First-order Commensurate-incommensurate Transition Research Articles

Unique Helical Magnetic Order and Field-Induced Phase in Trillium Lattice Antiferromagnet EuPtSi

Magnetic transition phenomena in cubic chiral antiferromagnet EuPtSi with $T_{\rm N}$=4.0~K were investigated by means of single crystal neutron diffraction. At 0.3~K in the ground state, magnetic peaks emerge at positions represented by an ordering vector ${q}_{1}$=$(0.2, 0.3, 0)$ and its cyclic permutation. Upon heating, an additional magnetic peak splitting with hysteresis was uncovered at around $T^*_{\rm N}{\sim}$2.5~K, indicating the presence of a first-order commensurate-incommensurate transition with ${q}^*_{1}$=$(0.2, 0.3, {\delta})$ (${\delta}_{\rm max}{\simeq}$0.04) at $T^*_{\rm N}$. A half-polarized neutron scattering experiment for polarization parallel to the scattering vector revealed that polarization antiparallel to the scattering vector has stronger intensity in both magnetic phases. This feature clarifies the single chiral character of the helical structure with moments lying perpendicular to the ordering vector in both ordered states. Under a vertical magnetic field of 1.2~T for ${B}{\parallel}$[1,1,1] at 1.9~K entering into the so-called $A$ phase, magnetic peaks form characteristic hexagonal patterns in the equatorial scattering plane around nuclear peaks. An ordering vector ${q}_{A}{\simeq}({\pm}0.09, {\pm}0.20, {\mp}0.28)$ of the $A$-phase has similar periodic length as $q_{1}$, and could be the hallmark of a formation of skyrmion lattice in EuPtSi.

Ordering of rare gas films on a decagonal Al–Ni–Co quasicrystal

This paper reviews recent progress in the study of rare gas films on quasicrystalline surfaces. The adsorption of Xe on the 10-fold surface of decagonal Al–Ni–Co was studied using low-energy electron diffraction (LEED). The results of these studies prompted the development of a theoretical model, which successfully reproduced the thermodynamic parameters found in the experiment. Grand canonical Monte Carlo (GCMC) simulations for Xe-produced structures that agreed with the experimental observations of the adsorption structures and provided a deeper insight into the nature of the ordering. A first-order commensurate–incommensurate transition, which involves a transition from a quasicrystalline five-fold structure to a periodic hexagonal structure, was discovered and characterized for the Xe monolayer. The five rotational domains of the hexagonal structure observed in the LEED study were shown in the GCMC study to be mediated by pentagonal defects, which are entropic in nature, and not by substrate defects. The GCMC study found an absence of any such transition for Kr, Ar and Ne on the same surface. A detailed analysis of this transition led to the conclusion that the formation of the hexagonal layer depends on matching the gas and substrate characteristic lengths.

Kinetic process of formation of discommensuration lattice in the first-order commensurate-incommensurate transition

Abstract The stability of the discommensuration lattice (DC lattice) and the kinetic process of the first-order commensurate-incommensurate transition (C-IC transition) have been studied theoretically and experimentally. At an early stage of the C-to-IC transition, the crystal is gradually covered with the clusters of DCs. After the crystal is covered with a DC lattice, its lattice spacing narrows to the final equilibrium value. At an early stage of the IC-to-C transition, the DC lattice remains to cover the crystal and only its lattice spacing widens. After the spacing has widened to the threshold value, the commensurate phase comes to appear and finally covers the whole crystal. This peculiar feature of the phase transition arises from the nature of the order parameter of the incommensurate system.

Kinetic Process of the Incommensurate-to-Commensurate Phase Transition in Rb2ZnCl4

The kinetic process of the incommensurate-to-commensurate phase transition in Rb 2 ZnCl 4 has been investigated by the measurements of the time evolution of polarization and dielectric constant during the field-induced transition. The following picture of the phase transition has been obtained. Under a sufficiently strong dc field, the discommensurations (DC's) are annihilated successively and the number of DC's decreases. The lattice spacing increases and at the same time the DC lattice becomes considerably disordered. When the lattice spacing has increased to a certain threshold value, regions of the commensurate phase start to appear, they grow and finally cover the whole crystal. The above picture agrees with the result of theoretical study based on the free energy of a system undergoing a first-order commensurate-incommensurate transition.

Time Evolution of Discommensuration Lattice During the First-Order Commensurate-Incommensurate Transition

On the basis of the free energy of an incommensurate system, a qualitative discussion is made of the time evolution of the spacing of discommensuration lattice during the first-order commensurate-to-incommensurate transition. It is shown that in the early stage of the transition when both phases coexist, the discommensuration lattice has a wider spacing l than the spacing l e in the final equilibrium state, and that after the crystal is covered with a lattice with l , the spacing is narrowed to l e by further nucleation of discommensurations.

On the influence of the reservoir and of the shape of the Fermi surface on the phase diagram of two-band models for Cr alloys

The Landau expansion around the triple point for a class of mean-field two-band modes including Rice's (1970) unequal-sphere model and the octahedron model of Shibatani et al (1969) is studied with the infinite-lifetime approximation and neglecting magnetostriction. It is shown that, when the reservoir is considered correctly, with reasonable values of the parameters one can obtain a phase diagram in agreement with experiment, i.e. a narrow first-order transition from the paramagnetic to an incommensurate antiferromagnetic phase for nearly pure Cr and a second order paramagnetic to commensurate phase transition accompanied at lower temperature by a wide first-order commensurate-incommensurate transition for Mn-rich Cr alloys. The temperature dependence of the wavevector in the incommensurate phase is also shown to be in qualitative agreement with experiment.

Discommensuration structure in a second-stage SbCl 5-graphite intercalation compound

Electron diffraction studies on the second-stage SbCl 5-graphite intercalation compound based on single crystals of Kish graphite are reported. At T cl (=210 K) the transition from the low-temperature glassy phase to the commensurate phase with (√7 × √7)R 19.11° structure occurs. At T cu (=230 K) the first-order commensurate-incommensurate transition occurs. The in-place structure above T cu is described in terms of the discommensuration structure in which the (√7 × √7)R 19.11° superlattice is modulated by the honeycomb domain structure.

Thermodynamic properties of Rb 2ZnCl 4 associated with three phase transitions

Transitions were observed by heat capacity measurements at 74.6 K, 195.2 K, and 303 K. They are a soft mode transition ( ΔH t = 30 J mol −1, ΔS t = 0.42 J K −1 mol −1), a first-order commensurate-incommensurate transition ( ΔH t = 6.2 J mol −1 ΔS t = 0.032 J K −1 mol −1), and a second-order incommensurate-normal transition, respectively.

Band-Structure Effects in Itinerant Antiferromagnetism

The effect of imperfect nesting of the band structure on itinerant antiferromagnetism is considered. A model band structure with spherical electron and hole pockets of unequal radii and a nonmagnetic reservoir is examined. A first-order commensurate-incommensurate transition is found, but the paramagnetic transition remains second order. The influence of generalized Kohn anomalies on the transition is investigated, and it is found that though they lead to anomalous behavior in the limit of vanishing antiferromagnetism, there is no change in the order of the transition.