Spectroscopic properties of ferroic superlattices

The interplay between charge, structure, magnetism, and orbitals leads to rich physics and exotic cross-coupling in multifunctional materials. Superlattices provide a superb platform to study the complex interactions between different degrees of freedom. In this dissertation, I present a spectroscopic investigation of natural and engineered superlattices including FexTaS2 and (LuFeO3)m/(LuFe2O4)1 under external stimuli of temperature and magnetic field as well as chemical substitution. Studying the phase transitions, symmetry-breaking, and complex interface interactions from a microscopic viewpoint enhances fundamental understanding of coupling mechanism between different order parameters and the exciting properties. In FexTaS2, we use optical spectroscopies to analyze the electronic properties. Strikingly, Fe intercalation dramatically changes the metallic character, revealing two separate free carrier responses in the Fe monolayer and TaS2 slabs, respectively. Signatures of chirality are deeply embedded in the electronic structures. These include a transition of electron density pattern from triangular to Kagome to honeycomb, a hole to electron pocket crossover at the K-point, and low energy excitations between spin split bands that cross the Fermi surface. These findings advance the understanding of intercalation and symmetry-breaking on the fundamental excitations in metallic chalcogenides, while at the same time, raise important questions about how the embedded metal monolayer affects vibrational properties due to the free-carrier response screened the infrared-active phonons. To address these issues, we extended this work using Raman scattering spectroscopy to reveal the vibrational properties. We particular focus on the coherent excitations in the Fe monolayer. The results reveal both in- and out-of-plane vibrational excitations at low frequencies in the intercalated Fe monolayer. Extending the measurements to other intercalated chalcogenides such as Cr1/3NbS2 and RbFe(SO4)2 reveals structural-property relations, which confirms the intercalated monolayer excitations are general and intrinsic. Furthermore, the intercalated monolayer excitations have a trend that depend upon the metal-metal distance, the size of the van der Waals gap, and the metal-to-chalcogenide slab mass ratio. A model for how mass ratio affects the frequencies of the monolayer excitations is developed as well,which excellently fits to our experimental trend. These findings suggest that external stimuli such as pressure and strain may be able to tune these intercalated monolayer excitations. In the (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices (m= 3, 7, and 9), we combined optical spectroscopy, magnetic circular dichroism, and first-principle calculations to uncover the origin of high temperature magnetism and charge-ordering states in a site-specific manner. Analysis of the dichroic spectra reveals optical hysteresis loops for different Fe sites. The site-specific coercivity vs. temperature curves are extracted from the optical hysteresis, which demonstrates that bulk magnetism derives principally from the LuFe2O4 layers. Magnetism emanating from the LuFe2O4 layer becomes more robust as the (3, 1) to (7, 1) to (9, 1) series progresses - a trend that correlates with increasing Lu-layer distortion. To understand this relationship more deeply, we extract the spectral signature of the interface for the (LuFeO3)m/(LuFe2O4)1 series (m = 3, 7 and 9). While the overall contribution of spin-down channel excitations is persistent over the sequence, enhanced Lu-layer distortion at the interface increases the contribution of the Fe2+ to Fe3+charge-transfer excitation in the spin-up channel. This amplifies LuFe2O4 layer magnetization and pinpoints the role of Fe2+. Key to this discovery is the ability of magneto-optical spectroscopy to provide direct, microscopic, site-specific information about interface magnetism in a two-dimensional material with multiple magnetic centers. Comparison of the theoretically predicted magnetic circular dichroism with the experimental spectrum also establishes the non-polar self-doped structure as the precise charge-ordering arrangement within the LuFe2O4 layer of the (3, 1) superlattice, thus resolving controversy regarding the many different isoenergetic charge states. In addition to introducing a remarkably powerful and versatile spectroscopic decomposition technique for revealing microscopic spin and charge character at the interface of a multiferroic superlattice with many different iron centers in a site-selective manner, this work provides a pathway to link bulk and interface properties in other engineered materials

SPECIFIC HEAT MEASUREMENTS ON STRONGLY CORRELATED ELECTRON SYSTEMS

Studies on strongly correlated electron systems over decades have allowed physicists to discover unusual properties such as spin density waves, ferromagnetic and antiferromagnetic states with unusual ordering of spins and orbitals, and Mott insulating states, to name a few. In this thesis, the focus will be on the specific heat property of these materials exhibiting novel electronic ground states in the presence and absence of a field. The purpose of these measurements is to characterize the phase transitions into these states and the low energy excitations in these states. From measurements at the phase transitions, one can learn about the amount of order involved [i.e. entropy: ΔS = ∫Δc p/T dT], while measurements at low temperatures illuminate the excitation spectrum. In order to study the thermodynamic properties of the materials at their phase transitions, a high sensitive technique, ac-calorimetry was used. The ac-calorimeter, workhorse of our low dimensional materials lab, is based on modulating the power that heats the sample and measuring the temperature oscillations of the sample around its mean value. The in-house ac-calorimetry set up in our lab has the capability to produce a quasi-continuous readout of heat capacity as a function of temperature. A variety of single crystals were investigated using this technique and a few among them are discussed in my thesis. Since many of the crystals that are studied by our group are magnetically active, it becomes useful for us to also study them in the presence of a moderate to high magnetic field. This motivated me to design, develop, and build a heat capacity probe that would enable us to study the crystals in the presence of non-zero magnetic fields and at low temperatures. The probe helped us not only to revisit some of the studied materials and to draw firm conclusions on the previous results but also is vital in exploring the untouched territory of novel materials at high magnetic fields (~ 14 T)

Chirality, symmetry-breaking, and chemical substitution in multiferroics

Multiferroic materials attract significant attention due to their potential utility in a broad range of device applications. The inclusion of heavy metal centers in these materials enhances their magnetoelectric properties, yielding fascinating physical phenomena such as the Dzyaloshinskii–Moriya interaction, nonreciprocal directional dichroism, enhancement of spin-phonon coupling, and spin-orbit-entangled ground states. This dissertation provides a comprehensive survey of magnetoelectric multiferroics containing heavy metal centers and explores spectroscopic techniques under extreme conditions. A microscopic examination of phase transitions, symmetry-breaking, and structure-property relationships enhances the fundamental understanding of coupling mechanisms. In A2Mo3O8 (A = Fe, Zn, Ni, and Mn), we use optical spectroscopy to analyze the electronic properties and compare our findings with first principles electronic structure calculations. We find that Fe2+ ions in the A site create a many-body effect from a valence band due to screening of the local moment – similar to a Zhang-Rice singlet. These findings advance the understanding of unusual hybridization with orbital occupation and the structure-property relationships in various metal-substituted systems. In a chiral, polar magnet, Ni3TeO6, we explore toroidal geometry to complete the set of configurations and develop structure-property relations by combining magneto-optical spectroscopy and first-principles calculations. The formation of Ni toroidal moments is responsible for the largest effects near 1.1 eV - a tendency that is captured by our microscopic model and computational implementation. Furthermore, we demonstrate deterministic control of nonreciprocal directional dichroism in Ni3TeO6 across the entire telecom wavelength range. This discovery will accelerate the development of photonics applications that take advantage of unusual symmetry characteristics. In Co4B2O9 (B = Nb, Ta), we combine variable temperature infrared spectroscopy, lattice dynamics calculations, and several different models of spin-phonon coupling to reveal the mechanism of magnetoelectricity. We reveal a spin-phonon coupling in Co2+ shearing mode near 150 cm−1 with coupling constants of 3.4 and 3.4 cm−1 for Co4Nb2O9 and the Ta analog, respectively. These coupling constants derive from interlayer exchange interactions, which contain competing antiferromagnetic and ferromagnetic contributions. Comparison to other contemporary oxides shows that spin-phonon coupling in this family of materials is among the strongest ever reported, suggesting an origin for magnetoelectric coupling

Comportamento magnético de sistemas híbridos granulares e modelagem da superfície de epicamadas MnAs/GaAs(111)B

Acompanha CD ROMInclui apêndiceOrientadores: Dante Homero Mosca e Victor Hugo EtgensTese (doutorado) - Universidade Federal do Paraná, Setor de Tecnologia, Programa de Pós-Graduaçao em Engenharia - PIPE e Université Pierre et Marie Curie. Defesa: Curitiba, 2007Inclui bibliografiaÁrea de concentraçao: Engenharia e ciencias de materiai

Studies of Molecular Precursors Used in FEBID Fabrication of Nanostructures

The adoption of nanotechnology is increasingly important in many aspects of our daily life influencing the clothes we wear and most of the electronic devices we use while also underpinning the development of drugs and medical techniques that we will need at some point in our lives. The methods by which nanoscale devices are fabricated is changing from a 'top down' etching based procedure to a 'bottom up' molecule by molecule deposition and assembly. The focus of the present research is the development, design, and analysis of new precursors for focused electron beam induced deposition (FEBID) and extreme ultraviolet nanolithography (EUVL) through a large pool of experimental and computational resources. The research is divided into two areas: gas - phase analysis of precursors (largely used for fragment and radicals' analysis, and molecular design) and surface and deposition science (physical deposition of precursors, simulation analysis of surface - molecule interactions and characterization of deposition processes to obtain optimal process parameters for molecular structures). It is necessary to collect data such as cross sections of electron - molecule interactions e.g., dissociative ionization (DI) and dissociative electron attachment (DEA) to provide accurate simulations that can be used to improve the FEBID and EUVL while understanding surface processes such as molecular absorption and diffusion to determine the structure and purity of the nanostructures formed by these methods. The objective of this thesis is to provide a gas - phase and deposition analysis of potential and widely used precursors for FEBID and EUVL at the nanoscale. To achieve this the experimental technique of velocity sliced map imaging (VsMI) was used in conjunction with theoretical tools such as density functional theory (DFT) simulations using Gaussian 16 software and evaluation of cross-section data for molecular dissociation at low electron energies of 0 - 20 eV using Quantemol-N. Results of the gas - phase analysis of negative ionic fragments formed by DEA and DI with their appearance, dissociation and ionization energies, angular distributions and kinetic energies, cross-sections for DEA fragmentation at low energy and excited states calculations at values up to 10 eV are presented. These results are used as the inputs to the models of the FEBID processes. The electronic, structural, and kinetic properties of several FEBID precursors are explored, and FEBID method used to create nanostructures using a Zeiss MeRiT SEM with GEMINI column operated at 20 kV. Analysis of the deposits was performed using EDX and atomic force microscopy (AFM) analysis as well as electron stimulated desorption (ESD) and temperature programmed desorption (TPD). Complementary simulations of the dynamics of processes at the surface were studied using MBN Explorer and surface - molecule interactions with great results in simulating the deposition process of islands and structures (results presented in Chapter 8)

Microscopy Conference 2017 (MC 2017) - Proceedings

Das Dokument enthält die Kurzfassungen der Beiträge aller Teilnehmer an der Mikroskopiekonferenz "MC 2017", die vom 21. bis 25.08.2017, in Lausanne stattfand

Microscopy Conference 2017 (MC 2017) - Proceedings

Das Dokument enthält die Kurzfassungen der Beiträge aller Teilnehmer an der Mikroskopiekonferenz "MC 2017", die vom 21. bis 25.08.2017, in Lausanne stattfand