and Fabri-Perot oscillations, confirming their uniformity and ballisticity. At fields of 1-2 Tesla, when Landau quantization is fully developed, regions of superconductivity can be observed on top of the conventional QH fan diagram. The measured supercurrent is very small, on a few nA scale, and periodic in magnetic field. I discuss possible mechanisms that could mediate supercurrent along the QH edge states.

Recent discovery of topological electronic states has shed light on the role of strong spin-orbit coupling in condensed mater. When the coupling is combined with electron correlation effect, their combination leads to a new class of topological states accompanying exotic magnetism. The 5d transition metal iridates have received considerable attention as the candidates that harness Kitaev quantum spin liquid, Wyle semimetal or Axion insulator phases. For the first part of this talk, I discuss resonant inelastic x-ray scattering (RIXS) investigation of a honeycomb Na2IrO3. The observation of diffuse magnetic scattering points to presence of short-range magnetic orders resulting from competition of bond-directional magnetic anisotropies. This validates the novel route to realize Kitaev spin liquid and evidences the proximity to the spin liquid phase. In the second part, I present RIXS study of magnetic excitation in a pyrochlore Eu2Ir2O7. Its metal-insulator transition driven by all-in-all-out (AIAO) magnetic order is regarded as possible realization of topological Weyl semimetal. We observe gradual softening of the magnon excitations in the entire Brillouin zone while warming, whose temperature evolution shows a direct relationship with the metal-insulator transition. This result suggests substantial change of magnetic exchange due to the varying electronic structure, and thereby classifies intermediate electron correlation strength: a requisite for realizing Weyl semimetal state.

The existence of strong spin-orbit coupling has brought the iridates to the forefront of materials research, whereas strong electronic correlation has proven to produce a plethora of novel properties within the manganites. The physical properties of interfaces between such materials were investigated by synthesizing a series of artificial superlattices consisting of the 5d paramagnetic metal SrIrO₃ and 3d antiferromagnetic insulators AMnO₃, where A = Sr or La. Through our experimental investigations by x-ray diffraction, SQUID magnetometry, dc-transport, x-ray circular dichroism, and polarized neutron reflectometry measurements, both novel magnetic and transport properties were observed, which drastically differ from those of the constituent materials and are highly sensitive to the degree of dimensional confinement within the superlattices. Here I will present these results and discuss the implications of these intriguing magnetic and electronic properties.