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Crystal-Field Symmetry and Ordered Phases in Arrays of Helical Xy Spin Chains

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Crystal-Field Symmetry and Ordered Phases in Arrays of Helical Xy Spin Chains University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1993 Crystal-Field Symmetry And Ordered Phases In Arrays Of Helical Xy Spin Chains O. Heinonen University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact [email protected]. Recommended Citation Heinonen, O., "Crystal-Field Symmetry And Ordered Phases In Arrays Of Helical Xy Spin Chains" (1993). Faculty Bibliography 1990s. 719. https://stars.library.ucf.edu/facultybib1990/719 PHYSICAL REVIEW B VOLUME 47, NUMBER 5 1 FEBRUARY 1993-I Crystal-Beld symmetry and ordered phases in arrays of helical XY spin chains O. Heinonen Department of Physics, University of Central Florida, Ortando, Ftorida M816' (Received 21 April 1992; revised manuscript received 21 July 1992) The e8'ect of crystal-field symmetry and strength on the nature of ordered phases in three- dimensional systems consisting of arrays of helical XY spin chains is investigated. The phase diagrams of chiral chains in arrays with twofold symmetry always exhibit a Lifshitz point, where paramagnetic, ferromagnetic, and incommensurate phases meet. In contrast, the phase diagrams of chains in arrays in which the crystal field has higher symmetry than twofold have no Lifshitz point, whether the chains are chiral or not. In these arrays, the initial phase transition is always paramag- netic to incommensurate. The existence of a Lifshitz point in the phase diagram of nonchiral chains in arrays with twofold symmetry depends sensitively on the interchain coupling. I. INTRODUCTION temperature is increased, the entropic contribution to the free energy increases, and commensurate structures other A variety of physical systems, such as magnetic sys- than the one which minimizes the internal energy, as well tems, binary alloys, and lipid bilayers, exhibit spa- as incommensurate structures, may become energetically tially modulated phases. These spatial modulations arise favorable. At elevated temperatures, there are polymer from competing interactions. Some systems with com- systems which have been observed to display phases with peting interactions have been modeled by, for exam- both commensurate and incommensurate structures. It ple, the anisotropic next-nearest-neighbor Ising model has also been observed experimentally that the individ- (ANNNI), ~ s and the chiral-clock model. ~ 4 The phase ual molecules in some crystalline polymers become disor- diagrams of these models in three dimensions are in gen- dered as the temperature is increased but remains below eral very complex, but are well described by mean-field the melting temperature of the lattice of chains. One ex- s theories and have the following qualitative features. ample is the polymer polytetrafiuoroethylene (PTFE). Let r be a parameter denoting the relative strength of The phase diagram of this polymer is displayed in Fig. the competing interactions. The phase diagram in the 1. In the high-temperature phases I and IV, the poly- (T —K) plane can in general be divided into two parts. mer chains form commensurate helices, and in phase I In one, the disorder-order phase transition is paramag- they have a large amount of helical disorder. ~o In the netic to ferromagnetic. In the other part of the phase low-temperature phase II, the chains are ordered incom- diagram, on the other hand, the transition is from a para- mensurate helices. In these three phases, the packing of magnetic phase to incommensurate one, with many sub- the chains is hexagonal. In the high-pressure phase III, sequent modulated phases as the temperature is further on the other hand, the unit cell is orthorhombic and the reduced. The paramagnetic, ferromagnetic, and incom- mensurate phases all meet at the Lifshitz point. Competing interactions also arise in many crystalline 0.6— polymers. In these systems, isolated chains have heli- cal conformations with a pitch incommensurate with the 0.5— monomer spacing along the chain, i.e., the displacement of torsional angle between successive monomers is not O a simple rational fraction of 2'. The reason for this is 0 0 4 that the pitch is determined by a complicated sum of interatomic forces. Unless the molecule itself has some K 0 3— particular the pitch resulting from this sum is CA symmetry, V) LU incommensurate with the monomer spacing. When the 0.2 polymer molecules are packed in a regular array, however, CL the interchain interactions typically favor a commensu- O. I rate ordering. The conformations of the molecules are then determined by the interplay between the competing intrachain and interchain interactions, as well as the tem- 40 80 t20 I 60 perature and external Beld. It has been shown that at TEMPERATURE ( 'C ) absolute zero of temperature, the polymer molecules in a regular array form a commensurate structure. 7 s As the FIG. 1. Phase diagram in the PT plane of PTFE. 47 2661 1993 The American Physical Society 2662 O. HEINONEN 47 molecules have planar zigzag conformations. spaced with a separation d along the z axis. Polymer In many cases, the physical properties of a polymer sys- molecules, for example, can be thought of as a set of di- tem depend on the helical conformation of the molecules. rected rnonomers pointing in the 2:y plane, and the spins It is therefore important to understand how this eonfor- represent the angles which the directed monomers make rnation is affected by intrachain and interchain interac- with respect to the 2: axis. The spins are free to rotate tions and temperature, as well as external fields. The in the xy plane and are positioned at r, ~, where the in- motivation for the work presented here was to qualita- dex i denotes the position along the z axis, and j is an tively describe the phase diagram of such systems. Most index denoting the chain. We will assume that r, ~ forms helical polymer molecules are degenerate in the sense a regular lattice with the property that z r; = i.r, . — ~ ~ that an isolated chain is equally likely to be a right- The spin at r, ,z is denoted S;,z —cos8, ,&x+ sin8, ,&y, handed helix or a left-handed helix. However, there exist where 8; ~ is the angle between S; ~ and the x axis. We whole classes of polymers which have a preferred chiral- write the Hamiltonian as a sum of intrachain and in- ity. We shall refer to such systems as chiral systems, terchain terms, H = Ho+ H;„~„. The specific form of in contrast to systems with no preferred handedness, the intrachain part Ho will depend on if the model is which we shall call nonchiral. Important examples of chiral or nonchiral. We will restrict the interchain part chiral rnolecules are biomolecules such as nucleic acids H;„t„ to include only nearest-neighbor interactions. In and n-amino proteins. s Recent developments in the addition to the nearest-neighbor spin-spin interactions, synthesis of polymers have also led to heliospecific poly- we expect that this part will contain a term describing merization in which chiral polymers are synthesized with the interaction of a monomer with its average crystalline a specific handedness. Therefore, it is important to un- environment. This term will explicitly contain the sym- derstand how chirality may aÃect the phase diagram of metry of the lattice of chains. We identify these two these systems. We have investigated how the symme- components of H;„t„by first writing the potential en- try and strength of the crystal field (the precise meaning ergy of in-plane nearest-neighbor interactions as a sum of the crystal field will be given later) affect the nature of pair potentials, and expand the monomer mass densi- of the phase diagram of three-dimensional systems con- ties in multipoles. ~4 To first order we obtain a term pro- — sisting of arrays of helical XY spin systems, both chiral portional to cos(8, ~ 8, ~'). Averaging over the angles and nonchiral. Preliminary results on the initial order- and summing over nearest neighbors removes the angular disorder transition for chiral systems in the limit of very dependence of all terms up to nth order, where n is the large crystal-Beld strength, appropriate for polymers, symmetry of the chain packing, i.e., n = 6 for hexagonal were published earlier. In the present paper, we present packing and n = 4 for a square lattice. To nth order, we a more complete and general account of both chiral and find a term proportional to cos(n8;, ~). This is the term nonchiral systems. Our main result is that for chiral he- that has the symmetry of the lattice. The Hamiltonian lices in crystal fields with twofold symmetry, for exam- is then ple, arrays with an orthorhombic structure, the disorder- — — order transition dramatically changes character as a func- H = NHO+p) cos(n8, ,) 2J~ ) cos(8, , 8, , ). tion of crystal-field strength. The transition changes from a high-temperature paramagnetic-incommensurate phase transition to a paramagnetic-ferromagnetic phase (2.1) transition. This crossover occurs for all values of the Here N is the number of chains, and j') means that preferred pitch of an isolated helix (although the exact (j, the summation extends over nearest-neighbor chains location in the phase diagram of the crossover depends j and j'. The parameter is a measure of what we shall on this The crossover implies the existence of a p pitch). here call the crystal-field strength. Note that both the Lifshitz point in the phase diagram. If the crystal field value of as well as the symmetry of the crystal field can has higher symmetry than twofold, on the other hand, p be affected by the application of, for example, isotropic the order-disorder transition occurs (almost) always s to pressure and uniaxial stress, as well as other external an initial incommensurate phase and there is no Lifshitz fieMs.
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