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Electronic phase diagrams and competing ground states of complex iron pnictides and chalcogenides: A Mössbauer spectroscopy and muon spin rotation/relaxation studyKamusella, Sirko 01 March 2017 (has links)
In this thesis the superconducting and magnetic phases of LiOH(Fe,Co)(Se,S), CuFeAs/CuFeSb, and LaFeP_1-xAs_xO - belonging to the 11, 111 and 1111 structural classes of iron-based arsenides and chalcogenides - are investigated by means of 57Fe Mössbauer spectroscopy and muon spin rotation/relaxation (μSR). Of major importance in this study is the application of high magnetic fields in Mössbauer spectroscopy to distinguish and characterize ferro- (FM) and antiferromagnetic (AFM) order. A user-friendly Mössbauer data analysis program was developed to provide suitable model functions not only for high field spectra, but relaxation spectra or parameter distributions in general.
In LaFeP_1-xAs_xO the reconstruction of the Fermi surface is described by the vanishing of the Γ hole pocket with decreasing x. The continuous change of the orbital character and the covalency of the d-electrons is shown by Mössbauer spectroscopy. A novel antiferromagnetic phase with small magnetic moments of ~ 0.1 μ_B state is characterized. The superconducting order parameter is proven to continuously change from a nodal to a fully gapped s-wave like Fermi surface in the superconducting regime as a function of x, partially investigated on (O,F) substituted samples.
LiOHFeSe is one of the novel intercalated FeSe compounds, showing strongly increased T_C = 43 K mainly due to increased interlayer spacing and resulting two-dimensionality of the Fermi surface. The primary interest of the samples of this thesis is the simultaneously observed ferromagnetism and superconductivity. The local probe techniques prove that superconducting sample volume gets replaced by ferromagnetic volume. Ferromagnetism arises from magnetic order with T_C = 10 K of secondary iron in the interlayer. The tendency of this system to show (Li,Fe) disorder is preserved upon (Se,S) substitution. However, superconductivity gets suppressed. The results of Mössbauer spectroscopy indicate that the systems tends to a secondary structural phase, where the local iron environment observed in pure FeS is absent. Moreover, two interlayer positions of the iron are identified. The absence of enhanced superconducting T_C in LiOHFeS thus is related to a structural instability.
Also, in CuFeAs the role of secondary iron at the Cu position turns out to be decisive for the observed magnetic behaviour. As in LiOHFeSe, it orders ferromagnetically at T_C ~ 11 K and superimposes with the magnetic instability of the main iron site. It is shown that a small charge doping of 0.1e/Fe, which is expected from (Cu,Fe) disorder, is sufficient to switch the system between a paramagnetic and an AFM ground state. Both magnetic orders are indistinguishable, because the magnetic order parameters are strongly coupled. This coupling was observed in the structurally identical CuFeSb, where the magnetic order parameters of both iron sites scale perfectly. The magnetically unstable CuFeAs and the ferromagnetic CuFeSb can be classified according to the theory of As height driven magnetism, predicting a change from paramagnetism to AFM and finally FM with increasing As height.:1 Acronyms and Symbols
2 Introduction
3 Iron-based arsenides and chalcogenides
3.1 Structural properties
3.2 Electronic properties
3.2.1 Magnetism
3.2.2 Superconductivity
3.2.3 Nematic phase
3.3 Investigated samples
4 Moessfit - a free Mössbauer fitting program
4.1 Aspects of program design
4.2 Errors
4.2.1 Uncorrelated
4.2.2 Hesse
4.2.3 MonteCarlo
4.2.4 Minos
4.3 Fitting algorithm
4.4 Maximum entropy method (MEM)
4.5 Kolmogorov-Smirnov confidence
5 Mössbauer spectroscopy
5.1 Mössbauer effect
5.2 Relativistic Doppler effect
5.3 Full static Hamiltonian
5.3.1 Quadrupole interaction
5.3.2 Isomer shift.
5.3.3 Zeeman splitting
5.3.4 Combined interaction
5.3.5 Transition probabilities
5.3.6 The magic angle
5.4 Transmission integral
5.4.1 Absorption area
5.4.2 Ideal thickness
5.4.3 Line width and line shape
5.4.4 Levelling
5.5 Applied field measurements of powder samples
5.5.1 Paramagnet, axial symmetric EFG in transverse field geometry 6
5.5.2 Uniaxial antiferromagnet, axial symmetric EFG in transverse field geometry 6
5.5.3 Paramagnet, axial symmetric EFG in longitudinal field geometry 6
5.5.4 Uniaxial ferromagnet, axial symmetric EFG in transverse field geometry 6
5.5.5 Polarised photons
5.5.6 Total absorption cross section
5.5.7 Polarised sources
5.6 Blume line shape model
6 μSR
6.1 Muon decay and detection
6.2 Magnetic order and dynamic relaxation
6.2.1 Magnetic order
6.2.2 Time dependent field distributions
6.2.3 Aspects of μSR in iron-based arsenides and chalcogenides
6.2.4 Weak transverse field (WTF)
6.3 Superconductivity - transverse field (TF) experiments
7 Intercalated FeSe
7.1 Bulk properties: XRD, susceptibility, resistivity
7.2 Structural characterization
7.3 LiOHFeSe - Mössbauer spectroscopy
7.3.1 Applied transverse field
7.4 LiOHFeSe - μSR
7.4.1 Zero field (ZF)
7.4.2 Pinning experiment
7.4.3 Transverse field (TF)
7.5 Mössbauer investigation of LiOHFe_1-yCo_ySe_1-xS_x.
7.6 Discussion
8 LaFeO(As,P)
8.1 Preliminary measurements and electronic structure calculations
8.2 Mössbauer spectroscopy
8.3 μSR
8.3.1 Magnetic characterization
8.3.2 Spin dynamics
8.3.3 Superconductivity
8.4 Discussion
9 CuFeAs and CuFeSb
9.1 Preliminary results of CuFeAs and CuFeSb
9.2 CuFeAs: Mössbauer spectroscopy
9.2.1 Zero field (ZF)
9.2.2 Longitudinal field (LF)
9.2.3 Transverse field (TF)
9.3 CuFeAs: μSR
9.3.1 Zero field (ZF)
9.3.2 Weak transverse field (WTF)
9.4 Further investigations on CuFeAs
9.4.1 Neutron scattering
9.4.2 Theoretical calculation
9.4.3 Local element analysis with EDX/WDX
9.5 CuFeSb: Mössbauer spectroscopy
9.5.1 Zero Field (ZF)
9.5.2 Transverse field (TF)
9.6 Discussion
10 Conclusion
11 Appendix
11.1 Derivation of the quadrupole interaction and isomer shift
11.2 Matrix form of the static nuclear Hamiltonian
11.3 Mössbauer line intensities
11.4 Blume line shape model
11.4.1 Special case: two states with diagonal Hamiltonians
11.5 Moessfit models
11.5.1 FeSe_1-xS_x(Li_1-zFe_zOH) ZF, standard
11.5.2 FeSe_1-xS_x(Li_1-zFe_zOH) ZF, 4 fractions
11.5.3 FeSe_1-xS_x(Li_1-zFe_zOH) Pinning
11.5.4 FeSe_1-xS_x(Li_1-zFe_zOH) TF
11.5.5 FeSe_1-xS_x(Li_1-zFe_zOH) CS-Vzz-MEM
11.5.6 LaFeP_1-xAs_x+ ferrocene, ZF
11.5.7 LaFeP_1-xAs_x+ ferrocene, LF
11.5.8 LaFeP_1-xAs_x+ iron foil, ZF
11.5.9 LaFeAsO ZF
11.5.10 LaFeAsO TF
11.5.11 CuFeAs + ferrocen, ZF
11.5.12 CuFeAs + ferrocen, ZF, high statistics
11.5.13 CuFeAs + ferrocen, LF
11.5.14 CuFeAs + ferrocen, TF
11.5.15 CuFeSb ZF
11.5.16 CuFeSb TF
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High-field electron spin resonance in low-dimensional spin systemsOzerov, Mykhaylo 04 May 2011 (has links)
Due to recent progress in theory and the growing number of physical realizations, low-dimensional quantum magnets continue to receive a considerable amount of attention. They serve as model systems for investigating numerous physical phenomena in spin systems with cooperative ground states, including the field-induced evolution of the ground-state properties and the corresponding rearrangement of their low-energy excitation spectra. This work is devoted to systematic studies of recently synthesized low-dimensional quantum spin systems by means of multi-frequency high-field electron spin resonance (ESR) investigations. In the spin- 1/2 chain compound (C6H9N2)CuCl3 [known as (6MAP)CuCl3] the striking incompatibility with a simple uniform S = 1/2 Heisenberg chain model employed previously is revealed. The observed ESR mode is explained in terms of a recently developed theory, revealing the important role of the alternation and next-nearest-neighbor interactions in this compound. The excitations spectrum in copper pyrimidine dinitrate [PM·Cu(NO3)2(H2O)2]n, an S = 1/2 antiferromagnetic chain material with alternating g-tensor and Dzyaloshinskii-Moriya interaction, is probed in magnetic fields up to 63 T. To study the high field behavior of the field-induced energy gap in this material, a multi-frequency pulsed-field ESR spectrometer is built. Pronounced changes in the frequency-field dependence of the magnetic excitations are observed in the vicinity of the saturation field, B ∼ Bs = 48.5 T. ESR results clearly indicate a transition from the soliton-breather to a spin-polarized state with magnons as elementary excitations. Experimental data are compared with results of density matrix renormalization group calculations; excellent agreement is found. ESR studies of the spin-ladder material (C5H12N)2CuBr4 (known as BPCB) completes the determination of the full spin Hamiltonian of this compound. ESR results provide a direct evidence for a pronounced anisotropy in this compound, that is in contrast to fully isotropic spin-ladder model employed previously for BPCB. Our observations can be of particular importance for describing the rich temperature-field phase diagram of this material. The frequency-field diagram of magnetic excitations in the quasi-two dimensional S = 1/2 compound [Cu(C4H4N2)2(HF2)]PF6 in the AFM-ordered state is studied. The AFM gap is observed directly. Using high-field magnetization and ESR results, parameters of the effective spin-Hamiltonian (exchange interaction, anisotropy and g-factor) are obtained and compared with those estimated from thermodynamic properties of this compound.
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Carrier Mobility And High Field Transport in Modulation Doped p-Type Ge/Si1-xGex And n-Type Si/Si1-xGex HeterostructuresMadhavi, S 03 1900 (has links)
Modulation doped heterostructures have revolutionized the operation of field effect devices by increasing the speed of operation. One of the factors that affects the speed of operation of these devices is the mobility of the carriers, which is intrinsic to the material used. Mobility of electrons in silicon based devices has improved drastically over the years, reaching as high as 50.000cm2/Vs at 4.2K and 2600cm2/Vs at room temperature. However, the mobility of holes in p-type silicon devices still remains comparatively lesser than the electron mobility because of large effective masses and complicated valence band structure involved. Germanium is known to have the largest hole mobility of all the known semiconductors and is considered most suitable to fabricate high speed p-type devices. Moreover, it is also possible to integrate germanium and its alloy (Si1_zGex ) into the existing silicon technology.
With the use of sophisticated growth techniques it has been possible to grow epitaxial layers of silicon and germanium on Si1_zGex alloy layers grown on silicon substrates. In tills thesis we investigate in detail the electrical properties of p-type germanium and n-type silicon thin films grown by these techniques. It is important to do a comparative study of transport in these two systems not only to understand the physics involved but also to study their compatibility in complementary field effect devices (cMODFET).
The studies reported in this thesis lay emphasis both on the low and high field transport properties of these systems. We report experimental data for the maximum room temperature mobility of holes achieved m germanium thin films grown on Si1_zGex layers that is comparable to the mobility of electrons in silicon films. We also report experiments performed to study the high field degradation of carrier mobility due to
"carrier heating" in these systems. We also report studies on the effect of lattice heating on mobility of carriers as a function of applied electric field.
To understand the physics behind the observed phenomenon, we model our data based on the existing theories for low and high field transport. We report complete numerical calculations based on these theories to explain the observed qualitative difference in the transport properties of p-type germanium and ii-type silicon systems. The consistency between the experimental data and theoretical modeling reported in this work is very satisfactory.
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Optimization of differential ion mobility and segmented ion fractionation to improve proteome coverageWu, Zhaoguan 09 1900 (has links)
La sensibilité et la profondeur de l'analyse protéomique sont limitées par les ions isobares et les interférences qui entravent l'identification des peptides de faible abondance. Lorsque nous analysons des échantillons de grande complexité, une séparation extensive de l'échantillon est souvent nécessaire pour étendre la couverture protéomique. Ces dernières années, la spectrométrie de mobilité ionique à forme d'onde asymétrique à haut champ (FAIMS) a gagné en popularité dans le domaine de la protéomique pour sa capacité à séparer les ions isobares, à améliorer la capacité de pic et la sensibilité de la spectrométrie de masse (MS). Nous rapportons ici l'intégration d'un appareil FAIMS Pro™ à un Q-Exactive HF™ ainsi qu'un spectromètre de masse Orbitrap Exploris 480™. Des expériences protéomiques sur des digestions d'extraits protéiques issues de cellules Hela à l'aide d'un spectromètre de masse avec FAIMS ont amélioré le rapport signal sur bruit (S/N) et réduit les ions interférents, ce qui a entraîné une augmentation du taux d'identification des peptides de plus de 42 %. FAIMS est également combiné avec le fractionnement ionique segmenté (SIFT), qui utilise tour à tour une fenêtre de 100 ~ 300 m/z au lieu de la large plage traditionnelle (700 ~ 800 m/z), augmentant ainsi la profondeur de la couverture protéomique tout en réduisant la proportion de spectres MS/MS chimériques de 50% à 27%. Dans l'analyse quantitative, nous démontrons l'application de FAIMS pour améliorer les mesures quantitatives lorsque le marquage peptidique isobare est utilisé. Par rapport aux expériences LC-MS/MS conventionnelles, la combinaison des expériences FAIMS et SIFT réalisées sur un modèle à deux protéomes a montré une amélioration de 65 % de la précision des mesures quantitatives. Les digestions tryptiques d'extraits protéiques de différentes lignées cellulaires du cancer colorectal ont été utilisées pour l'évaluation de stratégie combinée FAIMS et SIFT sur un spectromètre de masse Orbitrap Exploris 480™ offre un gain d'identification de 70 % par rapport à l'approche conventionnelle et combinée aux données transcriptomiques elle facilite l’identification de variants protéiques. / The sensitivity and depth of proteomic analysis in mass spectrometry (MS) is limited by isobaric
ions and interferences that hinder the identification of low-abundance peptides. For high
complexity samples, extensive separation is often required to expand proteomic coverage. In
recent years, high-field asymmetric waveform ion mobility spectrometry (FAIMS) has gained
popularity in the field of proteomics for its ability to resolve confounding ions, improve peak
capacity, and sensitivity. This thesis presents the integration of a FAIMS Pro™ interface with
electrical and gas embedded connections to a Q-Exactive HF™ as well as an Orbitrap Exploris
480™ mass spectrometer. Proteomic experiments on tryptic digests of HeLa cell line using a
FAIMS integrated mass spectrometer improved signal-to-noise ratio (S/N) and reduced the
occurrence of interfering ions. This enabled a 42% increase in peptide identification rate. Also,
FAIMS was combined with segmented ion fractionation (SIFT), which in turn scans with windows
of 100~300 m/z width instead of the traditional width (700~800 m/z), further increasing the depth
of proteome coverage by a reducing from 50% to 27% in terms of MS/MS chimeric spectra
numbers. The application of FAIMS gain improvement on quantitative measurements with TMT
labeling method is presented. Compared to conventional LC-MS/MS tests, the combination of
FAIMS and SIFT experiments showed a improvement by 65% in quantitative accuracy when
performed on a human-yeast two-proteome model. As an application of the method, the tryptic
digests from different colorectal cancer cell lines were used for the evaluation. FAIMS-SIFTcombined strategy on an Orbitrap Exploris 480™ mass spectrometer provides a 70% gain in
identification compared to the conventional LC-MS/MS approach for the same sample amount
and instrument time. This enhanced sensitivity facilitates single amino acid mutations confirmed
by RNAseq analyses.
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