Detecting Majorana Fermion Induced Crossed Andreev Reflection, 2017 The Graduate Center, City University of New York

#### Detecting Majorana Fermion Induced Crossed Andreev Reflection, Lei Fang

*All Graduate Works by Year: Dissertations, Theses, and Capstone Projects*

This dissertation is devoted to a study of detecting the Majorana fermion induced crossed Andreev reflection.

Majorana fermions are particles that constitute their own antiparticles. In condensed matter physics, Majorana fermions are zero energy modes that reside at edges or around vortices of topological superconductors. The special properties of Majorana fermions result in their potential to conduct topological quantum computation, which has been attracting a lot of current research. One of the most important issues in the field of the Majorana fermion physics now is to detect their existence in realistic systems. Among many classes of detecting methods, a transport ...

Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, 2017 University of Nothern Iowa

#### Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, Pavel V. Lukashev, Liuyan Zhao, Tula R. Paudel, Theanne Schiros, Noah Hurley, Evgeny Y. Tsymbal, Aron Pinczuk, Abhay Pasupathy, Rui He

*Pavel Lukashev*

The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In particular, we employ first-principles calculations and scanning Raman analysis to investigate the dependence of phonon frequencies in graphene on the concentration of nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory behavior as a function of nitrogen concentration. We analyze different mechanisms which could potentially be responsible for this behavior, such as Friedel charge oscillations around the localized nitrogen impurity atom, the bond length change between nitrogen impurity and its nearest neighbor carbon atoms, and the long-range interactions of the nitrogen point defects. We ...

Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, 2017 University of Nothern Iowa

#### Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, Pavel V. Lukashev, Liuyan Zhao, Tula R. Paudel, Theanne Schiros, Noah Hurley, Evgeny Y. Tsymbal, Aron Pinczuk, Abhay Pasupathy, Rui He

*Rui He*

The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In particular, we employ first-principles calculations and scanning Raman analysis to investigate the dependence of phonon frequencies in graphene on the concentration of nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory behavior as a function of nitrogen concentration. We analyze different mechanisms which could potentially be responsible for this behavior, such as Friedel charge oscillations around the localized nitrogen impurity atom, the bond length change between nitrogen impurity and its nearest neighbor carbon atoms, and the long-range interactions of the nitrogen point defects. We ...

Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, 2017 University of Nothern Iowa

#### Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, Pavel V. Lukashev, Liuyan Zhao, Tula R. Paudel, Theanne Schiros, Noah Hurley, Evgeny Y. Tsymbal, Aron Pinczuk, Abhay Pasupathy, Rui He

*Rui He*

The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In particular, we employ first-principles calculations and scanning Raman analysis to investigate the dependence of phonon frequencies in graphene on the concentration of nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory behavior as a function of nitrogen concentration. We analyze different mechanisms which could potentially be responsible for this behavior, such as Friedel charge oscillations around the localized nitrogen impurity atom, the bond length change between nitrogen impurity and its nearest neighbor carbon atoms, and the long-range interactions of the nitrogen point defects. We ...

Machine Learning Phases Of Strongly Correlated Fermions, 2017 San Jose State University

#### Machine Learning Phases Of Strongly Correlated Fermions, Kelvin Ch'ng, Juan Carrasquilla, Roger Melko, Ehsan Khatami

*Faculty Publications*

Machine learning offers an unprecedented perspective for the problem of classifying phases in condensed matter physics. We employ neural network machine learning techniques to distinguish finite-temperature phases of the strongly-correlated fermions on cubic lattices. We show that a three-dimensional convolutional network trained on auxiliary field configurations produced by quantum Monte Carlo simulations of the Hubbard model can correctly predict the magnetic phase diagram of the model at the average density of one (half filling). We then use the network, trained at half filling, to explore the trend in the transition temperature as the system is doped away from half filling ...

Machine Learning Phases Of Strongly Correlated Fermions, 2017 San Jose State University

#### Machine Learning Phases Of Strongly Correlated Fermions, Kelvin Ch'ng, Juan Carrasquilla, Roger G. Melko, Ehsan Khatami

*Ehsan Khatami*

Permeability And Kinetic Coefficients For Mesoscale Bcf Surface Step Dynamics: Discrete Two-Dimensional Deposition-Diffusion Equation Analysis, 2017 Iowa State University

#### Permeability And Kinetic Coefficients For Mesoscale Bcf Surface Step Dynamics: Discrete Two-Dimensional Deposition-Diffusion Equation Analysis, Renjie Zhao, James W. Evans, Tiago J. Oliveira

*James W. Evans*

A discrete version of deposition-diffusion equations appropriate for description of step flow on a vicinal surface is analyzed for a two-dimensional grid of adsorption sites representing the stepped surface and explicitly incorporating kinks along the step edges. Model energetics and kinetics appropriately account for binding of adatoms at steps and kinks, distinct terrace and edge diffusion rates, and possible additional barriers for attachment to steps. Analysis of adatom attachment fluxes as well as limiting values of adatom densities at step edges for nonuniform deposition scenarios allows determination of both permeability and kinetic coefficients. Behavior of these quantities is assessed as ...

Submonolayer Ag Films On Fe(100): A First-Principles Analysis Of Energetics Controlling Adlayer Thermodynamics And Kinetics, 2017 South University of Science and Technology of China

#### Submonolayer Ag Films On Fe(100): A First-Principles Analysis Of Energetics Controlling Adlayer Thermodynamics And Kinetics, Wei Li, Li Huang, James W. Evans, Yong Han

*James W. Evans*

Epitaxial growth of Ag on Fe(100) and postdeposition relaxation have been studied in several experiments. We provide a first-principles density functional theory analysis of key adatom interaction energies and diffusion barriers controlling growth and relaxation kinetics for the submonolayer regime, as these have not been assessed previously. A cluster expansion approach is used to obtain an extensive set of conventional lateral interactions between adatoms on fourfold hollow adsorption sites. We find robust oscillatory decay of pair interactions with increasing separation, and of trio interactions with increasing perimeter length. First- and second-nearest-neighbor pair interactions, as well as compact linear and ...

Diffusion Of Noninteracting Particles Into A Semi-Infinite Lattice, 2017 Iowa State University

#### Diffusion Of Noninteracting Particles Into A Semi-Infinite Lattice, James W. Evans

*James W. Evans*

Consider a gas of noninteracting particles diffusing into a semi-infinite lattice, Z+×ZD−1, subject to exclusion of double occupancy. An exact closed set of equations can be obtained and solved (in closed form) for the single-site occupancies or densities. Equations for the n-site occupancies couple only to such functions for n and n-1 sites, as for an infinite lattice. Their analysis reveals that negative pair correlations develop during this process, reflecting the propensity for diffusion to reduce density inhomogeneities and fluctuations.

Diffraction From Disordered One-Dimensional Islands With Domain Boundaries: Intensity Behavior For Various Statistical Approximations, 2017 Iowa State University

#### Diffraction From Disordered One-Dimensional Islands With Domain Boundaries: Intensity Behavior For Various Statistical Approximations, James W. Evans, R. S. Nord

*James W. Evans*

We consider the angular distribution of the diffracted intensity, I(q), for systems of disordered one-dimensional double-spaced islands with domain boundaries. Although I(q) is directly determined by the spatial pair correlations, it is often naturally reexpressed in terms of island size and separation distributions. We analyze the effect on I(q) of various approximate specifications of the island statistics. In particular, we highlight the approximations implicit in Guinier (-type) formulations, and provide a new very accurate approximation. Motivated by the scarcity of analysis for kinetically limited island growth (often seen in chemisorption), we consider such an irreversible cooperative filling ...

Characterizing Kinetics Near A First-Order Catalytic-Poisoning Transition, 2017 Iowa State University

#### Characterizing Kinetics Near A First-Order Catalytic-Poisoning Transition, James W. Evans, M. S. Miesch

*James W. Evans*

We conside the kinetics near a first-order poisoning transition for simple surface reaction models which exlude nonreactive desorption. Elucidation of the development and growth of the reactive steady state from a near-poisoned state is achieved via an ‘‘oxidation epidemic’’ analysis of the evolution of an empy patch embedded in the poisoned phase. Associated critical exponents vary strongly with reaction rate, and reflect slow kinetics. The increase in epidemic survival probability with initial patch size is smooth, and described by additional exponents, rather than indicating a critical size for survival.

Scaling Analysis Of Surface Roughness In Simple Models For Molecular-Beam Epitaxy, 2017 Iowa State University

#### Scaling Analysis Of Surface Roughness In Simple Models For Molecular-Beam Epitaxy, H. C. Kang, James W. Evans

*James W. Evans*

There is currently interest in simple models for molecular-beam epitaxy (MBE) which mimic the effect of thermal mobility by allowing immediate incorporation of deposited atoms at kink sites within a distance l of the deposition site. Scaling of the interface width, W, with mean film height, [h], of the form W approximately [h]beta, is analysed. Studies for a solid-on-solid geometry in d = 1 + 1 dimensions revealed a sudden transition from the T = 0 K (l = 0) behavior of beta = 1/2 to a new universality class for l greater-than-or-equal-to 1 with beta = 3/8. We consider the effect of ...

Kinetics Of Restricted Solid-On-Solid Models Of Film Growth, 2017 Iowa State University

#### Kinetics Of Restricted Solid-On-Solid Models Of Film Growth, James W. Evans, M. C. Bartelt

*James W. Evans*

We consider the kinetics of irreversible film growth in solid-on-solid models with various restrictions on the adsorption (or growth) sites. We show how the master equations for the probabilities of subconfigurations of filled sites can be analyzed exactly to obtain coverages and spatial correlations for the first several layers. These provide an efficient framework for analysis of the early-stage growth kinetics, and indicate rapid attainment of asymptotic behavior. We illustrate the (1+1)- and (2+1)-dimensional cases for the simplest restricted solid-on-solid condition, and various modifications.

Novel Half-Metallic And Spin-Gapless Heusler Compounds, 2017 University of Nebraska-Lincoln

#### Novel Half-Metallic And Spin-Gapless Heusler Compounds, Yunlong Jin

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

This thesis is devoted to experimental studies of Heusler compounds CoFeCrAl, CoFeCrX (X = Si, Ge) and Mn_{2}PtSn. These Heusler alloys present an interesting class of ferromagnetic materials for spintronic applications since they are predicted to be spin gapless semiconductors and have half-metallic properties with 100 % spin polarization at the Fermi level. In this thesis, the structural, magnetic, spin-polarization and electron- transport properties of the fabricated alloys were studied. CoFeCrAl thin films deposited on MgO exhibit nearly perfect epitaxy and a high degree of L2_{1} Heusler order. All considered types of chemical disorder destroy the spin-gapless semiconductivity of ...

Electronic Structure Of Rsb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu) Studied By Angle-Resolved Photoemission Spectroscopy, 2017 Iowa State University

#### Electronic Structure Of Rsb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu) Studied By Angle-Resolved Photoemission Spectroscopy, Yun Wu, Youngbin Lee, Tai Kong, Daixiang Mou, Rui Jiang, Sergey Bud'ko, Paul Canfield, Adam Kaminski

*Ames Laboratory Accepted Manuscripts*

We use high-resolution angle-resolved photoemission spectroscopy (ARPES) and electronic structure calculations to study the electronic properties of rare-earth monoantimonides *R*Sb (*R* = Y, Ce, Gd, Dy, Ho, Tm, Lu). The experimentally measured Fermi surface (FS) of *R*Sb consists of at least two concentric hole pockets at the Γ point and two intersecting electron pockets at the X point. These data agree relatively well with the electronic structure calculations. Detailed photon energy dependence measurements using both synchrotron and laser ARPES systems indicate that there is at least one Fermi surface sheet with strong three-dimensionality centered at the Γ point. Due ...

Nmr Studies Of The Incommensurate Helical Antiferromagnet Euco2p2 : Determination Of The Antiferromagnetic Propagation Vector, 2017 Ames Laboratory and Iowa State University

#### Nmr Studies Of The Incommensurate Helical Antiferromagnet Euco2p2 : Determination Of The Antiferromagnetic Propagation Vector, Nonoka Higa, Qing-Ping Ding, Mamoru Yogi, N. S. Sangeetha, Masato Hedo, Takao Nakama, Yoshichika Onuki, D. C. Johnston, Yuji Furukawa

*Ames Laboratory Accepted Manuscripts*

Recently, Q.-P. Ding *et al.* [Phys. Rev. B 95, 184404 (2017)] reported that their nuclear magnetic resonance (NMR) study on EuCo2As2 successfully characterized the antiferromagnetic (AFM) propagation vector of the incommensurate helix AFM state, showing that NMR is a unique tool for determination of the spin structures in incommensurate helical AFMs. Motivated by this work, we have carried out 153Eu, 31P, and 59Co NMR measurements on the helical antiferromagnet EuCo2P2 with an AFM ordering temperature TN=66.5 K. An incommensurate helical AFM structure was clearly confirmed by 153Eu and 31P NMR spectra on single-crystalline EuCo2P2 in zero magnetic ...

Crossover In The Magnetic Response Of Single-Crystalline Ba1−Xkxfe2as2 And Lifshitz Critical Point Evidenced By Hall Effect Measurements, 2017 Iowa State University

#### Crossover In The Magnetic Response Of Single-Crystalline Ba1−Xkxfe2as2 And Lifshitz Critical Point Evidenced By Hall Effect Measurements, Yong Liu, Thomas A. Lograsso

*Thomas A. Lograsso*

We report on the doping evolution of magnetic susceptibility χ(T) and Hall coefficient RH in high-quality Ba1−xKxFe2As2 (0.13≤x≤1) single crystals. It is found that the normal-state magnetic susceptibility of Ba1−xKxFe2As2 compounds undergoes a crossover from linear-T dependence in the undoped and underdoped samples into KFe2As2-type magnetic response in the overdoped samples with increasing K content. Although magnetic susceptibility χ(T) of optimally doped samples (0.34≤x≤0.47) still follows a monotonic increase with increasing temperature, a big hump around 300 K emerges. As x exceeds 0.53, a broad peak forms in ...

Neutron-Scattering Measurements Of Spin Excitations In Lafeaso And Ba(Fe0.953co0.047)2as2: Evidence For A Sharp Enhancement Of Spin Fluctuations By Nematic Order, 2017 Iowa State University

#### Neutron-Scattering Measurements Of Spin Excitations In Lafeaso And Ba(Fe0.953co0.047)2as2: Evidence For A Sharp Enhancement Of Spin Fluctuations By Nematic Order, Qiang Zhang, Rafael M. Fernandes, Jagat Lamsal, Jiaqiang Yan, Songxue Chi, Gregory S. Tucker, Daniel K. Pratt, Jeffrey W. Lynn, R. William Mccallum, Paul C. Canfield, Thomas A. Lograsso, Alan I. Goldman, David Vaknin, Robert J. Mcqueeney

*Thomas A. Lograsso*

Inelastic neutron scattering is employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe0.953Co0.047)2As2. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at TS, sets in well above the stripe antiferromagnetic ordering at TN. We find that the temperature-dependent dynamic susceptibility displays an anomaly at TS followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that ...

Upper Critical Field Of Kfe2as2 Under Pressure: A Test For The Change In The Superconducting Gap Structure, 2017 Iowa State University

#### Upper Critical Field Of Kfe2as2 Under Pressure: A Test For The Change In The Superconducting Gap Structure, Valentin Taufour, Neda Foroozani, Makariy A. Tanatar, Jinhyuk Lim, Udhara S. Kaluarachchi, Stella K. Kim, Yong Liu, Thomas A. Lograsso, Vladimir G. Kogan, Ruslan Prozorov, Sergey L. Bud'ko, James S. Schilling, Paul C. Canfield

*Thomas A. Lograsso*

#### ABSTRACT

We report measurements of electrical resistivity under pressure to 5.8 GPa, magnetization to 6.7 GPa, and ac susceptibility to 7.1 GPa in KFe2As2. The previously reported change of slope in the pressure dependence of the superconducting transition temperature Tc(p) at a pressure p∗∼1.8 GPa is confirmed, and Tc(p) is found to be nearly constant above p∗ up to 7.1 GPa. The T−p phase diagram is very sensitive to the pressure conditions as a consequence of the anisotropic uniaxial pressure dependence of Tc. Across p∗, a ...

Magnetic Excitations And Anomalous Spin-Wave Broadening In Multiferroic Fev2o4, 2017 Iowa State University

#### Magnetic Excitations And Anomalous Spin-Wave Broadening In Multiferroic Fev2o4, Qiang Zhang, Mehmet Ramazanoglu, Songxue Chi, Yong Liu, Thomas A. Lograsso, David Vaknin

*Thomas A. Lograsso*

We report on the different roles of two orbital-active Fe2+ at the A site and V3+ at the B site in the magnetic excitations and on the anomalous spin-wave broadening in FeV2O4. FeV2O4 exhibits three structural transitions and successive paramagnetic (PM)–collinear ferrimagnetic (CFI)–noncollinear ferrimagnetic (NCFI)/ferroelectric transitions. The high-temperature tetragonal/PM–orthorhombic/CFI transition is accompanied by the appearance of a large energy gap in the magnetic excitations due to strong spin-orbit-coupling-induced anisotropy at the Fe2+ site. While there is no measurable increase in the energy gap from the orbital ordering of V3+ at the orthorhombic/CFI–tetragonal ...