Archived Seminars

Lectures take place in the following locations:

Live seminar:
Louisiana State University - 1008B Digital Media Center

Broadcast:
LaTech – 122 Nethken
Tulane University - 600 Lindy Boggs
UNO – 234 Liberal Arts

 

 

Fei Han photoFei Han

Wednesday, August 9, 3:00 pm, 1008B Digital Media Center 

Dr. Han is a Postdoctoral Researcher at Carnegie Institution of Washington/Florida International University)

"Rational and Exploratory Synthesis of New Superconducting Materials"

ABSTRACT: Exploration of new superconductors plays a critical role in advancing research on superconductivity. Aiming to look for new superconductors, I employed solid state synthesis and crystal growth to design and explore new materials. Single crystal/powder X-ray diffraction and electronic transport probes were used to identify the structure of the new materials and reveal the existence of superconductivity or other novel properties in them. In this talk, I will introduce my results on discovering new materials, including the discoveries of various new superconductors and semiconductors. Moreover, a very interesting system with two superconducting domes, LaO1-xFxBiS2, will be discussed in detail.  Recently I have also utilized high-pressure techniques in my superconductivity research. With synchrotron X-ray diffraction and X-ray emission spectroscopy, we discovered a spin quenching assisted by a strongly anisotropic compression behavior in the first pressure-induced manganese-based superconductor MnP.

 

Jan Ilavsky photoJan Ilavsky

Wednesday, July 19, 3:00 pm, 435 Nicholson Hall

Dr. Ilavsky is a scientist in the X-Ray Science Division, Advanced Photon Source, Argonne National Laborabory

“Extended Range Ultra Small-Angle X-ray, Small-Angle, and Wide-Angle Scattering for Advanced Materials”

ABSTRACT:  Development of new high-performance materials, e.g., new aluminum or steel alloys, is critical for advances in energy production and utilization (and many other areas). These materials often exhibit complex microstructures spanning multiple length scales that control their performance.  In this context, it is important to simultaneously characterize, ideally in situ or in operando, various facets of the microstructure – for example precipitate shape and size, together with their phase and chemical composition. Combined Ultra-Small, Small, & Wide Angle X-ray Scattering (USAXS/SAXS/WAXS) facility [1] provides data spanning over 5 decades in microstructural size, which can be collected sequentially in 4 to 6 minutes, from the same volume during one in-situ experiment.

Presentation will show selected examples of research taking advantage of these unique capabilities. For example [2], we have studied the Al-Cu-Mg alloys, e.g., AA2024, at different aging conditions. The precipitate structure and precipitation kinetics in is alloy, aged at 190 °C, 208 °C, and 226 °C have been studied using ex situ TEM and in situ combined USAXS/SAXS/WAXS at the APS. TEM provided information concerning the nature, morphology, and size of the precipitates, while USAXS/ SAXS/WAXS provided qualitative and quantitative information concerning the time-dependent size and volume fraction evolution of the precipitates at different stages of the precipitation sequence.

This and other examples will show how the extended-range measurements simplify and speed up new advanced materials characterization and development. 

1. J. Ilavsky, P.R. Jemian, A.J. Allen, F. Zhang, L.E. Levine, G.G. Long, Journal of Applied Crystallography, 42, (2009), 469-479.
2. Zhang, F., L. E. Levine, A. J. Allen, C. E. Campbell, A. A. Creuziger, N. Kazantseva and J. Ilavsky, Acta Materialia 111, (2016), 385-398.

 

Wayne Saslow photoWayne Saslow

Thursday, July 6, 12:00 pm, 435 Nicholson Hall 

Dr. Saslow is a Professor of Physics at Texas A&M University

"Irreversible thermodynamics of uniform ferromagnets with spin accumulation: Bulk and interface dynamics"

ABSTRACT: Although ferromagnetic excitations were first studied without collisions -- as appropriate to neutron scattering -- collisions are necessary to produce  equilibrium and near-equilibrium behavior. Based on a two-band equilibrium picture, we consider the full small-amplitude non-equilibrium spin distribution function, having 10^{23} variables. We argue that, after a short time, collisions reduce the magnetic variables to only the usual magnetization M and the non-equilibrium spin accumulation m. We describe M and m thermodynamically, and then use irreversible thermodynamics to derive their dynamics, both in bulk and at interfaces, where they have distinct boundary conditions.

 

NelsonToby Nelson 

Friday, April 28, 2017 (joint LaCNS – Macromolecular Chemistry seminar):

Dr. Nelson is a Professor of Chemistry at Oklahoma State University

"Redefining Melanin: From Eumelanin-Inspired Materials to Structure-Property Relationship"

ABSTRACT: Melanin is a unique class of natural occurring pigments found in the hair, eyes, skin and the brain of mammals. Eumelanin is the black-brown variety of Melanin and understood to be a biosynthesized heterogeneous macromolecule containing 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxyic acid. Nature has chosen this substance with such complexity and undefined structure as the centerpiece for so many functions like coloration, radiation hardness, and neuron protector. This presentation will begin with an introduction to what we call Eumelanin-inspired materials. Here, we will explore the tunability of the Eumelanin-inspired indole core for applications such as sensors, OLEDs and antimicrobials. To follow, our progress toward understanding Eumelanin structure-property relationship to define function will be presented. 

 

BilheuxHassina Bilheux

Monday, April 17, 2017

Dr. Bilheux is a Lead Instrument Scientist at the Neutron Imaging Facility, Oak Ridge National Laboratory

"Neutron Imaging Capabilities at the Oak Ridge National Laboratory"

ABSTRACT: Oak Ridge National Laboratory is home to two of the most powerful neutron sources in the world: the 1.4 MW Spallation Neutron Source (SNS) and the 85 MW High Flux Isotope Reactor (HFIR). Neutron imaging is a non-destructive characterization technique capable of mapping light elements buried in high-Z materials. The technique has been used for defects in materials (fracture, porosity, etc.), in-situ time-dependent measurements such as water uptake in plant roots or Li distribution in battery electrodes, and archeological investigations (identification of a counterfeit object), to name a few.

This presentation gives an overview of recent applications at HFIR CG-1D imaging. Lately, the CG-1D team has prototyped qualitative polarized imaging using white beam neutrons and is currently working toward monochromatic capability to enable quantitative measurements of superconducting materials.

In preparation for the SNS VENUS imaging beamline, the team has developed Bragg edge imaging capabilities combined with neutron diffraction at the SNAP and VULCAN beamlines. Wavelength-dependent neutron transmission exhibits abrupt drops when the neutron wavelength is twice the value of d-spacing, allowing identification of crystalline planes. This technique has been applied to additively manufactured Inconel 718 with different crystalline orientations. Combination of the Bragg edge results with diffraction data is discussed, along with the microstructure interpretation of the results. An attempt at modeling Bragg edges is also presented. 

Finally, an overview of the jupyter notebook-based Python iBeatles user interface used to identify and fit Bragg edges is discussed.

 

KharlampievaEugenia Kharlampieva

Friday, March 10, 2017 (joint LaCNS – Macromolecular Chemistry seminar):

Dr. Kharlampieva is a Professor of Chemistry at the University of Alabama Birmingham

"Polymeric Nanocoatings for Drug Delivery and Cell Transplantation"

ABSTRACT: Bio-inspired fabrication of biologically-active and stimuli-sensitive nanostructured materials is of increasing interest in bio- and nanotechnology. This talk will focus on functional nanothin coatings and hollow nanothin microparticles (capsules) obtained by assembly of synthetic and biological macromolecules on inorganic templates and living cells. We will discuss pH-triggered changes in hydrogel film architecture probed with neutron reflectivity as well as shape transitions in hydrogel microcapsules to be used for controlled drug delivery. We will also address the application of these coatings in cell-based transplantation therapy. We will present immunomodulatory films with diminished inflammatory immune responses deposited on mammalian pancreatic islet cells. These materials provide prolonged cell viability and function to be used in diabetes treatment.

 

Yu-Shan LinYu-Shan Lin

Tuesday, February 21, 2017 (joint LaCNS – Physical Chemistry seminar)

Dr. Yu-Shan Lin is an Assistant Professor of Chemistry at Tufts University

"Understanding and Designing Cyclic Peptides"

ABSTRACT: Cyclic peptides (CPs) are highly sought after for several unique applications. For example, CPs can target protein surfaces with high affinity and selectivity, thereby inhibiting specific protein–protein interactions that cannot be easily targeted with other molecules. New inhibitors will enable mechanistic studies to dissect the functions of individual protein–protein interactions in the complicated cellular interactome. However, robust application of this fundamentally interesting class of molecules for these and other purposes is limited by our poor capacity to predict CP structures and the resulting inability to rationally design functional CPs. In this talk, we describe an efficient enhanced sampling method to simulate CPs, using which we aim to fill the knowledge gap of CP sequence–structure relationships, and enable rational design of CPs with desired structures.

 

Utpal ChatterjeeUtpal Chatterjee

Monday, February 20, 2017

Dr. Utpal Chatterjee is a Professor of Physics at the University of Virginia

"Electronic phase diagram of High Temperature Superconductors"

ABSTRACT: Even though High Temperature Superconductors (HTSCs) were discovered three decades ago, a microscopic theory is yet to be realized for this unique class of materials. An important step towards this is to characterize the normal state of the HTSCs in great detail. From our temperature (T) and carrier concentration (d) dependent Angle Resolved Photoemission Spectroscopy (ARPES) measurements on Bi2Sr2CaCu2O8+d (BISCO 2212) HTSCs, we find that unlike in conventional superconductors, where there is a single temperature scale Tc separating the normal from the superconducting state, HTSCs are associated with two additional temperature scales. One is the so-called pseudogap scale T*, below which electronic excitations exhibit an energy gap. The second is the coherence scale Tcoh, below which lifetimes of quasiparticles get enhanced. We observe that T*(d) and Tcoh(d) cross each other near optimal carrier concentration, i.e. the d for which Tc (d) attains its maximum value.  There is an unusual phase in the normal state where the electronic excitations are gapped as well as coherent. Quite remarkably, this is the phase from which the superconductivity with maximum Tc emerges. We also conduct direct comparison between the single-particle spectral functions from charge density wave systems and from the pseudogap phase of BISCO 2212 HTSCs. Our data do not seem to be consistent with the propositions that the energy gap for T< T* is due to some charge ordering. Our data are rather consistent with the presence of incoherent pairs in the pseudogap phase. Moreover, our experimental finding that the two crossover lines T*(d) and Tcoh(d) intersect is not compatible with the theories invoking “single quantum critical” point near optimal doping, rather it is more naturally consistent with theories of superconductivity for doped Mott insulators.

 

RamirezAnibal J. (Timmy) Ramirez-Cuesta

Monday, February 13, 2017

Dr. Ramirez-Cuesta is a Chemical Spectroscopy Group Leader in the Chemical and Engineering Materials Division, Neutron Sciences Directorate at Oak Ridge National Laboratory

"Neutrons and Numbers: The VISION challenge. The world’s first high throughput Inelastic Neutron Scattering Spectrometer"

ABSTRACT: Molecular spectroscopy is a very powerful tool to study the dynamical properties of solid, liquid and gases.  Inelastic Neutron scattering is a very powerful tool to study hydrogen-containing materials. With the development of neutron spallation sources, and the use of epithermal neutrons, inelastic neutron scattering can measure the vibrational spectra of materials on the whole range of vibrational motions (0-4400 cm−1) and effectively opening up the field of neutron spectroscopy. INS is a technique that was mostly used to study hydrogen-containing materials due to the high cross section of hydrogen.

The recently commissioned VISION spectrometer at the SNS in Oak Ridge Tennessee has an increased overall flux at low energy transfers up to 4000 times over its predecessors and it has unprecedented sensitivity. I will examine the limits of what is now possible in INS thanks to VISION. From the determination of INS spectra of publishable quality in minutes (for samples in the gram quantity range), measuring the signal of samples in the milligram range to the direct determination of the signal of 2 mmol of C02 adsorbed on functionalized catalysts.

Sample environment developments are a crucial part of an effective neutron scattering program, at VISION we have developed the world’s largest single crystal diamond anvil cell and measured the INS spectra of 1 mm3 of a HMB sample. Gas handling experiments are trivial to perform. A sample changer designed for VISION is being built, it is a whole new concept that will allow continuous operation for large number of samples (hundreds at a time) that will enhance the mail-in program for sample measurement. Recently, a simultaneous Raman and INS center-stick has been developed and tested in VISION measuring simultaneously the rotational spectra of hydrogen in the gas, liquid and in the solid state as function of the relative para-ortho hydrogen concentrations. We also have in-situ dielectric spectroscopy capabilities. There is a catalysis cell and gas handling equipment that is currently being built to perform in-situ chemical reactions.

Finally, the major challenges that we are facing will be discussed, in particular methods to automate data analysis and interpretation through computer modelling. We have recently commissioned VirtuES (VIRTUal Experiments in Spectroscopy), March 2016,  a computer cluster dedicated to analyse VISION data. We are developing the software to maximize the potential of the technique by generation of automated VDoS, generation of thermodynamic data, creation of databases etc.

 

Brian K. Long photo of Brian Long

Friday, February 3, 2017 (joint LaCNS – Macromolecular Chemistry seminar)

Dr. Long is a Professor of Chemistry at the University of Tennessee Knoxville

"Utilizing Coordination-Insertion Based Polymerizations for the Synthesis of Tailored Polyolefins and Gas Separation Membranes"

ABSTRACT: Coordination-insertion based polymerization methods provide a multitude of opportunities for enhanced control over catalytic activity, selectivity, and reactivity. Through tailored catalyst development and macromolecular design, the Long Research Group leverages these advantages to synthesize unique and/or tailored polymeric structures for a variety of applications. In this talk, we will demonstrate the potential power of these coordination-insertion based polymerization methods through two studies. First, we will provide fundamental evidence that redox-active olefin polymerization catalysts can be effectively used to modulate polyolefin microstructure and copolymer composition via simple in situ changes in a catalyst's oxidation-state. Second, we will demonstrate that careful catalyst selection can enable access to a unique class of polymers that was previously believed to be inaccessible, and that those materials are extremely attractive as highly efficient gas separation membranes.

 

 OhlMichael Ohl 

 Dec. 12, 2016. 3:00 pm, 1008B Digital Media Center

 Dr. Ohl is Lead Instrument Scientist at Spallation Neutron Source, Oak Ridge National Laboratory

"Neutron Spin Echo: The Technology and the Science"

ABSTRACT: When Neutrons were first used as probes to understand the properties of matter, it became immediately obvious that inelastic processes could be studied as well. The higher the energy- resolution of Neutron scattering instruments got, the better one could unravel slow dynamics.

Subsequently, Neutron scattering instruments were pushed more and more to higher resolution and more ideas were developed. The principle of Neutron Spin Echo (NSE) to encode and decode the energy transfer of Neutrons in the spin of the scattered Neutrons has been well known since 1971.

About eight years later IN11, the first NSE spectrometer worldwide, was built at the Institute Laue-Langevin in Grenoble, France and went into operation with first results. This was the start of many more NSE spectrometers to come later and up to now NSE spectrometers still possess the highest energy resolution in the field of Neutron scattering. As of today about six NSE spectrometers of the generic IN11 type are operated worldwide in Europe and the USA. The newest instrument is the NSE at the Spallation Neutron Source in Oak Ridge, TN, USA. A wide field of scientific questions can be addressed by this new type of instrument. They are mainly in the field of soft matter e.g. polymer main chain and side group motions, glass forming properties, internal dynamics of proteins, and functionalities of bacteria to name just a few. In this talk, the latest technology and newest scientific questions will be addressed.

 

HusseyDaniel Hussey 

Nov. 14, 2016. 3:00 pm, 1008B Digital Media Center

Dr. Hussey is Staff Scientist at the Neutron Imaging Facility, NIST

“Seeing the World with Neutron Vision”

ABSTRACT: In this talk, I’ll discuss how neutron imaging has benefited fuel cells, how one can image microstructural details in crystals and polymers, and introduce the idea of the neutron microscope. Neutrons primarily interact with matter via the strong nuclear force (as opposed to the electron density) and so provide a complementary view of the world compared to more conventional probes of matter. In particular, neutrons have a very high sensitive to hydrogen while being very insensitive to common metals such as aluminum. This has enabled neutron imaging to play a key role in understanding the water transport in hydrogen fuel cells. Because neutrons can also be thought of as both particle and wave, it is possible to create phase images which can resolve smaller changes than standard imaging, and also reveal structures that are much smaller than the spatial resolution of the image. An ongoing challenge in any neutron scattering or imaging measurement is the inherently low neutron intensity as compared to what is possible at modern x-ray synchrotrons. This is partly due to the difficulty in focusing neutrons as the refractive index differs from one by only 1-10 ppm. A new reflection base lens technology shows great promise to create the world’s first practical neutron microscope.

 

BanerjeeArnab Banerjee

Nov. 4, 2016. 3:30 pm, A101 Life Science Bldg

Dr. Banerjee is a Postdoctoral Fellow at Oak Ridge National Laboratory

"Search of Kitaev Physics and Majorana Fermions in a honeycomb magnet"

ABSTRACT: As dimensions get reduced, the effects of quantum fluctuations increase leading to interesting emergent quantum phenomena and new quantum states. The 2D Kitaev quantum spin liquid (QSL) is such a quantum state of matter which occurs when the three types of bonds of the honeycomb lattice have mutually incompatible Ising interactions. This Kitaev QSL is particularly intriguing since its magnetic excitations – which appear as a spectrum of Majorana Fermions - are theorized to realize a particular topological quantum computing technology in the solid-state. Here I describe the comprehensive characterization of the graphene-like Kitaev candidate material α-RuCl_3 using both real and reciprocal space measurements. This transition metal halide has a relatively high spin-orbit coupling in the presence of an octahedral crystal field ensuring a/ S/=1/2 ground state enhancing quantum fluctuations, which creates a conducive environment for the realization of Kitaev physics.
Using inelastic neutron scattering, we explicitly measure a thermally-resilient broad spinon continuum. This feature is independent of the long-range ordering and inexplicable by classical spin-wave theories, but matched the predictions of the high-energy 2D Majorana Fermions expected from exact Kitaev calculations, placing this material proximate to the true Kitaev QSL. We mapped the detailed energy-momentum resolved dispersion of this mode using single-crystal TOF neutron scattering, which would be the first measurement of its kind on a Kitaev continuum. Our measurements yield critical insights into /how /and /by how much /a real material departs from the ideal Kitaev behavior. Finally, I discuss our very recent field-dependent data which teases a particular route towards achieving the ideal QSL scenario.

 

JohnsonDuane Johnson 

Oct. 31, 2016. 3:00 pm, 1008B Digital Media Center 

Dr. Johnson is Chief Scientist at Ames Laboratory, U.S. Department of Energy, and F. Wendell Miller Professor of Materials Science & Engineering at Iowa State University

“Mapping and Manipulating Materials Transformation Pathways and Properties” 

ABSTRACT: Electronic-structure-based thermodynamic methods to explore, explain, and control materials phase transformations will be described with applications on novel, complex multicomponent alloys and responsive materials to reveal phenomena such as (i) transformations in shape-memory alloys and new structures, (ii) short- and long-order in high-entropy alloys, (iii) nanoalloy catalysts, (iv) magneto-structural collapse in magnets, and (v) Lifshitz transitions, and quantum critical points in iron-arsenide superconductors. To design multicomponent materials and tailor their functionality, we apply these unique techniques to quantify stability and properties by mapping global solid-solid transformations (e.g., order-disorder and competing long-range order (LRO) states) and local structural instabilities (e.g., short-range order, SRO). In particular, thermodynamic linear-response theory is used to predict SRO involving coupled electronic, chemical, magnetic, and structural fluctuations in complex N-component systems; the solid-state nudged-elastic band method was extended to map pathways (enthalpies and barriers) involving non-conserved order parameters responsible for, e.g., magneto-structural collapse.

 

BhartiBhuvnesh Bharti 

Oct. 3, 2016. 3:00 pm, 1008B Digital Media Center

Dr. Bharti is a Professor of Chemical Engineering at Louisiana State University

“Neutron Scattering for Investigating the Surface Aggregates Formed by Amphiphiles on Nanomaterials”

ABSTRACT: Nanoscale functional materials are of interest in many applications, ranging from catalysis, membrane processes, and chromatography to new areas such as microelectronics and medical diagnostics. Surfactants or related amphiphilic substances are involved in many of these fields, and understanding their surface-binding characteristics would enable the control and design of the assembly processes leading to the desired properties of the materials. An important characterization tool for addressing interfacial properties in aqueous soft matter systems is Small-Angle Neutron Scattering (SANS). To this end, we have applied of SANS to investigate the adsorption of non-ionic surfactants on spherical silica nanoparticles and in the periodic cylindrical pores of SBA-15 material. Using SANS at silica contrast matching H2O/D2O solution, we find that the self-assemblies formed by the surfactants on the silica nanomaterials are strongly dependent on their surface-binding energy. This binding energy between surfactant and silica can be altered by co-adsorbing modifier molecules (here lysine), which further change the characteristics of the dispersions and the surface aggregates formed by the amphiphiles. I will also present our recent study on the formation of fatty acid liquid nanocapillary bridges between iron oxide nanoparticles, where I will highlight the major challenges in SANS experiments and modeling of this magnetic-fatty acid composite materials.

 

ArgesChristopher Arges

Sept. 26, 2016. 3:00 pm, 1008B Digital Media Center

Dr. Arges is a Gordon A. and Mary Cain Professor of Chemical Engineering at Louisiana State University

“Molecular Level Engineering of Block Copolymer Electrolytes’ Structure via Directed Self-assembly”

ABSTRACT: Solid-state ion conducting polymers (i.e., polymer electrolytes) are found at the heart of numerous electrochemical processes that store and convert energy, synthesize chemicals, and purify water. One key functional property of these materials is their ion conductivity – a key transport property that governs the ohmic resistance in electrochemical devices. Block copolymer electrolytes are a subset of polymer electrolytes and they are attractive materials because their micro-phase separated architecture yields greater conductivity over their random copolymer counterparts. However, there is a poor understanding between molecular level structure and bulk material properties like ion transport.

In this work, the process of directed self-assembly controlled the micro-phase separated structure in block copolymer electrolytes with astonishing fidelity. Engineering the block copolymer electrolyte structure was achieved by first directing the self-assembly of the non-ionic variant block copolymer (poly(styrene-block-2-vinyl pyridine) using solvent vapor annealing on non-preferential layers or topographical patterned substrates. After self-assembly, an invasive, gas phase Menshutkin reaction with an alkyl halide converted the pyridine moiety to n-methyl pyridinium iodide – anion charge carriers. The reaction was benign to the self-assembled molecular structure. Complementary x-ray scattering (GI-SAXS and RSoXS) and electron microscopy (SEM and EDX mapping via STEM) substantiated the introduction of ionic groups without detriment to structural integrity. Key results from the ordered block copolymer electrolytes highlight that ion conductivity followed an exponential growth curve with respect to ion domain connectivity. Ion domain alignment to electrode surfaces with a tortuosity of 1 yielded a 4 order of magnitude improvement in ion conduction over anti-aligned ionic domains. The results of this work have far reaching implication for the rationale, molecular level design of block copolymer electrolytes.

 

RuparPaul Rupar 

Sept. 2, 2016. 12:30 pm, 214 Williams Hall

Dr. Rupar is an Assistant Professor of Chemistry at the University of Alabama

“The Living Polymerization of Aziridines and the Synthesis of Inorganic Element Containing Conjugated Polymers”

ABSTRACT: The first half of the talk will overview our work on the controlled polymerization of polyethyleneimine (PEI). PEI, a polymer with a -CH2CH2N(H)- repeat unit, is used in a wide variety of applications such as gene transfection, gas absorption/purification, and removal of metals from waste. PEI is typically formed by the cationic ring-opening polymerization (ROP) of aziridine, which produces a highly branched polymer (BPEI). The synthesis of BPEI is difficult to control, limiting the incorporation of PEI into intricate polymer structures such as block copolymers. We will demonstrate that the judicial selection of N-protecting groups enables the living anionic ROP of N-functionalized aziridines and subsequent conversion to linear PEI.  

The second part of the talk will discuss conjugated polymers containing boron and phosphorus atoms. The introduction of inorganic p-block elements into conjugated polymers is a powerful technique to create new functional materials. The incorporation of 3-coordinate boron is attractive as the boron empty p-orbital increases polymer electron affinity and provides the potential for polymers with inherent sensing applications. In the case of phosphorus containing polymers, the phosphorus atom is easily derivatizable and thus provides a handle for introducing new functionality into  conjugated polymers.

 

DasPinaki Das

Wednesday, July 6, 2016

Dr. Das is a Postdoctoral Researcher at Ames Laboratory, Iowa State University.

"Neutron scattering studies in quasicrystal and quasicrystalline approximants"

ABSTRACT: Quasicrystals, discovered by Dan Shechtman in 1982, are distinguished by the presence of sharp Bragg reflections with rotational point symmetries that are inconsistent with periodic translational order [1, 2]. Nevertheless, the resolution limited Bragg peaks observed for the best ordered icosahedral quasicrystals demonstrate that the atomic order is truly long-range, albeit aperiodic. In the 30 years since Shechtman’s discovery, significant advances have been made in our knowledge of the atomic scale structure, but less progress has been made in our understanding of the consequences of aperiodicity on electronic and magnetic properties. Here I will present our recent neutron scattering studies on the icosahedral quasicrystal i-Tb-Cd and its related quasicrystalline approximant TbCd6. Though TbCd6 shows a long-range antiferromagnetic order (TN = 24 K), only spin glass like behavior is observed in i-Tb-Cd with a spin freezing temperature of TF = 6 K. In the first part, I will show our detailed study of the elastic diffuse scattering observed in i-Tb-Cd associated with the short range magnetic correlations. In the second part, I will present our inelastic neutron scattering studies of the crystal field (CF) excitations in the approximant TbCd6, where the CF levels of the local Tb ion is close to a pentagonal symmetry.

[1] D. Shechtman et al., Phys. Rev. Lett. 53, 1951 (1984).
[2] A. I. Goldman, Sci. Technol. Adv. Mater. 15, 044801 (2014).

 

KumarSanat Kumar

Monday, April 25, 2016

Dr. Kumar is a Professor in the Chemical Engineering Department of Columbia University.

"Nanocomposites with grafted nanoparticles"

 ABSTRACT: A central area of research in the soft matter community is in inorganic/organic hybrid materials with nanoscale inorganic particles. These materials have been focused on due to their promise of having synergistic thermal, mechanical and optical properties relative to the pure materials. It is now accepted that the spatial distribution of the inorganic nanoparticles critically affects the properties of the resulting materials, but a grand challenge is to control the spatial distribution of the inorganic, hydrophilic nanoparticles in the organic, hydrophobic polymer matrix. I focus on one particular approach to controlling nanoparticle spatial dispersion, the use of polymer-grafted nanoparticles. In the case where the NP and the grafted polymer chains energetically “dislike” each other, we have an architecture akin to a microphase separated block copolymer or a surfactant. Analogous to these “surfactants” these grafted nanoparticles also assemble into a range of morphologies, thus giving us the unprecedented ability to control the particle dispersion state.

In this talk I first focus on the factors controlling this assembly and use this knowledge to consider the utility of other approaches to self-assembly – we show that the use of crystallizable polymers allows us to control nanoparticle order, in particular by varying the rate at which these materials crystallize. This allows us to mimic the growth of organisms such as nacre and oysters, whose shells combine the dual advantages of high strength and toughness. In a different vein, these grafted nanoparticles show the ability to creating membranes that have the potential to revolutionize the separation of hydrocarbons and in carbon sequestration.

 

KimchiItamar Kimchi

Monday, April 4, 2016

Dr. Kimchi is a Pappalardo Postdoctoral Fellow in the Department of Physics at Massachusetts Institute of Technology.

"How to identify and resolve beyond-geometrical frustration"

ABSTRACT: In this talk, we will discuss recent theoretical developments triggered by the experimental discoveries of iridium oxides α,β,γ-Li2IrO3. In these polytypes, spin-orbit-coupled J=1/2 moments form 2D and 3D lattices (honeycomb, hyperhoneycomb and stripyhoneycomb) which generalize the 2D honeycomb lattice. Scattering experiments on all three of these compounds have uncovered a peculiar non-coplanar incommensurate magnetic order, involving spirals which counter-rotate across neighboring sites. We discuss the emergence of this ordering, and the striking similarities visible across the three Li2IrO3 structures. 

The model Hamiltonians that capture the materials indicate strong magnetic frustration, which arises from spin-orbit coupling. Tuning the frustration, perhaps by just a 10% Hamiltonian perturbation, exposes a fractionalized phase: Kitaev's three-dimensional quantum spin liquid (QSL). What is its range of stability to the competing Hamiltonian terms which occur in the materials, such as antiferromagnetic Heisenberg exchange? The frustration prohibits direct computations. Instead, we demonstrate a viable approach by numerically solving the model in a fully quantum infinite-dimensional approximation, which captures both the magnetically ordered and the QSL phases. Finally, we discuss the phenomenology of the QSL phase, including the role of its emergent magnetic-like field lines in stabilizing its deconfined fermion excitations to finite temperatures. The resulting phase transition is a signature unique to three-dimensional fractionalization.

 

HjeimRex Hjelm

Monday, March 28, 2016

Dr. Hjelm is an Instrument Scientist at the Los Alamos Neutron Science Center, Los Alamos National Laboratory.

"Dispersion morphology of Nafion in the fabrication of fuel cell membrane electrode assemblies: relationships to electrolytic fuel cell durability and performance"

ABSTRACT: Modern polymer electrolyte fuel cells (PEFCs) require technologies that generate higher performance, durability and component structural integrity.  The membrane electrode assemblies (MEAs) of PEFCs typically contain a Pt/carbon black catalyst and an ionomer, such as Nafion. In one process MEAs are produced by solution casting from a dilute solvent dispersion of these components. We consider current hypotheses relating fuel cell durability and MEA toughness to changes in these components with multiple fuel cell voltage potential cycling: namely, that fuel cell durability is associated with the loss of electrochemical (Pt) surface area (ECSA) and that MEA mechanical properties are due to ionomer crystallinity. We weigh these hypotheses against an alternative that both fuel cell durability and MEA toughness are the result of dispersion particle morphology and its evolution during the course of drying to the MEA film. Small-angle neutron scattering (SANS) using the Lujan Center instrument, LQD, was used to show that different solvents determine the dispersion particle structure due to the amphiphilic nature of Nafion and that the morphology of the dispersion in different solvents follow different evolutionary paths on drying. The SANS results suggested that the ability of the dispersion particles to form an entangled network leading to a homogeneous, reversible gel alone was the determining factor for mechanical toughness of the MEA and fuel cell durability. It was found that neither toughness correlated with Nafion crystallinity nor was durability correlated with changes in the ECSA with voltage cycling. These results show the power of SANS as a characterization technique, when used along with other characterization methods and property and performance metrics, to address complex issues in materials science process, structure, properties and performance characteristics. Our work lends support to the hypothesis that MEA durability and toughness are affected by the dispersion morphology and are likely associated with the ability of Nafion to chain entangle.

 

DoChangwoo Do

Friday, March 11, 2016

Dr. Do is an Instrument Scientist at the Spallation Neutron Source, Oak Ridge National Laboratory.

ABSTRACT: Polymer self-assembly studies using small-angle neutron scattering

 

 

FaraoneAntonio Faraone

Monday, February 15, 2016

Dr. Faraone is a Postdoctoral Researcher at NIST Center for Neutron Research.

ABSTRACT: Quasielastic Neutron Scattering for the investigation of dynamics in soft condensed matter: polymer nanocomposites, molecular glass formers, and phospholipid vesicles Quasielastic neutron scattering (QENS) is a scattering technique for the investigation of relaxation dynamics with nanoscale space and time resolution. I will introduce the NIST Center for Neutron Research (NCNR) as a user facility with particular emphasis on its capabilities for QENS measurements on soft condensed matter. In the second part of my talk I will review, three scientific projects I have been involved recently with, which have extensively employed QENS. A model nanocomposite where miscible nanoparticle-bound polymer and matrix chains have a large difference in their glass-transition temperatures was investigated using different QENS techniques; the data clarify how the Rouse dynamics and the self-confinement of the chains are affected when cooling the bound polymer below its glass transition. Coherent QENS measurements allow investigating the dynamics of the nanoscopic structures present in associating molecular liquids; on the basis of the obtained results, the relation between the lifetime of the supramolecular associates and the structural relaxation is discussed. The elastic properties of phospholipid bilayers in unilamellar vesicles can be uniquely investigated using Neutron Spin Echo Spectroscopy; the effect of the addition of proteins on the properties of the membrane will be reviewed in two relevant cases. These examples provide an overview, necessarily partial, of the research efforts, currently being carried out at NCNR, on the dynamics of soft condensed matter systems.

 

Volker Urban

Monday, November 16, 2015

Volker Urban is active divison director and Energy and the Environment Group Leader.

"Unique Opportunities for Neutrons in Soft Materials and Biology"

ABSTRACT: Long-wavelength neutrons are an ideal probe for soft and biological materials because of their sensitivity to hydrogen, the possibilities of hydrogen/deuterium contrast variation and the fortunate combination of the neutron’s wavelength and energy. The seminar will review research questions in soft matter and biology that have used neutrons for extracting key information that was not accessible by other techniques. Examples include breakthroughs in our understanding of natural and man-made materials such as measuring the conformation of single polymer chain molecules in their natural melt or network environment, observing details of structural and compositional changes in cell walls during biomass industrial processing, and visualizing small proteins embedded in complex inorganic matrices. The unique capabilities of neutrons point to clear opportunities in soft matter and biology and suggest a science strategy that embraces development and investments for optimized neutron instruments and sample environments, computational analysis and experiment optimization, and chemical and biochemical deuterium isotope labeling.

 

Abhishek Pandey

Monday, November 9, 2015

Abhishek Pandey is a Postdoctoral Researcher at LSU Department of Physcis & Astronomy.

"Search for interesting behaviors beyond iron-based materials in tetragonal pnictides"

ABSTRACT: After the discovery of high-temperature superconductivity (SC) in the iron-based tetragonal compound LaFeAsO1-xFx in 2008, a worldwide effort began to understand the mechanism of SC and to discover other new superconductors in the related structures. This effort quickly led to the discovery of SC in structurally related 122-type iron-arsenides where the parent compounds with the composition of AFe2As2 (A = Ca, Sr and Ba) crystallize in tetragonal ThCr2Si2-type structure. Soon the interest expanded beyond the iron-based compounds and some exciting observations were made in other arsenide materials. Our stimulating observations of unexpected stripe-type antiferromagnetic correlations in SrCo2As2 and the discovery of a novel magnetic ground state in hole-doped BaMn2As2, where half-metallic itinerant ferromagnetism of doped holes coexists with a local-moment antiferromagnetism of Mn lattice, hint toward the abundance of possibilities contained in the transition metal-pnictide systems. I shall discuss some of our recent works on SrCo2As2 and hole-doped BaMn2As2 and their possible impact on the future research in this field. I will also briefly discuss about a new family of layered transition metal-pnictide materials recently discovered by us.

 

photo of Chetan DhitalChetan Dhital

Friday, May 8, 2015

Chetan Dhital is a Postdoctoral Researcher at Oak Ridge National Laboratory.

"Electronic phase separation and magnetic phase behavior in the Ru-doped spin-orbit Mott insulator Sr3Ir2O7"

ABSTRACT: Recent theoretical and experimental studies have predicted a very interesting electronic phase diagram in 5d iridate system arising due to interplay of spin orbit interaction and the electronic correlation. Spin-orbit Mott phase is one such electronic phase realized in Ruddelsden-Popper (RP) series [Srn+1IrnO3n+1] oxides. Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1) are two representative candidates of this series. Although their ground state properties are studied to some extent, very little is known regarding how the properties of their antiferromagnetic, insulating, parent states evolve upon carrier substitution. One way of experiencing the strength and relevance of electronic correlation in any condensed matter system is by doping charge carriers. The presence of electronic correlations in the host system determines the fate of the dopant and hence stabilizes a new electronic/magnetic ground state. I will discuss about importance of electronic correlations in one such doped system Sr3(Ir1-xRux)2O7 using combined neutron scattering, electric transport and magnetization techniques. Our studies show that the Mott insulating state of Sr3Ir2O7 is remarkably robust as the in-plane doped holes remain largely localized within a nanoscale phase-separated ground state and only generate a metal–insulator transition (MIT) near the two-dimensional (2D) percolation threshold. The resulting electronic phase diagram also reveals the surprising persistence of antiferromagnetic (AF) order deep into the metallic phase and suggests emergent itinerant magnetism at the interface between the AF-ordered spin-orbit Mott phase of Sr3Ir2O7 and the nearly magnetic Fermi liquid electronic phase of Sr3Ru2O7.

 

Joseph Strzalka

Monday, May 4, 2015

Dr. Strzalka’s research interests include proteins at interfaces, organic photovoltaics, grazing incidence x-ray scattering, and liquid surface scattering. He earned his PhD in Physical Chemistry and MS in Physics at the University of Pennsylvania.

"Elucidating the Structure-Performance Relationship in Organic Photovoltaics (OPVs) by Grazing Incidence X-Ray Scattering"

ABSTRACT: Since the introduction of the Bulk Heterojunction (BHJ) architecture in the mid 90s, organic photovoltaic devices have made steady progress toward improved power conversion efficiency, and are now poised to move from niche products to large scale commercial applications. In the BHJ, the photoactive layer consists of electron donor and acceptor materials in a bicontinuous phase blended on the nanoscale. Grazing incidence x-ray scattering, capable of characterizing thin film nanomorphology of surfaces and interfaces, has emerged as a key technique for investigating OPV materials. The hierarchical variety of lengthscales present in OPV materials requires both grazing incidence small- and wide-angle x-ray scattering, the latter recently enabled by improvements to the GISAXS instrument at 8-ID-E of the Advanced Photon Source. I will describe grazing-incidence studies at 8-ID-E that have contributed toward unraveling the complex relationship between OPV materials, processing and performance.

 

Yuen Yiu

Monday, April 27, 2015

Yuen Yiu is a Graduate Research Assistant in Condensed Matter Physics at the University of Tennessee & Oak Ridge National Lab

"Studies of Strongly Correlated Electron Systems Using Neutron Scattering"

ABSTRACT: Condensed Matter Physicists strive to continue to solve the puzzles at the frontier of material research. We look at 3 separate materials and present results of the studies from neutron scattering experiments.
First is an investigation of phase transitions in Ru/Fe substituted PrFeAsO. PrFeAsO is a member of the 1111 family of iron pnictides, in which superconductivity can usually be induced by suppressing the magnetic and structural transitions via carrier doping. We have used Neutron Powder Diffraction to investigate the effect of isoelectronic substitution on these transitions in Ru/Fe substituted PrFeAsO.
Second is the study of the helimagnetic ordering in Cr doped FeGe. Both CrGe and FeGe are in the B20 cubic structure, where CrGe exhibits no long range magnetic order down to at least 2K, and FeGe orders helimagnetically at 280K. We use Small Angle Neutron Scattering to study the doping dependence of helimagnetism of Cr doped FeGe.
Finally, we examine the lattice dynamics in the rocksalt structure compounds: UC and US. A recent inelastic neutron scattering experiment has revealed quantum harmonic oscillator behavior of N atoms in UN. We deduce that other uranium rocksalts should also exhibit such behaviors. We use Time-Of-Flight Inelastic Neutron Scattering to extend the study on the dependence of quantum harmonic oscillations in uranium rock salts.

 

Feng Ye

Monday, April 20, 2015

Dr. Ye’s research focuses on phase transition, magnetic ordering, and spin dynamics of strongly correlated electron materials such as magnetoresistive oxides, frustrated systems, and multiferroics materials using neutron scattering techniques.

"Tuning the magnetism in the multiferroic Mn1-xCoxWO4"

ABSTRACT: Multiferroic and magnetoelectric materials have attracted renewed attention since the magnetic orders breaks the spatial inversion symmetry and give rise to a ferroelectric state with a macroscopic polarization [1]. The frustrated nature of the magnetic state in multiferroic MnWO4 is the reason [2] for its extreme sensitivity of the ferroelectric/magnetic orders to small perturbations in form of magnetic and electric fields, external pressure, and chemical substitutions [3-5]. Using neutron diffractions, we have characterized the effect of cobalt substitution on the magnetic configuration and corresponding multiferroic behavior [6]. A complex phase diagram with multiple polarization flops upon increasing Co content is observed. Two critical concentrations separate the multiferroic phases with distinct spin structures and orientation of the ferroelectric polarizations. With application of hydrostatic pressure, we further examine the evolution of spin structures at various Co concentrations. Our neutron scattering results provide critical understanding of the pressure induced polarization-switching in the doped sample.

[1] T. Kimura et al., Nature 426, 55 (2003).
[2] F. Ye et al., Phys. Rev. B 84, 179901 (2011).
[3] R. P. Chaudhury et al., Phys. Rev. B 83, 014401 (2011).
[4] F. Ye et al., Phys. Rev. B 78, 193101 (2008).
[5] N. Poudel et al., Phys. Rev. B 89, 054414 (2014).
[6] F. Ye et al., Phys. Rev. B 86, 094429 (2012).

 

photo of Arun YethirajArun Yethiraj

Monday, April 13, 2015

Dr. Yethiraj is Professor of Chemistry at the University of Wisconsin-Madison, where he focuses on theoretical studies of soft condensed matter. His research has two components: the development of methods, and the application of these methods to understand the structure and dynamics of condensed phases. Some areas of interest are polymers in ionic liquids, gemini surfactants, and coarse-grained force fields for complex fluids. He is a Fellow of the American Physical Society and recipient of the National Science Foundation CAREER Award. He has served as Senior Editor of the Journal of Physical Chemistry since 2007.

"Self-assembly in complex fluids"

ABSTRACT: The self-assembly of molecules into nano-structured materials is a fascinating process because small changes in intermolecular interactions can have a large impact on the final mesoscopic structures. An interesting goal is the directed self-assembly of molecules where the chemical nature of the molecules drives the assembly into specific nanostructures. In this talk I will discuss two classes of molecules: lipid/peptide mixtures and Gemini surfactants. In the former specific interactions between the peptides and lipids causes lipid segregation and the formation of curved interfaces. Gemini surfactants form lyotropic liquid crystalline phases. Using computer simulation we show that both non-electrostatic and electrostatic interactions play an important role in the self-assembly of these systems, and both are promising candidates for chemistry directed self-assembly.

 

photo John AnknerJohn Ankner

Monday, March 23, 2015

Dr. Ankner is a Liquid Reflectometer Instrument Scientist at Oak Ridge National Laboratory.

"Studies of Polyelectrolyte Multilayers with the SNS Liquids Reflectometer"

ABSTRACT: Layer-by-Layer (LbL) assembly performed via alternating adsorption of water-soluble polymers at surfaces enables fabrication of films on almost any substrate, with nano-scale control over film composition, structure, and properties. Neutron reflectivity offers a window into the internal structure of Layer-by-Layer grown films. The dependence of neutron refractive index on nuclear rather than electronic scattering allows one to substitute deuterons (2H) for protons (1H) to highlight features of interest within a film. Deuterated precursors are available for a wide range of polyelectrolytes and the polymers themselves for commonly used species, such as poly(styrene sulfonate) and poly(methacrylic acid). By imposing rigorous mass balance and employing simplified block models, one can reduce the number of model parameters and extract meaningful structural information from reflectivity data. We will describe how to construct and constrain multilayer models and present application of these methods to various LbL structural problems, such as environmental response, the dependence of film quality on deposition parameters, adsorption of protein layers, asymmetric and salt-mediated diffusion, and the formation, structure, and pH-response of hydrogels.

 

 photo of Yuri LvovYuri Lvov

Friday, Feb. 13, 2015

Dr. Yuri M. Lvov is a Professor of Chemistry, Tolbert Pipes Eminent Endowed Chair on Micro and Nanosystems at the Institute for Micromanufacturing, Louisiana Tech University. He earned his Ph.D. in Physical Chemistry (protein crystallography) from #1 Russian university, M. Lomonosov's Moscow State University in 1979, then worked at the Institute of Crystallography, Russian Academy of Sciences where he earned a Doctor of Science in physics in 1991. After the break up of the Soviet Union, he worked in world famous research centers in Germany (Max Planck Institute for Colloids), Japan (National Institute of Materials Science, Tsukuba), and USA (Naval Research Laboratory, Washington DC). In 1999, Dr. Lvov came to the Institute for Micromanufacturing at LaTech from the Center for Biomolecular Science and Engineering, Naval Research Lab.

His area of specialization is nanotechnology including nanoassembly of ultrathin organized films, bio/nanocomposites, nano/construction of ordered shells on tiny templates (drug nanocapsules, shells on microbes and viruses), clay nanotubes for controlled release of anticorrosion and bioactive agents. Y. Lvov has 12 US, Australian and Japanese patents on nanoassembly. He was among pioneers of the polyelectrolyte layer-by-layer (LbL) assembly, - a nanotechnology method which, after his first papers in 1993, followed by many thousands publications by researchers all over the world. LbL nanoassembly already found industrial applications in HPLC, for eye lens modification, improvement of cellulose fiber for better fabric and paper, microcapsules for insulin sustained release, cancer drug nanocapsules, anticorrosion protection, and others. Yuri Lvov edited three books, published 22 book chapters, and 214 peer reviewed papers on nanosystems. Lvov’s total citation index is above 13,500 which is an outstanding result, and his h-index is 58.

His NSF, NIH, NASA, DoE grants and industrial contracts exceeded $9 million in the last ten years. In 2003, 2008, 2011 and 2013 he chaired American Chemical Society International Symposiums on Nanoassembly in New York, Philadelphia, Anaheim and New Orleans. Y. Lvov was named the Louisiana State’s Top Researcher in New Technologies (2007), and in 2008 he got Best of Small Tech National Innovator Award in recognition of achievements in micro and nanotechnology. In 2014, Yuri Lvov was awarded with prestigious international A. von Humboldt Prize for lifetime achievements in nanochemistry. Dr. Lvov has been invited to present talks at more than 140 national or international scientific conferences and has delivered lectures at more than 50 major universities and companies. Y. Lvov was elected as an honorable professor of Beijing University of Chemical Technology, China and Kazan Federal University, Tatarstan, Russia (2013).

"Nano/micro Containers for Sustained Release of Protective Agents and Drugs: Tubes and Capsules"

ABSTRACT: Nanotubes: Halloysite aluminosilicate nanotubes with a 15 nm lumen, 50 nm external diameter, and length of 800 ± 400 nm have been developed as an entrapment system for loading, storage, and controlled release of anticorrosion agents and biocides. Fundamental research to enable the control of release rates from hours to days is being undertaken.

Nanocapsules: Layer-by-layer (LbL) self-assembly of molecularly organized films was developed with linear polyions, nanoparticles, dye and proteins. The formation of alternate outermost layers of the opposite charge at every adsorption cycle is the key point of the procedure.

 

photo of Liusuo WuLiusuo Wu

Wednesday, Feb. 4, 2015

Liusuo Wu is a Postdoctoral Associate in Experimental Condensed Matter studying under Meigan Aronson at Stony Brook University. His interests include heavy fermions, frustrated magnets, and neutron scattering experiments.

"Spinon excitations in one dimensional magnet Yb2Pt2Pb"

ABSTRACT: Emergent fractional excitations in low dimensional magnets have attracted great interest in condensed matter physics. Many studies have focused on Heisenberg spin 1/2 systems, where quantum fluctuations are expected to be strongest. Here I will present measurements on a new low dimensional magnet Yb2Pt2Pb, where fractional, quantum spinon excitations are realized from one dimensional ladders with classical Yb Ising moments. Both elastic and inelastic neutron scattering data on Yb2Pt2Pb single crystals will be presented. In contrast to 1D Heisenberg spin 1/2 chains, the broad continuum of excitations observed in Yb2Pt2Pb is extended to much higher energies, indicating four spinon excitations are important in this system.

 

photo of Jyotsana LalJyotsana Lal

Monday, Feb. 2, 2015

Dr. Lal is a Researcher at Argonne National Laboratory focusing on the structure and internal dynamics of proteins and coherent diffraction imaging of complex polymer matrices. Her research interests include the development of novel methods for the use of x-rays and neutrons for the study of biological and soft condensed matter materials.

"Polymer chains in confined geometries"

ABSTRACT: Advanced neutron scattering techniques have well elucidated the structure and dynamics of polymer chains in bulk. Much less is known about the structure and dynamics of polymer chains undergoing deformation either mechanical or under shear. An easy way to deform polymer chains is to confine them. In this talk, I will discuss the conformation of single neutral and charged polymers confined in nanopores, also present the case of hydrophobically modified polymers confined to two-dimensional membranes and polymer confined in microemulsion droplets. These experiments depend on the use of techniques that are sensitive to the polymer conformations such as Small Angle Neutron Scattering (SANS) and Neutron Spin Echo (NSE). A brief introduction to neutron scattering techniques will be made. These techniques used by the soft matter community provide fundamental experimental insights into the structure, self-assembly and dynamics of soft materials.

 

photo of Zahid HussainZahid Hussain

Thursday, Jan. 29, 2015

Dr. Hussain is one of the 140 most cited physicists. He studies highly correlated electron systems with ultra-high energy and momentum resolution photoemission, resonant inelastic scattering spectroscopy and dynamics with coherent scattering.

"Instrumentation driven science for unraveling emergent phenomena"

ABSTRACT: Sharper and sharper experimental tools are often crucial for understanding of novel physical phenomena and making new discoveries. Today in condensed matter physics we are experiencing need for revolutionary new instrumentation for understanding interplay of many degrees of freedom interacting at different energy, length and time scales. These interactions lead to new phases of matter and emergent phenomena such as high temperature superconductors, topological insulators and thermoelectric materials, to name a few. My talk will focus, through various examples, upon the necessity for advanced techniques and instrumentation to elucidate the basic physics in the arena of soft x-ray synchrotron radiation and free electron laser.

 

photo of Qiang ZhangQiang Zhang

Monday, Jan. 26, 2015

Dr. Zhang is a Postdoctoral Researcher at Ames Laboratory. His research focuses on dielectric, ferroelectric, and multiferroic materials, Fe-based superconductors and complex magnetic materials exhibiting spin glass, magnetocaloric effects, magnetoresistance, exchange bias effects, and spin-orbit couplings, and strongly correlated electron behaviors.

"Magnetism and its coupling to structure and superconductivity in iron pnictides"

ABSTRACT: The discovery of high-temperature superconductivity in the fluorine-doped LaFeAsO has triggered intensive interest in superconductivity and also in itinerant magnetism in general in the "1111" and "122" families of layered iron pnictides. In this talk, I will present the complex magnetic structures in Fe and Ce sublattices determined by the elastic neutron scattering technique in CeFeAsO crystal. We found a spin-reorientation transition of Fe moments prior to long-range ordered Ce moments at lower temperatures, revealing a strong interplay between 4f Ce3+ and 3d Fe2+. The effect of the strong Ce-Fe coupling on the rearrangement of Fe ordering is yet another example of the vulnerability of the Fe spin density wave to perturbations such as minute doping or relatively low applied pressures.

While the previous studies have focused on the impact of superconductivity on the magnetic and orthorhombic phases, the interplay between these two ordered states has been a topic of intense debate and so far lacks a universal picture. Here, I will also present an evidence of sharp enhancement of the spin fluctuations, in particular the spin-spin correlation length below tetragonal-to-orthorhombic structural transition in LaFeAsO and underdoped Ba(Fe1-xCox)2As2 by inelastic neutron scattering measurements, in contrast with what one expects from a typical antiferromagnetic system. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides and also indicate the nematicity may help enhancing superconducting temperature in some circumstances in iron pnictides.

   

 

 

 Back to top