This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.

Le travail présenté est dédié à des problèmes d'EDP non linéaires. L'idée principale est de construire des solutions régulières á certaines EDPs elliptiques et hypo-elliptiques et étudier leur propriétés qualitatives. Dans une première partie, on considère un problème sur-critique du type $$-Delta u = lambda e^u$$ avec $lambda > 0$ posé dans un domaine extérieur avec conditions de Dirichlet homogènes. Une réduction en dimension finie permet de prouver l'existence d'un nombre infini de solutions régulières quand $lambda$ est assez petit. Dans une deuxième partie, on étudie la concentration de solutions d'un problème non local $$(-Delta)^s u = u^{p pm epsilon}, u>0, epsilon > 0$$ dans un domaine borné, régulier sous conditions de Dirichlet homogènes. Ici, on prend $0 < s < 1$ et $p:=(N+2s)/(N-2s)$, l'exposant de Sobolev critique. Une réduction en dimension finie dans des espaces fonctionnels bien choisis est utilisée. La partie principale de la fonction réduite est donnée en termes des fonctions de Green et Robin sur le domaine. On prouve que l'existence de solutions dépend des points critiques de la fonction susmentionnée augmentée d'une condition de non-dégénérescence. Enfin, on considère un problème non local dans le groupe de Heisenberg $H$. On s'intéresse à des propriétés de rigidité des solutions stables de $(-Delta_H)^s v = f(v)$ sur $H$, $s in (0,1)$. Une inégalité de type Poincaré connectée à un problème dégénéré dans $R^4_+$ est prouvée. Au travers d'une procédure d'extension, cette inégalité est utilisée pour donner un critère sous lequel les lignes de niveaux de la solution de l'EDP sont des surfaces minimales dans $H$. / This work is devoted to nonlinear PDEs. The aim is to find regular solutions to some elliptic and hypo-elliptic PDEs and study their qualitative properties. The first part deals with the supercritical problem $$ -Delta u = lambda e^u,$$ $lambda > 0$, in an exterior domain under zero Dirichlet condition. A finite-dimensional reduction scheme provides the existence of infinitely many regular solutions whenever $lambda$ is sufficiently small.The second part is focused on the existence of bubbling solutions for the non-local equation $$ (-Delta)^s u =u^p, ,u>0,$$in a bounded, smooth domain under zero Dirichlet condition; where $0<s<1$ and $p:=(N+2s)/(N-2s) pm epsilon$ is close to the critical exponent ($epsilon > 0$ small). To this end, a finite-dimensional reduction scheme in suitable functional spaces is used, where the main part of the reduced function is given in terms of the Green's and Robin's functions of the domain. The existence of solutions depends on the existence of critical points of such a main term together with a non-degeneracy condition.In the third part, a non-local entire problem in the Heisenberg group $H$ is studied. The main interests are rigidity properties for stable solutions of $$(-Delta_H)^s v = f(v) in H,$$ $s in (0,1)$. A Poincaré-type inequality in connection with a degenerate elliptic equation in $R^4_+$ is provided. Through an extension (or ``lifting") procedure, this inequality will be then used to give a criterion under which the level sets of the above solutions are minimal surfaces in $H$, i.e. they have vanishing mean $H$-curvature.

Orientador: Eduardo Miranda / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-01T08:41:01Z (GMT). No. of bitstreams: 1 Almeida_DalsonEloy_D.pdf: 2470369 bytes, checksum: 93d3b945f62f374cfd686217575dda95 (MD5) Previous issue date: 2017 / Resumo: Nesta tese estudamos a relação entre antiferromagnetismo e supercondutividade em pnictídeos à base de ferro. Este estudo será feito através da análise de uma energia livre de Ginzburg-Landau de parâmetros de ordem acoplados que será derivada de um modelo microscópico. Em particular, estamos interessados em saber se a transição entre os estados ordenados é de primeira ordem ou se as duas ordens podem coexistir. Para o caso de supercondutividade convencional as duas fases puras nunca coexistem. Entretanto, quando a supercondutividade é não-convencional e a condição de nesting perfeito não é satisfeita, pode haver um regime intermediário de coexistência microscópica das duas ordens. Nesta nova fase termodinâmica, as simetrias de rotação no espaço de spins, de reversão temporal e U(1) são quebradas simultânea e localmente. Logo, os canais de supercondutividade singleto e tripleto se misturam quanticamente. Em outras palavras, uma componente tripleto secundária do estado supercondutor é gerada. Os diagramas de fases do sistema são apresentados e analisamos também como flutuações magnéticas, acima da temperatura de Néel pura, afetam a temperatura de transição tripleto. Investigamos também o efeito da magnetização alternada no efeito Josephson, i.e., na supercorrente que flui através de uma junção entre dois supercondutores na fase de coexistência. Por fim, mas não menos importante, estudamos o efeito de proximidade em uma interface entre um supercondutor e um antiferromagneto. Veremos que os pares de Cooper podem penetrar a região magnética e em consequência, uma componente tripleto é induzida próximo da interface / Abstract: In this thesis, we study the interplay between antiferromagnetism and superconductivity in iron pnictides. This study will be done analyzing a free energy of coupled order parameters which will be derived from a microscopic model. In particular, we are interested if the phase transition between the ordered states is first order or if the two orders can coexist. For the case of conventional superconductivity, the two phases cannot coexist. However, when superconductivity is unconventional and the perfect nesting condition is not satisfied, there can exist an intermediary state of microscopic coexistence of the two orders. In this new thermodynamic phase, spin rotation, time reversal and U(1) symmetries are simultaneously and locally broken. Therefore, the singlet and triplet superconductivity channels are quantum mechanically mixed. In other words, a secondary triplet component is generated. The phase diagrams of the system are presented and we also analyze the effect of magnetic fluctuations above the pure Néel temperature on the triplet temperature transition. We also investigate the effects of the staggered magnetization on the Josephson effect, i.e., on the supercurrent that flows through a junction of two superconductors in the coexistence phase. Last, but not least, we study the proximity effect at an interface between a superconductor and an antiferromagnet. We will see that the Cooper pairs can penetrate the magnetic region and consequently a triplet component is induced near the interface / Doutorado / Física / Doutor em Ciências / 140834/2013-3 / 2342/15-4 / CNPQ / CAPES / BEX

Cette thèse montre le développement d’un code de calcul pour les simulations numériques directes d’écoulements diphasiques compressibles avec évaporation. Un couplage entre les méthodes Level Set et VOF est réalisé pour le suivi d’interface. Afin de résoudre les équations de la mécanique des fluides, une méthode basée sur la pression est employée et, pour découpler la vitesse de la pression, une méthode de projection est effectuée. Cette méthode permet l’implicitation des termes liés à l’acoustique et donc de diminuer la contrainte sur le pas de temps. Le liquide et le gaz sont traités de manière compressible permettant des variations locales des masses volumiques grâce à l’utilisation d’équations d’état. L’évaporation est simulée de deux manières différentes ; une première, où un taux d’évaporation constant est employé et une seconde, où ce taux est calculé par la thermique. Parallèlement à ce sujet, une étude de la distribution des courbures dans une injection de liquide est réalisée. Cette étude permet d’étendre le concept de distribution des tailles de gouttes dans un spray et d’améliorer les informations disponibles dans le modèle ELSA. Enfin, une autre étude est effectuée sur la recherche d’un critère, basé sur les courbures à l’interface, pour estimer la qualité d’une simulation. / This PhD thesis shows the development of a numerical method for solving two-phase flows with vaporisation. A coupling between Level Set and VOF methods is realised for the interface capturing. In order to solve fluid mechanics equations, a pressure based method is employed and, to decouple velocity and pressure, a projection method is performed. This method allows the implicitation of the acoustic terms and the time step constraint reduction. Liquid and gas are considered as compressible allowing local density variations with equations of state. The vaporisation is computed in two different ways ; a first one where the vaporisation rate is constant and a second one, where this rate is calculated by thermodynamics. Along with this topic, a study on curvature distribution in a liquid injection configuration is realised. This study allows to extend the drop size distribution concept in a spray and to improve available informations on ELSA model. Finally, an other study is performed on thedevelopment of a criterion, based on interface curvatures, which estimates the quality of a simulation.

In the study of strongly interacting condensed-matter systems controlled microscopic theories hold a key position. Spin-wave theory, large-N expansion, and $epsilon$-expansion are some of the few successful cornerstones. In this doctoral thesis work, we have developed a novel large-$d$ expansion method, $d$ being the spatial dimension, to study model Hamiltonians hosting a quantum phase transition between a paramagnet and a magnetically ordered phase. A highlight of this technique is that it can consistently describe the entire phase diagram of the above mentioned models, including the quantum critical point. Note that most analytical techniques either efficiently describe only one of the phases or suffer from divergences near the critical point. The idea of large-$d$ formalism is that in this limit, non-local fluctuations become unimportant and that a suitable product state delivers exact expectation values for local observables, with corrections being suppressed in powers of $1/d$. It turns out that, due to momentum summation properties of the interaction structure factor, all diagrams are suppressed in powers of $1/d$ leading to an analytic expansion. We have demonstrated this method in two important systems namely, the coupled-dimer magnets and the transverse-field Ising model. Coupled-dimer magnets are Heisenberg spin systems with two spins, coupled by intra-dimer antiferromagnetic interaction, per crystallographic unit cell (dimer). In turn, spins from neighboring dimers interact via some inter-dimer interaction. A quantum paramagnet is realized for a dominant intra-dimer interaction, while a magnetically ordered phase exists for a dominant (or of the same order as intra-dimer interaction) inter-dimer interaction. These two phases are connected by a quantum phase transition, which is in the Heisenberg O(3) universality class. Microscopic analytical theories to study such systems have been restricted to either only one of the phases or involve uncontrolled approximations. Using a non-linear bond-operator theory for spins with S=$1/2$, we have calculated the $1/d$ expansion of static and dynamic observables for coupled dimers on a hypercubic lattice at zero temperature. Analyticity of the $1/d$ expansion, even at the critical point, is ensured by correctly identifying suitable observables using the mean-field critical exponents. This method yields gapless excitation modes in the continuous symmetry broken phase, as required by Goldstone\'s theorem. In appropriate limits, our results match with perturbation expansion in small ratio of inter-dimer and intra-dimer coupling, performed using continuous unitary transformations, as well as the spin-wave theory for spin-$1/2$ in arbitrary dimensions. We also discuss the Brueckner approach, which relies on small quasiparticle density, and derive the same $1/d$ expansion for the dispersion relation in the disordered phase. Another success of our work is in describing the amplitude (Higgs) mode in coupled-dimer magnets. Our novel method establishes the popular bond-operator theory as a controlled approach. In $d=2$, the results from our calculations are in qualitative agreement with the quantum Monte Carlo study of the square-lattice bilayer Heisenberg AF spin-$1/2$ model. In particular, our results are useful to identify the amplitude (Higgs) mode in the QMC data. The ideas of large-$d$ are also successfully applied to the transverse-field Ising model on a hypercubic lattice. Similar to bond operators, we have introduced auxiliary Bosonsic operators to set up our method in this case. We have also discussed briefly the bilayer Kitaev model, constructed by antiferromagnetically coupling two layers of the Kitaev model on a honeycomb lattice. In this case, we investigate the dimer quantum paramagnetic phase, realized in the strong inter-layer coupling limit. Using bond-operator theory, we calculate the mode dispersion in this phase, within the harmonic approximation. We also conjecture a zero-temperature phase diagram for this model.

To understand the mechanism of a pressure-induced structural phase transition, it is important to know which bonding changes lead to the stabilization of the new structure. A useful approach in this regard is the quantum chemical topology, which provides a large variety of indicators for the characterization of interatomic interactions. In this work, a number of topological indicators are used to analyze the bonding changes during the pressure-induced phase transition from the cubic diamond (cd) to the β-Sn-type structure of the elements of the 14th group of the periodic table. The ability of these indicators to reflect the presence of the cd → β-Sn transition in experiment for Si, Ge and Sn and its absence for carbon is investigated. Furthermore, the effect of pressure on the interatomic interactions in the cd- and β-Sn-type structures is examined. It is observed that the energy change along the cd → β-Sn transformation pathway correlates with the evolution of certain parameters of the electron density and the electron localizability indicator (ELI-D). Accordingly, criteria of structural stability were formulated based on characteristics of interatomic interactions. These results can serve as guidelines for the investigation of other solid-state phase transformations by the topological methods.

Complex networks are employed as a mathematical description of complex systems in many different fields, ranging from biology to sociology, economy and ecology. Dynamical processes in these systems often display phase transitions, where the dynamics of the system changes qualitatively. In combination with these phase transitions certain components of the system might irretrievably go extinct. In this case, we talk about absorbing transitions. Developing mathematical tools, which allow for an analysis and prediction of the observed phase transitions is crucial for the investigation of complex networks. In this thesis, we investigate absorbing transitions in dynamical networks, where a certain amount of diversity is lost. In some real-world examples, e.g. in the evolution of human societies or of ecological systems, it is desirable to maintain a high degree of diversity, whereas in others, e.g. in epidemic spreading, the diversity of diseases is worthwhile to confine. An understanding of the underlying mechanisms for emergence and persistence of diversity in complex systems is therefore essential. Within the scope of two different network models, we develop an analytical approach, which can be used to estimate the prerequisites for diversity. In the first part, we study a model for opinion formation in human societies. In this model, regimes of low diversity and regimes of high diversity are separated by a fragmentation transition, where the network breaks into disconnected components, corresponding to different opinions. We propose an approach for the estimation of the fragmentation point. The approach is based on a linear stability analysis of the fragmented state close to the phase transition and yields much more accurate results compared to conventional methods. In the second part, we study a model for the formation of complex food webs. We calculate and analyze coexistence conditions for several types of species in ecological communities. To this aim, we employ an approach which involves an iterative stability analysis of the equilibrium with respect to the arrival of a new species. The proposed formalism allows for a direct calculation of coexistence ranges and thus facilitates a systematic analysis of persistence conditions for food webs. In summary, we present a general mathematical framework for the calculation of absorbing phase transitions in complex networks, which is based on concepts from percolation theory. While the specific implementation of the formalism differs from model to model, the basic principle remains applicable to a wide range of different models.

<p>The development of ultracold atom physics enables people to study fundamental questions in quantum mechanics within this highly-tunable platform. This dissertation focuses on several topics of the dynamical evolution of quantum systems.</p><p>Chapter 2 and 3 talk about Loschmidt echo, a simple quantity that reveals many hidden properties of a system’s time evolution. Chapter 2 looks for vanishing Loschmidt echo in the complex plane of time and the corresponding dynamical quantum phase transitions (DQPT) in the thermodynamic limit. For a two-site Bose-Hubbard model consisting of weakly interacting particles, DQPTs reside at the time scale inversely proportional to the interaction, where highly entangled pair condensates also show up. Chapter 3 discusses the revival of Loschmidt echo in a discrete time crystal, a Floquet system whose discrete temporal transition symmetry is spontaneously broken. We propose a new design and demonstrate its robustness against the fluctuations in the driving field. It can also be used in precision measurement to go beyond the Heisenberg limit. Experimental schemes are presented.</p><p>Out-of-time-order correlator (OTOC) is a more complicated variant of Loschmidt echo. Experimentally it requires reversing the time evolution. In Chapter 4, by exploiting the SU(1,1) symmetry of a weakly interacting BEC and connecting its quantum dynamics to a hyperbolic space, we obtain a geometric framework that enables experimentalists to manipulate the evolution with great freedom. Backward evolution is then realized effectively to measure OTOC of such SU(1,1) systems.</p><p>Chapter 5 discusses the decoherence of a spin impurity immersed in a spinor BEC. Our calculations show that by looking at the dynamics of the impurity’s reduced density matrix, the phase of the spinor BEC can be detected.</p>

In our experiments, the icing temperature of small water volumes placed on different pyroelectric single crystals (SrxBa1−xNb2O6, LiNbO3, and LiTaO3) was determined using two measurement setups: (1) the sessile droplet and (2) ring system method. In the first method, a free-standing water droplet was exposed to several external factors in the air environment. This was found to lead to higher icing temperatures compared to the second method where the water was more isolated from external factors such as evaporation. In the second method, the material of the ring system was found to be an important factor determining the freezing temperature of the enclosed water. A recommendation for the application of both methods is given, their advantages and disadvantages depending on the purpose of measurement, and their reproducibility for practical applications. In addition to this, the correlation between pyroelectricity and icing temperature, with regard to several types of internal and external factors affecting water freezing, is also discussed in the paper.

Verändern Photoreaktionen eingelagerter Substanzen die Klärtemperatur flüssigkristalliner Polyelektrolyt-Tensid-Komplexe? Komplexe (Salze) aus Polyaminen und Alkylsulfonsäuren bilden mit Wasser einen hexagonalen Flüssigkristall, der sich beim Erwärmen in eine optisch isotrope Phase umwandelt. In diese Demethylionenalkylsulfonate genannten Komplexe werden Bisthienylcyclopentene mit Alkylsulfonatseitenketten zwar ortsfest eingelagert, deren Photozyklisierung verschiebt das Gleichgewicht jedoch nur wenig von der hexagonalen zur isotropen Phase. Hingegen verläuft die Photozyklisierung der Bisthienylcyclopentene im festen Komplex teilweise deutlich langsamer als in Lösungen. / Can photoreactions of embedded substances change phase transition temperatures of polyelectrolyte surfactant complexes? Complexes made from polyamines and alkyl sulfonates form an hexagonal liquid crystal in presence of water, which is converted to optical isotropic phase by heating. These complexes resembling those of ionenes are called demethyl-ionene alkyl sulfonates. Bisthienylcyclopentenes bearing alkyl sulfonate side chains are incorporated into these complexes and keep their locations over prolonged periods of time, but the photocyclization of the chromophore only slightly shifts the equilibrium from hexagonal liquid crystal towards the optical isotropic phase. Conversely, the photocyclization of bisthienylcyclopentenes partially proceeds appreciably slower within the solid complex than in solution.