Condensed Matter Physics Commons^™

Evolution Of Magnetism In The Single-Crystal Honeycomb Iridates (Na1−XLiX)2Iro3, Gang Cao, Tongfei Qi, L. Li, Jsaminka Terzic, Vincent Shian Cao, Shujuan Yuan, M. Tovar, Ganpathy Murthy, Ribhu K. Kaul

Physics and Astronomy Faculty Publications

We report the successful synthesis of single crystals of the layered iridate (Na_1−xLi_x)₂IrO₃, 0 ≤ x ≤ 0.9, and a thorough study of its structural, magnetic, thermal, and transport properties. This compound allows a controlled interpolation between Na₂IrO₃ and Li₂IrO₃, while maintaining the quantum magnetism of the honeycomb Ir⁴⁺ planes. The measured phase diagram demonstrates a suppression of the Néel temperature T_N at an intermediate x, indicating that the magnetic orders in Na₂IrO₃ and Li₂IrO_{3 …}

Polarization-Coupled Transport Behavior In Ultrathin Ferroelectric Heterostructures, Haidong Lu

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Ferroelectric polarization-coupled resistive switching behavior in ferroelectric tunnel junctions (FTJs), the tunneling electroresistance (TER) effect, is a recently predicted new phenomenon, which attracts interest due to potential application in the next generation non-volatile ferroelectric random access memories (FeRAMs). In this dissertation, we demonstrate the TER effect in FTJ devices by means of scanning probe microscopy (SPM) techniques. We have investigated several device configurations for enhancement of polarization stability and for demonstration of the resistive switching behavior: (i) using the SPM probe as a top electrode; (ii) using heterostructures with engineered interfacial atomic terminations; (iii) using metal electrodes; (iv) adding an …

Magnetic Anisotropy And Exchange In (001) Textured Fept-Based Nanostructures, Tom George

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Hard-magnetic L1₀ phase FePt has been demonstrated as a promising candidate for future nanomagnetic applications, especially magnetic recording at areal densities approaching 10 Tb/in². Realization of FePt’s potential in recording media requires control of grain size and intergranular exchange interactions in films with high degrees of L1₀ order and (001) crystalline texture, including high perpendicular magnetic anisotropy. Furthermore, a write-assist mechanism must be employed to overcome the high coercivity of L1₀ FePt nanograins. The research described in this dissertation examines potential solutions to the aforementioned problems. Specifically, a nonepitaxial method of fabricating highly (001) textured …

Interface States In Cofe2o4 Spin-Filter Tunnel Junctions, Pavel V. Lukashev, John D. Burton, Alexander Smogunov, Julian P. Velev, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Spin-filter tunneling is a promising way to generate highly spin-polarized current, a key component for spintronics applications. In this paper we explore the tunneling conductance across the spin-filter material CoFe₂O₄ interfaced with Au electrodes, a geometry which provides nearly perfect lattice matching at the CoFe₂O₄/Au(001) interface. Using density functional theory calculations we demonstrate that interface states play a decisive role in controlling the transport spin polarization in this tunnel junction. For a realistic CoFe₂O₄ barrier thickness, we predict a tunneling spin polarization of about −60%. We show that this value …

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 …

Imaging Molecular Structures With Atomic Force Microscopy, Unurbat Erdenemunkh, Tyler Flanagan

Student Works

As part of the LEEP project we repaired the Atomic Force Microscopy (AFM), and wrote a user manual on using AFM. Then we scanned Self Assembly of Copolymer Films collaboration with Prof. Sergio Granados –Focil and Copper Oxide Nanocubes with Professor Prof.Luis Smith.

Characterization Of Polylactic Acid Films For Food Packaging As Affected By Dielectric Barrier Discharge Atmospheric Plasma, Shashi Pankaj, Luke O'Neill, N. Misra, Paula Bourke

Articles

Dielectric barrier discharge (DBD) air plasma is a novel technique for in-package decontamination of food, but it has not been yet applied to the packaging material. Characterization of commercial polylactic acid (PLA) films was done after in-package DBD plasma treatment at different voltage and treatment time to evaluate its suitability as food packaging material. DBD plasma increased the roughness of PLA film mainly at the site in contact with high voltage electrode at both the voltage levels of 70 and 80 kV. DBD plasma treatments did not induce any change in the glass transition temperature, but significant increase in the …

Wave Function For Time-Dependent Harmonically Confined Electrons In A Time-Dependent Electric Field, Yu-Qi Li, Xiao-Yin Pan, Viraht Sahni

Publications and Research

The many-body wave function of a system of interacting particles confined by a time-dependent harmonic potential and perturbed by a time-dependent spatially homogeneous electric field is derived via the Feynman path-integral method. The wave function is comprised of a phase factor times the solution to the unperturbed time-dependent Schrödinger equation with the latter being translated by a time-dependent value that satisfies the classical driven equation of motion. The wave function reduces to that of the Harmonic Potential Theorem wave function for the case of the time-independent harmonic confining potential.

Polarization Discontinuity Induced Two-Dimensional Electron Gas At Zno/Zn(Mg)O Interfaces: A First-Principles Study, Jesuan Betancourt, J. J. Saavedra-Arias, John D. Burton, Yasuyuki Ishikawa, Evgeny Y. Tsymbal, Julian P. Velev

Evgeny Tsymbal Publications

The discovery of a high-mobility two-dimensional electron gas (2DEG) in wurtzite ZnO/Zn(Mg)O heterostructures is promising for applications due to the high mobility of the carriers. In this paper, we study the formation and properties of the 2DEG at ZnO/Zn(Mg)O interfaces using first-principles calculations based on hybrid density functional theory. The 2DEG arises from the polarization discontinuity at the interface between the two materials. The uncompensated bound charge at the interface gives rise to an electric field in the bulk of ZnO which confines free carriers close to the interface. We find that the type of the confined carriers is determined …

Use Of Zwitterionic Molecules For Forming A Hole Or Electron Transport Layer, Bernard Doudin, Pierre Braunstein, Lucie Routaboul, Guillaume Dalmas, Zhengzheng Zhang, Peter Dowben

Peter Dowben Publications

The invention relates to the use of zwitterionic molecules for forming a hole or electron transport layer. The preferred zwitterionic molecules of the invention are derivatives of p-benzoquinonemonoimines. The invention is useful in the field of electronic devices in particular.

Ferroelectric And Dielectric Properties Of Electroactive Oligomers And Nanocomposites, Kristin Leigh Kraemer

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Polyvinylidene fluoride (PVDF) and its copolymers have been well established as ferroelectric polymers. The dielectric and ferroelectric properties for vinylidene fluoride (VDF) oligomer thin films were investigated. By synthesizing oligomers instead of long polymer chains, films with higher crystalinity can be formed and the locations of oligomers can be controlled for applications such as molecular electronics.

Evidence of ferroelectricity was observed in oligomer thin films evaporated onto room temperature substrates and by Langmuir-Blodgett (LB) deposition. Voltage and frequency dependence of the capacitance was measured. Oligomers functionalized with phosphonic acid formed self-assembled monolayers (SAM) on aluminum and mica substrates. Film thickness …

Magnetic Interactions In Low-Dimensional Iron Nanostructures, Rui Zhang

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Three systems involving low-dimensional magnetic nanostructures, namely the Kondo Effect in Isolated Cu(Fe) Clusters, Magnetization Reversal in Transition-Metal/Fe:SiO₂ Thin Films, and Anisotropy and Micromagnetism of Fe/CrPt Bilayers, have been investigated to understand the magnetic interactions in iron nanostructures.

Kondo Effect in Isolated Cu(Fe) Clusters —Iron impurities were added into copper clusters embedded in an insulating matrix to ensure that the Kondo effect is strictly confined by the size of the cluster. The Kondo temperature of our naoscale system is 0.7 K, which is greatly suppressed from its bulk value of 29 K and is consistent with our theory prediction. …

Anomalous And Spin Hall Effects In A Magnetic Tunnel Junction With Rashba Spin-Orbit Coupling, A. V. Vedyayev, M. S. Titova, N. V. Ryzhanova, M. Ye. Zhuravlev, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Anomalous and spin Hall effects are investigated theoretically for a magnetic tunnel junction where the applied voltage produces a Rashba spin-orbit coupling within the tunneling barrier layer. The ferromagnetic electrodes are the source of the spin-polarized current. The tunneling electrons experience a spin-orbit coupling inside the barrier due to the applied electrical field. Charge and spin Hall currents are calculated as functions of the position inside the barrier and the angle between the magnetizations of the electrodes. We find that both charge and spin Hall currents are located inside the barrier near the interfaces. The dependence of the currents on …

Tuning The JEff = 1/2 Insulating State Via Electron Doping And Pressure In The Double-Layered Iridate Sr3Ir2O7, L. Li, P. P. Kong, Tongfei Qi, C. Q. Jin, Shujuan Yuan, Lance E. Delong, P. Schlottmann, Gang Cao

Physics and Astronomy Faculty Publications

Sr₃Ir₂O₇ exhibits a novel J_eff = 1/2 insulating state that features a splitting between J_eff = 1/2 and 3/2 bands due to spin-orbit interaction. We report a metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La³⁺ for Sr²⁺) or application of high pressure up to 35 GPa. Our study of single-crystal Sr₃Ir₂O₇ and (Sr_1−xLa_x)₃Ir₂O₇ reveals that application of high hydrostatic pressure P leads to a drastic reduction in the electrical resistivity by as much …

Entropy Driven Crystal Formation On Highly Strained Substrates, John R. Savage, Stefan F. Hopp, Rajesh Ganapathy, Sharon J. Gerbode, Andreas Heuer, Itai Cohen

All HMC Faculty Publications and Research

In heteroepitaxy, lattice mismatch between the deposited material and the underlying surface strongly affects nucleation and growth processes. The effect of mismatch is well studied in atoms with growth kinetics typically dominated by bond formation with interaction lengths on the order of one lattice spacing. In contrast, less is understood about how mismatch affects crystallization of larger particles, such as globular proteins and nanoparticles, where interparticle interaction energies are often comparable to thermal fluctuations and are short ranged, extending only a fraction of the particle size. Here, using colloidal experiments and simulations, we find particles with short-range attractive interactions form …

Mechanical And Electrical Properties Of Single-Walled Carbon Nanotubes Synthesized By Chemical Vapor Deposition, Yuehai Yang

FIU Electronic Theses and Dissertations

Despite the tremendous application potentials of carbon nanotubes (CNTs) proposed by researchers in the last two decades, efficient experimental techniques and methods are still in need for controllable production of CNTs in large scale, and for conclusive characterizations of their properties in order to apply CNTs in high accuracy engineering. In this dissertation, horizontally well-aligned high quality single-walled carbon nanotubes (SWCNTs) have been successfully synthesized on St-cut quartz substrate by chemical vapor deposition (CVD). Effective radial moduli (E_radial) of these straight SWCNTs have been measured by using well-calibrated tapping mode and contact mode atomic force microscopy (AFM). It …

Electron Transport In Quantum Dot Chains: Dimensionality Effects And Hopping Conductance, V. P. Kunets, Mariama Rebello Sousa Dias, T. Rembert, M. E. Ware, Y. I. Mazur, V. Lopez-Richard, H. A. Mantooth, G. E. Marques, G. J. Salamo

Physics Faculty Publications

Detailed experimental and theoretical studies of lateral electron transport in a system of quantum dot chains demonstrate the complicated character of the conductance within the chain structure due to the interaction of conduction channels with different dimensionalities. The one-dimensional character of states in the wetting layer results in an anisotropic mobility, while the presence of the zero-dimensional states of the quantum dots leads to enhanced hopping conductance, which affects the low-temperature mobility and demonstrates an anisotropy in the conductance. These phenomena were probed by considering a one-dimensional model of hopping along with band filling effects. Differences between the model and …

Multiband S -Wave Topological Superconductors: Role Of Dimensionality And Magnetic Field Response, Shusa Deng, Gerardo Ortiz, Lorenza Viola

Dartmouth Scholarship

We further investigate a class of time-reversal-invariant two-band s-wave topological superconductors introduced earlier [Deng, Viola, and Ortiz, Phys. Rev. Lett. 108, 036803 (2012)]. Provided that a sign reversal between the two superconducting pairing gaps is realized, the topological phase diagram can be determined exactly (within mean field) in one and two dimensions as well as in three dimensions upon restricting to the excitation spectrum of time-reversal-invariant momentum modes. We show how, in the presence of time-reversal symmetry, Z2 invariants that distinguish between trivial and nontrivial quantum phases can be constructed by considering only one of the Kramers’ sectors …

Spin Switching And Magnetization Reversal In Single-Crystal Ndfeo3, Shujuan Yuan, W. Ren, F. Hong, Y. B. Wang, J. C. Zhang, L. Bellaiche, Gang Cao

Physics and Astronomy Faculty Publications

We report an experimental and computational study of single-crystal NdFeO₃, which features two inequivalent magnetic sublattices, namely, Fe and Nd sublattices that are coupled in an antiparallel fashion. This paper reveals that a strong interaction between 3d and 4f electrons of the two sublattices along with a spin-lattice coupling drives an extremely interesting magnetic state that is highly sensitive to the orientation and history of weak magnetic field. The following phenomena are particularly remarkable: (1) sharply contrasting magnetization M(T) along the a and c axes; (2) a first-order spin switching along the a …

Rapid Diffusion Of Electrons In Gamnas, Christopher P. Weber, Eric A. Kittlaus, Kassandra B. Mattia, Christopher J. Waight, J. Hagmann, X. Liu, M. Dobrowolska, J. K. Furdyna

Physics

We report ultrafast transient‐grating measurements, above and below the Curie temperature, of the dilute ferromagnetic semiconductor (Ga,Mn)As containing 6% Mn. At 80 K (15 K), we observe that photoexcited electrons in the conduction band have a lifetime of 8 ps (5 ps) and diffuse at about 70 cm²/s (60 cm²/s). Such rapid diffusion requires either an electronic mobility exceeding 7700 cm²/V s or a conduction‐band effective mass less than half the GaAs value. Our data suggest that neither the scattering rate nor the effective mass of the (Ga,Mn)As conduction band differs significantly from that …

Magnetic And Crystal Structures Of Sr2Iro4: A Neutron Diffraction Study, Feng Ye, Songxue Chi, Bryan C. Chakoumakos, Jaime A. Fernandez-Baca, Tongfei Qi, Gang Cao

Center for Advanced Materials Faculty Publications

We report a single-crystal neutron diffraction study of the layered Sr₂IrO₄. This work unambiguously determines the magnetic structure of the system and reveals that the spin orientation rigidly tracks the staggered rotation of the IrO₆ octahedra in Sr₂IrO₄. The long-range antiferromagnetic order has a canted spin configuration with an ordered moment of 0.208(3) μ_B/Ir site within the basal plane; a detailed examination of the spin canting yields 0.202(3) and 0.049(2) μ_B/site for the a axis and the b axis, respectively. It is intriguing that forbidden nuclear reflections …

Ab-Initio And Model Studies Of Spin Fluctuation Effects In Transport And Thermodynamics Of Magnetic Metals, James K. Glasbrenner

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Magnetic materials are vital to many devices and the manipulation of spins is central to the operation of novel devices such as spin transistors. It is important to understand the effect of spin fluctuations on such systems. In this dissertation, first-principles calculations and models further the understanding of spin fluctuation effects in the transport and thermodynamics of magnetic metals.

A simple classical spin-fluctuation Hamiltonian with a single itinerancy parameter is studied using the mean-field approximation, Monte Carlo simulations, and a generalized Onsager cavity field method. The results of these different methods are in agreement. It is found that the thermodynamics …

Hydroxyl-Decorated Graphene Systems As Candidates For Organic Metal-Free Ferroelectrics, Multiferroics, And High-Performance Proton Battery Cathode Materials, Menghao Wu, John D. Burton, Evgeny Y. Tsymbal, Xiao Cheng Zeng, Purusottam Jena

Evgeny Tsymbal Publications

Using a first-principles method we show that graphene based materials, functionalized with hydroxyl groups, constitute a class of multifunctional, lightweight, and nontoxic organic materials with functional properties such as ferroelectricity, multiferroicity, and can be used as proton battery cathode materials. For example, the polarizations of semihydroxylized graphane and graphone, as well as fully hydroxylized graphane, are much higher than any organic ferroelectric materials known to date. Further, hydroxylized graphene nanoribbons with proton vacancies at the end can have much larger dipole moments. They may also be applied as high-capacity cathode materials with a specific capacity that is six times larger …

Faraday Cup Designs For High Efficiency Determination Of Energy- And Angular-Resolved Charged Particle Fluxes, Kent D. Hartley

Senior Theses and Projects

Faraday cups provide a simple and efficient apparatus to measure the absolute magnitude of charge particle fluxes, and with the addition of a retarding field analyzer and defining apertures the capability to determine the energy and angular distributions of the fluxes. Through careful design of the electron optics, a Faraday cup can be tailored to meet specific requirements for detector size, minimum detectable flux, collection efficiency, absolute accuracy, energy discrimination, and angular resolution. This project explores optimal design concepts through electric field and charged particle trajectory simulations, theoretical analysis, and evaluation of experimental prototypes to develop compact, high efficiency Faraday …

Strain-Mediated Elastic Coupling In Magnetoelectric Nickel/Barium-Titanate Heterostructures, Robert Streubel, Denny Köhler, Rudolf Schäfer, Lukas M. Eng

Robert Streubel Papers

Multiferroic nanomaterials bear the potential for assembling a manifold of novel and smart devices. For room temperature (RT) applications, however, only the BiFeO3 single-phase perovskites are potential candidates to date. Nevertheless, vertical heterostructures separating magnetic and ferroelectric functionality into different layers are now widely proposed to circumvent this lack in materials' availability. We show here that the second approach is very profitable as illustrated by the strain-mediated coupling between such two layers, i.e., a ferroelectric barium titanate single-crystal (BTO) and a magnetostrictive nickel (Ni) thin film. Applying an electric field across the BTO substrate forces the magnetic easy axis in …

Diffusion Of Degenerate Minority Carrier In A P-Type Semiconductor, Christopher P. Weber, Eric A. Kittlaus

Physics

We report ultrafast transient-grating experiments on heavily p-type InP at 15 K. Our measurement reveals the dynamics and diffusion of photoexcited electrons and holes as a function of their density n in the range 2  ×  10¹⁶ to 6  ×  10¹⁷ cm⁻³. After the first few picoseconds, the grating decays primarily due to ambipolar diffusion. While, at low density, we observe a regime in which the ambipolar diffusion is electron-dominated and increases rapidly with n, it appears to saturate at 34 cm²/s at high n. We present a simple calculation that reproduces the main …

Adherent Cells Avoid Polarization Gradients On Periodically Poled Litao3 Ferroelectrics, Christof Christophis, Elisabetta Ada Cavalcanti-Adam, Maximilian Hanke, Kenji Kitamura, Alexei Gruverman, Michael Grunze, Peter A. Dowben, Axel Rosenhahn

Peter Dowben Publications

The response of fibroblast cells to periodically poled LiTaO₃ ferroelectric crystals has been studied. While fibroblast cells do not show morphological differences on the two polarization directions, they show a tendency to avoid the field gradients that occur between polarization domains of the ferroelectric. The response to the field gradients is fully established after one hour, a time at which fibroblasts form their first focal contacts. If suspension cells, with a lower tendency to establish strong surface contacts are used, no influence of the field gradients is observed.

Infrared Dielectric Anisotropy And Phonon Modes Of Rutile Tio2, Rafal Korlacki, Stefan Schöche, Tino Hofmann, Tom E. Tiwald, Mathias Schubert

Department of Electrical and Computer Engineering: Faculty Publications

Spectroscopic ellipsometry in the mid-infrared and far-infrared spectral range and generalized ellipsometry in the mid-infrared spectral range are used to investigate the anisotropic dielectric response of rutile TiO₂. The ordinary and extraordinary dielectric function tensor components and all infrared active phonon mode parameters of single crystalline rutile TiO₂ are determined with high accuracy for wavelengths from 3 μm to 83 μm. The data were acquired from samples of (001), (100), and (111) surfaces cut from bulk single crystals. A factorized model dielectric function is employed in order to determine the frequencies and damping parameters of the transverse …

Proton Transfer In Surface-Stabilized Chiral Motifs Of Croconic Acid, Donna A. Kunkel, James Hooper, Scott Simpson, Geoffrey A. Rojas, Stephen Ducharme, Timothy Usher, Eva Zurek, Axel Enders

Physics Faculty Publications

The structure and cooperative proton ordering of two-dimensional sheets of croconic acid were studied with scanning tunneling microscopy and first-principles calculations. Unlike in the crystalline form, which exhibits a pleated, densely packed polar sheet structure, the confinement of the molecules to the surface results in hydrogen-bonded chiral clusters and networks. First-principles calculations suggest that the surface stabilizes networks of configurational isomers, which arise from direct hydrogen transfer between their constituent croconic acid monomers. Some of these configurations have a net polarization. It is demonstrated through constrained molecular dynamics simulations that simultaneous proton transfer between any two molecules can occur spontaneously. …

Proton Transfer In Surface-Stabilized Chiral Motifs Of Croconic Acid, Donna A. Kunkel, James Hooper, Scott Simpson, Geoffrey Rojas, Stephen Ducharme, Timothy Usher, Eva Zurek, Axel Enders

Stephen Ducharme Publications

The structure and cooperative proton ordering of two-dimensional sheets of croconic acid were studied with scanning tunneling microscopy and first-principles calculations. Unlike in the crystalline form, which exhibits a pleated, densely packed polar sheet structure, the confinement of the molecules to the surface results in hydrogenbonded chiral clusters and networks. First-principles calculations suggest that the surface stabilizes networks of configurational isomers, which arise from direct hydrogen transfer between their constituent croconic acid monomers. Some of these configurations have a net polarization. It is demonstrated through constrained molecular dynamics simulations that simultaneous proton transfer between any two molecules can occur spontaneously. …