Condensed Matter Physics Commons^™

First-Principles Simulations Of Materials Under Extreme Conditions, Kien Nguyen Cong

USF Tampa Graduate Theses and Dissertations

The investigation of materials at extreme conditions of high pressure and temperature (high-PT), has been one of the greatest scientific endeavors in condensed mater physics, chemistry, astronomy, planetary, and material sciences. Being subjected to high-PT conditions, materials exhibit dramatic changes in both atomic and electronic structure resulting in an emergence of exceptionally interesting phenomena including structural and electronic phase transitions, chemical reactions, and formation of novel compounds with never-previously observed physical and chemical properties. Although new exciting experimental developments in static and dynamic compression combined with new diagnostics/characterization methods allow to uncover new processes and phenomena at high P-T conditions, …

Generation And Stability Of Structurally Imprinted Target Skyrmions In Magnetic Multilayers, Noah Kent, Robert Streubel, Charles Henri Lambert, Alejandro Ceballos, Soong Gun Je, Scott Dhuey, Mi Young Im, Felix Büttner, Frances Hellman, Sayeef Salahuddin, Peter Fischer

Robert Streubel Papers

Target Skyrmions (TSks) are extended topological spin textures with a constant chirality where the rotation of the z component of the magnetization is larger than π. TSks have topological charge 1 or 0, if the z component of the magnetization Mz goes through a rotation of nπwhere n is an odd or even integer, respectively. TSks with a rotation of the z component of up to 4πhave been imaged via high spatial resolution element-specific X-ray imaging. The TSks were generated by weakly coupling 30 nm thin Permalloy (Ni80Fe20, PY) disks with a 1 μm diameter to asymmetric (Ir 1 nm/Co …

Applied Symmetry For Crystal Structure Prediction, Scott William Fredericks

UNLV Theses, Dissertations, Professional Papers, and Capstones

This thesis presents an original open-source Python package called PyXtal (pronounced "pi-crystal") that generates random symmetric crystal structures for use in crystal structure prediction (CSP). The primary advantage of PyXtal over existing structure generation tools is its unique symmetrization method. For molecular structures, PyXtal uses an original algorithm to determine the compatibility of molecular point group symmetry with Wyckoff site symmetry. This allows the molecules in generated structures to occupy special Wyckoff positions without breaking the structure's symmetry. This is a new feature which increases the space of search-able structures and in turn improves CSP performance.

It is shown that …

Morphological Study Of Voids In Ultra-Large Models Of Amorphous Silicon, Durga Prasad Paudel

Dissertations

The microstructure of voids in pure and hydrogen-rich amorphous silicon (a:Si) network was studied in ultra-large models of amorphous silicon, using classical and quantum- mechanical simulations, on the nanometer length scale. The nanostructure, particularly voids of device grade ultra-large models of a:Si was studied, in which observed three-dimensional realistic voids were extended using geometrical approach within the experimental limit of void-volume fractions. In device-grade simulated models, the effect of void morphology; size, shape, number density, and distribution on simulated scattering intensities in small- angle region were investigated. The evolution of voids on annealing below the crystallization temperature …

Extended-Range Oscillations And The First Sharp Diffraction Peak In Amorphous Silicon: A Systematic Study, Devilal Dahal

Master's Theses

The first sharp diffraction peak (FSDP), which characterizes the static structure factor of many glassy systems near the wave vector region of 1-2 Å^-1 has been observed depending on the temperature, pressure and the degree of annealing of the system. The presence of the FSDP is indicative of the intermediate range order (IRO). In current work, we study the role of the extended- range oscillations on the parameters of the FSDP, i.e., intensity, position, area, and the full width at half maximum (FWHM) by using high-quality simulated models of amorphous silicon. The radial distance up to half of the …

Reconfigurable Ferromagnetic Liquid Droplets, Xubo Liu, Noah Kent, Alejandro Ceballos, Robert Streubel, Yufeng Jiang, Yu Chai, Paul Y. Kim, Joe Forth, Frances Hellman, Shaowei Shi, Dong Wang, Brett A. Helms, Paul D. Ashby, Peter Fischer, Thomas P. Russell

Robert Streubel Papers

Solid ferromagnetic materials are rigid in shape and cannot be reconfigured. Ferrofluids, although reconfigurable, are paramagnetic at room temperature and lose their magnetization when the applied magnetic field is removed. Here, we show a reversible paramagnetic-to-ferromagnetic transformation of ferrofluid droplets by the jamming of a monolayer of magnetic nanoparticles assembled at the water-oil interface. These ferromagnetic liquid droplets exhibit a finite coercivity and remanent magnetization. They can be easily reconfigured into different shapes while preserving themagnetic properties of solid ferromagnets with classic north-south dipole interactions. Their translational and rotational motions can be actuated remotely and precisely by an external magnetic …

New Heusler Compounds In Ni-Mn-In And Ni-Mn-Sn Alloys, Xingzhong Li, W.-Y. Zhang, Shah R. Valloppilly, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Rapidly quenched ternary Ni-Mn-T (T = In, Sn) alloys exhibit features associated with magnetic skyrmions, so that XRD, TEM, EDS, SAED and HREM investigations were carried out for structural characterization on the two alloy systems. In this paper, we report a new type of Mn-rich Heusler compound with a cubic unit cell, a = 0.9150 nm in Ni-Mn-In and a = 0.9051 nm in Ni-Mn-Sn, which coexist with a Ni-rich full-Heusler compound with defects, a = 0.6094 nm in Ni-Mn-In and a = 0.6034 nm in Ni-Mn-Sn. A further analysis of the experimental results reveals a close structural relationship between …

Toward Devices For Exploring Pt-Symmetry In Electronic Transport Of Graphene, Michael Carovillano

Senior Honors Papers / Undergraduate Theses

Parity-time symmetry, or PT -symmetry is the principle that in quantum mechanics a non- Hermitian Hamiltonian is capable of returning real eigenstates and real spectra.Recent research has demonstrated real world observation of PT -symmetry in electronics and optics. We aim to expand the regime of observed PT -symmetry through measurement of the electronic transport of graphene devices. Drawing from analogous experiments, we plan to use balanced ohmic resistance acting as both loss and relative gain to induce the required unbroken PT -symmetry regime. This paper analyzes techniques used in fabrication of such devices as well as the basis of the …

High-Pressure High-Temperature Exploration Of Phase Boundaries Using Raman Spectroscopy, Jasmine Kashmir Hinton

UNLV Theses, Dissertations, Professional Papers, and Capstones

Metastability of states can provide interesting properties that may not be readily accessible in a material’s ground state. Many materials show high levels of polymorphism, indicating a rich energy landscape and a potential for metastable states. Melt crystallization techniques provide a potential route to these states. We use a resistively heated diamond anvil cell (DAC) with fine control of a system’s pressure and temperature to explore these systems. Raman spectroscopy is used to track subtle structural changes across phase boundaries. Organic systems, such as glycine and aspirin, were our initial interest due to their high polymorphism and reported low melting …

Topological Insulating States In Photonics And Acoustics, Xiang Ni

Dissertations, Theses, and Capstone Projects

Recent surge of interest in topological insulators, insulating in their interior but conducting at the surfaces or interfaces of different domains, has led to the discovery of a variety of new topological states, and their topological invariants are characterized by numerous approaches in the category of topological band theory. The common features shared by topological insulators include, the topological phase transition occurs if the bulk bandgap is formed due to the symmetries reduction, the topological invariants exist characterizing the global properties of the material and inherently robust to disorder and continuous perturbations irrespective of the local details. Most importantly, these …

Origin Of Enhanced Anisotropy In Fept-C Granular Films Revealed By Xmcd, Robert Streubel, Alpha T. N'Diaye, Kumar Srinivasan, Antony Ajan, Peter Fischer

Robert Streubel Papers

We study the effect of carbon segregants on the spin and orbital moments of L10 FePt granular media using x-ray magnetic circular dichroism (XMCD) spectroscopy and report an effective decoupling of the structural film properties from the magnetic parameters of the grains. The carbon concentration reduces the grain size from (200 ± 160) nm2 down to (50 ± 20) nm2 for 40 mol. %C and improves sphericity and the order of grains, while preserving the crystalline order, spin and orbital moments, and perpendicular magnetocrystalline anisotropy. We identify the primary cause of enhanced saturation and coercive fields as the reduced demagnetization …

Magnetization Reversal And Local Switching Fields Of Ferromagnetic Co/Pd Microtubes With Radial Magnetization, Norbert Puwenberg, Christopher F. Reiche, Robert Streubel, Mishal Khan, Dipankar Mukherjee, Ivan V. Soldatov, Michael Melzer, Oliver G. Schmidt, Bernd Büchner, Thomas Mühl

Robert Streubel Papers

Three-dimensional nanomagnetism is a rapidly growing field of research covering both noncollinear spin textures and curved magnetic geometries including microtubular structures. We spatially resolve the field-induced magnetization reversal of free-standing ferromagnetic microtubes utilizing multifrequency magnetic force microscopy (MFM). The microtubes are composed of Co/Pd multilayer films with perpendicular magnetic anisotropy that translates to an anisotropy with radial easy axis upon rolling-up. Simultaneously mapping the topography and the perpendicular magnetostatic force derivative, the relation between surface angle and local magnetization configuration is evaluated for a large number of locations with slopes exceeding 45 degrees. The angle-dependence of the switching field is …

X-Ray Ptychography On Low-Dimensional Hard-Condensed Matter Materials, Xiaowen Shi, Nicolas Burdet, Bo Chen, Gang Xiong, Robert Streubel, Ross Harder, Ian K. Robinson

Robert Streubel Papers

Tailoring structural, chemical, and electronic (dis-)order in heterogeneous media is one of the transformative opportunities to enable new functionalities and sciences in energy and quantum materials. This endeavor requires elemental, chemical, and magnetic sensitivities at the nano/atomic scale in two- and three-dimensional space. Soft X-ray radiation and hard X-ray radiation provided by synchrotron facilities have emerged as standard characterization probes owing to their inherent element-specificity and high intensity. One of the most promising methods in view of sensitivity and spatial resolution is coherent diffraction imaging, namely, X-ray ptychography, which is envisioned to take on the dominance of electron imaging techniques …

Textured Heterogeneity In Square Artificial Spin Ice, J. C.T. Lee, S. K. Mishra, V. S. Bhat, R. Streubel, B. Farmer, X. Shi, L. E. De Long, I. Mcnulty, P. Fischer, S. D. Kevan, S. Roy

Robert Streubel Papers

We report evidence of spontaneous formation of a heterogeneous network of superdomains in two-dimensional square artificial spin ice nanostructures in externally applied magnetic fields. Coherent resonant soft-x-ray scattering from such textures gives rise to unique internal structure in Bragg peaks. The magnetic heterogeneity is locally disordered but has a zigzag texture at longer length scales. Our result suggests that the macroscopic magnetic texture is derived from the microscopic structure of the Dirac strings.

High Energy Product Of Mnbi By Field Annealing And Sn Alloying, Wenyong Zhang, Balamurugan Balasubramanian, Parashu Kharel, Rabindra Pahari, Shah R. Valloppilly, Xingzhong Li, Lanping Yue, Ralph Skomski, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Permanent-magnet materials are one cornerstone of today’s technology, abundant in disk drives, motors, medical equipment, wind gen- erators, and cars. A continuing challenge has been to reconcile high permanent-magnet performance with low raw-material costs. This work reports a Mn-Bi-Sn alloy exclusively made from inexpensive elements, exhibiting high values of Curie-temperature, magnetization, anisotropy, coercivity, and energy product. The samples are produced by field annealing of rapidly quenched Sn-containing MnBi alloys, where the improvement of the magnetic properties is caused by the substitutional occupancy of the 2c sites in the hexagonal NiAs structure by Sn. The substitution modifies the electronic structure of …

High Energy Product Of Mnbi By Field Annealing And Sn Alloying, Wenyong Zhang, Balamurugan Balasubramanian, Parashu Kharel, Rabindra Pahari, Shah R. Valloppilly, Xingzhong Li, Lanping Yue, Ralph Skomski, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Permanent-magnet materials are one cornerstone of today’s technology, abundant in disk drives, motors, medical equipment, wind generators, and cars. A continuing challenge has been to reconcile high permanent-magnet performance with low raw-material costs. This work reports a Mn-Bi-Sn alloy exclusively made from inexpensive elements, exhibiting high values of Curie-temperature, magnetization, anisotropy, coercivity, and energy product. The samples are produced by field annealing of rapidly quenched Sn-containing MnBi alloys, where the improvement of the magnetic properties is caused by the substitutional occupancy of the 2c sites in the hexagonal NiAs structure by Sn. The substitution modifies the electronic structure of the …

Boundary Conditions And Berry Phase In Magnetic Nanostructures, A. Ullah, B. Balamurugan, W. Zhang, D. J. Sellmyer, R. Skomski

Nebraska Center for Materials and Nanoscience: Faculty Publications

The effect of micromagnetic boundary conditions on the Berry curvature and topological Hall effect in granular nanostructures is investi- gated by model calculations. Both free surfaces and grain boundaries between interacting particles or grains affect the spin structure. The Dzyaloshinskii-Moriya interactions yield corrections to the Erdmann-Weierstrass boundary conditions, but the Berry curvature remains an exclusive functional of the local spin structure, which greatly simplifies the treatment of nanostructures. An explicit example is a model nanostructure with cylindrical symmetry whose spin structure is described by Bessel function and which yields a mean-field-type Hall-effect contribution that can be related to magnetic-force-microscopy images.

Radiation Tolerance In Nano-Structured Crystalline Fe(Cr)/Amorphous Sioc Composite, Qing Su, Tianyao Wang, Lin Shao, Michael Nastasi

Nebraska Center for Materials and Nanoscience: Faculty Publications

The management of irradiation defects is one of key challenges for structural materials in current and future reactor systems. To develop radiation tolerant alloys for service in extreme irradiation environments, the Fe self-ion radiation response of nanocomposites composed of amorphous silicon oxycarbide (SiOC) and crystalline Fe(Cr) were examined at 10, 20, and 50 displacements per atom damage levels. Grain growth in width direction was observed to increase with increasing irradiation dose in both Fe(Cr) films and Fe(Cr) layers in the nanocomposite after irradiation at room temperature. However, compared to the Fe(Cr) film, the Fe(Cr) layers in the nanocomposite exhibited ~50% …

Magnetic And Magnetocaloric Properties Of Pr2-Xndxfe17 Ribbons, Bishnu Dahal, Parashu Kharel, Thomas Ott, Wenyong Zhang, Shah R. Valloppilly, Ralph Skomski, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

The structural, magnetic and magnetocaloric properties of Fe deficient Pr_2-xNd_xFe₁₇ (x = 0.5, 0.7) alloys prepared by arc-melting and melt- spinning have been investigated. The room temperature x-ray diffraction patterns show that the samples are nearly single-phase and crystallize in the rhombohedral Th₂Zn₁₇-type crystal structure. The Curie temperatures determined from the thermomagnetic curves are 302 K and 307 K for Pr_1.5Nd_0.5Fe₁₇ and Pr_1.3Nd_0.7Fe₁₇, respectively. The peak magnetic entropy change and the relative cooling power at field change of 50 kOe …

Controlling The Magnetocrystalline Anisotropy Of E-Fe2o3, Imran Ahamed, Ralph Skomski, Arti Kashyap

Nebraska Center for Materials and Nanoscience: Faculty Publications

The magnetocrystalline anisotropy of pristine and Co-substituted ε-Fe₂O₃ is investigated by density functional calculations. The epsilon-iron oxide is the only polymorph of Fe₂O₃ magnetoelectric in its antiferromagnetic ground states other crystalline forms being α-Fe₂O₃ (hematite), β-Fe₂O₃, and γ-Fe₂O₃ (maghemite). The magnetizations of the four iron sublattices are antiferromagnetically aligned with slightly different magnetic moments resulting in a ferrimagnetic structure. Compared to the naturally occurring hematite and maghemite, bulk ε-Fe₂O₃ is difficult to prepare, but ε-Fe₂O₃ nanomaterials of different …

Quantum Phase Transition And Ferromagnetism In Co1+XSn, Rabindra Pahari, Balamurugan Balasubramanian, Rohit Pathak, Manh Coung Nguyen, Shah R. Valloppilly, Ralph Skomski, Arti Kashyap, Cai-Zhuang Wang, Kai-Ming Ho, George C. Hadjipanayis, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

The onset of ferromagnetism in cobalt-tin alloys is investigated experimentally and theoretically. The Co₁₊xSn alloys were prepared by rapid quenching from the melt and form a modified hexagonal NiAs-type crystal structure for 0.45 ≤ x ≤ 1. The magnetic behavior is described analytically and by density-functional theory using supercells and the coherent-potential approximation. The excess of Co concentration x, which enters the interstitial 2d sites in the hypothetical NiAs-ordered parent alloy CoSn, yields a Griffiths-like phase and, above a quantum critical point (x_c ≈ 0.65), a quantum phase …

Crystal Structure And Dzyaloshinski–Moriya Micromagnetics, Ahsan Ullah, B. Balamurugan, Wuzhang Fang, Shah R. Valloppilly, Xingzhong Li, Rabindra Pahari, Lanping Yue, Andrei Sokolov, David J. Sellmyer, Ralph Skomski

Nebraska Center for Materials and Nanoscience: Faculty Publications

The relationship between atomic-scale and micromagnetic Dzyaloshinski–Moriya (DM) interactions has been investigated. By analyzing the Lifshitz invariants for different point groups, we have found that there is no unique link between the absence of inversion symmetry and DM interactions. The absence of inversion symmetry is a necessary condition for a net DM interaction in crystals, but several noncentrosymmetric point groups have zero DM interactions. In many cases, the key consideration is whether the crystals are polar and/or chiral. For example, MnSi-type spin spirals, which violate helical spin symmetry, are caused by the insertion of chiral atomic-scale building blocks into an …

Comparative Study Of Topological Hall Effect And Skyrmions In Nimnin And Nimnga, Wenyong Zhang, Balamurugan Balasubramanian, Ahsan Ullah, Rabindra Pahari, Xingzhong Li, Lanping Yue, Shah R. Valloppilly, Andrei Sokolov, Ralph Skomski, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

A nonequilibrium rapid-quenching method has been used to fabricate NiMnIn and NiMnGa alloys that are chemically and morphologically similar but crystallographically and physically very different. NiMnGa crystallizes in a Ni2In-type hexagonal structure, whereas NiMnIn is a cubic Heusler alloy. Both alloys yield a topological Hall effect contribution corresponding to bubble-type skyrmion spin structures, but it occurs in much lower magnetic fields in NiMnIn as compared to NiMnGa. The effect is unrelated to net Dzyaloshinskii-Moriya interactions, which are absent in both alloys due to their inversion-symmetric crystal structures. Based on magnetic-force microscopy, we explain the difference between the two alloys by …

Structure And Magnetism Of Co2ge Nanoparticles, Onur Tosun, Frank M. Abel, Balamurugan Balasubramanian, Ralph Skomski, David J. Sellmyer, George C. Hadjipanayis

Nebraska Center for Materials and Nanoscience: Faculty Publications

The structural and magnetic properties of Co₂Ge nanoparticles (NPs) prepared by the cluster-beam deposition (CBD) technique have been investigated. As-made particles with an average size of 5.5 nm exhibit a mixture of hexagonal and orthorhombic crystal structures. Thermomagnetic measurements showed that the as-made particles are superparamagnetic at room temperature with a blocking temperature (T_B) of 20 K. When the particles are annealed at 823 K for 12 h, their size is increased to 13 nm and they develop a new orthorhombic crystal structure, with a Curie temperature (T_C) of 815 K. This …

Medium-Dependent Antibacterial Properties And Bacterial Filtration Ability Of Reduced Graphene Oxide, Alexander Gusev, Olga Zakharova, Dmitry S. Muratov, Nataliia S. Vorobeva, Mamun Sarker, Iaroslav Rybkin, Daniil Bratashov, Evgeny Kolesnikov, Aleš Lapanje, Denis V. Kuznetsov, Alexander Sinitskii

Nebraska Center for Materials and Nanoscience: Faculty Publications

Toxicity of reduced graphene oxide (rGO) has been a topic of multiple studies and was shown to depend on a variety of characteristics of rGO and biological objects of interest. In this paper, we demonstrate that when studying the same dispersions of rGO and fluorescent Escherichia coli (E. coli) bacteria, the outcome of nanotoxicity experiments also depends on the type of culture medium. We show that rGO inhibits the growth of bacteria in a nutrition medium but shows little effect on the behavior of E. coli in a physiological saline solution. The observed effects of rGO on E. …

Crystal Structure, Magnetism And Magnetocaloric Properties Of Mn2−XSn0.5Ga0.5 (X=0, 0.3, 0.5, 0.8) Alloys, H. Qian, Rabindra Pahari, K. Schroeder, Shah R. Valloppilly, Y. Huh, Pavel V. Lukashev, J. Hu, Parashu Kharel, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Magnetic refrigeration based on the magnetocaloric effect has attracted recent attention due to advantages such as high efficiency and environmental friendliness. We have investigated the structural, magnetic and magnetocaloric properties of Mn_2−xSn_0.5Ga_0.5 (x=0, 0.3, 0.5, 0.8) alloys prepared using arc-melting and meltspinning techniques with prospects for magnetic refrigeration. The Mn_2−xSn_0.5Ga_0.5 alloys, except for Mn_1.2Sn_0.5Ga_0.5, have a single-phase hexagonal crystal structure. The Mn_1.2Sn_0.5Ga_0.5 alloy also contains a small amount of MnSn₂ impurity phase. The Curie temperature and high-field (30 kOe) …

Theoretical Studies Of The Structure And Stability Of Metal Chalcogenide Crntem (1≤N≤6, 1≤M≤8) Clusters, Fnu Sweta Prabha

Theses and Dissertations

In the presented work, first principle studies on electronic structure, stability, and magnetic properties of metal chalcogenide, Cr_nTe_m clusters have been carried out within a density functional framework using generalized gradient functions to incorporate the exchange and correlation effects. The energetic and electronic stability was investigated, and it was found that they are not always correlated as seen in the cluster Cr₆Te₈ which has smaller gap between its HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) and a high electron affinity of 3.39 eV indicating lower electronic stability whereas higher fragmentation …