Physics Commons^™

An Efficiently Excited Eu3+ Luminescent Site Formed In Eu,O-Codoped Gan, Takenori Iwaya, Shuhei Ichikawa, Volkmar Dierolf, Brandon Mitchell, Hayley Austin, Dolf Timmerman, Jun Tatebayashi, Yasufumi Fujiwara

Physics & Engineering Faculty Publications

For the development of III-nitride-semiconductor-based monolithic micro-light-emitting diode (LED) displays, Eu,O-codoped GaN (GaN:Eu,O) is a promising material candidate for the red LEDs. The luminescence efficiency of Eu-related emission strongly depends on the local atomic structure of Eu ions. Our previous research has revealed that post-growth thermal annealing is an effective method for reconfiguring luminescent sites, leading to a significant increase in light output. We observed the preferential formation of a site with a peak at similar to 2.004 eV by the annealing process. In this study, we demonstrate that it is a previously unidentified independent site (OMVPE-X) using combined excitation-emission …

Ambipolar Ferromagnetism By Electrostatic Doping Of A Manganite, L. M. Zheng, X. Renshaw Wang, W. M. Lü, C. J. Li, Tula R. Paudel, Z. Q. Liu, Z. Huang, S. W. Zeng, Kun Han, Z. H. Chen, X. P. Qiu, M. S. Li, Shize Yang, B. Yang, Matthew F. Chisholm, L. W. Martin, S. J. Pennycook, Evgeny Y. Tsymbal, J. M. D. Coey, W. W. Cao

Evgeny Tsymbal Publications

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO₃, with electron–hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO₃ film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this …

Resonant Tunneling Across A Ferroelectric Domain Wall, M. Li, L. L. Tao, J. P. Velev, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Motivated by recent experimental observations, we explore electron transport properties of a ferroelectric tunnel junction (FTJ) with an embedded head-to-head ferroelectric domain wall, using first-principles density-functional theory calculations. We consider a FTJ with La_0.5Sr_0.5MnO₃ electrodes separated by a BaTiO₃ barrier layer and show that an in-plane charged domain wall in the ferroelectric BaTiO₃ can be induced by polar interfaces. The resulting V-shaped electrostatic potential profile across the BaTiO₃ layer creates a quantum well and leads to the formation of a two-dimensional electron gas, which stabilizes the domain wall. The confined electronic states …

Depairing Current Density In Nbtin Superconducting Films, Nahid Shayesteh Swails

Theses and Dissertations

In this work, we systematically investigate the superconducting properties of Niobium Titanium Nitride (NbTiN) superconducting films with different geometries, which at low temperature remain superconducting. NbT iN superconducting films with a few nm thickness are widely used in devices such as superconductor-insulatorsuperconductor (SIS) mixers, superconducting cavities and resonators and superconducting nanowire single-photon detectors (SNSPD). In all these applications, films of varying dimensions are required to achieve optimal performance. The initial thickness of our samples was 125 nm deposited on a silicon substrate. We change the thickness of our samples using lithography and ion milling. We measure and analyze the thickness …

Surface Impedance And Optimum Surface Resistance Of A Superconductor With An Imperfect Surface, Alex Gurevich, Takayuki Kubo

Physics Faculty Publications

We calculate a low-frequency surface impedance of a dirty, s-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities, or nonstoichiometric composition. We solved the Usadel equations self-consistently and obtained spatial distributions of the order parameter and the quasiparticle density of states which then were used to calculate a low-frequency surface resistance R_s (T) and the magnetic penetration depth λ(T) as functions of temperature in the limit of local London electrodynamics. It …

Reduced Dimensionality Effects In Gd-Based Magnetocaloric Materials, Hillary Faith Belliveau

USF Tampa Graduate Theses and Dissertations

Magnetic refrigeration based on the magnetocaloric effect (MCE) is a promising alternative to conventional gas compression based cooling techniques. Understanding impacts of reduced dimensionality on the magnetocaloric response of a material such as Gadolinium (Gd) or its alloys is essential in optimizing the performance of cooling devices, which is also the overall goal of this thesis. We have determined, in the first part of the thesis, that laminate structures of pure Gd produced by magnetron sputtering have several disadvantages. The target material (pure Gd), ultra-high vacuum components, and the electrical energy it takes to run the manufacturing process are all …

The Role Of Partial Surface Charge Compensation In The Properties Of Ferroelectric And Antiferroelectric Thin Films, Elena Glazkova

USF Tampa Graduate Theses and Dissertations

Ferroelectric and antiferroelectric ultrathin films have attracted a lot of attention recently due to their remarkable properties and their potential to allow for device miniaturization in numerous applications. However, when the ferroelectric films are scaled down, it brings about an unavoidable depolarizing field. A partial surface charge compensation allows to control the residual depolarizing field and manipulate the properties of ultrathin ferroelectric films. In this dissertation we take advantage of atomistic first-principles-based simulations to expand our understanding of the role of the partial surface charge compensation in the properties of ferroelectric and antiferroelectric ultrathin films.

The application of our computational …

Crystallographically-Oriented Carbon Nanotubes Grown On Few-Layer Graphene Films, Douglas R. Strachan, David P. Hunley

Physics and Astronomy Faculty Patents

A thermal and electrical conducting apparatus includes a few-layer graphene film having a thickness D where D≦1.5 nm and a plurality of carbon nanotubes crystallographically aligned with the few-layer graphene film.

Crystallographically-Oriented Carbon Nanotubes Grown On Few-Layer Graphene Films, David P. Hunley, Douglas R. Strachan

Physics and Astronomy Faculty Patents

A thermal and electrical conducting apparatus includes a few-layer graphene film having a thickness D where D≦1.5 nm and a plurality of carbon nanotubes crystallographically aligned with the few-layer graphene film.

Giant Enhancement Of Magnetic Anisotropy In Ultrathin Manganite Films Via Nanoscale 1d Periodic Depth Modulation, Anil Rajapitamahuni, L. Zhang, Mark A. Koten, V. R. Singh, John D. Burton, Evgeny Y. Tsymbal, Jeffrey E. Shield, Xia Hong

Evgeny Tsymbal Publications

The relatively low magnetocrystalline anisotropy (MCA) in strongly correlated manganites (La,Sr)MnO₃ has been a major hurdle for implementing them in spintronic applications. Here we report an unusual, giant enhancement of in-plane MCA in 6 nm La_0.67Sr_0.33MnO₃ (LSMO) films grown on (001) SrTiO₃ substrates when the top 2 nm is patterned into periodic stripes of 100 or 200 nm width. Planar Hall effect measurements reveal an emergent uniaxial anisotropy superimposed on one of the original biaxial easy axes for unpatterned LSMO along (110) directions, with a 50-fold enhanced anisotropy energy density of 5.6 × …

Seebeck Coefficient Enhancement Of Ald Pbte/Pbse Nanolaminate Structures Deposited Inside Porous Silicon Templates, Xin Chen, Pengtao Lin, Kai Zhang, Helmut Baumgart, Brian Geist, Vladimir Kochergin

Electrical & Computer Engineering Faculty Publications

In this paper, lead chalcogenide based thermoelectric nanolaminate structures were fabricated by alternately depositing PbTe and PbSe ALD layers on regular planar silicon wafers and on microporous silicon templates. Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C₁₁H₁₉O₂) ₂), plus (trimethylsilyl) telluride ((Me₃Si)₂Te) and (trimethylsilyl) selenide ((Me₃Si)₂Se) were used as the chemical ALD precursors for lead, tellurium and selenium, respectively. The Seebeck coefficient in horizontal direction (parallel direction to the surface) to the multiple layered PbTe/PbSe nanolaminate structures was measured by an MMR Seebeck system, and benchmarked against the Seebeck coefficient …

Probing Dynamics And Pinning Of Single Vortices In Superconductors At Nanometer Scales, L. Embon, Y. Anahory, A Suhov, D. Halbertal, J. Cuppens, A. Yakovenko, A. Uri, Y. Myasoedov, M. L. Rappaport, M. E. Huber, A. Gurevich, E. Zeldov

Physics Faculty Publications

The dynamics of quantized magnetic vortices and their pinning by materials defects determine electromagnetic properties of superconductors, particularly their ability to carry non-dissipative currents. Despite recent advances in the understanding of the complex physics of vortex matter, the behavior of vortices driven by current through a multi-scale potential of the actual materials defects is still not well understood, mostly due to the scarcity of appropriate experimental tools capable of tracing vortex trajectories on nanometer scales. Using a novel scanning superconducting quantum interference microscope we report here an investigation of controlled dynamics of vortices in lead films with sub-Angstrom spatial resolution …

Chemically Induced Jahn–Teller Ordering On Manganite Surfaces, Zheng Gai, Wenzhi Lin, John D. Burton, K. Fuchigami, Paul C. Snijders, T. Z. Ward, Evgeny Y. Tsymbal, J. Shen, Stephen Jesse, Sergei V. Kalinin, Arthur P. Baddorf

Evgeny Tsymbal Publications

Physical and electrochemical phenomena at the surfaces of transition metal oxides and their coupling to local functionality remains one of the enigmas of condensed matter physics. Understanding the emergent physical phenomena at surfaces requires the capability to probe the local composition, map order parameter fields and establish their coupling to electronic properties. Here we demonstrate that measuring the sub-30-pm displacements of atoms from high-symmetry positions in the atomically resolved scanning tunnelling microscopy allows the physical order parameter fields to be visualized in real space on the single-atom level. Here, this local crystallographic analysis is applied to the in-situ-grown manganite surfaces. …

Orientation And Surface Anchoring Of Nematic Liquid Crystals On Linearly Polymerized Photopolymers, H. G. Galabova, David W. Allender, J. Chen

David W Allender

The liquid crystal orientation on a poly(vinyl cinnamate) or poly(vinyl 4-methoxy-cinnamate) coated solid substrate exposed to linearly polarized UV light has been studied. The angular distribution of the polymer side chains and the photoreaction products was derived as a function of exposure time using a simple assumption about the probability for crosslinking of two side chains. The liquid crystal was assumed to be weakly anchored to the surface but have a degree of order and orientation determined from those of the polymer side chains. The polar and the azimuthal surface coefficients were estimated as a function of exposure time using …

A Cellulose Liquid Crystal Motor: A Steam Engine Of The Second Kind, Yong Geng, Pedro Lucio Almeida, Cheng Cheng, Peter Palffy-Muhoray, Maria Helena Godinho, Susete Nogueira Fernandes

Peter Palffy-Muhoray

The salient feature of liquid crystal elastomers and networks is strong coupling between orientational order and mechanical strain. Orientational order can be changed by a wide variety of stimuli, including the presence of moisture. Changes in the orientation of constituents give rise to stresses and strains, which result in changes in sample shape. We have utilized this effect to build soft cellulose-based motor driven by humidity. The motor consists of a circular loop of cellulose film, which passes over two wheels. When humid air is present near one of the wheels on one side of the film, with drier air …

Effect Of Director Distortions On Morphologies Of Phase Separation In Liquid Crystals, D. Voloschenko, Oleg P. Pishnyak, Sergij V. Shiyanovskii, Oleg Lavrentovich

Oleg Lavrentovich

We study phase separation from a nematic liquid crystal with spatially nonuniform director gradients. Particles of a phase-separated component, which is either an isotropic fluid (silicone oil) or a nonmesogenic photopolymer, accumulate in the regions with the strongest director distortions, thus reducing the overall energy of the system.

Anchoring-Mediated Interaction Of Edge Dislocations With Bounding Surfaces In Confined Cholesteric Liquid Crystals, Ivan I. Smalyukh, Oleg Lavrentovich

Oleg Lavrentovich

We employ the fluorescent confocal polarizing microscopy to image edge dislocations in cholesteric liquid crystals. Surface anchoring at the bounding plates determines the structure and behavior of defects. Two types of plates set in-plane director orientation but differ in the type of associated anchoring potentials. Plates with strong polar anchoring and nonzero azimuthal anchoring repel the dislocations, while plates with weak polar anchoring and no azimuthal anchoring allow the dislocations to escape through the boundary. To explain the results, we propose a coarse-grained model of cholesteric anchoring.

Polarization-Controlled Ohmic To Schottky Transition At A Metal/Ferroelectric Interface, Xiaohui Liu, Yong Wang, John D. Burton, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Ferroelectric polar displacements have recently been observed in conducting electron-doped BaTiO₃ (n-BTO). The coexistence of a ferroelectric phase and conductivity opens the door to new functionalities that may provide a unique route for novel device applications. Using first-principles methods and electrostatic modeling, we explore the effect that the switchable polarization of n-BTO has on the electronic properties of the SrRuO₃/n-BTO (001) interface. Ferroelectric polarization controls the accumulation or depletion of electron charge at the interface, and the associated bending of the n-BTO conduction band determines the transport regime across the interface. The …

Light-Induced Changes Of Optical And Electrical Properties In Bent-Core Azo Compounds, Antal Jakli, V. Prasad, D.S.S. Rao, Guangxun Liao, I. Janossy

Antal Jakli

We have studied the optical and electrical properties of two bent-core substances with an azo linkage in their cores. Pump-probe laser studies, direct textural observations, and spectrophotometric recordings show an initial decrease of light transmission, which at larger light intensities (similar to 1 mW/mm(2)) is followed by a bleaching. Simultaneously the electrical properties (electric conductivity, antiferroelectric polarization, switching threshold, and switching time) decreased monotonically with increasing light intensities. The monotonic decrease of electrical properties indicates that the darkening and bleaching have the same origin, namely, the photochemical isomerization of the azo linkage from the trans to the cis isomer. The …

Optical Studies Of The Liquid Crystal B2 Phase Formed By Banana-Shaped Molecules, D. A. Olson, M. Veum, A. Cady, M. V. D'Agostino, P. M. Johnson, H. T. Nguyen, Liang-Chy Chien, C. C. Huang

Liang-Chy Chien

Null-transmission ellipsometry and depolarized light microscopy have been performed on free-standing films of three achiral banana-shaped compounds in the B2 phase. Our results support a two-layer unit cell previously proposed to explain the observed antiferroelectricity in thin films and bulk samples. We have studied thicker films than previously reported and have found no deviations in the film structure from the earlier findings. Moreover, we can determine the layer spacing, the molecular tilt from the layer normal, and the three principal indices of refraction in the molecular reference frame.

Light Induced Structures In Liquid Crystalline Side-Chain Polymers With Azobenzene Functional Groups, O. Yaroschuk, Tatiana Sergan, J. Lindau, S. Lee, J. Kelly, Liang-Chy Chien

Liang-Chy Chien

We applied ellipsometry to study the distribution of azobenzene fragments in the films of two types of comblike polymers with azobenzene moieties in the side chains before and after ultraviolet ~UV! light irradiation. The polymer with an alkyl chain at the end of the azobenzene fragment forms structures with preferred homeotropic alignment of the fragments. Irradiation of this polymer with nonpolarized UV light at normal incidence induces a homeotropic alignment of azobenzene fragments. Oblique irradiation induces tilted structures. Polarized UV light irradiation at normal beam incidence induces biaxial structures with a fanlike distribution of azobenzene fragments and preferably out-of-plane alignment. …

Photoinduced Three-Dimensional Orientational Order In Side Chain Liquid Crystalline Azopolymers, O. Yaroshchuk, K. Kiselev, Yu. Zakrevskyy, T. Bidna, J. Kelly, Liang-Chy Chien, J. Lindau

Liang-Chy Chien

We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced three-dimensional (3D) orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, respectively. Using irradiation with the polarized light of two different wavelengths, we find that the kinetics of photoinduced anisotropy can be dominated by …

Material-Independent Determination Of Anchoring Properties On Rubbed Polyimide Surfaces, Bharat R. Acharya, Jae-Hoon Kim, Satyendra Kumar

Satyendra Kumar

A material-independent method for determining liquid-crystal (LC) anchoring energy on rubbed polyimide (PI) surfaces has been devised. This method exploits the changes in the easy axis of rubbed PI film induced by exposure to linearly polarized UV (LPUV) Light. The distribution of PI chains in a rubbed film is approximated by a Gaussian function and its width determined from the measured rotation of the LC easy axis as a function of exposure time. A quasimicroscopic free energy of the LC-substrate interface is used to model LC anchoring properties. The experimental and calculated values of the azimuthal anchoring energy are in …

Surface-Induced Discrete Smectic Order In The Isotropic-Phase Of 12cb In Cylindrical Pores, Germano S. Iannacchione, Joseph T. Mang, Satyendra Kumar, Daniele Finotello

Satyendra Kumar

No abstract provided.

Surface-Layer Phase-Transitions In Nematic Liquid-Crystals, Natasha Kothekar, David W. Allender, R. M. Hornreich

David W Allender

Surface-layer transitions in nematogenic materials characterized by a preferential planar surface interaction linear in the order parameter have been studied theoretically at temperatures above the bulk transition (T(NI)). The coupled Euler-Lagrange nonlinear differential equations obtained from the Landau-de Gennes free energy were solved exactly by numerical integration. This problem had been studied previously employing various limits and approximations with several differences in the phase diagram reported. The exact results allow one to determine which of these differences are artifacts of the approximations used and which are dependent upon the ratio of elastic constants. It is found, for physically relevant elastic …

Theory Of A Symmetry-Breaking Phase-Transition In A Surface-Layer Of Nematic Liquid-Crystal, Y. L'Vov, R. M. Hornreich, David W. Allender

David W Allender

For a surface interaction linear in the order parameter and favoring an orientation in which liquid-crystalline molecules lie parallel to the surface (but which is independent of whether the orientation is uniaxial or isotropic in the bounding plane), a symmetry-breaking phase transition in a surface layer is possible at temperatures above that of the bulk isotropic to nematic transition. At the surface transition, the high-temperature in-plane isotropic state becomes unstable with respect to a biaxial phase, the transition being continuous and therefore of-the Berezinskii-Kosterlitz-Thouless (BKT) type, It is expected to occur for intermediate surface couplings but not at very weak …

Synthesis Of Al2tio5 And Its Effect On The Properties Of Chitosan-Nh4scn Polymer Electrolytes, Siti Rohana Majid, Hassan F., Woo H. J., Aziz N. A., Kufian M. Z.

Siti Rohana Majid

In this paper, Al2TiO5 ceramic material has been synthesised and used as filler in polymer electrolyte system to enhance the conductivity. The precursor sintered at 1,050 A degrees C and contained 0.08 mole of aluminium nitrate gives the best and complete formation of Al2TiO5. Composite polymer electrolytes of chitosan-NH4SCN containing different amount of home-made Al2TiO5 were prepared by solution casting. The addition of filler has enhanced the conductivity of polymer electrolyte. The sample 57 wt% chitosan-38 wt% NH4SCN-5 wt% Al2TiO5 exhibited the highest electrical conductivity of 2.10 x 10(-4) S cm(-1) at room temperature. The presence of the Al2TiO5 creates …

Graphene Oxide And Its Electrochemical Performance, Siti Rohana Majid, Ban F. Y., Huang N. M., Lim H. N.

Siti Rohana Majid

In this study, graphene oxide (GO) was synthesized from graphite flakes using simplified Hummer's method. Field Emission Scanning Electron Microscopy (FESEM) image showed that the GO nanosheets had an average area 7000 mu m(2) with lateral dimension of up to 150 mu m. The X-Ray Diffraction (XRD) pattern revealed a (002) diffraction peak, signifying the successful synthesis of GO. GO solution was cast on an aluminum (Al) foil placed in a petri dish and left to dry to form an electrode made up of GO film on the Al foil (GO-Al). It was found that GO-Al exhibited equivalent series resistance …

Characteristics Of Methyl Cellulose-Nh4no3-Peg Electrolyte And Application In Fuel Cells, Siti Rohana Majid, Shuhaimi N. E. A., Alias N. A., Kufian M. Z., Arof A. K.

Siti Rohana Majid

We report the viability of methyl cellulose (MC) as a membrane in a polymer electrolyte membrane fuel cell (PEMFC). Methyl cellulose serves as the polymer host, ammonium nitrate (NH4NO3) as the doping salt and poly(ethylene glycol) (PEG) as plasticizer. Conductivity measurement was carried out using electrochemical impedance spectroscopy. The room temperature conductivity of pure MC film is is (3.08 +/- 0.63) x 10(-11)S cm(-1). The conductivity increased to (2.10 +/- 0.37) x 10(-6)S cm(-1) on addition of 25 wt.% NH4NO3. By adding 15 wt.% of PEG 200 to the highest conducting sample in the MC-NH4NO3 system, the conductivity was further …

Characteristics Of Methyl Cellulose-Nh4no3-Peg Electrolyte And Application In Fuel Cells, Siti Rohana Majid, Shuhaimi N. E. A., Alias N. A., Kufian M. Z., Arof A. K.

Siti Rohana Majid

We report the viability of methyl cellulose (MC) as a membrane in a polymer electrolyte membrane fuel cell (PEMFC). Methyl cellulose serves as the polymer host, ammonium nitrate (NH4NO3) as the doping salt and poly(ethylene glycol) (PEG) as plasticizer. Conductivity measurement was carried out using electrochemical impedance spectroscopy. The room temperature conductivity of pure MC film is is (3.08 +/- 0.63) x 10(-11)S cm(-1). The conductivity increased to (2.10 +/- 0.37) x 10(-6)S cm(-1) on addition of 25 wt.% NH4NO3. By adding 15 wt.% of PEG 200 to the highest conducting sample in the MC-NH4NO3 system, the conductivity was further …