京都大学 / 新制・課程博士 / 博士(農学) / 甲第25335号 / 農博第2601号 / 新制||農||1106(附属図書館) / 京都大学大学院農学研究科応用生命科学専攻 / (主査)教授 三芳 秀人, 教授 白井 理, 教授 森 直樹 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DGAM

This thesis presents the NMR measurements on the single crystals LiMnPO4 and Li0.9FeAs. Therefore, the thesis is divided into two separated sections. The first part reports on the competitive next generation cathode material LiMnPO4 with a stable reversible capacity up to 145 mAh/g and a rather flat discharge voltage curve at 4.1 V. For the basic understanding of the material the magnetic properties have been investigated by a Li and P NMR study in the paramagnetic and antiferromagnetic phase. LiMnPO4 shows a strong anisotropy of the dipolar hyperfine coupling due to the strong local magnetic moments at the Mn site. The corresponding dipole tensor of the Li- and P-nuclei is fully determined by orientation and temperature dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimentally determined values from the theoretical ones are discussed in terms of Mn disorder which could have an impact on the mobility of the Li ions. The disorder is corroborated by diffuse x-ray diffraction experiments which indicate a shift of the heavy elements in the lattice, namely the Mn atoms. Furthermore, the spin arrangement in the relative strong field of 7.0494 T in the antiferromagnetic state is understood by the NMR measurements. In order to obtain parameters of the Li ion diffusion in LiMnPO4 measurements of the spin lattice relaxation rate were performed. Due to the strong dipolar coupling between the Li-nuclei and the magnetic moments at the Mn site it is difficult to extract parameters which can characterize the diffusive behavior of the Li ions. The second section reports on the AC/DC susceptibility and NMR/NQR studies on Li deficit samples labeled as Li0.9FeAs. LiFeAs belongs to the family of the superconducting Pnictides which are discovered in 2008 by H. Hosono et al. In recent studies the stoichiometric compound reveals triplet superconductivity below Tc ∼ 18 K which demands ferromagnetic coupling of the electrons in the Cooper pairs. In Li0.9FeAs the Li deficit acts like hole doping which suppresses the superconductivity. Then ferromagnetism can arise which is very interesting because of the vicinity to the triplet superconductivity. With the microscopic methods NMR/NQR on the Li and As nuclei, it was investigated where the ferromagnetism can be located in Li0.9FeAs. Recent susceptibility, ESR and µSR studies reveal an internal field due to the ferromagnetism. In contrast, the internal field could not be used to perform zero field NMR measurements. Possible reasons for this discrepancy are discussed. In addition, the automatic insitu AC susceptibility technique by using the NMR radio frequency circuit has been tested by a reference compound Co2TiGa which shows itinerant ferromagnetism. Similar curves are observed for Li0.9FeAs which indicate the existence of itinerant magnetic moments in Li0.9FeAs. Furthermore, in order to determine the size of the dipolar contribution from the magnetic moments of the Fe the dipolar hyperfine coupling tensor was calculated from the crystal structure data. The comparison of the experimental and calculated hyperfine coupling elements reveals transferred hyperfine fields in LiFeAs.

At their discovery in 2008, iron pnictide superconductors (IPS) provoked tremendous scientific interest, comparable to the discovery of the cuprate superconductors. So far, IPS reached critical temperatures T c up to 56K. Typically, they show an antiferromagnetic (afm) spin density wave (SDW) which has to be suppressed by doping before superconductivity develops, which then is supported by further doping. Due to the close vicinity of the magnetic and the superconducting (sc) phase, magnetic fluctuations are discussed to be responsible for the sc pairing mechanism in IPS. A special member of the IPS is LiFeAs, because it does not need doping to become sc. It is a stoichiometric superconductor at a T c of 18K. In fact, doping is suppressing its T c . Also, there is no sign of an afm SDW present. Therefore, LiFeAs is a interesting material to study the properties of the IPS in an undisturbed material. In 2010, experiments of the Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden) revealed further surprising properties of LiFeAs. Samples with a Li deficiency undergo a ferromagnetic (fm) phase transition at 165K. Theoretical calculations suggest that fm fluctuations could induce triplet superconductivity in LiFeAs. This would cause a nonvanishing dynamic susceptibility below T c , which is supported by nuclear magnetic resonance (NMR) experiments. This thesis is discussing the results of the IFW Dresden experiments, and concludes that this ferromagnetism is of weak itinerant nature. The origin might be an increase of the density of states (DOS) at the Fermi level, which is causing an instability towards fm order, as proposed by the Stoner model. For further doping experiments, the synthesis procedure of polycrystalline LiFeAs was optimized to get samples with maximum T c and minimum impurities. Therefore, nuclear quadrupole resonance (NQR) was used. The NQR line width is a measure of impurities in the sample. By minimizing the NQR line width, optimal samples were synthesized. These samples are able to compete with the properties of single crystals. To investigate the doping behavior of LiFeAs, a scenario with four different kinds of impurities and deficiencies was performed with the optimized synthesis procedure. 24 different samples were analyzed, by means of NQR and electrical conductivity. It was found that in fact Fe excess is responsible for changing the physical properties of LiFeAs, and not Li deficiency. It is causing a shrinking of the unit cell volume, as seen by X-ray diffraction (XRD) measurements and it causes a decrease of T c . It also leads to a decrease of room temperature resistivity, which is supporting an increase of the DOS at the Fermi level. The NQR frequency is scaling with the amount of Fe excess and can be used to draw the sc and fm phase diagram of off-stoichiometric LiFeAs. At an amount between 3.2 and 3.6% o f Fe excess LiFeAs undergoes the fm transition.

No description available.

At their discovery in 2008, iron pnictide superconductors (IPS) provoked tremendous scientific interest, comparable to the discovery of the cuprate superconductors. So far, IPS reached critical temperatures T c up to 56K. Typically, they show an antiferromagnetic (afm) spin density wave (SDW) which has to be suppressed by doping before superconductivity develops, which then is supported by further doping. Due to the close vicinity of the magnetic and the superconducting (sc) phase, magnetic fluctuations are discussed to be responsible for the sc pairing mechanism in IPS. A special member of the IPS is LiFeAs, because it does not need doping to become sc. It is a stoichiometric superconductor at a T c of 18K. In fact, doping is suppressing its T c . Also, there is no sign of an afm SDW present. Therefore, LiFeAs is a interesting material to study the properties of the IPS in an undisturbed material. In 2010, experiments of the Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden) revealed further surprising properties of LiFeAs. Samples with a Li deficiency undergo a ferromagnetic (fm) phase transition at 165K. Theoretical calculations suggest that fm fluctuations could induce triplet superconductivity in LiFeAs. This would cause a nonvanishing dynamic susceptibility below T c , which is supported by nuclear magnetic resonance (NMR) experiments. This thesis is discussing the results of the IFW Dresden experiments, and concludes that this ferromagnetism is of weak itinerant nature. The origin might be an increase of the density of states (DOS) at the Fermi level, which is causing an instability towards fm order, as proposed by the Stoner model. For further doping experiments, the synthesis procedure of polycrystalline LiFeAs was optimized to get samples with maximum T c and minimum impurities. Therefore, nuclear quadrupole resonance (NQR) was used. The NQR line width is a measure of impurities in the sample. By minimizing the NQR line width, optimal samples were synthesized. These samples are able to compete with the properties of single crystals. To investigate the doping behavior of LiFeAs, a scenario with four different kinds of impurities and deficiencies was performed with the optimized synthesis procedure. 24 different samples were analyzed, by means of NQR and electrical conductivity. It was found that in fact Fe excess is responsible for changing the physical properties of LiFeAs, and not Li deficiency. It is causing a shrinking of the unit cell volume, as seen by X-ray diffraction (XRD) measurements and it causes a decrease of T c . It also leads to a decrease of room temperature resistivity, which is supporting an increase of the DOS at the Fermi level. The NQR frequency is scaling with the amount of Fe excess and can be used to draw the sc and fm phase diagram of off-stoichiometric LiFeAs. At an amount between 3.2 and 3.6% o f Fe excess LiFeAs undergoes the fm transition.

This thesis presents the NMR measurements on the single crystals LiMnPO4 and Li0.9FeAs. Therefore, the thesis is divided into two separated sections. The first part reports on the competitive next generation cathode material LiMnPO4 with a stable reversible capacity up to 145 mAh/g and a rather flat discharge voltage curve at 4.1 V. For the basic understanding of the material the magnetic properties have been investigated by a Li and P NMR study in the paramagnetic and antiferromagnetic phase. LiMnPO4 shows a strong anisotropy of the dipolar hyperfine coupling due to the strong local magnetic moments at the Mn site. The corresponding dipole tensor of the Li- and P-nuclei is fully determined by orientation and temperature dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimentally determined values from the theoretical ones are discussed in terms of Mn disorder which could have an impact on the mobility of the Li ions. The disorder is corroborated by diffuse x-ray diffraction experiments which indicate a shift of the heavy elements in the lattice, namely the Mn atoms. Furthermore, the spin arrangement in the relative strong field of 7.0494 T in the antiferromagnetic state is understood by the NMR measurements. In order to obtain parameters of the Li ion diffusion in LiMnPO4 measurements of the spin lattice relaxation rate were performed. Due to the strong dipolar coupling between the Li-nuclei and the magnetic moments at the Mn site it is difficult to extract parameters which can characterize the diffusive behavior of the Li ions. The second section reports on the AC/DC susceptibility and NMR/NQR studies on Li deficit samples labeled as Li0.9FeAs. LiFeAs belongs to the family of the superconducting Pnictides which are discovered in 2008 by H. Hosono et al. In recent studies the stoichiometric compound reveals triplet superconductivity below Tc ∼ 18 K which demands ferromagnetic coupling of the electrons in the Cooper pairs. In Li0.9FeAs the Li deficit acts like hole doping which suppresses the superconductivity. Then ferromagnetism can arise which is very interesting because of the vicinity to the triplet superconductivity. With the microscopic methods NMR/NQR on the Li and As nuclei, it was investigated where the ferromagnetism can be located in Li0.9FeAs. Recent susceptibility, ESR and µSR studies reveal an internal field due to the ferromagnetism. In contrast, the internal field could not be used to perform zero field NMR measurements. Possible reasons for this discrepancy are discussed. In addition, the automatic insitu AC susceptibility technique by using the NMR radio frequency circuit has been tested by a reference compound Co2TiGa which shows itinerant ferromagnetism. Similar curves are observed for Li0.9FeAs which indicate the existence of itinerant magnetic moments in Li0.9FeAs. Furthermore, in order to determine the size of the dipolar contribution from the magnetic moments of the Fe the dipolar hyperfine coupling tensor was calculated from the crystal structure data. The comparison of the experimental and calculated hyperfine coupling elements reveals transferred hyperfine fields in LiFeAs.

Nesta tese investigamos o estado normal (T > Tc) dos Yba2Cu3O6+x supercondutores de alto Tc. As técnicas da RMN e RQN foram empregadas para estudar as respostas estática e dinâmica do sistema de spins eletrônicos em função da temperatura, para diversas concentrações de oxigênio: x = 0.5 (cristal 1), 0.92 (cristal 2), 0.94 (cristal 3) e 1.0 (cristais 4 e 5). Focalizando diferentes sítios nucleares a RMN pode fornecer uma informação mais rica da susceptibilidade dinâmica, X (q.w), abrangendo diferentes regiões da Zona de Brillouin (ZB). Com os sítios do 17O(2,3) e do 89Y sensoreamos a região próxima ao centro do ZB, q == 0, enquanto que com o sitio do Cu(2) exploramos o extremo do ZB, (q == QAF). Foi realizado um estudo comparativo da RMN dos sítios do Cu(2), do O(2,3) e do Y sobre o cristal 1. Da comparação entre os deslocamentos hiperfinos magnéticos do Y e do O(2,3) extraímos as contribuições orbitais e as constantes de acoplamento hiperfino do Y. A razão entre as relaxações spin-rede do O(2,3) e do Y mostrou-se independente da temperatura, e próxima do esperado teoricamente a partir das constantes hiperfinas no cenário do hamiltoniano de Mila e Rice. Questões como a dependência da taxa de relaxação spin-rede (T1-1) com a intensidade do campo e a anisotropia da susceptibilidade estática de spin foram estudadas sobre o cristal 2. Acima de Tc, para o Cu(2), não foi encontrada nenhuma dependência de T1 com a intensidade do campo. Os resultados sobre o Y indicam uma anisotropia da susceptibilidade e do tensor de acoplamento hiperfino. Uma extensa comparação entre os resultados da RMN e da Difusão Inelástica de Nêutrons (DIN) foi realizada sobre as mesmas amostras (cristais 4 e 5), fato inédito ate então. Propõe-se um modelo gaussiano para X\"~q,o», compatível com as experiências de nêutrons, no qual o comprimento de correlação ~ foi fixado pelos resultados da DIN sobre o cristal 2. Analisando a relaxação spin-spin do Cu(2), 63(T2g)-1, em função deste modelo, extraímos a susceptibilidade estática X\'(q=QAF) em unidades absolutas e encontramos um fator de Stoner X\'(q=QAF) / X\'(q=O) ~ 10. Da análise conjunta de 63(T1T)-1 e 63 (T2g)-1 obtivemos a energia característica das flutuações AF, rAF::::3:0meV, em excelente acordo com a DIN. Estes mesmos resultados permitiram evidenciar, exclusivamente através do RMN, quando ocorre a abertura de um gap de spin. Neste trabalho mostramos que resultados da DIN são compatíveis com a RMN para o sitio do Cu(2), porem, sérias dificuldades são encontradas para explicar os resultados sobre os sítios do Y e, principalmente, do O(2,3). As alternativas para este problema são discutidas. Os resultados obtidos nesta tese levaram ao estabelecimento de um diagrama de fase, segundo a RMN, do estado normal dos Yba2Cu3O6+x supercondutores, cobrindo do regime sub-dopado ao regime sobre-dopado, passando pela composição de dopagem ótima. / The normal state(T > Tc) of the high Te superconducting Yba2Cu3O6+x was studied. The NMR and NQR techniques were employed in order to investigate the static and dynamic responses of the electronic spin system as function of temperature, for the following oxygen contents: x = 0.5 (crystal 1), 0.92 (crystal 2), 0.94 (crystal 3) e 1.0 (crystals 4 and 5). Focusing various nuclear sites the NMR could give a more rich information of the dynamical susceptibility, X(q,ffi), probing different regions of the Brillouin Zone (ZB). With 17O(2,3) and 89Y sites the BZ center, q:: 0, was studied, while with the Cu(2) site it was the BZ extreme, (q:: QAF) that was investigated. In crystal 1 we have compared the NMR of the Cu(2), O(2,3) and Y sites. From the comparison between the Y and O(2,3) magnetic hyperfine shift we have extracted the orbital contributions and the Y hyperfine coupling tensor. The ratio between the O(2,3) and Y nuclear spin lattice relaxation rate was found to be temperature independent, and its value was in fair agreement with what it was expected theoretically from the coupling constants within the picture of the Mila-Rice Hamiltonian. Problems such as the field intensity dependence of the spin-lattice -relaxation rate (T1-1) and the anisotropy of the static susceptibility where investigated in crystal 2. Above Tc, for the Cu(2) site, we have not found any field dependence of T1. Our results on the Y site indicate an anisotropy of both the static susceptibility and the Y coupling tensor. An extensive comparison between the NMR and the Inelastic Neutron Diffusion (IND) results was done in the very same samples, crystals 2 and 4. In accordance to neutron data a Gaussian model for X\"AF( q,ro) is proposed, in which the coherence length ~ was fixed by the IND results in crystal 2. Analyzing the Cu(2) nuclear spin-spin relaxation rate, 63(T2g)-1, as function of this model, we have extracted the staggered susceptibility X\'(q=QAF) in absolute units, finding a Stoner factor X \'( q=QAF) / X\'( q=O) ~ 10. From the analysis of 63(T1T)-1 together with 63(T2g)-1 we have obtained the AF characteristic energy, rAF ::::l 30meV, in excellent agreement with IND. These results provided us a powerful method to evidence the opening of a spin pseudo gap, exclusively from the NMR results. A consistent picture arise from the comparison between IND and the Cu(2) NMR. In contrast, it is hard to explain the Y and, specially, the O(2,3) results within this same picture. The theoretical alternatives to this puzzle are discussed. The whole set of results we have obtained led us to propose a NMR based phenomenological phase diagram for the normal state of the superconducting Yba2Cu3O6+x, covering from the under doped regime to the over doped regime, passing through the optimal doping.

Neste trabalho realizamos a implementação experimental de informação quântica (IQ) em um sistema de dois bits quânticos (q-bits) de spin 3/2 via ressonância quadrupolar nuclear (RQN). Foram implementadas portas lógicas quânticas que são necessárias para a criação e manipulação dos estados pseudo-puros (PPSs). Ademais, foi proposto um método de tomografia de estado quântico (TEQ) baseado na seleção de coerências de múltiplo quantum por ciclagem de fases. A TEQ foi empregada para avaliar os estados quânticos implementados experimentalmente. A amostra utilizada foi um monocristal de KClO3, o núcleo medido foi 35Cl, que possui spin 3/2. Neste sistema foi possível obter os quatro PPSs da base computacional. Sobre os PPSs foram aplicados portas lógicas quânticas CNOT e Hadamard, que produziram estados de sobreposição e estados emaranhados. Sobre os estados emaranhados foram analisados os conceitos de correlações clássicas e quânticas. A TEQ dos estados implementados experimentalmente mostrou altas fidelidades (maior de 90%). Também foi possível criar estados coerentes de spin aplicando rotações sobre os PPSs. Com base nos estados coerentes de spin foram gerados estados coerentes comprimidos mediante a aplicação de evoluções não lineares, presentes naturalmente em sistemas de RQN. Estes resultados promissores mostram que a RQN pode ser satisfatoriamente aplicada como uma ferramenta experimental em estudos de IQ. / In this work we describe the experimental implementation of quantum information processing (QIP) in a two spin qubits system by nuclear quadrupole resonance (NQR). We implemented quantum gates and their applications in the creation and manipulation of pseudo-pure state (PPS). Furthermore, we propose one method of Quantum State Tomography (QST) based on coherence pathways selected by RF phase cycling. QST is one of the tools used to evaluate QIP implementations, it allows to completely evaluate the quantum state of the spin system. We experimentally implemented NQR-QIP in a KClO3 single crystal and observing 35Cl, a spin 3/2 nucleus. It was possible to obtain all the four PPS associated with the computational basis and to apply the Controlled-not (CNOT) and Hadamard gates on them. The reading of the resulting states was performed by the proposed QST method, and resulted in experimental quantum state fidelities greater than 90%. It was also possible to create squeezed spin states. This states are generated by non linear interactions, which naturally arise in a NQR system. These are very promising results and they indicate that NQR can be successfully applied as an experimental tool for studying fundamental QIP theory.

La détermination des distributions de contrainte et de déformation dans des matériaux opaques est une question clé pour évaluer leur comportement sous des conditions de chargement souvent complexes. La plupart des méthodes expérimentales utilisées aujourd’hui pour traiter ce problème sont des méthodes invasives ou bien inefficaces pour les matériaux opaques. Dans ce contexte nous proposons d’utiliser le signal de Résonance Quadripolaire Nucléaire (RQN) de petits cristaux, utilisés comme charge, dans des matériaux peu ou pas conducteurs. Les contraintes transmissent par la matrice au cristal induisent une déformation de son réseau cristallin qui à son tour induit une modification de sa fréquence RQN. La RQN est, par conséquent, sensible aux déformations induites par une contrainte mécanique externe et le cristal agit comme une jauge de contrainte à l’échelle du micron. Nous appelons cette méthode la piézospectroscopie RQN.L’objectif de cette étude est de prouver que l’on peut utiliser la piézospectroscopie RQN pour mesurer des contraintes au sein de matériaux opaques et plus précisément au sein d’élastomères. Le lien fondamental entre la variation de fréquence RQN et le tenseur de contrainte appliqué au cristal a été étudié. Pour cela nous avons choisi le 63Cu dans la cuprite comme sonde RQN. Expérimentalement, la dépendance à la contrainte de la fréquence RQN est mesurée en utilisant différentes sollicitations mécaniques générées par des montages originaux intégrant mesure de contrainte et de RQN. Les résultats ainsi obtenus sont discuté en regard de calculs quantiques basés sur la DFT.En outre, comme preuve de concept de la piézospetroscopie RQN, nous avons réalisé des expériences supplémentaires en utilisant des conditions de chargement qui nous ont permis de démontrer, sans faire d’hypothèses sur les propriétés mécaniques du Cu2O, que la variation de fréquence RQN est représentative de la composante hydrostatique du champ de contrainte interne de l’élastomère. Nous avons ensuite étudié la capacité de cette méthode à cartographier un champ de contrainte hétérogène. / The determination of stress and strain distributions in opaque materials is a key issue to evaluate their mechanical behavior. However, most of the methods used today to measure stress are either extremely invasive or ineffective for opaque materials. Within this context, we propose to use the Nuclear Quadrupolar Resonance (NQR) signal of small crystals, embedded within non, or poorly, conductive materials. Stresses transferred by the matrix to the crystal induce a deformation of its lattice which in turn results in a modification of the Electric Field Gradient (EFG) at the nucleus of interest. NQR is, as a consequence, sensitive to deformations induced by external mechanical stress and the crystals act as local stress gauges at the micron-scale. We call this method NQR piezospectroscopy.The objective of this study is to prove that NQR piezospectroscopy can be used to measure stress within opaque materials and more precisely within elastomer. The fundamental link between the NQR frequency variation and the stress tensor applied to the crystal has been studied. For that purpose, 63Cu in cuprite has been selected as a NQR probe. The stress dependence of its NQR frequency is investigated experimentally using different mechanical loadings generated within an original integrated NQR – stress device and discussed in the light of DFT ab-initio calculations.In addition, as proof-of-concept of NQR piezospectroscopy, additional experiments were carried out using loading conditions proving, without hypothesis on cuprite’s mechanical properties, that its NQR frequency shift is representative of the hydrostatic component of the elastomer inner stress field. We then investigated the ability of this method to map an heterogeneous stress field.

Neste trabalho realizamos a implementação experimental de informação quântica (IQ) em um sistema de dois bits quânticos (q-bits) de spin 3/2 via ressonância quadrupolar nuclear (RQN). Foram implementadas portas lógicas quânticas que são necessárias para a criação e manipulação dos estados pseudo-puros (PPSs). Ademais, foi proposto um método de tomografia de estado quântico (TEQ) baseado na seleção de coerências de múltiplo quantum por ciclagem de fases. A TEQ foi empregada para avaliar os estados quânticos implementados experimentalmente. A amostra utilizada foi um monocristal de KClO3, o núcleo medido foi 35Cl, que possui spin 3/2. Neste sistema foi possível obter os quatro PPSs da base computacional. Sobre os PPSs foram aplicados portas lógicas quânticas CNOT e Hadamard, que produziram estados de sobreposição e estados emaranhados. Sobre os estados emaranhados foram analisados os conceitos de correlações clássicas e quânticas. A TEQ dos estados implementados experimentalmente mostrou altas fidelidades (maior de 90%). Também foi possível criar estados coerentes de spin aplicando rotações sobre os PPSs. Com base nos estados coerentes de spin foram gerados estados coerentes comprimidos mediante a aplicação de evoluções não lineares, presentes naturalmente em sistemas de RQN. Estes resultados promissores mostram que a RQN pode ser satisfatoriamente aplicada como uma ferramenta experimental em estudos de IQ. / In this work we describe the experimental implementation of quantum information processing (QIP) in a two spin qubits system by nuclear quadrupole resonance (NQR). We implemented quantum gates and their applications in the creation and manipulation of pseudo-pure state (PPS). Furthermore, we propose one method of Quantum State Tomography (QST) based on coherence pathways selected by RF phase cycling. QST is one of the tools used to evaluate QIP implementations, it allows to completely evaluate the quantum state of the spin system. We experimentally implemented NQR-QIP in a KClO3 single crystal and observing 35Cl, a spin 3/2 nucleus. It was possible to obtain all the four PPS associated with the computational basis and to apply the Controlled-not (CNOT) and Hadamard gates on them. The reading of the resulting states was performed by the proposed QST method, and resulted in experimental quantum state fidelities greater than 90%. It was also possible to create squeezed spin states. This states are generated by non linear interactions, which naturally arise in a NQR system. These are very promising results and they indicate that NQR can be successfully applied as an experimental tool for studying fundamental QIP theory.