Materiais que apresentam um acoplamento entre as propriedades elétricas e magnéticas, conhecidos como magnetoelétricos, estão despertando a atenção da comunidade acadêmica nos últimos anos principalmente devido ao seu grande potencial tecnológico no desenvolvimento de materiais multifuncionais. Sendo um maior esforço concentrado na obtenção de novos materiais multiferróicos, que são aqueles que possuem duas ou mais ordens ferróicas, em especial os que apresentam ferroeletricidade e ferromagnetismo. Este trabalho tem como objetivo uma investigação das propriedades dielétricas e magnéticas de um grupo de materiais ainda pouco estudado a fim de se compreender os mecanismos responsáveis pelo efeito magnetoelétrico, de forma a contribuir com a pesquisa nessa área. As amostras estudadas são os óxidos de titânio e cromo Nd(1-x)A(x)CrTiO5 (A=Y, La) e GdCrTiO5, todos na forma de cerâmicas policristalinas monofásicas. Foram discutidas de que forma a diluição da subrede magnética do neodímio afeta o caráter magnetoelétrico observado nesse material e qual a importância de cada uma das subredes (Nd3+/Gd3+ e Cr3+) nas propriedades dielétricas e magnéticas. Verificamos que o efeito magnetoelétrico é sensível a variações na rede do Nd3+. Entretanto, nossos resultados no composto GdCrTiO5 indicam que as subredes magnéticas são pouco acopladas, o que indica que o ion Cr3+ talvez seja o responsável por dirigir o efeito magnetoelétrico nesta família de materiais. / The coupling of the electric and magnetic properties in the magnetoelectric compounds is attracting a great deal of interest in the scientific community due to the huge potential for application based on new multifunctional materials. Of special attention are the so-called multiferroics, materials that simultaneously show ferromagnetism and ferroelectricity. The main purpose of this work is the experimental investigation of the dielectric and magnetic properties of a relatively less known class of materials with the aim to identify the origin of the magnetoelectric coupling. We study the single phase polycrystalline compounds Nd(1-x)A(x)CrTiO5 (A=Y, La) and GdCrTiO5 and discuss the effect of dilution of the Nd3+ ions and the importance of each magnetic sublattice on the samples behavior. We find that the magnetoelectric effect is susceptible to changes in the Nd3+ site. At the same time, our results indicate that the two magnetic sublattice (Nd3+/Gd3+ and Cr3+) are only weakly coupled indicating that the Cr3+ magnetic sublattice might be responsible for driving the magnetoelectric effect in this family of compounds.

A busca por dispositivos que possuam uma estrutura micrométrica ou nanomé- trica, tem sido um tópico relevante de pesquisa, pois esse tipo de estrutura permite o estudo de efeitos ainda em análise e com várias questões em aberto, como por exemplo, efeito magnetoelétrico, o qual pode estar atrelado à um efeito de superfície. Estes tipos de efeitos podem surgir em dispositivos elétricos de modo a alterarem suas proprieda- des. Neste trabalho, desenvolveu-se uma heteroestrutura capacitiva composta por dois ele- trodos metálicos (Al na base e Ag no topo) com Al2O3 nanoporoso como separador dielétrico. O NAM foi obtido a partir de uma folha de Al de alta pureza por meio do processo de anodização em duas etapas. Esta técnica permite que, naturalmente, um dos planos da folha original permaneça inalterado, preservando uma fina camada de Al. Antes de revestir o plano oposto com Ag (via pulverização catódica), os na- nofios (Nanofios(Nf)) foram eletrodepositados no arranjo poroso hexagonal (poros de aproximadamente 50nm de diâmetro e 3m de comprimento). A heteroestrutura foi caracterizada por difração de raios X e por Microscopia Eletrônica de Varredura. As medições de capacitância versus tensão para frequências entre 1KHz e 5MHz foram realizadas utilizando uma estação de sonda Cascade Microtech controlada por um ana- lisador B1500A e foram obtidas capacitâncias por unidade de área planar de cerca de 50 nF para esta nanoestrutura. A presença de Nf no capacitor permite estudar o com- portamento da capacitância em função da magnetização deles. Os testes de capacitân- cia foram realizados deixando os Nf em estados magnéticos bem definidos. Primeiro, mediu-se a capacitância com os fios no estado desmagnetizado e, em seguida, com eles em estado remanescente após a saturação magnética no plano. Por fim, mediu-se a capacitância com os fios no estado remanescente após a saturação magnética ao longo do eixo do Nf. Após essas medidas, constatou-se o aumento na capacitância quando a magnetização do Nf aumenta. Este efeito magnetoelétrico é uma evidência experimen- tal do acoplamento de interface entre um dielétrico e um metal polarizado por spin. / The search for devices that have a micrometric or nanometric structure, has been a relevant research topic, since this type of structure allows the study of effects still under analysis and with several open questions, such as, for example, magnetoelectric effect, which can be tied to a surface effect. These types of effects can arise in electrical devices in order to change their properties. In this work, we have developed a capacitive heterostructure consisting of two metal electrodes (Al at the base and Ag at the top) with nanoporous Al2O3 as the dielec- tric separator. The NAM was obtained starting from a high-purity Al sheet, using the two-step anodization process. This technique allows one of the planes of the origi- nal sheet to remain unaltered naturally, preserving a thin layer of Al. Before coating the opposite plane with Ag (via sputtering), nano-wires (NW) were electrodeposited in the hexagonal pore array (pores of approximately 60nm of diameter and 10m of length). The heterostructure was characterized by X-ray diffraction. Measurements of capacitance versus voltage for frequencies between 1KHz e 5MHz were performed using a Cascade Microtech probe station controlled by a B1500A Analyzer and there were obtained capacitances per unit of planar area of around 50 nF for this nanos- tructure. The presence of NW in the capacitor allows the study of the behavior of the capacitance as a function of the magnetization of the NW. The capacitance tests were performed by leaving the NW in well-defined magnetic states. First, the wires were measured in the demagnetized state, than with them in a remanent state after the in-plane magnetic saturation. Finally, measures of capacitance were taken when the wires were in the remanent state after the magnetic saturation along the axis of the NW. There was seen the increase in capacitance when the magnetization of the NW increases. This magnetoelectric effect is an experimental evidence of the interface cou- pling between a dielectric and a spin-polarized metal.

Materiais que apresentam um acoplamento entre as propriedades elétricas e magnéticas, conhecidos como magnetoelétricos, estão despertando a atenção da comunidade acadêmica nos últimos anos principalmente devido ao seu grande potencial tecnológico no desenvolvimento de materiais multifuncionais. Sendo um maior esforço concentrado na obtenção de novos materiais multiferróicos, que são aqueles que possuem duas ou mais ordens ferróicas, em especial os que apresentam ferroeletricidade e ferromagnetismo. Este trabalho tem como objetivo uma investigação das propriedades dielétricas e magnéticas de um grupo de materiais ainda pouco estudado a fim de se compreender os mecanismos responsáveis pelo efeito magnetoelétrico, de forma a contribuir com a pesquisa nessa área. As amostras estudadas são os óxidos de titânio e cromo Nd(1-x)A(x)CrTiO5 (A=Y, La) e GdCrTiO5, todos na forma de cerâmicas policristalinas monofásicas. Foram discutidas de que forma a diluição da subrede magnética do neodímio afeta o caráter magnetoelétrico observado nesse material e qual a importância de cada uma das subredes (Nd3+/Gd3+ e Cr3+) nas propriedades dielétricas e magnéticas. Verificamos que o efeito magnetoelétrico é sensível a variações na rede do Nd3+. Entretanto, nossos resultados no composto GdCrTiO5 indicam que as subredes magnéticas são pouco acopladas, o que indica que o ion Cr3+ talvez seja o responsável por dirigir o efeito magnetoelétrico nesta família de materiais. / The coupling of the electric and magnetic properties in the magnetoelectric compounds is attracting a great deal of interest in the scientific community due to the huge potential for application based on new multifunctional materials. Of special attention are the so-called multiferroics, materials that simultaneously show ferromagnetism and ferroelectricity. The main purpose of this work is the experimental investigation of the dielectric and magnetic properties of a relatively less known class of materials with the aim to identify the origin of the magnetoelectric coupling. We study the single phase polycrystalline compounds Nd(1-x)A(x)CrTiO5 (A=Y, La) and GdCrTiO5 and discuss the effect of dilution of the Nd3+ ions and the importance of each magnetic sublattice on the samples behavior. We find that the magnetoelectric effect is susceptible to changes in the Nd3+ site. At the same time, our results indicate that the two magnetic sublattice (Nd3+/Gd3+ and Cr3+) are only weakly coupled indicating that the Cr3+ magnetic sublattice might be responsible for driving the magnetoelectric effect in this family of compounds.

L'effet magnétoélectrique direct est défini par la modification de la polarisation électrique à partir de l'application d'un champ magnétique. Bien que cet effet existe de manière intrinsèque dans certains matériaux, nous étudions ici l'effet extrinsèque, où l'effet magnétoélectrique résulte d'un couplage intermédiaire entre deux phases distinctes. Dans ce cas, l'idée la plus répandue est de lier mécaniquement (par un collage) un matériau piézoélectrique à un matériau magnétostrictif. Ainsi, en appliquant un champ magnétique, le matériau magnétostrictif se déforme, transmet une contrainte au matériau piézoélectrique qui voit sa polarisation changer. Dans cette thèse nous nous intéressons à deux types de composites magnétoélectriques laminaires. Ceux employant les ferrites magnétostrictifs doux (ferrite de nickel) et ceux qui utilisent les ferrites semi-durs (ferrite de cobalt). Pour chacun des composites, on s'intéresse à optimiser l'effet magnétoélectrique en mettant en avant les paramètres qui l'influencent majoritairement. De ce fait, nous traitons différents aspects tels que l'influence de l'effet démagnétisant dans les multicouches, de la fraction volumique des composites, des phases secondaires, de la magnétostriction dynamique, de l'anisotropie magnétique uniaxe, et enfin de la fréquence et de l'amplitude du champ d'excitation magnétique sur l'effet magnétoélectrique. Grâce à la compréhension de ces phénomènes, nous avons pu optimiser le couplage magnétoélectrique de manière à développer un capteur de courant présentant des caractéristiques comparables aux capteurs de courant actuellement commercialisés et qui utilisent d'autres technologies (effet Hall, transformateur de courant). / The direct magnetoelectric effect is defined by the modification of the electric polarization induced by a magnetic field. Although this effect exists intrinsically in some materials, here we study the extrinsic effect, where the magnetoelectric effect results from an intermediate coupling between two distinct phases. In this case, the most common idea is to mechanically couple (by gluing) a piezoelectric material to a magnetostrictive material. Thus, by applying a magnetic field, the magnetostrictive material is deformed and transmits a stress to the piezoelectric material which makes its polarization change.In this thesis, we are interested in two types of laminar magnetoelectric composites: those using soft magnetostrictive ferrites (nickel ferrite) and those using semi-hard ferrites (cobalt ferrite). For each composites, we want to optimize the magnetoelectric effect by highlighting the parameters that mainly influence this coupling. As a result, we deal with different aspects such as the influence of the demagnetizing effect in multilayers, the volume fraction in the composites, the secondary phases, the dynamic magnetostriction, the uniaxial magnetic anisotropy, and finally the frequency and the amplitude of the magnetic exciting field on the magnetoelectric effect. Thanks to the understanding of the physical phenomena involved and the optimization of the resulting magnetoelectric coupling, we have been able to develop a current sensor with characteristics comparable to currently marketed current sensors that use other technologies (Hall effect, current transformer).

No description available.

Bismuth ferrite (BFO) is a multiferroic material with cross-correlation between magnetic and electric orders. With no applied external fields the spin structure of BFO is anitferromagnetic and cycloidal. This ordering prevents the detection of the weak ferromagnetism known to exist in the material. The application of magnetic and electric fields of suitable strength and direction is capable of compelling the Fe3+ spins to align in a homogeneous, antiferromagnetic fashion. This report details how numerical methods were used to simulate the spin alignment of a BFO system under different fields. The results were compiled into electric field-magnetic field phase diagrams of BFO to show the divide between cycloidal and homogeneous systems. / Graduate / 0607 / 0611 / marca@uvic.ca

Les systèmes multiferroïques magnéto-électriques sont des matériaux multifonctionnels très importants du point de vue des applications dans le domaine de l’électronique ou de la spintronique puisqu’ils présentent simultanément des ordres électriques et magnétiques généralement couplés. Ils peuvent donc répondre à la fois à l’application d’un champ magnétique et d’un champ électrique. L’une des familles de multiferroïques magnéto-électriques les plus étudiées est la série RMn₂O₅ où R est une terre rare. Ma thèse porte sur le rôle de la terre rare sur le caractère multiferroïque et sur les propriétés magnétiques de ces composés. De nombreux travaux ont déjà été publiés mais très peu concernent les composés SmMn₂O₅ et GdMn₂O₅, difficiles à étudier aux neutrons. Pourtant ce sont des composés clés, à la frontière entre des composés à terres rares légères qui ne sont pas multiferroïques et ceux à terres rares lourdes qui le sont. Je me suis donc intéressé à ces composés. Grâce à une étude théorique basée sur une analyse de symétrie et des calculs numériques ab initio tenant compte du fort couplage spin-orbite, nous avons pu prévoir un modèle pour l’ordre magnétique stabilisé dans Sm et Gd. Nous avons en parallèle étudié expérimentalement les structures magnétiques pour ces deux composés par le biais d’une analyse des donnés de diffraction de neutrons sur poudre utilisant des matériaux isotopes de Sm et Gd. Ma thèse a permis d’une part de valider le mécanisme d’échange striction comme origine du couplage magnéto-électrique dans cette série importante de multiferroïques. Elle a permis d’autre part de mettre en évidence l’existence d’une interaction d’échange supplémentaire dans GdMn₂O₅, à l’origine de la forte polarisation électrique dans ce membre de la série. Ces résultats amènent plus de clarté à la compréhension de la multiferroïcité dans ces systèmes. / The magneto-electric multiferroic systems are multifunctional materials very important for applications in the field of electronics or spintronics since they present simultaneously electrical and magnetic orders, which are generally coupled. They can thus respond to both the application of a magnetic field and an electric field. One of the most studied magneto-electric multiferroic families is the RMn₂O₅ series where R is a rare earth. My thesis deals with the role of rare earth on the multiferroic and the magnetic properties of these compounds. Numerous works have already been published, but very few concern the compounds SmMn₂O₅ and GdMn₂O₅, difficult to study with neutrons. Yet these are key compounds, on the border between light rare earth compounds that are not multiferroic and heavy rare earths that are. This explains my interest for these compounds. Using a theoretical study based on a symmetry analysis and numerical calculations ab initio taking into account the strong spin-orbit coupling, we were able to predict a model for the stabilized magnetic order in Sm and Gd. We have experimentally studied the magnetic structures for these two compounds by means of an analysis of the powder neutron diffraction data using Sm and Gd isotopes. On one hand, my thesis allowed to validate the mechanism of exchange striction as origin of the magnetoelectric coupling in this important series of multiferroics. On the other hand, it has made it possible to demonstrate the existence of an additional exchange interaction in GdMn₂O₅, at the origin of the strong electrical polarization in this member of the series. These results provide greater clarity to the understanding of multiferroicity in these systems.

Dirac fermions provide a prototypical description of topological insulators and their gapless boundary states, which are predicted by the bulk-boundary correspondence. Motivated by the unusual physical properties of these states, we study them in two different Hermitian quantum systems. In non-Hermitian systems, we investigate the failure of the bulk-boundary correspondence and show that non-Hermitian topological invariants impact a system’s bulk response. First, we study electronic topological insulators in three dimensions with time-reversal symmetry. These can be characterized by a quantized magnetoelectric coefficient in the bulk, which, however, does not yield an experimentally observable response. We show that the signature response of a time-reversal-invariant topological insulator is a nonlinear magnetoelectric effect, which in the presence of a small electric field leads to the appearance of half-integer charges bound to a magnetic flux quantum. Next, we consider topological superconducting nanowires. These feature Majorana zero modes at their ends, which combine nonlocally into a single electronic state. An electron tunneling through such a state will be transmitted phase-coherently from one end of the wire to the other. We compute the transmission phase for nanowires with broken time-reversal symmetry and confirm that it is independent of the wire length. Turning to non-Hermitian systems, we consider planar optical microcavities with an anisotropic cavity material, which may feature topological degeneracies known as excep- tional points in their complex frequency spectrum. We present a quantitative method to extract an effective non-Hermitian Hamiltonian for the eigenmodes, and describe how a pair of exceptional points arises from a Dirac point due to the cavity loss. Finally, we investigate generalized topological invariants that can be defined for non- Hermitian systems, but which have no counterpart (i.e. vanish) in Hermitian systems, for example the so-called non-Hermitian winding number in one dimension. Contrary to Hermitian systems, the bulk-boundary correspondence breaks down: Comparing Green functions for periodic and open boundary conditions, we find that in general there is no correspondence between topological invariants computed for periodic boundary con- ditions, and boundary eigenstates observed for open boundary conditions. Instead, we prove that the non-Hermitian winding number in one dimension signals a topological phase transition in the bulk: It implies spatial growth of the bulk Green function, which we define as the response of a gapped system to an external perturbation on timescales where the induced excitations have not propagated to the boundary yet. Since periodic systems cannot accommodate such spatial growth, they differ from open ones.

Tesis por compendio / [ES] El aumento de la esperanza de vida conlleva la aparición de problemas muscoloesqueléticos afectando a la calidad de vida de los pacientes. Las nuevas terapias regenerativas óseas se centran en el uso de las células madre mesenquimales, MSCs, encargadas de la regeneración del tejido in vivo. La inducción de un fenotipo osteogénico prediferenciado in vitro, previo a la implantación de las MSCs, resulta en una mejor capacidad de regeneración del tejido óseo. Habitualmente se han empleado medios de diferenciación osteogénica que contienen dexametasona. Estos métodos son poco eficientes, por lo que el uso de métodos físicos está adquiriendo relevancia. El hueso es un tejido con propiedades piezoeléctricas debido a las fibras de colágeno que forman parte de su matriz extracelular. Este estímulo ha sido relacionado con su capacidad de responder al estrés mecánico y autoregenerarse, donde juegan un papel importante las MSCs. Éstas se encuentran en un entorno electroactivo, y son precisamente estas señales físicas las que pueden influir en su proceso de diferenciación osteogénica pudiendo ser empleadas para su prediferenciación in vitro de forma efectiva. Para comprobar esta hipótesis, en la presente Tesis Doctoral se han diseñado soportes de cultivo piezoeléctricos en 2 y 3 dimensiones basados en el uso del polímero piezoeléctrico polifluoruro de vinilideno (PVDF) combinados con partículas magnetostrictivas de ferrita de cobalto (CFO). Esta combinación permite la estimulación de los soportes de cultivo aplicando un campo magnético con un biorreactor. Este campo magnético genera la deformación del componente magnetostrictivo, que es transmitida a la matriz polimérica, deformándola y generando un campo eléctrico. Ésta última es transmitida a las células cultivadas en estos soportes para estudiar su efecto sobre la diferenciación osteogénica. En el primer capítulo se desarrollaron y caracterizaron membranas electroactivas de PVDF fabricadas por el método de separación de fases inducida por no-solventes. Se empleó etanol como no-solvente, dando lugar a membranas homogéneas altamente porosas. Estas cristalizan en fase g. Se optimizó un recubrimiento basado en la técnica capa a capa (LbL), empleando recombinámeros similares a la elastina (ELRs) que contenían secuencias de adhesión celular RGD. Se estudió la respuesta celular inicial de las MSCs y se comparó con los mismos soportes recubiertos únicamente con fibronectina adsorbida. La presencia de los ELRs es necesaria para promover la adhesión inicial de las MSCs en este tipo de soportes. En el segundo capítulo se combinó el PVDF con CFO, usando agua como no-solvente. Las membranas eran no simétricas, con una superficie lisa, que fue empleada para cultivo celular, con una mayoría en fase b, la más electroactiva. Se recubrieron y caracterizaron las membranas mediante LbL con colágeno tipo I y heparina. Se estudió el comportamiento de las MSCs sobre el LbL, resultando esencial para la proliferación celular en el caso de las membranas PVDF-CFO. En el capítulo tres se desarrollaron films de PVDF y PVDF-CFO cristalizados en presencia del líquido iónico [Bmim][Cl]. La presencia de éste indujo la nucleación del PVDF en fase b. Las MSCs eran capaces de adherirse y proliferar. Se realizaron ensayos de estimulación piezoeléctrica empleando un biorreactor magnético. Las MSCs respondieron a la estimulación incrementado la longitud de sus adhesiones focales, así como reduciendo la vimentina en el citoplasma. Finalmente, se diseñaron soportes de cultivo piezoeléctricos en 3D. Para ello se desarrollaron microesferas de PVDF y PVDF-CFO mediante la técnica de electropulverizado. Las microesferas se encapsularon en hidrogeles de gelatina junto con las MSCs. Se estimularon y tras 7 días, se observó un incremento en la expresión del factor de transcripción RUNX2 en las muestras estimuladas demostrando que la estimulación piezoeléctrica es capaz de activar en mayor medida la diferenciación de las MSCs. / [CA] L'augment de l'esperança de vida comporta l'aparició de problemes muscoloesquelètics afectant la qualitat de vida dels pacients. Les noves teràpies regeneratives òssies es centren en l'ús de les cèl·lules mare mesenquimals, MSCs, encarregades de la regeneració del teixit in vivo. La inducció d'un fenotip osteogènic prediferenciat in vitro, previ a la implantació de les MSCs, resulta en una millor capacitat de regeneració del teixit ossi. Habitualment s'han fet servir mitjans de diferenciació osteogènica que contenen dexametasona. Aquests mètodes són poc eficients, per la qual cosa l'ús de mètodes físics està adquirint rellevància. L'os és un teixit amb propietats piezoelèctriques a causa de les fibres de col·lagen que formen part de la seva matriu extracel·lular. Aquest estímul ha estat relacionat amb la capacitat de respondre a l'estrès mecànic i autoregenerar-se, on juguen un paper important les MSCs. Aquestes es troben en un entorn electroactiu, i són precisament aquests senyals físics els que poden influir en el seu procés de diferenciació osteogènica podent ser emprats per a la seva prediferenciació in vitro de manera efectiva. Per comprovar aquesta hipòtesi, a la present tesi doctoral s'han dissenyat suports de cultiu piezoelèctrics en 2 i 3 dimensions basats en l'ús del polímer piezoelèctric polifluorur de vinilidè (PVDF) combinats amb partícules magnetostrictives de ferrita de cobalt (CFO). Aquesta combinació permet l'estimulació dels suports de cultiu aplicant un camp magnètic amb un bioreactor. Aquest camp magnètic genera la deformació del component magnetostrictiu, que és transmesa a la matriu polimèrica, deformant-la i generant un camp elèctric. Aquesta última és transmesa a les cèl·lules cultivades en aquests suports per estudiar-ne l'efecte sobre la diferenciació osteogènica. En el primer capítol es van desenvolupar i caracteritzar membranes electroactives de PVDF fabricades pel mètode de separació de fases induïda per no solvents. Es va emprar etanol com a no-solvent, donant lloc a membranes homogènies altament poroses. Aquestes cristal·litzen en fase g. S'optimitzà un recobriment basat en la tècnica capa a capa (LbL), emprant recombinàmers similars a l'elastina (ELRs) que contenien seqüències d'adhesió cel·lular RGD. Es va estudiar la resposta cel·lular inicial de les MSCs i es va comparar amb els mateixos suports recoberts únicament amb fibronectina adsorbida. La presència dels ELR és necessària per promoure l'adhesió inicial de les MSCs en aquest tipus de suports. En el segon capítol es va combinar el PVDF amb CFO, usant aigua com a no-solvent. Les membranes eren no simètriques, amb una superfície llisa, que va ser emprada per a cultiu cel·lular, amb una majoria en fase b, la més electroactiva. Es van recobrir i caracteritzar les membranes mitjançant LbL amb col·lagen tipus I i heparina. Es va estudiar el comportament de les MSCs sobre el LbL, resultant essencial per a la proliferació cel·lular en el cas de les membranes PVDF-CFO. Al capítol tres es van desenvolupar films de PVDF i PVDF-CFO cristal·litzats en presència del líquid iònic [Bmim][Cl]. La seva presència va induir la nucleació del PVDF en fase b. Les MSCs eren capaces d'adherir-se i proliferar. Es van realitzar assajos d'estimulació piezoelèctrica emprant un bioreactor magnètic. Les MSCs van respondre a l'estimulació incrementant la longitud de les seves adhesions focals, així com reduint la vimentina al citoplasma. Finalment, es van dissenyar suports de cultiu piezoelèctrics en 3D. Per això es van desenvolupar microesferes de PVDF i PVDF-CFO mitjançant la tècnica d'electropolveritzat. Les microesferes es van encapsular en hidrogels de gelatina juntament amb les MSCs. Es van estimular i després de 7 dies, es va observar un increment en l'expressió del factor de transcripció RUNX2 a les mostres estimulades demostrant que l'estimulació piezoelèctrica és capaç d'activar més la diferenciació de les MSCs. / [EN] Life expectancy increase entails the presence of musculoskeletal disorders producing a substantial impact on patient's quality of life. New bone regenerative therapies are focused on the use of mesenchymal stem cells (MSCs), the main effectors of bone regeneration in vivo. Over the years, it has been demonstrated that the induction of a pre-differentiated phenotype in vitro, before MSCs implantation, results in a better capacity for bone tissue regeneration. For this purpose, biochemical approaches based on the use of osteogenic differentiation medium containing dexamethasone have traditionally been used. These methods are not efficient, which has favoured the use of physical methods as an alternative. Bone is a piezoelectric tissue due to the collagen fibres that conform its extracellular matrix. This stimulus has been related to its ability to respond to mechanical stress and self-regenerate, a process in which MSCs play a key role. MSCs are subjected to an electroactive environment. It is hypothesised that these physical signals may influence their osteogenic differentiation process and be used to effectively pre-differentiate them in vitro. To test this hypothesis, along this Doctoral Thesis, piezoelectric cell culture supports have been designed in 2 and 3 dimensions based on the use of the piezoelectric polymer poly(vinylidene) fluoride (PVDF) combined with magnetostrictive cobalt ferrite oxide (CFO) nanoparticles. This combination allows the stimulation of culture supports by applying a magnetic field with a bioreactor. This magnetic field induces the deformation of the magnetostrictive component, which is transmitted to the polymeric matrix, generating a deformation and producing an electric field, which is transmitted to the MSCs to study its effect on their osteogenic differentiation. In the first chapter, electroactive PVDF membranes manufactured by the non-solvent induced phase separation technique were developed and characterised. Ethanol was used as a non-solvent, which gave rise to highly porous homogeneous membranes crystallised in the g phase. A coating protocol based on the layer-by-layer (LbL) technique, using elastin-like recombinamers (ELRs) containing RGD cell adhesion sequences, was optimised. MSCs' initial cellular response was studied and compared with the membranes coated with adsorbed fibronectin. The presence of the ELRs was necessary to promote MSCs' initial adhesion in this type of support. In the second chapter, PVDF was combined with CFO, using water as a non-solvent. The membranes were not symmetrical, with a smooth surface used for cell culture, with a majority in phase b, the most electroactive. Membranes were coated and characterised by LbL with type I collagen and heparin. The behaviour of MSCs on LbL was studied, essential for cell proliferation in the case of PVDF-CFO membranes. In chapter three, PVDF and PVDF-CFO films crystallised in the presence of the ionic liquid [Bmim][Cl] were developed. The presence of ionic liquid induced PVDF nucleation in the b phase. MSCs were able to adhere and proliferate. Piezoelectric stimulation tests were performed using a magnetic bioreactor. MSCs responded to stimulation by increasing the length of their focal adhesions and reducing vimentin in the cytoplasm. Finally, 3D piezoelectric culture supports were designed. For this, PVDF and PVDF-CFO microspheres were developed using the electrospray technique. The microspheres were encapsulated in gelatin hydrogels together with the MSCs. They were stimulated, and after 7 days, an increase in the expression of the transcription factor RUNX2 was observed in the stimulated samples, demonstrating that piezoelectric stimulation is capable of activating the differentiation of MSCs to a greater extent. / La presente tesis doctoral no podría haberse realizado sin la financiación del Ministerio de Economía y Competitividad a través de la beca para formación de personal investigador BES-2017-080398 y a la Agencia Estatal de Investigación a través de los proyectos PID2019-106000RB-C21 / AEI / 10.13039/501100011033, PID2019-106099RB- C41 y –C43 / AEI / 10.13039/501100011033. / Guillot Ferriols, MT. (2022). Electroactive Environments for Mesenchymal Stem Cells Osteogenic Differentiation [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/191003 / Compendio

Topological insulators are materials with their electronic band structure in bulk resembling that of an ordinary insulator, but the surface states are metallic. These surface states are topologically protected, meaning that they are robust against impurities. The topological phenomena of three dimensional topological insulators can be expressed within topological field theories, predicting axion electrodynamics and the topological magnetoelectric effect. An experiment have been suggested to measure the topological phenomena. In this thesis, the underlying theory and details around the experiment are explained and more detailed derivations and expressions are provided.