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A Cady

Publications and source records attributed to A Cady.

At least 19 recordsLinked to original sources
PubMed2006

Smectic-Calpha*-smectic-C* phase transition and critical point in binary mixtures.

We have investigated the smectic-Calpha*-smectic-C* (SmCalpha*-SmC*) transition in a series of binary mixtures with resonant x-ray diffraction, differential optical reflectivity, and heat capacity measurements. Results show that the phases are separated by a first-order transition that ends at a critical point. We report the observation of such a critical point. We have proposed the appropriate order parameter and obtained values of two critical exponents associated with this transition. The values of the critical exponents suggest that long-range interactions are present in the SmCalpha*-SmC* critical region.

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PubMed2005

Atomic origin of magnetocrystalline anisotropy in Nd(2)Fe(14)B.

The magnetic moment reversal at each of the two inequivalent Nd sites in a single crystal of ferromagnetic Nd(2)Fe(14)B is probed by dichroic resonant diffraction of circularly polarized x rays. The results, supported by theory, show that the c-axis intrinsic magnetic stability of this superior permanent magnetic material arises predominantly at one of the Nd sites (g). The other site (f) undermines magnetic stability by favoring a magnetic moment orientation in the basal plane.

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PubMed2004

Optical investigations on the biaxial smectic- A phase of a bent-core compound.

Several unique optical properties have been obtained from freestanding films of a bent-core compound. Our experimental results indicate the existence of the antiferroelectric biaxial smectic-A (Sm A) phase. The critical exponent associated with the biaxiality through the uniaxial-antiferroelectric biaxial Sm A transition has been measured to be 0.82+/-0.04 , which is in good agreement with our theoretical calculation. Our theoretical advances further demonstrate that the critical behavior of the optical biaxiality with the order parameter being a vector is described by the secondary-order parameter of the three-dimensional XY model. We also observe a remarkable even-odd layering effect exhibited by the surface layers of freestanding films under an applied electric field (approximately 20 V/cm) in the film plane.

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PubMed2004

Experimental investigations of one liquid-crystal compound exhibiting the no-layer-shrinkage effect near the Sm-A-Sm- C(*) transition.

Three experimental probes have been employed to investigate the nature of the smectic- A -smectic- C ( Sm-A-Sm- C(*) ) phase transition of one liquid-crystal compound showing almost no layer-shrinkage effect through the transition. Results from both x-ray diffraction and optical studies indicate that the compound exhibits a crossover behavior of different molecular packing arrangements within the bulk Sm-A phase window. The calorimetry results show a significant critical anomaly near the Sm-A-Sm- C(*) transition, although it was found to be weakly first order.

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PubMed2003

Unusual behavior of the surface-induced tilted layers in free-standing films of a non-layer-shrinkage liquid crystal compound.

Null-transmission ellipsometry has been conducted to study the molecular arrangements in free-standing films of one chiral compound above the bulk smectic-A-smectic-C* transition temperature. Upon cooling under a proper electric field, a nonplanar-anticlinic-synclinic or a nonplanar-synclinic transition has been observed. The nonplanar structure continuously evolves into the anticlinic or synclinic structures. Increasing electric field can induce a rare transition from a synclinic to an anticlinic structure.

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PubMed2003

Optical studies on free-standing films of an achiral smectic liquid crystal.

Employing null transmission ellipsometry and depolarized reflected light microscopy, we have studied two smectic phases, Sm-C1 and Sm-C2, of one achiral mesogen. Our results show that Sm-C1 and Sm-C2 are the synclinic Sm-C and anticlinic Sm-C(A) phases, respectively. We find no evidence to support recent claims that the Sm-C1 is chiral and ferroelectric [R. Stannarius et al., Phys. Rev. Lett. 90, 025502 (2003)].

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PubMed2003

Optical characterization of a nanoscale incommensurate pitch in a new liquid-crystal phase.

As a remarkable orientationally ordered soft-condensed matter system, the smectic-C*(alpha) phase exhibits an incommensurate nanoscale helical pitch of more than four smectic layers. Recent theoretical advances predict the existence of a new phase showing a helical pitch of less than four smectic layers (<16 nm). By applying a novel optical technique to the study of freestanding films with increments in thickness of one smectic layer, we have measured the size of this pitch using 633 nm HeNe laser light. Our results confirm the existence of the newly predicted phase in one unique compound.

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PubMed2003

Double reentrance of surface arrangements in freestanding liquid crystal films.

The molecular arrangement of tilted surface layers on freestanding films above the bulk smectic-A-smectic-C(*) transition has been studied by null transmission ellipsometry. In the presence of an applied in-plane electric field, a synclinic-anticlinic-synclinic-anticlinic double reentrance has been observed. Furthermore, we have characterized a novel transition from the synclinic arrangement to a structure in which the interior layers tilt in the same orientation and the outermost surface layers tilt oppositely with the interior layers.

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PubMed2002

Effect of enantiomeric excess on surface structures and phase sequences in free-standing liquid-crystal films.

We present differential optical reflectivity and null transmission ellipsometry data from several mixtures of MHPBC enantiomers. From this data, we have determined the chiral smectic phase sequence as a function of enantiomeric excess. The data suggest a change in the structure of the smectic-C(*)(alpha) phase and the surface tilt state as the enantiomeric excess is reduced. The results also show that the intermediate phases, smectic C(*)(FI2), smectic C(*)(FI1), and smectic C(*)(alpha), disappear sequentially on reducing the net chirality.

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PubMed2002

Molecular orientation arrangements in the smectic-C* variant liquid-crystal phases.

Recent experiments have identified three-layer and four-layer distorted helical structures in the smectic liquid-crystal phases Sm C*(FI1) and Sm C*(FI2), respectively. However, no theories have explained the existence of all these phases. A discrete phenomenological model of the free-energy is analyzed and found to predict the stability of distorted three-layer and four-layer structures, as well as simple helical solutions in smectic liquid crystals. A simple physical picture is provided to explain the stability of the phases exhibiting distorted helical structures.

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PubMed2002

Interlayer structures of the chiral smectic liquid crystal phases revealed by resonant X-ray scattering.

The structures of the liquid crystalline chiral subphases exhibited by several materials containing either a selenium or sulphur atom have been investigated using a resonant x-ray scattering technique. This technique provides a unique structural probe for the ferroelectric, ferrielectric, antiferroelectric, and SmC(*)(alpha) phases. An analysis of the scattering features allows the structural models of the different subphases to be distinguished, in addition to providing a measurement of the helical pitch. This paper reports resonant scattering features in the antiferroelectric hexatic phase, the three- and four-layer intermediate phases, the antiferroelectric and ferroelectric phases and the SmC(*)(alpha) phase. The helicoidal pitch has been measured from the scattering peaks in the four-layer intermediate phase as well as in the antiferroelectric and ferroelectric phases. In the SmC(*)(alpha) phase, an investigation into the helical structure has revealed a pitch ranging from 5 to 54 layers in different materials. Further, a strong resonant scattering signal has been observed in mixtures of a selenium containing material with as much as 90% nonresonant material.

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PubMed2002

Dramatic effect of an additional CH2 group on the temperature variation of the Sm-C(*)(alpha) short helical pitch.

High-resolution differential optical reflectivity was used to study the temperature evolution of the short helical pitch in the Sm-C(*)(alpha) phase of successive members from two liquid-crystal homologous series. With the addition of one CH2 group, the magnitude and temperature evolution of the pitch change dramatically, and the molecular arrangements between consecutive surface layers found in free-standing films change from being anticlinic to synclinic.

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PubMed2002

Observation of unusual surface ordering in a uniaxial Sm-A phase formed by a highly biaxial compound.

We have studied one banana-shaped compound using null-transmission ellipsometry. By studying free-standing films of various thicknesses we confirm that this compound exhibits the Sm-A phase. This is the first banana-shaped compound with a relatively small bend angle (approximately 140 degrees) in which a uniaxial phase has been observed. At the lower temperature end of the Sm-A phase we observe unusual ordering at the surface of the film.

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PubMed2001

Surface structures and transitions in the smectic-C* phase of one chiral liquid crystal compound.

Three optical probes have been employed on free-standing films to study surface structure and transitions of the SmC* phase of one liquid crystal compound. While the interior layers show the SmC* structure, the tilt in adjacent surface layers is found to be anticlinic. The number of anticlinic surface layers grows rapidly as the transition to the SmC*(FI2) phase is approached.

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PubMed2001

Detailed optical studies of several banana-shaped compounds in liquid crystal B2 phase.

Null-transmission ellipsometry has been performed on free-standing films of one compound in the liquid crystal B2 phase. We have studied films of thickness from 1 to 121 layers. One of the compounds used has an unusually wide 59 K window for the B2 phase. The tilt angle was investigated as a function of temperature and found to be constant over this temperature range to within our resolution of 1 degrees. The one-layer films studied exhibit the same structure as the thicker films, and have helped us to refine the optical model for the B2 phase. For thin films we find that modeling a single smectic layer as two uniaxial layers is a better description of the data than a single biaxial layer, but that for thick films the model used does not effect the simulated result appreciably. Preliminary results also find that the surface layers are less tilted than the interior layers, in contrast to rodlike liquid crystals, which show an enhanced surface tilt.

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PubMed2001

Orientational ordering in the chiral smectic-C*FI2 liquid crystal phase determined by resonant polarized x-ray diffraction.

High-resolution resonant polarized x-ray diffraction experiments near the sulfur K edge have been performed on free-standing liquid crystal films exhibiting the chiral smectic-C*FI2 phase. It is widely accepted that this phase has a four-layer repeat unit, but the internal structure of the repeat unit remains controversial. We report different resolved features of the resonant x-ray diffraction peaks associated with the smectic-C*FI2 phase that unambiguously demonstrate that the four-layer repeat unit is locally biaxial about the layer normal and that the measured angle, describing the biaxiality, is in good agreement with optical measurements.

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PubMed2001

Optical studies of the smectic-C(*)(alpha) phase layer structure in free-standing films.

The layer structure of the smectic-C(*)(alpha) phase of one liquid-crystal compound has been acquired from both differential optical reflectivity and ellipsometry measurements in the free-standing film geometry. The data from both techniques display characteristic oscillations as a function of temperature, which can be described by a model for the film consisting of surface anticlinic layers and an interior short-pitched azimuthal helix. These results are consistent with those found previously for another compound. Depolarized reflected light microscopy is used to study the films when the unique features of the aforementioned oscillations occur.

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