- Robert M. Stecher Collection of Darwiniana (x)
- Petschek R.G. (x)
- Search results
Search results
Show moreQuadrupolar and octupolar invariants may be used as methods to attain noncentrosymmetric materials for application in second order nonlinear optics. The structure-property relationships for the J = 2, or quadrupole rotational invariant, are investigated. Proper citation for the Technical Digest CD-ROM should be written in the following format: Ostrovekhov V., Ostroverkhova O., Petschek R.G., Singer K.D., Sukhomlinova L., Twieg R.J., “`Quadrupolar' molecular nonlinear optics,” in the Conference on Lasers and Electro-Optics (CLEO)/The International Quantum Electronics Conference (IQEC) (Optical Society of America, Washington, DC, 2009), Proceedings, 8-9
Show less
Show more[No abstract available] Appropriate credit for the requested material should be given as follows: "Reprinted (adapted) with permission from Langmuir 7:9, 1988-1990. Copyright 1991 American Chemical Society."
Show less
Show moreAn anomalously large flexoelectric effect is predicted in the nematic phase of thermotropic main-chain polymers, which have a defined orientation of subsequent monomers bonded on the chain. We identify two different ranges of order parameters. When the persistence length of the chain in the direction parallel to the order parameters is larger than a monomer length but not too long (small order parameter), the longitudinal flexoelectric coefficient f is proportional to this exponentially large persistence length, assuming that the polymers are many persistence lengths long. This result corresponds to the simple estimation that relates flexoelectric coefficients to the Frank elastic constants: fKs. When the nematic ordering approaches its saturation (strongly extended chains), the flexoelectric effect is essentially independent of the persistence length and is either determined by a competition between this effect and the tendency of the chain segments to align antiparallel, or the system is ferroelectric in the absence of deformations. © 1992 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Petschek R.G., Terentjev E.M., Physical Review A 45:8, 5775-5782 (1992). Copyright 1992 by the American Physical Society."
Show less
Show moreThe electroclinic effect and dielectric response have been investigated in the British Drug House material SCE12 over its entire 15-K smectic-A range. Near the smectic-Asmectic-C*-phase transition both the electroclinic coefficient and dynamic response behave normally, the former diverging and the latter exhibiting a critical slowing down on approaching TA-C* from above. Similar behavior is seen in the dielectric response. Approximately 10 to 12 K above TA-C*, however, it was found that the electroclinic relaxation time levels off and even begins to increase again on approaching the nematicsmectic-A transition from below. The magnitude of the electroclinic coefficient, however, decreases monotonically on approaching the nematicsmectic-A transition. © 1991 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Li Z., Akins R.B., Dilisi G.A., Rosenblatt C., Petschek R.G., Physical Review A 43:2, 852-857 (1991). Copyright 1991 by the American Physical Society".
Show less
Show moreAn investigation on the cyclic liquid crystalline trimer TPB-(c)9(3) was performed. The optical retardation and Fréedericksz techniques within a few tens of millikelvins of the superheating limit of the nearly second-order nematic isotropic phase transition were used for the investigation purpose. It was found that both the Fréedericksz bend threshold voltage and optical retardation were in good agreement with tricritical behaviour for the transition. Appropriate bibliographic citation and notice of the APS copyright must be included- "Syed I.M., Percec V., Petschek R.G., Rosenblatt C., Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 67:1, 117041-117044 (2003). Copyright 2003 by the American Physical Society."
Show less
Show moreFreedericksz measurements were performed on mixtures of a nematic mesogen and a V-shaped molecule. The bend elastic constant was found to decrease significantly with increasing concentration of the V-shaped molecule. The results are discussed theoretically, taking into account the detailed structure of the molecules. ©2000 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Dodge M.R., Rosenblatt C., Petschek R.G., Neubert M.E., Walsh M.E., Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics 62:4 B, 5056-5063 (2000). Copyright 2000 by the American Physical Society."
Show less
Show moreUsing an atomic force microscope to nanopattern a substrate for liquid crystal alignment, a bend distortion is imposed on a liquid crystal. In regions of large bend the smectic-A phase melts into the nematic phase, and the width of the melted region is measured as a function of temperature. The results are consistent with type-I superconducting (nematic-smectic-A) behavior, wherein a large magnetic field (bend or twist distortion) induces an order to disorder transition. A model that accounts for non-mean-field behavior is presented. © 2006 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Wang R., Syed I.M., Carbone G., Petschek R.G., Rosenblatt C., Physical Review Letters 97:16, 2006. Copyright 2006 by the American Physical Society."
Show less
Show moreThe equilibrium polymerization of chains and rings can be viewed as a bicritical point in a suitably augmented parameter space. The polymerization of chains in equilibrium with rings is described by the cross-over exponent, φ, for quadratic anisotropy in the O(n) vector model with n→1. While the total mass fraction of polymerized material is described by the Ising exponent α, the mass fraction of chains separately (and therefore also of rings) is described by the exponent αc=φ-1+α. © 1983 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Wheeler J.C., Petschek R.G., Pfeuty P., Physical Review Letters 50:21, 1633-1636, 1983. Copyright 1983 by the American Physical Society."
Show less
Show moreWe obtain upper bounds on the polymer-length distribution in a class of models for equilibrium polymerization. This rules out a recent claim for the existence of a collapsed phase in which a nonzero fraction of the monomers are in infinite polymer(s). We discuss the behavior of finite systems, in which a collapse of sorts does occur, and indicate how this collapse disappears in the thermodynamic limit. © 1992 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Wheeler J.C., Petschek R.G., Physical Review B 45:1, 171-175 (1992). Copyright 1992 by the American Physical Society."
Show less
Show moreIt is shown that energy/length scaling complicates maximizing the first hyperpolarizability of a single electron as a function of the potential. A more transparent formula for this hyperpolarizability is given. Examining this formula demonstrates that Zhou et al.1 have not proved that modulated conjugation results in large hyperpolarizability. © 2007 Optical Society of America. This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: [http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-32-8-942]. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.
Show less
Show more[No abstract available] Appropriate bibliographic citation and notice of the APS copyright must be included- "Al-Jishi R., Foldy L.L., Petschek R.G., Taylor P.L., Physical Review Letters 54:10, 1089, 1985. Copyright 1985 by the American Physical Society."
Show less
Show more[No abstract available] Appropriate bibliographic citation and notice of the APS copyright must be included- "Petschek R.G., Physical Review Letters 56:12, 1314, 1986. "Copyright 1986 by the American Physical Society."
Show less
Show more[No abstract available] Appropriate bibliographic citation and notice of the APS copyright must be included- "Petschek R.G., Physical Review Letters 57:4, 501, 1986. "Copyright 1986 by the American Physical Society."
Show less
Show moreWe investigated 1:1 mixtures of a variety of 4,4′-disubstituted diphenyldiacetylenes (PTTP series) and cyano-containing mesogens for possible complex formation Complexes were formed when alkyl/alkoxy cyanobiphenyls (CB series) were mixed with alkyl/alkoxy PTTP homologues. We found complexes having a melting temperature higher than that for either component, and complexes with lower melting temperatures. A fluoro substituent on the PTTP gave an enhanced nematic phase and a cyclohexane ring in place of a benzene ring yielded an enhanced smectic A phase, but a cyano on the PTTP gave poor liquid crystalline properties. Phase diagrams were constructed for PTTP-24/5-CB and PTTP-24/8O-CB mixtures. Both complexes formed had melting temperatures higher than those for either component. These complexes were recrystallized and shown by X-ray studies to be 1:1 complexes. The PTTP-24/5-CB complex showed only a monotropic nematic phase, but the PTTP-24/8O-CB complex showed induced smectic A and hexatic B phases in addition to an enantiotropic nematic phase. ©1999 Taylor & Francis Ltd.
Show less
Show moreIt has been suggested that ferroelectric smectic phases may be formed by fraternal-twin liquid-crystal molecules, consisting of two very different mesogens bonded together by an appropriate spacer [R. G. Petschek and K. M. Wiefling, Phys. Rev. Lett. 59, 343 (1987)]. We discuss the range in which these ferroelectric phases appear in a Monte Carlo simulation of a simple computer model for such mesogens. The mesogens consist of two identical flexible segments that are bonded to two rigid segments with very different lengths which, in turn, are bonded together by a flexible segment. In the computer model these monomers can move freely in one direction, but are constrained to lie on a hexagonal lattice of rods in the other two directions, roughly the packing expected in a smectic-B crystal. The flexible segments are modeled by springs, and there is an energy cost for the overlap of flexible and rigid segments on neighboring lattice rods. The region in which polar (flexible, short rigid, flexible, long rigid, flexible) smectic layers form will be discussed. © 1991 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Perchak D.R., Petschek R.G., Physical Review A 43:12, 6756-6770 (1991). Copyright 1991 by the American Physical Society."
Show less
Show moreThe distance R spanned by N monomers of a long chain in chemical equilibrium with other long chains and rings of all sizes varies like R approx. N**1**/**D with 1/D equals v(n)/ phi (n) where v(n) and phi (n) are respectively the correlation length critical exponent and the quadratic anisotropy crossover exponent of the O(n) model with n being a statistical preweighting factor associated with a ring.
Show less
Show moreWe describe improvements to the Kleinman-disallowed hyper-Rayleigh scattering (HRS) measurement technique, which can be used to measure all of the rotationally invariant figures of merit for the first molecular nonlinear optical hyperpolarizability tensor βijk. As multiphoton excited fluorescence continues to be an accuracy-limiting factor in any HRS experiment, we have implemented time-resolved single photon counting in HRS using a Ti:sapphire laser. We show, however, that spectral analysis is necessary to ensure that the fluorescence and second harmonic are emitted on different time scales in order to allow temporal separation and thus improved accuracy. We also demonstrate how an electrically controlled liquid-crystal phase retarder can be used to vary the polarization state of the probe light in order to determine the second-order nonlinear response tensor. It was found that results agree with those obtained using a rotating quarter-wave plate to control the polarization state and demonstrate improved precision and reproducibility. © 2008 Optical Society of America.
Show less
Show moreFinite-size-scaling analysis of a simulation of the critical behavior of a nonrandom three-dimensional frustrated system yields a tricritical-like behavior not predicted by a simple Landau-Ginzburg-Wilson theory. We argue that, because of the frustration, the free energy of many phases is close to that of the observed ordered phase and that a complete theory must take into account interactions between fluctuations of order parameters of all these phases. We suggest that these properties are generic to frustrated systems. Related transitions in the model are also discussed. © 1989 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Heinonen O., Petschek R.G., Physical Review B 40:13, 9052-9055 (1989). Copyright 1989 by the American Physical Society."
Show less
Show moreEquilibrium polymerization systems in which directed rings are formed correspond to O(2) vector models of magnetism and ones in which undirected rings form correspond to O(1) models. In this paper it is shown that a system in which polymers tend to form directed rings but there is a finite free energy for misdirected bonds corresponds to an O(2) model with a uniaxial anisotropy which causes it to crossover to an O(1) model. © 1985 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Petschek R.G., Physical Review A 31:3, 1943-1945 (1985). Copyright 1985 by the American Physical Society."
Show less
Show moreIt has been suggested that defects cause the central peak in SrTiO3. This paper presents a mean-field study of point defects which couple so strongly to the order parameter that nonlinear interactions in neighboring cells are important. Fair agreement with experimental results from electron paramagnetic resonance is found with a concentration of defects fitted to inelastic neutron scattering results. The hindered reorientation of domains is found to be consistent with this concentration of defects. © 1980 The American Physical Society. Appropriate bibliographic citation and notice of the APS copyright must be included- "Petschek R.G., Physical Review B 22:3, 1409-1416 (1980). Copyright 1980 by the American Physical Society."
Show less