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Competing Magnetic Interactions In The Antiferromagnetic Topological Insulator Mnbi2te4, Bing Li, J.-Q. Yan, D. Pajerowski, Elijah Gordon, Ana-Marija Nedic, Y. Sizyuk, Liqin Ke, Peter P. Orth, David Vaknin, Robert J. McQueeney 2020 Iowa State University and Ames Laboratory

Competing Magnetic Interactions In The Antiferromagnetic Topological Insulator Mnbi2te4, Bing Li, J.-Q. Yan, D. Pajerowski, Elijah Gordon, Ana-Marija Nedic, Y. Sizyuk, Liqin Ke, Peter P. Orth, David Vaknin, Robert J. Mcqueeney

Physics and Astronomy Publications

The antiferromagnetic (AFM) compound MnBi2Te4 is suggested to be the first realization of an AFM topological insulator. We report on inelastic neutron scattering studies of the magnetic interactions in MnBi2Te4 that possess ferromagnetic triangular layers with AFM interlayer coupling. The spin waves display a large spin gap and pairwise exchange interactions within the triangular layer are long ranged and frustrated by large next-nearest neighbor AFM exchange. The degree of frustration suggests proximity to a variety of magnetic phases, potentially including skyrmion phases, which could be accessed in chemically tuned compounds or upon the application of symmetry-breaking fields.


An Analysis Of The Debate Over Creation, Evolution, And The Timeline Of The Universe At An Ecumenical Christian University, Mason Pohlman 2020 Belmont University

An Analysis Of The Debate Over Creation, Evolution, And The Timeline Of The Universe At An Ecumenical Christian University, Mason Pohlman

Honors Theses

Throughout a significant portion of history and within modern culture, the fields of science and religion appear to be competing for the same holds in a person’s belief system. Universities are where academics and the sciences are the prevailing held truth, while in churches, the Bible reigns as supreme authority. However, in a Christian academic setting, the predominate school of thought in belief systems might turn into a little more of a melting pot. By analyzing gathered personal data (via surveys and interviews), one can begin to piece together the predominate thoughts on the apparent conflict between religion and ...


Mid-Ir Optical Refrigeration And Radiation Balanced Lasers, Saeid Rostami 2020 Doctoral Student, Optical Science and Engineering

Mid-Ir Optical Refrigeration And Radiation Balanced Lasers, Saeid Rostami

Optical Science and Engineering ETDs

This dissertation reports recent advances in mid-infrared (mid-IR) optical refrigeration and Radiation Balanced Lasers (RBLs). The first demonstration of optical refrigeration in Ho:YLF and Tm:YLF crystals as promising mid-IR laser cooling candidates is reported. Room temperature laser cooling efficiency of Tm- and Ho-doped crystals at different excitation polarization is measured and their external quantum efficiency and background absorption are extracted. Complete characterization of laser cooling samples is obtained via performing detailed low-temperature spectroscopic analysis, and their minimum achievable temperature as well as conditions to achieve laser cooling efficiency enhancement in mid-IR are investigated. By developing a Thulium-doped fiber ...


Spin Dynamics In Antiferromagnetic Oxypnictides And Fluoropnictides: Lamnaso, Lamnsbo, And Bamnasf, Farhan Islam, Elijah Gordon, Pinaki Das, Yong Liu, Liquin Ke, Douglas L. Abernathy, Robert L. McQueeney, David Vaknin 2020 Ames Laboratory

Spin Dynamics In Antiferromagnetic Oxypnictides And Fluoropnictides: Lamnaso, Lamnsbo, And Bamnasf, Farhan Islam, Elijah Gordon, Pinaki Das, Yong Liu, Liquin Ke, Douglas L. Abernathy, Robert L. Mcqueeney, David Vaknin

Ames Laboratory Accepted Manuscripts

Inelastic neutron scattering (INS) from polycrystalline antiferromagnetic LaMnAsO, LaMnSbO, and BaMnAsF are analyzed using a J(1)-J(2)-J(c) Heisenberg model in the framework of the linear spin-wave theory. All three systems show clear evidence that the nearest- and next-nearest-neighbor interactions within the Mn square lattice layer (J(1) and J(2)) are both antiferromagnetic (AFM). However, for all compounds studied the competing interactions have a ratio of 2J(2)/J(1) < 1, which favors the square lattice checkerboard AFM structure over the stripe AFM structure. The interplane coupling Jc in all three systems is on the order of similar to 3 x 10(-4)J(1), rendering the magnetic properties of these systems with quasi-two-dimensional character. The substitution of Sb for As significantly lowers the in-plane exchange coupling, which is also reflected in the decrease of the Neel temperature, T-N. Although BaMnAsF shares the same MnAs sheets as LaMnAsO, their J(1) and J(2) values are substantially different. Using density functional theory, we calculate exchange parameters J(ij) to rationalize the differences among these systems.


Gapless Dirac Surface States In The Antiferromagnetic Topological Insulator Mnbi2 Te4, Przemyslaw Swatek, Yun Wu, Lin-Lin Wang, Kyungchan Lee, Benjamin Schrunk, Jiaqiang Yan, Adam Kaminski 2020 Iowa State University and Ames Laboratory

Gapless Dirac Surface States In The Antiferromagnetic Topological Insulator Mnbi2 Te4, Przemyslaw Swatek, Yun Wu, Lin-Lin Wang, Kyungchan Lee, Benjamin Schrunk, Jiaqiang Yan, Adam Kaminski

Ames Laboratory Accepted Manuscripts

We used angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations to study the electronic properties of MnBi2Te4, a material that was predicted to be an intrinsic antiferromagnetic (AFM) topological insulator. In striking contrast to earlier literature showing a full gap opening between two surface band manifolds on the (0001) surface, we observed a gapless Dirac surface state with a Dirac point sitting at EB=−280meV. Furthermore, our ARPES data revealed the existence of a second Dirac cone sitting closer to the Fermi level. Surprisingly, these surface states remain intact across the AFM transition. The presence of gapless Dirac states ...


Magnetic Forces In The Absence Of A Classical Magnetic Field, Ismael L. Paiva, Yakir Aharonov, Jeff Tollaksen, Mordecai Waegell 2020 Chapman University

Magnetic Forces In The Absence Of A Classical Magnetic Field, Ismael L. Paiva, Yakir Aharonov, Jeff Tollaksen, Mordecai Waegell

Mathematics, Physics, and Computer Science Faculty Articles and Research

It is shown that, in some cases, the effect of discrete distributions of flux lines in quantum mechanics can be associated with the effect of continuous distributions of magnetic fields with special symmetries. In particular, flux lines with an arbitrary value of magnetic flux can be used to create energetic barriers, which can be used to confine quantum systems in specially designed configurations. This generalizes a previous work where such energy barriers arose from flux lines with half-integer fluxons. Furthermore, it is shown how the Landau levels can be obtained from a two-dimensional grid of flux lines. These results suggest ...


Manipulating Magnetism In The Topological Semimetal Eucd2as2, Na Hyun Jo, Brinda Kuthanazhi, Yun Wu, Erik I. Timmons, Tae-Hoon Kim, Lin Zhou, Lin-Lin Wang, Benjamin G. Ueland, Andriy Palasyuk, Dominic H. Ryan, Robert J. McQueeney, Kyungchan Lee, Benjamin Schrunk, Anton A. Burkov, Ruslan Prozorov, Sergey L. Bud’ko, Adam Kaminski, Paul C. Canfield 2020 Iowa State University and Ames Laboratory

Manipulating Magnetism In The Topological Semimetal Eucd2as2, Na Hyun Jo, Brinda Kuthanazhi, Yun Wu, Erik I. Timmons, Tae-Hoon Kim, Lin Zhou, Lin-Lin Wang, Benjamin G. Ueland, Andriy Palasyuk, Dominic H. Ryan, Robert J. Mcqueeney, Kyungchan Lee, Benjamin Schrunk, Anton A. Burkov, Ruslan Prozorov, Sergey L. Bud’Ko, Adam Kaminski, Paul C. Canfield

Physics and Astronomy Publications

EuCd2As2 is a magnetic semimetal that has the potential of manifesting nontrivial electronic states, depending on its low temperature magnetic ordering. Here, we report the successful synthesis of single crystals of EuCd2As2 that order ferromagnetically or antiferromagnetically depending on the level of band filling, thus allowing for the use of magnetism to tune the topological properties within the same host. We explored their physical properties via magnetization, electrical transport, heat capacity, and angle-resolved photoemission spectroscopy measurements and conclude that EuCd2As2 is an excellent, tunable system for exploring the interplay of magnetic ordering and topology.


Competing Pairing Interactions Responsible For The Large Upper Critical Field In A Stoichiometric Iron-Based Superconductor Cakfe4as4, Matthew Bristow, William Knafo, Pascal Reiss, William R. Meier, Paul C. Canfield, Stephen J. Blundell, Amalia I. Coldea 2020 University of Oxford

Competing Pairing Interactions Responsible For The Large Upper Critical Field In A Stoichiometric Iron-Based Superconductor Cakfe4as4, Matthew Bristow, William Knafo, Pascal Reiss, William R. Meier, Paul C. Canfield, Stephen J. Blundell, Amalia I. Coldea

Physics and Astronomy Publications

The upper critical field of multiband superconductors is an important quantity that can reveal details about the nature of the superconducting pairing. Here we experimentally map out the complete upper-critical-field phase diagram of a stoichiometric superconductor, CaKFe4As4, up to 90T for different orientations of the magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching values close to ∼3Tc at the lowest temperature, and the anisotropy decreases dramatically with temperature, leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the upper critical field can be well described ...


Size-Controlled Synthesis Of Nickel Nanoparticles Enclosed In Carbon Nanocages, Felicity Peebles, Grigorii Rudakov 2020 Univeristy of Louisville

Size-Controlled Synthesis Of Nickel Nanoparticles Enclosed In Carbon Nanocages, Felicity Peebles, Grigorii Rudakov

Undergraduate Arts and Research Showcase

We have demonstrated a simple, scalable, and tunable method of obtaining densely packed Ni Nanoparticles encapsulated in Carbon Nanocages (Ni@CNCs). Using a facile method, it was shown that via a simple annealing process of precursor based on nickel acetate and citric acid, Ni@CNCs with sizes varying from 5 to 20 nm can be synthesized by changing the heating ramp rate during the synthesis from 25 to 53 °C/min. The final temperature of 600 °C was held for 10 min, and was the same for all the samples. X-Ray Diffraction (XRD) multiple peaks analysis was performed to show ...


Light-Driven Raman Coherence As A Nonthermal Route To Ultrafast Topology Switching In A Dirac Semimetal, Chirag Vaswani, Lin-Lin Wang, Dinusha Herath Mudiyanselage, Q. Li, P. M. Lozano, G. D. Gu, Di Cheng, Boqun Song, Liang Luo, Richard H.J. Kim, Chuankun Huang, Zhiyan Liu, M. Mootz, I. E. Perakis, Yongxin Yao, Kai-Ming Ho, Jigang Wang 2020 Iowa State University and Ames Laboratory

Light-Driven Raman Coherence As A Nonthermal Route To Ultrafast Topology Switching In A Dirac Semimetal, Chirag Vaswani, Lin-Lin Wang, Dinusha Herath Mudiyanselage, Q. Li, P. M. Lozano, G. D. Gu, Di Cheng, Boqun Song, Liang Luo, Richard H.J. Kim, Chuankun Huang, Zhiyan Liu, M. Mootz, I. E. Perakis, Yongxin Yao, Kai-Ming Ho, Jigang Wang

Ames Laboratory Accepted Manuscripts

A grand challenge underlies the entire field of topology-enabled quantum logic and information science: how to establish topological control principles driven by quantum coherence and understand the time dependence of such periodic driving. Here we demonstrate a few-cycle THz-pulse-induced phase transition in a Dirac semimetal ZrTe5 that is periodically driven by vibrational coherence due to excitation of the lowest Raman active mode. Above a critical THz-pump field threshold, there emerges a long-lived metastable phase, approximately 100 ps, with unique Raman phonon-assisted topological switching dynamics absent for optical pumping. The switching also manifests itself by distinct features: nonthermal spectral shape, relaxation ...


Localized Singlets And Ferromagnetic Fluctuations In The Dilute Magnetic Topological Insulator Sn0.95mn0.05te, David Vaknin, Santanu Pakhira, Deborah L. Schlagel, Farhan Islam, Jianhua Zhang, D. M. Pajerowski, Cai-Zhuang Wang, David C. Johnston, Robert J. McQueeney 2020 Iowa State University and Ames Laboratory

Localized Singlets And Ferromagnetic Fluctuations In The Dilute Magnetic Topological Insulator Sn0.95mn0.05te, David Vaknin, Santanu Pakhira, Deborah L. Schlagel, Farhan Islam, Jianhua Zhang, D. M. Pajerowski, Cai-Zhuang Wang, David C. Johnston, Robert J. Mcqueeney

Ames Laboratory Accepted Manuscripts

The development of long-range ferromagnetic (FM) order in dilute magnetic topological insulators can induce dissipationless electronic surface transport via the quantum anomalous Hall effect. We measure the magnetic excitations in a prototypical magnetic topological crystalline insulator, Sn0.95Mn0.05Te, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our Sn0.95Mn0.05Te sample has no FM long-range order above a temperature of 2 K. However, we observe slow, collective FM fluctuations (<70μeV), indicating proximity to FM order. We also find a series of sharp peaks originating from local excitations of antiferromagnetically (AF) coupled and isolated Mn-Mn dimers with JAF=460μeV. The simultaneous presence of collective and localized components in the magnetic spectra highlight different roles for substituted Mn ions, with competition between FM order and the formation of AF-coupled Mn-Mn dimers.


Investigation Of Complex Magnetic Phenomena In Spinel Femn2o4, Mnfe2o4, And Nife2o4, Roshan Kumar Nepal 2020 Louisiana State University

Investigation Of Complex Magnetic Phenomena In Spinel Femn2o4, Mnfe2o4, And Nife2o4, Roshan Kumar Nepal

LSU Doctoral Dissertations

Frustrated magnetic systems, where all spin interactions cannot be simultaneously satisfied, have continued to attract interest due to a plethora of novel magnetic states that emerge in them due to frustration and their potential technological applications. Spinel oxides AB2O4, where A and B are metal ions) are an excellent testing ground for the exploration of frustrated magnetism. This dissertation presents the experimental investigation of complex magnetic phenomena in the spinel oxides FeMn2O4, MnFe2O4, and NiFe2O4.

FeMn2O4 and MnFe2O4 are members of the ...


Coherent Band-Edge Oscillations And Dynamic Longitudinal-Optical Phonon Mode Splitting As Evidence For Polarons In Perovskites, Zhaoyu Liu, Chirag Vaswani, Liang Luo, Di Cheng, X. Yang, Xin Zhao, Yongxin Yao, Z. Song, R. Brenes, R. J. H. Kim, J. Jean, V. Bulović, Y. Yan, Kai-Ming Ho, Jigang Wang 2020 Iowa State University and Ames Laboratory

Coherent Band-Edge Oscillations And Dynamic Longitudinal-Optical Phonon Mode Splitting As Evidence For Polarons In Perovskites, Zhaoyu Liu, Chirag Vaswani, Liang Luo, Di Cheng, X. Yang, Xin Zhao, Yongxin Yao, Z. Song, R. Brenes, R. J. H. Kim, J. Jean, V. Bulović, Y. Yan, Kai-Ming Ho, Jigang Wang

Ames Laboratory Accepted Manuscripts

The coherence of collective modes, such as phonons and polarons, and their modulation of electronic states is long sought in complex systems, which is a crosscutting issue in photovoltaics and quantum electronics. In photovoltaic cells and lasers based on metal halide perovskites, the presence of polarons, i.e., photocarriers dressed by the macroscopic motion of charged lattice, assisted by terahertz (THz) longitudinal-optical (LO) phonons, has been intensely studied yet is still debated. This may be key for explaining the remarkable properties of the perovskite materials, e.g., defect tolerance, long charge lifetimes, and diffusion lengths. Here we use the intense ...


Synthesis Of Nickel Selenide Thin Films For High Performance All-Solid-State Asymmetric Supercapacitors, Hao Li, Jiangfeng Gong, Jing-Chang Li, Xincheng Zhang, Chunmei Tang, Hongbing Yao, Qingping Ding 2020 Hohai University

Synthesis Of Nickel Selenide Thin Films For High Performance All-Solid-State Asymmetric Supercapacitors, Hao Li, Jiangfeng Gong, Jing-Chang Li, Xincheng Zhang, Chunmei Tang, Hongbing Yao, Qingping Ding

Ames Laboratory Accepted Manuscripts

As a significant semiconductor, nickel selenide shows enormous potential and extensive application prospects in the field of sensor, photocatalysis and supercapacitor. In this paper, nickel selenide (Ni3Se2, NiSe) thin films were successfully fabricated on stainless-steel sheet using a facile, effective electrodeposition technique. The morphologies, microstructures and chemical compositions of the thin films are characterized systematically. Electrochemical tests exhibit that the Ni3Se2 and NiSe possess high specific capacitance of 581.1 F/g and 1644.7 F/g, respectively. A flexible, all-solid-state asymmetric supercapacitor is assembled by utilizing NiSe film as positive electrode and activated carbon as negative electrode. The solid ...


London Penetration Depth At Zero Temperature And Near The Superconducting Transition, Vladimir G. Kogan, Makariy A. Tanatar, Ruslan Prozorov 2020 Ames Laboratory

London Penetration Depth At Zero Temperature And Near The Superconducting Transition, Vladimir G. Kogan, Makariy A. Tanatar, Ruslan Prozorov

Ames Laboratory Accepted Manuscripts

A simple relation is established between the zero-T penetration depth λ(0) and the slope of λ−2(T) near Tc, similar to Helfand-Werthamer's relation for Hc2(0) and the slope of Hc2(T) at Tc for the isotropic s-wave case with nonmagnetic scattering [E. Helfand and N. R. Werthamer, Phys. Rev. 147, 288 (1966)]. When the scattering parameter ρ=ℏv/2πTcℓ (v is the Fermi velocity and ℓ is the mean free path) varies from 1 to 10, the coefficient of proportionality between λ−2(0) and Tc(dλ−2/dT)Tc changes from 0.43 to 0 ...


Tip- And Plasmon-Enhanced Infrared Nanoscopy For Ultrasensitive Molecular Characterizations, Yilong Luan, Liam McDermott, F. Hu, Zhe Fei 2020 Iowa State University and Ames Laboratory

Tip- And Plasmon-Enhanced Infrared Nanoscopy For Ultrasensitive Molecular Characterizations, Yilong Luan, Liam Mcdermott, F. Hu, Zhe Fei

Ames Laboratory Accepted Manuscripts

We propose a method for ultrasensitive infrared (IR) vibrational spectroscopy of molecules with nanoscale footprints by combining the tip enhancement of a scattering-type scanning near-field optical microscope (s-SNOM) and the plasmon enhancement of breathing-mode (BM) plasmon resonances of graphene nanodisks (GNDs). To demonstrate this, we develop a quantitative model that is capable of computing accurately the s-SNOM signals of nanoscale samples. With our modeling, we show that the s-SNOM tip can effectively excite gate-tunable BM plasmonic resonances in GNDs with strong field enhancement and sensitive dependence on the size of GND. Moreover, we demonstrate that the intense electric field of ...


Evolution Of Two-Step Magnetic Transition On Nanogranular Gd5si1.3ge2.7 Thin Film, J. H. Belo, A. L. Pires, I. T. Gomes, J. B. Sousa, R. L. Hadimani, David C. Jiles, A. M. Pereira, J. P. Araújo 2020 Universidade do Porto

Evolution Of Two-Step Magnetic Transition On Nanogranular Gd5si1.3ge2.7 Thin Film, J. H. Belo, A. L. Pires, I. T. Gomes, J. B. Sousa, R. L. Hadimani, David C. Jiles, A. M. Pereira, J. P. Araújo

Electrical and Computer Engineering Publications

A multi-functional Gd5Si1.3Ge2.7 thin film deposited by pulsed laser ablation in the form of an ensemble of nanoparticles was studied for 18 thermal cycles via electron transport measurements together with structural and magnetic characterization. A general negative thermal dependency of the resistivity (ρ) is observed, which contrasts with the metallic-like behavior observed in bulk Gd5SixGe4-x compounds. This general trend is interrupted by a two-step, positive-slope transition in ρ(T) throughout the [150,250]K interval, corresponding to two consecutive magnetic transitions: a fully coupled magnetostructural followed by a purely magnetic order on heating. An avalanche-like behavior is unveiled ...


Si Nanocrystal Synthesis Via Double Implantation And Variable Implantation, James M. Gaudet 2020 The University of Western Ontario

Si Nanocrystal Synthesis Via Double Implantation And Variable Implantation, James M. Gaudet

Electronic Thesis and Dissertation Repository

Silicon (Si) nanocrystals (nc) precipitated from silicon-implanted silicon oxide (SiO2) are of interest as a novel light source for illumination, biomedical applications, optical computing, etc. They have some advantages over conventional III-V compound semiconductor nanocrystals produced by colloidal synthesis. They are compatible with Si/SiO2 based semiconductor processing, are stable, non-toxic at point of synthesis and consumption, and their luminescence falls with the infrared transmission window of biological materials. Unfortunately, synthesis of Si-nc embedded SiO2 is uneconomical and is not as amenable to precise control of the size distribution of nanocrystals as is the case for III-V ...


Density Functional Theory Study Of Two-Dimensional Boron Nitride Films, Pradip R. Niraula 2020 The Graduate Center, City University of New York

Density Functional Theory Study Of Two-Dimensional Boron Nitride Films, Pradip R. Niraula

Dissertations, Theses, and Capstone Projects

Since graphene was isolated in 2004, the number of two-dimensional (2D) materials and their scientific relevance have grown exponentially. Besides graphene, one of the most important and technolocially promizing 2D materials that has emerged in recent years is hexagonal boron nitride, in its monolayer or multilayer form. In my thesis work, I used density functional theory (DFT) calculations to investigate the properties of boron nitride films. In particular, I first studied the properties (i.e. formation energy, defect states, and structure) of point charged defects in monolayer and bilayer hexagonal boron nitride, and subsequently, I focused on the linear and ...


Ii-Vi Type-Ii Quantum Dot Superlattices For Novel Applications, Vasilios Deligiannakis 2020 The Graduate Center, City University of New York

Ii-Vi Type-Ii Quantum Dot Superlattices For Novel Applications, Vasilios Deligiannakis

Dissertations, Theses, and Capstone Projects

In this thesis, we discuss the growth procedure and the characterization results obtained for epitaxially grown submonolayer type-II quantum dot superlattices made of II-VI semiconductors. We have investigated the spin dynamics of ZnSe layers with embedded type-II ZnTe quantum dots and the use of (Zn)CdTe/ZnCdSe QDs for intermediate band solar cell (IBSC). Samples with a higher quantum dot density exhibit longer electron spin lifetimes, up to ~1 ns at low temperatures. Tellurium isoelectronic centers, which form in the ZnSe spacer regions as a result of the growth conditions, were also probed. A new growth sequence for type-II (Zn ...


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