Xiaoxing Xi (Temple): Cracking the Nanophysics of Oxide Interface and Heterostructures with Atomic Layer-by-Layer Laser MBE
Advancements in nanoscale engineering of oxide interfaces and heterostructures have led to discoveries of emergent phenomena and new artificial materials. Combining the strengths of reactive molecular-beam epitaxy and pulsed-laser deposition, we show that atomic layer-by-layer laser molecular-beam epitaxy (ALL-Laser MBE) significantly advances the state of the art in constructing oxide materials with atomic layer precision. Using Sr1+xTi1-xO3 as example, we demonstrate the effectiveness of the technique in producing oxide films with stoichiometric and crystalline perfection. With the growth of La5Ni4O13, a Ruddlesden-Popper phase with n = 4 that has never been reported in the literature, we demonstrate that ALL-Laser MBE allows us to push the equilibrium thermodynamic boundary further. By growing LaAl1+yO3 films of different stoichiometry on TiO2-terminated SrTiO3 substrate at high oxygen pressure, we show that the behavior of the two-dimensional electron gas at the LaAlO3/SrTiO3interface can be quantitatively explained by the electronic reconstruction mechanism. In LaNiO3 films on LaAlO3 substrate with LaAlO3 buffer layer, we observed the metal insulator transition in 1.5 unit cells, which is driven by oxygen vacancies in addition to epitaxial strain and reduced dimensionality.
As Interim Chair of Temple University’s Physics Department, May 20, 2015 was a normal day for me filled with work on my class, research, promotion of colleagues, and a university task force I was chairing. I had given a public lecture for “Pint of Science,” a science festival, at an Irish pub before picking up my wife at the airport, who was returning from an overseas conference trip. My elder daughter had come home a day earlier from college for a few days. We made a plan to visit a restaurant to try their famous Korean fried chicken. All of this was suddenly and forever changed a few hours later when I was awoken by the urgent pounding on my door. I was arrested by armed FBI agents and indicted by the U.S. government for sharing protected U.S. company technology with China. The indictment was dismissed in September after it had become clear that I did not share the protected technology with China. My case has raised serious concerns about international collaborations in science and technology, civil rights, and the long-term national security and economic future of the United States.
The Kagome Lattice Heisenberg Model is one of the simplest realistic spin models with a quantum spin-liquid ground state. We discuss the current status of our understanding of this well-studied model. The precise nature of the spin-liquid state and the existence of a spin-gap in the model still remain in dispute. We also discuss experimental studies of Herbertsmithite material Kagome-antiferromagnet ZnCu_3(OH)_6Cl_2. We focus on NMR measurements by Imai and collaborators, who have presented strong evidence for a spin-gap in the excitation spectra. Through a Numerical Linked Cluster (NLC) calculation of the frequency moments, we show that despite the existence of substitutional disorder in these materials, the high temperature nuclear relaxation rates are well described by the Heisenberg model.
The 2016 Nobel Prize in Physics to Kosterlitz, Thouless and Haldane honors a new set of ideas and theoretical formalism that has gradually become the mainstay of modern condensed matter thinking. Quantum Field Theory, Topology and the Renormalization Group, lie at the heart of present day theory of condensed matter. The Kosterlitz-Thouless theory of phase transitions and Haldane's conjecture on the spin dependence of spectrum of spin-chains were two of the most influential works in bringing these ideas together. These along with the Thouless-Kohmoto-Nightingale-Den Nijs-(TKNN) work on topological invariants of band structures were duly recognized by the Nobel committee. This talk will discuss how these theories defied existing paradigms and no-go theorems. And, how they continue to play a huge role in how we address quantum phases and phase transitions today. This Nobel Prize potentially sets the stage for many more prizes in the field of Topological Matter.
We provide a perspective on the recent emergence of “topological spintronics,” which relies on the existence of helical Dirac electrons in condensed matter. Spin- and angle-resolved photoemission spectroscopy shows how the spin texture of these electronic states can be engineered using quantum tunneling  or by breaking time-reversal symmetry . Inappropriately designed systems, broken time-reversal symmetry transforms helical Dirac states into chiral edge states, a realization of Haldane’s Chern insulator phase of matter. This is characterized by a precisely quantized Hall conductance and dissipationless edge transport without a magnetic field. We show how these edge states can be quantitatively characterized by analyzing their giant anisotropic magnetoresistance . At miilikelvin temperatures, the interplay between Chern states and disordered magnetism  results in surprising behavior, perhaps consistent with quantum tunneling out of a ‘false vacuum’ . Finally, we show how these helical Dirac electrons provide a possible pathway toward a spin device technology that works at room temperature [6,7].
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Axel Hoffmann (Argonne National Laboratory): Room Temperature Generation and Manipulation of Magnetic Skyrmions
The field of spintronics, or magnetic electronics, is maturing and giving rise to new subfields. An important ingredient to the vitality of magnetism research in general is the large complexity due to competitions between interactions crossing many length scales and the interplay of magnetic degrees of freedom with charge (electric currents), phonon (heat), and photons (light). One perfect example, of the surprising new concepts being generated in magnetism research is the recent discovery of magnetic skyrmions. Magnetic skyrmions are topologically distinct spin textures that are stabilized by the interplay between applied magnetic fields, magnetic anisotropies, as well as symmetric and antisymmetric exchange interactions. Due to their topology magnetic skyrmions can be stable with quasi-particle like behavior, where they can be manipulated with very low electric currents. This makes them interesting for extreme low-power information technologies, where it is envisioned that data will be encoded in topological charges, instead of electronic charges as in conventional semiconducting devices. Towards the realization of this goal we demonstrated magnetic skyrmions in magnetic heterostructures stable at room temperature, which can be manipulated using spin Hall effects. Furthermore, using inhomogeneous electric charge currents allows the generation of skyrmions in a process that is remarkably similar to the droplet formation in surface-tension driven fluid flows. However, detailed micromagnetic simulations show that depending on the electric current magnitude there are at least two regimes with different skyrmion formation mechanisms. Lastly, we demonstrated that the topological charge gives rise to a transverse motion on the skyrmions, i.e., the skyrmion Hall effect, which is in analogy to the ordinary Hall effect originating from the motion of electrically charged particles in the presence of a magnetic field.
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. Lithographic patterning was carried out at the Center for Nanoscale Materials, which is supported by DOE, Office of Science, BES (#DE-AC02-06CH11357).
Science2016 Susan Fullerton: Using Ions To Control Transport in Two-Dimensional Materials for Electronics
Two-dimensional (2D) materials are molecularly thin, layered materials held together by van der Waals forces. Because charge moves freely in the 2D plane, these materials have potential application in electronics; however, conventional doping strategies have not been developed for 2D materials. An alternative approach is to use electrolyte gating. Under an applied gate voltage, ions in the electrolyte create an electrostatic double layer (EDL) at the interface between the electrolyte and the semiconductor; the EDL can induce sheet carrier densities on the order of 1014 cm-2 for both electrons and holes–more than one order of magnitude larger than conventional gating techniques. I will describe our work using electrolytes to dope transistors and memory devices based on graphene and transition metal dichalcogenides (TMDs). Our group has developed a 2D electrolyte for use in memory devices based on 2D crystals, and the first device characteristics will be presented.
This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of six SRC STARnet Centers, sponsored by MARCO and DARPA, and NSF grant #ECCSGOALI-1408425.
Learning quantum mechanics is challenging, in part due to the non-intuitive nature of the subject matter. Our research shows that the patterns of reasoning difficulties in learning quantum mechanics are often universal similar to the universal nature of reasoning difficulties found in introductory physics. Our research also shows that students often have difficulty in monitoring their learning while learning quantum mechanics. To help improve student understanding of quantum concepts, we are developing quantum interactive learning tutorials (QuILTs) as well as tools for peer-instruction. The goal of QuILTs and peer-instruction tools is to actively engage students in the learning process and to help them build links between the formalism and the conceptual aspects of quantum physics without compromising the technical content. I will discuss the assessment of these learning tools.
Editor's Note: There were some technical difficulties at the beginning of the talk so the video begins a moment or two after the speaker begins. We apologize for this and hope you enjoy the seminar.
Molecular crystals have applications in nonlinear optics, organic electronics, and particularly in pharmaceuticals because most drugs are marketed as crystals of the pharmaceutically active ingredient. Molecular crystals are held together by van der Waals (vdW) interactions (also known as dispersion interactions) between molecules. Unlike chemical bonds, van der Waals interactions do not involve overlap of electron densities. Rather, they arise from quantum fluctuations of the electron density that lead to the formation of dipoles and higher order multipoles. The electrostatic interaction between these generates a weak but long-ranged attractive force. Owing to the weak nature of van der Waals interactions, a given molecule may crystallize in more than one structure. This is known as polymorphism. Polymorphic forms of the same molecule may possess markedly different physical and chemical properties. Crystal structure may profoundly influence the bioavailability, toxicity, manufacturability, and stability of drugs. In the context of technological applications, crystal structure affects the electronic and optical properties. We use computer simulations to perform structure prediction and design of molecular crystals from first principles, based solely on the knowledge of their elemental composition and the laws of quantum mechanics. We develop genetic algorithms, which are guided to the most promising regions of the configuration space by the evolutionary principle of survival of the fittest. Offspring are generated by combining structural “genes” of the fittest structures in the population to propagate desirable features, while random mutations are employed to maintain diversity. We are particularly interested in optimizing crystal packing for high-performance organic electronics and solar cells.
Ultrafast time-resolved spectroscopy, and in particular its extension to multidimensional techniques, can tell us a lot about solvation dynamics, structural dynamics and energy transfer processes of solution phase molecular systems. I will illustrate applications of these spectroscopies by discussing a couple of quite diverse examples that lie at the interface between Physics, Biology and Chemistry. In different ways, these examples highlight the importance of water as a very special substance. That is, I will start out with the ultrafast structural dynamics of bulk water and concentrated salt solution observed by THz photon echoes (Physics), continue with the catalytic cycle of an artificial photosynthetic system designed for light-driven water splitting (Chemistry), and finally discuss the response of an allosteric protein upon an external perturbation (Biology). Also the latter is in fact dictated by the dynamics of the water solvation layer.