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Optically Detected Magnetic Resonance Studies on π-conjugated semiconductor systems

Description: Optically Detected Magnetic Resonance (ODMR) techniques were used to investigate the dynamics of excitons and charge carriers in π-conjugated organic semiconductors. Degradation behavior of the negative spin-1/2 electroluminescence-detected magnetic resonance (ELDMR) was observed in Alq3 devices. The increase in the resonance amplitude implies an increasing bipolaron formation during degradation, which might be the result of growth of charge traps in the device. The same behavior of the negative spin-1/2 ELDMR was observed in 2wt% Rubrene doped Tris(8-hydroxyquinolinato)aluminium (Alq3) devices. However, with increasing injection current, a positive spin-1/2 ELDMR, together with positive spin 1 triplet powder patterns at {delta}m{sub S}={+-}1 and {delta}m{sub S}={+-}2, emerges. Due to the similarities in the frequency dependences of single and double modulated ELDMR and the photoluminescence-detected magnetic resonance (PLDMR) results in poly[2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenyl ene vinylene] (MEH-PPV) films, the mechanism for this positive spin-1/2 ELDMR was assigned to enhanced triplet-polaron quenching under resonance conditions. The ELDMR in rubrene doped Alq3 devices provides a path to investigate charge distribution in the device under operational conditions. Combining the results of several devices with different carrier blocking properties and the results from transient EL, it was concluded trions not only exist near buffer layer but also exist in the electron transport layer. This TPQ model can also be used to explain the positive spin-1/2 PLDMR in poly(3-hexylthiophene) (P3HT) films at low temperature and in MEH-PPV films at various temperatures up to room temperature. Through quantitative analysis, TE-polaron quenching (TPQ) model is shown having the ability to explain most behaviors of the positive spin-1/2 resonance. Photocurrent detected magnetic resonance (PCDMR) studies on MEH-PPV devices revealed a novel transient resonance signal. The signal may originate from the higher concentration of deep traps near cathode. A quantitative analysis based on this assumption was carried out and found to be consistent with the experimental ...
Date: December 6, 2011
Creator: Chen, Ying
Partner: UNT Libraries Government Documents Department

First principles analysis of lattice dynamics for Fe-based superconductors and entropically-stabilized phases

Description: Modern calculations are becoming an essential, complementary tool to inelastic x-ray scattering studies, where x-rays are scattered inelastically to resolve meV phonons. Calculations of the inelastic structure factor for any value of Q assist in both planning the experiment and analyzing the results. Moreover, differences between the measured data and theoretical calculations help identify important new physics driving the properties of novel correlated systems. We have used such calculations to better and more e#14;ciently measure the phonon dispersion and elastic constants of several iron pnictide superconductors. This dissertation describes calculations and measurements at room temperature in the tetragonal phase of CaFe{sub 2}As{sub 2} and LaFeAsO. In both cases, spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improves the agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase. In addition, we discuss a relatively new approach called self-consistent ab initio lattice dynamics (SCAILD), which goes beyond the harmonic approximation to include phonon-phonon interactions and produce a temperature-dependent phonon dispersion. We used this technique to study the HCP to BCC transition in beryllium.
Date: July 20, 2012
Creator: Hahn, Steven
Partner: UNT Libraries Government Documents Department

Structural and magnetic properties and superconductivity in Ba(Fe{sub 1-x}TM{sub x}){sub 2}As{sub 2}

Description: We studied the effects on structural and magnetic phase transitions and the emergence of superconductivity in transition metal substituted BaFe{sub 2}As{sub 2}. We grew four series of Ba(Fe{sub 1-x}TM{sub x}){sub 2}As{sub 2} (TM=Ru, Mn, Co+Cr and Co+Mn) and characterized them by crystallographic, magnetic and transport measurements. We also subjected Ba(Fe{sub 1-x}Cr{sub x}){sub 2}As{sub 2} and Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} to heat treatment to explore what changes might be induced.
Date: July 23, 2012
Creator: Thaler, Alexander
Partner: UNT Libraries Government Documents Department

Structural and magnetic properties of transition metal substituted BaFe{sub 2}As{sub 2} compounds studied by x-ray and neutron scattering

Description: The purpose of my dissertation is to understand the structural and magnetic properties of the newly discovered FeAs-based superconductors and the interconnection between superconductivity, antiferromagnetism, and structure. X-ray and neutron scattering techniques are powerful tools to directly observe the structure and magnetism in this system. I used both xray and neutron scattering techniques on di#11;erent transition substituted BaFe2As2 compounds in order to investigate the substitution dependence of structural and magnetic transitions and try to understand the connections between them.
Date: August 28, 2012
Creator: Kim, Min Gyu
Partner: UNT Libraries Government Documents Department

Electronic structure and f-orbital occupancy in Yb-substituted CeCoIn5

Description: The local structure and 4f orbital occupancy have been investigated in Ce{sub 1−x}Yb{sub x}CoIn{sub 5} via Yb L{sub III}-edge extended x-ray absorption fine structure (EXAFS), Ce and Yb L{sub III}-edge x-ray absorption near-edge structure (XANES), and angle-resolved photoemission spectroscopy (ARPES) measurements. Yb(III) (4f{sup 13}) is the hole analog of Ce(III) (4f{sup 1}). Yb is found to be strongly intermediate-valent in Ce{sub 1−x}Yb{sub x}CoIn{sub 5} throughout the entire doping range, including pure YbCoIn{sub 5}, with an f-hole occupancy for Yb of n{sub f} ≃ 0.3 (i.e. Yb{sup 2.3+}), independent of Yb concentration and independent of temperature down to T = 20 K. In contrast, the f-electron orbital occupancy for Ce remains close to 1 for all Yb concentrations, suggesting that there is no mutual influence on n{sub f} between neighboring Ce and Yb sites. Likewise, ARPES measurements at 12 K have found that the electronic structure along {Gamma} − X is not sensitive to the Yb substitution, suggesting that the Kondo hybridization of Ce f electrons with the conduction band is not affected by the presence of Yb impurities in the lattice. The emerging picture is that in Ce{sub 1−x}Yb{sub x}CoIn{sub 5} there are two networks, interlaced but independent, that couple to the conduction band: one network of Ce ions in the heavy-fermion limit, one network of Yb ions in the strongly intermediate-valent limit. The robustness of the local and electronic structure to doping suggests the absence of charge transfer between the Ce and Yb ions, and may explain the relative robustness of superconductivity for this Ce-site substitution as compared to the In-site substitution.
Date: May 3, 2011
Creator: Booth, C. H.; Durakiewicz, T.; Capan, C.; Hurt, D.; Bianchi, A. D.; Joyce, J.J. et al.
Partner: UNT Libraries Government Documents Department

Itinerant and localized magnetic moments in ferrimagnetic Mn{sub 2}CoGa thin films identified with x-ray magnetic linear dichroism: experiment and ab initio theory

Description: Epitaxial thin films of the half-metallic X{sub a}-compound Mn{sub 2}CoGa (Hg{sub 2}CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality #12;lms with a bulk magnetization of 1.95(5) {mu}{sub #22;}B per unit cell were obtained. The average Mn magnetic moment and the Co moment are parallel, in agreement with theory. The x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism allows to distinguish between itinerant and localized Mn moments. It is shown that one of the two Mn moments has localized character, whereas the other Mn moment and the Co moment are itinerant.
Date: August 8, 2011
Creator: Meinert, M.; Schmalhorst, J; Klewe, C.; Reiss, G.; Arenholz, E.; Bohnert, T. et al.
Partner: UNT Libraries Government Documents Department

Metal-insulator transition in low dimensional La{sub 0.75}Sr{sub 0.25}VO{sub 3} thin films

Description: We report on the metal-insulator transition that occurs as a function of film thickness in ultrathin La{sub 0.75}Sr{sub 0.25}VO{sub 3} films. The metal-insulator transition displays a critical thickness of 5 unit cell. Above the critical thickness, metallic films exhibit a temperature driven metal-insulator transition with weak localization behavior. With decreasing film thickness, oxygen octahedron rotation in the films increases, causing enhanced electron-electron correlation. The electronelectron correlations in ultrathin films induce the transition from metal to insulator in addition to Anderson localization.
Date: June 15, 2011
Creator: Dao, Tran M.; Mondal, Partha S.; Takamura, Y.; Arenholz, E. & Lee, Jaichan
Partner: UNT Libraries Government Documents Department

Widespread spin polarization effects in photoemission from topological insulators

Description: High-resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi{sub 2}Se{sub 3} using a recently developed high-efficiency spectrometer. The topological surface state's helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.
Date: June 22, 2011
Creator: Jozwiak, C.; Chen, Y. L.; Fedorov, A. V.; Analytis, J. G.; Rotundu, C. R.; Schmid, A. K. et al.
Partner: UNT Libraries Government Documents Department

Synthesis, Structure, and Physical Properties of YbNi{sub 3}Al{sub 9.23}

Description: The physical properties of YbNi{sub 3}Al{sub 9.23}, including the crystal structure, magnetization, specific heat, valence, and electrical resistivity, are reported. Single crystal X-ray diffraction reveals that the compound crystallizes with the rhombohedral space group R32 and has unit cell parameters a=7.2443(3) Å and c=27.251(3) Å with some crystallographic disorder on an Al site. The compound orders antiferromagnetically at T{sub N}=3 K despite the presence of strong ferromagnetic correlations, accompanied by a spin flop-like transition to a moment-aligned rate above 0.1 T. X-ray absorption spectroscopy and magnetic susceptibility measurements indicate a localized Yb{sup 3+} electronic configuration, while the Sommerfeld coefficient in the magnetically ordered state was determined to be approximately 135 mJ/mol-K{sup 2}, suggesting moderately heavy fermion behavior. Therefore, these data indicate a balance between competing Ruderman-Kittel-Kasuya-Yosida (RKKY) and Kondo interactions in YbNi{sub 3}Al{sub 9.23} with a somewhat dominant RKKY interaction that leads to a relatively high ordering temperature.
Date: November 4, 2010
Creator: Tobash, P. H.; Jiang, Y.; Ronning, F.; Booth, C. H.; Thompson, J. D.; Scott, B. L. et al.
Partner: UNT Libraries Government Documents Department

Magnetic frustration effects in uranium intermetallics

Description: The effect of geometrical frustration on the development of the heavy-fermion state and quantum criticality is studied in UAuCu{sub 4}, UAuPt{sub 4}, UAu{sub 3}Ni{sub 2} samples through measurements of their magnetic susceptibility, heat capacity, and electrical resistivity. In addition, since lattice disorder can play a large role in defining magnetic properties in frustrated systems, extended X-ray absorption fine structure (EXAFS) data have also been obtained. The local structure results show a strong correlation with the magnetic properties in these samples.
Date: June 10, 2010
Creator: Jiang, Yu; Booth, C. H.; Tobash, P. H.; Gofryk, K.; Torrez, M. A.; Ronning, F. et al.
Partner: UNT Libraries Government Documents Department

Epitaxial Fe{sub 3-x}Ti{sub x}O{sub 4} films from magnetite to ulvöspinel by pulsed laser deposition

Description: Epitaxial films along the Fe{sub 3-x}Ti{sub x}O{sub 4} (titanomagnetite) compositional series from pure end-members magnetite (Fe{sub 3}O{sub 4}) to ulvöspinel (Fe{sub 2}TiO{sub 4}) were successfully grown by pulsed laser deposition on MgO(100) substrates. Spectroscopic characterization including high resolution x-ray diffraction, x-ray photoelectron spectroscopy, and synchrotron-based x-ray absorption and magnetic circular dichroism consistently shows that Ti(IV) substitutes for Fe(III) in the inverse spinel lattice with a proportional increase in lattice Fe(II) concentration. No evidence of Ti interstitials, spinodal decomposition, or secondary phases was found in the bulk of the grown films. At the uppermost few nanometers of the Ti-bearing film surfaces, evidence suggests that Fe(II) is susceptible to facile oxidation, and that an associated lower Fe/Ti ratio in this region is consistent with surface compositional incompleteness or alteration to a titanomaghemite-like composition and structure. The surface of these films nonetheless appear to remain highly ordered and commensurate with the underlying structure despite facile oxidation, a surface condition that is found to be reversible to some extent by heating in low oxygen environments.
Date: July 21, 2011
Creator: Droubay, T.C.; Pearce, C.I.; Ilton, E.S.; Engelhard, M.H.; Engelhard, M.H.; Heald, S.M. et al.
Partner: UNT Libraries Government Documents Department

FeMn/Fe/Co/Cu(1,1,10) films studied using the magneto-optic Kerr effect and photoemission electron microscopy

Description: FeMn/Fe/Co/Cu(1,1,10) films were grown epitaxially and investigated using the magneto-optic Kerr effect and photoemission electron microscopy. We found that FeMn/Fe/Co/Cu(1,1,10) exhibits the same properties as FeMn/Co/Cu(1,1,10) for the ferromagnetic phase of the face centered cubic (fcc) Fe film but a different property for the non-ferromagnetic phase of the fcc Fe film. This result indicates that the characteristic property reported in the literature for FeMn/Co/Cu(001) comes from the FeMn spin structure and is independent of the ferromagnetic layer.
Date: July 31, 2011
Creator: Meng, Y.; Li, J.; Tan, A.; Park, J.; Jin, E.; Son, H. et al.
Partner: UNT Libraries Government Documents Department

Hole doping in high temperature superconductors using the XANES technique

Description: Superconducting and physical properties of F-doped HgPb-1223 and Ce-doped Tl-1223 systems were considerably improved through adjusting the hole content of the two systems. In this study, we have used the x-ray absorption near-edge structure (XANES) technique to investigate the electronic structure of the two systems by probing the unoccupied electronic states. For the F-doped Hg-1223 system, the O K-edge, Ca L{sub 2,3} and Cu L{sub 2,3}-edge structures were thoroughly investigated. The pre-edge features of O K-edge spectra, as a function of doping, reveal important information about the projected local density of unoccupied states on the O sites in the region close to the absorption edge, which is a measure of O 2p hole concentration in the valence band. In the originally under-doped Hg-1223, the results indicate that the number of O 2p holes in the CuO{sub 2} planes increases as fluorine was introduced up to an optimal value, after which it decreases. Furthermore, the Cu L{sub 2,3} absorption edge provides useful information about the valence state of Cu which is also related to the hole density in the CuO{sub 2} planes and confirms the same previous conclusion. The Ca L{sub 2,3}-edge shows the presence crystal field splitting in HgPb1223/F{sub x} which is similar to CaF{sub 2} and CaO in addition to the spin-orbit splitting of the Ca 2p core level electrons. These results ensure that fluorine goes into the structure of HgPb-1223/F{sub x} and it occupies the vacant interstitial oxygen site in the Hg-O plane, as was expected. In Ce-substituted Tl-1223, similar measurements were perfonned for samples with different Ce content. The pre-edge feature of the O K-edge spectra shows clearly the drastic decrease of the hole content in CuO{sub 2} planes of this originally over-doped system with increasing Ce content. This result is also confirmed from the chemical state ...
Date: September 1, 2008
Creator: Hamdan, Nasser & Hussain, Zahid
Partner: UNT Libraries Government Documents Department

Determination of the Fe magnetic anisotropies and the CoO frozen spins in epitaxial CoO/Fe/Ag(001)

Description: CoO/Fe/Ag(001) films were grown epitaxially and studied by X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD). After field cooling along the Fe[100] axis to 80 K, exchange bias, uniaxial anisotropy, and 4-fold anisotropy of the films were determined by hysteresis loop and XMCD measurements by rotating the Fe magnetization within the film plane. The CoO frozen spins were determined by XMLD measurement as a function of CoO thickness.We find that among the exchange bias, uniaxial anisotropy, and 4-fold anisotropy, only the uniaxial magnetic anisotropy follows thickness dependence of the CoO frozen spins.
Date: April 28, 2011
Creator: Meng, J. Li, Y.; Park, J. S.; Jenkins, C. A.; Arenholz, E.; Scholl, A.; Tan, A. et al.
Partner: UNT Libraries Government Documents Department

Direct Evidence of Two Gaps in Underdoped Bi2212

Description: This short paper summarizes the results we presented at the LEHTSC2007 conference. Recent doping and temperature dependence of angle-resolved photoemission data of underdoped superconducting cuprate Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} (Bi2212) have revealed the presence of two energy scales exhibiting distinct behaviours. One, which dominates the antinodal region, increases with underdoping and does not show obvious temperature dependence across Tc. This is a behaviour known for more than a decade and considered as the general gap behaviour in the underdoped regime. The other, which dominates the near-nodal regime, does not increase with less doping and opens near Tc via a BCS-like temperature dependence. This is a behaviour not previously observed in the single particle spectra. We propose a momentum space picture of these two energy scales or energy gaps that could resolve the seemingly contradictory gap measurements among different experimental techniques. Our results have also further constrained the theory for high-Tc superconducting cuprates.
Date: March 6, 2008
Creator: Tanaka, K.; Lee, W.S.; Hussain, Zahid & Shen, Z.-X.
Partner: UNT Libraries Government Documents Department

Crystal fields, disorder, and antiferromagnetic short-range order in (Yb{sub 0.24}Sn{sub 0.76})Ru

Description: We report extensive measurements on a new compound (Yb{sub 0.24}Sn{sub 0.76})Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L{sub 3} x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb{sup 3+} is split by the crystalline electric field (CEF) into a Γ{sub 7} doublet ground state and a Γ{sub 8} quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ{sub 6} excited state resides at 32 meV; however, the Γ{sub 8}/Γ{sub 6} transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch-Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the Γ{sub 7} ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.
Date: July 18, 2011
Creator: Klimczuk, T.; Wang, C. H.; Lawrence, J. M.; Xu, Q.; Durakiewicz, T.; Ronning, F. et al.
Partner: UNT Libraries Government Documents Department


Description: The 2012 Gordon Conference on Correlated Electron Systems will present cutting-edge research on emergent properties arising from strong electronic correlations. While we expect the discussion at the meeting to be wide-ranging, given the breadth of the title subject matter, we have chosen several topics to be the particular focus of the talks. These are New Developments in Single and Bilayer Graphene, Topological States of Matter, including Topological Insulators and Spin Liquids, the Interplay Between Magnetism and Unconventional Superconductivity, and Quantum Critical Phenomena in Metallic Systems. We also plan to have shorter sessions on Systems Far From Equilibrium, Low Dimensional Electron Fluids, and New Directions (which will primarily focus on new experimental methodologies and their interpretation).
Date: June 29, 2012
Creator: Kivelson, Steven
Partner: UNT Libraries Government Documents Department


Description: The meeting shall strive to develop and further the fundamental understanding of defects and their roles in the structural, electronic, optical, and magnetic properties of bulk, thin film, and nanoscale semiconductors and device structures. Point and extended defects will be addressed in a broad range of electronic materials of particular current interest, including wide bandgap semiconductors, metal-oxides, carbon-based semiconductors (e.g., diamond, graphene, etc.), organic semiconductors, photovoltaic/solar cell materials, and others of similar interest. This interest includes novel defect detection/imaging techniques and advanced defect computational methods.
Date: August 17, 2012
Partner: UNT Libraries Government Documents Department

Ultrafast Magnetism Dynamics Measure Using Tabletop Ultrafast EUV Sources

Description: In our work to date, we made two significant advances. First we demonstrated element-selective demagnetization dynamics for the first time, with a record time resolution for x-ray probing of 55 fs. Second, in new work, we were able to probe the timescale of the exchange interaction in magnetic materials, also for the first time. Our measurements were made using the transverse magneto-optic Kerr effect (T-MOKE) geometry, since the reflectivity of a magnetic material changes with the direction of the magnetization vector of a surface. In our experiment, we periodically reversed the magnetization direction of a grating structure made of Permalloy (Ni80Fe20) using an external magnetic field. To achieve maximum contrast, we used HHG light spanning the M-shell (3p) absorption edges of Fe and Ni. Our characterization of the static magnetization of a Permalloy sample shows high magnetic asymmetry at photon energies just above and below the absorption edges at 55 eV and 65 eV, respectively. This result is in excellent agreement with measurements done on the same using a synchrotron source.
Date: August 21, 2013
Creator: Silva, Thomas J. & Murnane, Margaret
Partner: UNT Libraries Government Documents Department