Reconstructions at the Interface in Complex Oxide Heterostructures with Strongly Correlated Electrons

Strongly correlated oxides exhibit a rich spectrum of closely competing orders near the localized-itinerant Mott insulator transition leaving their ground states ripe with instabilities susceptible to small perturbations such as lattice distortions, variation in stoichiometry, magnetic and electric fields, etc. As the field of interfacial engineering has matured, these underlying instabilities in the electronic structure of correlated oxides continue to be leveraged to manipulate existing phases or search for emergent ones. The central theme is matching materials across the interface with disparate physical, chemical, electronic, or magnetic structure to harness interfacial reconstructions in the strongly coupled charge, spin, orbital, and lattice degrees of freedom. In this dissertation, we apply the above paradigm to cuprate-manganite and cuprate-titanate interfaces. We examine ultrathin YBa2Cu3O7/La2/3Ca1/3MnO3 multilayers, where interfacial charge reconstruction modulates the distribution of charge carriers within the superconducting planes and thereby act as dials to tune through the cuprate doping phase diagram. The ultrathin nature of the cuprate layers allows the reconstructed states to be resolved free of a bulk admixture. The depleted carriers are observed to directly enter the CuO2 planes. With increasing manganite thickness, magnetic correlations are introduced, and coupling between interfacial Cu and Mn develops. The reconstructions in spin and electronic degrees of freedom found in cuprate-manganite heterostructures are expected to completely mask all other competing interactions. To this end, SrTiO,3 is incorporated as a spacer material in cuprate-titanate multilayers to reveal the role of dimensionality, interlayer coupling, and broken translational symmetry. At the unit cell limit, a decrease in carrier concentration is found that directly correlates with underdoping from lost charge reservoir layers at the interface, while increased titanate layer thickness is found to augment the carrier concentration with the charge reservoir layers but has no effect on the doping within the superconducting planes. Also spectroscopic evidence for charge transfer across the interface between Cu and Ti is shown to support a recent theoretical prediction of pre-doping at the cuprate-titanate interface in response to a polar discontinuity at the interface

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr2MnNiO6 is studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 2+ and 4+ states respectively. Using charge transfer multiplet analysis of Ni and Mn 2p XPS spectra, we find charge transfer energies {\Delta} of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the atomic values of 8.00 and 3.00 respectively thereby indicating the covalent nature of the system. The O 1s edge absorption spectra reveal a band gap of 0.9 eV which is comparable to the value obtained from first principle calculations for U-J >= 2 eV. The density of states clearly reveal a strong p-d type charge transfer character of the system, with band gap proportional to average charge transfer energy of Ni2+ and Mn4+ ions.Comment: 18 pages, 9 figure

Transition Metal Impurities in Semiconductors: Induced Magnetism and Band Gap Engineering

The main subject of this thesis is the study of electronic and magnetic properties of materials containing 3d transition metal atoms. Our motivation stems mainly from the modern fields of spintronic computing and solar energy conversion. The two primary goals of this work are to determine (i) why certain transition metal impurities in certain semiconductors can induce magnetic properties suitable for spintronic computing applications, and (ii) how transition metal impurities can be used to modify the electronic band gaps of semiconductors and insulators in ways useful for harnessing solar energy and for other applications. To accomplish these goals, we have applied both experimental and theoretical tools. We studied high quality materials prepared by advanced synthesis techniques using x-ray spectroscopy methods at synchrotron light sources. The results of these experiments were interpreted using a variety of theoretical techniques, primarily using computational software developed as part of this thesis and discussed herein. Regarding the study of introducing transition metal impurities into semiconductors to induce magnetic properties, we first developed and demonstrated a method to determine the location of impurity atoms within the host semiconductor lattice. This allowed to us explain the presence and absence of ferromagnetism in samples prepared under only slightly different synthesis conditions, which helped to address some long--standing issues in the spintronics field. We then studied an advanced and promising material -- indium (III) oxide with iron impurities -- to determine how magnetic ordering was maintained up to room temperatures. Our techniques unveiled that a portion of the iron atoms were coupled to oxygen vacancies in the material to create conditions which propelled the observed magnetism. This finding confirmed some earlier theoretical predictions by others in the field. For the study of electronic band gap modifications in semiconductors and insulators via the incorporation of transition metal atoms, we investigated a wide range of materials synthesized using different techniques. Again, we used experimental techniques to determine the location of impurity atoms within the materials, and used this to understand how band gaps were modified upon the introduction of the impurities. For Ti implantation into SiO2, Ni substitution into ZnO, and a new material, MnNCN, we have determined the electronic band gaps and used our techniques to explain how the values for the gaps arise. Finally, an additional outcome of this thesis work is a software program capable of simulating x-ray spectra using various advanced quantum models. We rewrote and built upon powerful existing programs and applied the result to the above studies. Our software was further applied in a collaborative effort with other researchers at the Canadian Light Source to study the differences in two experimental techniques for measuring x-ray absorption: partial and inverse partial fluorescence yields. By using the proper absorption and scattering formalisms to simulate each technique, we were able to explain the differences between the experimental spectra obtained from each. We explain fluorescence yield deviations using an analysis based on the spin configuration of different states, suggesting that the technique can be further extended as a quantitative spin state probe. These results could have significant implications for the field of soft x-ray absorption spectroscopy

Interplay of p-d and d-d charge transfer transitions in rare-earth perovskite manganites

We have performed both theoretical and experimental study of optical response of parent perovskite manganites RMnO_3 with a main goal to elucidate nature of clearly visible optical features. Starting with a simple cluster model approach we addressed the both one-center (p-d) and two-center (d-d) charge transfer (CT) transitions, their polarization properties, the role played by structural parameters, orbital mixing, and spin degree of freedom. Optical complex dielectric function of single crystalline samples of RMnO_3 (R=La, Pr, Nd, Sm, Eu) was measured by ellipsometric technique at room temperature in the spectral range from 1.0 to 5.0 eV for two light polarizations: E \parallel c and E \perp c. The comparative analysis of the spectral behavior of \varepsilon _1 and \varepsilon _2 is believed to provide a more reliable assignment of spectral features. We have found an overall agreement between experimental spectra and theoretical predictions based on the theory of one-center p-d CT transitions and inter-site d-d CT transitions. Our experimental data and theoretical analysis evidence a dual nature of the dielectric gap in nominally stoichiometric matrix of perovskite manganites RMnO_3, it is formed by a superposition of forbidden or weak dipole allowed p-d CT transitions and inter-site d-d CT transitions. In fact, the parent perovskite manganites RMnO_3 should rather be sorted neither into the CT insulator nor the Mott-Hubbard insulator in the Zaanen, Sawatzky, Allen scheme.Comment: 20 pages, 6 figure

Electronic structure investigation of GdNi using X-ray absorption, magnetic circular dichroism and hard x-ray photoemission spectroscopy

GdNi is a ferrimagnetic material with a Curie temperature Tc = 69 K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at T = 25 K in the ferrimagnetic phase. We analyze the Gd M$_{4,5}$-edge ($3d$ - $4f$) and Ni L$_{2,3}$-edge ($2p$ - $3d$) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd M$_{4,5}$-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized $4f$ states. On the other hand, a model cluster calculation for Ni L$_{2,3}$-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest neighbour Gd states, resulting in a $d$-electron count of 8.57. The Gd M$_{4,5}$-edge XMCD spectrum is consistent with a ground state configuration of S = 7/2 and L=0. The Ni L$_{2,3}$-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite magnetic moment ( $m_{tot}$ $\sim$ 0.12 $\mu_B$ ) with the ratio $m_{o}/m_{s}$ $\sim$ 0.11. Valence band hard x-ray photoemission spectroscopy shows Ni $3d$ features at the Fermi level, confirming a partially filled $3d$ band, while the Gd $4f$ states are at high binding energies away from the Fermi level. The results indicate that the Ni $3d$ band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni $3d$ and Gd $4f$ states.Comment: 9 pages, 6 figures, text and figures revise

Low temperature Terahertz Spectroscopy of LaFeO3_3, PrFeO3_3, ErFeO3_3, and LuFeO3_3: Quasimagnon resonances and ground multiplet transitions

We report on zone center THz excitations of non-Jahn Teller LaFeO$_3$, PrFeO$_3$, ErFeO$_3$, and LuFeO$_3$ distorted perovskites under external magnetic fields up 7 T. Low temperature-low energy absorptions of LaFeO$_3$ show antiferromagnetic and ferromagnetic quasimagnons at $\omega$AFM ~31.4 and $\omega$FM ~26.7 cm$^{-1}$ in the $\Gamma$4 (Gx, Ay, Fz) representation. LuFeO$_3$ is characterized by zero field magnetic resonances at $\omega$AFM ~26.3 cm$^{-1}$ and $\omega$FM ~22.4 cm$^{-1}$ in addition to Fe$^{3+}$ Zeeman-split crystal field (CF) 6A$_1$ ground transitions at ~10.4 cm$^{-1}$ triggered by structural deviations induced by smaller Lu 4f$^{14}$. This local non-centrosymmetric departure is also found in ErFeO$_3$ (Kramers 4f$^{11}$ Er$^{3+}$ (4I15/2); {\Gamma}2 (Fx, Cy, Gz) <TSR ~93 K), but with the ~4 cm$^{-1}$ Fe$^{3+}$ Zeeman branching strongly biased toward higher energies. Quasimagnons at $\omega$AFM ~31.5 cm$^{-1}$ and $\omega$FM ~21.5 cm$^{-1}$ in ErFeO$_3$ do not undergo field induced band splits but a 13-fold increase in the antiferro ($\omega$AMF) /ferro($\omega$AFM) intensity ratio. There is a remarkable field-dependent CF matching population balance between Fe$^{3+}$ higher and Er$^{3+}$ lower Zeeman branches. Antiferro- and ferro- resonances in PrFeO$_3$ turn much broader as non-Kramers Pr$^3$ introduces ligand changes at the A site leading into near degeneracy the antiferromagnetic mode and the lowest Pr$^{3+}$ CF transition. We conclude that low energy excitations in RFeO$_3$ (R=rare earth) strongly depend on the lanthanide ionic size. Minute lattice displacements also underlie considering non-centrosymmetric the most distorted RFeO$_3$ (R=rare earth). Changes triggered by the smaller rare earth and the nonlinear intrinsic oxygen ion polarizability provide grounds for interplay of ionic and electronic interactions yielding ferroelectric spontaneous polarization.Comment: Full Manuscript and Supplemental Material, 73 pages, 27 figure

A study of three transition metal compounds and their applications

The La5/8-yPryCa3/8MnO3 with colossal magnetoresistance (CMR) effect has a rich phase diagram and is well studied for its electronic phase separation phenomenon, where the ferromagnetic (FM) metal order co-exists and competes with the charge-ordered (CO) insulator phase. High Pr doping will favor the CO order, leading a sharp FM to CO dominated phase transition around Pr concentration of 0.3 and above. Hydrostatic pressure favors FM metallic order without damaging the sample and can be tuned continuously. In this study, pressurized-magnetic and resistivity measurements was done on a La0.25Pr0.375Ca0.375MnO3 single crystal. The sample, at first sitting in CO dominated phase, changed into FM upon a small amount of pressure. This transition was verified both by magnetic and resistivity measurement results. FeSe1-x is one of the newly discovered iron-based superconductors. As a binary transition metal compound, it is of great research interest due to the simple stacking 2d-layered structure. The itinerant or localized nature of the electrons in Fe2+ ion has been debated but not concluded. In this research, Raman scattering measurements on FeSe0.97 were applied within a temperature range from 5 K to 300 K. The excitation near 185 cm-1 was assigned to B1g phonon excitation. Broad and intensive excitation peaks were found in a wide region between 200 cm-1 and 700 cm-1, and they are classified as the Fe2+ crystal field excitations. These excitations suggest a low Hund coupling constant and thus support the itinerant nature of 3d electrons in Fe2+ ion indirectly. Evanescent wave was discovered to be able to tunnel through a negative reflectance index material and gets strengthened inside an alternating metal-high K material 1d photonic crystal structure. Where the regular light eventually fails in sub-micron photolithography due to diffraction limit, evanescent wave can carry the information of small structure below diffraction limit. In our study, HfO2, a transitional metal oxide widely used in IC fabrication, was used as the high-K material to construct a sub-wavelength length imaging device for nano scale photolithography applications