Abstract:

"Mixed-dimensional heterostructures are a unique class of material combining components of distinct dimensionalities through van der Waals interactions, e.g., zero-dimensional organic molecules self-assembled on two-dimensional (2D) materials. In addition to the versatile tunability of organic molecules and advantageous properties of 2D materials, the interface coupling has demonstrated exotic electronic and excitonic properties beyond those of the individual component, promising for next-generation electronic and optoelectronic applications. Here I will introduce some intriguing interfacial phenomena in mixed-dimensional heterostructures, our understanding on the physics and their potential applications. I will show that organic molecules and interlayer coupling provides an incredible avenue to engineer the band structure and interlayer exciton dynamics in mixed-dimensional heterostructures, such as that consisting of titanyl phthalocyanine molecules and monolayer MoS2. Finally, I will introduce a computational approach for the heterogeneous and incomplete screening at the interface for mixed-dimensional heterostructures, advancing our understanding and capabilities in this emerging field."

TBA

Somewhat to my surprise, two independent research projects we are pursuing take place in or with suspended atomically thin membranes. In the first case, we will review thermal transport measurements inspired by a search for physics of the Kitaev quantum spin liquid; and for the second, we'll show how suspended membranes can lead to quantum sensing of rotations with remarkable precision, sufficient for lab-scale sensing of gravitation and variations in the length of a day. 

In contemporary condensed matter physics and photonics, four length scales are fundamentally interesting and intertwined: 1) Polaritonic wavelength λ in infrared (IR) and terahertz (THz) frequencies ω (e.g. plasmon, phonon, exciton, or magnon polaritons), which defines the scale of the light confinement and light-matter interaction; 2) Magnetic lengths  l_B =√(ℏ/eB)=257Å/√(B[T]), (with B the magnetic field), which defines the restricted electron motion in a B field; 3) Diffusion length D of the hot carriers at interfaces and the edges, which defines the scale of energy relaxation, and 4) Periodicities of superlattices induced by moiré engineering, which defines the energy scale of emerging quantum phases. In this talk, I will report 1) A new type of optical near-field nanoscopy technique (Landau level nanoscopy) to tackle all four above-mentioned ‘lengths’ simultaneously; 2) A new type of infrared polaritons that can be tuned via magnetic field; 3) A nanoscale probe of the many-body physics through the excitations of magnetoexcitons in graphene across the allowed and forbidden optical transitions. Our approach establishes the Landau-level nanoscopy as a versatile platform for exploring magneto-optical effects at the nanoscale. Our preliminary research also sets the stage for future spectroscopic investigations of the topological and chiral photonic phenomena in complex quantum materials using low-energy photons.

In this talk, we will discuss experimental and theoretical results that suggest the occurrence of a new type of quasi-particle in nanomagnets. The persistent quasi-particle kinetics results into a highly viscous magnetic fluid state in two-dimensional nanomagnetic lattice. We argue that the vortex-shaped quasi-particle can be detected in any nanomagnet with geometrical dimension smaller than a characteristic length e.g. domain wall or coherence. The finding is expected to have implication to the design of next generation spintronic devices.

Two dimensional (2D) nanomaterials have been intensively investigated as emerging materials for future devices, including electronics, photonics, and electrochemical energy storage devices. The mechanical stability of each 2D component is critical to the reliability of the fabricated devices. Currently, research on experimental mechanics of 2D materials has been focused on quantifying mechanical properties and understanding fracture behaviors using different techniques. Confined to 2D geometry, cracks in 2D materials generally favor a brittle behavior with minimum plasticity at room temperature, which continues the dilemma of mutually exclusive fracture toughness and mechanical strength in bulk materials.

Considerable research has been devoted to improving fracture toughness of 2D materials. For example, carbon nanotubes (CNTs) were integrated into graphene as an extrinsic toughening strategy. The fabricated rebar graphene displays a zigzag fracture surface, guided and redirected by the embedded CNTs. Such toughening mechanism is similar to improving fracture resistance extrinsically by introducing fiber/lamella bridging, oxide wedging, transformation toughening, etc. In addition to rebar graphene, h-BN has been carefully investigated as it has the same structure of graphene but is composed of two elements. The fracture behavior of monolayer single crystalline h-BN has long been taken as an ideal brittle material subject to Griffith’s law. By combining computational analysis and in situ tensile test, the monolayer h-BN has an exceptionally high fracture toughness. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens h-BN tremendously and enables stable crack propagation not seen in graphene.

Abstract: The physics of low-dimensional magnetic systems has gained significant global attention over the last decade. Especially, two-dimensional (2D) layered magnetic systems are of present research interest due to their unusual magnetic properties, arising from the reduction in magnetic dimensionality and consequently, the geometrical spin frustrations. Such magnetic states are highly sensitive to the underlying magnetic lattice geometry. The talk will delve into the diverse magnetic properties of layered transition metal oxide compounds having variety of 2D magnetic lattices, viz., (a) triangular lattice [Na3Fe(PO4)2], (b) maple leaf lattice (Na2Mn3O7), and (c) honeycomb lattice [A2Ni2TeO6(A=Na/Li)] etc. The origin of unique long-range magnetic ground states, magnetic excitations in view of the spin-Hamiltonian of the system, 2D short range magnetic ordering, etc will be discussed. It will be demonstrated that how different types of 2D magnetic lattices (triangular lattice, maple leaf lattice, and honeycomb lattice) and their distortions result variations in geometrical spin frustrations, leading to the possibility of multiple spin structures. 

Furthermore, the layered materials are chosen in such a way that the magnetic layers are well-separated by the non-magnetic alkali-metal ions (A=Li/Na/K) alone. Such layered materials provide high ionic conduction and improved intercalation/de-intercalation properties, making them suitable for battery applications. The talk will discuss the role of underlying crystal structure on the ionic conduction properties within the context of functional battery applications. 

Diamond is not just a perfect gemstone. The tiny imperfections inside diamond can be turned into ultrasensitive nanoscale quantum sensors which can offer brand-new lenses to see through intricate phenomena spanning from atomic and molecular objects to events on a grand scale. In this talk, we will start with an overview of quantum sensing technologies based upon spin defects (e.g. nitrogen-vacancy centers) in diamond. We will then discuss our recent efforts at WashU to employ these diamond sensors for a wide range of applications covering condensed matter physics, biomedical imaging and geoscience. If time permits, we will present some of our results on developing a new generation of quantum sensors beyond diamond, specifically in two-dimensional materials.

Quantum materials with kagome lattice – comprised of corner-sharing triangles forming a hexagon in the crystal structure - have been studied as the potential playgrounds for exploring the interplay among parameters such as geometry, topology, electronic correlations, magnetic, and charge density orders. Niobium halides, Nb3X8 (X = Cl, Br, I), which are predicted to be two-dimensional magnets, have recently received attention due to their breathing kagome geometry. In this talk, I will discuss the electronic structure of Nb3X8 system revealed by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. ARPES results depict the presence of multiple flat and weakly dispersing bands. These bands are well reproduced by the theoretical calculations, which show that they have Nb d character indicating their origin from the Nb atoms forming the breathing kagome plane. These van der Waals materials can be easily thinned down via mechanical exfoliation to the ultrathin limit and such ultrathin samples are stable as shown from the time-dependent Raman spectroscopy measurements at room temperature. These results demonstrate that Nb3X8 system is an excellent material platform for studying breathing kagome induced flat band physics and its connection with magnetism. I will also discuss our recent results on topological quantum materials using ultrafast spectroscopy. 

About the speaker:

Dr. Neupane received his Ph.D. in Physics from Boston College, Boston, MA in 2010. He spent four years (2011-2014) as a postdoctoral research associate at Princeton University, Princeton, NJ and one year (2015-2016) as a Director’s Fellow at Los Alamos National Laboratory, Los Alamos, NM. He joined UCF in 2016 as an Assistant Professor and reached Associate Professor in 2020. He is the recipient of the Director’s Fellow at Los Alamos National Laboratory (2015), NSF Career Award (2019), UCF Luminary Award (2019), and UCF Research Incentive Award (2020). Neupane has been recognized as a highly cited researcher from 2019 to 2023 by analytics company Clarivate, based on data from Web of Science. His research focuses on the electronic and spin properties of new quantum materials. He utilizes various spectroscopic techniques to reveal the interesting electronic and spin properties as well as the momentum resolved dynamical properties of the bulk and symmetry-protected properties of the surface of these quantum materials.

Abstract: 

The manipulation of matter on the nanoscale can enable useful technologies with interesting underlying physics. Nano-patterned graphene facilitates standing-wave plasmonic behavior that can be used for surface-sensitive chemical detection. Owing to its versatility, this two-dimensional material can also be utilized in area-selective atomic layer deposition (AS-ALD) processes to make robust resistive memory cells. In turn, more technologically scalable routes of AS-ALD can be employed to make self-limiting electronic tunneling junctions. These diverse, but related, topics will be presented, and will hopefully be instructive to those interested in plasmonics, electronics, and materials science.

Glasses doped with rare-earth (RE) ions as possible opto-electronic materials have gained considerable interest. Heavy metal oxide glasses have been shown to improve the efficiency of RE ion’s fluorescence properties. The optical absorption and fluorescence properties of RE ions can be optimized by altering the chemical environment around the RE ions. Chemical environment of RE ions can be varied through the glass composition or by introducing metal/semiconducting nanoparticles (NPs) into the host glass matrix. In the present talk, I will discuss the effect of CdSe NPs on the fluorescence properties of praseodymium (Pr3+) ions. These NPs are grown in bismuth-boro tellurite glasses through controlled annealing for different durations. Our results show that the presence of CdSe NPs of certain specific size led to improved fluorescence efficiency of Pr3+ ions. 

About the speaker:

Dr. Mallur obtained her Ph.D. in Solid State Physics from the Indian Institute of Science, Bangalore (1996), India.  She did her post-doctoral work in University of Illinois at Urbana-Champaign. Currently she is a full professor at the department of physics, Western Illinois University. Her current research interests are in the synthesis of glasses doped with rare-earth ions and study their electronic and optical properties using optical absorption and fluorescence experiments. In addition, she also investigates the effect of nanoparticles on the optical properties of rare-earth ions in glasses. 

Transition metal dichalcogenide (TMD) based moiré materials have been shown to host various correlated electronic phenomena including Mott insulating states and fractional filling charge orders, quantum spin Hall effects, and (fractional) quantum anomalous Hall effects. To describe the low-energy states of long-wavelength moiré superlattice, we introduced the concept of moiré quantum chemistry, and developed transfer learning based large-scale first principle methods. In twisted bilayer TMD, we proposed the Mott ferroelectricity, kinetic magnetism and pseudogap metal from spin polarons. The pseudogap metal phase emerges at small doping below half filling and an intermediate range of fields, which exhibits a single-particle gap and a doping-dependent magnetization plateau.

I will also discuss the fractional quantum anomalous Hall effect in twisted homobilayer and its competing states. This series of works reveal the rich physics of semiconductor moiré superlattices as manifested in a variety of correlated and topological states. 

About the speaker:

Dr. Yang Zhang is an assistant professor in physics at the University of Tennessee, Knoxville, and visiting professor at Max Planck Institute Dresden. He received his BE degree from Tsinghua University and Ph.D. in Physics from Max Planck Institute Dresden, then he worked as a postdoc at MIT. His research interest lies in understanding the correlated/topological states, quantum transport, and light-matter interaction in quantum materials. Yang has received several awards, including the Tschirnhaus Medal from the Leibniz Association, and the Otto-Hahn Medal of the Max Planck Society.

Abstract: The breaking of time-reversal symmetry in topological insulators leads to novel quantum states of matter. One prominent example at the two-dimensional limit is the Chern insulator, which hosts dissipationless chiral edge states at sample boundaries. These chiral edge modes are perfect one-dimensional conductors whose chirality is defined by the material magnetization and in which backscattering is topologically forbidden. Recently, van der Waals topological magnet MnBi2Te4 emerged as a new solid-state platform for studies of the interplay between magnetism and topology. In this talk, I will present an overview of our progress toward controlling topological phase transitions and chiral edge modes in MnBi2Te4 as well as discovering new quantum states in this material family. First, I will establish how topological properties are intimately intertwined with magnetic states. I will then demonstrate electrical control of the number of chiral edge states and the discovery of chiral edge modes along crystalline steps between regions of different thicknesses and how these modes can be harnessed for the engineering of simple topological circuits. Finally, I will discuss the engineering of the superconducting state in topological insulators and demonstrate Pauli paramagnetic limit violation in atomically thin flakes of a topological superconductor candidate.

Bio: Dmitry Ovchinnikov earned his Ph.D. from the Institute of Electrical and Micro Engineering at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland in 2017. During his Ph.D., he conducted experiments on two-dimensional semiconductors and developed techniques to modulate disorder in low-dimensional systems. His thesis earned him the EPFL EDMI PhD thesis distinction award and the Gilbert Hausmann PhD thesis award. He received an early postdoc Swiss National Science Foundation (SNSF) mobility fellowship to research nanoscale van der Waals magnetic devices at the University of Washington with Prof. Xiaodong Xu. Currently, Dmitry is an Assistant Professor at the Department of Physics and Astronomy at the University of Kansas. His work involves exploring the fundamental physics and applications of topological magnets, superconductors, and correlated states in low-dimensional quantum materials.

“Phases and dynamics of dipolar gases and universality of ferromagnetic superfluids” 

This presentation consists of two separate parts. In the first, we will discuss the static properties and the dynamics of dipolar Dysprosium Bose-Einstein condensates subjected to a fastly rotating external magnetic field. The underlying phase diagram with respect to the atom number and relative interaction strengths for various field orientations is mapped out. Transitions  from a superfluid to a supersolid and then to arrays of dipolar droplets characterized by a non-vanishing global phase coherence will be elucidated. Following quenches, across the aforementioned phase transitions, we observe the dynamical nucleation of supersolids or droplet lattices. Three-body losses lead to self-evaporation of the ensuing structures, while the rotating magnetic field enables, for fixed values of the relative interactions, an enhancement of the droplet lifetimes. The second part will be devoted to  address universality in the non equilibrium dynamics of a two-dimensional ferromagnetic spinor gas subjected to modulations of the quadratic Zeeman coefficient. For short timescales we observe the spontaneous nucleation of topological defects (spin-vortices) which annihilate through their interaction giving rise to magnetic domains deeper in the evolution where the gas enters the universal coarsening regime. This is characterized by the spatiotemporal scaling of the spin correlation functions and the structure factor allowing to measure corresponding scaling exponents which depend on the symmetry of the order parameter and belong to distinct universality classes. These results represent a paradigmatic example of categorizing far-from-equilibrium dynamics in quantum many-body systems and reveal the interplay of topological defects for the emergent universality class.