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Josep Salud

Bio: Josep Salud is an academic researcher from Polytechnic University of Catalonia. The author has contributed to research in topics: Liquid crystal & Phase transition. The author has an hindex of 21, co-authored 61 publications receiving 1667 citations.


Papers
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Journal ArticleDOI
TL;DR: In this article, the structural properties of phase II and its glass transition were investigated from a structural point of view using x-ray diffraction and dynamic dielectric spectroscopy.
Abstract: –OH) is revealed to be a complexproblem and only two stable solid phases, denoted on cooling from the liquid as phases I and II, are foundusing static ~thermodynamic and x-ray diffraction! as well as dynamic ~dielectric spectroscopy! experimentaltechniques. Both solid phases are known to exhibit glass transitions if they are cooled down fast enough toprevent transition to ordered crystalline states. Although glass transitions corresponding to both phases hadbeen well documented by means of specific heat measurements, x-ray measurements constitute, as far as weknow, the first evidence from the structural point of view. In addition, a great amount of dielectric worksdevoted to phase I and its glass transition, were published in the past but next to nothing relating to thedielectric properties of phase II and its glass transition. The nature of the disorder of phase II will be discussed.DOI: 10.1103/PhysRevB.69.224202 PACS number~s!: 61.43.2j, 64.70.Pf, 65.60.1a, 77.22.2dI. INTRODUCTION

50 citations

Journal ArticleDOI
TL;DR: In this article, a tricritical point (TCP) composition at the second order SmA to N transition has been found for the binary mixtures of 8OCB and 9OCB.
Abstract: Modulated differential scanning calorimetry (MDSC) has been used to obtain specific heat measurements on octyloxycyanobiphenyl (8OCB), octylcyanobiphenyl (8CB) and nonyloxycyanobiphenyl (9OCB) liquid crystals and on several binary mixtures of 8OCB + 9OCB and 8CB + 9OCB. The order of the mesophase transitions, smectic A(SmA) to nematic(N) and N to isotropic (I) on pure components and on binary mixtures has been studied and, in addition, both binary mesophase diagrams have been built. For each set of mixtures, there exists a tricritical point (TCP) composition at the SmA–N transition. The TCP compositions have been found to be X9OCB = 0.63 for the system 8OCB + 9OCB and X9OCB = 0.42 for the system 8CB + 9OCB. In addition, the specific heat critical exponents (α) through second order SmA to N transition have been obtained between 8OCB(or 8CB) and the TCP composition for the two sets of binary mixtures. When these α-values were plotted against the normalised nematic range, a common and uniform crossover trend was found. Indeed, this fact suggests a uniform behaviour in terms of the McMillan ratio for the nCB and nOCB series of liquid crystals. Moreover, for these compounds there exists a common value of (TAN/TNI)TCP of 0.99 and also a value of (TAN/TNI)3D-XY of about 0.96.

48 citations

Journal ArticleDOI
TL;DR: The nematic-to-isotropic phase transition has been exhaustively studied and it has been concluded that the transition is first order in nature and follows the tricritical hypothesis.
Abstract: Broadband dielectric spectroscopy (103 to 1.8 × 109 Hz) and specific heat measurements have been performed on the odd nonsymmetric liquid crystal dimer α-(4-cyanobiphenyl-4′-oxy)-ω-(1-pyreniminebenzylidene-4′-oxy)undecane (CBO11O·Py), as a function of temperature. The mesogenic behavior is restricted to a nematic mesophase which can be supercooled down to its corresponding glassy state if the cooling rate is fast enough (no less than 15 K·min–1). Dielectric measurements enable us to obtain the static permittivity and information about the molecular dynamics in the nematic mesophase as well as in the isotropic phase and across the isotropic-to-nematic phase transition. Two orientations (parallel and perpendicular) of the molecular director with regard to the probe electric field have been investigated. In the nematic mesophase, the dielectric anisotropy is revealed to be positive. Measurements of the parallel component of the dielectric permittivity are well explained by means of the molecular theory of di...

42 citations

Journal ArticleDOI
TL;DR: In this article, the existence of a glass transition from the monoclinic low-temperature stable ordered phase (III) to its nonergodic state (associated with the freezing of exchange positions between Cl and Br atoms) is analyzed in terms of the asymmetry of the intermolecular interactions, and a new fingerprint for the glass transition is proposed on the basis of the aspherism index.
Abstract: The polymorphism of bromotrichloromethane (CBrCl 3 ) has been investigated by X-ray powder diffraction and high-pressure density experiments. Phase transitions as a function of temperature and pressure between the different phases have been characterized at normal pressure as well as at high pressures (up to 300 MPa). From the p-v-rdiagram (and the denved p-T diagram) the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray powder diffraction characterization. Special attention is given to the lattice symmetry of the orientational disordered phase II, characterized as rhombohedral (a = 14.639(8) A, α = 89.44(1)° at 240.2 K). The existence of a glass transition from the monoclinic low-temperature stable ordered phase (III) to its nonergodic state (associated with the freezing of exchange positions between Cl and Br atoms) is analyzed in terms of the asymmetry of the intermolecular interactions, and a new "fingerprint" for the glass transition is proposed on the basis of the aspherism index. Lattice parameters as a function of temperature were determined in order to build up the thermal expansion tensor.

42 citations

Journal ArticleDOI
TL;DR: It is proved that both phase transitions are weakly first order, displaying a nearly tricritical behavior, and the width of metastable regions seems to be dependent on the physical magnitude, although specific heat and volumetric determinations allow for comparable results.
Abstract: Different kind of measurements were performed on the liquid crystal nonyloxycyanobiphenyl (9OCB) to carry out a study of the molecular dynamics in the smectic A (SmA), nematic (N), and isotropic (I) phases as well as an exhaustive analysis of both the SmA-to-N and N-to-I phase transitions. For the dynamic study, broadband dielectric spectroscopy (102 to 1.8 × 109 Hz) was used. Two orientations (parallel and perpendicular) of the molecular director with regard to the probing electric field were investigated. From this study, the static dielectric permittivity was obtained in both alignments and, in addition, the molecular motions that contribute to each one were discussed. The static dielectric data together with specific heat and volumetric determinations were analyzed, proving that both phase transitions are weakly first order, displaying a nearly tricritical behavior. However, the width of metastable regions seems to be dependent on the physical magnitude, although specific heat and volumetric determina...

41 citations


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Proceedings Article
01 Jan 1999
TL;DR: In this paper, the authors describe photonic crystals as the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures, and the interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.
Abstract: The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

2,722 citations

Book
01 Jan 1996
TL;DR: A review of the collected works of John Tate can be found in this paper, where the authors present two volumes of the Abel Prize for number theory, Parts I, II, edited by Barry Mazur and Jean-Pierre Serre.
Abstract: This is a review of Collected Works of John Tate. Parts I, II, edited by Barry Mazur and Jean-Pierre Serre. American Mathematical Society, Providence, Rhode Island, 2016. For several decades it has been clear to the friends and colleagues of John Tate that a “Collected Works” was merited. The award of the Abel Prize to Tate in 2010 added impetus, and finally, in Tate’s ninety-second year we have these two magnificent volumes, edited by Barry Mazur and Jean-Pierre Serre. Beyond Tate’s published articles, they include five unpublished articles and a selection of his letters, most accompanied by Tate’s comments, and a collection of photographs of Tate. For an overview of Tate’s work, the editors refer the reader to [4]. Before discussing the volumes, I describe some of Tate’s work. 1. Hecke L-series and Tate’s thesis Like many budding number theorists, Tate’s favorite theorem when young was Gauss’s law of quadratic reciprocity. When he arrived at Princeton as a graduate student in 1946, he was fortunate to find there the person, Emil Artin, who had discovered the most general reciprocity law, so solving Hilbert’s ninth problem. By 1920, the German school of algebraic number theorists (Hilbert, Weber, . . .) together with its brilliant student Takagi had succeeded in classifying the abelian extensions of a number field K: to each group I of ideal classes in K, there is attached an extension L of K (the class field of I); the group I determines the arithmetic of the extension L/K, and the Galois group of L/K is isomorphic to I. Artin’s contribution was to prove (in 1927) that there is a natural isomorphism from I to the Galois group of L/K. When the base field contains an appropriate root of 1, Artin’s isomorphism gives a reciprocity law, and all possible reciprocity laws arise this way. In the 1930s, Chevalley reworked abelian class field theory. In particular, he replaced “ideals” with his “idèles” which greatly clarified the relation between the local and global aspects of the theory. For his thesis, Artin suggested that Tate do the same for Hecke L-series. When Hecke proved that the abelian L-functions of number fields (generalizations of Dirichlet’s L-functions) have an analytic continuation throughout the plane with a functional equation of the expected type, he saw that his methods applied even to a new kind of L-function, now named after him. Once Tate had developed his harmonic analysis of local fields and of the idèle group, he was able prove analytic continuation and functional equations for all the relevant L-series without Hecke’s complicated theta-formulas. Received by the editors September 5, 2016. 2010 Mathematics Subject Classification. Primary 01A75, 11-06, 14-06. c ©2017 American Mathematical Society

2,014 citations

Journal ArticleDOI
TL;DR: This Review focuses on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade, and the developed materials possess huge potential for applications in optics, photonics, adaptive materials, nanotechnology, etc.
Abstract: Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photoc...

576 citations

Journal ArticleDOI
TL;DR: This work experimentally demonstrates a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend.
Abstract: A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).

554 citations

Journal ArticleDOI
TL;DR: New exciting soft-matter structures distinct from the usually observed nematic, smectic, and columnar phases are presented, including multicompartment and cellular structures, periodic and quasiperiodic arrays of spheres, and new emergent properties, such as ferroelctricity and spontaneous achiral symmetry-breaking.
Abstract: Since the discovery of the liquid-crystalline state of matter 125 years ago, this field has developed into a scientific area with many facets. This Review presents recent developments in the molecular design and self-assembly of liquid crystals. The focus is on new exciting soft-matter structures distinct from the usually observed nematic, smectic, and columnar phases. These new structures have enhanced complexity, including multicompartment and cellular structures, periodic and quasiperiodic arrays of spheres, and new emergent properties, such as ferroelctricity and spontaneous achiral symmetry-breaking. Comparisons are made with developments in related fields, such as self-assembled monolayers, multiblock copolymers, and nanoparticle arrays. Measures of structural complexity used herein are the size of the lattice, the number of distinct compartments, the dimensionality, and the logic depth of the resulting supramolecular structures.

456 citations