Physics Commons^™

Laser-Induced Modifications In Two-Dimensional Materials, Tariq Afaneh

Graduate Theses and Dissertations

Atomically thin two-dimensional (2D) materials have attracted a growing interest in the lastdecade from the fundamental point of view as well as their potential applications in functional devices. Due to their high surface-to-volume ratio, the physical properties of 2D materials are very sensitive to the environmental factor such as surrounding media and illumination conditions (e.g. light-mater interaction). In the first part of this dissertation I will present recent advances in developing laser-assisted methods to tune the physical properties of 2D transition metal dichalcogenides (TMDs). We demonstrate laser-assisted chemical modification ultrathin TMDs, locally replacing selenium by sulfur atoms. The photo-conversion ...

Ferroelectric Polarization Control Of Magnetic Anisotropy In Pbzr0.2ti0.8o3/La0.8sr0.2mno3 Heterostructures, Anil Rajapitamahuni, L. L. Tao, Y. Hao, Jingfeng Song, Xiaoshan Xu, Evgeny Y. Tsymbal, Xia Hong

Evgeny Tsymbal Publications

The interfacial coupling between the switchable polarization and neighboring magnetic order makes ferroelectric/ferromagnetic composite structures a versatile platform to realize voltage control of magnetic anisotropy. We report the nonvolatile ferroelectric field effect modulation of the magnetocrystalline anisotropy (MCA) in epitaxial PbZr_0.2Ti_0.8O₃ (PZT)/La_0.8Sr_0.2MnO₃ (LSMO) heterostructures grown on (001) SrTiO₃ substrates. Planar Hall effect measurements show that the in-plane magnetic anisotropy energy in LSMO is enhanced by about 22% in the hole accumulation state compared to the depletion state, in quantitative agreement with our first-principles ...

Electronic And Thermal Transport Properties Of Engineered Organic-Inorganic Halide Perovskite Structures, Rahul Singh

Graduate Theses and Dissertations

Organic-inorganic halide perovskites have been on the radar of the photovoltaic community for a few years now. Significant research has been done on making the solar cell devices based on these materials more efficient. These perovskites have the following excellent photovoltaic properties : desirable band gap, high number of charge carriers, and high absorption coefficient. In this thesis, we aim to enhance these properties by using low-dimensional materials. We consider heterostructures of the organic-inorganic halide perovskites and characterize them for their thermo-electric properties. The heterostructures are constructed using the idea of low-dimensional structures.

In this work we engineer these structures with ...

Growth, Characterization, And Function Of Ferroelectric, Ferromagnetic Thin Films And Their Heterostructures, Mahesh Hordagoda

Graduate Theses and Dissertations

With recent trends in miniaturization in the electronics sector, ferroelectrics have gained popularity due to their applications in non-volatile RAM. Taking one step further researchers are now exploring multiferroic devices that overcome the drawbacks of ferroelectric (FE) and ferromagnetic (FM) RAM’s while retaining the advantages of both. The work presented in this dissertation focuses on the growth of FE and FM thin film structures. The primary goals of this work include, (1) optimization of the parameters in the pulsed laser deposition (PLD) of FE and FM films and their heterostructures, (2) development of a structure-property relation that leads to ...

Spin-Polarized Two-Dimensional Electron Gas At Gdtio3/Srtio3 Interfaces: Insight From First-Principles Calculations, J. Betancourt, Tula R. Paudel, Evgeny Y. Tsymbal, J. P. Velev

Evgeny Tsymbal Publications

Two-dimensional electron gases (2DEGs) at oxide interfaces have been a topic of intensive research due to their high carrier mobility and strong confinement. Additionally, strong correlations in the oxide materials can give rise to new and interesting physics, such as magnetism and metal-insulator transitions at the interface. Using first-principles calculations based on density functional theory, we demonstrate the presence of a highly spin-polarized 2DEG at the interface between the Mott insulator GdTiO₃ and a band insulator SrTiO₃. The strong correlations in the dopant cause ferromagnetic alignment of the interface Ti atoms and result in a fully spin-polarized 2DEG ...

Recent Advances In Electronic And Optoelectronic Devices Based On Two-Dimensional Transition Metal Dichalcogenides, Mingxiao Ye, Dongyan Zhang, Yoke Khin Yap

Department of Physics Publications

Two-dimensional transition metal dichalcogenides (2D TMDCs) offer several attractive features for use in next-generation electronic and optoelectronic devices. Device applications of TMDCs have gained much research interest, and significant advancement has been recorded. In this review, the overall research advancement in electronic and optoelectronic devices based on TMDCs are summarized and discussed. In particular, we focus on evaluating field effect transistors (FETs), photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and integrated circuits (ICs) using TMDCs.

Structure-Interaction Effects In Novel Nanostructured Materials, Nam B. Le

Graduate Theses and Dissertations

Recent advances in experimental and computational methods have opened up new directions in graphene fundamental studies. In addition to understanding the basic properties of this material and its quasi-one dimensional structures, significant efforts are devoted to describing their long ranged dispersive interactions. Other two-dimensional materials, such as silicene, germanene, and transition metal dichalcogenides, are also being investigated aiming at finding complementary to graphene systems with other "wonder" properties. The focus of this work is to utilize first principles simulations methods to build our basic knowledge of structure-interaction relations in two-dimensional materials and design their properties. In particular, mechanical folding and ...

Long-Range Electronic Reconstruction To A Dxz,Yz-Dominated Fermi Surface Below The Laalo3/Srtio3 Interface, Alexander Paul Petrović, A. Pare, Tula R. Paudel, K. Lee, S. Holmes, Crispin H.W. Barnes, A. David, T. Wu, Evgeny Y. Tsymbal, C. Panagopoulos

Evgeny Tsymbal Publications

Low dimensionality, broken symmetry and easily-modulated carrier concentrations provoke novel electronic phase emergence at oxide interfaces. However, the spatial extent of such reconstructions - i.e. the interfacial ‘‘depth’’ - remains unclear. Examining LaAlO₃/SrTiO₃ heterostructures at previously unexplored carrier densities n_2D 14 cm^-2 , we observe a Shubnikov-de Haas effect for small in-plane fields, characteristic of an anisotropic 3D Fermi surface with preferential d_xz,yz orbital occupancy extending over at least 100 nm perpendicular to the interface. Quantum oscillations from the 3D Fermi surface of bulk doped SrTiO₃ emerge simultaneously at higher n_2D. We distinguish ...

Polarization-Controlled Ohmic To Schottky Transition At A Metal/Ferroelectric Interface, Xiaohui Liu, Yong Wang, John D. Burton, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Ferroelectric polar displacements have recently been observed in conducting electron-doped BaTiO₃ (n-BTO). The coexistence of a ferroelectric phase and conductivity opens the door to new functionalities that may provide a unique route for novel device applications. Using first-principles methods and electrostatic modeling, we explore the effect that the switchable polarization of n-BTO has on the electronic properties of the SrRuO₃/n-BTO (001) interface. Ferroelectric polarization controls the accumulation or depletion of electron charge at the interface, and the associated bending of the n-BTO conduction band determines the transport regime across the interface. The interface exhibits ...

Mott Transition And Electronic Structure In Complex Oxide Heterostructures, Jian Liu

Theses and Dissertations

Strongly correlated electron systems, particularly transition metal oxides, have been a focus of condensed matter physics for more than two decades since the discovery of high-temperature superconducting cuprates. Diverse competing phases emerge, spanning from exotic magnetism to unconventional superconductivity, in proximity to the localized-itinerant transition of Mott insulators. While studies were concentrated on bulk crystals, the recent rapid advance in synthesis has enabled fabrication of high-quality oxide heterostructures, offering a new route to create novel artificial quantum materials.

This dissertation details the investigation on ultrathin films and heterostructures of 3d⁷(t_2g⁶e_g¹) systems with spin ...

Domain State Susceptibility In Fecl2/Copt-Heterostructures, Christian Binek

Christian Binek

Magnetic heterostructures with perpendicular anisotropy consisting of antiferromagnetic (111)-oriented FeCl2 single crystals and thin ferromagnetic CoPt-multilayers are investigated by ac-susceptometry after cooling in various axial magnetic fields to below the Néel temperature. The freezing process drives the system into a metastable antiferromagnetic domain state which gives rise to a huge excess contribution to the complex parallel susceptibility on heating in zero magnetic field. Its glow curve-like temperature dependence is modeled within the framework of a simple analytical approach. It involves the susceptibility of thermally activated clusters originating from antiferromagnetic stacking faults as well as the contribution of pairs of ...

Exchange Bias In Fe0.6zn0.4f2 Heterostructures, Christian Binek

Christian Binek

The previous exchange bias field, He, is measured in Fe0.6Zn0.4F2/Fe heterosystems prepared from Fe layers of 14 and 5 nm thickness which are deposited on top of the compensated (1 1 0) surface of the antiferromagnet. Deviations from a linear dependence of He on the magnetization of the Fe layer are attributed to ferromagnetic domains. Moreover, piezomagnetism and its influence on He are evidenced.

Thermodynamics Of Magnetic Multilayers, Tathagata Mukherjee

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

Our interest in thermodynamics of magnetic thin film heterostructure began by exploring the possibility to use magnetic nanostructures in the search for optimized magnetocaloric materials for potential room temperature refrigeration. In the present thesis magnetic thin film heterostructures are experimentally realized by Molecular Beam Epitaxy (MBE) and Pulsed Laser Deposition (PLD). Co/Cr and Fe/Cr superlattices were fabricated using mean-field theoretical concepts as guiding principles. The potential of artificial antiferromagnets for near room-temperature refrigeration is explored. Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. The effects of intra-plane and ...

Defect-Mediated Properties Of Magnetic Tunnel Junctions, Julian P. Velev, Mikhail Ye Zhuravlev, Kirill D. Belashchenko, Sitaram S. Jaswal, Evgeny Y. Tsymbal, T. Katayama, S. Yuasa

Evgeny Tsymbal Publications

Defects play an important role in the properties of metal oxides which are currently used as barrier layers in magnetic tunnel junctions (MTJs). We study the effect of O vacancies on the interlayer exchange coupling (IEC) and tunneling magnetoresistance (TMR) in Fe–MgO–Fe tunnel junctions. Measurements of IEC in fully epitaxial Fe–MgO–Fe(001) tunnel junctions show IEC is antiferromagnetic (AFM) for small MgO thickness but changes sign and then vanishes for large barrier thickness. First-principles calculations based on density functional theory demonstrate that the presence of neutral O vacancies (F-centers) in the MgO barrier can explain ...

Carrier Dynamics In Α‐Fe2o3 (0001) Thin Films And Single Crystals Probed By Femtosecond Transient Absorption And Reflectivity, Alan G. Joly, Joshua R. Williams, Scott A. Chambers, Gang Xiong, Wayne P. Hess, David M. Laman

All Faculty Scholarship for the College of the Sciences

Femtosecond transient reflectivity and absorption are used to measure the carrier lifetimes in α‐Fe₂O₃ thin films and single crystals. The results from the thin films show that initially excited hot electrons relax to the band edge within 300 fs and then recombine with holes or trap within 5 ps. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α‐Fe₂O₃)single crystals show similar but slightly faster dynamics leading to the conclusion that the short carrier lifetimes in these materials are due primarily to trapping to Fe ...

Monte Carlo Analysis Of Gan-Based Gunn Oscillators For Microwave Power Generation, R. P. Joshi, V. Sridhara, P. Shah, R. D. Del Rosario

Electrical & Computer Engineering Faculty Publications

Monte Carlo studies of transferred electron oscillators based on bulk wurtzite GaN are presented. Two structures have been examined: (i) devices with the conventional single notch structure, and (ii) repetitive structures with serial segments to fashion a "multiple domain" device. Wurtzite material has been chosen because of the higher drift velocity and because analytical expressions for the band structure have recently become available. Performance parameters of interest such as the operating frequency, output power, and conversion efficiency are calculated. Variations due to changes in temperature, biasing voltage, and device length are also included. It is shown that multidomain Gunn diodes ...

Exchange Bias In Fe0.6zn0.4f2 Heterostructures, Christian Binek

Christian Binek Publications

The previous exchange bias field, He, is measured in Fe0.6Zn0.4F2/Fe heterosystems prepared from Fe layers of 14 and 5 nm thickness which are deposited on top of the compensated (1 1 0) surface of the antiferromagnet. Deviations from a linear dependence of He on the magnetization of the Fe layer are attributed to ferromagnetic domains. Moreover, piezomagnetism and its influence on He are evidenced.

Domain State Susceptibility In Fecl2/Copt-Heterostructures, Christian Binek

Christian Binek Publications

Magnetic heterostructures with perpendicular anisotropy consisting of antiferromagnetic (111)-oriented FeCl2 single crystals and thin ferromagnetic CoPt-multilayers are investigated by ac-susceptometry after cooling in various axial magnetic fields to below the Néel temperature. The freezing process drives the system into a metastable antiferromagnetic domain state which gives rise to a huge excess contribution to the complex parallel susceptibility on heating in zero magnetic field. Its glow curve-like temperature dependence is modeled within the framework of a simple analytical approach. It involves the susceptibility of thermally activated clusters originating from antiferromagnetic stacking faults as well as the contribution of pairs of ...