TBD

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.