This Thesis focuses on different aspects of quantum computation theory: adiabatic quantum algorithms, decoherence during the adiabatic evolution and quantum simulators. After an overview on the area of quantum computation and setting up the formal ground for the rest of the Thesis we derive a general error estimate for adiabatic quantum computing. We demonstrate that the first-order correction, which has frequently been used as a condition for adiabatic quantum computation, does not yield a good estimate for the computational error. Therefore, a more general criterion is proposed, which includes higher-order corrections and shows that the computational error can be made exponentially small – which facilitates significantly shorter evolution times than the first-order estimate in certain situations. Based on this criterion and rather general arguments and assumptions, it can be demonstrated that a run-time of order of the inverse minimum energy gap is sufficient and necessary. Furthermore, exploiting the similarity between adiabatic quantum algorithms and quantum phase transitions, we study the impact of decoherence on the sweep through a second-order quantum phase transition for the prototypical example of the Ising chain in a transverse field and compare it to the adiabatic version of Grover’s search algorithm. It turns out that (in contrast to first-order transitions) the impact of decoherence caused by a weak coupling to a rather general environment increases with system size (i.e., number of spins/qubits), which might limit the scalability of the system. Finally, we propose the use of electron systems to construct laboratory systems based on present-day technology which reproduce and thereby simulate the quantum dynamics of the Ising model and the O(3) nonlinear sigma model.

No description available.

No description available.

The physics of iron pnictides has been the subject of intense research for half a decade since the discovery of superconductivity in doped LaFeAsO in 2008. By now there exists a large number of different materials that are summarized under the term "pnictides'' with significant differences in their crystal structure, electronic properties, and their phase diagrams. This thesis is concerned with the investigation of the various phase transitions that are observed in the underdoped compounds of the pnictide subgroups RFeAsO, where R is a rare-earth element, and AFe_2As_2, where A is an alkaline-earth element. These compounds display two closely bound transitions from a tetragonal to an orthorhombic phase and from a paramagnetic to an antiferromagnetic metal. Both symmetry-broken phases are suppressed by doping or pressure and close to their disappearance superconductivity sets in. The superconducting state is stabilized until some optimal doping or pressure is reached and gets suppressed thereafter. The central goal of this thesis is to improve our understanding of the interplay between these three phases and to describe the various phase transitions. We start from an itinerant picture that explains the magnetism as a result of an excitonic instability and show how the other phases can be included into this picture. This approach is based on the the observation that the compounds we are interested in have a Fermi surface with multiple nested electron and hole pockets and that they have small to intermediate interaction strengths. The thesis starts with a study of the doping dependence of the antiferromagnetic phase transition in four different five-orbital models. We use the random-phase approximation to determine the transition temperature, the dominant ordering vector, and the contribution of the different orbitals to the ordering. This allows us to identify the more realistic models, which give results that are in good agreement with experimental observations. In addition to the frequently made assumption of orbital-independent interaction potentials we study the effect of a reduction of the interaction strengths that involve the d_{xy} orbital. We find that this tunes the system between two different nesting instabilities. A reduction of the interactions that involve the d_{xy} orbital also enhances the tendency towards incommensurate (IC) order. For a weak reduction this tendency is compensated by the presence of the orthorhombic phase. However, for a reduction of 30%, as it is suggested by constrained random-phase-approximation calculations, we always find large doping ranges, where a state with IC order has the highest transition temperature. We continue the investigation of the magnetic phase transition by studying the competition of different possible types of antiferromagnetic order that arises from the presence of two degenerate nesting instabilities with the ordering vectors (pi,0) and (0,pi). We derive a Ginzburg-Landau free energy from a microscopic two-band model and find that the presence of the experimentally observed stripe phase strongly depends on the number and size of the hole pockets in the system and on the doping. We show that within the picture of a purely magnetically driven nematic phase transition, which breaks the C_4 symmetry and induces the orthorhombic distortion, the nematic phase displays exactly the same dependence on the model parameters as the magnetic stripe phase. We propose that in addition to the purely magnetically driven nematic instability there is a ferro-orbital instability in the system that stabilizes the nematic transition and, thus, explains the experimentally observed robustness of the orthorhombic transition. We argue that including a ferro-orbital instability into the picture may also be necessary to reproduce the transition from simultaneous first-order transitions into an orthorhombic antiferromagnetic state to two separate second-order transitions, which is observed as a function of doping. Finally, a study of the superconducting phase transition inside the antiferromagnetic phase that is observed in some pnictide compounds is presented. We present an approach to calculate the fluctuation-mediated pairing interaction in the spin-density-wave phase of a multiband system, which is based on the random-phase approximation. This approach is applied to a minimal two-band model for the pnictides to study the effect of the various symmetry-allowed bare on-site interactions on the gap symmetry and structure. We find a competition between various even- and odd-parity states and over a limited parameter range a p_x-wave state is the dominant instability. The largest part of the parameter space is dominated by even parity states but the gap structure sensitively depends on the bare interactions. We propose that the experimentally observed transition from a nodeless to a nodal gap can be due to changes in the on-site interaction potentials.

We propose a hypothesis and a numerical model for the formation of branching plagioclase textures visible at both macroscopic (∼cm to ∼m) and microscopic scale within melagabbro of the Isle of Rum, Scotland, based on macroscopic, microscopic observations and relevant geological history. The plagioclase crystals are typically linked as twins and form meshes of planar stellate structures (m-scale) with a large range in geometrical organization from patchy to radiating. Evidence of macroscopic crystal aggregation and alignment is attributed to interfacial free energy minimization at the microscopic scale during growth. Accordingly, a binary immiscible Lattice Boltzmann model was developed to simulate diffusion of simplified plagioclase in the melt phase. Isothermal phase transitions modelled via first order chemical reactions are subsequently coupled with stochastic dynamics at the crystal growth front to simulate energy minimization processes including twinning during crystallization in an igneous environment. The solid phase and the liquid phase are coupled with a temporal flexibility that sets the overall ratio between the rate of diffusion and chemical enrichment in the liquid state and the rate of crystallization. The parameter space of the model is explored extensively, followed by a reasonable transcription of physical parameters and an estimation of other parameters to construct realistic simulation scenarios yielding synthetic plagioclase stellates. The results are presented, analyzed and discussed. They appear to be in reasonable qualitative agreement with observations, and several aspects of the natural stellates such as the stellate spacing and long continuous stretches of plagioclase with epitaxial junctions seem to be in reasonable quantitative agreement with observations.

Η διασπορά κβαντικών τελειών ως πυρήνων αταξίας σε οργανωμένα συστήματα, έχει προσελκύσει επιστημονικό ενδιαφέρον και ερευνητικές δραστηριότητες, τόσο στο πεδίο της φυσικής στερεάς κατάστασης, όσο και σε αυτό της επιστήμης των υλικών. Δεν είναι σπάνια η διαπίστωση ότι υβριδικά συστήματα που προκύπτουν από συνδυασμούς όπως αυτός των υγρών κρυστάλλων και των νανοσωματιδίων, παρουσιάζουν αξιοσημείωτες και συχνά αναπάντεχες νέες ιδιότητες. Το θέμα της παρούσας διδακτορικής διατριβή κινείται σε δύο άξονες: το σχεδιασμό και την χημική σύνθεση κβαντικών τελειών επιφανειακά κατεργασμένων, για ελεγχόμενη αλληλεπίδραση με υγρούς κρυστάλλους, και τη μελέτη των φυσικών ιδιοτήτων των σύνθετων νανοδομημένων υγροκρυσταλλικών συστημάτων, που σχηματίζονται με διασπορά των νανοσωματιδίων αυτών σε χειρόμορφους θερμοτροπικούς υγρούς κρυστάλλους. Στο πρώτο μέρος της διατριβής παρουσιάζεται η χημική σύνθεση και επιφανειακή τροποποίηση κβαντικών τελειών CdSe, με υδρόφοβες επιφανειοδραστικές ομάδες τρι– οκτυλοφωσφίνης (TOP)/ελαϊκής αμίνης (ΟΑ), TOP/οκταδεκυλαμίνης, ΤOP/δωδεκυλα-μίνης, TOP/οκτυλαμίνης και τριφαινυλφωσφίνης/τριφαινυλαμίνης. Ο χαρακτηρισμός της δομής και των οπτικών ιδιοτήτων των νανοσωματιδίων έγινε με περίθλαση σκόνης ακτίνων Χ, φασματοσκοπία υπεριώδους–ορατού και φασματοσκοπία υπερύθρου, ενώ για τη διερεύνηση της μορφολογίας τους και τον προσδιορισμό των διαστάσεών τους, χρησιμοποιήθηκε ηλεκτρονική μικροσκοπία διέλευσης. Οι χημικές συνθέσεις, βασισμένες στην θερμολυτική διάσπαση οργανομεταλλικών ενώσεων, οδήγησαν σε επιτυχημένη παραγωγή σφαιρικών νανοσωματιδίων, μέσης διαμέτρου 3–4 nm, με στενή κατανομή μεγεθών και καλή διαλυτότητα σε οργανικούς διαλύτες. Στη συνέχεια μελετήθηκαν οι επιπτώσεις από τη διασπορά των νανοσωματιδίων CdSe με TOP και OA, με μέση διάμετρο 3.2 nm στη θερμοδυναμική και μοριακή οργάνωση χειρόμορφων υγρών κρυστάλλων, με τις τεχνικές της θερμιδομετρίας εναλλασσόμενης θερμικής εισόδου και με περίθλαση σκόνης ακτίνων Χ. Το ενδιαφέρον επικεντρώθηκε στη θερμοκρασιακή περιοχή των Μπλε φάσεων και στη μετάπτωση φάσης SmA–SmC*. Οι Μπλε φάσεις παρόλο που παρουσιάζουν εξέχουσες ιδιότητες για καινοτόμες εφαρμογές στη βιομηχανία οθονών και αισθητήρων, ωστόσο, είναι σταθερές σε πολύ στενά θερμοκρασιακά εύρη, μεταξύ Ισοτροπικής και Χειρόμορφης Νηματικής φάσης, γεγονός που δεν ευνοεί τη χρήση τους. Οι πειραματικές μετρήσεις αποκάλυψαν νέα φαινόμενα, όπως τη σταθεροποίηση της θερμοκρασιακής περιοχής της Μπλε φάσης ΙΙΙ (BPIII) σε μεγάλο θερμοκρασιακό εύρος, σε σχέση με τον αμιγή υγρό κρύσταλλο, και τη μετατόπιση των θερμοκρασιών μετάπτωσης σε μικρότερες τιμές. Η θεωρητική μελέτη που πραγματοποιήθηκε καταδεικνύει ισχυρή αλληλεπίδραση των νανοσωματιδίων με τις σειρές πλεγματικών ατελειών του υγρού κρυστάλλου. Παράλληλα τα αποτελέσματα φανερώνουν ότι ο χαρακτήρας Μέσου Πεδίου–Landau της μετάπτωσης SmA–SmC*, για το ίδιο σύστημα, δεν αλλοιώνεται. Η αλληλεπίδραση των κβαντικών τελειών CdSe με επιφανειακή δραστικοποίηση ΟΑ και TOP με χειρόμορφους υγρούς κρυστάλλους, παρέχει δυνατότητες και δημιουργεί σημαντικές προϋποθέσεις για νέες τεχνολογικές εφαρμογές και επιστημονικές εξελίξεις. / One dimensional semiconductor structures are intriguing materials for both fundamental research and industrial applications. On the other hand the long-range nature of the orientational order of liquid crystals is responsible for many fascinating optical, electromechanical and critical properties of these materials. Hybridization of these two fields may lead to novel materials with unusual optical and physical properties that are of considerable importance for technological applications as well as for basic physics studies on phase transitions and critical phenomena. In this context, complex soft materials were formulated that result from the dispersion of surface functionalized quantum dots in thermotropic chiral LCs. Special attention was paid to the synthesis and properties of the nanocrystals and to the dispersion state, as well as to the thermal and structural study of the composite materials. In the first part of this Thesis a chemical approach for the synthesis of semiconducting quantum dots is presented. The method, based on the thermolytic decomposition of organometallic compounds, leads to the production of spherical nanocrystalline particles highly soluble in organic media, with an average diameter of 3.2 nm, capped with a variety of amine and phosphine molecules. The as-synthesized nanoparticles were characterized by means of powder X-ray diffraction, ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy and transition electron microscopy. The second part of the Thesis is concentrated on the effects upon the Blue phases and smectic-A to chiral smectic-C* phase transition of the liquid crystal CE8, arising from the dispersion of CdSe quantum dots, surface-treated with oleylamine and trioctylphosphine. For this purpose, ac calorimetry and X-ray scattering studies have been carried out. Liquid– crystalline blue phases exhibit exceptional properties for applications in the display and sensor industry. However, in single component systems, they are stable only for a very narrow temperature range between the isotropic and the chiral nematic phase, a feature that severely hinders their applicability. The systematic high-resolution calorimetric studies revealed that Blue phase III is effectively stabilized in a wide temperature range by mixing surface-functionalized nanoparticles with chiral liquid crystals. The calorimetric measurements also revealed substantial downshifts of the transition temperature. Theoretical arguments show that the aggregation of nanoparticles at disclination lines is responsible for the observed effects. Furthermore, it was found that at the SmA–SmC* phase transition, as a function of increasing nanoparticle concentration, the heat capacity anomalies display an extended-mean-field to mean-field–like crossover behavior, while the temperature dependence of the tilt angle remains bulk-like with no occurrence of pretransitional effects. The interaction of CdSe quantum dots with the cores of disclination lines gains further support, as bound disclination lines are expected to affect smectic–smectic phase transitions in a very limited manner.

The thesis entitled “Structure-function control in organic co-crystals/salts via studies on polymorphism, phase transitions and stoichiometric variants” consists of five chapters. The main emphasis of the thesis is on two aspects, one to characterize co-crystal polymorphism in terms of propensity of intermolecular interactions to form co-crystals/salts or eutectics. The other aspect is to explore the feasibility of using such co-crystals/salts to exhibit properties like proton conduction, dielectric and ferroelectric behaviour. Gallic acid and its analogues possess functionalities to provide extensive hydrogen bonding capabilities and are chosen as the main component while the coformers are carefully selected such that they either accept or reject the hydrogen bonding offered. Such co-crystallization experiments therefore provide an opportunity to unravel the intricate details of the formation of crystalline polymorphs and/or eutectics at the molecular level. Further these co-crystal systems have been exploited to evaluate proton conductivity, dielectric and ferroelectric features since the focus is also on the design aspect of functional materials. In the context of identifying and utilizing Crystal Engineering tools, the discussions in the following chapters address not only the structural details but identify the required patterns and motifs to enable the design of multi-component co-crystals/salts and eutectics. In particular, the presence/absence of lattice water in gallic acid has been evaluated in terms of importing the required physical property to the system. Chapter 1 discusses the structural features of tetramorphic anhydrous co-crystals (1:1; which are synthon polymorphs) generated from a methanolic solution of gallic acid monohydrate and acetamide, all of which convert to a stable form on complete drying. The pathway to the stable form (1:3 co-crystal) is explained based on the variability in the hydrogen bonding patterns followed by lattice energy calculations. Chapter 2A studies the presence/absence and geometric disposition of hydroxyl functionality on hydroxybenzoic acids to drive the formation of co-crystal/eutectic in imide-carboxylic acid combinations. In Chapter 2B the crystal form diversity of gallic acid-succinimide co-crystals are evaluated with major implications towards the design and control of targeted multi-component crystal forms. The co-crystal obtained in this study shows a rare phenomenon of concomitant solvation besides concomitant polymorphism and thus making it difficult to obtain a phase-pure crystal form in bulk quantity. This issue has been resolved and formation of desired target solid form is demonstrated. Thus, this study addresses the nemesis issues of co-crystallization with implications in comprehending the kinetics and thermodynamics of the phenomenon in the goal of making desired materials. Chapter 3 focuses on the systematic co-crystallization of hydroxybenzoic acids with hexamine using liquid assisted grinding (LAG) which show facile solid state interconversion among different stoichiometric variants. The reversible interconversion brought about by varying both the acid and base components in tandem is shown to be a consequence of hydrogen bonded synthon modularity present in the crystal structures analyzed in this context. In Chapter 4A, the rationale for the proton conduction in hydrated/anhydrous salt/co-crystal of gallic acid - isoniazid is provided in terms of the structural characteristics and the conduction pathway is identified to follow Grotthuss like mechanism which is supplemented by theoretical calculations. Chapter 4B describes an extensive examination of the hydrated salt of gallic acid-isoniazid which unravels the irreversible nature of the dielectric property upon dehydration and suggests that the “ferroelectric like” behaviour is indeed not authenticated. This chapter brings out the significance role of lattice water in controlling the resulting physical property (dielectric/ferroelectric in this case). Chapter 5 describes the structural features of two hydrated quaternary salts of hydroxybenzoic acids-isoniazid-sulfuric acid and the phase transitions at both low and high temperatures are shown to be reversible. Single Crystal to Single Crystal (SCSC) in situ measurement corroborated by thermal and in situ Powder X-ray Diffraction studies proves the claim. Further, the properties exhibited by these materials are also governed by lattice water content.

VILELA, Rivelino Cunha. Estudo de transições de fase em cristais de l-alanina + ácido oxálico. 2013. 113 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2013. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-10-29T21:51:24Z No. of bitstreams: 1 2013_tese_rcvilela.pdf: 12577577 bytes, checksum: 4dc32419c1bbdde79375612c04227181 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-11-03T19:48:22Z (GMT) No. of bitstreams: 1 2013_tese_rcvilela.pdf: 12577577 bytes, checksum: 4dc32419c1bbdde79375612c04227181 (MD5) / Made available in DSpace on 2015-11-03T19:48:22Z (GMT). No. of bitstreams: 1 2013_tese_rcvilela.pdf: 12577577 bytes, checksum: 4dc32419c1bbdde79375612c04227181 (MD5) Previous issue date: 2013 / In the present word we have studied the effect of temperature on the Raman spectra of crystals of L-alanine + oxalic acid, C3H8NO2+.C2HO4-. Raman spectroscopy measurements were performed on polycrystalline samples at different temperatures varying in the range from room temperature to T = 20 K; a tentative assignment of all normal modes was furnished. In order to help the understanding of the crystal behavior we have also obtained X-ray diffractograms and studied the dependence of lattice parameters through dilatometry as a function of temperature in the 290 K – 93 K range. The three different techniques allowed us to construct a picture of the material under low temperature conditions. As a consequence we have realized that L-alanine + oxalic acid crystal undergoes three phase transitions at low temperatures. The splitting of a band at 90 cm-1 and an anomaly in one of the lattice parameters are the signature for the first phase transition that is observed at 250 K. At 150 K it was observed the appearance of two new bands in the Raman spectrum and, simultaneously, it was observed change in the curves of a and c lattice parameters. Additionally, it was verified the appearance of new peaks in the X-ray diffractogram at the same temperature, characterizing the second phase transition. At a temperature even lower, at about 43 K, it was verified the occurrence of the third phase transition that has as main characteristic the splitting of two bands that are associated to the lattice modes. Changes in the modes associated with CH3 and NH3+ during the cooling is discussed. An important behavior of the crystal with the cooling process was the red shift of the band of lower frequency, similar to the soft-mode vibration of ferroelectric materials, although the frequency of the mode in L-alanine + oxalic acid does not goes to zero. Based on the results on Raman spectroscopy, dilatometry and X-ray diffraction, and on the possible symmetry sites occupied by the molecules through the O=CC group in the various phases, it is suggested the following sequence of phase transitions D24  C2h5  Cs3  C23, which should occur at 250 K, 150 K and 43 K. / Neste trabalho, estudou-se o efeito da temperatura nos espectros Raman de cristais de L-alanina + ácido oxálico, C3H8NO2+.C2HO4-. Foram realizadas medidas de espectroscopia Raman em policristais a diferentes temperaturas no intervalo compreendido entre a temperatura ambiente e a temperatura de 20 K, sendo fornecida uma identificação tentativa para todos os modos normais de vibração observados. Para auxiliar o entendimento do comportamento do cristal também foram obtidos os difratogramas de raios-X bem como estudada a dependência dos parâmetros de rede em função da temperatura através de dilatometria no intervalo entre 290 K e 93 K. As três técnicas utilizadas em conjunto permitiram mostrar o comportamento estrutural do material em baixas temperaturas. Deste quadro foi possível inferir que os cristais de L-alanina + ácido oxálico apresentam três diferentes transições de fase durante o resfriamento. Em 250 K o aparecimento de um dubleto em 90 cm-1 e a anomalia num dos parâmetros de rede apontam para a ocorrência da primeira transição de fase. Em 150 K surgem pelo menos duas novas bandas no espectro Raman, ao mesmo tempo em que ocorrem bruscas mudanças de inclinação nas curvas que representam as dimensões dos eixos a e c do cristal. Também se verifica que, de forma semelhante ao que ocorre com os espectros Raman, aparecem novos picos no difratograma de raios-X em torno desta temperatura, caracterizando assim a segunda transição de fase. A uma temperatura ainda mais baixa, em torno de 43 K, foi verificada a ocorrência da terceira transição de fase, que tem como principal característica a separação de dois modos Raman associados a modos da rede. Mudanças nos ambientes dos grupos CH3 e do NH3+ durante o resfriamento são discutidas. Um importante aspecto apresentado pelos espectros Raman com o resfriamento da amostra foi o deslocamento da banda de mais baixa energia para menores valores de frequências, semelhantemente ao que ocorre com vibrações do tipo soft-mode em materiais ferroelétricos, embora a frequência do modo no cristal de L-alanina + ácido oxálico não tenha ido à zero. Baseado nos resultados acima e nos possíveis sítios de simetria ocupados pelas moléculas através do grupo O=CC nas diversas fases, sugere-se a seguinte sequência de transições de fase D24 para C2h5 para Cs3 para C23, que aconteceriam, respectivamente, nas temperaturas de 250 K, 150 K e 43 K.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho estudamos a dependência com o tamanho do sistema dos observáveis relacionados com a transição de fase de 1 ordem do rupo de calibre Z(2) em 4 dimensões. Foram realizadas simulações de Monte Carlo numa rede cúbica para diferentes valores da aresta, utilizando o método do Banho Térmico para sortear os elementos do grupo na rede. / In this work we study the dependence of the observable related to the size of the system to the phase transition of first order of the Z(2) gauge group in 4 dimensions. Monte Carlo simulations were performed on a cubic lattice for different values of the edge, using the method of the Heat Bath to randomize the group members in the lattice.

Au cours de cette thèse, nous avons étudié le comportement critique de chaînes de spins quantiques en présence de couplages désordonnés ou répartis de manière apériodique. Il est bien établi que le comportement critique des chaînes de spins quantiques d’Ising et de Potts est gouverné par le même point fixe de désordre infini. Nous avons implémenté́ une version numérique de la technique de renormalisation de désordre infini (SDRG) afin de tester cette prédiction. Dans un second temps, nous avons étudié la chaîne quantique d’Ashkin-Teller désordonnée par renormalisation de la matrice densité́ (DMRG). Nous confirmons le diagramme de phase précédemment proposé en déterminant la position des pics du temps d’autocorrélation intégré des corrélations spin-spin et polarisation-polarisation ainsi que ceux des fluctuations de l’aimantation et de la polarisation. Enfin, l’existence d’une double phase de Griffiths est confirmée par une étude détaillée de la décroissance des fonctions d’autocorrélation en dehors des lignes critiques. Comme attendu, l’exposant dynamique diverge à l’approche de ces lignes. Dans le cas apériodique, nous avons étudié les chaînes quantiques d’Ising et de Potts. En utilisant la méthode SDRG, nous avons confirmé les résultats connus pour la chaîne d’Ising et proposé des estimations de la dimension d’échelle magnétique. Dans le cas du modèle de Potts à q états, nous avons estimé l’exposant magnétique et observé qu’il était indépendant du nombre d’états q pour toutes les séquences dont l’exposant de divagation est nul. Toutefois, nous montrons que l’exposant dynamique est fini et augmente avec le nombre d’états q. En revanche, pour la séquence de Rudin-Shapiro, les résultats sont compatibles avec un point fixe de désordre infini et donc un exposant dynamique infini. / In the present thesis, the critical and off-critical behaviors of quantum spin chains in presence of a random or an aperiodic perturbation of the couplings is studied. The critical behavior of the Ising and Potts random quantum chains is known to be governed by the same Infinite-Disorder Fixed Point. We have implemented a numerical version of the Strong-Disorder Renormalization Group (SDRG) to test this prediction. We then studied the quantum random Ashkin-Teller chain by Density Matrix Renormalization Group. The phase diagram, previously obtained by SDRG, is confirmed by estimating the location of the peaks of the integrated autocorrelation times of both the spin-spin and polarization-polarization autocorrelation functions and of the disorder fluctuations of magnetization and polarization. Finally, the existence of a double-Griffiths phase is shown by a detailed study of the decay of the off-critical autocorrelation functions. As expected, a divergence of the dynamical exponent is observed along the two transition lines. In the aperiodic case, we studied both the Ising and Potts quantum chains. Using numerical SDRG, we confirmed the known analytical results for the Ising chains and proposed a new estimate of the magnetic scaling dimension.For the quantum q-state Potts chain, we estimated the magnetic scaling dimension for various aperiodic sequences and showed that it is independent of q for all sequences with a vanishing wandering exponent. However, we observed that the dynamical exponent is finite and increases with the number of states q. In contrast, for the Rudin-Shapiro sequence, the results are compatible with an Infinite-Disorder Fixed Point with a diverging dynamical exponent, equipe de renormalization