Magnetically and Optically Driven Topological Semimetals

In the past decade, there has been remarkable progress in our understanding of topological states of matter. A quantum state is topological if its ground state wave function bears a distinctive character that can be specified by a topological invariant—a discrete quantity that remains unchanged upon adiabatic deformations of the system. Since the 1980s, quantum Hall systems have been recognized to be topological, but it was long believed that such topological states are rather exceptional in nature and only exist in quantum liquids under extreme conditions (requiring high magnetic fields and low temperatures). However, after the discovery of topological insulators (TIs), it has come to be widely recognized that topological states of matter can actually be widespread. In this sense, TIs have established a new paradigm about topological materials and opened up an interdisciplinary field at the crossroad between physics,quantum chemistry and material science.

The goal of this research is to discover two new types of TSMs in magnetic and photo-driven systems respectively, and explore their novel properties. First, magnetic TSMs consist of itinerant electrons that form the Dirac/Weyl point in k-space and localized moment that generates magnetism. The exchange coupling between the two is expected to catalyze a plethora of novel correlated phenomena, including an anomalous Hall effect induced by Berry curvature, an axion electromagnetic response, tunable Weyl point creation/annihilation, and metal-insulator transitions. Despite intensive searches, magnetic TSMs have not been experimentally found. The PIs will combine theoretical and experimental efforts to search for magnetic TSMs in correlated electron systems.

Second, the PIs propose a new topological phase of matter in photon-electron hybrid states, which they term the Floquet topological semimetal. Recent experiments lead by the PI Gedik has demonstrated that Bloch states in solids can be “dressed” by intense laser light, forming “Floquet-Bloch states” that are completely coherent and behave like real bands. When these states form topologically robust band crossings in energy-momentum space, the photo-excited system is expected to become a Floquet topological semimetal. The PI Fu will theoretically determine the photon polarization and frequency required for this new phase, while Checkelsky will provide the necessary materials platform and Gedik use well-established time resolved photoemission techniques to uncover these Floquet TSMs.

I. Magnetic Topological Semimetals
An important class of TSMs, known as Weyl semimetals, is proposed to materialize in magnetic topological insulators, where time-reversal symmetry is spontaneously broken. Fu’s group will use first-principles calculations to identify such magnetically driven TSMs in candidate materials and predict their emergent phenomena. Checkelsky’s group will synthesize samples and characterize their transport and magnetic properties, which will then be studied by Gedik’s group using angle-resolved photoemission spectroscopy (ARPES) to directly observe the Dirac/Weyl point in the bulk and the topological surface states with Fermi arc. Gedik’s group will further perform optical and magneto-optical measurements to thoroughly characterize the electronic structure. In addition to its scientific value, the discovery of TSMs may enable new spintronic device application, where electrical properties can be efficiently tuned by magnetization.

II. Floquet Topological Semimetals
Besides the static optical and photoemission measurements probing the equilibrium, Gedik will also study the dynamics of the material in response to photoexcitation by light. Gedik has recently showed that by shining light with energy below the bulk band gap of a topological insulator, hybrid photon-electron states can be created. These states are called Floquet-Bloch states, which are the Bloch states “dressed” by the intense laser light. Surface electrons coherently can emit and absorb multiple laser photons giving rise to replicas of the original dispersion separated by integer multiples of photon energy. These new states are completely coherent and behave like real bands for practical purposes. When these bands cross they can hybridize and open up band gaps along certain directions. Considering the fact that material properties are largely determined by electronic dispersion, this ability to manipulate the electronic band structure with light is a novel way to engineer novel quantum phases of matter. There have been a number of proposals for inducing phase transitions using Floquet methods, such as inducing a Floquet topological insulator from a trivial insulator.

The Materials Research Science and Engineering Center (MRSEC) at Michigan State University focuses on sensing materials for control and diagnostics. The Center also provides seed funding for new opportunities in sensor materials. The Center supports education outreach efforts that include research experiences for undergraduates and outreach to the pre-college level through hands-on workshops for junior high school science teachers. The MRSEC also supports shared experimental facilities that are accessible to center participants and to outside users, and broad industrial outreach efforts.

Research in this MRSEC is organized into two interdisciplinary research groups. One group emphasizes optical probes of processes critical to engine diagnostics and sensing. A second group explores various transduction methods for transforming chemical and physical information into electrical signals. Participants in the Center currently include 21 senior investigators, 3 postdoctoral associates, 11 graduate students, 8 undergraduates, and one administrative support personnel. Professor Brage Golding directs the MRSEC.

The Materials Research Science and Engineering Center (MRSEC) at the University of Maryland supports interactive research in two interdisciplinary groups focusing on oxides, thin films, and novel surface spectroscopic probes. One of the research groups emphasizes fundamental materials issues in ferroelectric thin film heterostructures , related device problems of technological relevance, and fundamental materials physics of perovskite materials that exhibit unusually large ("colossal") magneto- resistance. The second group investigates the structure of surfaces on length scales from nanometers to microns, with the goal of developing a predictive understanding of surface morphology. The work may ultimately find practical application in micro-electronics, thin film growth, lubrication, catalysis, and other areas. A common theme for both groups is the development, optimization and utilization of novel surface sensitive tools to measure structural, magnetic, and electrical properties at microscopic length scales. The MRSEC supports the development, operation and maintenance of shared experimental facilities for materials research. It provides seed funding for exploratory research and fosters research participation by undergraduates. The Center is associated with an educational outreach program designed to enlighten pre-college and undergraduate students about science and the role of the center's research program in the modern world. The MRSEC also supports enhanced collaboration with industry, shared experimental facilities that also support research not directly funded by the MRSEC, and seed funding for exploratory research. The Center currently supports about 15 senior investigators, 7 postdoctoral research associates, 1 technician or other professional, 12 graduate students, and 8 undergraduates. The MRSEC is directed by Professor Ellen D. Williams. %%% The Materials Research Science and Engineering Center (MRSEC) at the University of Maryland supports interactive research in two interdisciplinary groups focusing on oxides, thin films, and novel surface spectroscopic probes. One of the research groups emphasizes fundamental materials issues in ferroelectric thin film heterostructures , related device problems of technological relevance, and fundamental materials physics of perovskite materials that exhibit unusually large ("colossal") magneto- resistance. The second group investigates the structure of surfaces on length scales from nanometers to microns, with the goal of developing a predictive understanding of surface morphology. The work may ultimately find practical application in micro-electronics, thin film growth, lubrication, catalysis, and other areas. A common theme for both groups is the development, optimization and utilization of novel surface sensitive tools to measure structural, magnetic, and electrical properties at microscopic length scales. The MRSEC supports the development, operation and maintenance of shared experimental facilities for materials research. It provides seed funding for exploratory research and fosters research participation by undergraduates. The Center is associated with an educational outreach program designed to enlighten pre-college and undergraduate students about science and the role of the center's research program in the modern world. The MRSEC also supports enhanced collaboration with industry, shared experimental facilities that also support research not directly funded by the MRSEC, and seed funding for exploratory research. The Center currently supports about 15 senior investigators, 7 postdoctoral research associates, 1 technician or other professional, 12 graduate students, and 8 undergraduates. The MRSEC is directed by Professor Ellen D. Williams.

The focus of IRG-2, Sustainable Nanocrystal Materials, is the design, synthesis, processing, and thin film properties of environmentally benign nanocrystal-based electronic and optoelectronic materials. The field is currently constrained by the use of toxic (e.g., Pb, Cd) and/or scarce (e.g., In, Te) elements, with serious environmental, health, and economic concerns. IRG-2 will overcome these barriers by discovering and developing nanocrystal-based electronic thin films made from nontoxic, abundant and sustainable materials using scalable, low-temperature processes. The IRG's research will pursue three closely linked, vertically integrated thrusts: (i) nanocrystal synthesis and characterization; (ii) quantum dot films and devices; and (iii) microcrystalline films and devices. This research aims to reinvent the scope of active materials for NC-based electronics and optoelectronics, which will ultimately enable energy efficient emissive or photovoltaic devices with sustainable materials choices.

 

Seeds 7 is working to design nano-structured materials for wide bandwidth operation that can be used to greatly enhance nonlinear interactions, including nonlinear frequency conversion (NFC) processes: frequency mixing between two or more photons.

Principal Investigators
Alejandro Rodriguez (Electrical Engineering)
Loren Pfeiffer (Electrical Engineering)
Claire Gmachl (Electrical Engineering)

* This seed is inactive. (Seed start/end date: April 1, 2016 - October 31, 2018)

Bottlebrush Hydrogels as Tunable Tissue Engineering Scaffolds
Senior Investigator: Robert Macfarlane, Assistant Professor, Department of Materials Science and Engineering

Tissue engineering (TE) is a promising method to grow artificial tissues for biological and biomedical applications, typically implemented using a porous, flexible, and biocompatible scaffold for cells so that, upon growth and proliferation, they ultimately form a continuous three-dimensional biomaterial1,2. However, living cells and tissues are complex constructs, and synthesizing scaffolds that properly interact with them remains a challenge; scaffolds need to simultaneously be (1) biocompatible, (2) mechanically matched to the native tissue, (3) porous enough to allow for nutrient flow and tissue development, and (4) capable of presenting molecular signals that promote cell growth and viability. Therefore, while hydrogels are a promising tool for medicine and biology, several key limitations in these biomedical technologies can only be addressed via advances in the field of materials science.

Here, we will develop methods to synthesize new BBP architectures, crosslink them into gels, and characterize how different design variables affect the resulting gel physical, chemical, and mechanical properties. Our lab is uniquely suited to study these materials, as we possess the requisite polymer
synthesis and characterization capabilities necessary, and have proven expertise in manipulating soft material structure at the nanoscale via controlled polymer synthetic strategies10. Additional support from other member of CMSE IRG II will aid in our characterization capabilities.

Principal Investigators:
Howard Stone (Mechanical & Aerospace Engineering)
Sujit Datta (Chemical and Biological Engineering)
Andrej Košmrlj (Mechanical & Aerospace Engineering)
Clifford Brangwynne (Chemical and Biological Engineering)
Bonnie Bassler (Molecular Biology)

Seed start and end dates: November 1, 2018 - October 31, 2019

An exploratory and promising research project based on a recent discovery — that polymers can regulate the structure and function of biological systems — is generating a new field of “living” soft matter. The researchers discovered that polymers can regulate the structure and function of biological materials, ranging from sub-cellular proteins to extracellular hydrogels to populations of cells, through entropic interactions. These results have generated a new field at the interface of biology, physics, and chemistry whose findings will enable the control and design of novel materials. The project goals are to define the principles underlying structural transition in intra-, extra-, and multi-cellular systems and to use this knowledge to control and design novel bio-inspired soft materials.

The research team is composed of Clifford Brangwynne (CBE), Howard Stone (MAE) and Andrej Košmrlj (MAE) who will focus on confined polymer dynamics and phase transitions. Other team members include Bonnie Bassler, (MolBio) and Sujit Datta (CBE) who will investigate structural transitions in biological and bio-inspired polymer liquids and polymer networks using experiments, theory, and simulations.

Highlights
Harnessing the Rules of Life to Enable Bio-Inspired Soft Materials (PDF)

Alexie M. Kolpak, Assistant Professor, Department of Mechanical Engineering

Water splitting over semiconductor photocatalysts using solar energy is a promising process for renewable hydrogen production, but an increase in conversion efficiency is required to make it economically viable. Increasing efficiency requires new materials with optimized (i) band alignment; (ii) visible light absorption; (iii) electron-hole separation; (iv) hydrogen and oxygen evolution activity; and (v) photo-corrosion resistance. We propose to use ab initio computations and classical molecular dynamics simulations to design novel core-shell catalysts to optimize these key metrics by taking advantage of interfacial effects. Our previous work showed that Si-oxide interface chemistry can induce a large electric field in an oxide thin film and a quasi-2D electron gas (Q2DEG) at the Si-oxide interface. We propose that in such a system, electrons (holes) will be driven to the Q2DEG (oxide surface), leading to a dramatic decrease in carrier recombination, and the field will also trap holes on the surface, enhancing catalytic activity and further increasing efficiency. The absorption spectrum, redox potentials, catalytic activity, transport properties, and field can be tuned by atomic-scale modifications (e.g., interfacial cation substitution), core diameter, shell thickness, and/or oxide choice. We will examine the coupling between these properties and the atomic structure, develop fundamental models of the interface chemistry, and design new high-efficiency photocatalysts. Both the physical insights and the new tools developed will be directly applicable to the design of tailored materials systems for other catalytic reactions, as well as for a wide variety of other applications in which interfaces play an important role, (e.g., photovoltaics, fuel cells, thermoelectrics).