Matching Items (19)
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- All Subjects: Physics
- Creators: Chen, Tingyong
- Member of: Theses and Dissertations
- Member of: Barrett, The Honors College Thesis/Creative Project Collection
Description
An electrical current with high spin polarization is desirable for the performance of novel spintronics devices, such as magnetic tunnel junction and giant magnetoresistance devices. The generation of spin polarized current can be from ferromagnetic materials or triplet superconductors.
Anomalous Hall effect (AHE) is an effective way to study the properties of magnetic structures. The scattering of electrons by the magnetic moments affects the change of resistance, which can be used to detect the magnetization. In this dissertation, AHE is used to study the perpendicular magnetic anisotropy (PMA) structures, including Co/Pt and Ta/CoFeB/MgO.
Domain walls exist in all ferromagnetic materials. This dissertation studies the domain wall movement in the Ta/CoFeB/MgO structure. A single domain is observed by measuring the anomalous Hall effect. On the other hand, a zero Hall step is successfully observed in a single layer of magnetic material for the first time, which can be used to fabricate advanced domain wall spintronics devices.
Besides the normal ferromagnetic material, the generation of spin polarized current in superconductor is also important for Spintronics. The electrons in superconductors form Cooper pairs. In this dissertation, Andreev Reflection Spectroscopy (ARS) is used to study the spin configuration in Cooper pairs.
Generally, ferromagnetism and superconductivity can not co-exist. In this dissertation, the Bi/Ni bilayer structure has been studied with ARS, and the measurement results show a triplet superconductivity below 4K. The appearance of superconductivity is believed to be attributed to the Bi-Ni interface, and the triplet Cooper pair makes it a promising candidate in superconducting spintronics.
Besides, a Bi3Ni single crystal is also studied with ARS. The measurements show a singlet superconductivity in this material, which further proves the importance of the Bi/Ni interface to achieve triplet superconductivity.
Finally, ARS is also used to study NbSe2 monolayer, a 2D superconductor. The monolayer is verified by the measurements of critical temperature and critical field, which are different from the values of multilayer or bulk. Andreev reflection results show that NbSe2 monolayer is a singlet superconductor and there is no node exist in the superconducting gap for a in plane magnetic field up to 58 kOe.
Anomalous Hall effect (AHE) is an effective way to study the properties of magnetic structures. The scattering of electrons by the magnetic moments affects the change of resistance, which can be used to detect the magnetization. In this dissertation, AHE is used to study the perpendicular magnetic anisotropy (PMA) structures, including Co/Pt and Ta/CoFeB/MgO.
Domain walls exist in all ferromagnetic materials. This dissertation studies the domain wall movement in the Ta/CoFeB/MgO structure. A single domain is observed by measuring the anomalous Hall effect. On the other hand, a zero Hall step is successfully observed in a single layer of magnetic material for the first time, which can be used to fabricate advanced domain wall spintronics devices.
Besides the normal ferromagnetic material, the generation of spin polarized current in superconductor is also important for Spintronics. The electrons in superconductors form Cooper pairs. In this dissertation, Andreev Reflection Spectroscopy (ARS) is used to study the spin configuration in Cooper pairs.
Generally, ferromagnetism and superconductivity can not co-exist. In this dissertation, the Bi/Ni bilayer structure has been studied with ARS, and the measurement results show a triplet superconductivity below 4K. The appearance of superconductivity is believed to be attributed to the Bi-Ni interface, and the triplet Cooper pair makes it a promising candidate in superconducting spintronics.
Besides, a Bi3Ni single crystal is also studied with ARS. The measurements show a singlet superconductivity in this material, which further proves the importance of the Bi/Ni interface to achieve triplet superconductivity.
Finally, ARS is also used to study NbSe2 monolayer, a 2D superconductor. The monolayer is verified by the measurements of critical temperature and critical field, which are different from the values of multilayer or bulk. Andreev reflection results show that NbSe2 monolayer is a singlet superconductor and there is no node exist in the superconducting gap for a in plane magnetic field up to 58 kOe.
ContributorsZhao, Gejian (Author) / Chen, Tingyong (Thesis advisor) / Bennett, Peter (Committee member) / Nemanich, Robert (Committee member) / Qing, Quan (Committee member) / Arizona State University (Publisher)
Created2018
Description
In this dissertation two kinds of strongly interacting fermionic systems were studied: cold atomic gases and nucleon systems. In the first part I report T=0 diffusion Monte Carlo results for the ground-state and vortex excitation of unpolarized spin-1/2 fermions in a two-dimensional disk. I investigate how vortex core structure properties behave over the BEC-BCS crossover. The vortex excitation energy, density profiles, and vortex core properties related to the current are calculated. A density suppression at the vortex core on the BCS side of the crossover and a depleted core on the BEC limit is found. Size-effect dependencies in the disk geometry were carefully studied. In the second part of this dissertation I turn my attention to a very interesting problem in nuclear physics. In most simulations of nonrelativistic nuclear systems, the wave functions are found by solving the many-body Schrödinger equations, and they describe the quantum-mechanical amplitudes of the nucleonic degrees of freedom. In those simulations the pionic contributions are encoded in nuclear potentials and electroweak currents, and they determine the low-momentum behavior. By contrast, in this work I present a novel quantum Monte Carlo formalism in which both relativistic pions and nonrelativistic nucleons are explicitly included in the quantum-mechanical states of the system. I report the renormalization of the nucleon mass as a function of the momentum cutoff, an Euclidean time density correlation function that deals with the short-time nucleon diffusion, and the pion cloud density and momentum distributions. In the two nucleon sector the interaction of two static nucleons at large distances reduces to the one-pion exchange potential, and I fit the low-energy constants of the contact interactions to reproduce the binding energy of the deuteron and two neutrons in finite volumes. I conclude by showing that the method can be readily applied to light-nuclei.
ContributorsMadeira, Lucas (Author) / Schmidt, Kevin E (Thesis advisor) / Alarcon, Ricardo (Committee member) / Beckstein, Oliver (Committee member) / Erten, Onur (Committee member) / Arizona State University (Publisher)
Created2018
Description
The structure of glass has been the subject of many studies, however some
details remained to be resolved. With the advancement of microscopic
imaging techniques and the successful synthesis of two-dimensional materials,
images of two-dimensional glasses (bilayers of silica) are now available,
confirming that this glass structure closely follows the continuous random
network model. These images provide complete in-plane structural information
such as ring correlations, and intermediate range order and with computer
refinement contain indirect information such as angular distributions, and
tilting.
This dissertation reports the first work that integrates the actual atomic
coordinates obtained from such images with structural refinement to enhance
the extracted information from the experimental data.
The correlations in the ring structure of silica bilayers are studied
and it is shown that short-range and intermediate-range order exist in such networks.
Special boundary conditions for finite experimental samples are designed so atoms
in the bulk sense they are part of an infinite network.
It is shown that bilayers consist of two identical layers separated by a
symmetry plane and the tilted tetrahedra, two examples of
added value through the structural refinement.
Finally, the low-temperature properties of glasses in two dimensions
are studied. This dissertation presents a new approach to find possible
two-level systems in silica bilayers employing the tools of rigidity theory
in isostatic systems.
details remained to be resolved. With the advancement of microscopic
imaging techniques and the successful synthesis of two-dimensional materials,
images of two-dimensional glasses (bilayers of silica) are now available,
confirming that this glass structure closely follows the continuous random
network model. These images provide complete in-plane structural information
such as ring correlations, and intermediate range order and with computer
refinement contain indirect information such as angular distributions, and
tilting.
This dissertation reports the first work that integrates the actual atomic
coordinates obtained from such images with structural refinement to enhance
the extracted information from the experimental data.
The correlations in the ring structure of silica bilayers are studied
and it is shown that short-range and intermediate-range order exist in such networks.
Special boundary conditions for finite experimental samples are designed so atoms
in the bulk sense they are part of an infinite network.
It is shown that bilayers consist of two identical layers separated by a
symmetry plane and the tilted tetrahedra, two examples of
added value through the structural refinement.
Finally, the low-temperature properties of glasses in two dimensions
are studied. This dissertation presents a new approach to find possible
two-level systems in silica bilayers employing the tools of rigidity theory
in isostatic systems.
ContributorsSadjadi, Seyed Mahdi (Author) / Thorpe, Michael F (Thesis advisor) / Beckstein, Oliver (Committee member) / Schmidt, Kevin E (Committee member) / Treacy, Michael Mj (Committee member) / Arizona State University (Publisher)
Created2018
Description
STEM education stands for science, technology, engineering and mathematics, and is necessary for students to keep up with global competition in the changing job market, technological advancements and challenges of the future. However, American students are lacking STEM achievement at the state, national and global levels. To combat this lack of achievement I propose that STEM instruction should begin in preschool, be integrated into the curriculum and be inquiry based. To support this proposal I created a month-long physics unit for preschoolers in a Head Start classroom. Students investigated the affect of incline, friction and weight on the distance of a rolling object, while developing their pre-math, pre-literacy and social emotional skills.
ContributorsGarrison, Victoria Leigh (Author) / Kelley, Michael (Thesis director) / Dahlstrom, Margo (Committee member) / Barrett, The Honors College (Contributor) / Division of Teacher Preparation (Contributor)
Created2015-05
Description
Over the past few years, the issue of childhood trauma in the United States has become significant. A growing number of children are experiencing abuse, neglect, or some other form of maltreatment each year. Considering the stressful home lives of maltreated children, the one sure sanctuary is school. However, this idea requires teachers to be actively involved in identifying and caring for the children who need it most. Traumatic childhood experiences leave lasting scars on its victims, so it is helpful if teachers learn how to identify and support children who have lived through them. It is unfortunate that teachers will most likely encounter children throughout their career who have experienced horrendous things, but it is a reality. With this being said, teachers need to develop an understanding of what traumatized children live with, and learn how to address these issues with skilled sensitivity. Schools are not just a place where children learn how to read and write; they build the foundation for a successful life. This project was designed to provide teachers with a necessary resource for helping children who have suffered traumatic experiences. The methodology of this project began with interviews with organizations specializing in working with traumatized children such as Arizonans for Children, Free Arts for Abused Children, The Sojourner Center, and UMOM. The next step was a review of the current literature on the subject of childhood trauma. The findings have all been compiled into one, convenient document for teacher use and distribution. Upon completion of this document, an interactive video presentation will be made available through an online education website, so that distribution will be made simpler. Hopefully, teachers will share the information with people in their networks and create a chain reaction. The goal is to make it available to as many teachers as possible, so that more children will receive the support they need.
ContributorsHanrahan, Katelyn Ann (Author) / Dahlstrom, Margo (Thesis director) / Kelley, Michael (Committee member) / Division of Teacher Preparation (Contributor) / Sanford School of Social and Family Dynamics (Contributor) / Barrett, The Honors College (Contributor)
Created2016-05
Description
Cubic boron nitride (c-BN) has potential for electronic applications as an electron emitter and serving as a base material for diodes, transistors, etc. However, there has been limited research on c-BN reported, and many of the electronic properties of c-BN and c-BN interfaces have yet to be reported. This dissertation focused on probing thin film c-BN deposited via plasma enhanced chemical vapor deposition (PECVD) with in situ photoelectron spectroscopy (PES). PES measurements were used to characterize the electronic properties of c-BN films and interfaces with vacuum and diamond. First, the interface between c-BN and vacuum were characterized with ultraviolet PES (UPS). UPS measurements indicated that as-deposited c-BN, H2 plasma treated c-BN, and annealed c-BN post H2 plasma treatment exhibited negative electron affinity surfaces. A dipole model suggested dipoles from H-terminated N surface sites were found to be responsible for the NEA surface. Then, Si was introduced into c-BN films to realize n-type doped c-BN. The valence structure and work function of c-BN:Si films were characterized with XPS and UPS measurements. Measurements were unable to confirm n-type character, and it is concluded that silicon nitride formation was the primary effect for the observations. Finally, XPS measurements were employed to measure the band offsets at the c-BN/diamond interface. Measurements indicated the valence band maximum (VBM) of c-BN was positioned ~0.8 eV above the VBM of diamond.
ContributorsShammas, Joseph (Author) / Nemanich, Robert J (Thesis advisor) / Ponce, Fernando (Committee member) / Chen, Tingyong (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2016
Description
This work presents analysis and results for the NPDGamma experiment, measuring
the spin-correlated photon directional asymmetry in the $\vec{n}p\rightarrow
d\gamma$ radiative capture of polarized, cold neutrons on a parahydrogen
target. The parity-violating (PV) component of this asymmetry
$A_{\gamma,PV}$ is unambiguously related to the $\Delta I = 1$ component of
the hadronic weak interaction due to pion exchange. Measurements in the second
phase of NPDGamma were taken at the Oak Ridge National Laboratory (ORNL)
Spallation Neutron Source (SNS) from late 2012 to early 2014, and then again in
the first half of 2016 for an unprecedented level of statistics in order to
obtain a measurement that is precise with respect to theoretical predictions of
$A_{\gamma,PV}=O(10^{-8})$. Theoretical and experimental background,
description of the experimental apparatus, analysis methods, and results for
the high-statistics measurements are given.
the spin-correlated photon directional asymmetry in the $\vec{n}p\rightarrow
d\gamma$ radiative capture of polarized, cold neutrons on a parahydrogen
target. The parity-violating (PV) component of this asymmetry
$A_{\gamma,PV}$ is unambiguously related to the $\Delta I = 1$ component of
the hadronic weak interaction due to pion exchange. Measurements in the second
phase of NPDGamma were taken at the Oak Ridge National Laboratory (ORNL)
Spallation Neutron Source (SNS) from late 2012 to early 2014, and then again in
the first half of 2016 for an unprecedented level of statistics in order to
obtain a measurement that is precise with respect to theoretical predictions of
$A_{\gamma,PV}=O(10^{-8})$. Theoretical and experimental background,
description of the experimental apparatus, analysis methods, and results for
the high-statistics measurements are given.
ContributorsBlyth, David (Author) / Alarcon, Ricardo O (Thesis advisor) / Ritchie, Barry G. (Committee member) / Comfort, Joseph R. (Committee member) / Schmidt, Kevin E (Committee member) / Arizona State University (Publisher)
Created2017
Description
A time-dependent semiclassical formalism is developed for the theory of incoherentdiffractive imaging (IDI), an atomically-precise imaging technique based on the principles of
intensity interferometry. The technique is applied to image inner-shell X-ray fluorescence
from heavy atoms excited by the femtosecond pulses of an X-ray free-electron laser (XFEL).
Interference between emission from different atoms is expected when the XFEL pulse duration
is shorter than the fluorescence lifetime. Simulations for atoms at the vertices of a simple
icosahedral virus capsid are used to generate mock IDI diffraction patterns. These are then
used to reconstruct the geometry by phase retrieval of the intensity correlation function between
photons emitted independently from many different atoms at two different detector
pixels. The dependence of the intensity correlation function on fluorescence lifetime relative
to XFEL pulse duration is computed, and a simple expression for the visibility (or contrast)
of IDI speckle as well as an upper bound on the IDI signal-to-noise ratio are obtained as a
function of XFEL flux and lifetime. This indicates that compact XFELs, with reduced flux
but attosecond pulses, should be ideally suited to 3D, atomic-resolution mapping of heavy
atoms in materials science, chemistry, and biology. As IDI is a new technique, not much has
yet been written about it in the literature. The current theoretical and experimental results
are reviewed, including a discussion of signal-to-noise issues that have been raised regarding
the idea that IDI is suitable for structural biology.
ContributorsShevchuk, Andrew Stewart Hegeman (Author) / Kirian, Richard A (Thesis advisor) / Schmidt, Kevin E (Committee member) / Weierstall, Uwe (Committee member) / Graves, William S (Committee member) / Arizona State University (Publisher)
Created2022
Description
Quantum Monte Carlo is one of the most accurate ab initio methods used to study nuclear physics. The accuracy and efficiency depend heavily on the trial wave function, especially in Auxiliary Field Diffusion Monte Carlo (AFDMC), where a simplified wave function is often used to allow calculations of larger systems. The simple wave functions used with AFDMC contain short range correlations that come from an expansion of the full correlations truncated to linear order. I have extended that expansion to quadratic order in the pair correlations. I have investigated this expansion by keeping the full set of quadratic correlations as well an expansion that keeps only independent pair quadratic correlations. To test these new wave functions I have calculated ground state energies of 4He, 16O, 40Ca and symmetric nuclear matter at saturation density ρ = 0.16 fm−3 with 28 particles in a periodic box. The ground state energies calculated with both wave functions decrease with respect to the simpler wave function with linear correlations only for all systems except 4He for both variational and AFDMC calculations. It was not expected that the ground state energy of 4He would decrease due to the simplicity of the alpha particle wave function. These correlations have also been applied to study alpha particle formation in neutron rich matter, with applications to neutron star crusts and neutron rich nuclei. I have been able to show that this method can be used to study small clusters as well as the effect of external nucleons on these clusters.
ContributorsPetrie, Cody L (Author) / Schmidt, Kevin E (Thesis advisor) / Shovkovy, Igor A. (Committee member) / Beckstein, Oliver (Committee member) / Alarcon, Ricardo O (Committee member) / Arizona State University (Publisher)
Created2019