Doctor of Philosophy, The Ohio State University, 2024, Chemistry

The design of next generation electrical and thermal transport materials is of far-reaching importance for myriad applications from thermoelectrics to dynamic transport switching. To that end, axis-dependent conduction polarity and thermal-switching materials hold significant promise. Axis-dependent conduction polarity (ADCP) is a phenomenon in which the electrons (n-type carriers) and holes (p-type carriers) are preferentially conducted along orthogonal directions in a crystal. The driving force for this phenomenon is a large (> 10x) anisotropy in the electron and hole mobilities between orthogonal directions. Herein is discussed the development of the first air-stable, wide bandgap (> 0.4 eV) semiconductor that displays ADCP, orthorhombic PdSe2. The anisotropy in the hole mobilities between the cross-plane and in-plane directions is > 100x, with holes preferentially conducting along the cross-plane direction. Additionally, the onset temperature of ADCP can be controlled via extrinsic doping with Ir and Sb as p-type and n-type dopants, respectively. When the chemical potential is near the valance band (Ir doping), ADCP is not observed up to 400 K. When it is mid-gap the onset temperature is about 350 K. But when it is near the conduction band (Sb doping), the onset temperature can be as low as 100 K. The dopant dependent onset temperature indicates the necessity for both the conduction and valance bands to be populated sufficiently to observe ADCP. Studies in this model system pave the way for further ADCP studies in semiconductors. Solid-state thermal switching is the rapid and reversible control over the thermal conductivity of a material between some low and high value without the need for physical phase changes or moving parts. Topologically non-trivial materials are promising candidates for solid-state thermal switching on account of their anomalous transport properties. Therefore, EuCd2As2 and MnBi2Te4 were studied for their thermal switching potential. Eu (open full item for complete abstract)

Committee: Joshua Goldberger (Advisor); Christine Thomas (Committee Member); Patrick Woodward (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2023, Physics

This thesis describes the role of magnetic materials in modern information technology research and application. Meanwhile, hybrid quantum systems are gaining great attention in the quantum computing area, which studies and manipulates the coupling physics between different platforms. Yttrium iron garnet (YIG) is one of the lowest magnetic damping materials and long has been studied for its potential in magnon-based information transmission, processing and storage. The integration of low damping YIG thin film in hybrid quantum systems is a promising research direction but has been hindered by its strongly increased damping due to its interaction with the gadolinium gallium garnet substrate, which YIG is commonly deposited on. In this thesis' work, improvement can be made by using a diamagnetic buffer layer or diamagnetic substrate, and a damping constant even lower than room temperature is obtained. Strong coupling in a hybrid system that incorporates a superconducting microwave resonator and the YIG film was demonstrated and proves that YIG epitaxial thin film can be an attractive candidate in low temperature applications.

Committee: Fengyuan Yang (Advisor); Richard Hughes (Committee Member); Jay Gupta (Committee Member); Nandini Trivedi (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2021, Physics

Time domain terahertz spectroscopy has allowed for a new way to analyze the properties of antiferromagnets. Since many materials have been explored using this technique, we took a different route for evaluating their properties. We evaluated how two different antiferromagnets (CaFe2O4 and TbMn2O5) interacted with metamaterials. CaFe2O4 was coupled to split ring resonators and TbMn2O5 was coupled to gammadion crosses. From the experiment performed on the CaFe2O4/split ring resonator sample, we did not find sufficient evidence indicating coupling between the sample and the metamaterial. For the TbMn2O5/gammadion sample, we observed an improvement in the efficiency of the electromagnon excitation compared to the bare sample. To understand why the expected anticrossing, an effect observed in coupled oscillator systems, was absent from either measurement, coupling effects between split ring resonators and a hypothetical antiferromagnet were analyzed more deeply utilizing numerical methods. From here we found that an anticrossing will occur when the spins in the crystal are parallel with the interface of the sample. This would allow for improved coupling between the magnetic moment of the split ring resonators and the antiferromagnet. From the data we were able to confirm the presence of an anticrossing. Following the metamaterial project, we began the development of an additional time domain terahertz technique, on chip terahertz, which allowed us to perform measurements on antiferromagnets that were not easily probed. This technique was applied to two different antiferromagnets, CaFe2O4 and MnPS3. For CaFe2O4, we observed a possible absorption in the spectrum that could be connected to on the magnon modes. For MnPS3, we detected three possible modes, one of which could be a low frequency magnon.

Committee: Rolando Valdes Aguilar (Advisor); Marc Bockrath (Committee Member); Ilya Gruzberg (Committee Member); Louis DiMauro (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2020, Mechanical Engineering

This dissertation is trying to elucidate the thermal transport of spin collective excitation in material systems that are either crystal structurally or magnetic structurally disordered. In magnetic ordered materials, magnon drag effect contributes significantly to thermopower, which has been quantitatively modeled and experimentally verified recently in elemental Fe, Co, and Ni. The main result of this thesis is to show how, in fact, the thermal excitation of spin ensemble, not only gives rise to magnon drag effect in magnetic ordered materials, but may also lead to similar effect in paramagnetic materials, as is the case of Li-doped MnTe. Even if the crystal structure displays significant disorder, the magnetic ordering could be unaffected and the thermally driven magnon flux still exists, which is the case for organic magnetic material V[TCNE]x. The starting point, which is detailed in chapter 2, is to verify the hydrodynamic theory of magnon-drag effect in binary Fe-rich, Fe-Co body-centered-cubic random alloys. The sign of the low temperature behavior of thermopower is explained well by the hydrodynamic theory for magnon-drag, informed by density functional theory calculations of the ground state of Fe-Co alloys. The high-temperature thermopower of some of the alloys, and indeed that of elemental iron, changes the sign, as previously observed. We propose a mechanism to elucidate this hitherto unexplained observation. Further, the power factor of Fe72Co28 peaks around 35 μV/cmK at 500 K, comparable to the standard thermoelectric material Bi2Te3. Because of their high thermoelectric power factor, Fe-Co alloys are potential candidate thermoelectric metals for active cooling of electronic devices. This work not only further verified the validity of the hydrodynamic theory, but also demonstrates a universal way of investigating magnon drag effect in magnetic alloy systems. The second part extends the existing research on magnon-drag into the paramagnetic regim (open full item for complete abstract)

Committee: Joseph Heremans (Advisor); Igor Adamovich (Committee Member); Roberto Myers (Committee Member); Wolfgang Windl (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2020, Materials Science and Engineering

The intrinsic spin Seebeck effect (SSE) describes the thermally driven spin currents within the bulk of materials. The intrinsic SSE current in yttrium iron garnet (YIG) drives a secondary bulk spin current at material boundaries and interfaces. The nonequilibrium magnons which carry this second spin current are parameterized by the magnon chemical potential, with a diffusion length of several microns in YIG. High speed transient optothermal measurements of SSE are performed on a platinum/yttrium iron garnet bilayer across a broad range of temperatures from 4 to 300 K. Interfacial SSE, driven by an electron magnon temperature difference develops over short time scales, ns, while the magnon chemical potential driven by intrinsic SSE develops over hundreds of microseconds. Time dependent SSE data are fit to a multi-temperature coupled spin-heat transport model using the finite element method (FEM). The nonequilibrium magnon lifetime, magnon-phonon thermalization time and an interfacial to intrinsic scaling ratio are used as free parameters to fit the data. The lifetime varies from 2 to 60 microseconds, and the low temperature drop is consistent with the relaxation of low energy magnons by impurities reported in ferromagnetic resonance measurements. The magnon-phonon thermalization time varies from 100 picoseconds to 20 nanoseconds. A nonmonotonic time dependence of SSE at low temperature is explained by the nonequilibrium between magnon and phonon temperatures, controlled by the thermalization time.

Committee: Roberto Myers (Advisor); Joseph Heremans (Committee Member); Fengyuan Yang (Committee Member) Subjects: Condensed Matter Physics; Energy; Materials Science; Solid State Physics

Doctor of Philosophy, The Ohio State University, 2020, Physics

Magnetic resonance is a powerful technique for detection and manipulation of spin dynamics. Resonance techniques are used in medical settings to non-invasively image patients in real time. In biology and chemistry settings resonance can be used to detect the local environment of spins which can reveal molecular structure and function. For condensed matter physicists the resonant response of novel materials can give insight about the underlying magnetic interactions by using the spins inside the sample as a probe of the local environment. My work focuses on using the nitrogen-vacancy defect center (NV center) in diamond to detect magnetic noise from nearby ferromagnetic thin films. NV centers enable optical detection of magnetic dynamics with nanoscale spatial resolution owing to their spin dependent fluorescence, high sensitivity to magnetic fields and atomic scale size. Ferromagnets are used in many applications like microwave resonators and filters owing to their wide frequency tunability with magnetic field. The damping parameter describes the relaxation of microwave-driven ferromagnetic samples and is akin to the quality factor of the ferromagnet as a resonator. Understanding the microscopic damping processes is in general challenging, but the application of NV centers to study magnetic field noise from nearby ferromagnetic layers can give valuable insight about the damping mechanisms. Magnetic field fluctuations associated with the damping processes in ferromagnets create stray dipolar fields which can couple to and relax nearby NV defects, allowing for local measurement of the ferromagnetic noise spectrum. I used NV centers to study the damping process in an organic magnetic sample vanadium tetracyanoethylene, a material of significant applications interest for its extremely low damping in combination with deposition on diverse substrates. We found that the magnon spectrum in this material is quite different than conventional low damping materials, which (open full item for complete abstract)

Committee: P. Chris Hammel (Advisor); Fengyuan Yang (Committee Member); Tin-Lun Ho (Committee Member); Amy Connolly (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2019, Physics

In crystalline materials, the symmetry of the crystal lattice imposes strict conditions on the observable properties of the material. These symmetry restricted conditions can be, in turn, probed by light via the electromagnetic interaction. Studying the electromagnetic excitations in solids can reveal many fundamental properties of these systems. A quick introduction and guide to symmetry in solids will be given, with an emphasis on how it can be used to interpret spectroscopic measurements. The measurement techniques used will also be described. Time domain Terahertz spectroscopy (TDTS) is the main technique used in this dissertation. Important experimental considerations pertaining to the construction of the THz spectrometer will be given. In the multiferroic Sr_2 FeSi_2O_7, we found multiple excitations in the few meV energy scale (THz), in the material's paramagnetic phase. Measurements with varying temperature and magnetic field revealed that these excitations are both electric and magnetic dipole active. By considering the ground state of the Fe 2+ magnetic ion in Sr 2 FeSi 2 O 7 , we concluded that our observation is coming from the spin-orbital coupled states of the ion. This realization demonstrated that spin-orbit coupling plays a crucial role in these exotic materials. Interestingly, these spin-orbital THz excitations persist into the magnetically ordered phase. The single-ion picture of the paramagnetic phase needs to be expanded theoretically to explain our observations. CaFe_2O_4 orders antiferromagnetically below ~ 200 K. Two co-existing magnetic structures (A and B phase) have been measured previously by neutron diffraction. The anti-phase boundaries between these two phases have been proposed to be the cause of the quantized magnetic excitations (magnons) measured by an inelastic neutron scattering study. We measured two antiferromagnetic resonances (magnons) with TDTS. Our observation can be explained by the orthorhombic crystal anisotropy of CaF (open full item for complete abstract)

Committee: Rolando Valdes Aguilar (Advisor); P. Chris Hammel (Committee Member); Nandini Trivedi (Committee Member); Douglass Schumacher (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2018, Physics

In this dissertation, we focus on topological phases protected or enforced by crystalline symmetries. The topological phase can appear in the ground state of fermion and interacting boson or in the excitation bands of the free boson such as phonon and magnon. Those topics are covered in three parts. In the second chapter, we study the Glide Symmetry Protected Topological (GSPT) phases of interacting bosons and fermions in three spatial dimensions with certain on-site symmetries. They are crystalline Symmetry Protected Topological (SPT) phases, which are distinguished from a trivial product state only in the presence of non-symmorphic glide symmetry. We classify these GSPT phases with various on-site symmetries such as $U(1)$ and time reversal, and show that they can all be understood by stacking and coupling two-dimensional short-range-entangled phases in a glide-invariant way. Using such a coupled layer construction we study the anomalous surface topological orders of these GSPT phases, which gap out the two-dimensional surface states without breaking any symmetries. While this framework can be applied to any non-symmorphic SPT phase, we demonstrate it in many examples of GSPT phases including the non-symmorphic topological insulator with "hourglass fermion" surface states. In the third chapter, we discuss the non-symmorphic crystalline symmetry enforced Quantum Spin Hall effect (QSHE). In the generic classification frame of SPT phases, the classification is $\mbz_2$ for the ground state of fermion in 2D with charge conservation and time-reversal (TR) symmetry. The ground state can be in either trivial state or QSHE state, which depends on the specific model. However, we prove that the gapped ground state must be QSHE other than trivial as long as given non-symmorphic symmetry present. We also propose a crystal structure as one realization, and make an effort on material searching based on the space group. This work can supply a new principle of QSHE mater (open full item for complete abstract)

Committee: Yuan-Ming Lu (Advisor); Tin-Lon Ho (Committee Member); Eric Braaten (Committee Member); Jay Gupta (Committee Member) Subjects: Physics; Theoretical Physics

Doctor of Philosophy, The Ohio State University, 2018, Mechanical Engineering

The majority of the world's energy comes from nonrenewable sources, with over 60% rejected as waste-heat. If waste-heat could be recovered, the effect on humanity would be equivalent to that of adding a renewable energy source, majorly increasing society's energy conversion efficiency. This can be accomplished through the use of thermoelectric materials, which convert a temperature gradient (like that from waste-heat) into a usable voltage output. Conventional thermoelectric materials have not increased in commercial efficiency in recent years, therefore a different approach is taken in this dissertation. Here, metals are explored as thermoelectric materials despite having a low thermoelectric efficiency. Magnon drag, an advective process utilizing magnetization dynamics in a temperature gradient to pull charge carriers through a crystal lattice, is shown to dominate thermoelectric transport in ferromagnetic transition metals. Experimental comparison with theory offers a pathway for increasing the thermoelectric efficiency in metals by increasing their magnon-drag thermopower. Additionally, novel transport is explored in the recently experimentally-realized class of materials called Weyl semimetals, where tuning the electronic band structure gives unique topological transport signatures. Predicted to have large transverse transport coefficients, NbP is experimentally proven to effectively convert a temperature gradient into a perpendicular output voltage via the Nernst effect. Transverse thermoelectric devices have technological advantages over conventional Peltier or Seebeck longitudinal modules (in which the applied temperature gradient is parallel to the output voltage), but they require an externally applied magnetic field. Further control over the band structure in Weyl semimetals offers a solution, where YbMnBi2 is experimentally predicted to effectively utilize a transverse geometry without the need for an external magnetic field. This effect is pr (open full item for complete abstract)

Committee: Joseph Heremans PhD (Advisor); Nandini Trivedi PhD (Committee Member); Fengyuan Yang PhD (Committee Member); Igor Adamovich PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2017, Electrical and Computer Engineering

Over the last two decades, spin transistors that operate using both charge and spin properties of electrons have motivated extensive studies of injection, detection and manipulation of electronic spin current in various material systems. Dilute magnetic semiconductors, in which the spin polarized charge carriers are coupled to the magnetic moment, are of particular interest due to their compatible lattice structures and similar growth methods to current Si and GaAs technology. The first part of this thesis focuses on the structural, magnetic and magnetotransport properties of magnetically doped GaN and 2D MoS2. The Gd doped AlN/GaN heterostructures are grown by plasma assisted molecular beam epitaxy. The Gd atoms are δ-doped at the AlN/GaN heterointerface where the two dimensional electron gas (2DEG) forms. These samples exhibit defect-induced room temperature ferromagnetism with an easy axis along the c-axis. However, the nonlinear Hall resistivity does not track the magnetization in these Gd doped samples indicating the lack of coupling between the conduction electrons in the 2DEG and the Gd-induced ferromagnetism. This makes Gd doped GaN not useful as a dilute magnetic semiconductor. Mn doped few-layer MoS2 samples synthesized via sulfurization of Mn thin film on sapphire are fabricated in the aim of realizing a 2D dilute magnetic semiconductor. However, these samples mainly show paramagnetism implying the lack of ferromagnetic coupling between the Mn dopants. In addition to the electronic spin current, magnonic spin current has recently received growing research interest since it serves as a new route for achieving novel thermoelectric generators and magnon transistors. The second part of the thesis focuses on the study of the transport properties of the thermally induced magnonic spin current via spin Seebeck effect. A nonlocal opto-thermal spin Seebck configuration is proposed and implemented to measure the spin diffusion length in yttrium iron garnet (YIG). (open full item for complete abstract)

Committee: Roberto Myers (Advisor); Joseph Heremans (Committee Member); Tyler Grassman (Committee Member) Subjects: Electrical Engineering; Materials Science