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T. Stoebe

Bio: T. Stoebe is an academic researcher from University of Minnesota. The author has contributed to research in topics: Hexatic phase. The author has an hindex of 1, co-authored 1 publications receiving 41 citations.
Topics: Hexatic phase

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TL;DR: In this article, the authors present an extensive review of the thermal properties associated with the phase transitions related to these liquid-crystal hexatic phases, and demonstrate that hexatic order is the only symmetry-breaking field associated with hexatic-B and smectic-I phases.
Abstract: Ever since the hexatic phase was proposed more than 14 years ago in the context of two-dimensional melting theory, the concept of hexatic order has offered unique insight into the nature of several liquid-crystal phases, namely the hexatic-B, smectic-I and smectic-F phases. Moreover, these liquid-crystal phases have offered numerous opportunities to explore and investigate the nature of hexatic order and the related phase transitions. In this paper we present an extensive review of the thermal properties associated with the phase transitions related to these liquid-crystal hexatic phases. Although both electron-beam and X-ray diffraction studies have indicated the importance of hexatic ordering in understanding these phases, the existing experimental data from numerous thermal measurements fail to demonstrate that hexatic order, which can be represented by a complex variable and therefore belongs to the XY universality class, is the only symmetry-breaking field associated with the hexatic-B—smect...

46 citations


Cited by
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TL;DR: In this paper, various modulation techniques for measuring specific heat, thermal expansivity, temperature derivative of resistance, thermopower, and spectral absorptance are reviewed, due to the periodic nature of temperature oscillations, high sensitivity and excellent temperature resolution are peculiar to all these methods.

95 citations

Journal ArticleDOI
TL;DR: The phase diagram of the Gay-Berne model with anisotropy parameters κ = 3, κ′=5 has been evaluated by means of computer simulations as mentioned in this paper.
Abstract: The phase diagram of the Gay–Berne model with anisotropy parameters κ=3, κ′=5 has been evaluated by means of computer simulations. For a number of temperatures, NPT simulations were performed for the solid phase leading to the determination of the free energy of the solid at a reference density. Using the equation of state and free energies of the isotropic and nematic phases available in the existing literature the fluid–solid equilibrium was calculated for the temperatures selected. Taking these fluid–solid equilibrium results as the starting points, the fluid–solid equilibrium curve was determined for a wide range of temperatures using Gibbs–Duhem integration. At high temperatures the sequence of phases encountered on compression is isotropic to nematic, and then nematic to solid. For reduced temperatures below T=0.85 the sequence is from the isotropic phase directly to the solid state. In view of this we locate the isotropic–nematic–solid triple point at TINS=0.85. The present results suggest that the high-density phase designated smectic B in previous simulations of the model is in fact a molecular solid and not a smectic liquid crystal. It seems that no thermodynamically stable smectic phase appears for the Gay–Berne model with the choice of parameters used in this work. We locate the vapor–isotropic liquid–solid triple point at a temperature TVIS=0.445. Considering that the critical temperatures is Tc=0.473, the Gay–Berne model used in this work presents vapor–liquid separation over a rather narrow range of temperatures. It is suggested that the strong lateral attractive interactions present in the Gay–Berne model stabilizes the layers found in the solid phase. The large stability of the solid phase, particularly at low temperatures, would explain the unexpectedly small liquid range observed in the vapor–liquid region.

86 citations

06 Oct 1990
TL;DR: The results confirm recent theoretical speculation that the positional order should be far more sensitive to disorder than the orientational order and that the low-temperature ordered phase of the flux lines in these systems might be a hexatic glass.
Abstract: We report observation of hexatic order in Abrikosov flux lattices in very clean crystals of the high {ital T}{sub {ital c}} superconductor Bi{sub 2.1}Sr{sub 1.9}Ca{sub 0.9}Cu{sub 2}O{sub 8+{delta}} (BSCCO). Our experiments consist of {ital in} {ital situ} magnetic decoration of the flux lattice at 4.2 K. Analysis of the decoration images shows that the positional order decays exponentially with a correlation length of a few lattice constants while the orientational order persists for hundreds of lattice constants and decays algebraically with an exponent {eta}{sub 6}=0.06{plus minus}0.01. Our results confirm recent theoretical speculation that the positional order should be far more sensitive to disorder than the orientational order and that the low temperature ordered phase of the flux lines in these systems might be an hexatic glass.

73 citations

Journal ArticleDOI
29 May 1998-Science
TL;DR: Data suggest the existence of two phases between the 2D solid and liquid: a hexatic phase and, at a higher temperature, an intermediate liquid phase with hexatic-like positional correlations but no long-range orientational order.
Abstract: An unexpected three-stage melting transition has been observed in two-dimensional (2D) free-standing liquid-crystal films by in situ electron-diffraction and optical-reflectivity measurements. These data suggest the existence of two phases between the 2D solid and liquid: a hexatic phase and, at a higher temperature, an intermediate liquid phase with hexatic-like positional correlations (∼40 angstroms) but no long-range orientational order. Previous high-resolution heat-capacity measurements have revealed a divergent-like anomaly at the hexatic-liquid transition that sharply contradicts the predictions of 2D melting theories. The observation of an intermediate isotropic phase may alter our understanding of 2D melting and lead to reconciliation between current experiments and theories.

59 citations

Journal ArticleDOI
TL;DR: In this paper, two new groups of azobenzene ester derivatives were synthesized: alkyl 4-(nonyloxy)phenyl]diazenyl]benzoates and 4-[4-(mene)diamyl]phenyl alkanoates, and the kinetic constants of the isomerization and relaxation processes were calculated.
Abstract: Two new groups of azobenzene ester derivatives were synthesised: alkyl 4-[4-(nonyloxy)phenyl]diazenyl]benzoates and 4-[4-(nonyloxy)phenyl]diazenyl]phenyl alkanoates. All 35 presented homologues are mesogenic. Moreover, some of the above-mentioned compounds exhibit rich liquid-crystalline polymorphism likewise tetramorphism. During this investigation by the use of polarising optical microscopy, differential scanning calorimetry and X-Ray studies, six types of mesophases were detected: nematic, smectics (A, C, I, F) and G. Furthermore, due to the presence of the photosensitive azo moiety, the E–Z isomerisation reaction is possible. This process, which is initiated by the UV irradiation, causes significant changes in the UV-Vis absorption spectra of investigated compounds. However, the photoisomerisation is a reversible process and in the dark the thermal relaxation of Z isomer takes place. Based on the achieved data, the kinetic constants of the isomerisation and relaxation processes were calculated. It sho...

39 citations