2012-2013 Colloquia Abstracts

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Thursday, Sept. 6

Polarization on the Prairie: The History of Early Polarimetry and Ellipsometry in Nebraska, 1900-1920

Ron Synowicki, J.A. Woollam Co., Inc.

For over 100 years Nebraska has a long tradition in polarized light! DeWitt Bristol Brace founded the UNL department of Physics in 1888 and the department of Electrical Engineering in 1895. Brace was an expert on polarized light and built a world-class research program at Nebraska. Clarence Skinner continued the polarized light research program started by Brace until 1919.

This presentation will highlight early work in polarimetry and ellipsometry at Nebraska by Brace, Skinner, and their students during the years 1900 to 1920. Particular emphasis will be placed upon the careers of Frederick John Bates and Arthur Quincy Tool. Both men used Brace’s invention of the sensitive-strip spectropolariscope to improve the sensitivity of their polarimeter and ellipsometer instruments to the best in the world.

Bates used polarimetry for the investigation of liquid solutions. Subsequently, Arthur Tool constructed the first ellipsometer in Nebraska; a fully variable angle and spectroscopic research instrument. Tool used the ellipsometer at Nebraska to investigate the optical properties of metals.

Frederick Bates and Arthur Tool went on to long, productive careers at the National Bureau of Standards, a career path followed by many early UNL Physics graduates. Bates became America’s foremost expert on polarimetry, developing improved polarimetry instrumentation and national standards for the sugar industry. Arthur Tool developed high-quality optical glass manufactured in the USA, using ellipsometry to characterize the quality of the glass. Today “Tool’s equation” is known throughout the glass industry for proper heat treatment during production of optical glass.

The UNL Physics department has preserved much of the original equipment from this classic era. Polarimeters from the time of Frederick Bates will be displayed, and Arthur Tool’s classic 1910 ellipsometer will be reassembled during this presentation.

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Thursday, Dec. 6

Demonstrations of Photo-Induced Magnetism in Metallic Nanocolloids Using Sunlight and Fridge Magnets

Luat Vuong, Queens College of CUNY

The focus of this talk is on nonlinear plasmonic vortex dynamics, which are far from understood and lead to appreciable photo-induced magnetic fields in metallic nanostructures.

We have recently experimentally, analytically, and numerically demonstrated the nonlinear photo-induced plasmon-assisted magnetic response that occurs with 80-nm gold particles in aqueous solution. The anomalously-large magnetic response — theoretically considered too small to observe at room temperature — was observed using light from a solar simulator and small (micro-to-milli-Tesla) magnetic fields. I will explain why the effect is observable using disperse nanocolloidal liquids, and present our theoretical model of an increased and anisotropic electrical conductivity, which yields modified absorption spectra in agreement with our experimental results.

This work, which is the first nano-demonstration of old physics, improves our fundamental understanding of surface charges in nanostructures and aids the development of broad-band photonics metamaterials, new polarization-encoded imaging methods, photocatalytic materials, photovoltaic devices, and sensors.

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Friday, Jan. 25

The Renaissance of Ferroelectric Memories

Professor James F. Scott, Cavendish Lab, Department Physics, Cambridge University

Electrical switching in ferroelectrics was discovered in 1920, but oddly enough, no commercial switching devices were made from them until 1984. Some modest industry use was achieved by 1994, culminating in application in the SONY Playstation 2. But this did not continue, and the Playstation 3 did not use ferroelectrics. In this talk I will explain why, describing both the basic physics (some of which goes back to Ku and Ullman in Lincoln) and technology transfer, with an up-date to include recent work on three-dimensional devices, resistive random access memories (RRAMs), and magnetoelectric switching (switching electrical polarization P not with an electric field E but with a small magnetic field H) at room temperature in a single-phase multiferroic crystal.

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Monday, Feb. 25

Understanding the Magnetic Ground States for Improper Multiferroic Materials

Jason Haraldsen, Los Alamos National Laboratory

Multiferroic materials have the unique multi-functionality of controlling magnetism through electric field and/or electric polarization through magnetic field, which presents possibilities for new technological advances and applications. To fully understand the connection between magnetism and electric polarization, one must have a full understanding of the underlying magnetic ground states within these materials. Through an investigation of the multiferroic material CuFeO2 and doped analogs, I examine the effects of anisotropy and magnetic field on the frustrated triangular lattice and determine the magnetic ground states. Through a rotational algorithm of the Holstein-Primakoff expansion for the spin Hamiltonian, the spin-wave dynamics for the multiferroic and high-magnetic-field phases are determined. With the dynamics of the multiferroic phase, I modeled the experimental data of doped CuFeO2. From this detailed analysis, it was concluded that the multiferroic ground state is that of a distorted incommensurate spiral, which provides insight into the effects of magnetic frustration within these materials. In closing, I will briefly discuss further research developments on the understanding of interfacial phenomena through magneto-electric coupling, and I will conclude with some future research directions in the pursuit of a full understanding multifunctional materials.

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Thursday, Feb. 28

Graphene: The Elastic Playground of Novel Electronic Phases

Pouyan Ghaemi, Institute for Condensed Matter Theory, University of Illinois at Urbana-Champaign

The discovery of massless Dirac fermions in graphene has generated a new area of science and technology in condensed matter systems where it is possible to observe and control charge carriers which behave relativistically. In comparison to other materials with relativistic band structure, graphene is truly two dimensional and this distinguishes it as the most tunable condensed matter system with Dirac band structure. In particular, recent experiments have shown strong interplay between lattice deformations of graphene and its electronic band structure. For example strain in graphene affects its electrons in a manner similar to applying a strong magnetic field, causing the formation of flat Landau levels. In this talk I will discuss the electronic properties of strained graphene and show that in this system, electron-electron interactions can lead to novel electronic phases, such as fractional topological states with preserved time reversal symmetry. These results show that graphene is a promising venue for experimental observation of many novel states of matter.

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Thursday, March 7

Intermediate Coupling Model of Correlated Systems

Tanmoy Das, Los Alamos National Laboratory

Understanding and modeling correlated electronic spectra has remained a constant theme of research for decades. We have relatively better modeling capabilities for systems residing either in the weakly or strongly correlated regimes. However, the intermediate coupling regime poses a challenge since both the Fermi-liquid or dynamical mean-field theories are inadequate here. Over the last few years, we have been working on developing a computational scheme for this problem. Our intermediate coupling model is based on materials-specific band structure, from which spin and charge correlations and corresponding self-energy correction are computed via self-consistent GW-like approach. In this talk, I will present results for high-temperature cuprate superconductors, and several representative actinide compounds in the Pu-115, and U-115 families. A common feature of intermediate coupling scenario is that the self-energy splits the electronic structure into low-energy coherent states, and high-energy localized state, yielding a coexistence of itinerant and localized states. The resulting electronic fingerprint reveals a universal ‘S’ or waterfall shape in the dispersion, and a peak-dip-hump feature in the density of states.

[1] Das, Zhu, Graf, Phys. Rev. Lett. 108, 017001 (2012)

[2] Das, et al. Phys. Rev. X 2, 041012 (2012).

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Friday, March 8

Just What the Heck Is a Higgs Boson?!

Dan Claes, University of Nebraska–Lincoln

Back to back announcements by both the Fermilab Tevatron and the CERN Large Hadron Collider provided evidence for a new particle that appears to be the long awaited Higgs boson. Good thing, else 50 years of theoretical work might have to be completely revised! In simple language Professor Dan Claes will give an accessible (to undergraduate physics majors anyway!) description of particle physics’ Standard Model and explain what this Higgs Boson is all about.

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Monday, March 11

Tuning Electrical and Optical Properties of 2D Atomic Crystals

AKM Newaz, Vanderbilt University

Two-dimensional (2D) atomic crystals are recently discovered materials that are only atoms thick, and yet can span laterally over millimeters. The diverse family of such materials includes graphene, a semimetal with massless relativistic charge carriers, and monolayer molybdenum disulfide (MoS2), a direct band gap semiconductor with strong spin-orbit interaction. Since every atom in these materials belongs to the surface, their physical properties are greatly affected by the immediate microenvironment. In my talk, I will demonstrate the wide tunability of the electrical and optical properties of both graphene and MoS2 and discuss some novel device applications.

In the first part of the talk, I will demonstrate the use of graphene field effect transistors (FETs) in sensing different physical parameters of nanometer-thick interfacial liquid volumes. I will demonstrate sensing of local liquid dielectric constant, mass flow velocity – with sensitivity 70nL/min, and ion concentration with sensitivity as low as 40 nM. I will also show that charge carrier scattering in graphene can be efficiently suppressed by placing graphene into a liquid environment. Overall, our results highlight the usefulness of graphene FETs for applications in ultra-precise fluidic sensing and as a potential replacement for silicon in next generation transistors.

In the second part of my talk, I will focus on mononalyer MoS2 and demonstrate that its optical properties, fluorescence quantum yield and transparency, can be tuned via electrical gating. In particular, we have observed a hundredfold modulation of excitonic photoluminescence from MoS2 at room temperature by varying the electric fields within ±1.7 MV/cm. Our findings demonstrate that MoS2 is the thinnest possible electroactive material and suggest the possibility of diverse applications ranging from nanoscale electro-optical modulators to quantum computing based on the spin degree of freedom.

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Thursday, March 14

Spin Mechanics: From Single Molecule Magnets to Majorana Fermions

Alexey Kovalev, University of California, Riverside

A nanomagnetomechanical system consisting of a mechanical resonator and a thin magnetic film has been predicted to display “magnetopolaritonic” modes that combine magnetic excitations and lattice deformations [1]. This phenomenon can be related to the well-known Barnett effect by involving a rotating frame of reference [2]. The “magnetopolaritonic” modes can be detected by splittings in ferromagnetic resonance spectra and mechanical modes. For sufficiently large mechanical actuation at certain resonance frequencies, the complete magnetization reversal can be performed by mere mechanical actuation [3]. Analogous phenomena can also happen in molecular magnets due to coupling of the macrospin quantum oscillations with the mechanical motion [4]. In our recent research [5], we theoretically address the quantum dynamics of a nanomechanical resonator coupled to the macrospin of a magnetic molecule by both instanton and perturbative approaches. Further generalizations that involve coupling of mechanical modes to Majorana ero energy modes and rely on magneto-Josephson effect are also discussed. This could result in new detection and manipulation schemes relying on magnetic resonance force microscopy. We also speculate on possible realizations of quantum control at a single-phonon level.

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Monday, March 25

One to Two, or to Nothing: Superconducting Spectroscopy for the Study of Electronic States

Xiaohang Zhang, Magnetic Materials Group, NIST

Single-electrons are spin-half particles, thus follow the Fermi-Dirac statistics. However, in superconductors, two single-electrons may be paired into a bosonic state, i.e., a Cooper pair. Because of the fundamental difference in their properties, when a single-electron in a metal heads to a paired state in a superconductor in a junction structure, the electron has to make a decision from the following two options: 1) to remain itself, but it will be reflected back to the metal; 2) to respect the custom, thus it will be paired with another electron with the opposite spin and enter the superconductor. In the first scenario, without getting into the superconductor, the reflected single-electron will not contribute to the conduction across the interface. In the second scenario, however, the conduction will be doubled as a paired state enters the superconductor. To retain the conservation of charge, spin, and momentum in the second scenario, a hole with the opposite spin will be created and reflected back into the metal, known as Andreev reflection. Under certain circumstances, spin freedom of electrons may also be an important factor in the decision-making process, which further allows the study of the spin-polarization of the electrons. In this talk, research examples using this unique spectroscopy tool to study the electronic states in iron-based superconductors and magnetic materials will also be provided.

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Thursday, March 28

Tunable Microwave Response and Other Emergent Magnetic Properties in Hybrid Nanostructures

Hari Srikanth, University of South Florida

Surface and interface effects in oxide nanoparticle assemblies have been increasingly found to play significant roles in controlling the magnetic properties. Modification of the surface spin structure in magnetic oxide nanoparticles can be achieved by controlling the particle shapes and forming hybrid structures. We discuss how these effects often lead to novel magnetic properties, useful for applications, such as tunable exchange bias (EB) and enhanced magnetocaloric effect (MCE). Exchange bias (EB)-like behavior in magnetic nanoparticles has been observed and reported in a number of systems. However the origin is not well understood and the results have often been misinterpreted in numerous reports in the literature. We have recently done systematic experiments to investigate these intriguing phenomena using a range of probes such as DC and AC magnetometry, RF transverse susceptibility, magnetocaloric effect and small angle neutron scattering (SANS). In this talk we will emphasize the need for systematic experimental studies to understand the origin and physics of magnetism in nanostructures and the correlation between surface anisotropy, freezing of surface and core spins with exchange bias. We will also present our recent advances in development of magnetic polymer nanocomposites that exhibit tunable microwave response. Performance of microstrip patch antennas fabricated with these nanocomposites will be demonstrated.

Research supported by Department of Energy and the Army Research Office.

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Friday, March 29

Single Phase Magnetoelectric Multi-Ferroics: From Mechanism to Applications

Xiaoshan Xu, Bryn Mawr

In the quest for new types of information processing and storage, complex oxides stand out as one of the most promising material classes. Multiple functionalities of complex oxides naturally arise from the delicate energy balance between the various forms of orders (structural, electronic, magnetic). In particular, multiferroic oxides which simultaneously exhibit more than one type of ferroic order have many advantages over other existing materials. Widespread practical applications will require a multiferroic material with a transition temperature that lies considerably above room temperature, large electric and magnetic polarizations, and strong coupling between ferroic orders, which unfortunately has not been realized in a single phase material. In this talk, I will give a brief review of the concept, known mechanism and application of multiferroics in the context of single phase complex oxides. I will also discuss the research on multiferroic materials including 1) elucidating the underlying mechanism of multiferroicity; 2) tuning known multiferroic materials to achieve desirable properties; 3) exploring applications of multiferroic materials by taking advantage of the multiferroic properties and interfaces with other materials; 4) discovering new multiferroic materials. Two prototypical multiferroic materials, LuFe2O4 and hexagonal LuFeO3 will be employed as examples.

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Monday, April 1

Magnetic Field Tuned Quantum Criticality of Heavy Fermion System YbPtBi

Eundeok Mun, National High Magnetic Field Laboratory, Los Alamos

Intensive study of strongly correlated electronic systems has revealed the existence of quantum phase transitions from ordered states to disordered states driven by non-thermal control parameters such as chemical doping, pressure, and magnetic field. Heavy fermion (HF) compounds provide the cleanest evidence for the quantum phase transition. In this talk, I will present the systematic measurements of the temperature and magnetic field dependences of the thermodynamic and transport properties of the Yb-based HF YbPtBi for temperatures down to 20 mK with magnetic fields up to 140 kOe to address the possible existence of a field-tuned quantum critical point. The face-centered cubic (fcc) YbPtBi is one of the few stoichiometric Yb-based HF compounds. An enormous low temperature Sommerfeld coefficient, g ∼ 8 J/mol K2, which corresponds to one of highest effective mass values among heavy fermion systems, is a characteristic of YbPtBi. This system manifests antiferromagnetic (AFM) ordering below TN = 0.4 K, below the estimated Kondo temperature of TK ∼ 1 K. The results of electrical resistivity and specific heat measurements suggested that a spin density wave (SDW) transition occurs below TN with small ordered moment of only ∼ 0.1mB. Measurements of magnetic field and temperature dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the AFM order can be suppressed by applied magnetic field of Hc ∼ 4 kOe. In the H-T phase diagram of YbPtBi, three regimes of its low temperature states emerge: (I) AFM state, characterized by SDW-like feature, which can be suppressed to T = 0 by the relatively small magnetic field of Hc ∼ 4 kOe, (II) field induced anomalous state in which the electrical resistivity follows r(T) ∼ T1.5 between Hc and ∼ 8 kOe, and (III) Fermi liquid (FL) state in which r(T) ∼ T2 for H > 8 kOe. Regions I and II are separated at T = 0 by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4ƒ states of Yb, region II may be a manifestation of a spin liquid state.

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Thursday, April 11

Five Decades of Lasers, Six Decades of Progress, and a Proposed Space Experiment to Test Einstein’s Assumptions

John Hall, JILA

Even though this is the 52nd year of the Laser, progress in its control and application in precision measurements is still accelerating. The Optical Frequency Comb technology exploded in 1999-2000 from the synthesis of advances in independent fields of Laser Stabilization, UltraFast Lasers, and NonLinear Optical Fibers, enabling a thousand-fold advance in optical frequency measurement, and searches (in the 17th digit) for time-variation of physical "constants." Current advances in ultra-precise locking are making possible stable optical frequencies defined by length and the speed of light, as well as by locking lasers to the resonant frequency of atoms. These two “clocks” represent our current prototypes of the clocks postulated by Einstein in 1905 in formulating the theory of Special Relativity, which can now be tested into the 18thdecimal in a proposed Space-based experiment now being planned by our Space-Time Asymmetry Research collaboration (STAR). An improvement in the modulation strategy may allow unexpectedly good frequency standard performance in a compact device, and so be useful on earth as well.

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Friday, April 19

Solar System Archaeology: What we Learn from Small Bodies in our Solar System

Susan Benecchi, Carnegie Institution

Since the discovery of the first Kuiper Belt object (KBO) in 1992 these objects have become key components to understanding the outer regions of our Solar System. Observations of both the dynamical and surface properties of these objects provide insight to the migration history of the giant planets. I will discuss various observational strategies for discovering and dynamically classifying KBOs and summarize our current understanding of the overall structure of the belt. Additionally, I will present the results from a compilation of studies on the colors (photometric properties), lightcurves and binary properties of sizable samples of KBOs in a variety of locations within the belt. Links between the dynamical and photometric properties of these objects may help to distinguish between various source populations and the range of conditions present in the protoplanetary disk.

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Friday, April 19

Visualization and Manipulation of Polarization and Screen Charges

Seungbum Hong, Argonne National Laboratory

Ferroelectric materials possess spontaneous polarization – net electric dipole moment per unit volume, of which magnitude and direction determine the surface charge density, and of which direction can be switched by electric field larger than a threshold called coercive field. As polycrystalline materials have grains with different crystallographic orientations and various grain boundaries dividing those grains, ferroelectric materials usually form domains with different polarizations and various domain boundaries dividing those domains. As such, ferroelectric domain structure and its dynamic behavior determine their macroscopic electric and piezoelectric properties. Furthermore, electric charges in various forms such as charged defects, electrons and ions interact with ferroelectric domains and their boundaries to influence the stability of domains and mobility of each domain boundary. Here I will present our efforts to develop angle-resolved piezoresponse force microscopy (AR-PFM) to visualize ferroelectric domains with polarization variants in 3D and scanning resistive probe microscopy (SRPM) to map the surface charge density (∼0.8 μC/cm2) with spatial resolution of 25 nm and temporal resolution of 125 μs, both of which are based on the system of atomic force microscopy (AFM). Furthermore, I will demonstrate how we could address the origin of domain structure showing polarization variants deviating from ferroelectric easy axes and electrostatically unstable charged domain boundaries in conjunction with the crystal nucleation and growth model. Lastly, the impacts of such domain structures on the local polarization switching behaviors will be discussed with the future implications for energy harvesting and memory devices.

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Thursday, April 25

Why are Ultrathin Films of Metallic Oxides Nonmetallic?

Jiandi Zhang, Louisiana State University

It has been discovered that many collective phenomena such as high-temperature superconductivity, "colossal" magnetoresistance, and quantum criticality, which do not appear in simple semiconductors, are emergent in complex correlated electron materials (CMEs). Even more surprisingly, many fascinating properties emerge at surfaces, interfaces, and artificial heterostructures of CEMs, the materials beyond mother nature. "The challenge is to understand how such collective phenomena emerge, discover new ones, and to determine which microscopic details are important and essential."

In contrast with the metallic or even superconducting phenomenon emerging at the interface of two insulating oxides such as LaAlO3/SrTiO3, several ultrathin films of metallic oxides exhibit nonmetallic behavior, challenging our understanding of these materials at interface and possible technological application. For such "dead layer" phenomena, the central question is: is this an intrinsic effect caused by dimensional confinement, or caused by strain, interface, segregation, impurity, or stoichiometry? We have systematically studied the thickness-dependence of structure/properties for La2/3Sr1/3MnO3 (LSMO) on SrTiO3(001) by using in-situ growth of laser MBE and characterization such as LEED, XPS and STM, and ex-situ transport measurements. With optimized growth conditions to minimize the oxygen deficiency (oxygen non-stoichiometry), we were able to focus on the other effects associated with the dead layer. In this talk, I will summarize our recent results on the dead layer of LSMO by showing that the dimensionality/structure effects play key role in determining the dead layer. With this optimized quality of ultrathin films, new critical behaviors emerge, such as the non-monotonic structure relaxation with thickness, the enhanced magnetoresistance effect and extreme sensitivity to strain at the critical thickness at ∼ 6 unit cells. These behaviors are proposed to correlate with subtle balance of different competing effects.

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